The sensitivity of photosynthesis to O2 and CO2 concentration identifies strong Rubisco control above the thermal optimum.

The biochemical model of C3 photosynthesis by Farquhar, von Caemmerer and Berry (FvCB) assumes that photosynthetic CO2 assimilation is limited by one of three biochemical processes that are not always easily discerned. This leads to improper assessments of biochemical limitations that limit the accuracy of the model predictions. We use the sensitivity of rates of CO2 assimilation and photosynthetic electron transport to changes in O2 and CO2 concentration in the chloroplast to evaluate photosynthetic limitations. Assessing the sensitivities to O2 and CO2 concentrations reduces the impact of uncertainties in the fixed parameters to a minimum and simultaneously entirely eliminates the need to determine the variable parameters of the model, such as Vcmax , J, or TP . Our analyses demonstrate that Rubisco limits carbon assimilation at high temperatures, while it is limited by triose phosphate utilization at lower temperatures and at higher CO2 concentrations. Measurements can be assigned a priori to one of the three functions of the FvCB model, allowing testing for the suitability of the selected fixed parameters of the model. This approach can improve the reliability of photosynthesis models on scales from the leaf level to estimating the global carbon budget.