Improved Temperature Response Functions for Modeling Photosynthetic Carbon Assimilation in the Context of Rising Carbon Dioxide
Predicting environmental responses of leaf photosynthesis is central for modeling changes in the future global carbon cycle and terrestrial biosphere. The steady-state biochemical model of C3 photosynthesis of Farquhar et al. (1980; Planta 149, 78--90) provides a basis for these larger scale predictions; but a weakness as currently parameterized has been inability to predict accurately carbon assimilation over an ecologically significant range of temperatures. The previous parameters were based on in vitro measurements made over a limited temperature range and require several assumptions of in vivo conditions. Inaccuracies in the functions predicting Rubisco- or RuBP-limited kinetic properties at different temperatures cause very significant error. Both anti-rbcS, for estimating Rubisco kinetics over a larger range of Ci, and wild-type tobacco, to estimate RuBP regeneration kinetics, were used in this study. The temperature functions required for Rubisco-limited photosynthesis were estimated in vivo from the rate of CO2 assimilation over a wide range of temperatures, and CO2 and O2 concentrations, using the anti-rbcS tobacco. Functions required for modeling RuBP-limited photosynthesis were estimated with gas exchange and fluorescence measurements on wild-type tobacco. The results differed substantially from previously published functions for Rubisco- and RuBP-limited photosynthesis. Gas-exchange measurements coupled with fluorescence were also made to determine the temperature response of mesophyll conductance for estimating Rubisco kinetics based on CO2 concentrations at the chloroplast level. These new functions were used to predict photosynthesis in greenhouse grown lemon and tobacco and found to faithfully mimic the observed temperature response. Finally, measurements of diurnal photosynthesis for three poplar clones and soybean plants exposed to current and future predicted levels of CO2 were made over the growing season in the PopFACE and SoyFACE research facilities located in Viterbo, Italy and Urbana, IL, respectively. These provided a rigorous test of the utility of the new functions in predicting the CO2 response of photosynthesis with daily and seasonal variation in temperature in contrasting environments and species. The results represent an improved ability to model leaf photosynthesis over a wide range of temperatures (10--40°C) necessary for predicting carbon uptake by terrestrial C3 systems of the world.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign