Leaf area, senescence and uniform development as favorable treits, in the adaptation of maize cultivars with contrasting genetic variability to crowding

The increment in plant density is an alternative to maximize solar radiation interception and to increase maize grain yield. However, it can also reduce the crop photosynthetic activity and limit its efficiency to convert carbohydrates to grain production. Increases in maize hybrid s tolerance to crowding have been reported in different production areas around the world. Two traits that may contribute to this feature are the slower leaf senescence during grain filling and the higher uniformity in plant growth and development during the crop cycle. This work was carried out aiming to quantify the effects of plant population increment on the leaf area, grain yield and the variation coefficient of agronomic traits of maize cultivars with contrasting genetic variability. The trial was set in Lages, SC, Brazil. A randomized block experimental design disposed in split-plots was used. Three cultivars were evaluated in the main plot: an open-pollinated variety (Fortuna), a double-cross hybrid (Ag 303) and a single-cross hybrid (P30F53). Five plant populations were tested in the split-plot: 25,000, 50,000, 75,000, 100,000, and 125,000 plants ha-1. The experiment was sowed on 10-26-2005, with a row spacing of 0.70 m. When the crop had four expanded leaves, 10 plants of each split-plot that were in the same growth stage were labeled. These plants were used to measure leaf area and to follow the crop growth, determining the variation coefficient of leaf area, growth stage and plant height. These variables were estimated 10 times, at the growth stages of V4, V8, V12, V16, R1 (silking) and 14, 28, 42, 56 and 70 days after silking. After harvesting, the variation coefficient and grain production per plant were determined based on the 10 selected plants. In addition to that, grain production and yield components were determined. The data were submitted to the variance analysis by the F test, at the 5.0% significance level (P<0.05). The means were compared by the Tukey s test (P<0.05), by polynomial regression and linear correlation. The response of grain yield to increment in plant population was quadratic, regardless of cultivar. Grain yield of the singlecross hybrid (SCH) was higher and more responsive to increase in plant density than grain yield of the double-cross hybrid (DCH) and the open-pollinated variety (OPV). The plant densities that optimized grain yield were 86,665 plants ha-1, 53,044 plants ha-1 and 85,000 plants ha-1 for the SCH, DCH and OPV, respectively. The SCH presented higher leaf area values than the other cultivars 56 days after silking, when plant populations greater than 50,000 plants ha-1 were used. The highest variation coefficient value for grain production per plant was registered at 125,000 plants ha-1 and the lowest at 25,000 plants ha-1. The SCH presented smaller variation coefficient than the DCH and OPV for plant height and leaf area at silking, and for grain yield per plant. The higher morphologic uniformity of the SCH probably contributed to reduce intra-specific competition for water, light and nutrients, establishing a stronger sink after flowering. The SCH greater demand for photosynthetic products possibly favored the maintenance of leaf activity for a higer period, delaying senescence. Therefore, the slower leaf senescence and the more uniform plant growth contributed to the SCH higher grain yield and to the higher productivity differences between the SCH and the other two cultivars in the highest plant densities (75,000, 100,000 e 125,000 plants ha-1)O incremento na densidade de plantas é uma forma de maximizar a interceptação da radiação solar e o rendimento de grãos do milho. Contudo, ele também pode reduzir a atividade fotossintética da planta e a eficiência de conversão dos fotoassimilados à produção de grãos. Aumentos na tolerância de híbridos de milho ao adensamento têm sido reportados em diferentes regiões produtoras no mundo. Duas características que podem contribuir para isto são a senescência foliar mais lenta no enchimento de grãos e a maior uniformidade no crescimento e desenvolvimento das plantas durante o ciclo da cultura. Este trabalho foi conduzido com os objetivos de quantificar o efeito do incremento da população de plantas sobre a área foliar, o rendimento de grãos e o coeficiente de variação de características agronômicas de cultivares de milho com bases genéticas contrastantes. O experimento foi conduzido no município de Lages-SC. O delineamento experimental utilizado foi o de blocos casualizados dispostos em parcelas subdivididas. Na parcela principal foram testadas três cultivares de milho: uma variedade de polinização aberta (Fortuna), um híbrido duplo (Ag303) e um híbrido simples (P30F53). Cinco populações de plantas foram avaliadas nas subparcelas: 25.000, 50.000, 75.000, 100.000 e 125.000 plantas ha-1. O experimento foi implantado no dia 26 de outubro de 2005, com espaçamento entre linhas de 0,70 m. Quando a cultura estava com quatro folhas expandidas, marcou-se 10 plantas de cada subparcela que estivessem no mesmo estádio fenológico. Estas plantas foram utilizadas para as avaliações de área foliar, bem como para o acompanhamento do crescimento, determinando-se o coeficiente de variação para área foliar, estádio fenológico e estatura de planta. Estas variáveis foram avaliadas 10 vezes, nos estádios V4, V8, V12, V16, R1 (espigamento) e aos 14, 28, 42, 56 e 70 dias após o espigamento. Após a colheita, foram determinados o coeficiente de variação e a produção de grãos por planta, nas 10 plantas selecionadas. Além disso, determinou-se a produção de grãos na área útil, bem como os componentes do rendimento. Os dados foram submetidos à análise de variância pelo teste F, ao nível de significância de 5% (P<0,05). As médias foram analisadas pelo teste de Tukey, a 5% de probabilidade de erro (P