New insights into the covariation of stomatal, mesophyll and hydraulic conductances from optimization models incorporating nonstomatal limitations to photosynthesis

Optimization models of stomatal conductance (g_s) attempt to explain observed stomatal behaviour in terms of cost--benefit tradeoffs. While the benefit of stomatal opening through increased CO₂ uptake is clear, currently the nature of the associated cost(s) remains unclear. We explored the hypothesis that g_s maximizes leaf photosynthesis, where the cost of stomatal opening arises from nonstomatal reductions in photosynthesis induced by leaf water stress. We analytically solved two cases, CAP and MES, in which reduced leaf water potential leads to reductions in carboxylation capacity (CAP) and mesophyll conductance (g_m) (MES). Both CAP and MES predict the same one-parameter relationship between the intercellular : atmospheric CO₂ concentration ratio (c_i/c_a) and vapour pressure deficit (VPD, D), viz. c_i/c_a ≈ ξ/(ξ + √D), as that obtained from previous optimization models, with the novel feature that the parameter ξ is determined unambiguously as a function of a small number of photosynthetic and hydraulic variables. These include soil-to-leaf hydraulic conductance, implying a stomatal closure response to drought. MES also predicts that g_s/g_m is closely related to c_i/c_a and is similarly conservative. These results are consistent with observations, give rise to new testable predictions, and offer new insights into the covariation of stomatal, mesophyll and hydraulic conductances.