Transpiration efficiency of tropical maize (Zea mays L.)

Transcription

1 Reserch Collection Doctorl Thesis Trnspirtion efficiency of tropicl mize (Ze mys L.) Author(s): Cmp, Krl-Heinz Publiction Dte: 1996 Permnent Link: Rights / License: In Copyright - Non-Commercil Use Permitted This pge ws generted utomticlly upon downlod from the ETH Zurich Reserch Collection. For more informtion plese consult the Terms of use. ETH Librry

2 Diss. ETH No Trnspirtion Efficiency of Tropicl Mize (Ze Mys L.) A disserttion submitted to the Swiss Federl Institute of Technology Zurich for the degree of Doctor of Nturl Sciences presented by Krl-Heinz Cmp Dipl. Ing. Agr., Universitt Kiel born Jnury 1,1964 in Rtingen (Germny) ccepted on the recommendtion of Prof. Dr. Peter Stmp Dr. Urs Schmidhlter Dr. Christin R. Jensen Zurich 1996

3 Contents Generl Introduction Drought tolernce nd trnspirtion efficiency Outline of the thesis 2 Estimtion of Mize Cnopy Trnspirtion Bsed on Porometric Mesurements 21 Abstrct 2 2 Introduction 2 Mterils nd Methods Technicl equipment used in field nd growth chmber expenments Growth chmber experiments 5 2 Field experiments Results nd Discussion Devitions in trnspirtion rtes between cuvette enclosed nd non-enclosed mize leves Temperture effects Boundry lyer effects A smpling design for porometer mesurements to compre cultivrs nd extrpolte to cnopy trnspirtion in field experiments Contribution of dxil nd bxil lef surfces to whole lef trnspirtion Verticl distribution of trnspirtion rtes Diurnl course of trnspirtion rtes Comprison of porometriclly derived trnspirtion rtes with reference methods in growth chmber nd field experiments Vribility of trnspirtion rtes nd stomtl conductnce in the field nd limits to detect genotypic differences Conclusions 2 Minerl Element Content in Lef Tissue s n Indictor of Trnspirtion nd Trnspirtion Efficiency in Mize with Prticulr Considertion of Silicon 25 1 Abstrct 25 2 Introduction 25 Mterils nd Methods 27

4 ..1 Growth conditions nd experimentl lyout Anlysis of minerl elements 28.4 Results Pot experiments Field experiments 1.5 Discussion 4 4. Growth, Stomtl Conductnce, Trnspirtion nd Trnspirtion Efficiency of Tropicl Mize Cultivrs nd Responses to Drought Stress t Vegettive Stges Abstrct Introduction Mterils nd Methods Lyout of the experiments Genotypes Soil Meteorologicl dt Culturl prctices Soil wter prmeters Plnt prmeters Dry mtter ccumultion nd grin yield Stomtl conductnce nd trnspirtion rtes Sttisticl nlyses nd clcultions Results Totl shoot biomss nd shoot biomss ccumultion Grin yield nd hrvest index Stomtl conductnce during vegettive growth Response of stomtl conductnce to soil wter limittion Response of stomtl conductnce to tmospheric humidity Genotypic vribility in stomtl conductnce Cnopy trnspirtion Trnspirtion efficiency Trnspirtion efficiency nd wter limittion Genotypic vribility for trnspirtion efficiency Reltions between trnspirtion efficiency, stomtl conductnce nd yield prmeters Discussion Shoot biomss nd grin yield Stomtl conductnce Cnopy trnspirtion Trnspirtion efficiency under drought 74

5 5. Diurnl nd Sesonl Chnges of Lef Wter Sttus nd Reltions to Stomtl Conductnce Abstrct Introduction 77 5 Mterils nd methods Lyout of the experiments Genotypes 79 5 Lef wter prmeters Results Sesonl course of the wter sttus of trnspiring leves Genotypic vribility in lef wter reltions Diurnl course of lef wter reltions, stomtl conductnce nd trnspirtion rte Discussion Generl Discussion nd Conclusions Summry References 100 Appendix 110 Zusmmenfssung Acknowledgements Curriculum Vite

6 1 Generl Introduction 1.1 Drought Tolernce nd Trnspirtion Efficiency Insufficient wter supply is one of the mjor limittions to mize production in the tropicl nd subtropicl regions, nd the use of dpted vrieties my be the only ffordble option for smll frmholders on rmfed cultivted res In the semind regions unpredictble spells of drought occur frequently during the growing seson, hence, drought escpe by choice of sowing dtes or erly mturing vneties is not suitble s e g in the Mediterrnen climte with typiclly terminl drought events In the rnge of physiologicl trits relted to the bility of crops to cope with wter stress wter-use efficiency, s n obvious determinnt of growth with limited wter supply, hs received erly scientific interest (see Bnggs nd Shntz 191) While wter-use efficiency in hydrologicl or gronomicl mening is commonly defined s the rtio of bove-ground biomss ccumultion to the totl wter input, we will refer in the following to trnspirtion efficiency in physiologicl mening s the rtio of boveground biomss ccumultion to the mount of wter trnspired The efficiency of wter use is of prticulr relevnce in non-irrigted crops but nonetheless incresingly emphsized in irrigted lnd with respect to ecologicl nd economic effects like groundwter depletion nd incresing pumping energy costs Wter-use efficiency might be enhnced by gronomic mens such s, plnting dtes, irrigtion technique nd scheduling, nutrient supply etc, but s well s by breeding, since genotypicl vrition for trnspirtion efficiency hs been shown in different species (see Boyer 1996) Significnt genotypic vrition for trnspirtion efficiency in mize ws found either with gs-exchnge mesurements or grvimetric determintion of trnspirton in green-house studies (Sobrdo 1990b, Sin 199) In the ltter study trnspirtion efficiency under wter stress vried between 2 nd 4 2 g kg 1 within 12 vrieties nd showed high nd positive correltion to the dry weight ccumultion This investigtion hs been done with potted mize seedlings Due to methodologicl problems, little informtion is currently vilble bout genotypic vrition for trnspirton efficiency nd the relevnce for drought tolernce under field conditions Hence, we investigted in this thesis two spects, which hve to be considered for the improvement of drought tolernce in tropicl mize by enhncing trnspirtion efficiency (i) to find relible screening methods for trnspirtion efficiency suitble for field expenments nd (ii) to determine if there is genotypicl vribility for trnspirtion efficiency worth of being included in breeding progrms 1

7 1.2 Outline of the Thesis Trnspirtion is strongly controlled by stomtl conductnce t the lef level Due to the close link between crbon dioxide nd wter flux, stomtl conductnce will lso ffect trnspirtion efficiency At the whole-plnt nd cnopy level stomtl control of both is medited by erodynmic conductnces, which re prtly dependent on morphologicl chrcteristics, like cnopy structure, but minly on environmentl conditions However, stomtl conductnce nd stomtl response to wter deficits re potentil trits for selection in rnge of cultivrs At first, we will investigte the possibility nd relibility of cnopy trnspirtion estimtes bsed on porometnc mesurements of stomtl conductnce The grvimetnc determintion of trnspirtion in potted plnts nd the trnspirtion determined by soil wter blnce in field experiments will serve s reference methods (Chpter 2) In the next chpter, we will study the potentil of n integrtive pproch for estimting trnspirtion nd trnspirtion efficiency by n nlysis of minerl element contents in plnt tissue Bsed on the hypothesis tht minerl elements, in prticulr silicon, re tken up nd trnsported to the shoot by the trnspirtion strem, the concentrtion s the rtio of cumulted minerl uptke to biomss produced should be correlted to the trnspirtion efficiency (Chpter ) Eight tropicl mize vrieties, exposed to wter stress t the erly nd lte vegettive stge, were investigted in two yer field expenments during the dry seson in Thilnd Genotypic vribility nd dpttions to limited wter supply in growth, stomtl conductnce, trnspirtion, depth of wter uptke nd trnspirtion efficiency were evluted The prospects of selection for stomtl conductnce, depth of wter uptke nd trnspirtion efficiency for improving the drought tolernce in mize will be discussed (Chpter 4) Lef wter reltions re closely involved in the regultion of stomtl conductnce Genotypic vribility in the interdependences of lef wter sttus nd stomtl conductnce nd the dpttions under drought, e g in the osmotic potentil, will be exmined with respect to their significnce for improving drought tolernce of mize (Chpter 5) 2

8 2 Estimtion of Mize Cnopy Trnspirtion Bsed on Porometric Mesurements 2.1 Abstrct Trnspirtion efficiency s the rtio of biomss ccumultion to trnspirtion is n importnt plnt ttribute to produce high yield with limited wter supply, but sufficiently precise determintion of trnspirtion of different genotypes is difficult to chieve in field experiments. In growth chmber nd field experiments with wellwtered nd wter stressed mize plnts the estimtion of whole plnt nd cnopy trnspirtion, bsed on porometric mesurement of stomtl conductnce nd lef re determintion ws investigted. The porometriclly determined trnspirtion ws compred to grvimetric mesurements of trnspirtion of potted plnts. Reference trnspirtion in the field ws clculted from soil wter blnce, bsed on neutron probe mesurements of soil wter content. The devition in lef nd ir temperture due to cuvette enclosure of leves resulted in n overestimtion of cnopy trnspirtion of bout 10% in the field. Whole plnt trnspirtion in mize stnd could be described s function of whole plnt lef re, the noon trnspirtion rte of the youngest fully developed lef t the bxil lef surfce, nd coefficient describing the verticl distribution of trnspirtion rte nd lef re. The sphericl distribution of lef re in the cnopy nd the cuvette temperture effect cused some limittions in the estimtion of cnopy trnspirtion under field conditions. The vribility of trnspirtion rte ws high, with CV vlues of 10% in well-wtered plnts, incresing to more thn 0% in wter stressed plnts. Porometric mesurements my hence llow detection of genotypic differences in trnspirtion rtes in well-wtered plnts. For wter-stressed plnts, however, the method is not likely to detect potentilly importnt genotypic differences in trnspirtion nd, consequently, trnspirtion efficiency in common designs of field experiments. 2.2 Introduction Trnspirtion efficiency, the rtio of biomss ccumultion to the mount of wter trnspired, is n importnt plnt ttribute to produce high yield with limited wter supply. Theoreticl derivtion of its relevnce ws given by Pssiour (1977), who expressed yield s the result of the three trits: wter used, trnspirtion efficiency nd hrvest index. An increse in ny of these, presumbly independent fctors should result in better performnce of crops or vrieties under drought conditions. Severl uthors hve rgued tht there is little prospect in finding genotypic differences in trnspirtion efficiency within one species becuse of the close reltion between

9 crbon ssimiltion nd trnspirtion (Krieg 198b, Sinclir et l. 1984, Jones 199). In field experiments the smll but nevertheless potentilly importnt genotypic differences my be msked by the high vribility in the estimtes of trnspirtion. Over the previous yers crbon isotope discrimintion hs successfully been used to revel genotypic vribility for trnspirtion efficiency in species with C ssimiltion pthwy (Frquhr nd Richrds 1984, Hubick nd Frquhr 1989, Mrtin nd Thorstenson 1988, Ismil nd Hll 1992). However, for C4 plnts like mize the discrimintion is low nd precise method to screen for differences in trnspirtion in the field is still lcking. Porometry is widely used method to mesure trnspirtion rte nd stomtl conductnce in the field. Modern diffusion porometers re portble, fst nd esy to use. To minimize the effect of incresing humidity on the stomtl perture during the mesurement stedy-stte porometers hve been developed. These porometers blnce the trnspirtion with strem of dry ir into the cuvette, so tht n initilly set reltive humidity is mintined. Different designs of porometers hve been described nd possible errors due to misclibrtions nd opertionl problems hve thoroughly been discussed elsewhere (McDermitt 1990, Monteith nd Cmpbell 1988). Objective of this study ws to develop method to extrpolte porometric mesurements of stomtl conductnce to whole plnt nd cnopy trnspirtion. The method should be precise enough to revel genotypic differences in trnspirtion, which we expected to be importnt in the rnge of 5-10%. A high cpcity in order to compre resonble number of entries is required. The method should be pplicble in well-wtered nd wter stressed plnts, since the response in trnspirtion efficiency to limited wter supply might in prticulr express the genotypicl dptibility. We evluted the precision of estimted trnspirtion rtes with respect to the level of significnce for genotypic differences nd suggest smpling procedure to determine trnspirtion nd trnspirtion efficiency of mize vrieties in field experiments. 2. Mterils nd Methods 2..1 Technicl Equipment nd Procedures Used in Field nd Growth Chmber Experiments All porometric mesurements were conducted with LI-1600 stedy-stte porometer (Li-Cor, Lincoln, Neb. USA). Lef tempertures were mesured with thermocouples (lumel/chromel 0.2 mm), ir temperture nd humidity with portble thermometer/hygrometer device (Rotronic AG, Zurich, Switzerlnd). Thermometers nd thermocouples were equlized with temperture clibrtor (Jofr Clibrtor D40, Ametek Instruments, Frum, Denmrk). Lef boundry lyer conductnces within the cnopy were estimted using lef replics of double folded wet filter pper (Whtmn No.4) connected to wter reservoir nd mounted on tripod. Trnspirtion rte (mmol m"2 s'1) ws determined 4

10 hile* grvimetnclly nd boundry lyer conductnce gu (mmol m'2 s1) ws clculted from P E 9»-Te p, (2.1) where e^, ewa nd Pre lef nd ir wter vpour pressure nd tmospheric pressure (kp), respectively ir tempertures nd reltive ir humidity Lef nd ir wter vpour pressure ws clculted from lef nd enclosed lef segment ws determined nlogously Boundry lyer conductnce for the cuvette 2..2 Growth Chmber Experiments Mize plnts were grown with one plnt per 1 5 L pot up to the fifth to sixth lef stge in growth chmbers (400 umol m'2 s1 PPFD, 12 h photopenod, 24/20 C dy/night temperture, 50/80% dy/night rel humidity) The soil ws illitic-chlontic silt lom (fine mixed mesic Aquic Ustifluvent) (Schmidhlter et l 1994) For hlf of the pots wtering ws withheld nine dys fter plnting t the three lef stge to induce vribility in trnspirtion The pots were covered with polyethylene foil to void evportion from the soil Plnts were widely spced in the growth chmber to prevent grdients of light intensity nd humidity t different lef levels During porometric mesurements the exhled ir of the expenmenter ws ejected out of the growth chmber to prevent stomtl closure nd decresed trnspirtion rtes due to n increse in C02 concentrtion in the growth chmber (Cmp nd Schmidhlter 1994) Determintion of trnspirtion from dxil nd bxil lef surfces For the grvimetnc determintion of one-sided lef trnspirtion rtes, ll but one lef of mize plnts were wrpped with luminium foil to prevent trnspirtion We mesured then grvimetnclly the trnspirtion rte of the single uncovered lef nd fterwrds the trnspirtion of the lef with either one of the lef surfces seled with vrnish, ech period lsting for two to three hours The porometric mesurements were done for both lef surfces on the sme lef The dimensionless fctor Cb, relting the bxil trnspirtion rte to whole lef trnspirtion ws clculted from C* Li (2.2) where 4 nd i)re the reltive contribution of dxil nd bxil trnspirtion to whole lef trnspirtion rte Comprison of porometric nd grvimetric trnspirtion rtes Dunng penod of three to five hours trnspirtion rte of the bxil lef surfce 5

11 Et, ws mesured three times with the porometer on ll leves except the oldest two, which were too smll to cover the perture of the cuvette Additionlly, trnspirtion ws mesured grvimetnclly by weighing the pots t the beginning nd the end of the experiment Lef re ws determined from ruler mesurements of length nd mximum width of single leves, using fctor of 0 7 to clculte lef re This fctor ws determined in previous expenments under the sme growth conditions by destructive lef re mesurements, using lef re meter (LI-000 A, Li-Cor, Lincoln, Nebrsk, USA) An totl men of ll mesured trnspirtion rtes of bxil lef surfces per single plnt (9 to 12 mesurements three times on three to four leves) ws used to clculte whole plnt trnspirtion ccording to p/w - LAplnl Ct (2-) where Ep[nV E~^, LAplnt nd Cb re the trnspirtion rte of the whole plnt (mmol s"1), men trnspirtion rte of the bxil lef surfces (mmol m'2 s'1), whole plnt lef re (m2) nd the dimensionless coefficient Cb, relting the bxil lef surfce trnspirtion rte to whole lef trnspirtion rte 2.. Field Experiments Experimentl site nd lyout The field experiments were conducted t the Ntionl Corn nd Sorghum Reserch Center, Frm Suwn, Ksetsrt University in Thilnd (14 5 N lt) The climte in this re is semi-nd with low verge rinfll from November to Mrch The soil ws n ustic, isohyperthermic, kolinitic oxisol Cly content ws 56,85 nd 88% in 5,0 nd 60 cm, respectively (Neidhrt 1994) The plnt vilble wter ws 12 to 15 vol % in the rnge of soil mtric potentil from -5 to kp, s determined from wter retention curves (see Chpter 4, Fig 4 1) Four experiments were conducted during the dry sesons 199/94 nd 1994/95 Eight tropicl mize vrieties nd two wter regimes were rrnged in split-plot design with four replictions Detiled informtion bout the vrieties nd culturl prctises re given in Chpter 4 A brief description is given below Ech yer wter stress ws induced by withholding wter t the erly vegettive stge (erly stress experiments) nd in second experiment t the lte vegettive stge (lte stress experiments) The control plots received weekly furrow irrigtion of estimted 70 mm Observtions were strted fter the first irrigtion in the control, tht mens when wter ws withheld on stressed plots for t lest one week Plnt prmeters Six to twelve plnts per plot nd week were hrvested nd dry mtter weight of 6

12 " green leves nd stem/senescent leves were determined Lef re ws mesured with portble, non destructive lef re meter (LI-000 A, Li-Cor, Lincoln, Nebrsk, USA) on the hrvested plnts in one repliction Specific lef weight of green leves ws determined nd green lef re for the other replictions ws clculted bsed on this specific lef weight Porometric mesurements for genotypic comprisons were mde from to 1 00 h in 199/94 nd from to h locl time in 1994/95 Smpling techniques were s follows (S1) In 1994/95 we mesured on two genotypes in the erly stress nd on four genotypes in the lte stress experiment with six mesurements per plot in four replictions, four to six dys per week (S2) In the erly stress expenment 1994/95 six genotypes were mesured with two mesurements per plot in three replictions on four to six dys per week (S) In 199/94 we mesured eight genotypes with one repliction per dy t four to five dys ech week in the erly nd lte stress experiments The fctor repliction ws replced in this cse by repliction/dy (rep/dy) combintion were mde in ech plot Six mesurements on rndomly chosen plnts Lef boundry lyer conductnce ws mesured dily for the whole experiment during the period of porometric mesurements from to h, nd ws previously determined for the porometer cuvette Trnspirtion rte E (mmol m 2 s 1) ws reclculted from the stomtl conductnce gs to ccount for lef boundry lyer conductnce in situ gbl ccording to E _L.1. 9t,i 1 <»!. O p 9s ' (2-4) where gw g^ e^, ewa nd Pre lef boundry lyer nd stomtl conductnce (both in mmol m 2 s 1), lef nd ir wter vpour pressure nd tmosphenc pressure (both in kp), respectively Determintion of verticl distribution of trnspirtion rte nd lef re The verticl distribution of trnspirtion rte nd lef re ws determined in the field expenment t 52 DAE (dys fter emergence) The trnspirtion rte of ech lef position ws mesured porometriclly on 15 plnts of ech wter supply tretment rrnged in three replictions The lef re of ech lef position ws mesured destructively on six plnts per genotype nd wter supply tretment in four replictions An verge lef re distnbution of ll genotypes in the wter supply tretment ws used to clculte the dimensionless coefficient Cvd describing the verticl distribution of trnspirtion Whole plnt trnspirtion cn be Ep/n( described s function of whole plnt lef re M^,, the trnspirtion rte of the youngest fully developed lef 0nd the coefficient Cvd. This coefficient is the sum of the reltive trnspirtion rtes 7

13 ^0 weighted by the frctionl lef re for ll lef positions Eplnt " ^pfnl ^vd (2.5) with i L"plnl where Ere/i) nd LA(i) re trnspirtion rte of lef (i), reltive to trnspirtion rte on the youngest fully developed lef, nd lef re of lef (i), respectively Soil wter prmeters Soil mtnc potentils were monitored t 20, 40, 60 nd 80 cm soil depth using tensiometers Volumetric wter content ws mesured with neutron probe (Model 50DR, CPN Co, Mnnez, Cliforni, USA) in 20 cm increments down to 100 cm soil depths on 2 plots with one ccess tube per plot in the lte stress experiment 1994/95 Mesurements were mde three times per week in stressed plots, in wtered plots just before nd one dy fter irrigtion nd dditionlly t DAE 5 fter rinfll event Evportion ws estimted grvimetnclly with smll lysimeters (12 cm dimeter, 20cm depth), which received weekly 60 ml wter in the well-wtered plot tretment Deep percoltion ws estimted by monitoring the soil wter depletion on six experimentl plots, where wter loss by evpotrnspirtion ws prevented On this plots, the mize plnts where removed nd the soil surfce ws covered immeditely fter n irrigtion Soil wter content ws then mesured for the next five dys with the neutron probe Sttisticl nlysis of error mrgins Tbles for nlysis of vnnce including plot smpling re given below men squres were derived ccording to Schultz (1955) Expected Tble 2.1 Tble of nlysis of vnnce for weekly mens of trnspirtion rtes in 199/94, seprtely for ech wter tretment Source df MQ Expected men squre rep/dy (r) r-1 MQr O^ + no^+ngo2, genotype (g) 9-1 MQg n2 no2gr nr62g + + genotype*rep/dy (9-D(r-1) MOgr n2 no2gr + plnts within plots (n) (n-1)gr MQn On2 Totl ngr-1 8

14 ? Zl Tble 2.2 Tble of nlysis of vnnce for weekly mens of trnspirtion rte in 1994/95 seprtely for ech wter tretment with plot smpling The repeted mesurements re rrnged in split-plot design (genotype s minplot nd dy s subplot fctor) Source d.f. MQ Expected men squre repliction (r) r-1 MQr n + no2rd(g) + + r>gdc?r genotype (g) g-1 MQg n2 + no2rd(g) +nro2gd +ndd2rg + nrd^g repliction'genotype (r-1)(g-1) MQrg %2 + no2^) + ndtj2rg dy (d) d-1 MQd n2 + no2rd(g; + + "^d genotype'dy (g-1)(d-1) MQgd n2 + n^rdfg) + nr<^gd repliction'dy within genotype (M)(d-1)g MQrd(g) "n2 + no2rd(g) plnts within plots (n) (n-1)grd MQn On2 Totl ngr-1 According to equtions (2.7) nd (2.8) we determined the mrgins of error (Gomez nd Gomez 1984) Dg/94 nd Dg4/g5 for the estimted genotypic weekly men vlue of trnspirtion rte: *». - ( ^. ^ > (2.7) where D x mrgin of error for tretment men estimte s frction of men, totl men within the wter tretment,?.z2-(4*%*-^). ndr dr r (2-8) n number of mesurements per plot, Z stndrdized norml vrite t level of significnce, ' ^kj). % estimted components of vrince, r number of replictions (94/95) or number of rep/dy combintions (9/94), d g number of dys per week on which mesurements were mde, number of genotypes. The components of vrince were estimted ccording to restricted mximumlikelihood method (PROC VARCOMP, SAS Institute Inc. 1988). The error mrgin Dis relted to the lest significnt difference for genotypic differences ccording to: 9

15 '"g/2 /2 njij Jn _ X Zl2 q (2.9) where f^is the /2-percentile of the t-distnbution with the degrees of freedom of the respective error term ccording to the tble of nlysis of vrince 2.4 Results nd Discussion Devitions in Trnspirtion Rtes between Cuvette Enclosed nd Non- Enclosed Mize Leves Wter vpour loss from lef plced in the porometer cuvette is determined s the trnspirtion rte by mesuring the flow rte of dry ir, which is necessry to mintin constnt reltive humidity inside the cuvette The mbient reltive humidity is commonly used s the reference null-point Lef nd ir temperture nd reltive humidity re mesured nd wter vpour pressure is clculted for lef nd ir ssuming 100% humidity for the lef intercellulr spce Stomtl conductnce gs cn be clculted from the eqution (2 4) Devitions between the cuvette nd the nonenclosed lef environment in temperture nd humidity grdients, wind speed, nd eventully perceived rdition will led to errors when we extrpolte mesured trnspirtion rtes for non-enclosed leves Assuming tht stomtl conductnce is not ffected by plcing the lef into the porometer cuvette, trnspirtion rtes for non cuvette enclosed leves cn be reclculted from ir humidity, lef nd ir temperture nd boundry lyer conductnce for the non cuvette enclosed lef However, in field expenments monitoring of lef nd ir temperture nd humidity grdients independently of the porometer is imprcticl for huge number of mesurements Therefore quntittive informtion bout the possible devitions under field conditions is needed Altertions in stomtl perture, mostly s stomtl closure in response to the enclosure of the lef into the ventilted cuvette, hs been observed (McDermitt 1990) Stomtl closure is evidenced during the mesurement by the inbility to stbilize the flow of dry ir into the cuvette lthough the reference humidity is reched We observed stomtl closure for the cuvette enclosed lef of mize plnts occsionlly in growth chmbers, but seldom in the field The frequent experience of strong winds nd mechnicl stress in the field might cuse n dpttion The bility of hrdening in this respect hs not been systemticlly investigted, but for the growth chmber grown plnts the incresed ventiltion in the porometer cuvette ws probbly much more exceptionl thn for field grown mize plnts For the older nd mture leves in mize the higher frequency of hirs might lso be fctor to explin the lower sensitivity in comprison to growth chmber grown mize plnts Hirs re n effective wy to 10

16 increse boundry lyer thickness nd hence my reduce stomtl responses to fluctutions in humidity nd wind speed (Schuepp 199) Temperture Effects McDermitt (1990) showed reltive errors in estimted stomtl conductnce s functions of errors in lef nd ir temperture. Estimted stomtl conductnce is prticulrly susceptible to errors in the lef to ir temperture grdient. Therefore, temperture differences between cuvette enclosed nd non-enclosed leves were mesured under field conditions on severl dys during the experiment. Air nd lef tempertures were incresed by 2- C for porometer enclosed leves, with comprtively smll effects on the mesured lef to ir temperture grdient (Fig. 2.1 A, B, C). For severely stressed plnts, when leves were much wrmer thn the surrounding ir (AT > 2 C), the effective temperture grdient ws lowered fter enclosure into the porometer cuvette. Due to the higher tempertures in the cuvette the lef to ir wter vpour pressure grdient ws higher for cuvette enclosed leves. Independent of the wter supply the grdient ws overestimted by bout 10% using the tempertures mesured by the porometer (see Fig. 2.1 D). According to eqution (2.4) the trnspirtion rte is proportionl to the lef to ir wter vpour pressure grdient, hence we must expect for the investigted mesurements n overestimtion of the trnspirtion rte by the sme extent. Idso et l. (1988) reported much higher devitions of temperture grdients for cuvette enclosed nd non-enclosed leves nd the devition incresed with ir sturtion deficit. Our observtions were mde in rnge of sturtion deficits from 1.8 to.2 kp, but we neither detected significnt effect of the sturtion deficit in the ir nor of the lef to ir wter vpour pressure difference. McDermitt (1990) emphsized the possible wrming of the cuvette during continuous use in full sun. Before mesurements were tken the porometer cuvette ws llowed to equilibrte with ir temperture nd humidity within the cnopy. Nevertheless, the cuvette temperture ws considerbly incresed by rdition lod in comprison to the cnopy ir temperture. After equilibrtion no significnt rise of cuvette tempertures with operting time ws detected. Hence, we conclude tht the temperture devitions ffect the extrpolted trnspirtion rte for the non-enclosed lef environment, but do not distort the comprison of genotypes or tretments within the sme experiment. 11

17 A / / - /*. i i i Atr temperture cuvette < C) Lef temperture cuvette ( C) o c % rr 5= 2 8/ re "O o u n x ^ = 0 87 <D 1 * i i i Lef to tr temperture grdient in the cuvette ( C) Lef to irvpd in the cuvette (kp) o wter stressed well-wtered Fig. 2.1 Air temperture (A), lef temperture (B), lef to ir temperture grdients (C) nd wter vpor pressure grdients (D) of mize leves s mesured in situ nd enclosed in porometer cuvette in well-wtered nd wter stressed plnts Dshed lines indicte equlity; the solid line in (D) refers to the regression Boundry Lyer Effects Porometers re well ventilted to prevent tht differences in stomtl conductnce between plnts re conceled by high erodynmic resistnces. Lef boundry lyer resistnces cn be clculted from the energy blnce ccording to the Penmn- Monteith eqution (Monteith 1965), from mesurements of wind speed nd dimensions of the lef (Nobel 198), or empiriclly from trnspirtion with ief replics mde of wet filter pper. The wet filter pper method llows one to determine nlogously the erodynmic resistnces for the compred environments. 12

18 In growth chmbers we mesured boundry lyer conductnce for lef replics, which ws less thn one sixth of the boundry lyer conductnce for lef segment inside the cuvette (growth chmber gu = 0. mol m"2 s"1, cuvette gw = 2 mol m'2 s"1). Estimted lef boundry lyer conductnce in the field ws in the rnge of 1 to 0.5 mol m"2 s"1 nd showed high vribility for the erly stge, but with the more closed cnopy in the lte vegettive stge both the men nd the dy to dy vribility decresed (Fig. 2.2). In contrst to temperture effects the different boundry lyer conductnces resulted in nonproportionl bis in extrpolted trnspirtion rtes. Stomtl resistnce nd boundry lyer resistnce re dditive Fig Dys fter emergence Lef boundry lyer conductnce in the cnopy of field-grown mize (see eq. 2.4). For wter stressed plnt with rther high stomtl resistnce the contribution of the comprtively low boundry lyer resistnce to the lef resistnce is low. Therefore, the trnspirtion rte for the non-chmber enclosed lef is for wter stressed plnt not severely ffected by the difference in the boundry lyer resistnce. However, for well-wtered plnts with stomtl resistnce being closer to the mgnitude of boundry lyer resistnce, the in situ trnspirtion rte differs from tht in the porometer cuvette. For both environments, growth chmber nd field, the overestimtion of trnspirtion in the observed rnge of stomtl conductnces incresed from 5% for severely stressed plnts to more thn 25% for highly trnspiring well-wtered plnts. Hence, the djustment of trnspirtion rtes nd reclcultion of stomtl conductnce is necessry, even if only reltive comprisons re intended nd not the extrpoltion of trnspirtion rtes. For the field environment, effective erodynmic resistnce to trnspirtion includes dditionlly to the lef boundry lyer resistnce n cnopy boundry lyer resistnce to wter vpour trnsfer. The estimtion of lef boundry lyer conductnce enbles one to correct the trnspirtion for the pthwy between lef nd cnopy ir. The extrpolted trnspirtion to cnopy level is vlid under the ssumption of constnt cpcitnce of the cnopy for wter vpour, indicting tht the flow through the lef boundry lyer nd cnopy boundry lyer is continous A Smpling Design for Porometer Mesurements to Compre Cultivrs nd Extrpolte to Cnopy Trnspirtion in Field Experiments For comprisons of genotypes the environmentl effects on trnspirtion rtes 1

19 growth - growth must be minimized Therefore, we chose the trnspirtion rte for period round noon (12 00 to h) of non-shded lef segment on the bxil lef surfce of the youngest fully developed lef (lefpos=0) s the definition of the reference lef segment Cnopy trnspirtion might be described bsed on this well-defined trnspirtion rte ssuming interreltions between trnspirtion from different lef surfces, for different levels in the cnopy, nd for different times of the dy The crop nd experimentl site specific coefficients of the bove reltions were determined with respect to eventul modifictions under wter stress Trnspirtion of genotypes might differ becuse of different trnspirtion rtes on the reference lef segment, but lso becuse of differences in sphencl lef ngle distribution or rtio of dxil to bxil trnspirtion Although these differences eventully influence the reltion between mesured lef segment nd cnopy trnspirtion, the coefficients for upscling were not systemticlly investigted for genotypic differences Contribution of Adxil nd Abxil Lef Surfces to Whole Lef Trnspirtion The contribution of dxil nd bxil lef surfces to whole lef trnspirtion re bout one third nd two thirds, respectively (Tble 2 ) As n verge Cb (eq 2 we used 1 5 in our clcultions The porometric mesurements showed somewht lower contribution of the dxil lef surfce in comprison with the grvimetric method Tble 2. Contribution of dxil nd bxil lef surfce to whole lef trnspirtion s mesured by porometry or grvimetry (in % ± stndrd devition) Method of determintion dxil lef surfce bxil lef surfce " grvimetry porometry - porometry - porometry chmber 4 8± ± 8 9 chmber 28 6±29 714±29 field 1 6 ± ± field, lef inversion* 417±60 58 ±60 the lef ws inverted for mesurement of the dxil lef surfce Porometnc mesurement of the dxil lef surfce requires either shding of the mesured lef segment with the porometer cuvette or inverting the lef, wht likewise mens chnge in the light interception of the mesured lef segment Trnspirtion rte of mize leves decresed rpidly fter shding (in less thn 0 s) nd incresed within one minute fter incresing light intensity s could be shown grvimetnclly (dt not shown) This susceptibility of the mesured lef segment to chnges in light environment might explin the reduced contribution of the dxil lef surfce to whole lef trnspirtion if it is mesured in situ position, nd n incresed contribution if the lef is inverted Leves of control plnts, which hd hlf of the upper nd hlf of the 14

20 lower lef surfce seled, trnspired bout hlf s much s leves without ny seling (49 ± 1 5%, men ± one stndrd devition) Moreover, no deleterious side effects of the seling were found, since trnspirtion ws constnt for severl hours fter vrnishing The rtio of dxil nd bxil trnspirtion is well supported by ntomic investigtions of stomt frequencies nd dimensions of mize leves of comprble tropicl cultivrs (Sin 199) About 40% of the stomt were found on the dxil nd 60% on the bxil lef surfce with minor chnges under limited wter supply (42 nd 58%, respectively) Stomt were pproximtely 10% smller on the dxil lef surfce for both wter supply tretments Pospisilov nd Solrov (1980) concluded, tht the rtio of dxil nd bxil trnspirtion is not significntly chnged under wter stress for rnge of species Verticl Distribution of Trnspirtion Rtes In dense cnopy, grdients in light interception nd humidity will led to different trnspirtion rtes on different lef positions By compring the trnspirtion rtes t different lef positions, ccounting for the lef re t the respective position, we cn describe the verticl distribution of trnspirtion within cnopy Verticl distribution of trnspirtion rte nd lef re for mize plnts in drughted nd non-droughted cnopy is depicted in Fig 2 For both tretments the trnspirtion rte incresed with higher light interception t higher levels in the cnopy The increse ws more pronounced in well-wtered plnts compred to wter stressed plnts With incresing wter stress trnspirtion rte nd increse of trnspirtion rtes t higher lef positions decresed In severely stressed plnts trnspirtion rtes re very low except for the very youngest leves (dt not shown) The high trnspirtion rte of the youngest fully developed lef (lefpos=0) is due to the definition of the reference position s non-shded lef segment Clculted reltive trnspirtion rtes, relting trnspirtion rtes on ech lef position to the trnspirtion rte on the youngest fully developed lef, showed tht incresing senescence led to lower reltive trnspirtion rtes of the older leves, while reltive trnspirtion rtes were higher on the youngest leves Wter stress chnges the rchitecture of the cnopy by enhncing senescence of the older leves nd by depressing elongtion of the growing leves (Acevedo et l 1979) Thirteen dys fter withholding wter, the whole plnt lef re ws not very much reduced, except the lef re of the youngest leves (Fig 2 B) Chnges in the distribution of trnspirtion rte nd lef re together resulted in higher frction of whole plnt trnspirtion, contributed by leves t positions round the youngest fully developed lef However, the pttern of the verticl distribution of trnspirtion ws similr for both wter regimes (Fig 2 4) 15

21 10 I I, 1, 1, 1, Trnspirtion rte (mmol m s ') Lef re (cm 2 ) Fig. 2. Verticl distnbution of trnspirtion rtes (A) nd lef re (B) of well-wtered (ww) nd wter stressed (ws) field-grown mize plnts t DAE 52 For wter stressed plnts imgtion ws withheld since DAE 8 Error brs denote one stndrd devition Lef position 0 mrks the youngest fully developed lef.10 I i i i ' ' i I > ' i i i i Trnspirtion rte (mg plnt 1s1) Frction of whole plnt trnspirtion Fig. 2.4 Verticl distribution of bsolute trnspirtion rtes (A) nd expressed s frctions of whole plnt trnspirtion rte (B) in well-wtered (ww) nd wter stressed (ws) fieldgrown mize t DAE 52 For wter stressed plnts irrigtion ws withheld since DAE 8 Lef position 0 mrks the youngest fully developed lef 16

22 We determined Cvd of 0 72 (see eq 6) s men of both wter supply tretments (in well-wtered Cvd= 0 75 ± 0 05 nd in wter stressed plnts 0 69 ± 0 09, men ± one stndrd devition) t DAE 52, which ws in midst of the lte stress period Verticl distribution of lef re nd trnspirtion rte is dynmic nd hs to be considered with respect to the developmentl stge, nd intensity nd history of the wter stress The verge trnspirtion rte of wter-stressed plnts ws t this time decresed by 50% in compnson to the control coefficient my chnge Rochette et l With more severe stress situtions this (1991) compred different models for the upschng of stomtl conductnce to cnopy conductnce The best performing model included, besides the verticl distnbution of lef re in the cnopy, the ssumption of sphericl distnbution of lef ngles Cnopy trnspirtion is therefore likely to be overestimted, when the distribution of lef re with respect to the lef ngles is not ccounted for in the clcultion However, informtion bout the ctul distnbution, which is likely to vry between genotypes of erect nd lx lef position, is hrdly vilble Including the ssumption of spencl lef ngle distribution is thus not exspected to fortify the comprison of genotypes Diurnl Course of Trnspirtion Rtes Comprisons of trnspirtion rte nd stomtl conductnce for rnge of vrieties in field expenments re hmpered by fst chnges of tmosphenc conditions Most studies compring stomtl conductnce or gs-exchnge of genotypes or tretments in the field re conducted round mid-dy becuse light intensity nd temperture during this period of the dy is rther stble For both yers diurnl chnges of trnspirtion rtes, stomtl conductnce nd the meteorologicl prmeters rdition, lef nd ir temperture, nd humidity were recorded from DAE 15 to 60 with different intensities of wter stress to estblish reltion between the trnspirtion rte t noon nd trnspirtion rte integrted over the dy Typicl ptterns of the diurnl course of stomtl conductnce, trnspirtion rte, nd lef to ir wter vpour pressure grdient for well-wtered nd wter stressed plnts re shown in Fig 2 5 The highest stomtl conductnce ws observed in the morning when the wter vpour pressure difference from lef to ir ws still Stomtl conductnce decresed throughout the dy for wter stressed nd wellwtered plnts Trnspirtion rtes showed more symmetricl pttern, with the pek of trnspirtion occurring t noon Trnspirtion rtes of severely wter stressed plnts reched the mximum erlier nd decresed grdully throughout the dy courses hve been estblished for different genotypes low Diurnl but the pttern of stomtl conductnce throughout the course of the dy ws similr (compre lso Chpter 5) 17

23 to E Time of dy Time of dy well-wtered - 15 dys fter lst irrigtion s Q_ - dys fter lst irrigtion. > 5 Q Time of dy Fig. 2.5 Diurnl chnge of stomtl conductnce (A), trnspirtion rte (B) nd lef to ir wter vpour pressure difference VPD (C) of field-grown mize plnts with different wter supply. Error brs indicte one stndrd error of men (n=6) The whole dy trnspirtion rtes were linerly relted to the noon trnspirtion rtes. We found unique reltion independent of wter supply, genotype, ge of the crop nd yer (Fig. 2.6), which ws used to clculte dily integrtes of trnspirtion rte. No interction of diurnl courses nd cnopy rchitecture on trnspirtion hs been tested. Turner nd Begg (197) nd Turner (1974) observed diurnl chnges for leves in ll levels of the cnopy, but the rnge ws smller for the lower levels. Since the contribution of the lower lef levels to whole plnt trnspirtion ws reltively smll, the ssumption of no interction is not likely to produce big error in the clcultion of cnopy trnspirtion. 18

24 & to o CO 5 <" CO 1 CO O) O E T ~ «O) CD Trnspirtion rte t noon (mmol m 2 s 1) Fig 2.6 Regression between trnspirtion rte t noon nd integrted dily trnspirtion rte of well-wtered (ww) nd wter stressed (ws) mize t different dys (20 to 60 DAE) The experiments were conducted over two yers 2.4. Comprison of Porometriclly Derived Trnspirtion Rtes with Reference Methods in Growth Chmber nd Field Experiments Grvimetric nd porometric determintion of trnspirtion for mize plnts in growth chmbers greed well when in the clcultion ws ccounted for the differences in lef boundry lyer conductnce between porometer cuvette nd growth chmber (Fig 2 7) For field experiments the correltion of trnspirtion ssessed by soil wter blnce nd mesurements of noon trnspirtion rtes for seventeen-dy-period is depicted in Fig 2 8 Porometric trnspirtion is bsed on six mesurements per plot t ech of ten dys during this period The correltion reveled systemtic overestimtion with porometry Errors in the clcultion of the soil wter blnce, especilly the ssumptions bout wter uptke from the top soil, deep percoltion nd/or wter uptke from more thn 1 m depth, my hve contributed to the different estimtes of trnspirtion in the field Profiles of soil wter uptke indicted, t lest for one genotype in the stressed tretment, wter uptke from more thn one meter soil depth (compre Chpter 4, Fig 4 10) When we estimted the deep percoltion decrese of both volumetnc wter content nd soil mtnc potentil ws found for more thn five dys in the percoltion plots This indictes tht there is no well-defined field cpcity threshold in this soil 19

25 = y x r2 = 0.87"* Moreover, there ws no difference in the soil wter depletion under mize cnopy nd the covered percoltion plots for the first five dys fter irrigtion. For clcultion of the soil wter blnce we ssumed therefore, tht ll wter ' ' i_ Grvimetric trnspirtion rte (gh"1) Fig. 2.7 Trnspirtion rtes of mize plnts in Fig. 2.8 growth chmber s mesured by porometry nd grvimetry..' <5><r' y= 1.01 x o r2 = 0.62"' _i 1 i 1 i 1 i Trnspirtion by soil wter depletion (kg m"2) Cumulted trnspirtion of mize cnopy in the field for the lte stress period in 1994/95 s ssessed by porometry nd soil wter blnce. remining in the soil 24 hours fter irrigtion ws, lthough slowly drining, fully vilble for plnt wter uptke. This ssumption ws supported by the chrcteristics of the soil hydrulic conductivity. While hydrulic conductivity ws high in sturted soil (11 ± 9.7 cm dy'1; men ± one stndrd devition), it decresed rpidly when the soil moisture decresed (see Chpter 4, Fig. 4.2). The lck of well-defined field cpcity hs been observed in mny soils nd is common problem in the estimtion of evpotrnspirtion from soil wter records (McGown nd Wilims 1980). It is of prticulr relevnce for rther short term observtions with frequent nd high wter input, s the conditions hve been in the well-wtered tretment of our experiments. Neutron probe mesurements of soil wter depletion re commonly crried out with one ccess tube instlled per plot. The repeted mesurements on this tube describe minly the soil wter uptke of the two surrounding mize plnts, which hve to represent the whole plot. In contrst, porometric mesurements hve been crried out on 0 to 50 plnts throughout the experimentl period. Sptil heterogeneity in the soil wter depletion within one plot ws reveled by mesurements of soil wter potentil using tensiometers, where two for ech depth were instlled within distnce of 0.75 cm. The verged CV for the soil mtric potentil between these two djcent sites ws 1%. In clibrting the neutron probe, we found rther strong correltion of the count rtio to bulk soil density. Since not for every plot the bulk soil density ws mesured, n verging clibrtion curve for ll plots hd to be used. Most of the sctter in the 20

26 comprison of trnspirtion estimted by porometry nd soil wter blnce is therefore likely to be cused by the ltter method Vribility of Trnspirtion Rtes nd Stomtl Conductnce in the Field nd Limits to Detect Genotypic Differences Although we predefined the mesured lef segment with respect to time of dy, developmentl stge nd position of the lef in the cnopy, the vribility of trnspirtion rte nd stomtl conductnce of lef segments ws high, with CV vlues of 10% in well-wtered plnts, incresing to more thn 0% in wter stressed plnts This incresed vribility will cuse decrese in the gin of selection, when these trits will be selected for under wter stress The precision of the estimtes of plot nd genotypic men trnspirtion rtes depends on the smpling technique nd the experimentl lyout The number of entries tht cn be evluted for trnspirtion is limited by the required time for mesurements with rpidly chnging environmentl conditions in the field the desired precision of estimtes We Hence one hs to find n optimum for the number of entnes nd performed sttisticl nlysis to estimte the experimentl error vrince nd the vnnce between plnts within one plot determined the precision of the estimtes of genotype mens in different smpling designs, which were prtly chosen on the bsis of the first yer results, prtly becuse of brek-down of the equipment Since breeders re minly interested in reltive differences precision ws expressed s frction of the men Clcultions were conducted for ech wter tretment seprtely, becuse nonhomogeneity of vrinces ws found for the wter tretments For stomtl conductnce vrition within plots s well s experimentl error vrince ws higher in well-wtered tretments, while for the trnspirtion rte the higher vnnce within plots ws often found for wter stressed plnts We No heterogeneity of vrinces ws found for genotypes, indicting tht compnsons between genotypes cn be done with stndrd procedures on originl dt The error mrgin D, s proposed by Gomez nd Gomez (1984) describes the precision for the estimted mens, bsed on expenmentl dimensions Error mrgins of men trnspirtion rtes re given for ll experimentl periods nd different smpling dimensions in Tble 2 4 An error mrgin of 10% mens tht with 95% probbility the popultion men trnspirtion rte of the genotype is within rnge of ± 10% of the estimte The LSD for the comprison of two genotypes (in e g smpling design (S1), including four genotypes) is then bout 16 %, s pproximted from eqution (2 9) Precision of estimted men stomtl conductnce ws in generl lower nd showed similr trends for tretments nd expenmentl periods For wter stressed plnts the error mrgins were higher nd incresed with incresing stress intensity This effect ws pronounced for the lte stress expenments the frction of men ws not pnmnly due to incresing Decrese of precision, expressed s vnnces but due to 21

27 lte decresing mens of trnspirtion rte nd stomtl conductnce (see Chpter 4, Fig 4 5) The dimensions of smpling ffected the precision of estimtes for wter stressed plnts more strongly thn for well-wtered plnts Tble 2.4 Mrgins of error D^^g nd Dg/94 in percent of the estimted genotypic men trnspirtion rte for expenmentl penods nd different smpling dimensions in wellwtered (ww) nd wter stressed (ws) mize plnts smpling design wter tretment erly stress stress DAE (S1) 1994/95 n=6, d=4, r=4 WW WS (S2) 1994/95 WW n=2, d=4, r= WS DAE (S) 199/94 WW n=6, d/r=4 WS n = number of mesurements per plot d = number of dys per week, r = number of replictions (J) d/r = number of rep/dy combintions From the definition of the error mrgin D (eq 2 7 nd 2 8) it cn be seen tht the most efficient wy to optimize precision for genotypic comprisons of trnspirtion rtes is to increse the number of replictions for the experiment nd secondly the number of dys on which mesurements re conducted Increse of the number of mesurements within the plot is, s fr s we re mostly interested in precise genotype informtion, much less promising Expected error mrgins for the genotype mens of trnspirtion rte with different expenmentl dimensions bsed on the estimted components of vrince re shown in Fig 2 9 It is evidenced, tht for relible estimtes of cnopy trnspirtion efficiency which requires observtions for longer period of growth it is necessry to mesure frequently, t best every dy However, lso for experiments including six replictions the error mrgin for wter stressed plnts is not likely to be smller thn 14% The error mrgin for plot men trnspirtion rte is more dependent on the number of mesurements per plot If we intend to compre wter consumption on plot level, s we hve done in the comprison of soil wter blnce nd porometry, it however resonble to increse the precision by high number of mesurements per plot is 22

28 o ) c 01 to Number of mesurements per plot n Number of dys per week d Number of replictions r Fig. 2.9 Mrgins of error for estimted genotypic men trnspirtion rte for weekly penod in well-wtered (closed symbols) nd wter stressed plnts (open symbols) s function of (i) number of mesurements per plot with d=4 nd r=4 (circles), (n) number of dys per week d with n=4 nd r=4 (tringles) nd (in) number of replictions r with n=4 nd d=4 (squres) 2.5 Conclusions Porometnclly determined trnspirtion devited only under field conditions from the trnspirtion ssessed by reference methods Two resons for the porometric overestimtion of cnopy trnspirtion hve been identified in the clcultion, we did not tke ccount of the sphericl lef re distribution in the mize stnd nd the incresed temperture in the porometer cuvette Both re of prticulr relevnce for field expenments Heting of the cuvette ws insignificnt in the growth chmber with rther low rdition Relibility of the correction of trnspirtion rtes to the non-cuvette environment concerning the higher boundry lyer resistnce ws evidenced by the high greement between porometric nd grvimetric determined trnspirtion rtes of mize plnts in growth chmbers A gret frction of rdition in smll growth chmber hs been reflected t the wlls This should diminish the effect of the lef ngle distribution on trnspirtion rtes Hence, whole plnt trnspirtion cn be described with mesuring trnspirtion rtes on well-defined lef position, since consistent coefficients for the extrpoltion hve been found However, limittions exist under field conditions where we re likely to overestimte cnopy trnspirtion by extrpolting porometric mesurements Reltive compnsons of genotypes or tretments should nevertheless be relible if trnspirtion rtes re djusted to the erodynmic conductnces in the field 2

29 Porometnc mesurements llow the detection of genotypic differences in trnspirtion rtes in the mgnitude of 10% for well-wtered mize plnts However, for wterstressed plnts nd common designs of field expenments with 4 to 6 replictions, differences between genotypes must exceed 20% to be significnt, which might be beyond resonble expecttions With respect to the high vribility found for lef trnspirtion rtes, the porometric method is not likely to detect potentilly importnt genotypic differences in trnspirtion efficiency in field expenments 24

30 Minerl Element Content in Lef Tissue s n Indictor of Trnspirtion nd Trnspirtion Efficiency in Mize with Prticulr Considertion of Silicon.1 Abstrct An integrtive method of determining trnspirtion efficiency is still lcking prticulrly in C4 crops, where crbon isotope discrimintion is low. Therefore, the reltions between concentrtions of minerl elements nd HCI non-soluble sh (nhcish), nd trnspirtion nd trnspirtion efficiency were investigted in mize in pot nd field experiments. Mize plnts, grown with different wter supply, were nlyzed for minerl elements nd nhci-sh, s n indictor of Si. Trnspirtion ws mesured grvimetriclly for potted mize plnts nd porometriclly in the field experiments including four tropicl mize vrieties. In the pot experiment trnspirtion ws highly correlted with totl K nd Si content in the plnt, but mss flow could only be ssumed for Si. Trnspirtion efficiency ws only wekly correlted to the concentrtion of minerl elements in the pot experiments. A high, non-liner correltion ws obtined between nhci-sh concentrtion in leves nd trnspirtion rte during previous perid of wter stress in the field experiments. No genotypic differences were found in this reltion, wheres K nd C concentrtions vried genotypiclly nd trnspirtion rte ws highly correlted to the respective concentrtion only within the genotypes. In contrst to nhci-sh nd C, the concentrtion of K ws negtively correlted with trnspirtion rte. Trnspirtion efficiency ws negtively correlted to nhci-sh concentrtion on the bsis of wter tretment nd repliction mens. Conclusively, the concentrtions of most minerl nutrient elements in the plnt tissue re not suitble indictor of trnspirtion nd trnspirtion efficiency due to the non-homogeneous distribution in the soil, the selective nd specific uptke kinetics, nd genotypic differences in the uptke. However, the concentrtion of nhci-sh, implicitly Si, showed some potentil to describe trnspirtion nd trnspirtion efficiency in mize. Further evlution of the method is needed before it cn be used to evlute trnspirtion efficiency for broder rnge of mize genotypes..2 Introduction Estimtion of trnspirtion is essentil to describe wter uptke nd trnspirtion efficiency, nd both re mjor fctors for the evlution of drought tolernce in rnge of cultivrs. Porometric mesurements of trnspirtion re tedious, extremely timeconsuming nd difficult to extrpolte for longer experimentl periods. Soil wter 25

31 blnce or spflow mesurements re often imprcticl in field expenments with lrge number of entries Over previous yers extensive use hs been mde of crbon isotope discrimintion to estimte trnspirtion efficiency This method, which is integrtive over the growing period, hs reveled genotypic vribility for trnspirtion efficiency in severl plnt species (Frquhr nd Richrds 1984, Hubick nd Frquhr 1989, Mrtin nd Thorstenson 1988, Ismil nd Hll 1992) nd hs not been shown to be correlted to trnspirtion efficiency Discrimintion is low for C4 plnts A comprble integrtive pproch to crbon isotope discrimintion my be provided by the minerl element concentrtion in plnt tissue Mss flow nd diffusion re the min mechnisms for movements of nutrients from the soil solution to the root cell wlls nd to the ctul plce of ion uptke t the plsm membrne Minerl elements re further trnsported from the root to the shoot vi the trnspirtion driven mss flow Therefore n effect of trnspirtion rte on the minerl content in the shoot is likely nd my provide mens to mesure trnspirtion Ion uptke is chrcterized by selectivity nd certin minerls re tken up preferentilly, while others re discnmmted or lmost excluded (Mrschner 1995) Hence expected reltions of sh or minerl element concentrtion to trnspirtion depend on the plnt species specific kinetics of trnsport nd uptke nd ion Controversil results of correltions between sh or minerl element concentrtion nd trnspirtion or trnspirtion efficiency hve been reported Correltions between lef minerl content nd trnspirtion rtio were found in greenhouse studies with whet, sunflower nd sorghum (Msle et l 1992) For ot (Jones nd Hndreck 1965) nd whet (Hutton nd Nornsh 1974) high correltions between trnspirtion nd Si uptke or concentrtion were reported Mylnd et l (1991) showed prtil ctive uptke of Si for whet nd hence concluded, tht Si cnnot be used to estimte wter use in whet Stronger correltions between Si concentrtion nd trnspirtion efficiency of brley in the field, s compred to the greenhouse, emphsized environmentl effects on the chrcteristics of Si (Wlker nd Lnce 1991) uptke Since n integrtive method of determining trnspirtion efficiency is of prticulr importnce in C4 crops we investigted the potentil of sh nd minerl element concentrtion to indicte trnspirtion nd/or trnspirtion efficiency of mize under well-wtered nd wter stressed conditions in pot nd field experiments Results of porometnclly determined trnspirtion efficiency indicted no significnt genotypic vntion Therefore, environmentlly induced vrition in trnspirtion efficiency ws lso investigted 26

32 . Mterils nd Methods..1 Growth Conditions nd Experimentl Lyout Pot experiments Mize (cv. KTX 101) ws grown with one plnt per 1.5 L pots contining n illiticchloritic silt lom up to the fifth to sixth lef stge. These pots were either plced in growth chmbers (400 umol m"2 s'1 PPFD, 12 h photoperiod, 24/20 C dy/night temperture, 60/80% dy/night rel. humidity) or outdoor (except for riny dys, when they were plced in the greenhouse), where the minimum nd mximum dily ir temperture verged 12.1 nd 21.5 C nd the minimum nd mximum dily ir humidity verged 67 nd 9%, respectively. In ech environment wter ws withheld nine dys fter emergence for hlf of the pots. The pots were covered with polyethylene foil to void evportion. Trnspirtion ws mesured grvimetriclly. Above-ground biomss ws determined 18 or 22 dys fter emergence (DAE). Anlysis of minerl element content ws conducted seprtely for leves nd stem nd the verge concentrtion in the plnt biomss ws clculted. The environmentl conditions of growth were vried to crete vrition in trnspirtion nd trnspirtion efficiency. Only the results of the correltion nlysis which ws crried out on single plnt vlues, irrespective of wter tretment, growth environment nd ge, re reported. Field experiments The field experiments were performed t the Ntionl Corn nd Sorghum Reserch Center, Frm Suwn, Ksetsrt University in Thilnd (14.5 N lt.), on n ustic, isohyperthermic, kolinitic oxisol. More detiled informtion bout meteorologicl dt, culturl nd experimentl prctises nd genotypes re given in Chpter 4. A brief description of the experiments is given below. Two experiments were conducted during the dry seson 1994/95 with four replictions including two wter regimes rrnged in split-plot design. In ll experiments wter stress ws induced by withholding wter for pproximtely four weeks t the erly vegettive stge (erly stress, from DAE 9 to 7) or t the lte vegettive stge (lte stress, from DAE 7 to 61). Observtions were strted fter the first irrigtion in the control, when wter hd been withheld on stressed plots for t lest one week. The control plots received weekly furrow irrigtion of bout 50 mm. The plots included six rows, ech of which were 0.75 m prt nd the plot size ws 6 m2. All mesurements were restricted to the inner four rows. The experiments included two tropicl mize cultivrs in the erly stress experiment (KS 6 nd DK 888) nd four in the lte stress experiment (dditionlly Crgill 922nd Tuxpeno seq. Cg). Dry mtter weight of leves nd stems s well s lef re were determined weekly on 6 nd 12 well-bordered plnts for the well-wtered nd wter stressed tretment, respectively. Men lef re nd dry mtter ccumultion were clculted for weekly periods. Porometric mesurements of trnspirtion rtes were conducted from 12:00 to 14:00 h with LI-1600 stedy-stte porometer (Li-Cor, Lincoln, Nebrsk, USA). Only bxil trnspirtion rtes of the youngest fully developed lef ws 27

33 mesured. Six mesurements on rndomly chosen plnts per plot were performed on four to six dys per week nd verged to weekly men trnspirtion rtes. Cnopy trnspirtion ws clculted s previously described (Chpter 2). A weighted men trnspirtion rte E (mmol m'2 s"1) for the whole experiments, consisting of severl weeks were clculted for ech genotype nd wter tretment to tke into ccount the development of lef re nd trnspirtion rte in time:., n LA E-i-&! *> <-1) where LA nd is the lef re nd trnspirtion rte of week /. The weight for ech weekly men trnspirtion rte ws the rtio of the lef re of the respective week to the lef re t the end of the experimentl period...2 Anlysis of Minerl Elements Pot experiments C, Cu, Fe, K, Mg, Mn, N, P, Si nd Zn concentrtions were mesured with n Inductively Coupled Plsm Atomic Emission Spectrometer, ICP-AES, (Liberty 200, Vrin Austrli Pty. Ltd., Mulgrve, Victori, Austrli) fter wet-shing with modified two-stge microwve digestion procedure (Fridlund et l. 1994). Lef nd stem biomsses were dried t 65 C nd finely ground. A 250 mg smple ws plced into HTFE-vessel. The orgnic mtter ws destroyed with 2 ml of concentrted HN0 nd 6 ml of 0% H202 solution in microwve system (MLS 1200 Meg, MLS GmbH, Leutkirch, Germny). These smples were llowed to cool down nd in second microwve digestion step silic ws dissolved with 0.75 ml of 40% HF. The smples were diluted to finl volume of 50 ml using 2000 ppm boron solution to neutrlize excess HF. Field experiments Lef nd shoot dry mtter were nlyzed seprtely for B, C, Cu, Fe, K, Mg, Mn, N, P, Si, Zn nd HCI non-soluble sh. The leves of plnts from the lte stress experiment were divided into the six youngest leves, ssumed to be minly grown under limited wter supply, nd older leves. The smples were dried t 70 C for 6 h nd finely ground. To void problems of dust contmintion in the smples, different nlysis of minerl elements ws conducted for field-grown plnts. A 500 mg dry mtter smple ws plced into pltin combustion vessel nd shed t 540 C for 6 h in muffle oven. The sh ws dissolved in 10 ml H20 nd 10 ml concentrted HCI, plced on hot wter bth, until the solution hd completely evported. The residue ws dissolved in 5 ml concentrted HCI nd 25 ml H20 nd filtrted through n shfree 28

34 filter The filter nd the combustion vessel were nnsed with wter nd the filtrte ws filled up to finl volume of 100 ml The filter ws shed in muffle oven for 2 h t 540 C nd the residue ws determined grvimetnclly s HCI non-soluble sh (nhci-sh) B, C, Cu, Fe, K, Mg, Mn, N, P nd Zn concentrtions were mesured by ICP-AES in the HCI-extrct To discnminte between endogenous morphous or polymenzed silic nd contmintions with dust in form of qurtz, concentrted NgCOg solution (10 g per 40 ml wter) in wter bth nhci-sh ws dissolved in hot, The solution ws filtrted, the vessel nd the filter nnsed nd the filtrte ws filled up to 100 ml fter dding 0 4 ml 10% NOH The Si concentrtion of this filtrte ws mesured by ICP- AES Contmintion ws determined s the residul between Si in the nhci-sh, nd Si mesured in the N2C0-extrct, nd ws found to vry between 0 1 nd 0 7% of the dry weight (DW) However, strong correltion for grvimetnclly determined nhcish nd Si, mesured in the N2C0-extrct, provides chep nd simple wy of estimting the concentrtion of Si in mize dry mtter smples (Fig 1) HCI non-soluble sh (% of DW) Fig.1 Reltion between grvimetnclly determined HCI non soluble sh nd Si concentrtion mesured in hot N2C0-extrct of field-grown mize leves.4 Results.4.1 Pot Experiments For potted plnts correltions of minerl element uptke, s the totl mount in the bove-ground biomss, nd minerl element concentrtion with trnspirtion nd trnspirtion efficiency were significnt for severl mcro- nd micronutnents (Tble 1) Since none of the micronutnents except B were relted to trnspirtion in the field expenment, the correltions for micronutnents re not shown The highest correltions to trnspirtion were found for Si nd K The liner reltion with non-significnt 29

35 intercept between trnspirtion nd totl Si in the plnt, indicted tht Si ws minly tken up pssively with the trnspirtion strem (Fig 2) Si concentrtion ws positively correlted with trnspirtion (Tble.1) In contrst, the positive nd significnt intercept in the reltion between trnspirtion nd K uptke indictes tht mss flow cnnot descnbe the uptke of K sufficiently (Fig 2) The sme ws found for C nd P (dt not shown) Tble.1 Correltion coefficients of trnspirtion nd trnspirtion efficiency with uptke (totl mount) nd concentrtion of minerl elements in the bove ground biomss nd the rtio of Si nd K concentrtion (Si/K) for potted mize plnts grown under different temperture, humidity, light nd wter supply (n=0, level of significnce is given in brckets) C K Mg p Si Trnspirtion 0 (0075) 0 7 (0 0001) 0 2 (0212) 0 77 (0 0001) 0 89 (0 0001) Trnspirtion efficiency 0 08 (0 67) -011 (0 57) 0 14 (0 46) 019 (0 1) -0 5 (0 058) concentrtion - C K Mg P Si Si/K Trnspirtion (0 0001) -0 8 (0 0001) 0 76 (0 0001) 01 (052) 0 68 (0 0001) 0 86 (00001) Trnspirtion efficiency 0 56 (0 001) 0 57 (0 001) 0 50 (0 001) 0 15 (0 44) -0 1 (0 10) (0 01) Fig..2 K nd Si content in the shoots of potted mize plnts in reltion to cumulted whole-plnt trnspirtion Trnspirtion (g plnt ) 0

36 The concentrtion of K in the plnt decresed with incresing trnspirtion Negtive correltions between minerl element concentrtion nd trnspirtion were lso found for C nd Mg Only wek correltions were found between trnspirtion efficiency nd the concentrtion of minerl elements Except for Si nd the rtio of Si to K, ll correltions were positive This result is contrdictory to the theoreticl expecttion.4.2 Field Experiments For the field experiments, only the concentrtion of minerl elements t the end of the expenmentl period nd not the uptke during the expenmentl period ws evluted Lef minerl element concentrtions were in generl more closely relted to trnspirtion thn minerl element concentrtions of stem dry mtter For the lte stress expenment correltions between trnspirtion nd minerl concentrtions were higher if only the upper leves were considered, those which were ssumed to be minly grown under limited wter supply (dt not shown) bsed on plot mens of minerl concentrtion in young Correltion coefficients, leves nd wter use prmeters for the lte stress experiment, re given in Tble 2 The highest, nd non liner, correltion ws obtined for nhci-sh nd trnspirtion rte (Fig ) The correltions of K nd C to trnspirtion were wek, if ll plots were compred, but were comprble to the correltions for nhci-sh when correltions where clculted seprtely for ech genotype (verged for the four genotypes r = for K nd r = CO y = ln(x) I 5 2 g 4 CO ^ = 074 v<*]gtf, #/ tf s A o V J* V V y v Mo rjb - 7* O V A Crgill 922 DK8S8 KS6 D Tuxpeno CO o o o B AA A $> 8 v * o A B O 0 B & S <s B B B 1 I J _i J 1 1 u 1 l, l 04 HCI non-soluble sh C Fig.. Concentrtion of HCI non-soluble sh, K nd C (in % of dry weight) of the six youngest leves of four field-grown mize genotypes t flowenng stge in reltion to the weighted men trnspirtion rte (in mmol m s 1) for previous period with differing wter supply 1

37 0 74 for C) Significnt genotypic differences were found for the concentrtion of the mcronutnents C, K, Mg nd P in the nlysis of vnnce when cumulted trnspirtion ws included s covnte, wheres no genotypic differences were indicted for nhci-sh or Si in this covnnce nlysis Hence the trnspirtion in different wter supply tretments for single genotype might be described by the concentrtion of K nd C, but cross different genotypes differences in minerl element concentrtion cnnot solely be ttributed to differences in trnspirtion The concentrtion of K tends to decrese with incresing trnspirtion, while C, B nd P concentrtions were incresed Significnt correltions were lso found for B nd P No significnt correltion to trnspirtion rte ws found for Cu, Fe, Mg, Mn, N nd Zn No significnt correltions between ny of the investigted elements nd trnspirtion efficiency were obtined in these clcultions bsed on plot mens Tble.2 Coefficients of correltion of trnspirtion, weighted men trnspirtion rte, nd trnspirtion efficiency to minerl element concentrtions nd the rtio of nhci-sh nd K concentrtion of the six youngest leves of field grown mize (n=2, four genotypes two imgtion regimes four replictions) concentrtion B C K Mg P nhcish nhcish/k Trnspirtion 0 47 (0 007) 0 14 (0 44) (0 0001) (0 146) 0 46 (0 01) 0 75 (0 0001) 0 76 (00001) Weighted men trnspirtion rte 0 8 (0 0) 0 1 (0 085) (0 0001) 0 05 (0 77) 04 (0 014) 0 8 (00001) 0 8 (0 0001) Trnspirtion efficiency 0 0 (0 88) (0 40) 0 27 (015) -018 (02) -012 (0 5) (0 2) 0 19 (0 2) The reltion between trnspirtion efficiency nd nhci-sh, bsed on genotype mens for well-wtered nd wter stressed plnts, is depicted in Fig 4 No significnt genotypic vrition ws found for trnspirtion efficiency nd nhci-sh (see lso Chpter 4) However, negtive reltion between trnspirtion efficiency nd concentrtion of nhci-sh ws indicted for the limited number of genotypes nd within the limited vrition in trnspirtion efficiency The CV for porometnclly estimted trnspirtion efficiency nd nhci-sh ws comprble (29 nd 28%, respectively) in wter stressed plnts, but ws considerbly smller for nhci-sh (2 nd 14%, respectively) in well-wtered plnts Higher vrition in trnspirtion efficiency occurred, when not genotype mens but the mens of experimentl replictions nd different tretments of both expenments were compred (Fig 5) In this cse high correltions were found for the nhci-sh concentrtion nd weighted men of trnspirtion Trnspirtion efficiency showed significnt, negtive correltion to nhci-sh concentrtion Especilly for the rtio of nhci-sh to K unique reltions to the weighted men trnspirtion rtes for both experiments were found 2

38 . y - _ - - y=-024x+4 " o well-wtered wter stressed ±:~ \ T erly stress Fig..4. Genotypic mens of trnspirtion efficiency nd concentrtion of HCI non-soluble sh of ll mize leves for the erly (left grph) nd the six youngest mize leves for the lte stress experiment (right grph). Error brs indicte stndrd errors of men (n=4). HCI non-soluble sh (% of DW) -. "/ = 0 59 X _ CO 'p r2 = 0 75 D y^* y^». y=142x+11 y< D K&S CPtf ^ = 087 ' *c S e D D O " ^^""""""^--i-. o*-*^. o ^^**~-^ - - Q^> ^^0 48 y 0 49 = x r* = 0 9 K t 1 ' HCI non-soluble sh (% of DW), i i i HCI non-soluble sh / K o erly stge ws erly stge ww lte stge ws lte stge ww Fig..5 Reltions between weighted men trnspirtion rte nd trnspirtion efficiency of wter stressed nd well-wtered field-grown mize plnts nd lef concentrtion of HCI non-soluble sh nd the rtio of HCI nonsoluble sh nd K concentrtion.

39 .5 Discussion Cn minerl element concentrtions reflect trnspirtion efficiency? A negtive liner correltion between concentrtion of minerl elements or the sum of minerl concentrtions nd trnspirtion rtio (rtio of wter trnspired to dry mtter, hence the reciprocl of trnspirtion efficiency) hs been suggested (Msle et l 1992), with the rtio of minerl uptke per unit of trnspirtion being the fctor of proportionlity However, constnt rtios of uptke per unit of trnspirtion cn not generlly be ssumed for minerl nutrient elements This prerequisite is especilly questionble when wter supply is limited, the environment where trnspirtion efficiency is of prticulr interest The reltions between minerl element concentrtions nd trnspirtion efficiency hve not yet been investigted under field conditions with limited wter supply We obtined for for none of the mcronutnent elements the hypothesized reltion in mize, comprble to results of Msle et l (1992) for sunflower, sorghum nd whet, wheres for nhci-sh concentrtion, implicitly Si, some evidence for reltion to trnspirtion efficiency ws found in the present experiments For severl resons the concentrtion of mcronutnents in the shoot is not suitble indictor of trnspirtion Trnsport nd uptke of nutrient minerl elements in soil nd plnt Mss flow cn ccount for nutrient trnsport to the plce of uptke t the cell plsm membrne nd for the trnsport vi xylem into the shoot but not for the uptke into the symplst Mss flow is the min trnsport mechnism to the root for C nd Mg (Brber 1995), s ws evidenced by the positive, liner correltion between C concentrtion nd trnspirtion in our experiment K nd P re mostly trnsported to the root by diffusion (Brber 1995) Decresing wter content in the soil decreses the effective diffusion coefficient by reducing the cross sectionl re of wter filled soil pores for ion diffusion The trnsport of K nd P is then not likely to be limited due to decresed trnspirtion rte but rther due to slower diffusion in the soil Moreover, the uptke of K is closely coupled with metbolic ctivity nd not directly influenced by wter flux cross the membrne (Hsio nd Luchli 1986) Differences in the effect of wter stress on K concentrtions were observed for the erly nd lte stress expenment In the lte stress experiment wter stress ws less severe nd the higher K concentrtions were found in the leves of stressed plnts This might indicte tht metbolic ctivity in the roots ws not s severely ffected s wter uptke nd growth Specific nd selective uptke kinetics thus hinder the use of K nd P concentrtion s n indictor of trnspirtion efficiency s evidenced by the insignificnt or even positive correltions with trnspirtion efficiency K is, for rnge of species, the mjor minerl 4

40 element ccounting for up to 50% of the sum of minerl element content In our experiments, the correltions for sh concentrtion nd trnspirtion or trnspirtion efficiency were very similr to the correltions with K (dt not shown) Therefore, we my hve to exclude sh content s n possible indictor of trnspirtion nd trnspirtion efficiency likewise Except B, C nd Si, ll minerl nutrients re known to be retrnslocted in the plnt nd the concentrtion in the roots is believed to be signl of shoot nutnent sttus nd my thereby regulte ion uptke of roots This cycling of nutrients decouples concentrtions in the shoot from trnspirtion nd uptke rte Trnsloctions will certinly be of mjor importnce for determintion of trnspirtion efficiency in mture plnts or t the reproductive stge The concentrtions of minerl elements in kernels hve therefore been correlted to crbon isotope discrimintion, s n indictor of trnspirtion efficiency in brley (Febrero et l 1994) However, expecttions of correltions re not strightforwrd since the pthwy of trnsloction to the kernels is in the phloem nd hence not dependent on trnspirtion Distribution of minerl elements nd wter uptke in the soil The concentrtions of the minerl nutrients in the soil solution vries becuse they depend on fctors like soil moisture, soil depth, ph, ction-exchnge cpcity, redox potentil, quntity nd distribution of orgnic mtter, microbil ctivity nd fertilizer ppliction (Asher 1978) For field, nerby the experimentl site, high grdients in ph, cly content nd nutrient concentrtions (42, 8 nd 8 mg kg 1 P, 98, 5 nd 2 mg kg 1 1 K, 180,126 nd 98 mg kg Mg, 42,16, nd 11 mg kg 1 Fe nd 0 27, 017,0 09 mg kg1 B in 5,0 nd 60 cm soil depth) were observed (Neidhrt 1994) In drying soils the wter is successively tken up from deeper lyers (see Chpter 4, Fig 4 10), therefore, the uptke of nutrients per unit of wter will decrese nd introduce bis in the estimtion of trnspirtion bsed on minerl concentrtions A negtive correltion between sh nd wter use efficiency ws reported by Mylnd et l (199) for field experiment with crested whet grss, where clones were plnted outdoors in pots to llow grvimetric determintion of trnspirtion been found with the roots exposed to the nturlly occurring therefore be questionble If this reltion would hve grdients in the soil might Since the minerl concentrtion in the shoot is to be used for the screening of genotypes with respect to trnspirtion nd trnspirtion efficiency, the genotypic differences in the reltions of C nd K concentrtion nd trnspirtion re of prticulr relevnce Estimtes of trnspirtion bsed on concentrtions of C nd K re likely to be confounded with genotypic differences in nutrient uptke, which re not relted to trnspirtion In contrst, concentrtion of nhci-sh did not vry between genotypes when differences in trnspirtion ws ccounted for 5

41 Silicon s n indictor of trnspirtion Due to the uptke of Si nd B s unchrged molecules (monosilic cid, boric cid) both re thought to move pssively cross biologicl membrnes The ccumultion of Si is gretly influenced by trnspirtion rte (Rven 198) In contrst to most minerl elements remobihztion of Si is very restricted (Smuels et l 1991) nd deposition of Si predominntly occurs t the sites of high trnspirtion, nmely in the lef epiderml cells (Rven 198) Si uptke nd concentrtion vnes mong plnt species (Jones nd Hndreck 1967, Tkhshi nd Miyke 1977, vn der Vorm 1980) Within the grmine, strong ccumultors hve been found with Si concentrtions of more thn 5% in the dry mtter (e g rice nd sugrcne), wheres for ot Si uptke ws perfectly relted to the expected uptke s ws clculted bsed on trnspirtion (Jones nd Hndreck 1965) Tkhshi nd Miyke (1977) found n verge of 1 96% Si in the dry mtter of 4 Siccumultor species In our expenments the mize leves hd Si concentrtions of 1 48 nd 1 04% under well-wtered nd wter stressed conditions Mize might therefore be described s wek Si ccumultor Correltions between Si uptke nd trnspirtion hve been found in field experiments with brley (Wlker nd Lnce 1991), whet (Hutton nd Nornsh 1974, Schultz nd French 1976) nd crested whetgrss (Mylnd et l 199) However, contrdictory results hve lso been reported for whet (Mylnd et l 1991) nd other species mostly in greenhouse nd pot studies (Msle et l 1992, Mylnd et l 199) Considerble sctter in the reltion between the porometnclly determined trnspirtion nd trnspirtion, estimted from the soil wter blnce, ws evidenced in our experiments (see Chpter 2) nd resulted in lower correltions between nhcish nd trnspirtion, when trnspirtion ws ssessed by the soil wter blnce compred to the porometnclly denved estimte These methodologicl limittions my lso hve contnbuted to the wek correltions between trnspirtion efficiency nd Si concentrtion found for whet in the field experiments of Schultz nd French (1976) Precise determintion of trnspirtion efficiency in the field is difficult to chieve, however pot studies cnnot simulte the field sitution, especilly under drought when genotypes vry in root prolifertion nd uptke of wter nd minerls Silicon uptke Mylnd et l (199) concluded high correltions between trnspirtion nd Si uptke to be ssocited with 1 1 rtio of Si concentrtion in soil solution nd xylem sp However, the necessry conditions for correltion between trnspirtion efficiency nd Si concentrtion re tht both the uptke rtio, s the rtio of uptke exspected from mssflow nd ctul uptke, nd the concentrtion in the soil solution re constnt For brley shift from ctive to pssive uptke of Si with incresing 6

42 trnspirtion rte cused by decresing tmospheric humidity ws found by Brber nd Shone (1966) in solution culture experiments nd similrly evidenced for soil-grown plnts by Wlker nd Lnce (1991). This shift in the uptke rtio might be vlid lso for mize, since in our pot study with rther low evportive demnd the correltion for Si concentrtion nd trnspirtion ws wek in comprison to the field, indicting tht Si ccumultion ws less dependent on trnspirtion. The low or insignificnt correltions between Si concentrtion nd trnspirtion rtio reported by Msle et l. (1992) were lso derived from greenhouse experiments with presumbly rther low wter vpour pressure deficits. The impct of trnspirtion rte might be explined by feedbck control of Si uptke. Low concentrtion of Si in the plnt, when Si uptke is low due to low trnspirtion, might induce n ctive uptke. For rice, Jones nd Hndreck (1967) suggested tht rther the Si trnsport to the shoot is dependent on trnspirtion rte thn the uptke. However, for the trget environment in the field we will most often find the required conditions of high evportive demnd. Silicon in the soil solution With incresing Si concentrtion in the solution the rtio of ctul to mssflowexpected uptke decreses in hyperbolic fshion (Jones nd Hndreck 1969, vn der Vorm 1980). A strong decrese for the uptke rtio ws found with low concentrtions, but with concentrtions of more thn 50 ppm Si(OH)4 in the solution the contribution of mss-flow uptke ws rther constnt for ll species except the strong ccumultor rice. Due to its bundnce in the soil the concentrtion of monosilic cid in the soil solution is presumbly less vrible thn for other nutrient elements. Reltively smll vrition in Si concentrtion of the soil solution t eight different field sites ws lso ssumed by Hutton nd Norrish (1974), who found for ll sites unique reltion of the Si concentrtion in whet husks to trnspirtion. While the solubility of morphous Si in wter is unffected in the rnge of 2 to 9 ph, concentrtions in soil solutions re lrgely controlled by ph dependent dsorption rections (Jones nd Hndreck 1967). Hence, vribility in the Si concentrtion cn not be excluded. No mesurements of Si concentrtions in the soil solution were mde in our study. Depending on the soil type the concentrtion of Si(OH)4 ws found to vry between nd 70 ppm in soil solutions for the rnge of 5 to 9 ph (Scheffer nd Schchtschbel 1992). Higher uptke of Si per unit of trnspirtion with incresing soil wter supply ws found by Mylnd et l. (1991) nd explined with higher Si concentrtion in the soil solution tken up by plnts of the wettest tretment. The overproportionl rise in Si concentrtion with incresing trnspirtion rte might lso be explined s the result of lower trnspirtion efficiency for well-wtered plnts, compred to wter stressed plnts, s ws shown in Chpter 4. If the specific lef weight nd rtio of lef to stem dry weight re considered constnt we my express the trnspirtion rte s the trnspirtion divided by dry 7

43 weight, which is the trnspirtion rtio. A rising trnspirtion rtio, hence decresing trnspirtion efficiency with incresing trnspirtion rte will result in n overproportionl rise in cumulted trnspirtion nd Si uptke per unit of dry weight nd hence overproportionl rise in the Si concentrtion. The vrition in Si concentrtion in plnts grown with different nitrogen or phoshorus supply were likewise explined by the effects of nutrient supply on trnspirtion efficiency (Jones nd Hndreck 1967). Both, the non-liner reltion between nhci-sh concentrtion nd trnspirtion rte (Fig..), nd the significnt correltions between trnspirtion efficiency nd nhci-sh concentrtion of different tretments (Fig..5) indicte, tht for mize Si concentrtions my provide mens of estimting trnspirtion efficiency. No genotypic vrition ws found neither for porometnclly determined trnspirtion efficiency nor for the Si concentrtion in leves. Further evlution of the method is needed, especilly with respect to Si concentrtions in the soil solution nd uptke kinetics in mize before trnspirtion efficiency cn be evluted for broder rnge of genotypes vi n nlysis of Si content. 8

44 Growth, Stomtl Conductnce, Trnspirtion 4 nd Trnspirtion Efficiency of Tropicl Mize Cultivrs nd Responses to Drought Stress t Vegettive Stges 4.1 Abstrct Stomtl conductnce, cnopy trnspirtion nd trnspirtion efficiency, s the rtio of bove-ground dry mtter ccumultion nd trnspirtion, were investigted for eight tropicl mize cultivrs under field conditions dunng the dry seson in Thilnd Responses to wter stress in either of these trits were nlyzed with respect to the potentil for improving drought tolernce in mize Field experiments were conducted in two yers, where wter stress ws imposed either t the erly or t the lte vegettive stge by withholding the weekly furrow irrigtion for period of four weeks Stomtl conductnce ws mesured frequently with porometer nd used to clculte the cnopy trnspirtion Trnspirtion efficiency during the vegettive growth ws clculted fter determintion of dry mtter ccumultion Additionlly, the soil wter depletion ws monitored t 20, 40, 60, 80 nd 100 cm soil depth with neutron probe mesurements Wter stress reduced bove-ground dry mtter yield t the end of the stress period to 28 nd 41 % of the control in the erly nd to 70 nd 82% of the control in the lte vegettive stge Grin yield ws less reduced by wter stress t the lte vegettive stge, wht ws concluded rther to be the result of lower stress intensity thn lower sensitivity to wter stress in lte vegettive stge compred to erly vegettive stge Hrvest index ws incresed by stress t the erly vegettive stge nd slightly decresed by wter stress t the lte vegettive stge Genotypic differences were significnt for dry mtter ccumultion nd grm yield but no genotype by wter supply interction week fter wter ws withheld Stomtl conductnce ws strongly depressed within one The initil decline in stomtl conductnce ws highly correlted to the soil mtric potentil t 20 cm depth for the erly stress experiments, nd to the soil mtric potentil in deeper lyers for the lte vegettive stge Despite of generlly high error vrinces genotypicl differences in stomtl conductnce were indicted in both irrigtion tretments nd growth stges Stomtl conductnce ws generlly low in KTX101, sensitive to drought in KS6 nd the lest ffected by drought in DK888 The mintennce of high stomtl conductnce ws beneficil for growth under wter stress Cumulted trnspirtion ws to very lrge extent determined by the differences in lef re nd only slightly ffected by genotypic differences in stomtl conductnces within the irrigtion tretments With incresing 9

45 stress intensity soil wter ws tken up succeedingly from deeper lyers nd genotypic differences were indicted for soil wter uptke long the profile The genotypic shift in soil wter uptke to deeper lyers under drought corresponded with the mintennce of stomtl conductnce nd trnspirtion under wter stress Trnspirtion efficiency ws incresed under wter stress with the increse being relted to stress intensity The highest trnspirtion efficiency ws found when trnspirtion ws decresed to bout 50% of the control No significnt genotypic differences were found in trnspirtion efficiency However, under well-wtered conditions, DK888 showed consistently the highest trnspirtion efficiency Correltions of genotype mens between trnspirtion efficency nd dry mtter ccumultion were insignificnt within the wter stressed tretment nd positive within the control Therefore, not trnspirtion efficiency but rther the cumulted trnspirtion, determined by the bility to explore the wter from greter soil volume ws the mjor chrcteristic of drought tolernce in mize However, for well-wtered conditions we do not hve ny evidence, tht high trnspirtion efficiency hs ny negtive effects on growth 4.2 Introduction Insufficient wter supply is one of the mjor limittions to mize production For mny frmers the use of drought tolernt cultivrs is the only ffordble option in drought-prone res Tolernce to drought events is governed by severl intercting physiologicl trits nd the contribution of specific trits hs to be considered with respect to the specific drought scenrio While for mediterrnen climte drought occurs mostly t the end of the growing seson, spells of drought re frequent in the semind subtropicl region throughout or prticulr t the beginning of the growing seson Sensitivity of grin yield to limited wter supply in mize is correlted to the growth stge, with drought t or ner nthesis being the most sensitive stge Westgte nd Boyer (1986) concluded tht physiologicl processes ssocited with nthesis nd erly grin development re especilly vulnerble to wter deficit However, the sensitivity of this stge might lso be explined by the coincidence of the pre-nthesis growth stge with the stge of mximum biomss ccumultion nd wter use, s ws concluded by Sinclir et l (1990) Biomss ccumultion during penods of wter shortge might be less reduced in vrieties which exhibit high efficiency of wter-use or re ble to increse the efficiency of wter-use with limited wter supply Genotypic vnbihty for wter-use efficiency of mize plnts ws shown in greenhouse experiments bsed on gs-exchnge mesurements (Sobrdo 1990b) or dry mtter ccumultion (Sin 199) The ltter found for twelve tropicl mize cultivrs strong positive reltion between wter-use efficiency nd dry mtter production under limited wter supply Due to methodologicl problems in evluting trnspirtion efficiency in the field, little is known bout the contribution of trnspirtion efficiency to drought tolernce of 40

46 vneties nd the reltion between trnspirtion-efficiency nd growth prmeters under field conditions Stomt hve key function in regulting the reltion between growth nd wter consumption According to their stomtl behviour, i e the sensitivity of responses to soil wter limittion in the soil, plnts my be chrcterized s wter spenders or wter svers In the context of the reltion between ABA concentrtion nd drought tolernce some uthors suggested sensitive stomt regultion to be mjor chrctenstic of drought resistnt genotypes (Pekic nd Qurne 1987) The inevitble cost of prtil stomtl closure will be lower crbon ssimiltion nd hence it is questionble if sensitive response of stomt, implying wter sving chrcteristic, is dvntgeous for gronomic crops Stomtl conductnce is likely to influence wter-use efficiency The expected effect on trnspirtion efficiency is contested nd depends on the rtio of stomtl nd non-stomtl limittions of photosynthesis (Sinclir et l 1975, Nobel 198, Kneg 198, Sinclir et l 1984, Ludlow nd Muchow 1990) The objectives of this study were (i) to determine stomtl conductnce nd trnspirtion rtes of tropicl mize cultivrs during vegettive growth under sufficient nd limited wter supply, (n) to estimte trnspirtion efficiency of field-grown mize plnts with sufficient nd limited wter supply, (in) to investigte genotypic vribility in stomtl behviour nd trnspirtion efficiency, (iv) to descnbe the reltion between stomtl behviour, trnspirtion efficiency nd growth prmeters nd the impct of drought on these reltions These informtions should llow one to evlute the potentil of improving drought tolernce by selection for specific stomtl behviour or improved trnspirtion efficiency 4. Mterils nd Methods 4..1 Lyout of the Experiments Four experiments were conducted during the dry seson 199/94 nd 1994/95 with four replictions including eight tropicl mize cultivrs nd two wter regimes rrnged in split-plot design In ll experiments wter stress ws induced by withholding wter for pproximtely four weeks t the erly vegettive stge (199/94 from dys fter emergence (DAE) 5 to nd 1994/95 from DAE 9 to 7) or t the lte vegettive stge (199/94 from DAE to 61 nd 1994/95 from DAE 7 to 61) lter referred to s "erly stress" nd "lte stress" experiments All plots received two (199/94) or three (1994/95) sprinkler irrigtions fter plnting until withholding wter for the drought tretment The tretments in 1994/95 were delyed for few dys to provide better seedling estblishment The lte stress expenment 1994/95 ws shortened to 17 dys to prevent influences of wter stress t flowering The control plots received weekly furrow irrigtion of bout 70 mm The plots included eight rows (199/94) nd six rows (1994/95) which were 0 75 m prt The plot size ws 6 m2 All mesurements were restricted to the inner six or four rows, respectively 41

47 4..2 Genotypes The expenments included eight tropicl mize cultivrs (open pollinted vrieties: two originting from the Ksetsrt University (KU), Thilnd KS 6 nd Suwn nd two from CIMMYT, Mexico. Tuxpeno seq. C0nd Tuxpeno seq C# four modern hybnds frequently used in Thilnd- Crgill 922, DeKlb 888, Hercules 40 (Cib-Geigy G5440) nd KU hybrid KTX Soil The field experiments were conducted t the Ntionl Corn nd Sorghum Reserch Center, Frm Suwn, Ksetsrt University in Thilnd (14 5 N lt.) The soil ws n ustic, isohyperthermic, kolinitic oxisol It is chrcterized by high cly content (56,85 nd 88% in 5,0 nd 60 cm, respectively) nd grdient in ph vlues (7 5, 6 8 nd 5. in 5, 0 nd 60 cm, respectively), probbly due to 20 yer long irrigtion with clcium crbonte ennched ground wter (Neidhrt 1994) Bulk soil density ws 1 27, 1 2,1 17 g cm' in 5,0 nd 60 cm soil depth for the field used in 199/94 nd 1 29, 1.25 nd 1 2 g cm' in 20, 40 nd 60 cm soil depth for the field used in the expenments 1994/ Meteorologicl Dt The expenmentl site hs semi-rid climte with low long-term verge rinfll from November to Mrch Minimum nd noon tempertures, humidity nd vpour pressure deficit t noon for the experimentl periods re given in Tble mm nd mm occurred on DAE 5 nd DAE 55 in 1994/95 Rinfll of Tble 4.1 Meteorologicl conditions (or the experimentl penods in both yers 199/94 DAE Rel humidity t noon (%) Air temperture t noon ( C) Vpour pressure deficit t noon (kp) Mm temperture ( C) /95 DAE Rel humidity t noon (%) Air temperture t noon ( C) Vpour pressure deficit t noon (kp) Mm temperture ( C)

48 Mulem 4..5 Culturl Prctices Agriculturl prctices were ccording to the locl stndrd. After ploughing, 60 kg N s mmonium sulfte, P205 nd KjO per h were brodcsted with mixed fertilizer ( ) nd incorported into the soil. In 199/94 ll plots of the lte stress experiment received 50 kg N h'1 in form of ure during the irrigtion t DAE. In 1994/95 residul N-content in the soil, three weeks fter emergence, ws high (120 kg N) nd no dditionl fertilizer ws pplied. Plots were prepred in the ridge nd furrow system with ridges being 0.75 m prt nd 0.25 m high. Two seeds were sown mnully per seed position, the distnce between two seed positions ws 0.2 m in 199/94 nd 0.25 m in 1994/95. After thinning to one plnt per seed position the plnt density ws 6.6 (199/94) nd 5. plnts per squre meter (1994/95). Chemicl weed control ws pplied with pre-emergence herbicides, trzin nd lchlor, t the rtes of kg h'1 nd L h"1, respectively. Therefter weed control ws crried out mnully. Monocrotophos (750 ml s 60 % solution) ws pplied ginst europen com borer (Ostrini nublis) nd frit fly (Oscinell frit) t the three-lef stge Soil Wter Prmeters Wter retention curves were determined on 100 cm soil cores in Richrds pressure plte pprtus (Fig. 4.1). The frction of plnt vilble wter ws found to be low (12 to 15 % in the rnge of soil mtric potentil from -5 to kp). Sturted hydrulic conductivity ws mesured with the constnt hed method (Klute nd Dirksen 1986) nd the unsturted hydrulic conductivity ws estimted using the Vn Genuchten - model (Vn Genuchten 1980), bsed on one-step pressure outflow experiments. The model prmeters sturted 6S the residul wter content 6r nd the empiricl fitting prmeters nd n were estimted using the computer Fig. 4.1 Wter retention curves for different soil depths of the experimentl fields E9 in 199/94 nd G11 in 1994/ Volumetric soil wter content 6 (err? cm ') 4

49 progrm SFIT (Kool nd Prker 1987) The reltive hydrulic conductivity ws expressed s function of the soil mtnc potentil Sturted hydrulic conductivity ws -10 cm dy1 nd reltive unsturted conductivity ws chrcterized by shrp decrese t high soil mtric potentils (Fig 4 2) Soil mtnc potentil during the experimentl periods in the field ws monitored with tensiometers t four depths (20, 40,60 nd 80 cm) except in erly stress 199/94, where no mesurements were tken t 80 cm soil depth Soil wter depletion could well be observed with tensiometers since more thn two thirds of the totl plnt vilble wter ws used before the soil mtric potentil exceeded -80 kp 25 cm soil depth 50 cm soil depth Fig 4 2 Unsturted reltive hydrulic conductivity of the soil in the expenmentl field E9 in 199/94 t 25 nd 50 cm soil depth estimted from one-step pressure outflow experiments Soil mtnc potentil (kp) The volumetnc wter content ws mesured with neutron probe (Model 50DR CPN Co, Mnnez, Cliforni, USA) in 20 cm increments down to 1 m soil depth on 2 plots with one ccess tube per plot for the lte stress experiment 1994/95 Mesurements were mde two to three times per week, in wtered plots before nd one dy fter irrigtion nd dditionlly fter the rinfll t DAE 54 Evportion ws determined grvimetnclly from smll lysimeters (12 cm dimeter, 20 cm depth), which were wtered ccording to the plot tretment Deep percoltion ws estimted by monitoring dily soil wter depletion fter n irrigtion on expenmentl plots, where plnts were removed nd the soil surfce ws covered to prevent wter loss by evpotrnspirtion 4..7 Plnt Prmeters Dry Mtter Accumultion nd Grin Yield In weekly intervls till flowenng (DAE 12,19,26,,40,47,54,61 in 199/94 nd DAE 16, 2, 0, 7, 44, 51 nd 61 in 1994/95) 6 to 12 well-bordered plnts per plot were hrvested nd prtitioned into green leves nd stems/senescent leves Dry weight ws determined fter drying t 65 C for 6 hours Lef re of green leves 44

50 ws mesured with portble, non destructive lef re meter device (LI-000 A, Li- Cor, Lincoln, Nebrsk, USA) on the hrvested plnts in one repliction Men specific lef weight for this repliction ws clculted, seprtely for the wter supply tretments, nd used to determine green lef re for the other replictions bsed on the green lef dry weight Totl shoot biomss, grin yield, hrvest index nd yield components (ers per plnt, kernels per er, kernel rows per er, 100-kernel weight) were determined on twelve well-bordered plnts per plot in the yer 1994/ Stomtl Conductnce nd Trnspirtion Rtes All porometric mesurements were conducted with LI-1600 stedy-stte porometer (Li-Cor, Lincoln, Nebrsk, USA) Mesurements were mde from to 1 00 h in 199/94 nd to h in 1994/95 Only the bxil trnspirtion rtes of the youngest fully developed leves were mesured Smpling techniques were s follows (i) In 199/94 eight genotypes were mesured, with one repliction per dy t four to five dys ech week, in erly nd lte stress expenments Six mesurements on rndomly chosen plnts were performed (n) In 1994/95 two genotypes (DK888 nd KS6) in the erly stress expenment nd four genotypes (Crgill922, DK888, KS6, Tuxpeno C0) in the lte stress experiment were mesured with six mesurements per plot, in four replictions on four to six dys per week (in) In the erly stress experiment 1994/95 the remining six genotypes were mesured with two mesurements per plot in three replictions on four to six dys per week Due to the brek-down of one porometer no mesurements hve been mde on the four remining genotypes in the lte stress experiment 1994/95 Porometncl mesurements for the wter stress tretment were strted fter the first irrigtion in the control, which mens when wter ws withheld on stressed plots for t lest one week Lef boundry lyer resistnce in the field ws mesured nd ll trnspirtion rtes nd stomtl conductnces were djusted for differences in lef boundry lyer resistnce between cuvette enclosed nd non-enclosed leves s descnbed previously in Chpter Sttisticl Anlyses nd Clcultions Anlysis of vrinces nd men comprisons for bove-ground dry mtter yield nd trnspirtion were erned out on log-trnsformed vlues, since vrinces were not homogeneous over yers nd tretments Mens of originl vlues re given in the tbles with results of multiple men comprisons crried out on trnsformed vlues The expenmentl penods were split into pproximtely weekly durtions for clcultion of trnspirtion nd trnspirtion efficiency Men lef re ws determined, s the rithmetic men of lef re t the beginning nd the end of weekly period, nd dry mtter ccumultion s the difference in shoot biomss between two hrvests Anlysis of vnnce nd men comprisons for porometnc mesurements of trnspirtion rte nd stomtl conductnce for ech expenmentl period were 45

51 clculted ccording to split-plot design including plot smpling in 199/94, with wter tretment s the min-plot fctor nd genotype s the sub-plot fctor The fctor repliction ws replced by repliction/dy (rep/dy) combintion since only one repliction ws mesured per dy In 1994/95 clcultions were crried out ccording to split-split-plot design including plot smpling, with wter tretment s the min-plot fctor, genotype s the sub-plot fctor nd dy of mesurement s the sub-sub-plot fctor Cnopy trnspirtion ws clculted s previously described (Chpter 2), using empiriclly determined fctors to tke into ccount the rtio of bxil nd dxil trnspirtion, the verticl distribution nd the diurnl chnges of trnspirtion rte Some clcultions were performed on weighted mens x of lef re, stomtl conductnce or trnspirtion rte, referring to the whole expenment consisting of severl weeks in order to tke into ccount the development of lef re, stomtl i "LA 4^ " <"> conductnce nd trnspirtion rte in time where LA, is the lef re nd xt the trit under considertion of week / The weight for ech weekly men ws the rtio of the lef re of the respective week to the lef re t the end of the expenment Reltive stomtl conductnce ws expressed ccording to the susceptibility index of Fischer nd Murer (1978), dpted for the weighted conductnce in ech experiment The index is clculted s men stomtl SC SC -S5=- (4.2) SC where SCwt, scm, sc^ nd sc^ re the genotype men stomtl conductnce for well-wtered nd wter stressed conditions nd verge stomtl conductnce under well-wtered nd wter stressed conditions, respectively 4.4 Results Totl Shoot Biomss nd Shoot Biomss Accumultion The men dry weight ccumultion nd trnspirtion for weekly periods of the four 46

52 199/ /95 Fig. 4. Totl men dry weight ccumultion (upper grphs) nd trnspirtion (lower grphs) for the erly nd lte stress expenments (mens of eight genotypes, except for the lte vegettive stge 1994/95, where only four genotypes were included) Error brs denote one stndrd error of men (n=4). experiments re shown in Fig. 4.. Wter stress in the erly stge reduced dry mss ccumultion to 28% in 199/94 nd 41% of the control in 1994/95. Shoot dry weight ws significntly ffected t the second hrvest 10 nd 16 dys fter withholding wter in 199/94 nd 1994/95, respectively. Only smll significnt genotypic differences were found t the end of the erly stress experiments in shoot biomss (Tble 4.2). A vigorous erly growth of KS6 ws found in both yers nd tretments, while Tuxpeno C0 nd Tuxpeno were C8 comprtively low in shoot dry weight production for both yers. A significnt genotype by wter supply interction ws found only for the erly stress experiment 199/94 with KS6 nd DK888 showing the lowest reduction in dry mtter ccumultion under wter stress. For the erly stress experiment the nlysis of shoot dry mtter increse during the experimentl period gve very similr results s the nlysis of shoot dry weight t the end of the erly stress experiment, these dt re therefore not shown. Also for wter stress in the lte vegettive stge, bove ground dry mtter yield ws significntly reduced in both yers (Tble 4.). Due to rinfll nd the shorter experimentl period, wter stress ws less severe in 1994/95 nd shoot biomss verged for ll genotypes ws still 82% of the control, while it ws 70% in the yer 47

53 letters cpitl different by indicted re differences Tretment 1 Student-Newmn-Keuls-Test to ccording level probbility 1 0 the t different significntly re letter sme the by followed not column in Mens B 5 59 A B Averge1 (7) (8) b(6) (6) 9 52 (6) 0 1 b(8) 1 14 b(8) 9 5 (8) (7) b(8) (7) 6 52 (8) 1141 b(7) 15 b(7) 55 (5) (5) (2) (8) 5 51 (5) b(6) 19 (1) 96 (6) (6) b(6) (5) 1 5 (7) (4) b 4 20 (5) 7 77 (D (1) (1) (4) 62 (2) (1) 5 2 (2) 2 92 (2) (2) () (1) 6 71 (1) () b 7 21 () 2 85 (4) (4) b(5) () 62 () (2) b 6 24 (6) 0 72 () () b(4) (2) 6 69 (4) (5) b 7 20 (4) 4 80 m mtter dry g men totl s1 s1 s1 C8 Tuxpeno C0 Tuxpeno Suwn KTX101 KS6 40 Hercules DK Crgill tretment Irrigtion CO nlysis combined in rnk 7 DAE 1994/95 DAE 199/94 emergence fter Dys tretments irrigtion nd yers over nd yers, over combined vnnce of nlysis to ccording rnks nd conditions (s1) stressed wter nd (ww) well-wtered under experiment stress erly the of end the t cultivrs mize eight of yield mtter dry ground Above 4.2 Tble

54 letters cpitl different by indicted re differences Tretment 1 Student-Newmn-Keuls-Test. to ccording level probbility 0.1 the t different significntly re letter sme the by followed not column in Mens B A 1 65 B A Averge1 (7) dc b(7) b(6) (7) b 465 (7) be 577 (6) 86 b(4) 571 C Tuxpeno (8) d b(8) b(8) b(8) 440 c(8) 54 (8) 7 b(6) 55 C0 Tuxpeno (5) be (4) b(7) (1) 575 (5) bc 619 (5) 402 b(8) 58 Suwn (6) be b(6) b(4) (6) b 49 (6) bc 622 (7) 56 ) b KTX () bc (D b(5) () b 58 (4) bc 650 (1) 485 b(5) 551 KS6 (2) b (2) b(2) (2) 564 b() 660 () 445 (2) b Hercules (1) () (1) (4) b 58 (1) 725 (2) 466 (1) 718 DK888 (4) bc (5) b() (5) b 522 (2) b 69 (4) 419 b(7) Crgill me totl s2 WW s2 m"2 mtter WW gdry s2 WW tretment Irrigtion nlysis combined in rnk 61 DAE 1994/95 61 DAE 199/94 emergence fter Dys tretments. irrigtion nd yers over nd yers, over combined vrince of nlysis to ccording rnks nd conditions (s2) stressed wter nd (ww) well-wtered under stge flowering t cultivrs mize eight of yield mtter dry ground Above 4. Tble

55 before. Significnt genotypic differences in dry weight t 61 DAE (flowering stge) were found in both yers for well-wtered nd wter stressed conditions. A higher shoot dry weight with high wter supply ws found for the hybrids (esp. DK888, Hercules40 nd Crgill922) compred to open-pollinted vrieties. In contrst, two open pollinted vrieties hd the highest shoot dry weight under limited wter supply (KS6 for 199/94 nd Suwn in 1994/95), lthough these differences were not significnt. No genotype by wter supply interction ws significnt. Combined over yers nd wter supply, DK888 hd significntly the highest, nd both cycles of the Tuxpeno popultion the lowest, shoot dry weight t the flowering stge. Dry mtter increse during the lte stress experimentl period ws reduced to 57% of control in 199/94 nd 68% of control in 1994/95 s men for ll genotypes. Bsiclly similr genotype rnking ws found for dry weight t the flowering stge nd dry weight increse during the lte stress experiments, but less significnt differences were found, gin the dt re not shown. KS6 showed high shoot dry weight t the flowering stge in spite of reltively low dry weight increse during the lte vegettive period. In contrst we found for the improved popultion of Tuxpeno C8, reltively higher shoot dry weight increse thn shoot dry weight Grin Yield nd Hrvest Index Wter stress t the erly vegettive stge resulted, in 1994/95, in higher yield reduction thn wter stress t the lte vegettive stge (Tble 4.4). Plnts of the erly stress tretment could not compenste for the reduction in biomss ccumultion during the stress period. Dry mtter t flowering, four weeks fter stress relese ws still only 52% nd t mturity 6% of the control. The grin yield ws 71% of the control. Hrvest index ws incresed (Tble 4.5). In the cse of wter stress t lte vegettive stge, the reduction compred to the control ws quite similr for the dry weight t the time of stress relese t flowering, for the dry weight t mturity, nd for the grin yield (18-20%). Hrvest index ws decresed by wter stress in the lte vegettive stge. Significnt genotypic differences in grin yield existed in ll irrigtion tretments. Anlysis of vrince for grin yield did not revel ny genotypic differences in the response to wter supply. The rnking of genotypes ws similr for totl biomss ccumultion t flowering, t mturity nd for grin yield. Genotypic differences were lso found for the hrvest index (Tble 4.5) nd yield components. Especilly importnt in our investigtions is the hrvest index, since cultivrs which produce more grin yield per totl biomss re underestimted in their performnce on biomss bsis during the vegettive growth. Crgill922 nd KTX101 hd high hrvest index nd their rnking for grin yield is higher thn for growth rtes under wter stress during vegettive growth nd totl biomss t mturity. 50

56 Tble 4.4 Grm yield nd rnking of eight mize cultivrs in 1994/95 with weekly irngtion nd with wter stress imposed t erly or lte vegettive stge well-wtered erly stress lte stress totl men grin yield (g m 2) Crgill (2)b 262 (1) 290 (1) (2) DK (1) 252 (2)b 42 (2)b (1) Hercules (5) bc 217 (4) bc 27 () bc ()b KS6 261 (6) be 184 (6) be 201 (6)dc (6)cd KTX () bc 198 (5) bc 240 (4) bed (4) be Suwn 0 (4) bc 221 () bc 221 (5) bed (5) be Tuxpeno C0 222 (8)c 170 (7)0 169 (7)d (8)d Tuxpeno C8 249 (7)c 169 (8)c 158 (8)d (7)d Averge1 294 A 209 C 27 B Mens in column not followed by the sme letter re significntly different t the 0 1 probbility level ccording to Student-Newmn-Keuls-Test 1 Tretment differences re indicted by different cpitl letters Tble 4.5 Hrvest index of eight tropicl mize cultivrs in 1994/95 nd rnking with weekly irngtion nd with wter stress imposed t erly or lte vegettive stge well-wtered erly stress lte stress totl men Crgill (2)b (1) (l) (1) DK (4) bc (5) be 051 (2) ()b Hercules () bc (4) be 0 48 (4) (4)b KS (8)c () be (5) be (5) be KTX (1) (2)b () (2) Suwn (7)c (8)d (6) be (8)d Tuxpeno C (6) be 0 52 (7)cd (7)c (7)cd Tuxpeno C (5) bc 0 56 (6) be 0 94 (8)c (6)cd Averge B A C Mens in column not followed by the sme letter re significntly different t the 0 1 probbility level ccording to Student-Newmn-Keuls-Test 1 Tretment differences re indicted by different cpitl letters 4.4. Stomtl Conductnce During Vegettive Growth Response of Stomtl Conductnce to Soil Wter Limittion The soil wter potentil t four depths dunng the experimentl periods is shown 51

57 in Fig 4 4 for well-wtered nd wter stressed plots, respectively The lter onset of wter stress nd lower plnt density in the expenment 1994/95 led to more grdul decrese in the soil mtnc potentil In lter growth stges wter stress developed much fster nd the decline of soil mtric potentil ws steeper The decrese of the soil mtnc potentil in well-wtered plots ws strongly dependent on the growth stge In the lte vegettive stge soil mtric potentil t 40 nd 60 cm depth ws decresed, while in the erly vegettive stge only the soil mtric potentil of the upper soil lyer ws significntly ffected within the weekly irrigtion cycle Well wtered Wter stressed 20 cm A 40 cm o- 60 cm o- 80 cm v Dys fter emergence Dys fter emergence Fig 4 4 Soil mtnc potentil mesured by tensiometers in the well-wtered nd wter stressed plots for erly nd lte stress experiments Drk tringles indite dys of irrigtion the rrows indicte rinfll (12 mm) on DAE 52 in 1994/95 Stomtl conductnce verged for il genotypes during the experiments is depicted in Fig 4 5 Stomtl conductnce ws strongly depressed by wter stress right from the beginning in ll experiments, indicting tht rewtenng fter one week of withholding wter resulted in significnt increses in stomtl conductnce, even in the erly vegettive stge, where plnt wter uptke is still low Better seedling estblishment in the erly stress expenment 1994/95 led to more grdul decrese in stomtl conductnce throughout the expenmentl period compred to the first yer, where stomtl conductnce ws strongly decresed fter the first week of observtion In lter growth stges wter stress developed much fster nd the initil decline in stomtl conductnce ws more drstic In the erly stress experiments decline in dily mens of stomtl conductnce of wter stressed plnts were highly correlted to the soil wter potentil t 20 cm depth (r=0 89***, compred to r=0 55** for 40 nd r=0 50** 52

58 for 60 cm depth). When wter stress occurred in the lte vegettive stge, stronger correltions to the soil mtric potentil t deeper lyers (r=0.55, r=0.79***, r=0.62** nd r=0.66* for 20, 40, 60 nd 80 cm soil depth, respectively) indicted tht stomtl conductnce ws more sensitive to the soil wter sttus t deeper lyers. The estimted prmeters for liner regressions re given in the ppendix A1. The soil mtric potentil t the end of weekly irrigtion period in well-wtered plots is, especilly in the lte vegettive stge, reduced to levels lower thn -40 kp, which is likely to induce prtil stomtl closure, in the lte stress experiments, significnt correltions of stomtl conductnce nd trnspirtion rtes were found with comprtively still high soil mtric potentils of bout -41, -5, -25 nd -22 kp t 20, 40, 60 nd 80 cm soil depth, respectively o 4 0 o o 1 00 oo E % V-^cftXIo w\ Dys fter emergence erly stress lte stress well-wtered rel humidity Fig. 4.5 Stomtl conductnce nd ir humidity during the erly nd lte stress experiments. The rrow indictes rinfll (12 mm) t DAE 52 in 1994/95. The experiments included eight or four genotypes Response of Stomtl Conductnce to Atmospheric Humidity Vrition of stomtl conductnce ws ffected by ir humidity, especilly for well-wtered plnts (Fig. 4.6). The lte stress experiment in 199/94 hs been excluded in the nlysis of the response of stomt to ir humidity becuse the 5

59 - - - r ' II O ws WW t _ > y=419x+717 ^/ y - j*t ««n* o x o Sj^O^q' 11 c-jct^q CO Fig. 4.6 Reltion between ir humidity nd stomtl conductnce of wellwtered (ww) nd wter stressed (ws) mize Ech point represents the dily mens mesured t noon The experiments included eight or four genotypes 50 o Frctionl reltive humidity mesurements of humidity were not mde within the cnopy of well-wtered nd wter stressed plots seprtely nd high grdients, in ir humidity between both tretments, re likely for dense cnopies with high differences in trnspirtion Stomtl response to humidity led to decoupling of trnspirtion rte from stomtl conductnce This decoupling ws complete for well-wtered plnts (Fig 4 7), wheres ten percent increse in stomtl conductnce due to incresed humidity for wter stressed plnt (gs= 100 mmol cm 2 s'1) resulted in six percent increse in trnspirtion e 5 e 6 6 " r2 4 2 = y x V = 0 01 *#<% </# & ~~*7o+, A* * 8/ y 0015 = x r2 = WS Fig. 4.7 Reltion between stomtl conductnce nd trnspirtion rte of well-wtered (ww) nd wter stressed (ws) mize Ech point represents dily mens mesured t noon The expenments included eight or four genotypes WW Stomtl conductnce (mmol m 2s ') For the lte stress expenment in 1994/95 ll genotypes hd the sme principl response of stomtl conductnce to wter vpour pressure difference (Fig 4 8) However, Tuxpeno C0 showed t ny wter vpour pressure difference higher 54

60 - ~ - - stomtl conductnce thn the other genotypes, which showed rther close reltion o 00 c m conduc w M E o <n (0 fc 150 t- o w ,\0 ^? J\ o Crgill 922 (r*-0 87) V ^W Xs DK 888 (r*=0 88) - A KS6(rJ»0 91) H Tuxpeno Cq (r*-0 91) $LV 0 i 1 1 VPD (kp) Fig. 4.8 Reltion between stomtl conductnce nd lef to ir wter vpour pressure difference (VPDtef t0 ir) of four mize vrieties under well-wtered conditions during the lte vegettive stge in 1994/1995 Ech point represents dily mens of mesurements t noon Genotypic Vribility in Stomtl Conductnce Genotypic men stomtl conductnces for the genotypes in ll experimentl penods re given in Tbles 4 6 nd 4 7 for the erly stress experiments nd Tble 4 8 nd 4 9 for the lte stress experiments In generl, error vrinces were high nd the coefficients of vrition nsed to more thn 50% for severely wter-stressed plnts However, in both irrigtion tretments nd especilly in lter growth stges, some genotypic differences were significnt nd consistent genotypicl chrcteristics were detected For Tuxpeno C0, under well-wtered conditions, we found low stomtl conductnce t the erly growth stge nd high stomtl conductnce in the lter growth stge, while stomtl conductnce of Tuxpeno C8 ws low throughout the vegettive period In erly growth stges reltively high stomtl conductnces were observed for KS6 nd Hercules40 KTX101 showed consistently over tretments, penods nd yers, low stomtl conductnce For the genotypes DK888 nd KS6, there ws never significnt difference in stomtl conductnce However, in ll weekly penods of ll four experiments except for the lst week in lte stress 199/94, when wter stress ws most severe nd stomt were virtully closed (conductnce of 55

61 t-newmn-keuls-test. B A 4 40 (2) 119 (8) 15 (1) 18 (4) 40 (6) 90.0 (2) 55 (8) 84.0 (7) 2 (4) 92.0 (5) 9 (7) 7 88 () 50 () 11 (6) 8 (5) 1 91 (1) 68 s1 ww DAE 27 - to ccording level probbility 0.1 B 81.9 A 18.7 () 4 86 (5) b 215 (4) 80.6 (4) b 220 (5) 74 (2) 26 (7) 72.0 b(8) 19 (8) 65.5 (7) b 210 (6) 2 74 (6) b 21 (1) 106 (1) 26 (2) 9 () b 22 s"1 m"2 mmol s1 WW DAE letters. cpitl different t different significntly re letter me B A (7) b 157 (7) b 28 (4) b 166 (6) b 242 (5) b 160 (4) 255 b (6) b 157 (5) b 244 () b 166 (2) 271 b(8) 146 (1) 276 (1) 195 () b 261 b(2) 191 (8) b 227 s1 WW DAE b indicted re differences Tretment 1 the by followed not column in Mens Averge1 C8 Tuxpeno C0 Tuxpeno Suwn KTX101 $ KS6 40 Hercules DK Crgill tretment Irrigtion emergence fter Dys 199/94. experiment stress erly the for conditions (s1) stressed wter nd (ww) well-wtered under cultivrs mize tropicl eight of rnkings nd conductnce Stomtl 4.6 Tble

62 letters. cpitl different by indicted re differences Tretment 1 Student-Newmn-Keuls-Test. to ccording level probbility 0.1 the t different significntly re letter sme the by followed not column in Mens B 94.4 A 26 B 11.7 A B 0 11 A Averge1 b(4) (1) 111 (6) 224 (1) 140 (6) 227 b(4) 14 b(7) 20 C8 Tuxpeno (2) 25.6 (2) 110 (2) 248 (4) 11 (4) 244 b(5) 10 b(6) 21 C0 Tuxpeno b(5) (5) 90.2 (5) 27 (8) 96.5 (8) 220 (1) 162 (4) b 240 Suwn b(8) 20.7 (8) 76.8 (8) 215 (6) 109 (7) 226 b(8) 4 90 b(8) 208 KTX101 b() 22.7 (6) 86. (1) 254 (7) 105 () 248 b(6) 128 (2) b 250 KS6 (D (7) 86.2 (4) 241 (5) 111 (1) 274 b{7) 97 () b Hercules b(6) () 102 (7) 222 () 120 (2) 258 b() 16 (5) b 29 DK888 b(7) 21.2 (4) 94.8 () 244 (5) 26 (2) 158 (1) Crgill (2) 122 s"1 m"2 mmol ww s1 ww s1 ww s1 ww tretment Irrigtion DAE 8-44 DAE DAE 24-0 DAE 16-2 emergence fter Dys 1994/95. experiment stress erly the for conditions (s1) stressed wter nd (ww) well-wtered under cultivrs mize tropicl eight of rnkings nd conductnce Stomtl 4.7 Tble

63 letters cpitl by different indicted re differences Tretment 1 Student-Newmn-Keuls-Test to ccording level probbility 1 0 the t different significntly re letter sme the by followed not column in Mens B 9 59 A 1 06 B 4 70 A B A 02 Averge1 (6) 0 5 (7) 289 (4) b 2 70 (7) be 255 (4) b 209 (6) 287 C8 Tuxpeno (4) 581 (1) 25 (2) b 2 80 (2) b 2 b() 21 (2) 26 C0 Tuxpeno (8) 6 46 (5) 07 (5) b 8 69 (4) 296 bc cd(7) 154 (4) 12 Suwn () 7 61 (4) 14 b(8) 2 29 (6) 284 bc d(8) 124 (8) KTX (1) 5 92 () 20 b(7) 42 8 (5) 287 bc (6) bed 161 (5) 298 KS6 (7) 6 48 (6) 29 () b 1 77 b() 19 (2) 247 () 1 40 Hercules (2) 9 61 (2) 27 (1) 109 (1) 68 (1) 251 (1) 27 DKK 888 (5) 8 56 (8) 274 (6) b 4 67 c(8) 218 (5) be 189 (7) Crgill 1 s m"2 mmol s2 ww s2 ww s2 ww tretment Irrigtion DAE DAE DAE emergence fter Dys 199/94. expenment stress lte the for conditions (s2) stressed wter nd (ww) well-wtered under cultivrs mize tropicl eight of rnkings nd conductnce Stomtl 4.8 Tble

64 letters. cpitl different by indicted re Tretment differences 1 Student-Newmn-Keuls-Test. to ccording level probbility 1 0 the t different significntly re letter sme the by followed not column in Mens B 9.0 A 26. B A 27.2 Averge1 g (4) 86.6 (1) 00 (4) 144 (1) 275 C0 Tuxpeno () 90.8 (4) b 246 () 150 b() 22 KS6 (1) 97. () b 252 (1) 181 (2) b 24 DK888 (2) 97.1 (2) b 256 (2) 166 (4) b Crgill s'1 m"2 mmol s2 ww s2 ww tretment Irrigtion DAE DAE emergence fter Dys 1994/95. experiment stress lte the for conditions (s2) stressed wter nd (ww) well-wtered under cultivrs mize tropicl four of rnkings nd conductnce Stomtl 4.9 Tble

65 wter stressed plnts ws bout 20% of the control), DK888 showed 10 to 0% higher stomtl conductnce thn KS6 This my indicte tht there is higher stimulus for stomtl closure by soil dryness in KS6 nd tendency to mintin reltively high stomtl conductnce under wter stress for DK888 For the experimentl period when wter stress ws most severe (lst week in lte stress 1994/95), the highest conductnce ws found for KS6 nd KTX101, which might be result of wter svings by erly stomtl closure during the development of wter stress Sensitivity of stomt to wter stress might be expressed in terms of reltive stomtl conductnce compred to the well-wtered control A genotypic susceptibility index (see eq 4 2), dpted from Fischer nd Murer (1978) for the weighted men stomtl conductnce, is given in Tble 410 An index higher thn 1 indictes higher reduction of stomtl conductnce thn the verge of ll genotypes, wheres n index lower thn 1 indictes reltively smll reduction Tble 4.10 Susceptibility index ccording to Fischer nd Murer (1978) for the weighted men stomtl conductnce for ech experiments erly stress experiment lte stress experiment Crgill DK Hercules KS KTX Suwn Tuxpeno C Tuxpeno C Within single expenment the differences between genotypes in the susceptibility index ws never significnt, however in ll experiments we found for the genotype DK888 comprtively low nd KTX101 high reduction of stomtl conductnce under stress High tendency for stomtl closure ws observed for Hercules40 nd KS6 in both yers t the erly vegettive stge No consistent clssifiction is suggested for the other vrieties since the rection vried from expenment to expenment Cnopy Trnspirtion Cnopy trnspirtions for the erly nd lte stress experiments, bsed on men trnspirtion rtes clculted from the mesurements of stomtl conductnce nd lef 60

66 letters cpitl re indicted by different Tretment differences 1 Keuls-Test Student-Newmn to ccording level probbility 1 0 the t different significntly re letter sme the by followed not column in Mens B 140 A 0 46 B 74 A 5 4 Averge1 b(5) b(8) (5) 14 (8) 1 40 b(8) 59 d(8) 8 25 C8 Tuxpeno b(8) b(7) (7) 125 (7) 4 40 b(7) 6 d(7) 7 27 C0 Tuxpeno b(6) b() (6) 125 (5) 5 42 b(5) 71 () b 6 9 Suwn b(7) b(6) (8) 12 (6) 40 9 b(6) 66 (4) be 5 KTX101 (1) (2) (2) 149 (2) 8 50 d) 96 (1) 2 42 KS6 b(4) (1) (4) 145 (1) 5 57 b(4) 74 (2) b Hercules b(2) b(5) () 144 () 1 48 b(2) 85 (6) cd 2 0 DK888 b() b(4) (1) 165 (4) (5) be Crgill b() - 2 m wter kg s1 ww s1 ww s1 ww tretment Irrigtion nlysis combined in rnk 16-7 DAE 1994/ DAE 199/94 emergence fter Dys yers over combined vrince of nlysis to ccording rnks nd experiments stress erly the during conditions (s1) stressed wter nd (ww) well-wtered under cultivrs mize eight of Trnspirtion 4.11 Tble

67 letters cpitl different by indicted re differences Tretment 1 Student-Newmn-Keuls-Test to ccording level probbility 1 0 the t different significntly re letter sme the by followed not column in Mens (4) b (4) b (4) () b (2) b () (1) (1) (1) (2) b () b (2) s2 ww nlysis combined in rnk B 0 68 b(4) 5 () 71 (1) 7 7 (2) 6 7 s2 A 1159 (4) b (2) () (1) 12 B 717 b(7) 8 64 b(6) 5 67 b(5) 9 69 c(8) 48 (2) b 2 79 () b 1 79 (1) 9 90 b(4) 2 74 m' wter kg s2 A 16 b(8) b(7) 149 b(2) 169 6b() 168 2b(6) 158 8b(5) 165 (1) b(4) DAE 1994/ DAE 199/94 Averge1 C8 Tuxpeno C0 Tuxpeno Suwn KTX101 KS6 40 Hercules DK Crgill tretment Irrigtion emergence fter Dys o> tretments irrigtion nd yers over nd yers, over combined vrince of nlysis to ccording rnks nd experiments stress lte the during conditions (s2) stressed wter nd (ww) well-wtered under cultivrs mize eight of Trnspirtion 4.12 Tble

68 " "" re determintions (see Chpter 2), re shown in Tbles 4.11 nd 4.12, respectively. Significnt genotypic differences were found in ll experiments except for the erly stress experiment in 1994/95. Genotypic rnking for trnspirtion ws similr to the rnking for dry mtter ccumultion. Cumulted trnspirtion within the irrigtion tretment ws to very lrge extent explined by the lef re. Modifictions due to differences in stomtl conductnce nd trnspirtion rtes within the wter supply tretments were only smll. For the lte vegettive stge, when high lef re hd been developed before wter ws witheld, cumulted trnspirtion ws more closely correlted with trnspirtion rte thn with lef re, s is shown by comprison of the correltions of lef re nd trnspirtion rtes with cumulted cnopy trnspirtion (Tble 4.1). To ccount for the development in time of lef re nd trnspirtion rte the correltions were clculted on weighted mens of either trit. The weight for ech weekly men ws the rtio of ctul lef re to the mximum lef re of the experimentl period (see eq. 4.1). Tble 4.1 Correltions of genotype mens between cumulted trnspirtion nd () weighted men lef re nd (b) weighted men trnspirtion rte nd the utocorreltion between both ltter prmeters (n=8, except lte vegettive stge 1994/95, where n=4) vegettive erly lte stge yer 199/ /95 199/ /95 irrigtion WW S1 both WW S1 both WW S2 bottl WW s2 both () NS NS NS NS (b) 0 08 NS 0 04 NS NS NS utocorreltion NS NS NS NS NS 0 47 NS 0 18 NS NS 0 19 NS significnt with < 0 05 significnt with < 0 01 significnt with < 0001 NS riot significnt Genotypic differences in soil wter uptke for the lte vegettive stge were lso found with the soil wter blnce bsed on neutron probe mesurements of soil wter content. The zone of mximum wter uptke shifted down the soil profile with incresing stress intensity (Fig. 4.9). For well-wtered plnts most wter ws tken up from the most upper lyer. Mximum rte of wter uptke ws found t 40 to 50 cm soil depth for the first week of wter stress nd 60 to 80 cm soil depth in the second period of wter stress. The profiles of soil wter uptke showed genotypic differences in the depth of mximum wter uptke under wter stress. While Crgill922 showed the highest uptke from 60 to 80 cm soil depth with considerble uptke from 100 cm, 6

69 (n=4) men of error stndrd one denote brs Error 1994/95 experiment stress lte the in line) (stright dys 17 fter nd line) (dotted dys 8 fter conditions stressed wter nd well-wtered under vrieties four for depletion wter soil cumulted of Profile %) (Vol wter depletion Soil Well-wtered - 1 ^ " * - 1 / 1 K*H - no 80 -»< t*i 60 " *( i*i Crgill i i i 20 stressed Wter

70 mximum uptke ws restricted to 40 to 60 cm soil depth for Tuxpeno C0. DK888 nd KS6 showed mximum uptke lso from 60 to 80 cm, but took up less wter thn Crgill922 t 100 cm soil depth Trnspirtion Efficiency Trnspirtion Efficiency nd Wter Limittion Trnspirtion efficiency ws incresed in ll experiments under wter stress conditions (Tbles 4.14 nd 4.15). The higher trnspirtion efficiency, seems however to be of minor importnce with respect to the differences in totl trnspirtion nd dry mtter ccumultion between well-wtered nd wter stressed tretments (Fig. 4.10). E _ cu CO E wter stressed, erly stge - o well-wtered, erly stge D wter stressed, lte stge well-wtered, lte stge n. % 200 #D y=121+27x r r2 = 0 94*" t/j Fig Reltion between trnspirtion nd dry mtter ccumultion of wellwtered nd wter-stressed mize genotypes t n erly nd lte vegettive stge. The genotypic mens of four field experiments, conducted in two yers, including eight genotypes re presented. Trnspirtion (kg m"2) The chnge in trnspirtion efficiency with limited wter supply ws relted to stress intensity, e.g. expressed s the trnspirtion reltive to the control. Trnspirtion efficiency ws similr compred to the control for moderte nd severe stress levels, but incresed nerly 40% for intermedite levels of stress intensity, when trnspirtion ws decresed by bout 40-50% (Fig A). A significnt contribution of incresed trnspirtion efficiency to dry mtter production for wter stressed plnts is then shown by the reltion between reltive dry mtter ccumultion nd reltive trnspirtion (Fig. 4.11B). For mximized trnspirtion efficiency, dry mtter ccumultion ws still bout 65% of the control despite reltive trnspirtion of bout 50%. 65

71 1 D " - g 15 ICI *= 114* CO C 2 1 T.y- -*---, ' o ir O -o. 12 CO c _p o CD o % 10 CO CD rr 09 erly stress o 08 v lte stress CO * tr.- ' V T V.*.»,.«'''./'» X /' 05 s CO 04 > /'.*'/" cu 0 > 1 <^'' 02. A B, Fig Reltive trnspirtion nd reltive trnspirtion efficiency (A) nd reltive dry mtter ccumultion (B) of wter-stressed mize. Ech point represents the men of eight or four vrieties within one repliction of four field experiments. Dotted lines re fitted by eye. The stright line in Fig. B indictes expected dry mtter ccumultion, if trnspirtion efficiency would not hve been chnged with limited wter supply. r\ 1 j 1 1 1, _ Reltive trnspirtion Genotypic Vribility for Trnspirtion Efficiency For the erly stress experiments genotypic differences were found under wellwtered conditions (Tble 4.14). DK888 showed the highest nd Tuxpeno C0 the lowest trnspirtion efficiency. In cse of wter limittion no consistent genotypic differences were found. KS6, which used the wter most efficient in 199/94, showed the lowest rnking for trnspirtion efficiency in 1994/95. The low mount of wter trnspired in the erly vegettive stge compred to lter stges clerly emphsizes the higher importnce of trnspirtion efficiency in lter vegettive stges. No significnt genotypic differences for trnspirtion efficiency were found in the lte vegettive stge under both well-wtered nd wter stressed conditions (Tble 4.15). However, for well-wtered conditions DK888 reveled the highest trnspirtion efficiency (1% higher thn the men of the well-wtered tretment), confirming the results from erly stress nd first yer experiments. With limited wter supply the results re contrdictory. Tuxpeno C0, combining low rnking in growth nd low trnspirtion efficiency in the yer 199/94, ws superior in trnspirtion efficiency under wter stress in 1994/95. 66

72 letters cpitl indicted by different re Tretment differences 1 Student-Newmn-Keuls-Test to ccording level probbility 1 0 the t different significntly re letter sme the by followed not column in Mens B 22 4 A 22 B 2 4 A 212 Averge1 (7) (8) b(6) (5) 17 4 (2) 41 b(8) 0 2 (7) 98 1 C8 Tuxpeno (8) (7) b(8) (6) 14 4 (8) 91 2 b(7) 10 2 (8) 91 1 C0 Tuxpeno (4) (6) b(2) (4) 27 4 (5) 0 (5) b 2 (1) 4 2 Suwn (1) (1) b() (2) 52 4 (1) 51 (2) b 72 2 (5) 210 KTX101 () () b(5) (8) 61 (6) 08 (1) 02 (4) 210 KS6 (6) (2) b(7) (1) 80 4 (7) 01 (4) b 44 2 (6) Hercules (2) (4) (1) (7) 89 () 21 () b 54 2 (2) 2 2 DKK 888 (5) (5) b(4) () 8 4 (4) 0 (6) b 1 2 () Crgill wter)1 (kg mtter dry g s1 ww s1 ww s1 ww tretment Irrigtion nlysis combined in rnk 16-7 DAE 1994/ DAE 199/94 emergence fter Dys tretments irrigtion nd yers over nd yers, over combined vrince of nlysis the to ccording rnks nd experiments stress erly the during tretments (s1) nd wter stressed (ww) under well-wtered cultivrs mize eight of efficiency Trnspirtion 4.14 Tble

73 letters cpitl different by indicted re differences Tretment 1 Student-Newmn-Keuls-Test to ccording level probbility 1 0 the t different significntly re letter sme the by followed not column in Mens 10 A 2 A 88 2 A 9 A B 67 2 Averge1 (2) 08 (4) 27 (2) 89 2 (1) 95 (4) 84 2 (4) 01 (2) 4 (4) 67 2 (4) 02 (2) 90 2 (1) 0 () (1) 26 (4) 0 (1) 4 () 71 2 men totl s2 nlysis combined in rnk (2) 6 (1) 49 () 25 (2) 11 wter)1 (kg mtter dry g - - s2 ww DAE 1994/95 (2) 96 (8) 60 2 (6) 29 (1) 42 4 () 66 (7) 20 (5) 1 (4) 4 (1) 09 () 94 2 (8) 25 2 (5) 57 2 (6) 45 2 (4) 68 2 (2) 0 (7) 1 2 CB Tuxpeno C0 Tuxpeno Suwn 101 KTX KS6 40 Hercules DK Crgill s2 ww tretment Irrigtion DAE 199/94 emergence fter Dys CO en tretments irrigtion nd yers over nd yers, over combined vrince of nlysis to ccording rnks nd experiments stress lte the during tretments (s2) stressed wter nd (ww) well-wtered under cultivrs mize eight of efficiency Trnspirtion 4.15 Tble

74 1994/ Reltions between Trnspirtion Efficiency, Stomtl Conductnce nd Yield Prmeters Correltions between growth nd trnspirtion efficiency Trnspirtion efficiency ws negtively correlted with growth trits when genotype mens of both wter supply tretments were considered, since trnspirtion efficiency tends to increse while growth ws depressed under limited wter supply (compre Tble 4.2, 4.,4.14 nd 4.15). Within either irrigtion tretment correltions between trnspirtion efficiency nd dry mtter ccumultion, bsed on genotype mens, were mostly positive (Tble 4.16), especilly for well-wtered conditions. Positive nd prtilly high correltions between trnspirtion efficiency nd growth were found in both wter supply tretments, when correltions were bsed on plot mens in either wter supply tretment. Tble 4.16 Correltions between trnspirtion efficiency nd dry mtter ccumultion bsed on genotypic mens (n=8, except for lte vegettive stge 1994/95, where n=4) nd plot mens (n=2, except for lte vegettive stge 1994/95, where n=16) vegettive stge eny lte yer 199/ /95-199/94 - irrigtion ww s1 ww s1 ww s2 WW s2 genotypes 0 59NS 0 91*" 0 02NS 0 8NS 0 72* 016NS 0 88NS -0 45NS plot 0 87*" 0 90*** 0 40* 0 55** 0 81"* 0 79*** 0 90*" 0 2NS NS significnt with < 0 05 significnt with < 0 01 significnt with < not significnt Negtive, but not significnt correltions were found between the susceptibility index for stomtl conductnce nd dry mtter ccumultion under wter stress in bsolute vlues or s percentge of the control tretment (dt not shown). The rther similr rnking in biomss ccumultion of DK888 nd Hercules40 or KS6 for the erly vegettive stge (Tble 4.2) nd the rther contrsting rnkings in reltive stomtl conductnce (Tble 4.10) indicted, tht the sensitivity of stomt ws not mjor chrcteristic for the growth of the vrieties under drought. Correltions with grin yield High correltions of genotype mens between shoot biomss t flowering nd grin yield were found for the well-wtered (r=0.89***, n=8) nd erly stress tretment (r=0.90***, n=8), wheres the correltion ws less strong for the lte stress tretment (r=0.57 ns, n=8). Positive, but only for the well-wtered tretment in the lte stress experiment significnt correltions were found for dry mtter ccumultion during the experimentl period nd finl grin yield (Tble 4.17). Both dry weight ccumultion 69

75 nd dry weight t flowering were only wekly correlted to grin yield for wter stress t lte vegettive stge This my stress the relevnce of the genotypic bility to recover fter wter stress for the finl grin yield Tble 4.17 Correltions of genotype mens between grin yield nd dry weight ccumultion during the expenmentl period (n=8) nd trnspirtion efficiency during the expenmentl period (n=4) well-wtered wter stressed erly lte erly lte dry weight ccumultion 0 5 ns 0 81 * 0 60 ns 0 19 ns for experimentl period trnspirtion efficiency 0 47 ns 0 89 ns 0 8 ns -0 8 ns for experimentl period NS significnt with cc< 0 05 not significnt 4.5 Discussion Shoot Biomss nd Grin Yield Response to pre-nthesis drought Anthesis nd the erly grin filling period re the most vulnerble stges to drought stress, presumbly due to the high sensitivity of reproducing tissues with respect to wter nd crbohydrte sttus (Westgte nd Grnt 1989, Westgte nd Boyer 1985) Therefore, most investigtions considered the effects of wter deficits during nthesis or grin filling period Little ttention in reserch ws given so fr to wter stress during vegettive growth, lthough mize grin yield cn be severely ffected when wter deficits occur during vegettive growth (Muchow 1989, Weerthworn et l 1992, Neidhrt 1994) Sobrdo (1990) reported grin yield decreses of 10 to 25% nd 50 to 70% fter withholding wter for 25 nd 40 dys before flowering, compred to yield reduction of 29 nd 20% fter 28 nd 2 dys without irrigtion during erly nd lte vegettive stge in our expenments Sinclir et l (1990) found close reltion between biomss t mturity nd grin yield when wter stress ws pplied for different periods during nthesis Experiments of Sobrdo (1990) long with our results showed close reltion for dry weight or lef re t flowering nd grin yield in cse of pre-nthesis drought events In the study of Sobrdo wter stress ws relesed few dys before nthesis Correltions of genotype mens between vegettive growth until flowering nd grin yield were low within stress tretments, s found by Sobrdo (1990) nd in our lte stress expenment Higher correltions were reveled, when period of recovery fter drought ws included (compre the correltions for the erly stress tretment) This finding indictes tht the recovery fter wter stress nd the genotypic vribility for this bility is probbly s importnt s ctul growth under limited wter supply in determining grin yield 70

76 Oevelpomentl stge Incresed resistnce to wter stress of mize with geing of the crop ws previously reported. When wter ws withheld either in the vegettive stge or t tssel initition nd grin filling period, the lef wter potentil, where ssimiltion nd stomtl conductnce declined to zero ws higher for the erlier growth stge (Humn et l. 1990, Ackerson 198). For both periods of pre-nthesis drought in our experiments no chnge in the sensitivity of vegettive growth to wter stress ws found, since reduction in dry mtter ccumultion could be described s function of the stress intensity, mesured s reduction in trnspirtion. The higher grin yield reduction in 1994/95 fter wter stress in the erly vegettive stge ws most probbly due to the higher stress intensity in terms of trnspirtion deficits. By wter stress t the erly vegettive stge the hrvest index ws considerbly incresed. Increse of the hrvest index fter pre-nthesis wter stress ws lso found by Siri (199) nd Neidhrt (1994), wheres stble or decresing hrvest index is reported from other investigtions, when wter stress ws pplied either t tsselling or in lter growth stges (Denmed nd Shw 1960, McPherson nd Boyer 1977, Bolfios nd Edmedes 199, Cruford nd Pecock 199). Depression of erly vegettive growth probbly cused bigger proportion of ssimiltion ctully tking plce, fter reproductive tissue ws generted. A smller proportion of the totl ssimiltes finlly found in the grin needed therefore to be trnslocted from the reserves lredy present t nthesis. Selection for drought tolernce The growth of n open-pollinted vriety (KS6) ws comprtively vigorous under severe wter stress. In 1994/95 the high rnking in biomss fter relese of erly wter stress, ws mintined in comprison to the hybrids until the finl grin hrvest. A reltively high grin yield of n open-pollinted vriety under severe wter stress ws reported previously nd seemed to be relted to high wter-use efficiency (Sobrdo 1990b). KS6, too, showed high wter-use efficiency for the severe stress situtions in the erly nd lte stress experiment in 199/94. However, no significnt genotype by wter tretment interction occurred, therefore specific drought tolernce ws not reveled in our experiments, neither for vegettive growth under wter stress nor for grin yield. Similr results were reported for five mize hybrids with highly diverse genetics bckground (Undersnder 1987). Bolfios nd Edmedes (199) could not find significnt cultivr by wter supply interction for the yield of different cycles of the Tuxpeno popultion, where significnt improvement of drought tolernce hd been chieved. It seems therefore, tht selection for drought tolernt cultivrs must not necessrily be undertken in drought environments. It ws in the ltter study however concluded, tht the selection under wter stressed conditions is dvntgeous for the tolernce to drought events t the flowering stge, since for the nthesis-silking 71

77 intervll significnt interction between the gin of selection nd selection environment (well-wtered or wter stressed) ws found (Byrne et l. 1995, Bolfios nd Edmedes 199) Stomtl Conductnce The importnce of the investigtion of the stomtl behviour during vegettive growth stges is emphsized by Ackerson nd Kneg (1977), who observed only limited stomtl regultion of wter loss t the reproductive stge Reported vlues of mximum stomtl conductnces of 0 6 to 0 8 cm s"1 (Komer et l 1979, Ackerson nd Kneg 1977, Ackerson 198) gree resonbly well with our mesurements in wellwtered plnts, lthough ccurte comprisons of conductnces in velocity units require knowledge bout lef nd ir tempertures Response to soil wter sttus Response in stomtl conductnce to wter stress depends like other physiologicl effects very much on intensity nd history of the stress event Compnsons of genotypes re possibly confounded by different stress intensities, for exmple induced by fster growth nd consequently fster development of wter deficit Therefore, prmeters re needed to describe the wter stress sttus of plnts Differences in lef wter potentil between well-wtered nd wter stressed plnts do not necessrily occur in field expenments Differences in stomtl conductnce cn often not be ccounted for by dytime or pre-dwn lef wter potentils nd re rther explined by chemicl root signl like ABA or other phytohormones (Golln et l 1986, Trdieu et l 1991, see lso Chpter 5) Soil wter sttus might be used s n independent descriptor of stress intensity Due to problems in estimting soil wter retention, trnsmission properties, nd sptil grdients in soil wter potentil, the wter vilble to plnts is not lwys reflected ccurtely in soil-bsed mesurements However, the initil decline in stomtl conductnce of wter stressed plnts ws in ll experiments strongly correlted with the bulk soil mtnc potentil, s mesured by tensiometers After the initil decrese, stomtl conductnce vried from dy to dy but did not show nymore decline long with further soil wter depletion Stomtl conductnce ws t minimum, before the bulk soil mtric potentil reched -80 kp This is rther high soil mtric potentil compred to permnent wilting point of bout to kp Shrp nd Dvies (1985) showed, tht the rte of root prolifertion nd wter uptke decresed, when the bulk soil mtric potentil fell below -500 kp The wter vilbility in the investigted field site is most probbly not primrily limited by the bulk soil mtric potentil, but by the extremely decresed hydrulic conductivity with decresing soil wter content nd insufficient replenishment of wter to the soil ner to the root surfce The effective soil mtric potentils close to the roots re probbly much lower thn indicted by tensiometers 72

78 Response to ir humidity Prtil stomtl closure with decresing humidity or incresing lef to ir wter vpour pressure difference hs extensively been studied in mny crops in the context of stomtl nd environmentl control of trnspirtion (Lnge et l 1971, Losch nd Tenhunen 1981) It is supposed, tht trnspirtion rte or epiderml wter loss is sensed nd exhibits control of stomtl gurd cell behviour, rther thn humidity t the lef surfce or wter vpour pressure difference between lef nd ir is sensed directly (Bunce 1985, Mott nd Prkhurst 1991) Adpttion of stomtl conductnce to humidity in sesonl or dily time scle cn enhnce the verge trnspirtion efficiency becuse ssimiltion is shifted to periods of lower vpour pressure deficit (Sinclir et l 1984) Prtil stomtl closure hs been observed in severl crops for the time of highest evportive demnd t noon (Frquhr nd Shrkey 1982, Ferreir nd Kterji 1992) This stomtl regultion is believed to be mechnism to optimize trnspirtion efficiency since it prevents excessive trnspirtion with incresed wter vpour pressure difference (Cown nd Frquhr 1977, Frquhr et l 1980) The prllel decrese in conductnce with incresing wter vpour pressure deficit for ll genotypes in Fig 4 8 indictes little genotypic vrition in the dpttion to diurnl vrition in wter vpour pressure deficits With limited wter supply, there ws little or no correltion between stomtl conductnce nd wter vpour pressure deficit, nd the genotypic differences diminished Therefore, the chnces to improve trnspirtion efficiency by dpttion of stomtl response to cnopy ir humidity re low, especilly for conditions of limited wter supply, where high trnspirtion efficiency should be of prticulr relevnce The low trnspirtion efficiency of Tuxpeno C0 under well-wtered conditions compred to the other genotypes ws indeed result of higher stomtl conductnce t ny wter vpour pressure difference Genotypic vrition The susceptibility index for stomtl conductnce cn be used to describe genotypes with respect to the sensitivity of stomtl closure There ws no consistent reltion to growth under wter stress, but low sensitivity ws found for the high yielding hybrids DK888 nd Crgill922 In other investigtions limited to two genotypes, the drought tolernt vriety ws found to mintin higher stomtl conductnce under wter stress (Ackerson 198, Lorens et l 1987, O'Regn et l 199) It seems therefore, tht sensitive stomtl closure is not necessrily trit chrcterizing drought tolernce of cultivrs selected for high yield insted of survivl It might well be importnt for drought events occurring under mediterrnen climte, where reduced trnspirtion in vegettive stge my sve wter for post-nthesis growth with beneficil effect on grin yield nd hrvest index, s ws proposed for whet (Pssiour 1977) 7

79 4.5. Cnopy Trnspirtion Genotypic differences in trnspirtion nd positive correltions between trnspirtion nd growth under limited wter supply were found. The profiles of soil wter uptke showed differences in the depth of mximum wter uptke between wellwtered nd wter stressed plnts s well s between genotypes within the wter stressed tretment. Shrp nd Dvies (1985) showed tht soil wter depletion in deeper soil lyers corresponded with both root density nd the rte of wter uptke per unit root length. Lorens et l. (1987b) showed higher root length densities nd O'Regn et l. (199) higher percentge of root dry weight in deeper soil lyers in the more drought tolernt hybrid. Similr results were observed in sorghum (Wright nd Smith 198). The genotypicl bility to penetrte into deeper soil lyers nd to explore greter soil volume significntly determines trnspirtion nd growth under limited wter supply. The genotypicl shift in soil wter uptke to deeper lyers under drought corresponded well with reltive stomtl conductnce. This indictes tht wter uptke enbles the plnt to mintin higher stomtl conductnce s ws climed by O'Regn et l. (199). On the other hnd, high stomtl conductnce is prerequiste for ssimiltion nd hence enbles the plnt to invest ssimiltes in the rooting system Trnspirtion Efficiency under Drought Trnspirtion nd trnspirtion efficiency in field experiments re mostly estimted either bsed on soil wter blnce or gs-exchnge mesurements. Our estimtes of trnspirtion efficiency re composed of mesurements of lef trnspirtion rtes nd dry mtter ccumultion. The upscling of lef trnspirtion mesurements ws described nd shown to represent soil wter depletion (see Chpter 2). Gs-exchnge mesurements of ssimiltion rtes or instntneous wteruse efficiencies re difficult to extrpolte to the gronomicl meningful wter-use efficiency in terms of shoot biomss or grin yield per unit of wter trnspired becuse of respirtory losses nd prtitioning of gined ssimiltes between shoot nd roots (Mrtin nd Thorstenson 1988). However, most investigtions, of genotypic differences in trnspirtion efficiency, consider the instntneous gs-exchnge wter-use efficiency, becuse of the methodologicl problems in estimting cnopy trnspirtion on plot level. Comprison to gs-exchnge trnspirtion efficiency The theory of gs-exchnge in species with C4 ssimiltion pthwy predicts either incresed or decresed instntneous trnspirtion-efficiency with prtil stomtl closure due to wter stress, depending on the reltion between stomtl nd mesophyll resistnce nd their chnges under wter stress (Krieg 198, Nobel 198, Sinclir et l. 1975). Under controlled environmentl conditions, constnt rtio of ssimiltion nd stomtl conductnce ws reported, hence leding to constnt trnspirtion efficiency even with decresing stomtl conductnce (Wong et l. 1979, 74

80 6 Wong et l 1985, Tked et l 1978), but lso incresed trnspirtion efficiencies were found in drughted mize (Evequoz 199) nd sorghum plnts (Premchndr etl 1994) Moreover, Sobrdo (1990b) mesured genotypiclly vrying response of trnspirtion efficiency to decresed lef wter potentils in mize Percy (198) pointed out, tht pretretment nd velocity of stress development determines the contribution of stomtl nd non-stomtl limittions of photosynthesis For slowly imposed wter stress nd cycles of wter stress, the non-stomtl inhibition is ssumed to be smll nd higher dehydrtion is needed to cuse rise in the mesophyll resistnce The rther slow development of wter stress in the field sitution, compred to pot studies, might explin the incresed trnspirtion efficiency in our field expenments Incresed vlues of trnspirtion efficiency under wter stress, bsed on biomss ccumultion nd soil wter depletion in the field hve recently been reported with vlues of g kg"1 for well-wtered nd g kg'1 for wter stressed mize (Otegui et l 1995) In Chpter 2, we stted n overestimtion of trnspirtion with the porometer This overestimtion might lso explin the lower vlues of trnspirtion efficiency obtined in our experiments compred to the results of Otegui etl (1995) intensity The enhncement of trnspirtion efficiency ws strongly depending on stress For severely stressed plnts negtive effects on trnspirtion efficiency re prtly the result of increses in wter vpour pressure grdients from lef to ir when decresed trnspirtion prohibits sufficient cooling of the lef since trnspirtion efficiency is negtively ssocited with the wter vpour pressure grdient between lef nd ir (Sinclir et l 1984, Jones 1985) The effect of n incresed wter vpour pressure grdient due to higher lef temperture is pronounced in the field with high rdition compred to greenhouse nd growth chmber experiments For severely stressed plnts wter loss vi the cuticul, ssumed to be negligible for well-wtered plnts, mkes up higher proportion of totl wter loss Estimtes of cuticulr conductnce for wter vpour rnge from 1 to 20 mmol m"2 s"1 (Schonherr 1982) The upper bound estimte is bout hlf of the stomtl conductnce of severely stressed mize plnts in our experiment The incresed frction of epiderml wter loss my then contribute to decresed trnspirtion efficiency with severe wter stress, becuse it hs no link to C02 trnsport to the site of crboxyltion Prtitioning of ssimiltes Differences in the prtitioning of ssimiltes in response to wter limittions might lso result in lower trnspirtion efficiency defined on bove-ground biomss Enhnced root growth nd incresed root-shoot rtio under limited wter supply hs erly been reported (Muller-Thurgu 1875) No ttempt hs been mde to estimte produced root dry mtter in the field study Close vlues for wter-use efficiency, bsed on lef gs-exchnge nd mesured shoot biomss nd wter use of well-wtered field-grown sorghum plnts, indicted little devition in the crbon blnce due to root biomss nd respirtionl losses in sorghum (Peng nd Kneg 1992) Ludlow nd 75

81 Muchow (1990) concluded the crbon costs of n incresed rooting system under drought to be of minor importnce with respect to the smll rtio of root nd shoot biomss of plnts in lter growth stges However, due to the decy of root biomss, this rtio is likely to underestimte the crbon cost for the rooting system The pronounced trnsfer of ssimiltes into root growth s n erly rection of wter stress nd the negtive effect on trnspirtion efficiency is lso evidenced by the genotypic profiles of soil wter uptke Trnspirtion efficiency ws not incresed under the moderte stress intensity in the lte stress experiment 1994/95 for the vrieties Crgill922, DK888 nd KS6, vrieties, which ll showed strong response to wter stress in wter depletion of deeper soil lyers nd therefore presumbly strong response in root growth The high trnspirtion efficiency of Tuxpeno C0, which did not show deep root prolifertion, could hence prtly be consequence of shoot pronounced prtitioning of ssimiltes Prospects for the breeding of drought tolernt mize In the lte vegettive stge the totl mount of trnspired wter is high nd the efficient use of wter should be especilly importnt for plnts under drought conditions However, for this period no significnt genotypic correltions were found between trnspirtion efficiency nd growth prmeters under limited wter supply, indicting tht trnspirtion efficiency ws probbly not the most importnt trit for growth under wter stress The high positive correltion between wter-use efficiency nd growth under limited wter supply, found in the greenhouse study of Sin (199), could not be observed for drought tolernce in the field In the greenhouse study limittions for growth under stress my indeed exist in the efficiency of wter-use since in smll pots genotypic bility for soil wter extrction from deeper lyers is not essentil In the field, the totl mount of trnspirtion, nd hence the efficiency of the rooting system in extrcting the soil wter, is be more limiting Trnspirtion efficiency, however, ws positively correlted with growth under well-wtered conditions Therefore we do not hve ny evidence, tht by selecting for high trnspirtion efficiency, we my select for genotypes with wter sving strtegy nd negtive effect on growth, s ws ssumed by Jones (199) Also in whet it hs been shown, tht trnspirtion efficiency ws positively correlted with growth nd yield trits (Siddique etl 1990, Lopez-Cstend nd Richrds 1994) The positive ssocition might be especilly vlid for erly vegettive growth, since in fst developing cnopies the verge boundry lyer resistnce of the cnopy is higher with beneficil effect on trnspirtion efficiency vi lower cnopy temperture nd wter vpour pressure deficit The mintennce of stomtl conductnce is hence not only of relevnce for ssimiltion but will certinly help in the efficient use of the sprse stressed cnopies wter in wter 76

82 c ^^k Diurnl nd Sesonl Chnges of Lef Wter \J Sttus nd Reltions to Stomtl Conductnce 5.1 Abstrct The mintennce of stomtl conductnce t low lef wter sttus my enble drought resistnt genotypes to mintin ssimiltion nd wter uptke to higher soil wter deficits. Therefore, the reltion between lef wter sttus nd stomtl conductnce of tropicl mize vrieties hs been investigted. In field experiments during the dry seson in Thilnd sesonl nd diurnl courses of lef wter potentil nd components, stomtl conductnce nd trnspirtion rte hve been mesured during the period of vegettive growth. Wter stress ws pplied either t the erly or t the lte vegettive stge. The erly fternoon lef wter potentil declined througout the vegettive growth nd ws linerly correlted with the lef to ir wter vpour pressure grdient nd the stomtl conductnce. Sensitivity of the lef wter potentil to chnges in either of these prmeters decresed t the lte vegettive stge, but ws not ffected by the wter supply tretment. After onset of wter stress the lef wter potentil declined, but ws lter stbilized due to stomtl closure. No significnt difference in the lef osmotic potentil between well-wtered nd wter stressed plnts ws found. Diurnl courses of lef wter potentil reveled tht differences between the wter supply tretments could either be found erly in the morning or during the fternoon nd night, but diminished for the hours of mximum trnspirtion t noon. Mximum stomtl conductnce ws found erly in the morning nd did not chnge substntilly for the first hours despite of steep drop in lef wter potentil. The hysteresis in the reltion between lef wter potentil nd stomtl conductnce could prtly be ccounted for by differences in temperture nd wter vpour pressure deficit, but might dditionlly be cused by n "fternoon ftigue" in soil wter supply to the root, prticulrly in soils with low unsturted hydrulic conductivity. One vriety (KS6) showed consistently lower lef wter potentil, which ws not ssocited with lower stomtl conductnce. The lower wter potentil might enble this genotype to mintin wter uptke to the lower soil wter potentil. 5.2 Introduction Stomtl conductnce nd trnspirtion rte of tropicl mize vrieties under different wter supply were previously evluted with respect to the effects on trnspirtion efficiency nd growth prmeters (Chpter 4). A substntil effect of the chnging stomtl conductnce with incresing stress intensity ws observed. Trnspirtion efficiency ws initilly incresed, until under severe wter stress nonstomtl limittions of photosynthesis nd/or incresed wter vpour pressure 77

83 grdients between lef nd ir, cused decrese in trnspirtion efficiency Two mjor feedbck loops re involved in stomtl regultion By the C02 loop, stomtl perture is regulted by the internl C02 concentrtion s the result of C02 influx through the stomt nd the photosynthetic cpcity of the mesophyll (Willmer nd Fncker 1996) Lef tissue wter potentil s the result of wter lost by trnspirtion nd the vilbility of wter from other plnt tissue or the soil, is the centrl prt in the second feedbck loop Physicl (hydropssive) nd/or chemicl (hydroctive) signls trnsmit lef wter sttus to the gurd cells, which ctully exhibit control of stomtl perture In mny drought prone regions deficits in the soil wter supply occur coincidentlly with dry nd hot tmospheric conditions A plnt hs then to dpt to both low soil wter potentil nd high wter vpour sturtion deficits in the ir For rnge of species including mize, stomtl conductnce is ssumed to be lrgely independent of lef wter potentil until criticl level is reched When lef wter potentils drops below this threshold, which is modified by environmentl fctors nd stress history stomtl closure is observed (Hsio nd Brdford 198) Osmotic djustment, s the net ccumultion of solutes in response to wter deficits, hs been shown in rnge of species It is considered to mintin stomtl opening, photosynthesis nd the growth of roots nd leves to lower soil nd lef wter potentil (Turner 1986) Adpttions in the interdependencies between lef wter reltions nd stomtl conductnce under wter stress my so provide mens of discnmintion between drought sensitive nd drought susceptible vrieties Informtion bout the reltion between lef wter potentil nd stomtl conductnce is of further interest since the lef wter potentil is frequently included in the modelling of stomtl conductnce (Losch et l 1992, Rochette et l 1991) Therefore, we investigted in this study the reltions between lef wter sttus nd stomtl conductnce s influenced by wter stress nd high evportive demnd in the field 5. Mterils nd Methods 5..1 Lyout of the Experiments Four experiments were conducted during the dry seson 199/94 nd 1994/95 with four replictions including eight tropicl mize cultivrs nd two wter regimes rrnged in split-plot design In ll experiments wter stress ws induced by withholding wter for pproximtely four weeks t the erly vegettive stge (199/94 from 5 to dys fter emergence (DAE) nd 1994/95 from 9 to 7 DAE) or t the lte vegettive stge (199/94 from to 61 DAE nd 1994/95 from 7 to 61 DAE) lter clled "erly stress" nd "lte stress" experiments All plots received two (199/94) or three (1994/95) spnnkler irrigtions fter plnting until the withholding of wter for the 78

84 drought tretment The control plots received weekly furrow irngtion of bout 70 mm The plots included eight (199/94) or six rows (1994/95) which were 0 75 m prt nd the plot size ws 6 m2 All mesurements were restricted to the inner six or four rows Culturl prctises nd detiled description of soil wter prmeters is given in Chpter Genotypes The expenments included eight tropicl mize cultivrs (open pollinted vrieties two originting from Ksetsrt University (KU), Thilnd KS 6 nd Suwn nd two from CIMMYT, Mexico Tuxpeno seq C0 nd Tuxpeno seq Cg, four hybrids frequently used in Thilnd Crgill 922, DeKlb 888, Hercules 40 (Cib-Geigy G5440) nd KU hybrid KTX Lef wter prmeters Porometnc mesurements were conducted with LI-1600 stedy-stte porometer (Li-Cor, Lincoln, Neb USA) Lef wter potentils were mesured with portble pressure chmber (PMS 1002, PMS Inc, Corvllis, Oregon, USA) The distl prt (pproximtely one third) of the youngest fully developed lef ws cut nd immeditely inserted into the pressure chmber For older leves with prominent midnbs the lef ws cut so tht bout 2 cm of the lef blde without midrib could be plced into the rubber stopper of the pressure chmber top immeditely seled into plstic bg nd plced After the mesurement, the lef ws on ice in the field until ll mesurements were finished. Afterwrds the smples were stored in deep freezer Tht prt of the lef not used for the mesurement of the osmotic potentil ws seled into plstic bg for determintion of the wter content Leves were thwed for bout 1-2 hours before cell sp ws expressed nd osmotic potentils were mesured with vpour pressure osmometer (Wescor 5500, Wescor Inc, Logn, Uth, USA) Lef osmotic potentil hs been corrected for the dilution by poplstic wter which occurs when sp is expressed from lef tissue The poplstic wter frction ws ssumed to be 15%, ccording to estimtes bsed on concomitnt mesurements of the osmotic potentil by vpour pressure osmometry of mixed sp, nd by modified pressurevolume curve mesurements (Wenkert 1980) Leves were not rehydrted Thus, reported vlues represent the lef osmotic potentil t the ctul reltive wter content in the field for both pre-dwn nd dytime mesurements Sesonl time courses of stomtl conductnce, trnspirtion rte, lef wter potentil nd components were estblished by porometer mesurements from to 1 00 h nd lef wter sttus mesurements from 1 15 to 14 0 h throughout the expenments in the yer 199/94 All porometric mesurements were crried out on the bxil surfce of the youngest fully developed lef used Only non-shded leves were On ech of four to five dys per week one repliction ws mesured with six 79

85 mesurements on rndomly chosen plnts per plot. The results of the nlysis of vrince represents weekly mens. In the yer 1994/95 we mesured pre-dwn lef wter potentils nd components t the end of the stress periods in ll replictions with three leves per plot. Diurnl chnges of stomtl conductnce, trnspirtion rte, lef wter potentil nd components were mesured pproximtely once week on one or two genotypes. Ech point represents the men of four to six leves per time of dy, genotype nd tretment. 5.4 Results Sesonl Course of the Wter Sttus of Trnspiring Leves In Fig. 5.1 the trnspirtion rte, wter potentil, osmotic potentil nd turgor potentil of the youngest fully developed lef re depicted for the course of both experiments in 199/94. For the well-wtered plnts lef wter potentil declined from -0.8 to -1. MP with geing of the plnt. Turgor potentil decresed concomitntly, while lef osmotic potentil showed vrition from dy to dy, but did not exhibit significnt sesonl trend. The lef wter potentil in wter stressed plnts declined to -1.5 MP t the end of the lte stress experiment. The decrese of lef wter potentil occured minly t the beginning of the stress period, nd ws lter on stbilized by stomtl closure nd low trnspirtion rtes. Except for two dys in the erly stress experiment the difference in lef osmotic potentil between the well-wtered nd wter stressed plnts ws low. During the lte stress experiment positive turgor potentil ws not mintined in the leves of wter stressed plnts. The decline of lef wter potentils throughout the experiments ws t lest prtly explined by the higher wter vpour pressure deficit during the lte stress experiment, since lef wter potentils decresed with incresing wter vpour pressure difference between lef nd ir (Fig. 5.2). With incresing ge the sensitivity for chnges in humidity decresed. The developmentl stge ffected lso the reltion between lef wter potentil nd stomtl conductnce (Fig. 5.). Lef wter potentil decresed with decresing stomtl conductnce nd the lef wter sttus of plnts in the lte vegettive stge ws significntly lower thn in the erly vegettive stge for similr vlues of stomtl conductnce. Extrpoltions of the regressions suggest complete stomtl closure with lef wter potentil of bout -1.5 MP, independent of the developmentl stge nd wter tretment. Wter stress did not lter the reltion between wter vpour pressure grdient, respectively stomtl conductnce, nd lef wter potentil significntly, therefore, regressions were clculted for both developmentl stges. The effect of developmentl stge ws similrly evidenced in the reltion of lef wter potentil nd trnspirtion rte (dt not shown). The difference in the lef wter potentil between erly nd lte vegettive stge for similr 80

86 C CO gcm E CO CI) t S E o "o / ^o ^ ^ cl7 N* / <X>>v O- 2 (0 4.* --o-- WS WW VPD O 0---OV h -1 ' / / O -<> <*>-<> %o<>o-./o _1 y /I 1 1 1_ h o.. *4 'oo' o-^y _ /A Dys fter emergence Fig. 5.1 Erly fternoon wter potentil C4\), osmotic (4J,,), turgor potentil (MJp) nd noon trnspirtion rtes of well-wtered nd wter stressed mize leves nd noon wter vpour pressure difference The expenments were conducted dunng vegettive growth nd included eight genotypes The totl mens of wter supply tretments re presented with error brs indicting one stndrd error of men (n=2) 81

87 - -08 r= 0 74" (erly) o eriy ws eriy ww lte ws lte ww r= 0 66** (lte) 4 5 VPDLeltor(kP) Fig. 5.2 Reltion between erly fternoon lef wter potentil nd lef to difference (VPD,e(,0 ir) ir wter vpour pressure for wellwtered (ww) nd wter stressed (ws) mize t the erly (y= x) nd lte (y= x) vegettive stge Ech point represents men of eight genotypes Regressions re clculted for ech developmentl stge nd include both wter supply tretments E o r/> y=19x (lte) / ^ = 0 82* * / - St 7 " f mml _/ / * / f / y 0 62 = x (erly) /qoo* *> ^ = 081"* i i i VL (MP) o erly ws erly ww m ltews lte ww Fig. 5. Reltion between lef wter potentil nd stomtl conductnce for well-wtered (ww) nd wter stressed (ws) mize t the erly nd lte vegettive stge Ech point represents men of eight genotypes Regressions re clculted for ech developmentl stge nd include both wter supply tretments vlues of stomtl conductnce ws therefore not solely due to the incresed wter vpour pressure difference in lter vegettive stges Genotypic Vribility in Lef Wter Reltions Genotypic mens of wter potentil, osmotic nd turgor potentil of leves, mesured, in the erly fternoon, during the lte stress experiment 199/94 re given in Tble 5 1 Some genotypic differences were found for the lef wter potentil nd osmotic potentil but not for the turgor potentil KS6 showed consistently the lowest wter nd osmotic potentil The difference in osmotic potentil between well-wtered 82

88 nd wter stressed plnts ws very smll nd not significnt, except for the intermedite period of the erly stress experiment. The pre-dwn lef wter potentil, mesured in the expenments 199/94, ws significntly higher for the well-wtered tretment (shown for the end of the lte stress period in Tble 5 2) Osmotic potentil nd turgor potentil of leves were only insignificntly decresed in wter stressed plnts Apprent osmotic djustment, s the difference in osmotic potentil in well-wtered nd wter stressed plnts t mximum rehydrtion ws bout 0 08 MP nd, in mesurements, not significnt osmotic djustment ws found ccordnce with the results of dytime No significnt genotypic vrition for the pprent Lef wter potentil nd stomtl conductnce of the mens of genotypes were negtively correlted (Fig 5 4) KS6 showed the lower lef wter potentil despite similr vlues of stomtl conductnce in comprison to ll other genotypes in both wter supply tretments This potentilly different behviour of KS6 ws further investigted in the yer 1994/95 by mesunng stomtl conductnce, trnspirtion rte nd lef wter sttus in temperture response to the diurnl course of rdition, humidity nd Tble 5.2 Pre-dwn lef wter potentil (UJL) osmotic (tpn) nd turgor potentil (i Jp) of the lte stress experiment 1994/95 (DAE 60) for four tropicl under well-wtered (ww) nd wter stressed (s2) conditions (in MP) t the end mize cultivrs wter supply WW s2 WW S2 WW s2 -ml---- -Yrr "Vp Crgill DKK KS Tuxpeno C c Averge1 A B A A A A Mens in column not followed by the sme letter re significntly different t the 0 1 probbility level ccording to Student Newmn-Keuls-Test 1 Tretment differences re indicted by different cpitl letters 8

89 """" - Tble 5.1 Lef wter potentil (i JL), osmotic potentil (tj/n), nd turgor potentil (ifjp) t noon nd rnkings of eight tropicl mize cultivrs under well-wtered (ww) nd wter stressed (s2) conditions during the lte stress expenment 199/94 (in MP) wter supply WW s2 WW s2 WW s DAE Wl"--- " " " " "...""" HI n M»p Crgill (7) -1 8 (6) b (6) b (7) 0 1 (6) 0 04 (5) DKK (2) -1 8 (5) -1 4 b (2) -1 42b(6) 017 () 0 05 () Hercules (1) -1 9 (7) (1) -1 8b(2) 0 09 (7) 0 00 (7) KS6-1 6 (8) -1 8 (4) -1 5 b (8) b (5) 017 (4) 0 0 (6) KTX (5) -1 4 (2) -1 4b(7) -1 2 (1) 0 18 (D (8) Suwn () -1 5 () -1 8b(5) () 0 18 (2) 0 05 (4) Tuxpeno C (4) (1) -1 5b(4) (4) 0 14 (5) 011 (2) Tuxpeno C (6) (8) -1 4b() b (8) 0 08 (8) 012 (l) Averge1 A B A A /\ ;\ DAE Crgill b (7) -1 6 (1) (8) -1 8 () 0 10 (4) 0 02 () DKK (1) () -1 4 () -1 6 (2) 0 25 (1) (7) Hercules b(2) -1 4 (5) -122(1) (4) 0 02 (6) (6) KS6-1 9 b (8) (8) -1 8 (6) b (8) (7) (5) KTX b(5) (7) -1 6 (4) -1 (1) 0 06 (5) (8) Suwn -1 5 b (6) (6) (2) (5) (8) -0 0 (4) Tuxpeno C b () -1 9 (2) -1 9 (7) (6) 0 18 (2) 0 0 (2) Tuxpeno C8-1 22b (4) -14 (4) -1 7 (5) (7) 0 15 () 0 0 (1) Averge1 A B A A A B DAE Crgill (6) (4) -1 5 (6) (1) 0 2 () (8) DKK (5) (6) (4) -1 6 () 012 (7) (7) Hercules (4) (7) -1 9 () (6) 0 10 (8) (5) KS6-1 5 (8) (8) (8) -1 5 (7) 0 2 (2) () KTX (2) (1) -1 (1) (5) 01 (6) 0 00 (2) Suwn (1) (2) -1 (2) -1 6 (2) 0 17 (5) (4) Tuxpeno C0-1 2 () (5) (5) -1 8 (4) 0 21 (4) (6) Tuxpeno C8-1 (7) -145 () (7) -1 5 (8) 0 2 (D 008 (1) Averge1 A ;k A A A A Mens in column not followed by the sme letter re significntly different t the 0 1 probbility level 1 ccording to Student-Newmn-Keuls-Test Tretment differences re indicted by different cpitl letters 84

90 " - o E E c co D "O c o o "r r E o co well-wtered -T- ; -+- * -f wter stressed i i i i i A Crgill 922 T DK888 0 Hercules 40 KS6 o KTX 101 D Suwn Tuxpeno C0 Tuxpeno C i /L (MP) Fig. 5.4 Reltion between lef wter potentil nd stomtl conductnce for eight well-wtered nd wter stressed mize vrieties t noon. Points represent the mens for the experimentl period t the lte vegettive stge. Error brs indicte one stndrd error of men referring to four replictions 5.4. Diurnl Course of Lef Wter Reltions, Stomtl Conductnce nd Trnspirtion Rte Typicl diurnl courses of wter potentil, osmotic nd turgor potentil, stomtl conductnce nd trnspirtion rte of leves, re shown in Fig. 5.5 for the genotypes KS6 nd DK888 t the end of the lte stress experiment 1995 (DAE 60), long with the diurnl courses of lef nd ir tempertures, photosynthetic photon flux density PPFD nd wter vpour pressure deficit in the ir VPD. For the hours of mximum trnspirtion the lef wter potentil ws not different in wter stressed nd wellwtered plnts. The recovery of plnt wter sttus throughout the fternoon nd the night ws significntly delyed in wter stressed plnts nd lef wter potentil t pre dwn were significntly lower. Osmotic potentil decresed in the course of the dy from bout -1 MP t pre-dwn to -1.5 MP t the erly fternoon with no significnt differences between the wter supply tretments. Mesurements of wter content indicted, tht the concentrtion of solutes due to dehydrtion cn not ccount for the decresed osmotic potentil during the dy s consequence of dehydrtion (dt not shown). The turgor potentil declined from bout 1 MP t pre-dwn to zero t the erly 85

91 _ jz iiu -," 100 E O si «E O \ /-\ ffl g 5-1 J Time of dy DK888 ws DK888 ww KS6ws KS6ww / f\ VPD PPFD \ V J I I I I I L o 2 0. > Time of dy Fig. 5.5 Diurnl course for lef wter sttus (left grphs), stomtl conductnce, trnspirtion rte, lef temperture for two genotypes t the end of the lte stress period long with ir temperture, wter vpour pressure deficit VPD nd photosynthetic photon flux density PPFD (right grphs) Ech point represents the men of four to six mesurements 86

92 fternoon for both well-wtered nd wter stressed plnts Significntly higher lef turgor potentils were observed in well-wtered plnts during the lte fternoon nd the night Stomtl conductnce in well-wtered plnts ws high in the morning when both VPD nd PPFD were low nd declined throughout the fternoon when PPFD decresed but VPD ws still high Trnspirtion rte showed more symmetricl pttern throughout the dy nd ws mximum t noon or erly fternoon The time of mximum stomtl conductnce in wter stressed plnts ws dependent on stress intensity, but ws generlly reched erlier in the morning nd declined throughout the dy Lef tempertures were significntly higher thn mbient for wter-stressed plnts nd lower for well-wtered plnts during the hours of trnspirtion These differences in lef tempertures for the different wter supply tretments incresed significntly the lef to ir wter vpour pressure grdient nd consequently trnspirtion rte (compre Chpter 2) KS6 showed typiclly for the hours of mximum trnspirtion slightly lower lef wter nd lef osmotic potentil compred to DK888, wht confirms the results of the sesonl mesurements in the preceding yer In contrst, diurnl courses of trnspirtion rte, stomtl conductnce nd lef tempertures did t none of the investigted dys indicte ny significnt genotypic differences In the diurnl course of the reltion between stomtl conductnce or trnspirtion rte nd lef wter potentil strong hysteresis effect ws found (Fig 5 6) For the morning hours stomtl conductnce did not chnge substntilly despite of the drop in lef wter potentil, nd for the trnspirtion rte significntly higher lef wter potentil ws observed in the morning compred to the fternoon These reltions were shifted to lower vlues of lef wter potentil nd stomtl conductnce under wter stress but the pttern of hysteresis ws mintined From six diurnl courses, three from the lte stress experiment in the yer 199/95 nd three from the lte stress experiment in the yer 1994/95, dt set ws extrcted with observtions of lef wter potentil, stomtl conductnce nd ir temperture, humidity nd wter vpour pressure grdient mesured t similr PPFD (1200 to 1700 umol m2s1) t bout nd 14 0 h in both the well-wtered nd the wter stressed tretment Simple nd multiple regressions were clculted to revel if the chnge in temperture nd ir humidity cn ccount for the hysteresis in the reltion of lef wter potentil nd stomtl conductnce Coefficients of determintion for the different models re given in Tble 5, prmeter estimtes for the models on the whole dt set re listed in the ppendix A2 About hlf of the vrition in stomtl conductnce for the whole dt set could be explined by the prmeters lef wter potentil nd temperture The sme degree of determintion ws found for the stomtl conductnce of wter stressed plnts with the lef wter potentil lone Atmospheric conditions, nd especilly temperture, 87

93 J*^*~~. M Fig. 5.6 Diurnl hysteresis in the reltion of lef wter potentil to stomtl 120 conductnce (upper grph) nd 100 trnspirtion rte (lower grph) of E E E \f-- -<>M / E well-wtered (ww) nd wter stressed (ws) mize plnts t the lte vegettive stge M indictes the first mesurement t 7 0 h in the morning nd E the lst 0 5 O ws WW mesurement t h in the evening Ech point represents the men of eight to twelve Vv mesurements on two genotypes <7- E Tble 5. Coefficients of determintion (r2) for simple nd multiple regressions of lef wter potentil (ijil), wter vpour pressure deficit in the ir (VPD) nd ir temperture (T) on stomtl conductnce Dt sets included either ll observtions (ll), or only observtions in the morning (AM), or fternoon (PM), or only on well-wtered (ww) or on wter stressed (s2) plnts Levels of significnce re given in brckets, n is the number of observtions model ml ipl nd VPD ipl nd T dt set n ll (0 009) 0 2 (0 04) 0 47 (000) AM (0 1*6) 0 6 (0 14) 0 47 (0 057) PM 10 0 (0 082) 0 (0 242) 0 9 (0180) ww (o 120) 0 25 (0 27) 0 50 (006) s (0012) 0 5 (0 049) 0 65 (oois) could t lest prtly ccount for the hysteresis since the coefficients of determintions incresed substntilly for ll dt sets, except for the observtions in the fternoon 88

94 For this time of the dy, stomtl conductnce cn not sufficiently be explined by lef wter potentil nd meteorologicl prmeters, indicting tht the hysteresis in the reltion between stomtl conductnce nd lef wter potentil is not solely due to the chnge of meteorologicl prmeters In generl, the coefficients of determintion were rther low By including the bulk soil wter potentil (verged over ll depths) into the model with lef wter potentil nd temperture, the determintion ws significntly incresed for wter stressed = plnts (r2 0 84, p=0 004), nd for observtions in the morning (r2 = 0 60, p=0 050), but not for the other dt sets Rdition did not significntly ffect stomtl conductnce in the selected dt set Also Bethenod et l (1990, in Trdieu nd Dvies 199) reported the mximum stomtl conductnce of field-grown mize being reched with less thn 1000 pmol m'2 s'1, moreover, stomtl conductnce ws usully mximum in the morning nd strted to decrese despite tht PPFD ws still rising 5.5 Discussion Lef wter potentil nd stomtl conductnce on sesonl time scle The higher sensitivity of lef wter potentils to humidity were found in the erly vegettive stge, compred to the lter vegettive stge, wht might be the result of higher boundry lyer resistnce in the lter vegettive stge (Chpter 2, Fig 2 2) nd higher degree of decoupling the lef wter reltions from tmospheric conditions Throughout the vegettive development lef wter potentils t noon decresed for both well-wtered nd wter stressed mize plnts without consistent decrese in lef osmotic potentils However, the lef wter potentil, where stomtl conductnce declined to zero, ws not significntly reduced in the lte vegettive stge compred to the erly vegettive stge In other investigtions, the lef wter potentil, where stomtl conductnce or gs-exchnge zeroed, ws lower t nthesis or grinfilling compred to the vegettive stge This indictes higher resistnce to wter stress in these lter stges for mize (Humn et l 1990, Ackerson 198, McPherson nd Boyer 1977), whet (Tere etl 1982), or perl millet (Henson etl 198) Decrese in lef wter potentils with geing of the crop ws lso found in other field experiments with mize (Fereres et l 1978, Wolfe etl 1988) The lowering of the lef wter sttus in growing mize cnopy my indicte tht root prolifertion into deeper soil lyers nd the increse of wter uptke does not sufficiently compenste for the higher trnspirtionl wter loss of fully developed mize cnopy This is supported by the finding of decresing root-shoot rtio for field-grown mize between 20 to 100 dys fter emergence (Foth 1962) Bolfios et l (199) observed n decrese of lef wter potentil from 50 to 100 dys fter emergence Mximum lef re ws presumbly reched erlier nd they postulted n incresed resistnce to wter uptke with geing of the crop Stomtl closure t the threshold lef wter potentil is ssumed to be mechnism to prevent cvittions in the xylem (Shendr et l 1995) The 89

95 crops might then be less susceptible to xylem cvittions t lter growth stges The lower minimum lef wter potentils my likewise indicte high priority of mintining ssimiltion t reproductive stge, even with the risk of decresed conductivity in the xylem, nd uptke of wter nd nutrients from the soil The response in lef wter potentil to differences in VPD ws not chnged by the wter supply tretment The sme ws found for the reltion between lef wter potentil nd stomtl conductnce, both mesured in the erly fternoon, suggesting role of the lef wter reltions in regulting stomtl conductnce independent of the bulk soil wter potentil Stomtl responses to tmospheric humidity do, however, not necessrily need chnge in the bulk or mesophyll lef wter potentil, but re probbly induced by cuticulr trnspirtion of the gurd cells nd neighbouring cells, so clled pen-stomtl trnspirtion, which minly ffects epiderml wter potentil (Lnge etl 1971, Hsio nd Brdford 198) Lef wter potentil nd stomtl conductnce on dily time scle On sesonl time scle the vlues of stomtl conductnce nd lef wter potentil mesured t the eriy fternoon were highly correlted, wheres on diurnl time scle lef wter potentil cn by fr not ccount for the chnge of stomtl conductnce In contrst to growth chmber expenments, where lower lef wter potentils in wter stressed mize plnts mirrors the lower vilbility of soil wter lso throughout the dy, this is no longer vlid under high evportive demnd in the field Here the threshold lef wter potentil, where stomtl closure occurs, is reched during the hours of high evportive demnd in both well-wtered nd wter stressed plnts Alredy erly in the morning the difference in stomtl conductnce nd trnspirtion between well-wtered nd wter stressed plnts cnnot be explined by differences in lef wter potentil, or requires pre-conditioning of stomtl conductnce by the difference in pre-dwn lef wter potentil Low correspondence between lef wter potentil nd stomtl conductnce hs frequently been shown (Btes nd Hll 1981, Golln et l 1986, Henson et l 1989b, Trdieu etl 1991) ABA synthesis in dehydrting roots nd its trnsport to the shoot vi the xylem is believed to override the lef wter sttus in regulting stomtl conductnce (Wlton etl 1976, Dvies nd Zhng 1991, Trdieu nd Dvies 199) Krmer (1988) considered the expenmentl pproches, where prt of the roots or the whole root system ws exposed to dry soil, when experimentlly high shoot wter sttus ws mintined, s being little relted to the field sitution Here wter deficits my occur in leves s consequence of trnspirtion despite rther high bulk soil mtnc potentil Our field expenments evidenced the regultion of stomtl conductnce without chnges in lef wter potentils even for rther high evportive demnd A constnt lef wter sttus with decresing stomtl conductnce requires feedforwrd regultion of stomtl conductnce so tht the reduced trnspirtion then 90

96 llows the prtil recovery of lef wter sttus Besides the dehydrtion of roots in dry soil, chnges in the hydrulic resistnce in the root hve been suggested to be involved in the stomtl control of trnspirtion (Sliendr nd Meinzer 1989) For mize nd sugrcne the hydrulic resistnce in the root ws found to increse grdully with decresing soil wter potentils (Sliendr nd Meinzer 1989, Schmidhlter 1995) The diurnl vrition in stomtl conductnce my be influenced by decrese of the soil wter potentil, ner to the root, ffecting root wter sttus or root hydrulic conductnce, s consequence of the wter uptke during the dy, which cn not sufficiently be replenished Diurnl chnges of the soil wter potentil, ner to the root, might be prticulrly high in this soil with low unsturted hydrulic conductivity pttern of hysteresis in the reltion between lef wter potentil nd stomtl conductnce ws observed for both well-wtered nd wter stressed plnts, but the The diurnl rnge of stomtl conductnce decresed with decresing soil wter sttus No dt bout diurnl decrese in soil/root wter potentil for the observed field sitution re vilble, but with the higher rte of wter uptke the diurnl drop in soil wter sttus ner to the root might be higher for the well-wtered plnt nd cuse the greter decline of stomtl conductnce in well-wtered plnts The diurnl hysteresis in the reltion of lef wter potentil nd stomtl conductnce due to n "fternoon ftigue" in soil wter supply my lso be vlid for other sites, depending on soil hydrulic prmeters, nd cn then provide n explntion for lck of greement between modelled nd mesured vlues of diurnl courses of stomtl conductnce, bsed on lef wter potentil, rdition nd meteorologicl prmeters, e g the overestimtion of stomtl conductnce in the fternoon, s observed by Losch et l (1992) Osmotic djustment A decrese of osmotic potentil ws observed during the dy, but not on sesonl time scle, neither with mesurements t noon nor with pre-dwn mesurements The mesurements of osmotic potentils t noon re likely to be confounded by the concentrtion of solutes due to decrese in wter content, cellulr shrinkge, chnge in lef tissue elsticity, or reltive prtitioning between symplstic nd poplstic wter frctions (Girm nd Kneg 1992) At pre-dwn the lef tissue experiences mximum rehydrtion nd the lef wter potentil is ssumed to be in equilibrium with the soil wter potentil in the wettest prt of the soil (Schmidhlter 1994) Sesonl chnges of osmotic djustment should rther be elborted from pressure-volume curves, where informtion bout reltive wter content, lef tissue elsticity, nd prtitioning between poplstic nd symplstic frction ll re vilble However, since the reltion between lef wter potentil nd stomtl conductnce hs not been modified by wter stress, significnt effect of osmotic djustment on 91

97 stomtl behviour ws not evidenced in our expenments The rdition dependent build-up of solutes induces the decrese of lef osmotic potentils dunng the morning hours of the dy (Turner nd Begg 197, Acevedo et l 1979, Wenkert 1981) In contrst to our results, Turner (1974) reported significntly higher diurnl chnge of osmotic potentils for mize plnts t lower soil mtric potentils The influence of the velocity of wter stress development ws emphsized by Turner nd Jones (1980) nd the rther fst development of wter stress, prticulrly for the lte stress experiment with the trnspirtion rte decresing more thn 50% within 10 dys, might then explin the lck of osmotic djustment However, Premchndr et l (1989) reported substntil decrese of osmotic potentil (-0 5 nd -0 8 MP) lredy ten nd twenty dys fter withholding wter The time of smpling in the ltter study rnged from to 1 00 h Therefore, the increse might prtly be confounded with diurnl djustment In ccordnce with our results Fereres et l (1978) reported osmotic djustment only for moderte stress intensity nd the erly vegettive stge Limited cpcity nd genetic vnbility for osmotic djustment in tropicl germplsm ws lso reported from other expenments including brod rnge of cultivrs nd popultions (Bolhos nd Edmedes 1991, Bolhos et l 199, Guei nd Wssom 199) The osmotic djustment observed in sorghum ws not ssocited with the mintennce of stomtl conductnce nd ssimiltion rte under wter stress (Girm nd Kneg 1992b, Flower et l 1990) We only focused on the impct of osmotic djustment on the mintennce of stomtl conductnce Osmotic djustment certinly ffects growth under wter stress vi other mechnisms, e g effects of mintined turgor on cell expnsion For droughted plnts the solutes ccounting for the decresed osmotic potentil re minly orgnic (Evequoz 199) The vlue of osmotic djustment for drought tolernce, in the gronomic mening of higher growth under drought, hs then been questioned, since the solutes ccounting for it hve to be diverted from other processes like protein nd cell wll synthesis (Munns 1988) In conclusion, osmotic djustment seems not to be substntil tool for incresing drought tolernce in mize Genotypic vribility in the interdependences of plnt wter reltions nd stomtl conductnce Some genotypic vntion for lef wter nd lef osmotic potentil ws evidenced Prticulrly for the wter stressed tretment nd in tendency lso for the well-wtered tretment, KS6 hd the lower lef wter potentil nd lower osmotic potentil compred to the other vrieties but did not devite in stomtl conductnce In Chpter 4 it ws shown, tht KS6 showed n erly stomtl closure fter onset of wter stress, but hd reltively high stomtl conductnce with severe wter stress Comprtively high growth under wter stress ws found for this vriety in the erly vegettive stge, which is, with respect to totl dry mtter ccumultion, less importnt thn the lte vegettive 92

98 stge. A somewht lower wter potentil might enble KS6 to mintin wter uptke to the lower soil wter potentil. McGown et l. (1984) estimted n increse in cumulted wter uptke by bout 25 mm for whet, s consequence of osmotic djustment nd decresed lef wter potentils by bout 0.5 MP. KS6 differed by bout 0.15 MP from the other vrieties. For our field experiments, wter uptke to soil wter potentil of bout 0.15 MP below the verge, would increse wter uptke by bout 2 mm (A0=O(.11MP) nd 0(.1-15 MP), 1m depth). In reltion to the totl wter uptke for the stress period, this cn be n dvntge for the erly vegettive stge, but is certinly of minor relevnce for the lte vegettive stge with wter consumption of up to 50 mm per week (see Chpter 4, Fig. 4.) In comprison, dditionlly 15 mm wter might be gined by incresing the depth of wter uptke by 10 cm (15% plnt vilble wter, wter uptke of 1 m depth). Both theoreticl considertions will hold, for severe nd prolonged periods of wter stress, nd hve restricted mening for spells of drought, where extrction of plnt vilble wter is frequently not complete. However, it my indicte some limits for improving wter consumption nd consequently growth under wter stress with selection for the lower lef wter potentil. The lower lef wter potentil cn dditionlly be dvntgeous due to the higher uptke of wter from the upper soil lyers with higher concentrtions of minerl nutrients. Besides the mount of totl extrctble wter the incresed grdient between soil nd plnt wter potentil will lso increse the wter uptke rte. In comprisons of drought tolernt nd drought susceptible genotypes, the mintennce of higher stomtl conductnce to the lower lef wter potentil ws found for the drought resistnt genotype by Skretkowicz nd Thurtell (198). The ltter reltion ws lso found to be ssocited with higher drought resistnce in sorghum (Wright et l. 198, Premchndr et l. 1994). Lorens et l. (1987b) reported the higher lef wter potentil long with higher stomtl conductnce for the resistnt genotype. The common ttribute of the drought tolernt vriety ws the higher root length density deeper in the soil profile for mize nd sorghum (Lorens et l. 1987b, Wright nd Smith 198). Wright nd Smith (198) found for the drought resistnt genotype in sorghum 65 mm higher soil wter depletion t the end of the seson due to both more extensive utiliztion of stored wter nd the higher uptke in deeper soil lyers. The beneficil effect of n enhnced soil wter explortion in deeper soil lyers for mintennce of growth nd stomtl conductnce ws lso emphsized in experiments (see Chpter 4), but the combintion of both trits ws not evidenced in the investigted vrieties. The lower lef wter potentil observed for KS6 ws not ssocited with the higher plnt wter stress intensity, since stomtl conductnce ws mintined nd therefore ssimiltion ws t lest not prevented by stomtl closure. Hence, the lef wter potentil ws in this investigtion less suitble tool to monitor wter stress intensity for hours of mximum trnspirtion. A difference in the lef wter potentil between well-wtered nd wter stressed plnts could either be found t pre-dwn or our 9

99 in the lte fternoon. Therefore, if we re interested in even smller genotypic differences, we my rther hve to mesure during the fternoon nd t the phse of recovery thn during the time of mximum trnspirtion. Obviously, with respect to the rther steep rise in lef wter potentil in the fternoon this is imprcticl for higher number of entries. The informtion bout the plnt wter stress sttus ws in our experiments rther provided by the mesurement of stomtl conductnce. 94

100 ^mj Generl Discussion nd Conclusions In the Chpter 2 nd different methods hve been evluted to estimte trnspirtion nd trnspirtion efficiency in field experiments (porometric determintion, soil wter blnce, minerl element content). With neither of these methods significnt genotypic differences in trnspirtion efficiency were indicted, prticulrly for wter stressed conditions. There re other methods, which were not considered in this study. Meteorologicl prmeters cn be used to clculte cnopy trnspirtion ccording to the Monteith combintion eqution. Common plot size in field experiments mostly does not fulfill requirements for representtive meteorologicl mesurements, however with respect to the uncertinty in the soil wter blnce it my serve s the reference method in the field nd llows furthermore short term mesurements on diurnl time scle (Rochette et l. 1991, Losch et l. 1992, Jensen et l. 199). Other redy vilble systems monitor either the trnsduction of het pulses in the stem, or the het blnce for stem segment, s estimtes of sp flow velocity. These sp flow mesurements reveled in different woody nd herbceous species high precision for single plnt trnspirtion rtes (Cohen et l. 1990; Steinberg et l. 1989, Ishid et l. 1991). For mize however, severe limittions were found. For the het blnce method the heterogeneous distribution of conducting vessels in the stem leds to poor representtion of temperture chnges in the stem cross-sectionl re becuse only the stem surfce temperture is mesured (Cohen nd Fuchs 1989, Cohen et l. 199). For the het pulse method the low sp flow rtes, especilly for wter-stressed plnts, cused poor ccurcy. Moreover, this pproch is rther pproprite for precise mesurements in few plnts thn for long term observtions in field experiments, since for every plnt to mesure well-fitting guges hve to be instlled. The mesurement of ssimiltion nd instntneous gs-exchnge trnspirtion efficiency does not necessrily represent the trnspirtion efficiency in n gronomicl mening of bove-ground dry mtter yield per unit of trnspirtion. The porometric method hs been questioned becuse of possible rtifcts due to the cuvette enclosure of the lef, but lso becuse of the limited stomtl control of trnspirtion in dense crop stnd with low boundry lyer conductnce (Jones 1987). However, resonble greement ws found for porometnclly determined trnspirtion nd trnspirtion ssessed by the soil wter blnce in this study. Moreover, trnspirtion rte of well-wtered crop 95

101 stnd ws indeed not strongly dependent on stomtl conductnce, wheres for wter stressed plnts stomtl conductnce ws mjor determinnt of trnspirtion (Fig 4 7) In conclusion, the choice of method is unlikely to be responsible for the insignificnt genotypic vrition for trnspirtion efficiency in the field It my either not exist t ll, or t lest not in mgnitude, which is significnt in field experiments nd plys mpr role for the dry mtter ccumultion of genotypes under spells of drought during vegettive growth The bove methods for determintion of trnspirtion (porometry, gs-excgnge mesurements, soil wter blnce) hve, however, potentil in evluting trits, which re relted to drought tolernce The impct of wter stress on trnspirtion efficiency is still under debte (see contrsting results of gs-exchnge mesurements) The mesurement of gs-exchnge cn show whether the limittion of ssimiltion in wter stressed plnts is minly stomtl, s we concluded from the incresed trnspirtion efficiency, or is rther locted in the photosynthetic cpcity in wter stressed plnts Moreover, the reltion between crbon ssimiltion nd bove-ground biomss my provide informtion bout the root-shoot prtitioning of ssimiltes of different genotypes Peng nd Kneg (1992) ssumed tht the ssimilted crbon, which is not found in shoot biomss hs been trnslocted to the root system The neutron probe mesurements of soil wter depletion were found to describe very decisive trit for the performnce under drought The profile of wter uptke indictes the desired trit of wter uptke nd not the root growth per se, which ws speculted to be potentilly higher thn optimum (Pssiour 198) The genotypic correspondence of growth under drought nd soil wter extrction t deeper soil lyers suggests tht the investment into the root system does not imply deficits in shoot growth Fischer et l (198) suggested in view of the inherent difficulties in mesunng root growth nd wter uptke tht the selection for n improved lef wter sttus would result in n improved root system The higher root length density nd higher lef wter potentil ws found for the drought resistnt genotype by Lorens et l (1987b) However, the higher lef wter sttus cn be misleding in mize, considering our results where differences in the lef wter sttus did not hve mening for stomtl conductnce under wter stress Extrction of soil wter in deeper soil lyers ws rther mirrored in mintined stomtl conductnce The investigtion of soil wter uptke ws in this study limited to four genotypes Since reltive stomtl conductnce corresponded with the genotypic profiles of soil wter uptke further studies re required to mnifest the potentil screening of rooting chrcteristics by this mesurement on the esy ccessible prt of the plnt 96

102 Genotypic vribility for stomtl response to wter stress hs previously been shown in mize (Ackerson et l. 1980, Skretkowicz nd Thurtell 198, Lorens et l. 1987b, Humn et l. 1990, O'Regn et l. 199). These experiments were mostly done with potted plnts nd were ll limited to two genotypes. In this study the limited pplicbility of stomtl conductnce mesurements in rnge of genotypes under field conditions ws evidenced with the insufficient precision nd thus imprcticl high number of mesurements which re required. The high vribility in stomtl conductnce with chnging environmentl conditions hs not been sufficiently considered nd is often likely to be responsible for the low correltions with more integrtive trits of performnce under drought (see e.g. Tuberos et l. 1994). For selection the vribility does not only restrict the gin of selection but my lso increse the risk for discrding potentilly good lines. With respect to the high vribility of stomtl conductnce we my therefore look for other trits chrcterising specific stomtl behviour with less environmentlly medited vrition. The concentrtion of ABA in lef tissue or xylem sp is such potentil trit, lthough ABA is known to induce mny chnges in the physiology nd development of crops (see e.g. Qurrie 1991). Sensitive stomtl closure ws correlted with high ABA concentrtion nd suggested to be ssocited with drought tolernce (Pekic nd Qurrie 1987, Lebreton et l. 1994). Our results of stomtl behviour nd performnce under drought suggest tht rther the mintennce of stomtl conductnce is trit chrcterising drought tolernce thn sensitive stomtl behviour. Lndi et l. (1995) found delyed pollen shed, reduced plnt height nd higher grin yield reduction under wter stress in hybrids with the higher ABA concentrtion in leves nd concluded the concentrtion of ABA rther to be n indictor of plnt wter stress thn cusl gent cpble of limiting yield reduction under drought stress. Thus, it is not yet consistently shown which designtion of stomtl behviour nd ABA concentrtion is chrcterising drought tolernce in n gronomicl mening of higher yield under drought, nd is certinly dependent on the drought scenrio studied. It might lso be possible tht the drought tolernce enhncing effect of ABA is not primrily linked to the stomtl behviour, but to the enhncement of root growth with consequences for the totl wter uptke. When genotypic vrition in trnspirtion efficiency does not exist the mount of totl wter uptke determines dry mtter yields under limited wter supply. 97

103 Summry Spells of drought dunng the growing seson re mjor limittions to mize production in semind regions In the rnge of physiologicl trits relted to drought tolernce, trnspirtion efficiency, s the rtio of dry mtter produced per unit of trnspirtion, hs received erly scientific interest However, nd t lest prtly due to methodologicl problems, little informtion is currently vilble bout trnspirtion efficiency of field-grown mize plnts nd its potentil for improving drought tolernce Therefore, we investigted (i) methodologicl pproches to determine trnspirtion efficiency nd (n) genotypic vribility in trnspirtion, trnspirtion efficiency nd the relted trits stomtl behviour nd lef wter reltions Responses to wter stress in either of these trits were nlyzed with respect to the potentil for improving drought tolernce in mize In growth chmber nd field experiments dunng the dry seson in Thilnd coefficients hve been determined to mesure plnt nd cnopy trnspirtion bsed on porometric ssessment of stomtl conductnce Grvimetnc determintion of wter loss from potted plnts nd soil wter blnce, bsed on neutron probe mesurements, served s the reference methods Above-ground dry mtter ccumultion, stomtl conductnce, cnopy trnspirtion nd trnspirtion efficiency, nd lef wter reltions were investigted for eight tropicl mize cultivrs during the vegettive growth The field experiments were conducted for two yers dunng the dry seson in Thilnd Wter stress ws imposed either t the erly or t the lte vegettive stge by withholding the weekly furrow irrigtion for period of four weeks Additionlly, the soil wter depletion ws monitored t 20, 40, 60, 80 nd 100 cm soil depth with neutron probe mesurements Trnspirtion of mize cnopy could be described by the stomtl conductnce t defined lef position nd the whole plnt lef re Some limittions of the method cused n overestimtion of trnspirtion with porometry for field conditions, but reltive compnsons were relible Sttisticl nlysis of the vribility of trnspirtion rtes in the field reveled tht porometric mesurements my llow detection of genotypic differences for well-wtered plnts, but with wter stressed plnts differences must exceed 20% Another more integrtive pproch for the determintion of trnspirtion efficiency hs been investigted For minerl elements, tken up by mss-flow, the totl minerl 98

104 element content in the plnt should be correlted with cumulted trnspirtion, nd consequently, the concentrtion of the minerl element to the trnspirtion efficiency Mize plnts were grown, either in pots or in the field, under different wter supply The concentrtion of minerl elements nd hydrochloric cid non-soluble sh (nhci-sh) s n indictor of Si in the plnt tissue ws mesured uptke of Si, could be described ssuming mss-flow In the pot experiments the In the field high, non-liner correltion ws found for the concentrtion of nhci-sh in the leves nd trnspirtion rte, verged for the preceeding period In ccordnce with the porometric result there ws no significnt genotypic differences in trnspirtion efficiency indicted by the concentrtion of nhci-sh When vntion in trnspirtion efficiency ws incresed by considenng tretment mens significnt correltions were obtined The concentrtion of other minerl elements ws not suitble for evluting trnspirtion nd trnspirtion efficiency in mize Wter stress reduced dry mtter ccumultion, stomtl conductnce nd trnspirtion With incresing stress intensity wter ws tken up succeedingly from deeper soil lyers Trnspirtion efficiency incresed initilly under wter stress to bout 140% of the control, nd decresed with higher stress intensity Diurnl courses of lef wter potentil reveled tht differences in lef wter potentil between wter supply tretments cn either be found erly in the morning or during the fternoon nd night, but diminished for the hours of mximum trnspirtion t noon Genotypic differences were significnt for dry mtter ccumultion nd trnspirtion but not for the response to wter stress Despite generlly high error vrinces genotypicl differences in stomtl conductnce nd in the response to wter stress were indicted Differences in cumulted trnspirtion were to very lrge extent determined by the lef re nd only slightly ffected by genotypic differences in stomtl conductnces within the irrigtion tretment The genotypicl shift in soil wter uptke to deeper soil lyers under drought corresponded with the mintennce of high stomtl conductnce under wter stress One vriety showed consistently lower lef wter potentil in both wter supply tretments The lower lef wter potentil ws not ssocited with higher plnt stress, since stomtl conductnce ws mintined Genotype mens of dry mtter ccumultion were positively correlted with trnspirtion efficiency in well-wtered plnts, but were insignificntly correlted within the wter stressed tretment No significnt genotypic differences were found for trnspirtion efficiency in eight tropicl mize cultivrs Therefore, not the trnspirtion efficiency but rther the cumulted trnspirtion, determined by the bility to explore wter from greter soil volume ws the mjor chrctenstic of drought tolernce in mize 99

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111 POSPISILOVA, J nd J SOLAROVA 1980 Environmentl nd biologicl control of diffusive conductnces of dxil nd bxil lef epidermes Photosynthetic 14, PREMACHANDRA, G S,D T HAHN, nd R J JOLY 1994 Lef wter reltions nd gs-exchnge in two grin sorghum genotypes differing in their pre- nd post-flowering drought tolernce J Plnt Physiol 14, PREMACHANDRA, G S, H SANEOKA, M KANAYA, nd S OGATA 1989 Responses of reltive growth rte, wter reltions nd solute ccumultion to incresing wter deficits in mize J Plnt Physiol QUARRIE, S A 1991 Implictions of genetic differences in ABA ccumultion for crop production Pp in Abscisic Acid physiology nd biochemistry Eds W J Dvies nd H G Jones Oxford Bios Scientific RAVEN, J A 198 The trnsport nd functions of silicon in plnts Biol Rev 58, ROCHETTE, P, E PATTEY, R L DESJARDINS, L M DWYER, D W STEWART, nd P A DUBE 1991 Estimtion of mize cnopy conductnce by scling up lef stomtl conductnce Agnc For Meteorol 54, SALIENDRA, N Z nd F C MEINZER 1989 Reltionship between root/shoot hydrulic properties nd stomtl behvior in sugrcne Aust J Plnt Physiol 16, SALIENDRA, N Z,J S SPERRY, nd J P COMSTOCK 1995 Influence of lef wter sttus on stomtl response to humidity, hydrulic conductnce, nd soil drought in Betul occidentls Plnt 196, SAMUELS, A L,A D M GLASS, D L EHRET, nd J G MENZIES 1991 Mobility nd deposition of silicon in cucumber plnts Plnt Cell Environ 14, SAS INSTITUTE INC 1988 SAS/STATtm Guide for Personl Computers, Relese 6 0 Edition Cry, NC, SAS Institute Inc SCHEFFER, F nd P SCHACHTSCHABEL 1992 Lehrbuch der Bodenkunde 1 ed Stuttgrt, Ferdinnd Enke Verlg SCHMIDHALTER, U 1995 Does soil nutrient uptke control drought perception in plnts? Pp MIA, 5 pges in Proc Int Congr on Drought Tolernce of Higher Plnts Montpelher, Frnce SCHMIDHALTER, U 1994 Mesuring nd modeling wter uptke by roots t sptilly vrible soil mtric potentils Pp in Proc Third Congr Europ Soc Agron Pdov, Itly SCHMIDHALTER, U, H M SELIM nd J J OERTLI 1994 Mesuring nd modeling root wter uptke bsed on 6chlonde discrimintion in silt lom soil ffected by groundwter Soil Set 158, SCHONHERR, J 1982 Resistnce of plnt surfces to wter loss Trnsport properties of cutin, subenn nd ssocited lipids Pp in Physiologicl plnt ecology II Wter reltions nd crbon ssimiltion vol Encycl Plnt Physiol Vol 12 B, eds O L Lnge, P S Nobel, C B Osmond, nd H Ziegler Berlin Springer Verlg 106

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115 Appendix Tble A1. Estimted prmeters for liner regression models of soil mtnc potentil in the rnge of -5 to -80 kp s mesured by tensiometers, on stomtl conductnce (in mmol m"2 s'1) Dt from Fig 4 4 nd 4 5 Soil mtnc potentil t n intercept ±SE regression coefficient ±SE r2 20 cm depth 6 262*" ± "* ± *"* 40 cm depth '" ± *" ± *" 60 cm depth *" ± "" ± *** 80 cm depth* 24 08"* ± *** ± *" 20 cm depth 6 2"* ±14 4 7*"* ± *" 40 cm depth 41 0"* ±12 9*" ± *** 60 cm depth 41 12*** ± *" ± *" 80 cm depth 7 69"* ± *" ± *" significnt with < # only for one yer {1994/95) nd one field Tble A2. Estimted prmeters nd coefficients of determintion (r2) for simple nd multiple deficit in the regressions of lef wter potentil (ijjl in MP), wter vpour pressure ir (VPD in kp) nd ir temperture (T in C) on stomtl conductnce in (gs mmol m"2 s'1) Dt set consisted of mesurements on six different dys t lte vegettive stge t bout nd 14 0 h nd similr PPFD (1200 to 1700 umol nvv1) in both well-wtered nd the wter stressed mize plnts (n=22) ±SE b±se C±SE r2 model gs = + b i il 421"* ± ** ± ** model gs = + b hil + c VPD 45"* ± * ± NS± * model gs = + bipl + ct 148NS± " ± * ± ** model gs = b gil + c T 457*" ± *" ±4 0 8*" NS significnt with < significnt with < 0 01 significnt with < 0 05 not significnt 110

116 20 ZUSAMMENFASSUNG Ertrgsusflle im Misnbu werden in tropischen und subtropischen Regionen hufig durch ds Ausbleiben von Niederschlgen fur emige Wochen whrend der Vegettionspenode verurscht Die Trnspirtionseffizienz, ls ds Verhltms produzierter Biomsse zur Gesmttrnspirtion, ist ernes der moglichen physiologischen Merkmle, um die Trockenheitstolernz verschiedener Misorten zu beemflussen Dss die Trnspirtionseffizienz und ihre Bedeutung fur die Trockenheitstolernz verschiedener Sorten dennoch bisher selten untersucht wurde, ist im wesentlichen uf Probleme bei der przisen Bestimmung der Trnspirtion unter Feldbedingungen zuruckzufuhren Dher wurden in der voriiegenden Studie verschiedene methodische Anstze zur Bestimmung der Trnspirtion und Trnspirtionseffizienz im Feld untersucht Weiterhm wurde in einem Sorftment von cht tropischen Tieflndkulbvren die Vnbilitt bezuglich der Merkmle Trnspirtion und Trnspirtionseffizienz sowie der moglichen Einflussfktoren stomtre Leitfhigkeit und Blttwssersttus ermittelt Die Rektionen dieser Merkmle uf em limitiertes Wsserngebot wurden im Hinblick uf ihr Potentil zur zuchtenschen Verbesserung der Trockenheitstolernz von Mis nlysiert In Experimented die sowohl in Klimkmmern wie uch uf dem Feld durchgefuhrt wurden, sind Koeffizienten bestimmt worden, um die Trnspirtion von Einzelpflnzen und Bestnden, bsierend uf der porometnschen Messungen der stomtren Leitfhigkeit n einer definierten Blttposition, zu bestimmen Die ermittelte Trnspirtion wurde emerseits mit Wgungen von getopften Pflnzen sowie einer Bodenwsserbilnz im Feld uberpruft Die Bodenwsserbilnz wurde dbei nhnd von Neutronensondenmessungen des Bodenwssergehltes und Bestimmung der Evportion und Tiefensickerung erstellt In zweijhngen Feldversuchen whrend der niederschlgsfreien Zeit in Thilnd wurden die Trockenmssenbildung, stomtre Leitfhigkeit, Bestndestrnspirtion und -trnspirtionseffizienz whrend der vegettiven Entwicklung ermittelt Dbei unterbheb die in der Kontrolle wdchentlich durchgefuhrte Furchenbewsserung in den Przellen der Wsserstressbehndlungen entweder zu einem fruhen (wenige Tge nch dem Auflufen) Oder einem spteren Entwicklungsstdium (c funf Wochen nch dem Auflufen) fur jeweils c vier Wochen Neben der tensiometrischen Bestimmung der Bodenwssersugspnnung (in 20,40,60 und 80 cm Tiefe) wurde in einem der vier Expenmente die volumetnschen Bodenwssergehlte (in 10 - cm Schntten bis zu einer Tiefe von einem Meter)

117 whrend der Versuchsperiode bestimmt. Die Trnspirtion eines Misbestndes konnte nhnd der stomtren Leitfhigkeit n der Blttunterseite des jungsten vollentwickelten Bldes und der Messung der Gesmtblttflche beschrieben werden. Eine Ueberschtzung der Gesmttrnspirtion unter Feldbedingungen Cuvettentempertur und die nichterfsste, wr whrscheinlich uf die erhohte rumliche Verteilung der Blttflche im Hinblick uf den Sonneneinstrhlungswinkel zuruckzufuhren. Trotz dieser Abweichungen ist ein reltiver Vergleich von Sorten bzw. Wsserbehndlungen mittels Porometermessungen mdglich, wenn die Unterschiede im Grenzschichtswiderstnd der Bltter in der Cuvette und im ungestorten Bestnd berucksichtigt werden. Eine sttistische Anlyse der Vribilitt der gemessenen stomtren Leitfhigkeit und Trnspirtionsrte ergb, dss bei guter Wsserversorgung Sortenunterschiede in der Grossenordnung von 10% ls signifiknt Wsserversorgung hingegen Doppelte betrgen. mussten dzu die erknnt werden konnen, bei limitierter Sortenunterschiede mehr ls ds Aufgrund der unbefriedigenden Przision der porometrischen Bestimmung wurde dher ein mehr integrtiver Anstz zur Bestimmung der Trnspirtionseffizienz untersucht. Die ufgenommene Menge n Minerlelementen ist proportionl zur Gesmttrnspirtion der Pflnze, wenn die Aufnhme der Minerlelemente im wesentlichen mittels Mssenfluss verluft. Unter dieser Vorussetzung ist dnn die Konzentrtion des Minerlelement.es in der Trockensubstnz mit der Trnspirtionseffizienz korreliert. Diese Hypothese wurde n getopften Pflnzen und in Feldexperimenten bei guter und limitierter Wsserversorgung fur eine Reihe von Minerlelementen und der nicht slzsureloslichen Asche - Indiktor fur Silicium - ls leicht bestimmbrem untersucht. Dbei konnte die Gesmtufnhme von Silicium bei getopften Pflnzen gut mit der Trnspirtion beschrieben werden, eine Korreltion zur Trnspirtionseffizienz ergb sich jedoch nicht. Im Feld deutete eine hohe nichtlinere Korreltion der Konzentrtion nicht slzsureloslicher Asche mit der Trnspirtionsrte eine uch porometrisch bestimmte erhohte Trnspirtionseffizienz von Pflnzen n, die einer limitierten Wsserversorgung usgesetzt sind. Genotypische Unterschiede in der Trnspirtionseffizienz wurden jedoch, in Uebereinstimmung mit dem porometrischen Ergebnis, nicht gefunden. Als weitere Unterstutzung der Hypothese konnte eine signifiknte Korreltion der Konzentrtion der nicht slzsureloslichen Asche zur Trnspirtionseffizienz gefunden werden, wenn die Mittelwerte von Wsserbehndlungen bzw. verschiedenen Experimenten (Entwicklungsstdien) betrchtet wurden. Andere Minerlelemente en/viesen sich ls ungeelgnet zur Ermittlung der Trnspirtionseffizienz.