Grain jeld and kernel weight of two maize genotypes differing in nitrogen use efficiency at variuos levels of nitrogen and carbohydrate availability during flowering and grain filling

Grain yield per plant (GYP) and mean kernel weight (KW) of maize (Zea mays L.) are sensitive to changes in the environment during the lag phase of kernel growth (the time after pollination in which the potential kernel size is determined), and during the phase of linear kernel growth. The aim of this study was to assess genotypic differences in the response to environmental stresses associated with N and/or carbohydrate shortage at different phases during plant development. The rate and timing of N and carbo- hydrate supply were modified by application of fertilizer, shading, and varying the plant density at sow- ing, at silking or at 14 d after silking. The effects of these treatments on the photosynthetic capacity, grain yield and mean kernel weight were investigated in two hybrids differing in N use efficiency. The total above-ground biomass and grain yield per plant of the efficient hybrid responded little to altered environmental conditions such as suboptimal N supply, enhanced inter-plant competition, and shading for 14 d during flowering, when compared to the less efficient genotype. We conclude that grain yields in the efficient genotype are less sensitive not only to N stress, but also to carbohydrate shortage before grain filling. Shading of N deficient plants from 14 d after silking to maturity did not significantly reduce grain yield in the non-efficient genotype, indicating complete sink limitation of grain yield during grain filling. In the efficient genotype, in contrast, grain yield of N-deficient plants was significantly reduced by shading during grain filling. The rate of photosynthesis declined with decreasing foliar N content. No genotypic differences in photosynthesis were observed at high or low foliar N contents. However, at high plant density and low N supply, the leaf chlorophyll content after flowering in the efficient genotype was higher than that in the non-efficient genotype. Obviously, the higher source capacity of the efficient geno- type was not due to higher photosynthetic N use efficiency but due to maintenance of high chlorophyll contents under stressful conditions. In the efficient genotype, the harvest index was not significantly affected by N fertilization, plant density, or shading before the grain filling period. In contrast, in the non-efficient genotype the harvest index was diminished by N deficiency and shading during flowering. We conclude that the high yielding ability of the efficient genotype under stressful conditions was associated with formation of a high sink capacity of the grains under conditions of low carbohydrate and N availability during flowering and with maintenance of high source strength during grain filling under conditions of high plant density and low N availability

Kernel set in maize genotypes differing in nitrogen use efficiency in response to resource availability around flowering

Environmental conditions affect grain yield in maize (Zea mays L.) mainly by altering the kernel number per plant (KNP). This number is determined during a critical period of about 2 weeks around silking. The objectives of this study were to assess how the rate and timing of nitrogen (N) fertilizer applications affect biomass partitioning and KNP in two genotypes with different N use efficiency, and to compare kernel set of these genotypes under varying regimes of carbohydrate and N availability during the criti- cal period for kernel set. In the first field experiment, plant density and the rate of N supply per plant were varied independently. In the second field experiment, N availability was controlled via the applica- tion of N fertilizer, and carbohydrate availability was controlled by shading or thinning at silking. In both experiments, low rates of N supply reduced KNP more strongly in the non-efficient genotype when compared to the efficient genotype. The genotypic differences in kernel set were neither associated with N uptake into the above-ground biomass at maturity, nor above-ground biomass at silking. In the non- efficient genotype, application of N fertilizer at silking increased KNP. This increase was not associated with an increase in plant growth but with increased partitioning of biomass towards the reproductive organs during the critical period for kernel set. The genotype which had been selected for its high N use efficiency also showed higher kernel set at high plant density and shading during flowering when com- pared to the non-efficient genotype. Under conditions of restricted resource availability per plant, plant and ear growth rates during the critical period of about 14 days after onset of flowering declined com- pared with non-limiting conditions. However, these growth rates were less reduced in the efficient geno- type. Pooling treatments of different plant density and different available N, each hybrid showed linear responses of KNP to plant growth rate and to ear growth rate. Furthermore, in the efficient genotype KNP was reduced to a lesser extent in response to decreasing growth rates. We conclude that higher kernel set of the efficient genotype compared to the non-efficient genotype under stressful conditions was associated with low sensitivity of plant growth and dry matter distribution towards reproductive organs to low assimilate availability during the critical period of kernel set, and particularly with low sensitivity of kernel set to decreasing plant and ear growth rates