MODELING DAILY NET CANOPY PHOTOSYNTHESIS AND ITS ADAPTATION TO IRRADIANCE AND ATMOSPHERIC CO2 CONCENTRATION

A model to calculate the daily rate of net canopy photosynthesis in re sponse to irradiance and atmospheric CO2, incorporating adaptation to growth conditions, is presented. Adaptation is treated by assuming tha t the plant protein nitrogen content varies so as to maximize daily ne t photosynthesis through its effect on gross photosynthesis and respir atory losses. Net photosynthesis is calculated using daily mean direct solar and difuse-sky radiation components, estimated from total daily solar radiation. The behaviour of the model is consistent with patter ns observed in the literature. For maximum net canopy photosynthesis, nitrogen levels in C-4 plants are predicted to be significantly lower than for C-3. Large changes in single-leaf photosynthetic capacity may not translate into large variation in net canopy photosynthesis. The optimum protein nitrogen level is predicted to decrease with increasin g atmospheric CO2. The model implies that the long-term optimum value of the maximum rate of leaf gross photosynthesis will be less than tha t observed in the short term, and that maintenance respiration will de cline in the long term, due to a decrease in the optimum level of plan t protein nitrogen at higher CO2 levels. The approach presented is sim ple enough to be readily incorporated into crop, pasture and ecosystem models, while allowing for representation of several important physio logical responses.