Soil water stress and coupled photosynthesis–conductance models: Bridging the gap between conflicting reports on the relative roles of stomatal, mesophyll conductance and biochemical limitations to photosynthesis

Various plant responses to water stress have been reported, but conflicting reports as to which limiting process is the most important and ecophysiologicaly relevant during water stressed periods make it difficult to confidently model terrestrial CO2 and water flux responses. It has become increasingly accepted that mesophyll conductance could play a role in regulating photosynthesis during periods of water stress. We adapt the Farquhar-BB-type canopy photosynthesis–conductance model coupling to incorporate mesophyll conductance, embed it in an ecophysiological forest model and use it to simulate the effects of seasonal soil water stress on canopy CO2 and water fluxes at a Mediterranean Quercus ilex forest. Tests of various hypotheses regarding the relative roles of stomatal conductance limitations (SCL), mesophyll conductance limitations (MCL) and biochemical limitations (BL) confirmed that during water stressed periods, applying only BL allows for the accurate simulation of CO2 and water fluxes. Neither SCL nor MCL alone could accurately reproduce the observed CO2 and water fluxes. However, a combination of both MCL and SCL was successful at reproducing water stress induced reductions in CO2 and water fluxes, suggesting that mesophyll conductance could bridge the gap between conflicting reports on the processes behind responses to water stress in the field.