The sensitivity of C3 photosynthesis to increasing CO2 concentration: a theoretical analysis of its dependence on temperature and background CO2 concentration

ABSTRACT The atmospheric CO2 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 C3 plants is strongly dependent on CO2 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 CO2 concentration. A simple biochemically based model of photosynthesis was coupled to a model of stomatal conductance to calculate photosynthetic responses to ambient CO2 concentration. In the combined model, photosynthesis was much more responsive to CO2 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 CO2, whereas at 5°C, 77% of the CO2 non‐limited rate was attained. Relative CO2 sensitivity also became smaller at elevated CO2, as CO2 concentration increased towards saturation. As photosynthesis was far from being saturated at the current ambient CO2 concentration, considerable further gains in photosynthesis were predicted through continuing increases in CO2 concentration. The strong interaction with temperature also leads to photosynthesis in different global regions experiencing very different sensitivities to increasing CO2 concentrations.