Abstract: LI-6400R portable photosynthesis system was used to measure the light-response curves of winter wheat at different growth stages,diurnal changes in photosynthetic rate of both sunlit and shaded leaves at different canopy levels and in the related factors.Results showed that the maximum photosynthetic rate appeared at booting stage,the modeled light-response curve was in good agreement with the observed,and PAR was the main reason for the difference both in the photosynthetic rate and in the other physiological progresses between sunlit and shaded leaves.Moreover,the sunlit leaves had a midday depression in photosynthesis under highly strong sunlight,and the shaded leaves were low in photosynthesis due to limited PAR,and water use efficiency was increased from the bottom to the top in canopy,and the sunlit leaves had much higher WUE than that of the shaded ones.

Key words: Winter wheat, Winter wheat, Light-response curve, Sunlit and shaded leaves, Photosynthetic characteristics, Water use efficiency(WUE)