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Four wheat cultivars were used to study the effects of delayed nitrogen topdressing on popula-tion structure, grain yield and photosynthetic characteristics of flag leaves under high-yielding conditions. Theresults showed that the nitrogen topdressing delayed from overwintering-turning green stage (N1) to turninggreen-jointing stage(N2 ) and jointing-booting stage(N3 ) decreased ineffective tiller in spring, retarded the de-cline of green leaf area and chlorophyll content in late life span, promoted photosynthetic ability of flag leavesand significantly increased the grain yield. N3 treatment was suitable for most wheat cultivars except the geno-types with higher single spike productivity and less ears in unit area.