Response mechanism of photosynthetic characteristics of winter wheat to CO₂ concentration and soil water content

Photosynthesis can reveal the response of plant to the environment.By fitting the light response curve, the physiological mechanism of plant under different growth conditions could be clarified.Under four treatments of two CO₂ concentrations (400 and 750 μmol·mol^-1) and two soil water contents45%(drought stress) and 75%(suitable water condition) of field capacity, Li-6400 portable photosynthetic system was used to measure the light response curves of flag leaves of winter wheat.Five photosynthetic light response models, including rectangular hyperbolic model (RHM), non-rectangular hyperbolic model (NRHM), rectangular hyperbolic modified model (RHMM), exponential model (EM) and modified exponential model (MEM), were used to fit the light response curve of winter wheat.The effects on photosynthesis of winter wheat of different treatments were analyzed.The results showed that the light saturation point (LSP) and the max net photosynthetic (P_nmax) fitted by the rectangular hyperbolic model and the non-rectangular hyperbolic model were larger than the measured values.The rectangular hyperbolic model and the non-rectangular hyperbolic model could not fit the photoinhibition.Meanwhile, the exponential model could simulate well of LSP and P_nmax while could not show the photoinhibition, either.Although the modified exponential model could reveal the photoinhibition, there were serious bias in the parameters of light response curve, and the parameters were lower than the other models.In general, the rectangular hyperbolic modified model was the best model for the simulation of light response curve and its parameters.The increase of CO₂ concentration effectively improved the apparent quantum efficiency, light saturation point and maximum net photosynthetic rate of winter wheat, and reduced the light compensation point and dark respiration rate of winter wheat.Meanwhile, winter wheat energy conversion rate, maximum net photosynthetic rate, light use scope and initial photosynthetic efficiency were increased, which suggested that the increase of CO₂ concentration could effectively enhance the photosynthetic capacity of winter wheat.When the soil moisture content was low, the effects of drought stress was partly offset by increased initial photosynthetic efficiency of winter wheat, but the apparent quantum efficiency, light saturation point and maximum net photosynthetic rate were reduced obviously than the appropriate soil moisture content, namely the photosynthetic ability of winter wheat decreased significantly under drought stress.In addition, the increase of CO₂ concentration had a certain compensation effect on the decrease of photosynthesis caused by drought stress, and the enhancement effects of CO₂ concentration increase on photosynthetic capacity of winter wheat under drought stress was greater than that under appropriate soil moisture content.