One of current questions is to increase the yield of agricultural plants [1-3]. One of the ways to increase the yield is selection of varieties with high efficiency of photosynthesis and create a crop structure favorable for light absorption [2-5]. In the current situation, it is important to study changes in photosynthesis and other processes of accumulation of plant biomass in a wide range of meteorological conditions. Also important is the forecast of changes in the intensity of photosynthesis in trees. This relevance is caused by an increase in the concentration of carbon dioxide in the atmosphere and an increase in its temperature. The prognosis is the basis for predicting the consequences of global climate change on the planet. This paper considers a model for prognosis of processes in plants during photosynthesis. We consider a possibility of changing the processes occurring in the plant with a change in the conditions of photosynthesis. We presents an analytical approach for analysis the above model with account changing of their parameters, as well as its nonlinearity.