The sensitivity of C₃photosynthesis to increasing CO₂concentration: a theoretical analysis of its dependence on temperature and background CO₂concentration

The atmospheric CO 2 concentration has increased from the pre‐industrial concentration of about 280 μmol mol −1 to its present concentration of over 350 μmol mol −1 , and continues to increase. As the rate of photosynthesis in C 3 plants is strongly dependent on CO 2 concentration, this should have a marked effect on photosynthesis, and hence on plant growth and productivity. The magnitude of photo‐synthetic responses can be calculated based on the well‐developed theory of photosynthetic response to intercellular CO 2 concentration. A simple biochemically based model of photosynthesis was coupled to a model of stomatal conductance to calculate photosynthetic responses to ambient CO 2 concentration. In the combined model, photosynthesis was much more responsive to CO 2 at high than at low temperatures. At 350 μmol mol −1 , photosynthesis at 35°C reached 51% of the rate that would have been possible with non‐limiting CO 2 , whereas at 5°C, 77% of the CO 2 non‐limited rate was attained. Relative CO 2 sensitivity also became smaller at elevated CO 2 , as CO 2 concentration increased towards saturation. As photosynthesis was far from being saturated at the current ambient CO 2 concentration, considerable further gains in photosynthesis were predicted through continuing increases in CO 2 concentration. The strong interaction with temperature also leads to photosynthesis in different global regions experiencing very different sensitivities to increasing CO 2 concentrations.