Internal coordination between hydraulics and stomatal control in leaves

The stomatal response to changing leaf-atmospheric vapour pressure gradient (Dl) is a crucial yet enigmatic process that defines the daily course of leaf gas exchange. Changes in the hydration of epidermal cells are thought to drive this response, mediated by the transpiration rate and hydraulic conductance of the leaf. Here, we examine whether species-specific variation in the sensitivity of leaves to perturbation of Dl is related to the efficiency of water transport in the leaf (leaf hydraulic conductivity, K leaf). We found good correlation between maximum liquid (K leaf) and gas phase conductances (gmax) in leaves, but there was no direct correlation between normalized Dl sensitivity and Kleaf. The impact of Kleaf on Dl sensitivity in our diverse sample of eight species was important only after accounting for the strong relationship between Kleaf and gmax. Thus, the ratio of gmax/Kleaf was strongly correlated with stomatal sensitivity to Dl. This ratio is an index of the degree of hydraulic buffering of the stomata against changes in Dl, and species with high gmax relative to Kleaf were the most sensitive to D l perturbation. Despite the potentially high adaptive significance of this phenomenon, we found no significant phylogenetic or ecological trend in our species. © 2008 The Authors.