Abstract: Canopy structure has strong effects on photosynthesis and grain yield in maize (Zea mays L.). Planting density is one of the most important factors that can regulate canopy structure. Many researches have shown that leaf area index (LAI) and leaf area duration (LAD) increase accordantly with the proper increase of planting density, but the percent transmission decreases sharply under excessive high density, resulting in uneven light distribution within canopy and photosynthesis reduction. We need to know which proper planting densities for cultivars lead to little influence on photosynthesis and higher grain yield in summer maize. However, so far few reports on this topic have been found and no quantitative criteria can be used in evaluating canopy structure of different maize cultivars. Therefore, we conducted an experiment with three cultivars and three planting densities in the field having medium soil fertility and application of 180 kg N ha^-1 in Wuqiao Experimental Station (37°41′02″N，116°37′23″E) of China Agricultural University in 2006 to establish such quantitative criteria for high yielding cultivars in North China Plain. The split plot design was employed with main plot of plant density (low, medium, and high respectively), sub-plot of cultivar (CF008, Zhengdan 958, and Jinhai 5 respectively), and three replicates in each sup-plot. According to plant type, the densities of three cultivars were 9.75×104 (low), 11.25×104 (medium), and 12.45×104 (high) plants ha-1 for CF008; 8.25×104 (low), 9.75×104 (medium), and 11.25×104 (high) plants ha-1 for Zhengdan 958; and 6.75×104 (low), 8.25×104 (medium), and 9.75×104 (high) plants ha-1 for Jinhai 5. The high-yielding canopy structure and photosynthesis were obtained under both low and medium densities of the three cultivars. Percent transmission, leaf angle, and stem diameter decreased with the increase of plant density. The chlorophyll relative content (SPAD) and the rate of net photosynthesis rate (Pn) were the smallest under high density because of the uneven light distribution within canopy. For CF008, SPAD value of ear leaf and the third leaf under ear decreased sharply in later growth stages. LAD and LAI values before mid-filling stage were greater under medium or high densities, while different tendency occurred in the maturity stages. Moreover, the proportion of after-silking LAD was greater under low or medium densities, showing that canopy structure was unsuitable under high density due to early senescence. For Zhengdan 958 and Jinhai 5, LAD after silking was greater than that before silking, which was benefit for high grain yield. Our results confirmed that proper planting density can establish high-yielding canopy structure and improve population photosynthesis and yield in maize. We also obtained a series quantitative criteria for high-yielding canopy structure based on the data from Wuqiao area: percent transmission of 13.4%–19.45% in silking stage and 16.19%–21.48% in mid-filling stage under low or medium densities; LAI of 5.59–6.75 in silking stage and 2.24–3.68 in maturity stage, especially highen in middle and upper leaf layers in maturity stage under low or medium densities; Pn of 33.6–43.8 μmol CO2 m-2 s-1 in middle and upper layer leaves in silking stage under low or medium densities; higher LAD after silking under low or medium densities, with 172.01–235.91 m2 d m-2 under medium density.

Key words: Summer maize, Density, Leaf areaindex, Percent transmission, Photosynthesis