For plants and ecosystems, greater stomatal apertures result in higher stomatal conductance (gs), a major determinant of photosynthetic productivity, water use, and drought response. Stomatal closure with declining plant water status dynamically influences gas exchange and productivity in ecosystems worldwide. Based on biophysical mechanisms and life history theory, we hypothesized that across species, this response would be constrained by a trade-off between gs in hydrated leaves (gmax) and the leaf water potential at which gs is reduced by 50% (Ψgs50). We tested for the gmax- Ψgs50 trade-off in a controlled experiment on 15 diverse species of southern California determining the response of gs to dehydration, and in novel analyses of a compiled database of published stomatal responses for diverse species. We additionally tested hypotheses that might singly or in combination explain the trade-off.
Results/Conclusions
Across the 15 study species, those with higher gmax had stomatal closure more sensitive to leaf dehydration, i.e., less negative values of Ψgs20, Ψgs50, and Ψgs80. In our analyses of the published literature, we found strong support for the commonness of this trade-off. Mechanistically, the trade-off could be explained as a result of variation in stomatal size, or cell osmotic concentration, either of which would influence both gmax and sensitivity of stomatal closure to leaf dehydration. Additionally, the trade-off may arise due to selection for effective plant hydraulic design such that species with high gmax would more stringently protect against drought-induced damage to their high capacity hydraulic system. Our findings provide new, strong insight into the coordination of stomatal traits, wilting traits, leaf anatomy, and structure. Stomata are well recognized as the primary determinants of plant water fluxes, and also the first defense against drought tolerance. Our findings show that these two fundamental, contrasting roles of stomata are intrinsically linked, with important consequences for the function of plants and ecosystems.