Effects of Long-term Shading on Canopy and Single Leaf Photosynthetic Properties of Winter Wheat(Triticum aestivum L.)

[Objective] The aim was to study the effects of long-term low radiation on canopy apparent photosynthesis(CAP) and photosynthetic properties of the top three leaves of winter wheat,especially the flag leaf.[Method] Two winter wheat cultivars,differing in shading(low radiation) tolerance,Yangmai 158 and Yangmai 11 were shaded to evaluate the impacts of low radiation on CAP and photosynthetic properties of the top three leaves,chlorophyll content of the flag leaf and grain yield.Three treatments were designed as non-shading(S0),22% less intercept radiation(S1) and 33% less intercept radiation(S2) from jointing to maturity.[Result] With shading intensity enhancing,CAP of both cultivars dropped significantly,while net photosynthetic rate(Pn) of flag leaf decreased significantly,no obvious effect on Pn of the second leaf from the top and significantly increased in that of the third leaf.The decrease of Pn in flag leaf was partially compensated by the increase of Pn in the third leaf.The compensation efficiency changed in different shading intensity and cultivars:It was greater in S1 than that in S2,while greater in Yangmai 158 than that in Yangmai 11.Long-term shading declined total chlorophyll,chl a and b content and chl a/b value of flag leaf in its photosynthetic active duration.[Conclusion] Shading between jointing and maturity reduced chl a and chl a/b of flag leaf and then to its Pn.Thus,the CAP was decreased,which resulted in significant decline in grain yield.This research provides an important theoretical basis for wheat production in the middle-down area of the Yangtze River.