Abstract: 【Objective】The purpose of this study was to investigate the evolution characteristics of main agronomic traits and the responses to density of maize hybrids and their parents released in different eras, thus providing the critically important information for further developing the promising maize cultivars.【Method】Sixteen elite parents and their maize hybrids widely popularized and utilized in China released in 1960s, 1980s and 2000s were used to analyze the changes in main agronomic and physiology traits associated with yields in genetic improvement made during the past four decades in 2007-2008.【Result】The results showed that compared with 1980s and 1960s hybrids and their parents, the modern varieties had higher grain yield with the eras evolution, the grain yield increased by 25.7%, 35.8% and 16.3%, 29.7%, respectively. The 100-kenerl-weight increased significantly, but no significant difference was observed in heterosis index of maize hybrid under high density. However, plant height, ear position height and ear position height/plant height of hybrids released in different eras did not change significantly with eras evolution, while plant stem diameter increased, and leaf orientation value was higher significantly, the plant density-resistance and lodging-resistance enhanced significantly. Compared with 1980s and 1960s hybrids, the contemporary hybrids and their parents had obviously higher dry matter accumulation, post-anthesis dry matter accumulation/total dry matter accumulation and harvest index, especially the distribution ratio in post-anthesis was significantly increased of dry matter of contemporary hybrids. Also, the contemporary hybrids photosynthesis capacity (mean leaf index, chlorophyll contents, and mean net assimilation rate) had obviously advantages under high density, and 2000s hybrids had higher grain-leaf ratio, and leaf area per unit showed more contribution to their yields. The correlation analysis indicated that yield was positively correlated with 100-kernel weight, plant stem diameter and leaf orientation value, but not significantly correlated with plant height, ear height and plant height/ear height, positively significantly correlated with biomass, ratio of post-anthesis dry matter accumulation and mean net photosynthetic rate. The grain-leaf ratio and obtained grain yield of leaf area per unit of the contemporary hybrids positively significantly correlated with its grain yield, but not positively with their parents. So physical density-resistance of high-yielding maize population was significantly ameliorated.【Conclusion】These results demonstrated that maize grain yield enhancing mainly attributed by synergistically increased plant morphology and physiological density tolerance improvement in the progress of genetic improvement for maize in China.

Key words: single hybrid maize , parents , eras , traits evolution , plant density