A novel optimization approach incorporating non-stomatal limitations predicts stomatal behaviour in species from six plant functional types

The primary function of stomata is to minimise plant water loss while maintaining CO2 assimilation. Stomatal water loss incurs an indirect cost to photosynthesis in the form of non-stomatal limitations (NSL) via reduced carboxylation capacity (CAP) and/or mesophyll conductance (MES). Two optimal formulations for stomatal conductance (gs) arise from the assumption of each type of NSL. In reality both NSL could coexist, but one may prevail for a given leaf ontogenetic stage or plant functional type (PFT), depending on leaf morphology. We tested the suitability of two gs formulations (CAP vs. MES) on species from six PFTs (C4 crop, C3 grass, fern, conifer, evergreen and deciduous angiosperm trees). MES and CAP parameters (the latter proportional to the marginal water cost to carbon gain) decreased with water availability only in deciduous angiosperm trees while there were no clear differences between leaf ontogenetic stages. Both CAP and MES formulations fit our data in most cases, particularly under low water availability. For ferns, stomata only appeared to operate optimally when subjected to water-stress. Overall, the CAP formulation provided a better fit across all species, suggesting that sub-daily stomatal responses minimise NSL by reducing carboxylation capacity predominantly, regardless of leaf morphology and ontogenetic stage.