NITROGEN AND DENSITY INFLUENCES ON SILK EMERGENCE, ENDOSPERM DEVELOPMENT, AND GRAIN-YIELD IN MAIZE (ZEA-MAYS L)

Grain yield in maize (Zea mays L.) can be limited by supplies of carbon and/or nitrogen through reduced kernel number, due to dow growth of silks, preventing pollination, and through reduced kernel size due to fewer and/or smaller endosperm cells. A field experiment was conducted under irrigation to evaluate effects of changes in nitrogen and carbon supplies on development of yield components in plants having similar ear size at anthesis. Attention was given to silk extrusion of distal ovaries and to endosperm growth and development. Two levels of nitrogen fertilizer at planting (0 and 167 kg N ha(-1); n and N) and two plant densities (36 600 and 73 200 plants ha(-1); d and D), imposed at initiation of silking, were used to manipulate nitrogen and carbon supplies. Although nitrogen stress reduced whole-plant nitrogen concentration ([N]) and leaf area, phenology and aboveground dry mass per plant at silking were not affected. Spikelet number and initial mass of developing kernels were also similar among treatments. Nitrogen stress led to fewer kernels due mainly to reduced emergence of distal silks through less cell division; subsequent abortion was more density dependent. Unfertilized and high-density treatments resulted in less kernel mass per ear and smaller kernel [N]. Only density affected individual-kernel mass. In treatments where distal kernel mass varied, maximum endosperm length, 25 days after silking (DAS), was correlated with kernel volume and individual-kernel dry mass. The greater endosperm cell number in high-nitrogen treatments was accompanied by a smaller cell size. In most cases, maximum cell number in endosperm of proximal kernels was apparently not achieved by 25 DAS. Constant carbon/nitrogen ratio in apparent fluxes of substrates to the ear during the exponential phase of kernel growth was observed with all treatments. Crop parameters were strongly affected by density, and compensatory growth was evident in the grain yield of the low-density treatments. Harvest indices were similar.