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The relationship between photosynthetic rate and yield formation processes of the newer and older maize hybrids were investigated. Leaf area at flowering (source) and kernel number (sink) of the newer hybrids were greater than the older ones although their light-saturated photosynthetic rate (Psat) were not greater than the older ones before flowering. After flowering, Psat and chlorophyll content of the newer hybrids declined more slowly than the older ones. They not only distributed almost all photosynthates produced after flowering to grain but also reallocated some reserved photosynthates produced before flowering to grain. The newer hybrids exhibited greater grain mass than the older ones mostly because they could optimally regulate the photosynthetic rate and yield formation processes to maximize grain mass.