Responses of Leaf-level Water Use Efficiency of Winter Wheat to CO_2 Enrichment in Different Canopy Layers

Elevated atmospheric CO2 concentration will bring about a series of environmental problems to earth ecosystem.Plant photosynthesis and transpiration can response to the environmental change through stomatal regulation.The water use efficiency(WUE) was taken as the breakthrough point to evaluate the correlation of plant photosynthesis and transpiration with study object of winter wheat.The responses of leaf-level photosynthesis and transpiration to CO2 concentration enrichment were measured by using a LI-6400 portable photosynthesis system after dividing the canopy height into three layers of top,middle and bottom,during the grain filling stage in 2008.The results showed that with the CO2 concentration enrichment,the leaf net photosynthesis rate of every layer had a rectangular hyperbolic increase with no standard difference(P0.05),while the parameters of photosynthetic characteristics,including carboxylation efficiency,maximum net photosynthesis rate and respiration rate decreased with canopy depth.The both leaf transpiration rate and stomatal conductance of each layer decreased with standard difference(P0.01),which had a significant relationship(P0.01).The net photosynthetic rate increased and transpiration rate decreased which led to an increase in leaf-level WUE due to CO2 enrichment.