INFLUENCE OF PLANT HEIGHT AND OF LEAF NUMBER
ON MAIZE PRODUCTION AT HIGH PLANT DENSITIES'
LUIS SANGOI' and RICARDO JONATI-IAN SALVADOR'
ABSTRACT - This experiment was conducted in Ames, Iowa, US, with lhe purpose of veriing if reduction in plant height, through the use of dwarfing genes, or leaf number, by growing short season genolyjes, can be a usefui strategy to reduce barrenness and improve maize tolerance to drought and high piant population strcsses. Five genotypes were tested: a fuil season hybrid, a short season hybrid, and three dwarf lines, containing lhe homozygous recessive genes d3, br2 and dl. Each genotype was planted at four plant populations: 25,000, 50,000, 75,000 and 100,000 plants.hr'. There was no sig-nificant drought stress during lhe entire growing season of 1994. High plant population decrcased number of grains per ear of dwarf lines and did not affect this variable for hybrids. Consequently, difíerences in yield between hybrids and dwarves were greater at the higher plant populations. In-breeding depression, unimproved genelic background and poor plant architecture limited lhe response ofdwarfgrain yie!d tolhe increase ia plant population.
Index lerms: Zea mays, plant population, grain yie!d.
INFLUÊNCIA DA ESTATURA DA PLANTA E DO NÚMERO DE FOLHAS NA PRODUÇÃO DE MILHO EM ALTAS DENSIDADES POPULACIONAIS RESUMO - Este trabalho foi conduzido em Ames, Iowa, Estados Unidos, com o objetivo de verificar se a redução na estatura, mediante a utilização de genes de nanismo, ou do número de folhas produzi-das por planta, através do cultivo de materiais de ciclo superprecoce, podem contribuir para aumentar o rendimento de grãos de milho, em ambientes de alta densidade de semeadura e restrição hldrica. Cinco genótipos foram avaliados: um híbrido de ciclo normal, um híbrido superprecoce e três linha-gens anãs contendo os genes d3, br2 e dl. Cada genótipo foi semeado em quatro densidades, equiva-lentes a 25.000, 50.000, 75.000 e 100.000 plantas.hr'. Não houve incidência de deficiência hídrica durante o período experimental. A utilização de densidades elevadas reduziu o número de grãos por espiga das linhagens anãs e não interferiu significativamente nesta variável nos híbridos. Conseqüen-temente, as diferenças em rendimento de grãos entre híbridos e linhagens anãs foram maiores nas populações mais densas. Depressão endogâmica, falta de uma melhor base genética e presença de uma arquitetura de plantas inapropriada limitaram a resposta do rendimento de grãos das linhagens anãs ao • incremento na densidade populacional das plantas. -
Ternos para indexação: Zea mays, população de plantas, rendimento de grãos.
INTRODUCTION
• Population density, whether operating directly on the plant or indirect!y on biotic factors associated with plant density, is one of the most important
fac-'Accepted for publication on September 24, 1997.
2 Eng. Agr., Ph.D., Dep. de Fitotec., Univ. do Estado de Santa
Catarina(JDESC), Caixa Postal 281,CEP 88520-000 Lages, SC. Bolsista do CNPq.
3 Eng. Agr., Ph.D., Dep. of Agronomy. Iowa State University,
1126 Agronomy Hall, 50011, Ames, Iowa, United States ol' America.
tors in determining grain yield and other important agronomic attributes of a crop (Meyer, 1970). The com plant is less capable of adjustment to a poor stand than other members of the grass family. Mod-em maize varieties do not tilIler much, even at Iow plant densities, and very often produce only one ear per plant. Therefore, maize does not have the flex-ibility of most crop species, which cnn increase leaf area and number of reproductive units by branching at low crop densities (Gardner et ai., 1985).
On the other hand, lhe use of high popuiations may be detrimental to fmal yie!d by stimulating api-
cai dominance and inducing barrennei. As thenum- shouid reduce water costs per piant for vegetaive ber of plants in a pianting pattem increases, distance growth. and maintenance; resuiting in greater re- between piants decreases and competition among source avaiiabiiity for grain production.
individuais increases (Duncan, 1984). At lower than : Two ways to reduce piant height and leaf num- maximum yieid popuiation, adding more plants com- ber are 1) dwarfing genes, and 2) selection for ear- pensates for the iowered grain yieid per piant dueto .• iiness. A number of dwarí and semi-dwarfmutants fite incitased crowding. Above some popuiation, are availabie and the trait is simpiy inherited (Neuffer however, the effect of rapidiy increasing crowding et ai., 1968). Most of the experiments reported in dueto fite cidsei piant spacing caü't be offset by tite 1 the iiterature to evaluáte dwarf maize were per- yieid of the added plaiits. Consequetti>Ç grain yieid formed in fite iate 50s and eariy 60s by Leng (1957), perunit arca beyond this point decreases as a conse- Leng & Ross (1959), Pendieton & Seif (1961, 1962), quence ofthe sharp reduction in kernel number and and Sowell et ai. (1961). The main objective at that kernei size produced per piant (Lemcoff& Loomis, time was to deveiop a piant with a better standabiiity 1994). . . . and that couid withstand high piant popuiations un-
For each production situation, there is a popuia- der favorabie edaphic and ciimatic conditions. tion that maximizes the utilization of the resources No report lii the hterature was found of an ex- avaiiabie, especiaiiy iight, water and nutrients, ai- periment with the specifzc purpose of evaiuàting the iowing the production of maximum grain yieid. , possibieadvantages of short staiked cuitivars over Optintum com popiflation for maximiim ààonàmic normal maize at high piant popuiation and dry envi- grain yieid varies with cultivar, row width, soil fer- ronments. Tliere is some indirect evidence derived tility, sou water and ciimatic effects (Dungan et ai.; . from experiments with growth retardants that a re- 1958; Larson & Hanway; 1977). . duction in piant height has the potentiai for improv- Two important factors te consider in defining ing resistance to drought in com (Shanahan & optimum popuiation for ntaize are piant architec- .. Nieisen 1987; Kaseie et ai., 1994). In support to ture and water availability. Generaiiy speaking,
si
this approach, Ackerson (1983) observed, in a and ieaf cuitivars require iow densities to saxi comparison of two drought resistant hybrids, that mize grain yieid per arca (Aidrich et ai., 1986). it is the more resistant material was shorter and producedleaves.
also weil known that increasing piant density in- fewer, smaiier and shorter In addition to that,
Kriedemann
creases leaf arca index and, consequently, water
potential evapotranspiration to potentiai photosyn- consumption (Tetio-Kagho & Gardner, 1988). The
thesis increases with height in high insoiation and use of high plant popuiations under iimited water . windy environments. Consequently, shorter piants suppiy may increase plant stress and reduce grain are better adapted than taiier plants te such condi- yield dramaticaily, specialiy if the water shortage tions.
coincides with the period of 2-3 weeks bracketing rtiusworkwasdesignedtoevaivate ifreductions siiking (Ciassen & Shaw, 1970; Frey, 1981; in piant height, through the use of dwarfing genes, Wetgate, 1994. Therefore, drouáht stress, particu- or ieaf number, by growing short season genotypes, lriy when combined with high piant density, can may be an efl'ective way to reduce barrenness and cause cdmpiete ioss of grain production, if severe to increase grain yield under high piam popuiation stress occurs during the tasseiing and siiking stage and limited water availabilityl
of reproduction (Herrero & Johnson, 1981; Edmeades et al.,'1993).
MATERIAL AND MET!I9!.S.J Theoreticaiiy, with a smaiieraiid Iess iea& piant,
thà levei of competition of each individuai over the The experiment was performed during lhe growing . .
others should be iower. flus, more individuais can season of 1994, in Ames, iowa, US. The ciimate of lhe be pianted per unit of iand arca and fewer inputs per region is classifled as Dfa, coid with a moist winter and a piant shouid be required. In a water iimited envi- hot summer. Study site soU was a Nicollet ioam (Fine- ronment the lower amount of vegetative biomass loamy, mixed, mesic Aquic 1-lapludoli).
INFLUENCE OF PLANT HEIGHT AND OF LEAF NUMBER 299
The trial was designed as a factorial combination of two factors: cultivar and plant density. The experimental design was a split plot with the main plots arranged in a randomized complete biock. Each trealmenl was replicated four times. In lhe main plot, five genotypes were evalu ated: one fuil season hybrid adapted lo Central Iowa (Northrup king 4525), one short season hybrid adapted lo norlhern Minnesota (Cargill 1077) Iwo no-tiliering (156-A, 117-A) and one liliering inbred lines (302-E), containing, respectively, lhe homozygous reces-sive dwarfing genes d3, br2 and dl.
In lhe split plol, four plant populations were tested: 25,000,50,000,75,000 and 100,000 plants.ha. Each split plol was constituted by four 6 meter rows, spaced 0.50 cm equidistantly. The space between two adjacent plants within each row was 0.8, 0.4. 0.267 and 0.2 m for the densities of 25,000, 50,000, 75,000 and
100,00 plants.ha4 , respectively.
Ferlilizer was applied according to soil tesl recommen-dations from the ISU soil testing laboralory. The experi-ménts were hand-planted on May 3, 1994. Three seeds were dropped per hill to assure the desired stand on each treatment. When plants were aI stage V4 (Ritchie & Hanway, 1982), thinning was performed to adjust lhe popuiation lo desircd leveis. Plots were also hand-hoed and wheel-hoed to control weed competition.
t:.At anthesis, five plants randomly chosen within each splil piol were used lo delermine number of leaves pro-duced per planl. Earlier in lhe season, the tips of the fourth and eighth leaf were marked with a non-washable ink, serving as a reference point to correctly determine the lo• tal number of cxpanded leaves. Plant height was also measured on five planls in lhe two central rows of the piot whenlhey reached R3, lhe milk stage (Ritchie & 1-lanway, 1982).
Harvesling was done by hand when the leaves of each variety senesced enlirely. The two central rows of each split plol were harvested, representing an arca of 6 m 2.
Ears were dehusked, dried, shelled and weighed. To de-termine grain yield, values were converted to an arca of one heclare and adjusted lo a standard moisture of 15.5%. A sub•sample of 200 grains was taken and re-weigbed. The value obtained was mulliplied by 5 and converted to amoisture of 15.5% to express lhe weight of 1,000 grains. The number of grains per ear was estimated indireclly through lhe relationship between weight of 200 grains, weight of tola) number of kernels and number of ears bar-vested within each split plot.
An analysis of variance was performed using lhe Gen-eral Linear Modeis procedure of lhe Statistical Analysis System(SAS), version 6.07 for Unix Systems (SAS
Institute, 1987). F values for main treatmenl efTects and their inleractions were considered significant aI lhe 1' <0.05 levei. Whenever a particular factor or interaction offactors significantly influenced a variable, means were separated using Fischer's LSD test at lhe 0.05 probability levei, foliowing methodology presented in Little & Hillls (1978). To provide bater understanding of the main ef-fects and lheir interactions, specific orthogonal contrasta were calculaied.
The foliowing contrasts were examined: Cultivar effect -
ai: hybrids (NK4525 + C 1077) x dwarves (156-A + +117-A + 302-E);
a2: full season hybrid (NK 4525) x short season hybrid (C 1077);
a3: lillering dwarf (302-E) x non-tillering dwarves (156-A+ 117-A);
a4: non-liliering dwarf 1 (156-A) x non-tiliering dwarf 2 (117-A).
Density eftect bI: linear effect; b2: quadratic effect.
•A regression anaiysis was performed and lhe linear and quadratic effccts for each significant contrast were calculated. The equations thal provided higher coefficients ofdetermination, and that beller expiained lhe biological behavior af the genotypes, were chosen to summarize lhe information gathered for each variable. To describe lhe results, plant densities of 25,000, 50,000, 75,000 and 100,000 pl.ha' are represented by lhe numbers 1, 2, 3 and 4, respeclively. Linear and quadratic equations were also calculated wilh lhèse values ofx.
RESULTS AND DISCUSSION Overali pecipitation in 1994 svas stightiy below average. Howéver, no significant water deficit was observed probably due to the great soil-moisture reserve carriéd fromtheprèvious season (1993 was an year with a record amount of precipitation in Iowa), and also due to lhe cooler temperatures ex-perienced during periods of potentially high evapo-rativedemand.
• No effect of piant density was detected on rate of grbwth and development of the planta. Hybrids ger-minated faster than dwarf inbreds (Table 1).No dif-ference in lhe amount of time required for emer-gence was observed between the two hybrids or among the three dwarves. Plants of the early hybrid
TABLE 1. Phenological development ef a fuil season hybrid (NK 4525), a short season hybrid (C 1077), a tiliering (302-E) and two no- -tiltering (117-A e 156-A) dwarf inbred unos, averaged at four plant populations, Ames, Iowa, 1994.
Plicnological NK 4525 C 1077 302-E 156-A 117-A stag&
Duratiori of phenotogical stages (days)'
PI-Em .12 12 .: 16,'16. 16
Em - Si! 57
. .
46 68 67 66Sil- Har 77 64 54 56 62
PI-Har 146 122 138 139 144
P1 - planting; Em - emergence; Si! - silking; Har - harvesting.
2 Number of da>'s when aI least 30% of lhe plants on each split piot
reached lhe specific phcnologycal stage.
silked iii approximateiy 20% Iess time afler emer-gence, when compared to fite hybrid adapted to cen-trai Iowa conditions. Dwarves showed siower veg-etative development, reaching the silking stage more than a week after fite fuil season hybrid. Little dif-ference among fite dwarves was noticed in time re-quired te reach sillcing. Despite their slow vegeta-tive development, dwarves had quicker leaf senes-cence after anthesis than hybrids. This contributed to speed their grain development, decreasing the amount of time between silking and harvesting in comparison to hybrids. The fulI season hybrid was thé cultivar with fite slowést rates of leaf senescence. Considering each cultivar individually, the theo-reticai population to accompiish maximum yield (optimal popuiation) was rather different. NK 4525 reached maximum grain yieid at 99,000 pl.ha', whereas the ideal population for the short season hybrid could not be estimated because its yield in-creased linearly within the range of densities used (Fig. 1). The maximum potential yield ofthe dwarves was achieved between 70,000 and 80,000 pl.ha', depending ou the material. Little difference in grain yield within the dwarves was observed, regardless ofplant density. Cànsidering mean values ofthe four plant populations tested in the trial, hybrids yielded 10.12 Mg.ha' whereas dwarves produced 3.64 Mg.ha*
Hybrids and dwarves responded differently te the increase in plant population (Fig. 2): Within the range of plant densities evaluated, hybrids increase
grain yield by 1.9 Mg.ha' for each increment of 25,000 pl.ha'. In contrast, dwarf grain yield went quadratically up te 82,500 p1.ha4 and then declined; Therefore, difíerences in yield between dwarves and
22
-
r2.O.78
, •
14K 4323 Y 4.9+
$4x-
O. 6&2•
C 1070 Y-243+2,0x 12 .8320-
À 302-E y-3+0.38x +o.oIx2r2_O,34 18
-
iii
U11 6 2 0-, '1 2 ''' . 3 '., .
4Plant population (p1.ha 4) x 25,000. FIG. 1. Grain yield of a fuil season hybrid (NK4525), a short season hybrid (C 1077), a tiliering (302- E) and two no-tiliering (117-A e 156-A) dwarfinbred finos, at four ptant poputations, Ames, Iowa, 1994. 12- i10_ . 8,- '' • Hybrids Y-5.37+l.90xr0.93
"-1
6' 21 2 "':. 4PIaM population (p1.ha) x 25,000 FIG. 2. Grain yield of hybrids and dwarf tines at four
ptant poputations, Ames, Iowa, 1994.
INFLUENCE OF PLANT HEIGHT AND OF LEAF NUMBER 301
hybrids were more accentuated at higher plant popu-lations.
Weight of 1,000 grains was affected signiflcantiy by cultivar and by the contrast al(hybrids x dwarves)* b2(quadratic effect of ptant density). NK 4525 had grains that were 22% heavier than C 1077 (Tabie 2). Non-tiliering dwarves 156-A and 117-A produced heavier grains than the tiliering short stalked une 302-E. Regardiess ofpiant density, weight of 1,000 grains was higher for hybrids than for dwarf tines (Fig. 3). Moreover, hybrids were abie to increase grain weight with increase in poputation up to 60,000 pI.ha'. In contrast, piant popuiations above 25,000 pl.ha promoted a decrease in weight of 1,000 grains ofdwarves.
Simiiariy, number of grains produced per ear was higher iii the fuil season hybrid than in the short sea-son hybrid, and iii the non-tiliering dwarves than in the tiiiering line (Tabte 2). Iiybrids did not vary number of grains produced per ear significantly at the range of p&pulations evaivated (Fig. 4). On the other hand, this yieid component was reduced by 21 grains each time dwarf piant poputation was in-creased by 25,000 pt.ha*
Number of grain bearing ears was affected dif-ferentiy in each genotype by the increase in piant population. The tiliering dwarf-line 302-E produced
more ears per piam flian the other materiais, par-ticuiarly at lhe tower piant popuiations (Fig. 5). This tine tiitered profuse!y at 25,000 and 50,000 pl.ha -'. Some of the tiiiers produced were weti deveioped and had grain bearing ears, which contribuited to
300 29,1 270- -22 e llybridsY246+36x.tsx r .95 260- o Dwazv Y= 247. 11.7x + 1.25,? ?. 0.98 S 250 - o 240- 230- 220- ... - 1 --- 210 - 2 3 4
Plant popuiation (pl.ha) x 25,000 FUI 3. Weight of 1,000 grains of hybrids and dwarf
tines aI four plant poputations, Ames, Iowa, 1994.
500
TABLE 2. Yield components, plant height and num-bar of laves par plant of a fult season hy-brid (N}C 4525), a short season hyhy-brid (C 1077), a tiliering (302-E) and two no--tiliering (117-A e 156- A) dwarf inbred tines, averaged aI four ptant populations, Ames, Iowa, 19941.
Genotype Grains par Weight of Plant 1_caves
cal(a°) I,000grains height perplant
(g) (cm) (rio)
NK4525 592a 310a 243 a 138h
C1077 356h 254h 143h 15.4e
302-E 245€! 211d 135e 20.1a
156-A 269c 232c 112€! 20.0a
117-A 274c 241e 134c 19Mb
LSDAmeans 27 II 5.6 0.5
C.V. A (%) 10.1 5.5 4.7 3.8
c.v.Br/,) 11.5 7.0 5.0 2.9
For each vaziable, means followed by the sarne letier ia the colunm are not significantüy different by d'e LSD test (P0.05).
450- 400 - Hybrida Y-464+3,9x 2=0.40 4) 350 - Dwarve Y-315.212x 12..0,94 300- 250- 200- 1 2 3 4
Plant popuiation (p1.ha 4 ) x 25,000 FUI 4. Grains par ear of hybrids and dwarf lines at
four plant populations, Ames, Iowa, 1994.
the highnumber df fruiting sites observed at iow piant popuiations. Ali cultivars decreased number of eàxs por plant with increase iii piant population The ratos of decrease were more aceentuated mi thé tiiiering dwart intermed iate iii the hybrids and very smaii for the non-tiliering dwarves (Fig.5). Conse-quentiy, there were more significant differences in the number of viabie ears among materiais at the lower than at the higher piant popuiations.
Plant height was significantly influenced by the single effects of cultivars and piant density. Aver-aged of ali cuitivars, each increase in 25,000 pl.ha' promoted an increase of 2.7 cmin plant height. The cultivars can be ciassified iii four categories mn terms of thefr stature, when averaged across plant densi-ties (Table 2). The iong season hybrid had talier plants than the short season hybrid, which, in turri, was talier than the other dwarves. Among the dwarves, 156-A had smaiier piants than the other two materiais.
Number of leaves produced per plant was only affected significantly by cultivar. The short season hybrid had the iowest number of leaves per plant of
4 • NK 4525 Y2.8 - lOIx+Ol4x2 ?=0s9 3
.4
C 1070 Y..2.3-0.67x+0.09x2 r2=O.83'5'
1
A 302-E Y3.8-1.54x + 0.21x2 r2 081 156A Y-1.I .07.002x 2 2061 - Z. 2j4 11:A:a.0004 x2 r2 =0; 01 1 - - 2 3 4- Piant popiiiation (pi.ha') x 25,000
FIG. S. Ears per plant of a hill season hybrid
-
- - (NK4525), a short sanou hybrid (C 1077), a titiering (302-E) ind two no-tittering (117-A -
- - e 156-A) dwarf inbred tines, at four plant
- - populations, Ames, Iowa, 1994. --
Pesq. agropec, bras., BrasIlia, v.33, n.3, p.297-306, mar. 1998
au materiais, when averaged across piant densities - (Table 2). Dwarves produced more ieaves than hybrids regardiess of plant density. - - - - - The responso of grain yieid to the increase mn piant popuiation observed in this experiment was higher thaii what has been reported by Stickier (1964), Andrew & Peek (1971), Oison & Sanders (1988) and Toiienaar (1992). Most pubiished results from various states in the US have' shown few mnstances where 'ieid continues to inerease above 70,000 pl.ha-' unless com was irrigated. In the present study, hybrids improved grain production per area above 70,000 pl.ha' without irrigation (Fig. 1 and 2).:
One of the main causes of yieid reduction at high piam densities is an increase iii barrenness. High interpiant competition for iight, water and nutrients may decrease grain yield per plant to the extent that the addition of more plants can't compenate for the Ioss of production on an individual piant basis, re suiting in iower grain production per area (Duncan; 1984). Usuaiiy the yield component that is affected most negatively by greater than optimuni popula-tion is the number of grains per ear (Lemcoff & Loomis, 1986, 1994; Jacobs & Pearson,-1991). In contrast to what has been reported in the literature, the increase in piam population did not decrease the number of kernels set per ear for the hybrids (Fig. 4) which contributed to their positivo response to plant density.
- Several factàrs can be advanced to explain the higher than average response of grain yieid to piant density. First, 1994 was a season with very favor-abie cliinatic conditions for com growth and devei-opment in Central Iowa, registering record yieids iii the whoie state. No significant periods of water
defi-cit were observed during the most criticai stages of the crop ontogenyl lncreasinj piam popuiation in-creases water use and - thereby generaiiy enhances the potential for water stress (DàwnéyÇ 1971). Adequate water avaiiabiiity undoubtediy piayed an important role in improved yieid response to piant density. Associated with favorabie weather, the fer-tiie soil ofthe experimental area probabiy aiso heiped support higher than average piant popuiations Additionaiy, a narrow row spàcing may have pro-vided better distribution of piants and better utiliza-tion of solar radiautiliza-tion. Reducing row width to pro-
INFLUENCE OF PLANT HEIGIIT AND OF LEAF NUMBER 303
vide a more equidistant spatial pattern may shift optimum piant popuiation to a higher value, particu-lariy in eariy piantings, with high fertiiity leveis, and good water availability (Pasziewick, 1994).
The higier yield per area of hybrid NK 4525, relative to C 1077, regardless of plant density (Fig. 1), was mainly dueto higher number ofgrains per ear and to the production of heavier grains (Table 2). The short season hybrid reached anthesis 11 days earlier than lhe fuil season hybrid (Tabie 1). A positive relation between number of days to flow-ering and yield has been reported for Hallauer et ai. (1967), Toilenaar (1977) and Oison & Sanders (1988). Short season hybrids hasten lhe period be-t-ween ear differentiation and siiking. The shorter time available for ear development and spikelet dif-ferentiation may negatively impact lhe number of potential spikeiets availabie lo be fertilized, decreas-ing kemels set per ear. On lhe other hand, lhe lower number of leaves produced by these genotypes (Table 2) may also represent a source iimitation con-tributing lo limit their abiiity to filia higher number ofkemeis. Milbom (1977), Hunler (1980) and Sal-vador (1984) have pointed oul that the source ca-pacity of lhe piant may be lhe major factor limiting yield of maize cultivars grown at high latitudes.
The short season hybrid had a shorler silking--harvesting period than lhe fuli season-hybrid. Even lhough physioiogical maturily was nol preciseiy determined lii this experiment, it appears lhat C 1077 also had a shorler grain fiuiing period than NK 4525, which might have contributed lo decrease 1 ,000-grain weight and yieid. A strong positive cor-relation between fiuiing period duration and final yieid has been reporled by Crosbie & Mock (1981) and Dwyer eI ai. (1994). Therefore, under favorabie environmentai conditions, specialiy lack ofdrought stress, reduclion in ieaf number lhrough eariiness was not an efficient strategy to improve maize grain yield aI high planl densities in reiation to a fuli sea-son hybrid adapted lo Central Iowa.
As in eariier reports of Pendieton & Seif (1961), dwarf lhes yieided significantiy less per area than hybrids (Fig. 1 and 2). Contrary to the initiai hy-pothesis, reduction in planl size through introduc-tion of dwarfing genes did not improve the perfor-mance of the piants at higher piant popuiations. The
higher absolute vaiues of grain yieid presenled by lhe hybrids over lhe dwarf lines was an expected resuil since almosl always hybrids yield better than inbred lines regardiess of using dwarfing genes. However, lhe expectation was lhat lhe relalive dif-ference in yield between hybrids and dwarf iines should be lower at lhe higher piant populations which did nol happen. In fact, lhe higher lhe population lhe greater lhe difference in yieid per area belween lhe hybrids and lhe dwarf lines. The superiority iii yieid expressed by lhe hybrids was mainly due to lheir heavier grains and greater number of grains per ear, regardiess of plant density (Table 2 and Fig. 3 and 4).
Genetic and physioiogicai constraints probabiy iimited a belter agronomic performance oflhe dwarf iines used in lhe lriai aI lhe high plant popuiatiõns. II was nol possible to obtain any elite source of dwarfbess lhat could be compared to its normal coun-terpart. Wilh lhe eariy disappointing resulls derived from experiments carried oul in lhe iate 50s and eariy 60s, lhe whoie dwarfing program was abandoned by most seed companies and today it is almost im-possible to find commerciai source of dwarf elite genotype. The genetie background of hnes 117-A, 302-E and 156-A was a mixture of old inbreds, in-ciuding W23, M13 and L 317, which are not used commercialiy any ionger. Those inbreds were im-portant parents ofmidwestern hybrids in lhe 60s and 70s and have been maintained by selfing and sib-crossing for many years. Therefore, inbreeding de-pression and unimproved genetic background are important factors expiaining the poorer relative per-formance of lhe dwarves in comparison to lhe hy-brids.
Severai measured attributes of the dwarves ac-counted for their grain yieid variation. Dwarves re-quired more time to reach siiking than hybrids (Tabie 2). For normal com cultivars, lhere is usu-aily a positive correialion between ienglh of the pe-riod emergence-flowering and the size of lhe female inflorescence. This did nol happen wilh lhe dwarves. Their large number of leaves wouid indicate thatlhey take more time to differentiate reproductive struc-tures. They might aiso have had lower rales of ear deveiopment after differentiation. Both lhings have been observed by Stephens (1948) and Stein (1955),
and may help to explain the lack of a better associa-tion between the lengthofemergence-sitking period and the size of the dwarf ear.
The dwarves used in this experiment presented the sarne kind of ptant architecture problems reported by Katta & Castro (1970) and Castro (1975). lii spite oftheir smatl stature, their number of leaves was at least similar to the fblI season hybrid (Table 2). The combmnation of these two factors resutted in very short intemodes and the emergence of leaves in a single vertical plane. The presence of short intern-odes and the uniform directional alignment ofleaves may have prevented better penetration of solar ra diation into the canopy, even at the lower plant den-sities. Therefore, dwarfplant architecture probably increased intrapiant competition for light, which in turn may have reduced source potential for provid-ing assimilates to the developprovid-ing reproductive struc-tures. Besides promoting heavy shading, the dose superposition of leaves around the ear node may also have created a physicai barrier to pollination. With the tight arrangement of leaves around the ear, p01-len could not have reached silks as efficiently as it should, contributing to reduce kernel set.
The early senescence of dwarf leaves (Table 1) is evidence that the failing period was shorter for dwarves than for hybrids. The shorter filling period may have contributed to decrease 1,000 grain weight ofthe dwarves. Lemcoff& Loomis (1986) and Sal-vador & Pearce (1995) have pointed out that difíer-ences iii fmal gramn weight among com genotypes are determined mainly by differences in effective filting period. ..
-, lhe shorter fiuing period of dwarves can be linked
to timitations in source potential, sink strength, or both. Fischer (1975), Stay (1976), Tottenaar (1977) and Salvador (1984) have suggested that both sink and source iirnitations may occur in crop piants and the particular combination of genotype and environ-ment determines which one predominates. One could hypothesize that limitations in photosynthesis pro-moted by poor canopy architecture and unimproved genetic background lcd to limiting ear sink capacity of the dwarves. There is evidence that maize sink strength is, to some extent, related to assimilate sup-plyto the ear during the pre-flowering and fiower-ing period. In addition, leaf photosynthetic rates may
be affected by grain demand,(Moss, 1962; Thiagarajah eta!,, 1981). When fertilization ofthe ears is prevented totally or partiatly, photosynthetic rates decline contmnuously. This correlation between leafassimilation and sink demand during grain fihi-ing has also been reported in other crops by fome & Kolter (1974). So, it is possible that early senes-cence, shorter grain fihling period and the produc-tion of lighter grains in the dwarves are ali conse• quences of the weak sink establishment at the be-ginning of the fitling phase... . .
Alrnost alt the information reported in the lítera-tire, pius the data generated herein, indicate that reduction in ptant height alone, through the use of dwarfing genes, wili not improve maize tolerance to high piant population. It is also important to re-duce the riumber of ieaves and to select for opti-mum teaf arrangement. In addition, these features should be mncorporated into an improved genetic background. The presence ofdrought stress and the development of isogenic elite dwarf inbred lines are important points to observe, so that a better com-parison can be made between dwarves and their normal counterparts in terms of tolerance to high plant population and reduction in barrenness.
1. Reduction in plant height through the use of dwarfíng genes does not decrease barrenness or im-prove relative grain yieid of maize inbred lines at high ptant poputations, in relation to a full season hybrid adapted to Central Iowa.
1 Inbreeding depression, the lack of a better ge-netic background and the dose superposition of numerous teaves around the ear node prõvent better agronomic performance of the dwarf lines at high plant densities. .
REFERENCES
ACKERSON, R.C. Comparative physiology and water reiatíons of two com hybrids during water stress. Crop Scicnce, Madison, v.23, ai, p.278-283. lan. - 1983.
ALDRICH, S.R.; SCOTI, W.O.; HOEFT, R.G. Modern com production. Champaign: A and L Pubi., 1986. 386p.
INFLUENCE OF PLANT IIEIGHT AND OF LEAF NUMBER 305 ANDREW, LII.; PEEK, J.W. Influence of cultural
practices and field cnvironment on consistency of com yields ia northern arcas. Agronomy Journal, Madison, v.63, n.2, p.628-633, Mar. 1971. CASTRO, M.G. Erect leafed super dwarfcorn for high
productivity. SaltilIo: Univ. Antonio Narro, 1975. 35p.
CLASSEN, M.M.; SHAW, R.H. Water deflcit effects on com. 11. Grain components. Agronomy Journal, Madison, v.62, n.2, p.652-655, Mar. 1970. CROSBIE,T.M.; MOCK, J.J. Changes in physiological
traiu associated with grain improvcmcnts in thrcc maize breeding programs. Crop Science, Madison, v.21, n.I, p.255-259, San. 1981.
DOWNEY, L.A. Water use by maize at three plant populations. Journat of Experimental Agriculture, v.7, n.I, p.I6I-I69, San. 1971.
DUNCAN, W.G. A thcory to explain the relationship between com population and grain yield. Crop Science, Madison, v.24, n.4, p.II41-II'15, Suly 1984.
DUNGAN, 0.11.; LENO, A.L.; PENDLETON, J.W. Com piant population in relation to soil productivity. Advances in Agronomy, Cambridge, v.I0, p.435--473, 1958.
DWYER, L.M.; EVENSON, L.; HAMILTON, L.I. Maizc physiological traits reiated to grain yieid and harvest moisture in mid lo short season environments. Crop Science, Madison, v.34, n.3, p.985-992, May 1994. EDMEADES, 0.0.; BOLANOS, .1.; LAFFITE, A.R. Progress in breeding for drought tolerance in maize. In: ANNUAL CORN AND SORGIIUM RESEARCII CONFERENCE, 47., 1993, Chicago. Proceedings... Chicago: American Sccd Company Association, 1993. p93-1 II.
FISCHER, R.A. Yieid potential in dwarf spring wheat and thc effect of shading. Crop Science, Madison, v.I5, n.2, p.607-613, Mar. 1975.
FREY, N.M. Dry matteraccumulation in kernels ofmaize
(Zea mays L.). Crop Science, Madison, v.21, n.1, p.I18-122, San. 1981.
GARDNER, F.P.; PEARCE, R.B.; MITCHELL, R.L.
Physiology of crop planta. Ames: Iowa State Univ. Press, 1985. 478p.
HALLAUER, AR.; HUTCHCROFF, CD.; IIILLSON, M.T.; fIGOS, R.L. Relation among three maturity measurements and yield of grain in com. Iowa State Journat ofScience, Ames, v.42, n. I, p. 121-136, San.
1967. -
HERRERO, M.P.; SOHNSON, R.R. Drought stress and
its efíects on maize reproductive systems. Crop Science, Madison, v.21, n.1, p.I05-1I0, San. 198I. HIJNTER, R.B. Increased leaf arca (source) and yieid of maize in short season arcas. Crop Science, Madison, v.20, n.2, p.571-574, Mar. 1980.
IACOBS, B.l.; PEARSON, C.J. Potential yield ofmaize determined by rates of growth and development of ears. Field Cropa Research, Amsterdan, v.27, n.1, p.281-298, San. 1991.
KASELE, i.N.; NYIRENDA, F.; NIELSEN, D.C.;
ANDRIA, R. Etcphon altcrs com growth, water use and grain yield under water strcss. Agronomy Journal, Madison, v.86, n.2, p.283-288, Mar. 1994. KATTA, S.S. CASTRO, M.G. Some reasons for depresscd yield in dwarf com. Maize Genetics News LeUer, v.48, p.21-25, 1970.
KRIEDMANN, P.E.; BARRS, H.D. Photosynthetic adaptation to water stress and implications for drought resistance. In: RAPER SUNIOR C.D.; KRAMER, P.J. (Eds.). Crop reactions lo water and temperature atresses ia humid and temperate climates. Boulder:WcstviewPress, 1983. p.2OI-230. LARSON, W.E.; HANWAY, 1.1; Com produclion. In:
SPRAGUE, G.F. (Ed.) Corn and corn
improve-ment. Madison: American Sociely of Agronomy, 1977. 878p.
LEMCOFF, 1H.; LOOMIS, R.S. Nitrogen influences on yield dctermination in maize. Crop Science, Madison, v.26, n.5, p.I017-1022, Sept. 1986. LEMCOFF, 1H.; LOOMIS, R.S. Nitrogen and density
influences on silk emergence, endosperm deveiopment, and grain yield of maize (Zea incps
L.). Field Crops Reseamch, Amsterdan, v.38, nt, p.63-72, lan. 1994.
LENG, E.R. Gcnctic production of short stalked hybrids. In: ANNUAL CORN AND SORGI-IUM
RESEARCH CONFERENCE, 11, 1957, Chicago.
Proceedings... Chicago: Amcrican Seed Company Association, 1957, p8049.
LENO, E.R.; ROSS, G.L. Performance of commerciai cora hybridsin Illinois. IllinoisAgricultureExperimdfl-tal Station Bullctin, Urbana, v.651, p.l-l5, 1959. LITTLE T.M.; HILLS, F.J. Agricultural experimenta-tion: design and analysis. New York: Willey, 1978. 456p.
MEYER, D.W. Use of male sterility for increasing the population tolerance ofcorn (Zea mays L). Ames: Iowa State Univ., 1970. Ph.D. Dissertation. Univ. Microfllm Order, 70.25808.
MILBORN, G.M. Yieid potential of maize in different regions of the world. Annals of Applied Biology, New York, v.87, n.2, p242-245. Feb. 1977. MOSS, D.N. Photosynthesis and barrenness. Crop
Science, v.2, n.2, p.366-367, Mar. 1962.
NEUFFER, M.G.; JONES, L.; ZUBER, M.S. The mutants of maize. Madison: American Society of - Agronomy, 1968. 276p.
OLSON, R.A.; SANDERS, D.H. Com production. In: SPRAGUE, G.F.; DUDLEY, J.W. Com and com improvement. 3.ed. Madison: American Society of Agronomy. 1988. p675-698.
PASZIEWICK, S. Com yield increases in narrow rows. Crop Insights, v.37, n.12, p.660-665, Dec. 1994. PENDLETON, J.W.; SEIF, R.D. Piam population and row
spacing studies with brachytic-2 dwarf com. Crop • Science, Madison, v.1, n.3, 433.435, May 1961. PENDLETON, 5.; SEIF, R.D. Role of piant height lii com
competition. Crop Science, Madison, v.2, n.1, • p.154-156, 1962.
RITCHIE. S.W.; UANWAY, J.J. How a coro plant develops. Ames: Iowa State University of Sciencc and Technoiogy, 1982. 26p. (Special Report, 48). SALVADOR, R.J. Assimilate uptake by maize (Zea
,nays L.) kernels as affected by source-sink manipulations. Ames: Iowa State Univ., 1984. 184p. M.Sc. Thesis.
SALVADOR, Ri.; PEARCE, R.B. Proposcd standard system of nomenclature for maize grain filling events and concepts. Maydica, v.40, n.l, p.l41-I46, San.
1995.
SAS INSTITUTE. SAS user's guide. Cary, 1987. 732p. SHANAHAN, J.F.; NIELSEN, D.C. Influence ofgrowth retardants (anti-gibereilins) on com vegetative growth, water use and grain yield under different
Pesq. agropcc. bras., Brasilia, v.33, n.3, p297-306, mar. 1998
leveis of waterstress. Agronomylournal, v.79, n.1, p.103-IO9. San. 1987.
SOWELL, W.F.; OHLRAGGE, Ai.; NELSON, O.E. Growth and fruiting of compact and Hy normal com types under a high population stress. Agronomy Joumnal, v.53, n.1, p.25-28, San. 1961.
STAY, V. Source and sink properties as related to yield in different bariey genotypes. In: INTERNA-TIONAL BARLEY GENETICS SYMPOSIUM, 3.,
1976, Melboume. Proceedings... Melbourne, 1976. p441-648.
STEIN, O.L. Rates of leaf initiation in two mutants of
Zea Inays, dwarf-1 and brachytic-2. American
Journal of Botany, v.42, p.885-892, 1955. STEPHENS, S.G. A comparative deveIopmental study of
a dwarf mutant ia maize, and its bearing on lhe interpretation of tassel and ear structure. Annais of lhe Missouri Botanical Garden, St. Louis, v.35, p.289-299, 1948.
STICKLER., F.C. Row width and plant population studies with com. Agronomy Journal, Madison, v.56, n.2, p.438-44I, Mar. 1964.
TETIO-KAGHO, E.; GARDNER, F.P. Responses of maize to plant population density. 1. Canopy development. Iight relationships, and vegetative growth. Agronomy Journal, Madison, v.80, n.5, p.930-935, Sept. 1988.
THIAGARAJAH, M.R.; HUMID, L.A.; MAHON, ID. EfTects of position and age on leaf photosynthesis ia com (Zea mays L.). Canadian .Joumnal of Botany, v.59, ai, p.28-33, San. 1981.
THORNE, J.H.; KOLLER, H.R. Influence of assimilate - demand on photosynthesis, diffusive resistance, transiocation, and carbohydrate leveis of soybean • leaves. Ptant Fhyslology. Bethesda, v.54, n.2,
p.20 1-20'7, Feb. 1974.
TOLLENAAR, M. Is low plant density a stress ia maize? Maydica, v.37, p.305-31 1, 1992.
TOLLENAAR, M. Sink-source relationships during reproductive development ia maize: a review. Maydica, v.22, p.49-75. 1977.
WESTGATE, M.E. Seed formation in maize during drought. Ia: BOOTE K.S.; BENNET,J.M.; SINCLAIR, T.R.; PAULSES, G.M. (Eds.). Physiotogy and determination of crop yield. Madison: American Society of Agronomy, 1994. p.36I-36l.
