J. ISSAAS Vol. 23, No. 2: (2017)

J. ISSAAS Vol. 23, No. 2: 192-22 (217) EFFECTS OF AERENCHYMA FORMATION ON THE ROOT POROSITY, LATERAL ROOT DEVELOPMENT, TOTAL WATER UPTAKE AND SHOOT BIOMASS ACCUMULATION OF GLOBAL MAGIC RICE UNDER SEEDLING- STAGE DROUGHT STRESS Vince Angelo G. Gicry 1,2, Nin M. Cdiz 2 nd Ameli Henry 3 1 BS Affilite Thesis Scholrship Progrm, Crop nd Environmentl Sciences Division, Interntionl Rice Reserch Institute, Los Bños, Lgun, Philippines 2 Institute of Biologicl Sciences, College of Arts nd Sciences, University of the Philippines Los Bños, Lgun, Philippines 3 Crop nd Environmentl Sciences Division, Interntionl Rice Reserch Institute, Los Bños, Lgun, Philippines *Corresponding uthor: vggicry@up.edu.ph (Received: August 6, 217; Accepted: November 19, 217) ABSTRACT Drought stress negtively ffects rice productivity due to the lck of wter necessry for growth nd development. Previous studies hve shown tht the formtion of root corticl erenchym (RCA) cn influence the development of lterl roots, which id in the uptke of wter nd ccumultion of biomss during drought. The present study evluted erenchym formtion in severl genotypes of globl Multi-Prent Advnced Genertion Intercross (MAGIC) popultion in rice, nd its reltionship with lterl root development, root porosity, totl wter uptke, nd shoot biomss ccumultion during seedling-stge drought stress. The following methods were used in the study: erenchymtous regions were mesured in pixels using GIMP v.2.8.14 (GNU Imge Mnipultion Progrm); percent lterl roots were determined using root scnner coupled by Winrhizo v. 25, nd root porosity ws estimted using the buoyncy method. Hlf of the genotypes studied exhibited 2-35% RCA development under drought-stress (), compred with 7-25% under the well-wtered () tretment. Men percent RCA ws lso higher in (14.3%) thn in (12.98%), but ws not significntly different t p=.53. In ddition, high RCA genotypes, like MG- 6865, did not show highest or lowest shoot biomss nor totl wter uptke for both tretments. Thus, RCA formtion in the globl MAGIC rice genotypes did not influence totl wter uptke, shoot biomss ccumultion, root porosity, nd the production of lterl roots. Under seedling-stge during severe drought stress, formtion of erenchym ws observed to be mere response to the bsence of wter through cell lysis. Therefore, root trits other thn RCA might be more correlted with droughttolernce in the studied genotypes. Key words: globl MAGIC rice, biomss, wter uptke INTRODUCTION Drought is mjor fctor limiting crop growth nd development nd negtively ffects the griculturl sector. It is the primry reson for mjor production losses round the world. The Asi- Pcific Region produces nd consumes bout 9% of the totl rice in the world (Serrj et l. 25, FAO 2), unfortuntely, pproximtely 2% of Asi s rice fields re severely ffected by drought (Pndey nd Bhndri 28). In 216 lone, the Philippines lost 81 million pesos worth of griculturl 192

Effects of erenchym formtion on the root porosity. produce due to the onslught of El Niño (Interntionl Federtion of Red Cross nd Red Crescent Societies 216). The occurrence of drought events is expected to increse, bsed on current climte projections (IPCC 212) nd competition mong griculturl, urbn nd industril sectors (Hro von Mogel 213). Interntionl interest on drought studies hs incresed through the yers, focusing minly on root biology (Coms et l. 213; Gowd et l. 211). As the primry orgn for wter nd nutrient uptke, root morphology, topology, distribution nd its entire rchitecture limit shoot functioning (Lynch 1995, Nrdini et l. 22, Sperry et l. 22). Thus, expnding our knowledge nd understnding on how the root system intercts with different soil-wter regimes, especilly during drought, would id in further improving drought-tolernt rice vrieties nd ultimtely, globl food security. A vriety of root dpttions of rice under drought stress hve been identified, such s the development of well-brnched roots (Yoshid nd Hesgw 1982) nd incresed mximum root depth vi verticl elongtion (Singh et l. 2, Gowd et l. 211, Henry et l. 211, Lynch 213, Wsson et l. 212). Rice roots re typiclly chrcterized by the presence of corticl erenchym which disintegrtes the root cortex to provide spce for the diffusion of oxygen during flooding nd nerobic conditions (Evns 24). Aerenchym tissue lso lowers the metbolic costs of respirtion to mximize growth (Zhu et l. 2), nd nutrient nd wter uptke during drought (Colmer et l. 25). Rice under trnsient drought-to-wterlogged conditions ws found to induce the production of lterl roots through the development of erenchym tissues, with positive correltions between root porosity nd lterl root production (Niones et l. 213). Although chnging wter regimes ffects the development of erenchym in rice roots, its ctul function in wter uptke during drought is poorly understood (Henry et l. 212). The present study investigted erenchym formtion under drought stress; determined whether biomss ccumultion nd wter uptke re ffected by erenchym formtion; nd determined if genotypes with high percentge of root corticl erenchym (RCA) hve greter root porosity nd lterl roots. We used genotypes from the globl Multi-prent Advnced Genertion Intercross (MAGIC) rice popultion, chrcterized by its diverse genetic bckground nd contrsting shoot biomss under drought. The globl MAGIC popultion ws developed by crossing eight prentl lines from n indic MAGIC popultion nd eight prentl lines from jponic MAGIC popultion mking the globl MAGIC popultion representtion of 16 prents from totl of 15 multiple crosses. The popultion ws clled s "globl MAGIC" becuse it "encompsses the indic nd jponic rice ecotypes tht re known globlly"; lso, it hs undergone multiple crosses tht boosted genetic diversity, which is sid to be higher thn the two prentl lines (Bndillo et l. 213). The study ws conducted t the Drought Physiology greenhouse nd lbortory of the Interntionl Rice Reserch Institute from My-September 214. MATERIALS nd METHO Plnt Mterils Five hundred forty genotypes from the globl MAGIC popultion were grown in the field under drought conditions. Ten genotypes with high shoot biomss nd ten with low shoot biomss were selected from the previous screening. The genotypes re s follows: (High shoot biomss) MG- 6157, MG-6638, MG-616, MG-6781, MG-6148, MG-68, MG-6863, MG-6865, MG-686 nd MG-6164; (Low shoot biomss) MG-6415, MG-649, MG-6411, MG-6419, MG-6261, MG-6353, MG-642, MG-6462, MG-6485 nd MG-6256. Experimentl Design The greenhouse experiment ws rndomized complete block design with 2 x 2 fctoril rrngement of tretments. Ech genotype ws replicted five times nd ws exposed to two tretments: drought-stress () nd well-wtered (). 193

J. ISSAAS Vol. 23, No. 2: 192-22 (217) Growth Conditions Seeds were germinted Petri dishes with filter pper (Whtmn, No.41) moistened with tp wter, with ech dish hving bout 5 seeds. Ech Petri dish ws lbeled ccording to genotype nd plced inside n incubtor t 37 C for 3 dys. Wter ws continully dded until the rdicle nd smll portion of the epicotyl emerged. Soil ws collected, sieved, sterilized in the oven for 6-7 hours, nd sun-dried until moisture ws completely removed. Two hundred Mylr tubes were filled with 95 g of soil ech, with its height in the tube djusted to 4 cm. Plstic cylinders were plced inside the tubes to hold the soil. Cheese cloth ws plced t the bottom of ech plstic cylinder to llow wter to seep through. These Mylr tubes were kept in boxes ssisted with wire rcks on top of tble t the IRRI greenhouse. The tubes were sturted with wter for 2 hours nd were left overnight to rech the field cpcity. Both drought-stressed nd well-wtered tretments were observed strting t field cpcity. Ech Mylr tube ws then plnted with three pre-germinted seedlings nd were thinned out to one fter complete emergence of the epicotyl. The plnts were llowed to grow under both tretments strting t field cpcity for 21 dys. Totl Wter Uptke Prior to plnting, tubes were soked with wter for 2 h nd were drined overnight to rech field cpcity. The tubes were then weighed in top loding blnce to serve s bsis for the mesurement of wter uptke by weight. The mesurement of wter uptke for both nd conditions strted t field cpcity; however, for the, the soil ws left to dry until the durtion of the experiment wheres for, lost wter ws brought bck with the field cpcity weight of the tube s the bsis. Hence, if the field cpcity weight of the tube is 16 g nd weighed 15g fter two dys, then g of wter should be dded bck to the tube so s to mintin the in field cpcity. The tubes were weighed three times week (MWF) for 21 dys. Totl wter uptke ws computed using the formul below. Totl Wter Uptke (TWU in g) = (Tube weight t Field Cpcity, g)- (Tube weight t hrvest, g) Percent Lterl Roots Root systems were seprted from the shoot systems nd were thoroughly clened. A root scnner coupled by Winrhizo v. 25 (Regent Instruments, Quebec Cnd) ws used to mesure root length (cm), volume(cm 3 ), nd clssify root dimeter clsses (mm<dimeter<1mm). Percent lterl roots were computed using the formul below: % Lterl roots= (sum of root length with dimeters rnging up to.2mm) totl root length (cm) 194 x % Root Porosity Porosity, or the reltive volume of internl gs spces in root tissue, is importnt in determining resistnce to flooding (Visser nd Bogemnn 23). This prmeter cn lso be dopted in investigting root response to drought. Root porosity ws estimted using the buoyncy method (Rskin 1983, Thomson et l. 199, Visser nd Bogemnn 23). From the root bse, 3-cm nodl root ws cut into 3 segments nd plced in smll bottle contining distilled wter. The soked roots were blotted dry using bsorbent pper for 2 seconds to remove dhering wter. The dried roots were weighed nd then plced in 1.5 mlglss tube. The glss tubes contining the roots were then plced in vcuum chmber for 45 minutes. The roots were removed from the bottle nd were rpidly blotted with bsorbent pper. The roots were then weighed. Using the weight (g) of the root before(g, w1) nd fter (g, w2)the vcuum, the porosity of the roots ws clculted s: (w2 w1)x Specific Weight (SW)of wter %Root Porosity= x % w2 Root porosity my not be n entire representtion of erenchym formtion, s rice roots grown in well-drined condition typiclly show -12% porosity (Armstrong 1971). Hence, this

% Root Corticl erenchym (RCA) Effects of erenchym formtion on the root porosity. mesurement is prone to overestimtion. However, since there re no estblished methods to estimte erenchym formtion for the entire root system, both the buoyncy nd pixel methods were used. Root Corticl Aerenchym In procedure, similr to tht of Zhu et l. (2) 1-cm nodl roots from the bse were cut freehnd. A thin cross section ws mde using blde under dissecting microscope with three replictes mde per smple. Sections were viewed under DP 7 Olympus microscope t -2x mgnifiction, depending on the size of the section. Imges were tken nd nlyzed using GIMP v 2.8.14. The re of the stele, whole root section, nd erenchymtous regions were mesured using GIMP v.2.8.14 (GNU Imge Mnipultion Progrm). The number of pixels (px) for ech prmeter ws then recorded. Percent root corticl erenchym ws computed using the formul below. %RCA= erenchym re (px) whole root re (px) stele re (px) x % Biomss Both the shoot nd the root systems were plced on seprte envelopes nd were dried for five dys t 6 C. The smples were then weighed using top loding blnce. Sttisticl Anlyses Dt were nlyzed in R v. 3.1.2 (R Development Core Tem 214) using Person correltion, nlysis of vrince (ANOVA), nd Tukey s Honest Significnt Difference (HSD) for men comprison of prmeters. RESULTS AND DISCUSSION Aerenchym Formtion under nd Conditions Root corticl erenchym cn be induced through vriety of conditions, such s NPKdeficiency (Postm nd Lynch 211), hypoxi (Evns 24) nd mechnicl impednce (Striker et l. 27). Rice lines under drought were lso observed to hve less erenchym (Henry et l. 212, Gowd et l. 211). Men %RCA ws higher under (14.3%) thn (12.98%), but ws not significntly different t p=.53 (Fig. 1). Percentge vrition in RCA development rnged from 35% to 2% under, nd from 25% to 7% under conditions (Fig. 2). Hlf of the genotypes produced more erenchym tissue under thn in. Under, erenchym tissue incresed for MG-6865 (75%) nd MG-6419 (61%), while under, percentge RCA incresed for MG-68 (89%) nd MG-6353 (77%). Percent RCA ws significntly different cross genotypes under (p=.14), but not for (p=.38). Under, MG-6865 hd the highest % RCA (35.4%) while MG-68 hd the lowest (1.98%). On the other hnd, under, MG-6863 hd the highest % RCA (25.39%) while MG-6462 hd the lowest (7.13%). 15 5 Fig. 1. Men % RCA under nd conditions (not significntly different t p=.53). Tretment 195

% Root Corticl Aerenchym (RCA) 4 35 3 25 2 15 5 J. ISSAAS Vol. 23, No. 2: 192-22 (217) b b b b b b b b b b b b b b b b b b b Genotypes Fig. 2. Men % RCA of ech genotype under nd conditions. Brs indicte the stndrd error. Mens with the sme letter re not significntly different t P<.5 within tretments. Percent root corticl erenchym decreses during drought (Henry et l. 212). Representtive genotypes from the globl MAGIC rice popultion under hd higher men RCA vlues thn those under conditions. However, results were vrible s hlf the genotypes hd greter RCA under thn under conditions. The high genetic vribility of the popultion must hve contributed to such vrible results (Fig.3). In mize, RCA formtion reduces root respirtion by converting corticl cells to gs spces (Zhu et l. 2). Fig. 3. Phenotypic vrition in root corticl erenchym formtion per genotype with three replictes: (1 st row) MG-6261 (highest shoot dry weight, SDW nd wter uptke under ); (2 nd row) MG-649 (lowest SDW nd wter uptke under ); (3 rd row) MG-6863 (highest SDW nd wter uptke under );(4 th row) MG-6462 (lowest SDW nd wter uptke under ). 196

% Root Porosity % Root Porosity Effects of erenchym formtion on the root porosity. Reltionship of Percentge RCA with Root Porosity nd Percentge Lterl Roots Formtion of root corticl erenchym is positively correlted with root porosity (i.e increse in percent RCA increses the volume of gs-filled spces in roots) (Colmer 23, Mlik et l. 23). This enhnces the bility of roots, under nerobic conditions, to trnsport oxygen to the shoots (Justin nd Armstrong 1987, Evns 24), but porosity my differ ccording to the root type, whether seminl or nodl (Thomson et l. 199). Men percent porosity ws higher under (46.7%) thn under (45.1%) conditions, but there ws no significnt difference (Fig. 4). incresed root porosity of MG-6148 by 42% nd of MG-6419 by 3%; wheres conditions incresed root porosity of MG-686 by 18% nd of MG-642 by 17%. Percent root porosity ws not significntly different cross genotypes under both (p=.91) nd (p=.24). Under, the highest nd lowest percent porosity ws present in MG-6148 (55.3%) nd MG-6256 (38.9%), respectively. Under, on the other hnd, the highest nd lowest present porosity ws present in MG-6353 (35.5%) nd MG-6148 (31.9%), respectively. No correltion between root porosity nd erenchym development ws present (Fig. 5). 5 4 3 2 Tretment Fig. 4. Men percentge root porosity vlues under nd conditions (not significntly different t p=.43). 6 5 4 3 2 2 3 4 % Root Corticl Aerenchym (RCA) Liner () Liner () Fig. 5. Root porosity nd percent RCA under nd conditions. Correltion ws not significnt using Person s correltion t p=.64. Percentge lterl roots rnged from 76-84% under nd from 76-82% under condition, with significnce level of p<.1. Lterl roots represent lrger portion of the entire root system (Wng et l. 29); thus, their density my ffect root system functioning, especilly in improving contct with shrinking wter columns in the soil (Henry et l. 212). Moreover, erenchym development influences lterl root production under wterlogged-drought conditions 197

Percentge % Lterl Roots J. ISSAAS Vol. 23, No. 2: 192-22 (217) (Niones et l. 213). However, there ws no correltion between percentge of lterl roots nd percent RCA (Fig. 6). Furthermore, either n increse or decrese in percent root corticl erenchym did not ffect percentge of lterl roots for ll genotypes studied (Fig. 7). Hence, RCA formtion ws not ssocited with lterl root development under for this study. 86 84 82 8 78 76 74 2 4 % Root Corticl Aerenchym Liner () Liner () Fig. 6. Percent lterl roots nd % RCA under nd conditions. Correltion not significnt using Person s correltion. (not significntly different t p=.67). 9 8 7 6 5 4 3 2 bc bc b bc bc bc b bc b bc bc bc bc bc bc bc bc c b b b b b b b b b b b b b b b b b b b b RCA LR Genotypes Fig. 7. Men % RCA ginst men % lterl roots (LR) for ech genotype under tretment. Br indictes the error dt. Mens with the sme letter re not significntly different t P<.5 Reltionship of Percent Root Corticl Aerenchym (%RCA) with Totl Wter Uptke nd Shoot Biomss Shoot dry weight (g) nd totl wter uptke were both highly significnt cross ll genotypes under both (P<.1) nd (p<.1) nd in between tretments. genotypes exhibited 54% greter shoot biomss thn genotypes under. Shoot biomss responds significntly to vrying soil moisture conditions with genotypes under exhibiting 7-79% reduction in shoot dry weight reduction (Surlt nd Ymuchi 28). Genotypes under tretment exhibited 48% more totl wter uptke thn under. These results indicte RCA formtion ws not correlted with either wter uptke nd biomss ccumultion under nd tretments (Fig.8). 198

% Root Corticl Aerenchym % Root Corticl Aerenchym Effects of erenchym formtion on the root porosity. 4 3 2 Liner () Liner ().2.4.6.8 1 Shoot Dry Weight (g) 4 35 3 25 2 15 5 2 3 4 Totl Wter Uptke (g) Liner () Liner () Fig. 8. Reltion between % RCA nd totl wter uptke (TWU) nd between % RCA nd shoot dry weight (SDW). SDW nd % RCA re not correlted under (p=.43) nd (p=.11). TWU nd % RCA re lso not correlted under (p=.75) nd (p=.18). Dt ws nlyzed using Person Correltion t p>.5 significnce level. Most studies on the reltionship between biomss nd RCA support strong positive correltion between the two, especilly under nutrient deficient nd hypoxic conditions. The efficient use of metbolic resources impcts biomss, soil explortion, nd resource cquisition (Lmbers et l. 22). RCA formtion in soyben dpts nitrogen metbolism nd biomss prtilly during prolonged periods of flooding (Thoms et l. 25). In phosphorus-deficient soils, RCA development is induced to reduce the plnt s phosphorous requirements nd root respirtion (Fn et l. 23). Under nitrogendeficient soil, RCA enhnces root depth by 15 to 31%, increses shoot biomss by up to 66%, nd boosts grin yield by 58% (Sengwili et l. 214). Under drought, high RCA in mize hybrid lines exhibited high biomss nd yield compred togenotypes with low RCA (Zhu et l. 2). Under drought-to-wterlogged conditions, RCA in rice hybrids promotes lterl root production, which contributes to high dry mtter production (Niones et l. 213). RCA formtion decreses wter uptke in rice plnts under drought conditions, s it impedes the rdil trnsport of wter (Yng et l. 212). As our results show, RCA formtion in the globl MAGIC rice genotypes did not ffect totl wter uptke, shoot biomss ccumultion, nd the production of lterl roots. Under seedling-stge severe drought stress, the formtion of erenchym ws observed to be mere response to the bsence of wter through cell lysis. Therefore, other root trits besides RCA my be more correlted with drought-tolernce for the genotypes studied. CONCLUSION Erlier studies suggest tht erenchym tissue is formed in roots to decrese the metbolic costs of root respirtion, nd to increse the root system cpcity for uptke, by inducing lterl root formtion. In this study, erenchym formtion in rice roots under drought ws not correlted with root porosity, lterl root development, totl wter uptke, nd shoot biomss. The high erenchym 199

J. ISSAAS Vol. 23, No. 2: 192-22 (217) formtion observed in genotypes exposed to drought my hve been due to cell lysis, s response to the bsence of wter. Hence, erenchym formtion does not id the studied genotypes in dpting to drought stress. Thus, the vrible results in the trits observed my be due to the popultion s high genetic diversity. High RCA-producing lines in the popultion need to be subjected to further drought studies to improve our understnding on the dynmics of erenchym formtion in rice roots exposed to drought stress. RECOMMENDATIONS As the study is n initil report on erenchym formtion under drought stress for the globl MAGIC rice, it is therefore recommended to conduct sesonl replictions, to further estblish the vrious prmeters mesured. In ddition, identified genotypes with high RCA formtion should be studied further on their drought response. ACKNOWLEDGMENT The reserchers wish to thnk the IRRI Undergrdute Scholrship Progrm for the funding of the reserch nd IRRI s Drought Physiology group. REFERENCES Armstrong, W. 1971. Rdil oxygen losses from intct rice roots s ffected by distnce from the pex, respirtion nd wterlogging. Physiology of Plnts 25: 192-197. Bndillo, N., Rghvn, C., Muyco, P., Sevill, M., Lobin, I., Dill-Ermit, C., Tung, C.W., McCouch, S., Thomson, M., Muleon, R., Singh, R.K., Gregorio, G., Redon, E. nd H. Leung. 213. Multi-prent dvnced genertion inter-cross (MAGIC) popultions in rice: progress nd potentil for genetics reserch nd breeding. Rice: SpringerOpen Journl, 6:11. Colmer, T. 23. Long-distnce trnsport of gses in plnts: perspective on internl ertion nd rdil oxygen loss from roots. Plnt, Cell nd Environment 26: 17-36. Colmer, T., Greenwy, L. nd M. Rivs-Crbo. 25. Oxygen trnsport, respirtion nd nerobic crbohydrte ctbolism in rots in flooded soils. In. Plnt Respirtion: From Cell to Ecosystem. H. Lmbers nd M. Ribs-Crbo. (eds.) Netherlnds. Springer. p. 137-154. Coms, L., Becker, S., Cruz, V., Byme, P. nd D. Dierig. 213. Root trits contributing to plnt productivity under drought. Frontiers in Plnt Science 4:442. doi:.3389/fpls.213.442. Evns, D. 24. Aerenchym Formtion. New Phytologist161: 35-49. Fn, M., Richrds, J., Brown, K. nd J. Lynch. 23. Physiologicl roles for erenchym in phosphorus-stressed roots. Functionl Plnt Biology 3: 493-56. FAO. 2. Bridging the Rice Yield Gp in the Asi-Pcific Region. Bngkok, Thilnd: Food nd Agriculture Orgniztion of the United Ntions. Gowd, V., Henry, A., Ymuchi, A., Shshidhr, H. nd R. Serrj. 211. Root biology nd genetic improvement from drought voidnce in rice. Fields Crop Reserch 122: 1-13. Hro von Mogel, K. 213. Interctions key to beting future droughts. CSA News 58: 4-9. 2

Effects of erenchym formtion on the root porosity. Henry, A., Cl, A., Btoto, T., Torres, R. nd R. Serrj. 212. Root ttributes ffecting wter uptke of rice (Oryz stiv) under drought. Journl of Experimentl Botny 63: 4751-4763. Henry, A., Gowd, V., Torres, R., McNlly, K. nd R. Serrj. 211. Vrition in root system rchitecture nd drought response in rice (Oryz stiv); phenotyping of the OryzSNP pnel in rinfed lowlnd fields. Fields Crop Reserch 44: 139-15. [IPCC] Intergovernmentl Pnel on Climte Chnge. 212. Mnging the risks of extreme events nd dissters to dvnce climte chnge dpttion. 32 Avenue of the Americs, New York, NY 13-2473, USA: Cmbridge University Press. Interntionl Federtion of Red Cross nd Red Crescent Societies. 216. Philippines: Drought nd Dry Spells. Informtion Bulletin n 1 GLIDE n DR-216-17-PHL 29 April 216. 5 p. Justin, S. nd W. Armstrong. 1987. The ntomicl chrcteristics of roots nd plnt response to soi flooding. New Phytologist 6: 465-495. Lmbers, H., Atkin, O. nd F. Millenr. 22. Respirtory ptterns in roots in reltion to their functioning. In Y. Wisel, A. Eshelnd nd K. Kfkki. Plnt Roots, the Hidden Hlf (pp. 521-552). New York City, USA: Mrcel Dekker Inc. Lynch, J. 1995. Root rchitecture nd plnt productivity. Plnt Physiology. 9:7-13. Lynch, J. 213. Steep, chep nd deep: n ideotyype to optimize wter nd cquisition by mize root systems. Annls of Botny 112: 347-357. Mlik, A., Colmer, T., Lmbers, H. nd M. Schortemeyer. 23. Aerencym formtion nd rdil oxygen loss long dventitious roots of whet with only the picl root portion exposed to oxygen deficiency. Plnt, Cell nd Environment 26: 1713-1722. Nrdini, A., Slleo, S. nd M. Tyree. 22. Ecologicl spects of wter permebility of roots. In Y. Wisel, A. Eshel nd U. Kfkfi, The Hidden Hlf 3rd ed.(pp. 683-698). New York: NY Mrcel Dekker Inc. Niones, J., Surlt, R., Inuki, Y. nd A. Ymuchi. 213. Roles of root erenchym development nd its ssocited QTL in dry mtter production under trnsient moisture stress in rice. Plnt Production Science 16(3): 215-216. Pndey, S. nd H. Bhndri. 28. Drought: economic costs nd reserch implictions. In R. Serrj, J. Bennettnd B. Hrdy, Drought Frontiers in Rice: Crop Improvement for Incresed Rinfed Production. World Scientific Publishing nd Los Bños (Philippines): Interntionl Rice Reserch Institute, Singpore. Postm, J. nd J. Lynch. 211. Root corticl erenchym enhnces the growth of mize on soils with suboptiml vilbility of nitrogen,phosphorus, nd potssium. Plnt Physiology 156: 119 121. R Development Core Tem. 214. A lnguge nd environment for sttisticl computing. R Foundtion for Sttisticl Computing, Vienn, Austri Rskin, I. 1983. A method for mesuring lef volume, density, thickness, nd internl gs volume. Hortscience 18: 698-699. 21

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