Maize kernel weight response to postflowering source-sink ratio

In maize (Zea mays L.), the negative effects of increased stand densities on final kernel weight (KW) are attributed to reductions in the effective grain-filling period, and not in kernel growth rate. This suggests that competition for assimilates among kernels only occurs at the last stages of grain filling. To test this hypothesis, two commercial hybrids of different KW were grown at two stand densities (3 and 9 plants m(-2)) during 1998 to 1999 and 1999 to 2000. Pollination treatments were performed in order to modify kernel number per plant (KNP) and to obtain a range of source-sink ratios. Pollination treatments altered KNP, and negative relationships were established between KW and KNP, with no differences between years. On the basis of regression analysis of the response of KW to changes in KNP, KW increased between 0.09 to 0.28 mg kernel(-1) per unit decrease in KNP, depending on stand density and genotype. The theoretical potential KW was independent of preanthesis plant population effects, which affected ear growth significantly (P < 0.01). Kernel weight was closely related to variations in kernel growth rate during the effective grain-filling period (r(2) = 0.84; P < 0.001), and not to modifications in the duration of this stage. Within each hybrid, the plant source-sink ratio established during the postflowering period explained KW response to modifications in KNP, independently of stand density. Hybrids differed in the capacity to transform biomass produced at the postflowering period into KW. This was in agreement with differences between hybrids in the capacity to sustain KW when the source-sink ratio was reduced. It is concluded that assimilate limitations to kernel growth occur during the whole grain-filling period.