PLANT POPULATION INFLUENCES ON MAIZE PHYSIOLOGICAL RESPONSES TO NITROGEN APPLICATION

PLANT POPULATION INFLUENCES ON MAIZE PHYSIOLOGICAL RESPONSES TO NITROGEN APPLICATION C.R. Boomsm nd T.J. Vyn Purdue University, West Lfyette, Indin Astrt Pst geneti improvements in mize (Ze mys L.) hve led to the retion of modern hyrids tht re inresingly tolernt to vriety of stresses nd, in prtiulr, to high plnt popultions. However, high plnt densities n dversely ffet overll grin yield due to greter intrspeifi (within speies) ompetition for limiting resoures. This ompetition inreses rrenness nd plnt-to-plnt vriility for grin yield (GY CV ) nd other morpho-physiologil trits. In this study, we hypothesize tht nitrogen (N) pplition will redue intr-speifi ompetition for N, thus deresing GY CV while inresing per-unit-re grin yield (GY A ). We further suggest tht redutions in GY CV will e more pronouned t higher thn t lower plnt popultions. In 2005 nd 2006, hyrid Pioneer 31G68 ws grown t densities of 54,000, 79,000, nd 104,000 plnts h -1. Ure Ammonium Nitrte (UAN) (28-0-0) ws pplied vi side-dressing t rte of 170 kg N h -1 one (V3), twie (V3, V5), or not t ll. Individul plnts were mesured for numerous morpho-physiologil trits inluding lef N ontent (SPAD) (2005, 2006), R6 totl iomss (TB P ) (2006), hrvest index (HI) (2006), nd grin yield (2005, 2006), nd numer of nopy-level mesurements were tken. The pplition of 170 kg N h -1 generlly inresed GY A, per-plnt grin yield (GY P ), HI, TB P, nd lef N ontent reltive to the zero N rte regrdless of plnt popultion. Corresponding with the intensity of intr-speifi ompetition during vegettive nd reprodutive stges, GY CV ws gretest t the highest plnt density when no N ws pplied nd smllest t the lowest plnt popultion when 340 kg N h -1 ws pplied. The positive GY A response to N pplition ws muh igger t higher plnt popultions. Without suffiient N t high plnt densities, intr-speifi ompetition for soil minerl N ws severe. Limittions in ville N led to redued folir N onentrtions nd erlier lef senesene tht limited TB P nd GY P. Overll, this study demonstrtes tht N pplition plys ritil role in deresing plnt-to-plnt vriility whih ws more pronouned when mize ws grown t high plnt densities.. Introdution Geneti improvements in mize (Ze mys L.) hve led to modern hyrids tht re inresingly tolernt to vriety of stresses nd, in prtiulr, to high plnt popultions (Tollenr nd Wu, 1999; Duvik, 2001). One result of higher plnt density tolerne is tht the optimum plnt popultion for mximum grin yield hs inresed stedily over the pst 70 yers (Duvik nd Cssmn, 1999). However, pst yield gins nnot e ttriuted to geneti improvements lone. Insted, they re the produt of n improved geneti y gronomi-mngement intertion (Tollenr nd Lee, 2002). The doption of superior gronomi-mngement prties hs inluded the pplition of eonomilly nd environmentlly pproprite rtes of inorgni nitrogen (N) fertilizers. All

mize hyrids rely on dequte N inputs to reh mximum yield potentil, nd modern hyrids re generlly more N effiient (i.e. grin yield per unit N pplied) thn their older ounterprts (Tollenr nd Wu, 1999). However, even when density-tolernt hyrids nd N pplitions re employed, high plnt popultions n dversely ffet overll grin yield due to inresed plntto-plnt vriility for grin yield (GY CV ) nd higher inidene of rrenness (Toktlidis nd Koutrous, 2004). Both mngement prties nd environmentl onditions typilly use inresed GY CV nd deresed uniformity for other morpho-physiologil trits. Some mngement-relted uses of plnt-to-plnt vriility inlude devitions in plnting depth nd seed sping, uneven nutrient pplition nd rop residue distriution, wheel-trk omption, nd tillge/rottion prties (Boomsm nd Vyn, 2007). Environmentl-relted uses inlude vritions in inset feeding nd disese pressure long with inherent soil sptil vriility (Andrde nd Ate, 2005). In modern mize prodution systems, enhned plnt-to-plnt vriility often results from inresed ompetition mong individul plnts t progressively higher plnt densities for limiting resoures suh s N, inident photosynthetilly tive rdition (IPAR), nd soil moisture. Pst studies hve often emphsized tht stnd uniformity is essentil for high produtivity levels, nd tht inresed plnt-to-plnt vriility (determined nd expressed using vriety of mize growth nd developmentl prmeters) redues per-unit-re mize grin yields (GY A ) through redued stress tolerne (Tollenr nd Wu, 1999). Therefore, t higher plnt popultions, resoure vilility must e dequte to help mintin uniform growth, development, nd grin yield of djent plnts in mize nopy. Historilly, numerous growth nd developmentl prmeters suh s plnt height nd V-stge hve een used to investigte the morpho-physiologil progression of mize hyrids throughout the growing seson. Plnt height hs lso een used to mesure plnt-to-plnt vriility, lthough orreltions etween the height of individul plnts t prtiulr growth stges nd their respetive grin yields hve often een wek (Muldoon nd Dynrd, 1981; Dynrd nd Muldoon, 1983). Other per-plnt morpho-physiologil prmeters tht hve enled n enhned understnding of intr-speifi ompetition, per-plnt vriility, nd resulting effets on grin yield inlude lef N onentrtion, totl iomss, nd hrvest index (HI). Previous studies hve exmined numer of these per-plnt morpho-physiologil trits with respet to intr-speifi ompetition nd plnt-to-plnt vriility t multiple plnt densities (e.g. Edmedes nd Dynrd, 1979; Veg et l., 2000; Veg nd Sdrs, 2003; Pgno nd Mddonni, 2007). However, no study to dte hs investigted the morpho-physiologil effets of N vilility on intr-speifi ompetition in mize t multiple plnt densities. Overll, this study seeks to understnd the effets of plnt popultion nd N rte on mize intrspeifi ompetition nd growth nd developmentl uniformity t oth the per-plnt nd nopy levels of orgniztion through the mesurement of numerous morpho-physiologil trits t multiple stges of vegettive nd reprodutive development. We hypothesize tht () minerl N vilility my e more essentil for proper mize growth nd development t high plnt popultions thn t low plnt popultions nd () grin yield loss due to inresed plnt-to-plnt vriility t higher plnt popultions will e redued when N rtes re inresed. We lso suggest tht with inresed N rtes t higher plnt densities, GY A my e inresed due to deresed GY CV, improved per-plnt totl iomss prodution (TB P ) (i.e. enhned photo-

ssimilte prodution), greter prtitioning of iomss to the grin (i.e. improved HI), nd oth delyed nd redued lef senesene during the grin-filling period. Mterils nd Methods Experimentl Design nd Mngement Prties Reserh in 2005 nd 2006 ws onduted t Purdue University s Agronomy Center for Reserh nd Edution (ACRE) ner West Lfyette, Indin (40 28 N Lt.). The soil, whih hd developed under pririe vegettion, ws Chlmers (fine-silty, mixed, mesi Typi Endoquoll) silty ly lom with pproximtely 4.0% orgni mtter ontent in the top 30 m of the soil profile. The experimentl re hd less thn 2% slope nd ws systemtilly tile drined. In oth yers, mize ws grown following soyens (Glyine mx L. Merrill). Striptillge ws performed in the fll. The study ws nlyzed s split-plot design with four loks. Plnt popultion (whole plot) nd N pplition rte (suplot) served s the two tretment ftors. Hyrid Pioneer 31G68 ws plnted t rtes of 58,000, 83,000, nd 108,000 seeds h -1 to hieve the desired finl plnt popultions of 54,000, 79,000, nd 104,000 plnts h -1. Strter fertilizer (10-34-0) ws pplied t rte of 185 L h -1 5 m to the side nd 5 m elow the seed. Ure Ammonium Nitrte (UAN) (28-0-0) ws pplied vi side-dressing t rte of 170 kg N h -1 one (V3), twie (V3, V5), or not t ll, depending upon eh plots presried N pplition rte. Soil smples (0-30 m) were tken t V4 nd V8 in plots reeiving only strter fertilizer to determine soil nitrte (NO 3 - ) nd mmonium (NH 4 + ) onentrtions. Eh plot onsisted of 6 rows (76-m row sping) nd ws 4.6 m in width nd 27.5 m or 30.5 m in length in 2006 nd 2005, respetively. Individul Plnt Non-destrutive nd Destrutive Mesurements In oth yers, smpling res (4 m in length) were estlished in rows 3 nd 4 of eh 6-row plot. Within eh of the designted smpling res, individul plnts ( 36-70 per smpling re depending upon plnt popultion, 1,8 in totl yr -1 ) were tgged with rodes tht were individully numered. The growth nd development of eh of the tgged plnts ws mesured throughout the growing seson using the following prmeters: emergene dte; pre- (V15) nd post-silking (R3 nd R6) stlk dimeter (6 th internode); pre-silking (V5 nd V15) plnt height nd V-Stge; finl plnt height (R6); pre- (V15; 12 th lef) nd post-silking (R1, R3, nd R5; erlef) SPAD (i.e. lef greenness mesured with hlorophyll meters); erlef position; totl lef numer; plnt sping; nd nthesis (2006) nd silking (2005, 2006) dte. In 2006, the LAI of eh tgged plnt ws determined t R1 using modified proedure from Vlentinuz nd Tollenr (2006). In oth yers, individul ers of eh tgged plnt were hnd-hrvested, shelled, nd mesured for moisture ontent, totl kernel weight, nd totl kernel numer. In 2006, the R6 TB P nd resulting HI of 6 onseutive, representtive, tgged plnts from eh smpling re ws determined. All plots were mhine hrvested fter hnd-hrvesting of the smpling res. Yields were orreted to 15.5% moisture ontent. For this pulition, only suset of the previously mentioned individul plnt mesurements ws nlyzed. Although results presented here re for only two yers nd single hyrid, this

experiment involves four yers of dt (2004-2007) with two hyrids eh yer; hyrids were the first split, plnt popultion the seond split, nd N rte the third split in ll these tests. Sttistil Anlyses Anlysis of vrine (ANOVA) ws performed using SAS (SAS Institute, 2004) PROC GLM or PROC MIXED for lned or unlned dt, respetively. The whole-unit error ws pooled with the su-unit error. A omined yer nlysis (2005 nd 2006) ws performed, with ll yer tretment intertions pooled. When tretment effets were signifint, lest signifint differene (LSD) men seprtion tests nd lest-squres men (LS-men) seprtion tests (t-test) were performed for lned nd unlned dt, respetively. Results As shown in Figure 1, GY A inresed for eh plnt popultion with n initil pplition of 170 kg N h -1, however, no inrese in GY A resulted from seond 170 kg N h -1 pplition t eh plnt density. The prtiulrly low GY A for the 104,000 plnts h -1, 0 kg N h -1 tretment omintion likely resulted from intense intr-speifi ompetition for soil minerl N. Slightly greter GY A t 79,000 plnts h -1 thn t 104,000 plnts h -1 for the mid nd high N rtes likely resulted from the highest plnt density eing ove tht for mximum GY A for Pioneer 31G68 (Pioneer Hi-Bred Intl., In., 2005). Although GY A ws reltively similr ross plnt densities for the 170 nd 340 kg N h -1 rtes, GY A deresed drmtilly with inresing plnt popultion for the 0 kg N h -1 rte. This potentilly suggests tht N pplition ws more importnt for sustining GY A t higher plnt densities thn t lower plnt densities. Per-Unit-Are Grin Yield (Mg h -1 ) 20 18 16 14 12 10 8 6 4 2 0 kg N h -1 Figure 1. Effet of plnt popultion nd N rte 170 kg N h -1 340 kg N h -1 on per-unit-re grin yield (GY A ). Mens with different letters indite sttistilly signifint differenes t P 0.05 within eh plnt popultion. The plnt popultion N- rte intertion ws signifint t P 0.05. Plnt Popultion (plnts h -1 ) As shown in Figure 2A, inreses in plnt popultion resulted in dereses in men per-plnt grin yield (GY P ) s result of intensifying intr-speifi ompetition. While the first pplition of 170 kg N h -1 signifintly inresed GY P, seond pplition of N t the sme rte hd no signifint effet on GY P regrdless of plnt popultion (Figure 2B). A lk of inrese in GY A (Figure 1) nd GY P from seond pplition of 170 kg N h -1 (Figure 2B) suggests tht other resoures (e.g. IPAR nd soil moisture) my hve een more limiting thn minerl N fter the initil side-dress pplition. The greter GY A t 79,000 plnts h -1 thn t 104,000 plnts h -1 for the mid nd high N rtes (Figure 1) likely resulted from limited per-plnt

resoure vilility during the period rketing silking t the highest plnt density (Pgno nd Mddonni, 2007). Per-Plnt Grin Yield Men (g) 225 (A) 200 175 1 125 100 75 25 0 Plnt Popultion (plnts h -1 ) Per-Plnt Grin Yield Men (g) 225 200 175 1 125 100 75 25 0 (B) 0 170 340 Nitrogen Rte (kg h -1 ) Figure 2. Effet of plnt popultion (A) nd N rte (B) on men per-plnt grin yield (GY P ). Mens with different letters indite sttistilly signifint differenes t P 0.05. The plnt popultion N-rte intertion ws not signifint t P 0.10. Figure 3 shows tht per-plnt grin yield vriility (GY CV ) generlly inresed with inreses in plnt popultion regrdless of N rte (with exeption to the 170 kg N h -1 rte t 54,000 plnts h -1 nd 79,000 plnts h -1 ). An initil N pplition of 170 kg h -1 deresed GY CV t 54,000 plnts h -1 nd 79,000 plnts h -1 ut not t 104,000 plnts h -1. Although seond N pplition of 170 kg h -1 did not inrese GY A (Figure 1) nd GY P (Figure 2) t the highest plnt density, it did result in signifint derese in GY CV. The effet of N pplition ws smllest t 54,000 plnts h -1 due to redued intr-speifi ompetition for pplied nd residul soil N. In ordne with the degree of intr-speifi ompetition, the tretment omintion of 104,000 plnts h -1, 0 kg N h -1 hd the highest GY CV, while the tretment omintion of 54,000 plnts h -1, 340 kg N h -1 hd the lowest GY CV. Per-Plnt Grin Yield CV (%) 40 35 30 25 20 15 10 5 0 0 kg N h -1 170 kg N h -1 340 kg N h -1 Plnt Popultion (plnts h -1 ) Figure 3. Effet of plnt popultion nd N rte on per-plnt grin yield vriility (GY CV ). GY CV is expressed s the oeffiient of vrition (CV) of per-plnt grin yield. Mens with different letters indite sttistilly signifint differenes t P 0.05 within eh plnt popultion. The plnt popultion N-rte intertion ws signifint t P 0.10. Figure 4A shows tht HI ws signifintly less t 104,000 plnts h -1 thn t 54,000 plnts h -1 nd 79,000 plnts h -1. It ws lso signifintly less for the 0 kg h -1 N rte thn the 170 kg h -1 nd 340 kg h -1 N rtes (Figure 4B). Mize HI typilly dereses t ove-optimum plnt densities s intr-speifi ompetition redues iomss prtitioning to the er (Pgno nd Mddonni, 2007). At the highest plnt popultion in this study, intense intr-speifi ompetition led to greter proportionl redutions in grin yield thn totl ove-ground dry

mtter umultion, inidentlly resulting in redued HI. A lk of N pplition similrly resulted in disproportionl llotion of photossimiltes to non-reprodutive tissues. HI 0.60 0.55 0. 0.45 0.40 0.35 0.30 (A) Plnt Popultion (plnts h -1 ) HI 0.60 0.55 0. 0.45 0.40 0.35 0.30 (B) 0 170 340 Nitrogen Rte (kg h -1 ) Figure 4. Effet of plnt popultion (A) nd N rte (B) on hrvest index (HI). Mens with different letters indite sttistilly signifint differenes t P 0.05. The plnt popultion N-rte intertion ws not signifint t P 0.10. Dt is from 2006 only. Figure 5 indites tht t eh N rte, inreses in plnt popultion resulted in dereses in R6 TB P. A single pplition of 170 kg N h -1 signifintly inresed TB P t eh plnt density. A seond pplition of 170 kg N h -1 inresed TB P t only the lowest plnt popultion. This suggests tht fter n initil pplition of 170 kg N h -1, N vilility limited TB P t the low ut not t the mid nd high plnt densities. The highest TB P ourred for the 54,000 plnts h -1, 340 kg N h -1 tretment omintion, while the lowest TB P ws present for the 104,000 plnts h -1, 0 kg N h -1 tretment omintion. This suggests plstiity for TB P in response to resoure vilility for Pioneer 31G68. Per-Plnt Totl Biomss (g) 400 3 300 2 200 1 100 0 kg N h -1 Figure 5. Effet of plnt popultion nd N 170 kg N h -1 340 kg N h rte on R6 per-plnt totl iomss (TB -1 P ). Mens with different letters indite sttistilly signifint differenes t P 0.05 within eh plnt popultion. The plnt popultion N-rte intertion ws signifint t P 0.10. Dt is from 2006 only. 0 Plnt Popultion (plnts h -1 ) Enhned N uptke s result of greter N vilility n result in lower N moiliztion from plnt leves, delys in lef senesene (i.e. sty-green ), nd, resultntly, prolonged umultion of dry mtter (Mddonni nd Otegui, 2004). Chlorophyll meters hve een routinely used to estimte lef N ontent nd exmine ptterns of lef senesene in mize through the mesurement of reltive lef greenness (i.e. SPAD) (e.g. M et l., 1996; M nd Dwyer, 1997). When used in this study on per-plnt sis, SPAD vlues delined with inresing plnt popultion (Figure 6A) nd deresing N rte (Figure 6B) t eh growth stge. For ll three popultions, SPAD inresed from V15 to R1, peked etween R1 nd R3, nd

deresed from R3 to R5 (Figure 6A). Vlues for SPAD t V15, R1, R3, nd R5 did not differ for the 170 kg N h -1 nd 340 kg N h -1 pplition rtes (Figure 6B), inditing tht the seond N pplition did not improve lef N ontent or derese the rte of lef senesene for these tretments. However, the ontriution of n initil N pplition of 170 kg h -1 did signifintly improve oth lef N ontent nd sty-green for ll three plnt popultions in similr fshion. While plnts reeiving 170 kg h -1 or 340 kg h-1 of N showed inreses in lef N from V15 to R1 nd from R1 to R3 nd dereses in lef N from R3 to R5, plnts reeiving no N pplition exhiited deresing lef N ontent from V15 to R5. The rte of lef senesene ws lso mrkedly greter t 0 kg N h -1 thn t 170 kg N h -1 nd 340 kg N h -1 (Figure 6B). SPAD 60 55 45 40 35 (A) 54,000 plnts h -1 79,000 plnts h -1 104,000 plnts h -1 V15 R1 R3 R5 Growth Stge SPAD 65 60 55 45 40 35 30 25 20 (B) 0 kg N h -1 170 kg N h -1 340 kg N h -1 V15 R1 R3 R5 Growth Stge Figure 6. Effet of plnt popultion (A) nd N rte (B) on lef greenness t V15, R1, R3, nd R5 growth stges. Mens with different letters indite sttistilly signifint differenes t P 0.05. The plnt popultion N-rte intertion ws not signifint t P 0.10. Summry Plnts tht experiene high levels of intr-speifi ompetition due to poor resoure vilility hve een shown to hve redued ility for resoure pture tht results in redued vegettive nd reprodutive iomss prodution. Deresed TB P nd inresed GY CV re therefore expeted responses to redued resoure vilility in mize (Mddonni nd Otegui, 2004). In this study, GY CV ws gretest nd GY A ws lowest when N vilility ws most limiting (104,000 plnts h -1, 0 kg N h -1 ) (Figures 3 nd 1, respetively). At the highest plnt popultion in this study (104,000 plnts h -1 ), lk of N pplition lso limited TB P umultion (Figure 5). Sine improved TB P umultion is ssoited with the intereption nd utiliztion of IPAR (Tollenr nd Lee, 2006), lower TB P vlues likely resulted from redued lef N ontent nd inresed folir senesene (Figures 6A nd 6B). These folir responses to low N vilility likely limited per-plnt photosyntheti rtes nd indued soure limittions during the grinfilling period tht resultntly restrited oth GY A nd GY P (Figures 1 nd 2, respetively). Intr-speifi ompetition in mize for limited resoures leds to the formtion of plnt hierrhies tht onsist of dominted nd dominting plnts (Mddonni nd Otegui, 2004). Low nopy-level HI vlues t high plnt densities hve een shown to e relted to redutions in the er iomss of dominted plnts t physiologil mturity (Mddonni nd Otegui, 2006). Inresed ompetition t the highest plnt density nd lowest N rte in this study lso

redued HI vlues (Figures 4A nd 4B), likely inditing limittion mong dominted plnts exposed to severe N stress to prtition photossimiltes to the developing er. In onlusion, the vilility of N is more ritil for mintining GY A t higher plnt popultions thn t lower plnt popultions. Without suffiient N t high plnt densities, intrspeifi ompetition for soil N is severe. Limittions in ville N led to redued folir N onentrtions nd erlier lef senesene tht limit TB P nd GY P. These redutions in GY P nnot e ompensted for y inreses in plnt density, thus resulting in deresed GY A. Aknowledgements Funding for this reserh ws provided y Pioneer Fellowship in Plnt Sienes (2006- present), Purdue University Andrews Foundtion Fellowship (2004-2006), nd the Purdue Reserh Foundtion. Pioneer Hi-Bred Intl., In. provided the seed for this study, nd Deere & Compny provided the 1780 plnter sine 2001 nd JD 7930 trtor with RTK guidne system in 2006. We express our thnks to the Cropping Systems Tem memers inluding grdute students Mtís Cánep nd Yning Xi, reserh gronomist Terry West, field tehniins Adm West nd Rfel Slerno, nd volunteer Cthy Vyn for olleting muh of the dt for the projet. Thnks lso to Judy Sntini for her sttistil onsulting nd mnusript reviews. Referenes Andrde, F.H., P.E. Ate. 2005. Response of mize nd soyen to vriility in stnd uniformity. Agron. J. 97:1263-1269. Boomsm, C.R., T.J. Vyn. 2007. Plnt-to-plnt uniformity is essentil for optimum yield in notill ontinuous orn. Purdue Extension pulition AY-329-W. http://www.es.purdue.edu/extmedi/ay/ay-329-w.pdf Dynrd, T.B., J.F. Muldoon. 1983. Plnt-to-plnt vriility of mize plnts grown t different densities. Cn. J. Plnt Si. 63:45-59. Duvik, D.N. 2001. Biotehnology in the 1930s: the development of hyrid mize. Nture Rev. Geneti. 2:69-74. Duvik, D.N., K.G. Cssmn. 1999. Post-green revolution trends in yield potentil of temperte mize in the north-entrl United Sttes. Crop Si. 39:1622-1630. Edmedes, G.O., T.B. Dynrd. 1979. The development of plnt-to-plnt vriility in mize t different plnting densities. Cn. J. Plnt Si. 59:561-576. M, B.L., L.M. Dwyer. 1997. Determintion of nitrogen sttus in mize senesing leves. J. Plnt Nutr. 20:1-8. M, B.L., Morrison, M.J., L.M. Dwyer. 1996. Cnopy light refletne nd field greenness to ssess nitrogen fertiliztion nd yield of mize. J. Plnt Nutr. 88:915-920.

Mddonni, G.A., M.E. Otegui. 2004. Intr-speifi ompetition in mize: erly estlishment of hierrhies mong plnts ffets finl kernel set. Field Crops Res. 85:1-13. Mddonni, G.A., M.E. Otegui. 2006. Intr-speifi ompetition in mize: Contriution of extreme plnt hierrhies to grin yield, grin yield omponents nd kernel omposition. Field Crops Res. 97:155 166. Muldoon, J.F., T.B. Dynrd. 1981. Effets of within-row plnt uniformity on grin yield of mize. Cn. J. Plnt Si. 61:887-894. Pgno, E., G.A. Mddonni. 2007. Intr-speifi ompetition in mize: erly estlished hierrhies differ in plnt growth nd iomss prtitioning to the er round silking. Field Crops Res. 101:306-320. Pioneer Hi-Bred Intl., In. 2005. Agronomy Reserh Summry: Centrl Mize Belt. SAS Institute, 2004. SAS/STAT User s Guide, Version 9, SAS Institute, Cry, NC. Toktlidis, I.S., S.D. Koutrous. 2004. A review of mize hyrids dependene on high plnt popultions nd its implitions for rop yield stility. Field Crops Res. 88:103-114. Tollenr, M., J. Wu. 1999. Yield improvement in temperte mize is ttriutle to greter stress tolerne. Crop Si. 39:1597-1604. Tollenr, M., E.A. Lee. 2002. Yield potentil, yield stility nd stress tolerne in mize. Field Crops Res. 75:161-169. Tollenr, M., E.A. Lee. 2006. Dissetion of physiologil proesses underlying grin yield in mize y exmining geneti improvement nd heterosis. Mydi 51:399-408. Vlentinuz, O.R., M. Tollenr. 2006. Effet of genotype, nitrogen, plnt density, nd row sping on the re-per-lef profile in mize. Agron. J. 98:94-99. Veg, C.R.C., Sdrs, V.O., Andrde, F.H., S.A. Uhrt. 2000. Reprodutive llometry in soyen, mize nd sunflower. Ann. Bot. 85:461-468.