Abstract:Maize grain yield was closely associated with kernel number at maturity, and kernel number was affected by apical kernel abortion mainly occurred at early grain filling stage. The objectives of this work were (i) to evaluate the (effect) of N on grain yield and its components, and to determine the relationship between grain yield and above-ground (dry) matter accumulation; (ii) to analyze the difference of kernel development between apical and mid-basal kernels at grain filling stage, and explore the possible reasons affecting kernel development. The field experiment with summer maize (Zea mays L.) hybrid Zhengdan 958 were carried out at Wuqiao experiment station of China Agricultural University in 2003. The treatments consisted of different crop densities( 52500 plant/ha, 67500 plant/ha, 82500 plant/ha) and nitrogen supply rates (0 kg/ha, 120 kg/ha, 240 kg/ha, 360 kg/ha). Grain yield and its components, above-ground dry matter accumulation, grain filling and content of soluble carbohydrates in summer maize were studied. The results showed that the grain yield per plant was mainly depended on the number of kernels per ear at maturity while the kernel weight was relatively constant. The grain yield and kernel number were closely associated with above-ground dry matter accumulation apparently improved by nitrogen supply. Grain filling and metabolism of soluble carbohydrates in apical and mid-basal kernels were apparently different. In 5—20 day after pollination(DAP), the size of fresh grain, grain dry weight, grain filling rate, total content of soluble sugar, content of sucrose and starch in apical kernels were lower than these in the mid-basal kernels, while in apical kernels these were higher in nitrogen supply treatments than in treatments without nitrogen supply. The grain filling rate in apical kernels began to (decrease) in 15—20 DAP while kernel abortion usually occurred. The results suggested that one of the main reasons affecting development of apical and mid-basal kernels was assimilate supply. Assimilate supply in apical kernels could be improved by nitrogen supply and kernel abortion were inhibited.