Interactions between Carbon Dioxide and Water Deficits Affecting Leaf Photosynthesis: Simulation and Testing

Abstract

Atmospheric CO2 concentration is known to have contrasting effects on photosynthesis, particularly in C3 plant species. These effects may be produced mathematically through the combination of a biochemically based submodel of CO2 fixation in leaf chloroplasts with a physically based submodel of CO2 and H2O diffusion through leaf surfaces and atmospheric boundary layers. Water‐deficit effects may be represented in both submodels. The behavior of a combined model was tested against data recorded under atmospheric CO2 concentrations of 330 and 660 μmol mol−1 in the Soil—Plant—Atmosphere Research Units at the University of Florida. The model reproduced changes in the quantum requirements of CO2 fixation and in diurnal CO2 fluxes of soybean [Glycine max. (L.) Merr.] leaves at these different concentrations. Simulated CO2 fixation at midday responded more to increased CO2 concentration at high irradiance (22.9 vs. 42.0 μmol m−2 s−1 at 1050 μmol m−2 s−1) than at low (17.1 vs. 19.7 μmol m−2 s−1 at 330 μmol m−2 s−1), as found experimentally. Under water deficits (leaf water potential ψl = −1.55 MPa), light saturation of CO2 fixation occurred at lower irradiance, again consistent with experimental findings. Interaction between the water deficit effects in each submodel allowed the combined model to simulate a higher sensitivity of CO2 fixation at 660 μmol mol−1 (41.1 vs. 19.3 μmol m−2 s−1) than at 330 (22.9 vs. 14.2 μmol m−2 s−1) when ψl was reduced to −1.55n from −0.60 MPa, as observed experimentally. The theoretical basis for CO2 effects on photosynthesis is well established, allowing them to be reproduced with confidence in the absence of water deficits. However, the theoretical basis for water deficit effects on photosynthesis is incomplete, such that the reproduction of CO2 effects on photosynthesis in the presence of water deficits must still be regarded as speculative.