A perspective on optimal leaf stomatal conductance under CO2 and light co-limitations

To describe stomatal response to micro-environmental variations, optimization theories for canopy gas exchange are often used as alternatives to empirical or mechanistic but complex models of stomatal function. Solutions for optimal stomatal conductance have been proposed assuming leaf photosynthesis is limited by either Rubisco activity (and hence by CO2 at the photosynthetic site) or ribulose-1,5-biphosphate (RuBP) regeneration rate (and hence light availability). These contrasting assumptions result in different relations between the marginal water use efficiency λ (the key optimization parameter) and atmospheric CO2 concentration (ca). Contrasting predictions of stomatal responses to elevated ca ensue, begging the question as to which approach is most suitable. Here, it is proposed that stomatal aperture is optimized for shifting limitations, motivating the development of a framework where Rubisco activity and electron transport co-limit photosynthesis. This approach attempts to reconcile the two previously proposed optimality solutions. Based on a minimalist model of photosynthesis that accounts for both limitations, optimal stomatal conductance is derived as a function of photosynthetic parameters, λ, and leaf micro-environmental conditions. The optimal stomatal conductances resulting from the different formulations of photosynthesis and functional dependencies of λ on ca are compared for varying environmental conditions, with reference to often observed patterns and scaling relationships. The results suggest that short-term (e.g., sub-daily) fluctuations in ca trigger small adjustments in stomatal aperture at a constant λ, while long-term (e.g., growing season or longer) elevated ca may elicit acclimation mechanisms, potentially resulting in changes in λ.