The article describes a mathematical model of photosynthesis and photorespiration of C 3 -plants in the natural environment. The model is sort of physiological one because it takes into account the effect of nitrogen deficiency on the intensity of these processes. A generating solution describing, according to the constructed model, the dynamics of the processes of photosynthesis and photorespiration of C 3 -plants was found. It is proved that the obtained solution is an asymptotic approximation of the solution of the dynamic system and is sufficient for the calculation of the total daily rate of photosynthesis with satisfactory accuracy (for the problems of forecasting the yield). The results of numerical experiments with daily model of photosynthesis are presented which confirm that the model adequately reflects the daily dynamics of the intensities of the processes of photosynthesis and photorespiration of C 3 -plants in sowing.