The difference in leaf photosynthesis and senescence characteristics of lowyielding and low Nefficiency, highyielding and medium Nefficiency, highyielding and high Nefficiency rice cultivars was investigated using six representative japonica varieties under their optimum N levels, respectively. The results were as follow: compared with lowyielding varieties, the highyielding ones showed higher population leaf area index (LAI), higher leaf area rates of productive tillers and top 3 leaves, higher flag leaf chlorophyll content (SPAD value)and net photosynthetic rate, and slower senescence of leaves. Among highyielding varieties, as the nitrogen efficiency increased, the population leaf area index (LAI) reduced at the early and middle growth stages (Nn, jointing, and full heading), and increased at maturing. Though with no significant changes during early grain filling stage (0-10 days after full heading), the net photo synthetic rate remarkably increased in middle and late grain filling stages (20-40 days after full heading), which could mainly be attributed to the less breakingdown of chlorophyll, ensuring the higher CO2 assimilation capacity of leaves, the higher SOD activity providing strong antioxidant capacity of the plant, and the lower MDA content implying the less damage degree of membrane lipid. It could be one promising method for further improvement in nitrogen efficiency of highyielding rice varieties to reasonably control the growth of nonproductive and inefficient leaf area before heading, thus properly reducing population leaf area scale before heading and effectively delaying leaves senescence after heading to ensure sustained higher chlorophyll content and net photosynthetic rate of leaves.
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