Physiological adaptations of five poplar genotypes grown under SRC in the semiarid Mediterranean environment

Transcription

1 Physiologicl dpttions of five poplr genotypes grown under SRC in the semirid Mediterrnen environment Alejndr Nvrro, Ginni Fcciotto, Psqule Cmpi & Mrcello Mstrorilli Trees Structure nd Function ISSN Volume 28 Numer 4 Trees (2014) 28: DOI /s

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3 Author's personl copy Trees (2014) 28: DOI /s ORIGINAL PAPER Physiologicl dpttions of five poplr genotypes grown under SRC in the semi-rid Mediterrnen environment Alejndr Nvrro Ginni Fcciotto Psqule Cmpi Mrcello Mstrorilli Received: 11 Novemer 2013 / Revised: 19 Mrch 2014 / Accepted: 25 Mrch 2014 / Pulished online: 11 April 2014 Ó Springer-Verlg Berlin Heidelerg 2014 Astrct Key messge The genotype Nev under high plnt density showed the highest iomss yield nd optiml physiologicl strtegies nd could e the most suitle choice under semi-rid environment Astrct The poplrs (Populus spp.) re the most sensitive plnts to wter deficit conditions mong the woody species utilized for iomss production for energetic purposes; their productivity is ssocited with wter vilility in the soil. In the Mediterrnen environment, crops re minly limited y evpotrnspirtive demnd tht is not lnced y rinfll supply. As new hyrids with high growth rtes nd resistnce to wter stress re selected, the use of poplr s n energy crop my increse in Southern regions of Mediterrnen Europe. The growth dynmics nd physiologicl chrcteristics of poplr hyrid genotypes hve een monitored for 2 yers t site with Mediterrnen climte, Apuli region, tht could e used for energy crops. Unrooted cuttings of three recently selected genotypes of poplr ( Nev, Dvin nd Len ) nd two trditionl genotypes ( Luis Avnzo nd Bellini ) were plnted in the spring of 2010 t two different densities: () low plnt density = 1,667 cuttings h -1 (LPD); () high plnt density = 6,667 cuttings h -1 (HPD). The genotypes Len nd Dvin showed the lowest survivl rtes nd the poorest growth mong the hyrid poplr tested. The genotype Bellini hd low stomtl sensitivity to soil wter content nd moderte productive performnce. The genotypes Luis Avnzo nd Nev hd good degree of rooting nd sprouting, high vlues of lef reltive wter content (RWC l ) nd low vlues of stomtl conductnce (g s ) during the summer months. In Nev, these chrcteristics were ssocited with the est yields (4 t h -1 ) in HPD. Keywords Mediterrnen environment SRC Physiologicl trits Biomss Drought tolernce Communicted y H. Rennenerg. A. Nvrro (&) P. Cmpi M. Mstrorilli Consiglio per l Ricerc e l sperimentzione in Agricoltur- Reserch Unit for Cropping Systems in Hot nd Arid Environments (CRA-SCA), Vi Celso Ulpini 5, Bri, Itly e-mil: lejndr.nvrrogrci@entecr.it P. Cmpi e-mil: psqule.cmpi@entecr.it M. Mstrorilli e-mil: mrcello.mstrorilli@entecr.it G. Fcciotto Consiglio per l Ricerc e l sperimentzione in Agricoltur- Reserch Unit for Intensive Wood Production (CRA-PLF), Strd Frssineto 35, Csle Monferrto, Itly e-mil: ginni.fcciotto@entecr.it Introduction According to the Directive 2009/28/EC, the Europen Union will hve to produce renewle energy equl to 20 % of the finl energy consumption y In the EU- 27 (Europen Union of 27 Memer Sttes), the shre of renewle energy in gross finl energy consumption hs grown stedily from 8.1 % in 2004 to 12.5 % in 2010 (Svov 2012). The iomss nd wste use in the EU-27 in 2009 ws Mtoe, which represented for 6.7 % of the EU-27 finl energy consumption (Eurostt 2011). The most importnt source of renewle energies in the EU-27 ws iomss nd wste, which ccounted for 67.7 % of primry renewle production in 2009, nd their totl solute

4 Author's personl copy 984 Trees (2014) 28: contriution is expected to grow significntly (Eurostt 2011). The cultivtion of tree species using the short rottion coppice (SRC) technique of lignocellulosic iomss production hd spred following the first oil crisis. The min gronomic nd economic chrcteristics of SRC re high growth rte, dequte sprouting of the stool ed nd n dpttion to su-optiml environmentl conditions. In Mediterrnen Europe, SRC is especilly developed in the Northern regions. Here, the most common cultivted species in experimentl trils nd in commercil stnds re s follows: poplr (Populus spp.), willow (Slix spp.) nd lck locust (Roini pseudocci L). Other species such s Euclyptus (Euclyptus spp.) re cultivted in smller stnds in Southern regions. To extend the use of SRC in Southern regions, chrcterized y low wter vilility nd to verify their production in these environments, specific reserch progrms hve een funded y the EC (Europen Community) nd y ntionl nd locl gencies. Poplrs re mong the fstest growing trees in temperte ltitudes, nd their cultivtion cn led to high iomss production under dequte conditions. This high productivity is ssocited with high wter requirements; s consequence, productivity my e strongly limited y wter vilility (Zsuff et l. 1996; Tschplinski et l. 1994). Although ll species elonging to the Populus genus hve n overll sensitivity to drought, wide diversity in drought tolernce levels nd response ptterns to wter deficit hs een reported (Brignols et l. 2000; Mrron et l. 2002, 2003; Zhng et l. 2004; Monclus et l. 2006; Silim et l. 2009). Most of these studies hve een conducted in controlled conditions (i.e., growth chmers, greenhouses nd nurseries), where the wter stress ws provoked y withholding wter nd successively the effects on plnt growth, wter reltions, gs exchnge nd moleculr nd metolic ctivities hve een oserved. These oserved effects represent n djustment of plnt functioning in response to wter constrints under controlled conditions while, under nturl conditions, when wter vilility is limited, the plnt exercises n dpttion strtegy to the drought. Considering the effects of climte chnge, the selection criteri used for commercil poplr genotypes need to tke into ccount the resistnce to iotic stresses in generl nd to drought in prticulr. The selection of poplr genotypes in the future must focus on comining optiml wter use with mximum iomss production (Mrron et l. 2008). This is especilly importnt in Southern regions of Mediterrnen Europe, or other similr semi-dry res, where drought represents severe limittion to the future development of SRC with poplrs. Approprite poplr genotypes for ioenergy production must e identified if we wnt to cultivte them without dditionl wter supply in these res. Therefore, the purpose of this study ws to test five hyrid poplr genotypes t two plnt densities (1,667 nd 6,667 cuttings h -1 ) to find the most suitle choice in semi-dry re of Mediterrnen. Two criteri will led to this choice: the highest iomss yield nd the development of optiml physiologicl strtegies. Chnges in growth dynmics, iomss production nd sesonl effects on physiologicl trits (wter reltions nd stomtl conductnce) were evluted nd relted with their ility to withstnd dverse conditions during the vegettion seson in field conditions. Mterils nd methods Plnt mteril, experimentl design nd growth conditions Five hyrid poplr (Populus L. spp.) genotypes of different prentge (Tle 1) were plnted t two plnt densities: low plnt density (LPD) = 1,667 cuttings h -1, with interrow distnces of 3 m nd spcing of 2 m etween cuttings within the row, nd high plnt density (HPD) = 6,667 cuttings h -1, with inter-row distnces of 3 m nd spcing of 0.5 m etween cuttings within the row, on n experimentl field of 0.40 h. The estlishment of the plnttion y mnul plnting ws completed in Mrch The plnting mteril consisted of unrooted hrdwood cuttings (22 cm in length, cm in dimeter) with three or more lterl uds. The poplr genotypes re ctegorized s new SRC genotypes ( Nev, Dvin nd Len ) or s trditionl genotypes ( Luis Avnzo nd Bellini ). The genotypes were selected y the Reserch Unit for Intensive Wood Production, CRA-PLF (Csle Monferrto, AL, Itly) sed on their rpid growth nd their tolernce to mjor folir diseses nd to Woolly Aphid Phloeomyzus psserinii (Sign.). Cuttings from the five genotypes were plnted in clonl locks with four replictions t oth plnt densities. The locks elonging to the HPD hd 64 cuttings, while those pertining to the LPD contined only 16. Tretments were rrnged in split-plot design with plnt densities (HPL nd LPD) s min plots nd poplr genotypes ( Luis Avnzo, Nev, Bellini, Dvin nd Len ) s split-plots. The experimentl trils were conducted t the experimentl frm of the Reserch Unit for Cropping Systems in Dry Environments (C.R.A. S.C.A.) in Rutiglino (lt: , long: , lt: 147 m.s.l.) in Southern Itly. During the two growing sesons within the experimentl period (2010 nd 2011), the mximum nd minimum tempertures were similr (Fig. 1). Mximum men vlues of temperture (T mx ) were recorded in the summer months of 2011 (30 C) nd minimum men vlues (T min ) in

5 Author's personl copy Trees (2014) 28: Tle 1 List of Populus genotypes used in this study, with species, gender, prentge nd origin Genotype nme Species Gender Prentge Origin Luis Avnzo Nev Bellini P. 9 cndensis Mönch P. 9 cndensis Mönch P. 9 cndensis Mönch F P. deltoides 3479/985 9? (open pollintion) Stoneville (USA) 9 Bgni di Tivoli (Itly) F P. deltoides P. nigr PI Illinois (USA) 9 Lucc (Itly) M P. deltoides Chutgne 9? (open pollintion) Frnce 9 Bgni di Tivoli (Itly) Dvin P. deltoides Brtr M P. deltoides 9? (open pollintion) Knss (USA) 9 Csle Monferrto (Itly) Len P. deltoides Brtr M P. deltoides 9? (open pollintion) Illinois (USA) 9 T ( C) Precipittion T min T mx Jn Fe Mr Apr My Jun Jul Aug Sep Oct Nov Dec Jn Fe Mr Apr My Jun Jul Aug Sep Oct Nov Dec Fig. 1 The men mximum nd minimum monthly tempertures (T mx nd T min ) nd the precipittion (mm) registered from Jnury 2010 to Decemer 2011 Jnury nd Ferury of 2011 (3.5 C). The totl nnul precipittion ws 530 mm in oth 2010 nd 2011 (Fig. 1). Precipittion minly occurred during the utumn winter period (314 mm of rinfll etween utumn 2010 nd winter 2011) nd ws reduced or sent during the spring summer period (149 mm of rinfll etween spring nd summer 2011). This precipittion does not meet the tmosphere s ET requirements; the nnul wter deficit is 560 mm (Cmpi et l. 2005). To ensure the rooting of poplrs, the field ws supplied with supplementl irrigtion of 175 mm during the first months fter plnttion estlishment (My August 2010). Fertiliztion ws performed with mmonium nitrte (26 %) t dose of 150 kg h -1 (39 kg h -1 of N) nd ws pplied in the first yer fter estlishment. The field ws prepred y ploughing to depth of 0.3 m. The field ws then refined y hrrowing just efore plnting. In ll plots, weeds were controlled y mechnicl hrrowing (with shrp teeth) etween the rows nd mnul hoeing on the rows. This process ws conducted twice during the first growing seson; the mechnicl processes were repeted fter coppicing Precipittion (mm) Chemicl weed control ws mde shortly fter plnting with pre-emergent hericide (ctive ingredient: pendimethlin) t dose of 3 kg h -1, nd once yer during the dormnt seson (Ferury) with post-emergence hericide (ctive ingredient: glyphoste) t rte of 4Lh -1. Pest control ws required due to the ppernce of Cryptorhynchus lpthi L. (poplr orer) nd ws mde fter coppicing (Mrch 2012) with synthetic pyrethroid (ctive ingredient: deltmethrin) t dose of 2 g hl -1. The soil ws found to hve the sme chrcteristics t oth plnt densities. This soil ws clssified s cly (42 % cly, 37 % silt nd 21 % snd) ccording to the soil texture tringle (USDA Clssifiction), with field wter cpcity of 30 %, wilting point of 18 % (mesured using Richrds pltes on dry soil weight), nd ulk density of 1.15 g m -3. Becuse the soil profile is shllow (0.6 m), ville soil wter cpcity ws moderte (83 mm). Climtic prmeters (ir temperture, precipittion, solr rdition nd ir reltive humidity) were mesured hourly t n gro-meteorologicl sttion locted ner the experimentl site. The ir temperture (T) nd reltive humidity (RH) were mesured using thermo-hygrometers (model CS500, Cmpell Scientific, Inc. USA; ccurcy: ±0.2 C for ir temperture nd ±1.5 % for RH). Solr rdition (R s ) ws mesured using pyrnometer (WE300, Glol Wter Instrumenttion, USA; ccurcy: ±1 %). The vpor pressure deficit (VPD) ws clculted using ir reltive humidity nd ir temperture mesurements (ccording to the FAO56 procedure to derive VPD) nd shows smll differences in the protected re. Growth mesurements nd hrvesting To evlute the dpttion of poplr genotypes to semi-rid conditions, we recorded survivl rtes nd iometric prmeters. The numer of living plnts in ech smpling re ws mesured s rooting percentge t the end of the yer 2010 nd s sprouting percentge t the eginning of the yer 2012 (fter hrvesting of the poplr genotypes t HPD).

6 Author's personl copy 986 Trees (2014) 28: Tle 2 List of the growth vriles mesured t the end of second growing seson (winter 2011/2012) in ech clonl lock t oth HPD nd LPD Growth vriles Shoots Numer of shoots per stool Numer of living shoots with height [150 cm Numer of living shoots with height \150 cm Percentge of rnched shoots Height (m) Men totl height Height of the dominnt shoot Men height of the secondry shoots Brest height stem dimeter t 130 cm (cm) Men dimeter t rest height Dimeter t rest height of the dominnt shoot Men dimeter t rest height of the secondry shoots Stem dimeter t 10 cm ove ground (cm) Men dimeter t 10 cm Dimeter t 10 cm of the dominnt shoot Men dimeter t 10 cm of the secondry shoots Arevition N SS N LS [150 cm N LS \150 cm % BS MTH DH SH M D130 D D130 S D130 M D10 D D10 S D10 A set of growth vriles (Tle 2) ws recorded t the end of second growing seson in winter 2011/2012 (fter lef fll nd efore iomss hrvesting). These prmeters were mesured in 16 trees per clonl lock t ech plnt density. In HPD, 16 centrl trees were mesured nd in LPD, the totl numer of trees per lock. The clculted oveground iomss (stem? rnches) production (C ABP; t h -1 ) ws clculted using the following exponentil function: W ¼ e ðd130þ where W is totl shoot dry weight, D130 is the stem dimeter t rest height (t 130 cm ove ground), nd = nd = re constnts. To ssess the effective oveground iomss (stem? rnches) production (E ABP; t h -1 ), t the end of the second growing seson (first iennil rottion, Mrch 2012), smple of ten trees per clonl lock (only t HPD) ws mechniclly hrvested t cutting height of pproximtely cm from the soil surfce. The shoot fresh weight ws mesured in the field immeditely fter the cut using lnce with n ccurcy of 20 g. Smples were then oven-dried t 100 C until constnt weight to otin dry weight mesurements nd determine moisture content. Dry mtter content ws mesured for stem nd rnch smples from the poplr genotypes to determine their contriution to the totl oveground iomss (iomss prtition on dry sis). Wter reltions Tree wter sttus ws determined during the poplrs vegettion seson. Middy lef wter potentil (W l ; MP), middy stem wter potentil (W x ; MP) nd lef reltive wter content (RWC l ; %) were mesured throughout 2011 on My 5 (DOY 125), July 8 (DOY 189) nd Septemer 13 (DOY 256). These prmeters were mesured etween 10:00 nd 12:00 h in six trees per tretment (poplr genotype 9 plnt density). Mesurements of W l were recorded using sunlit leves of similr ge nd position in the cnopy (8 12 leves from the pex). W x ws mesured in non-trnspiring leves tht were covered with oth plstic sheet nd luminum foil for t lest 2 h efore mesurement. Bgging prevents lef trnspirtion; therefore, lef wter potentil equlled stem wter potentil (Begg nd Turner 1970). The leves smpled for W l nd W x were cut nd enclosed in plstic g (Turner 1988), which ws immeditely plced in pressure chmer (Mod. 3000, Soil Moisture Equipment Co., Snt Brr, USA) ccording to Scholnder et l. (1965), nd the pressure ws rised using nitrogen gs t rte of 0.03 MP s -1. The RWC l ws mesured ccording to the eqution descried y Brrs nd Wetherley (1962): RWC l ¼ ðfw DW = TW DWÞ x 100 where FW, DW, nd TW re the fresh, dry nd turgid weights, respectively, of the whole lef. All leves were weighed immeditely fter collection to determine the fresh weight (FW). The cut end of ech lef ws plced in distilled wter nd kept in dim light t 4 C for h efore the turgid weight (TW) ws recorded. Dry weight (DW) ws mesured fter leves were dried for h t 75 C. The diurnl course of lef stomtl conductnce (g s, mmol m -2 s -1 ) ws mesured on the xil surfce of sun-exposed leves using diffusion porometer (Delt-T AP4, Delt-T Devices Ltd, Burwell, Cmridge, UK); mesurements were tken from sunrise to sunset, t intervls of 2 3 h. The mesurements were tken on the sme dy nd in the sme trees s the W l nd W x mesurements. Two to four mture leves of similr ge were chosen t rndom from the mid-cnopy level of the tree. To exmine the reltionships etween W x nd g s,12 plnts from ech clone t ech density were studied during the vegettion seson. After mesuring stomtl conductnce t middy, the lef ws excised nd the corresponding W x ws determined immeditely. The stomtl responsiveness to decresing W x ws determined y compring W x mens for ech genotype (t oth high nd low plnt densities) to the stomtl conductnce (g s ) decresing y 75 % nd y B15 % of the mximum conductnce

7 Author's personl copy Trees (2014) 28: vlues. The mximum stomtl conductnce vlues were ssumed for ech genotype nd density in spring, when the mximl vlue of stomtl conductnce ws oserved. The stomtl conductnce ws considered to e t 75 % when g s ws etween 70 nd 80 % of the mximum vlues, nd ws considered to e B15 % when g s ws \15 % of the mximum vlues (Tle 2). The men W x t 75 % g s ws considered to e the threshold W x (W x75 % g s ) t which stomt egin to close, nd the W x t B15 % g s ws the W x (W x B 15 % g s ) t which stomt were considered to e closed. The W x rte to chieve the stomtl closure ws represented s the difference in MP etween W x75 % nd W x B 15 %. Sttisticl nlysis For growth prmeters, wter-reltion prmeters, clculted oveground iomss production, rooting percentge, W x75 % g s, W x B 15 % g s nd W x75 % - W x B 15 %, twowy ANOVA with the Type III sum of squres ws fit to test for differences etween densities, genotypes, nd their interctions. To test for differences in the stomtl conductnce mesurements within ech seson, these prmeters were nlyzed seprtely for ech seson dte y dte; one-wy ANOVA ws fit to test for differences etween the genotypes for ech plnt density. Tretment mens were seprted using the Duncn s multiple rnge test (P B 0.05). For effective nd clculted oveground iomss production nd sprouting percentges, one-wy ANOVA ws fit to test for differences etween the genotypes. Tretment mens were seprted using the Duncn s multiple rnge test (P B 0.05). These sttisticl nlyses were performed using the Sttgrphics Plus 5.1 softwre. Percentge dt were rcsine squre root trnsformed efore the sttisticl nlyses to ensure the homogeneity of vrince. Results Growth Growth dt differed significntly etween densities (Tle 4; Fig. 2). Trees in HPD (6,667 cuttings h -1 ) plots were fewer rnched nd presented smller dimeter (oth D130 nd D10) thn in LPD (1,667 cuttings h -1 ) ones. The higher vlues of C ABP were chieved with the highest density. Among the poplr genotypes, Nev stood out s hving the higher rooting nd sprouting percentges (Tle 3), long with high rmifiction, the gretest height, the lrgest D130 nd D10 of the dominnt shoot (Tle 4) nd Tle 3 Effects of density (D), poplr hyrid genotypes (G) nd their interction (D 9 G) in rooting percentge (% rooting) t the end of the yer 2010 (i.e., fter plnting) nd sprouting percentge (% sprouting) t the eginning of the yer 2012 (i.e., fter plnttions with 6,667 trees h -1 were hrvested) for ech hyrid poplr genotype Tretments % Rooting % Sprouting Density (D) HPD 87 LPD 79 Poplr genotypes (G) Luis Avnzo Nev Bellini c Dvin 79 c 71 c Len 66 c 64 c P D ns C ** *** D 9 G ns **, *** nd ns denote sttisticl significnce t the 0.01 nd levels nd the sence of significnce, respectively Different letters in the columns indicte significnt differences etween tretments within the sme fctor, ccording to the Duncn s test (P \ 0.05) the gretest C ABP t oth densities (4.10 nd 2.32 t h -1 for HPD nd LPD, respectively; Fig. 2). Dvin nd Len were chrcterized s hving the lowest percentges of rooting nd sprouting (Tle 3) nd low vlues of height nd rmifiction (Tle 4). Len hd the lowest C ABP (2.24 nd 0.71 t h -1 for HPD nd LPD, respectively; Fig. 2). The gretest E ABP ws otined in the genotypes Nev, Luis Avnzo nd Bellini, with 4 t h -1 nd the lowest in the genotype Len, with 2.4 t h -1 (Fig. 2). Significnt differences in the iomss distriution were found etween poplr genotypes. The stem iomss ccounted for 73 % (highest vlue) of the totl iomss for Nev, 70 % for Luis Avnzo nd Dvin, 65 % for Bellini nd finlly 62 % for Len ; vice vers to the rnches iomss (Fig. 2). Wter reltions nd gs exchnge Figure 3 outlines the following plnt wter sttus indictors: middy lef wter potentil (W l ), middy stem wter potentil (W x ), nd lef reltive wter content (RWC l ). These mesurements indicted tht the plnts hd good wter sttus during the spring with high vlues of W l nd W x (Fig. 3 d) nd high of RWC l (Fig. 3e, f). Due to the high evpotrnspirtive demnd nd low rinfll in the following months (Figs. 1, 4h), soil wter reserves were

8 Author's personl copy P D ns ** ns *** ns ns ns ** ** ns *** *** *** 988 Trees (2014) 28: Tle 4 Effects of density (D), poplr hyrid genotypes (G) nd their interction (D 9 G) in growth prmeters s defined in the Tle 2 Tretments Shoots Height (m) D130 (cm) D10 (cm) N SS % BS N LS [150 cm N LS \150 cm MTH DH SH MD130 DD130 SD130 MD10 DD10 SD10 Density (D) HPD LPD Poplr genotypes (G) Luis Avnzo Nev Bellini Dvin c Len c C ** * ** ns * ** ns ns * ns ns * ns D 9 G ns ns ns ns ns ns ns ns ns ns ns ns ns *, **, *** nd ns denote sttisticl significnce t the 0.05, 0.01 nd levels nd the sence of significnce, respectively Different letters in columns indicte significnt differences etween the tretments within the sme fctor, ccording to the Duncn s test (P \ 0.05) ABP (t h -1 ) % 30% B 73 % 27% A stems E ABP rnches in HPD C ABP in HPD C ABP in LPD Luis Avnzo Nev Bellini Dvin Len Poplr genotypes Fig. 2 Effective oveground iomss production (E ABP) in t h -1 nd iomss prtition on dry sis (proportion of rnches nd stems to totl oveground iomss) t HPD nd clculted oveground iomss production (C ABP) in t h -1 t oth HPD nd LPD. Dt were collected for the five poplr genotypes t the end of the second growing seson. Ech histogrm represents the men of 10 trees per 4 clonl locks, nd verticl rs indicte stndrd errors. Different letters in histogrms indicte significnt differences etween genotypes within the sme plnt density, ccording to the Duncn s test (P \ 0.05) depleted nd the wter sttus of the plnts worsened. We found tht in summer nd utumn, DOY 189 nd 256, W l in the HPD nd W x in oth plnt densities ecme more negtive (Fig. 3, c, d), nd RWC l in oth plnt densities decresed (Fig. 3e, f). 35% 65% B 30 % 70% B 38 % 62% B W l nd W x nd RWC l were differently ffected y plnt density nd poplr genotypes. In My nd July, the W l ws lower for LPD plots thn for HPD plots, while in Septemer it ws greter for LPD plots (Fig. 3, ). The W x ws lower for trees in LPD plots throughout the mesurement period (Fig. 3c, d). No differences in the RWC l etween the two plnting densities were found (Fig. 3e, f) in My nd Septemer, while in July these vlues were higher in LPD. Among the poplr genotypes compred, Nev presented the higher vlues of W l nd W x nd Bellini the most negtive (Fig. 3 d). There were no differences etween the genotypes for RWC l in My, while in July nd Septemer, RWC l vlues were higher in L. Avnzo nd Nev compred to other poplr genotypes, especilly in HPD plots (Fig. 3e, f). During the 2011 growing seson, temperture (T), solr rdition (R s ), nd VPD were recorded in three representtive dys of typicl Mediterrnen climte conditions, DOY 125, 189, nd 256 (Fig. 4). DOY 125 (My) ws chrcterized y mild tempertures mx. 17 C nd min. 10 C (Fig. 4), high solr rdition (Fig. 4d) nd low VDP mx. vlue 1.08 kp (Fig. 4g); DOY 189 (July) hd the highest hourly tempertures mx C nd min. 19 C (Fig. 4), high solr rdition (Fig. 4e) nd high VDP mx. vlue 2.5 kp (Fig. 4g); nd on DOY 256 (Septemer), the tempertures remined high (Fig. 4c), the solr rdition from 9:00 to 14:00 h decresed y 47 % compred to DOY 189 (Fig. 4f), nd VPD ws similr to tht of DOY 125 mx vlue 1.15 kp (Fig. 4i).

9 Author's personl copy Trees (2014) 28: HPD (3 x 0.5 m) LPD (3 x 2m) -0.6 l (MP) Ψ *** *** *** c c c c d x (MP) Ψ *** *** *** c c c Bellini L Avnzo Nev Dvin Len c RWC l (%) ns ns *** c c c e c c c f My 5, 2011 July 8, 2011 Sept 13, 2011 (DOY 125) (DOY 189) (DOY 256) My 5, 2011 July 8, 2011 Sept 13, 2011 (DOY 125) (DOY 189) (DOY 256) Fig. 3 Lef wter potentil (W l ;, ), stem wter potentil (W x ; c, d), nd lef reltive wter content (RWC l ; e, f) t middy in the five hyrid poplr genotypes t HPD (, c, e) nd LPD (, d, f) on dys 125, 189, nd 256 of Ech histogrm represents the men of 6 vlues, nd verticl rs indicte stndrd errors. Different letters in histogrms indicte significnt differences etween genotypes within the sme dy, ccording to the Duncn s test (P \ 0.05). The circle, tringle, nd squre represent the men vlue of the five poplr genotypes for dys 125, 189, nd 256 of 2011, respectively. **, *** nd ns denote sttisticl significnce t the 0.01 nd levels nd the sence of significnce, respectively, etween plnt density tretments t the sme dt The diurnl ptterns of stomtl conductnce (g s ) were mesured t these three dtes (DOY 125, 188, nd 256) (Fig. 5).The highest vlues of g s for the poplrs werefoundinthespringnddecresedinthesummer nd utumn. Bellini, Dvin nd Len reched the minimum g s in Septemer (Fig. 5k, n, o). The genotypes L. Avnzo nd Nev reched minimum g s vlues in July (Fig. 5g, h) nd incresed in Septemer (Fig. 5l, m). The lowest g s in My nd Septemer were otined in the genotypes Dvin nd Len (Fig. 5d,e, n, o) nd in July were found in L. Avnzo nd Nev (20 mmol m -2 s -1 Fig. 5g, h). Bellini hd the gretest g s mesurements in the spring (1,660 mmol m -2 s -1 Fig. 5) nd summer (234 mmol m -2 -s -1 Fig. 5f) nd Nev reched the mximum g s vlue in Septemer (183 mmol m -2 s -1 Fig. 5m). The g s mesured in most genotypes did not differ significntly etween the two plnt densities. In the genotype Bellini, g s ws higher with LPD thn with HPD (Fig. 5, f, k). In the genotype Nev, the mximum stomtl

10 Author's personl copy 990 Trees (2014) 28: My 5, 2011 (DOY 125) July 8, 2011 (DOY 189) Septemer 13, 2011 (DOY 256) c 25 T ( C) ,5 3,0 d e f R s (MJ m -2 ) 2,5 2,0 1,5 1,0 0,5 0,0 2,8 2,4 g h i 2,0 VPD (kp) 1,6 1,2 0,8 0,4 0, Time (h) Fig. 4 Diurnl courses of temperture (,, c), solr rdition (R s ; d, e, f) nd vpor pressure deficit (VPD; g, h, i) registered on dys 125 (, d, g), 189 (, e, h), nd 256 of 2011 (c, f, i) opening ws delyed in My nd Septemer with LPD respect to HPD (Fig. 5c, m). There were some differences noticele in the trends dopted y the poplr genotypes for their diurnl courses of g s, therefore Bellini, Dvin, nd Len in My nd July presented the mximum vlues of g s in the erly morning (until 10:30 h), which decresed in the centrl hours of the dy, nd remined low nd constnt until lte fternoon (Fig. 5, d, e, f, i, j). In Septemer, these differences etween the g s found in the erly morning nd those of the fternoon were ttenuted, nd throughout the dy these vlues remined low nd constnt (Fig. 5k, n, o). Insted, the diurnl ptterns of g s for the genotypes L. Avnzo nd Nev show in My stomtl ehvior similr to tht reported y the genotypes Bellini, Dvin, nd Len with mximum pek of g s in the erly morning nd drop of this vlue t middy, tht continued low nd constnt until fternoon (Fig. 5, c), ut in July nd Septemer this ehvior chnged regrding the others poplr genotypes. In July the g s of Luis Avnzo nd Nev ws low throughout the dy (Fig. 5g, h) nd it ws incresed it in Septemer (Fig. 5l, m), ut the diurnl pttern of g s chnged etween the two genotypes. In the genotype L. Avnzo, the g s ws high in the erly morning (until 10:30 h) with drop t middy nd lter increse in the erly fternoon (Fig. 5l), while Nev hd high vlues of g s until 13:00 h tht decresed in the fternoon (Fig. 5m). The threshold W x t which stomt strted to close (W x75 % g s ) occurred when g s vlues ttined 75 % of the mximum rte, nd the W x threshold vlue for stomtl closure (W x B 15 % g s ) occurred when g s vlues ttined B15 % of the mximum rte. The threshold W x75 % ws highest in Nev (-0.88 nd MP t LPD nd HPD, respectively) nd in L. Avnzo plnts (-1.04, though only t HPD) nd it ws lowest in Dvin nd Len plnts under HPD (-1.20 nd MP). No differences were found in the stomtl closure threshold (W x B 15 % )

11 Author's personl copy Trees (2014) 28: My 5, 2011 (DOY 125) g s (mmol m -2 s -1 ) Bellini L. Avnzo Nev c d e HPD (3 x 0.5 m) LPD (3 x 2 m) Dvin Len Septemer 13, 2011 (DOY 256) July 8, 2011 (DOY 189) g g s (mmol m -2 s -1 s (mmol m -2 s -1 ) ) Time f g h i k l m n Time Time Time Time j o Fig. 5 Diurnl courses of stomtl conductnce (g s, mmol m -2 s -1 ) of the poplr genotypes Bellini (, f nd k), Luis Avnzo (, g nd l), Nev (c, h nd m), Dvin (d, i nd n), nd Len (e, j nd o), on dys 125 (,, c, d nd e), 189 (f, g, h, i nd j), nd 256 of 2011 (k, l, m, n nd o). Solid nd dshed lines represent the HPD nd LPD, respectively. Verticl rs represent stndrd error Tle 5 Stem wter potentils (W x ; MP) when the stomtl conductnce (g s ) of Bellini, Luis Avnzo, Nev, Dvin, nd Len plnts t HPD or LPD, reched 75 % (W x \ 75 %, threshold W x )or15%(w x B 15 % ) of the stedy-stte vlues nd the W x rte to chieve the stomtl closure (W x75 % - W x B 15 % ) Hyrid poplr genotypes W x75 % g s (MP) W x B 15 % g s (MP) W x75 % - W x B 15 % (MP) HPD LPD HPD LPD HPD LPD Populus cndensis L Luis Avnzo Nev * * Bellini * 0.40 Populus deltoides L. Dvin * * Len * * Mens represent the men of 3 10 mesurements Different letters in columns indicte significnt differences etween genotypes, ccording to the Duncn s test (P \ 0.05) * Within ech genotype, men vlues followed y * indicte significnt differences etween plnt density tretments etween genotypes under HPD (from to MP), while under LPD, Nev, Dvin, nd Len hd higher W x B 15 % (from to MP) thn L. Avnzo nd Bellini (-1.30 nd MP). The W x rte to chieve the stomtl closure (W x75 % - W xb15 % ) lso differed mong clones, in P. 9 cndensis genotypes occurred fter decrese in W x of 0.31 MP (men vlue) t oth plnt densities, wheres this vlue ws pproximtely 0.18 MP for P. deltoides ones (Tle 5). Discussion Plnt density effects in the typicl Mediterrnen crops such s olives, lmonds nd grpevines (Guerfel et l. 2010; Hunter 1998) hve een widely studied in the drylnd conditions, where these crops were spced widely to tke full dvntge of the stored soil wter from winter rins for lter growth in spring nd summer. In these crops, n optimum reltionship etween quntity nd

12 Author's personl copy 992 Trees (2014) 28: qulity of fruit is required, while the cultivtion of tree species using the SRC technique ims only to iomss production. Thus in this tril, we hve tested high plnt density (6,667 cuttings h -1 ) nd low plnt density (1,667 cuttings h -1 ), oth lower thn those commonly used in Northern Europe, round 10,000 12,000 cuttings h -1 (Dnfors, 1992; Lureysens et l. 2003, 2005; Armstrong et l. 1999; Mitchell et l. 1999), nd in Northern Itly, round 10,000 cuttings h -1 (Fcciotto et l. 2006). Nonetheless, LPD t tree level produced more rnched nd lrger dimeter trees thn HPD, in terms of yield (t h -1 ) the highest C ABP ws otined with the highest density (HPD) due to the higher numer of cuttings per plot (6,667 cuttings h -1 ) respect to LPD (1,667 cuttings h -1 ). The productivity of the poplr genotypes studied here ws modest, considering tht in 2 yers these genotypes produced the sme s they chieved in 1 yer in the environments where the soil wter content does not represent limiting fctor (Fcciotto et l. 2006). The genotype Nev ws introduced in Chin from Itly in the mid-80s, nd is the most promising vriety for fforesttion nd woody production in the rid nd semi-rid res of this country (Du et l. 2012), where it is known s genotype I In our trils, the ABP otined for the poplr genotype Nev (4.10 t h -1 ), t HPD (6,667 cuttings h -1 ) nd under the semi-rid conditions of the Apuli region, represented one-third of the verge production otined for this genotype in Northern Itly (round 14 t h -1 in 2 yers Fcciotto nd Zmruno 2004) where soil wter constrints re not prolem. Production of the genotype L. Avnzo ws similr to tht of Nev, while in the genotype Bellini ws lower in vlue (Fig. 2). The production of L. Avnzo ws equl or slightly higher thn those of genotype I-124 0, the most cultivted vriety in Itly (Avnzo 1982). The P. deltoides genotypes Dvin nd Len re not recommended s SRC species in semi-rid climtic conditions due to the low rtes of rooting nd sprouting nd low iomss production otined in this study. Dvin nd Len re strongly dependent on rinfll nd soil wter vilility. Yields of these genotypes (Fcciotto et l. 2006) t plnt density of 10,000 cuttings h -1 were 25 t h -1 in high nnul precipittion region (t Csle Monferrto, Northern Itly), nd 6 t h -1 in region with less nnul rinfll (t Bgni di Tivoli, Centrl Itly). In this study, the wood iomss produced y these genotypes ws hlved (3 t h -1 ) compred to Bgni di Tivoli due to the lower plnt density (6,667 cuttings h -1 ) nd low nnul precipittion (530 mm), together with higher evpotrnspirtion demnd of the tmosphere nd modest wter storge cpcity of the soil occurred t Rutiglino, Southern Itly. The vlues of W l nd W x (Fig. 5) did not chnge s much s those of g s (Fig. 3) over the course of the growing seson. This ehvior of poplr plnts is typicl mong isohydric species. In these, lef wter potentil does not differ gretly with differing levels of soil wter content. According to Trdieu nd Simonneu (1998), this is ecuse isohydric plnts progressively close stomt s wter uptke decreses due to limited soil wter content. These plnts mintin nerly constnt lef wter potentil nd control of flow rtes is chieved vi stomtl control, while in the nisohydric species, oth lef wter potentil nd stomtl conductnce decrese s the soil wter ecomes limiting nd flow rtes re controlled through vritions in lef wter potentil. Despite the stomtl control oserved in poplr trees, Bellini hd greter susceptiility to wter stress due to reduced stomtl closure compred to the other poplr genotypes (Fig. 5,, c). Bellini tended to dispel the wter reservoir of the soil, even when it ws t minimum levels during summer months (Fig. 1), s reflected y the mximum pek of g s otined in the erly morning in July (Fig. 5). This ws lso confirmed y the more negtive vlues of W l nd W x nd the low vlues of RWC l otined for this genotype during July nd Septemer (Fig. 3). The stomtl ehvior of L. Avnzo nd Nev leves suggests these genotypes re etter dpted to the wter conditions of the soil. These vrieties djusted their stomtl opening rtes with chnging soil wter vilility nd climte conditions. The recorded g s vlues were constnt nd high in the spring (Fig. 5d, g) due to high rdition, low temperture nd low VPD (Fig. 4, d, g). With the rrivl of high tempertures nd high VPD (Fig. 4, h), these genotypes djusted the rte of their stomtl opening, i.e., the stomt tended to close from the erly hours of the morning, nd the trnspirtion rte ws reduced (low vlues of g s Fig. 5e, h). When the VPD egn to decrese in Septemer (Fig. 4i), the g s vlues in these genotypes incresed, lthough they were not s high s in the spring due to decresed solr rdition nd high tempertures (Fig. 4c, f). The decrese in g s of the L. Avnzo nd Nev plnts prtilly compensted the effect of rdition on trnspirtion t middy; this ws reflected in etter plnt wter sttus in these genotypes, less negtive vlues of W l nd W x nd higher vlues of RWC l during summer conditions (Fig. 3), especilly t HPD. Stomtl regultion could e useful mechnism to reduce the loss of wter through trnspirtion nd optimize wter resources when the evpotrnspirtive demnd is high (Tenhunen et l. 1990; Nvrro et l. 2009). It indictes tht the genotypes L. Avnzo nd Nev re well suited to semi-rid conditions. Responses similr to those seen in the genotype L. Avnzo under drought conditions hve een descried y Mrron et l. (2002), who recorded

13 Author's personl copy Trees (2014) 28: comprle RWC l nd W l vlues nd the erliest drop in g s with the increse of drought in the genotype L. Avnzo, regrding nother drought-tolernt poplr genotype Dorskmp. Jio (2008) listed the genotype Nev s one of the more drought-resistnt vrieties, in study where n evlution index ws used to pprise drought resistnce nd eco-physiologicl dptilities in seven poplr genotypes. The erly stomtl closure in response to soil drying in oth L. Avnzo nd Nev genotypes my protect trees from ctstrophic xylem cvittion (Hrvey nd vn den Driessche 1997) nd susequent lef scission (Liu nd Dickmnn 1992). L. Avnzo nd Nev lso mintined high RWC l, especilly when wter potentil decreses (in summer months) nd this ility is considered indictive of drought tolernce (Irigoyen et l. 1992; Kimni et l. 1994; Mrron et l. 2002). Lef wter potentil (W l ), s mesured on single lef, reflects mny fctors: the locl lef wter demnd (VPD, lef-intercepted rdition), soil wter vilility, internl plnt hydrulic conductivity nd stomtl regultion. Stem wter potentil (W x ) is mesured in non-trnspiring lef (Begg nd Turner 1970). Dily W x is the result of whole plnt trnspirtion, nd hs een successfully pplied s wter deficit indictor for pech nd plum orchrds s well s grpevines (Grnier nd Berger 1985; McCutchn nd Shckel 1992; Choné et l. 2001). We considered W x to e noticele indictor of plnt wter deficit, s this mesurement ws generlly less vrile thn W l. This lower vriility in W x improved the ility of this mesure to detect smll (0.1 MP) ut sttisticlly significnt differences etween poplr genotypes under oth plnt densities (Fig. 3). Due to the sensitivity nd reltive predictility of environmentl VPD, W x ws closely relted to stomtl conductnce to otin the threshold W x 75 % g s t which the stomt egin to close, nd the W x B 15 % g s t which stomt were considered to e closed. The reltively high threshold of W x75 % nd the more grdul pttern of stomtl closure (W x75 % - W x B 15 %.) in the P. cndensis genotypes L. Avnzo nd Nev could e ssocited with more conservtive wter use. Furthermore, P. cndensis genotypes ut especilly L. Avnzo nd Nev could e considered more drought tolernt thn those exhiiting rpid stomtl closure, such s the genotypes Dvin nd Len (Bssmn nd Zwier 1991; Giorio et l. 1999; Nsh 2009). Conclusions The high plnt density tested (6,667 cuttings h -1 ) for poplrs grown under the semi-rid conditions of the Mediterrnen gve us etter outcomes thn the low plnt density, since wood iomss production ws higher under HPD nd stomtl conductnce mesured in plnts grown t HPD (6,667 cuttings h -1 ) do not significntly differ from those in poplrs grown t LPD (1,667 cuttings h -1 ). The results shown here re derived from the first cycle of SRC production; results from successive cycles re required to evlute the effects of plnt density on soil wter vilility nd soil fertility. Among the five genotypes tested, Len nd Dvin demonstrted poor growth nd low percentges of rooting nd sprouting. The genotype Bellini hd low stomtl sensitivity to the wter conditions of the soil (i.e., high g s during greter evpotrnspirtive demnd) nd modest production performnce. The genotype Nev hd the highest percentges of rooting nd sprouting, long with high growth nd iomss production. In Nev, these chrcteristics hve een ssocited with specific physiologicl strtegies for production under semi-rid environments: n effective stomtl regultion, high W x75 % threshold, grdul pttern of stomtl closure nd constnt nd high RWC l vlues. Although further field experiments re needed to evlute the potentil for using poplrs genotypes s SRC in Mediterrnen climtes, the genotype Nev ppers to e etter dpted to reduced soil moisture during the summer seson (with high rdition nd evpotrnspirtive demnd). Acknowledgments This reserch ws conducted with finncil support from the FAESI project, funded y the Ministero delle Politiche Agricole Alimentri e Forestli (Itly). The uthors re grteful, prticulrly to the technicins of the experimentl frm of the Agriculturl Reserch Council Reserch Unit for Cropping Systems in Dry Environments (C.R.A. S.C.A.) in Rutiglino (Itly). Conflict of interest of interest. 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