Improved temperature response functions for models of Rubisco-limited photosynthesis

Predicting the environmental responses of leaf photosynthesis is central to many models of changes in the future global carbon cycle and terrestrial b iosphere. The steady-state biochemical model of C-3 photosynthesis of Farqu har et al. (Planta 149, 78-90, 1980) provides a basis for these larger scal e predictions; but a weakness in the application of the model as currently parameterized is the inability to accurately predict carbon assimilation at the range of temperatures over which significant photosynthesis occurs in the natural environment. The temperature functions used in this model have been based on in vitro measurements made over a limited temperature range a nd require several assumptions of in vivo conditions. Since photosynthetic rates are often Rubisco-limited (ribulose, 1-5 bisphosphate carboxylase/oxy genase) under natural steady-state conditions, inaccuracies in the function s predicting Rubisco kinetic properties at different temperatures may cause significant error. In this study, transgenic tobacco containing only 10% n ormal levels of Rubisco were used to measure Rubisco-limited photosynthesis over a large range of CO2 concentrations. From the responses of the rate o f CO2 assimilation at a wide range of temperatures, and CO2 and O-2 concent rations, the temperature functions of Rubisco kinetic properties were estim ated in vivo. These differed substantially from previously published functi ons. These new functions were then used to predict photosynthesis in lemon and found to faithfully mimic the observed pattern of temperature response. There was also a close correspondence with published C-3 photosynthesis te mperature responses. The results represent an improved ability to model lea f photosynthesis over a wide range of temperatures (10-40 degreesC) necessa ry for predicting carbon uptake by terrestrial C-3 systems.