Internal coordination between hydraulics and stomatal control in leaves

The stomatal response to changing leaf-atmospheric vapour pressure gradient (D-l) 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 D-l 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 (g(max)) in leaves, but there was no direct correlation between normalized D-l sensitivity and K-leaf. The impact of K-leaf on D-l sensitivity in our diverse sample of eight species was important only after accounting for the strong relationship between K-leaf and g(max). Thus, the ratio of g(max)/K-leaf was strongly correlated with stomatal sensitivity to D-l. This ratio is an index of the degree of hydraulic buffering of the stomata against changes in D-l, and species with high g(max) relative to K-leaf 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.