Ecofisiologia do milho sob diferentes épocas de semeadura e densidades de plantio

Abstract

The photosynthetically active radiation, accumulation of thermal units and the spatial arrangement of plants have great influence on growth, development and yield of crops when the water availability in the soil is not limited. Therefore, the objective of this study is to evaluate maize growth, development and yield in agrometeorological conditions at different times of sowing and plant densities. The experiment was conducted in the experimental área of the Centro de Ciências Agrárias (CECA) of the Federal University of Alagoas (UFAL), na area of 2,736 m². The research was composed of four sowing dates: [E1 (June 28), E2 (July 08), E3 (July 18) and E4 (July 28), 2013] and four maize planting density: D1 (125,000); D2 (87,500); and D4 (37,500) plants per hectare under drip irrigation. The experimental design was a randomized block in split plots (4×4), with four replications. The spacing was 0.80 m, and each plot was composed by the sowing dates and the subplots composed by plant densities. The subplot was formed with five rows of seven meters. Leaf area was determined by the indirect method from four plants randomly selected in each subplot, drawn shortly after sowing, the measurements were made weekly. Leaf area index was determined by the ratio between total leaf area by the area occupied by plants. The photosynthetic solar irradiance (Rf) was estimated as 43% of global solar irradiance (Rg), which was measured by an Eppley pyranometer (model 848, B & W) located in the agrometeorological station of the CECA situated next to the experiment. Photosynthetic irradiance transmitted (Rf_T) was measured using a linear Quatum sensor (LI-191SA) placed diagonally on the soil surface between the planting rows. The season 4 showed the highest mean air temperature comparing to other times and this caused a reduction in crop cycle. According to the soil water content measurements, treatments with different plant population densities showed distinct water demand. The accumulated potential evapotranspiration ranged from 522 mm in the density 2 and 398 mm in density 3. The denser cultivation provided a smaller stem diameter, shorter ear length and diameter and lower grain weight per ear. The maximum yield (6.1 Mg ha-1) was obtained by density of 50,000 plants ha-1, while the population with 125,000 plants ha-1 had the lowest grain yield (4.7 Mg ha-1). The sowing dates were not significantly different for yield. The extinction coefficient (k) of light is inversely proportional to the LAI. The fraction of transmitted irradiance decreased over cultivation. At a density of 125,000 plants ha-1 in V7 growth stage, approximately 60% of irradiance was transmitted to the soil, whereas in D4 it was approximately 80%. All densities intercepted more light in maximum LAI. D1 showed the highest estimated LAI (5.68), while D4 showed the lowest estimated LAI (2.98) and the highest k (0.66). Water and radiation use efficiency, for biomass production, was always higher in denser treatments. The Al bandeirante variety responds negatively to the increased plant densities, for grain yield.