hydraulic conductance of Fraxinus ornus leaves is constrained by soil water availability and coordinated with gas exchange rates

Leaf hydraulic conductance (K(leaf)) is known to be an important determinant of plant gas exchange and photosynthesis. Little is known about the long-term impact of different environmental factors on the hydraulic construction of leaves and its eventual consequences on leaf gas exchange. In this study, we investigate the impact of soil water availability on K(leaf) of Fraxinus ornus L. as well as the influence of K(leaf) on gas exchange rates and plant water status. With this aim, K(leaf), leaf conductance to water vapour (g(L)), leaf water potential (psi leaf) and leaf mass per area (LMA) were measured in F. ornus trees, growing in 21 different sites with contrasting water availability. Plants growing in arid sites had lower K(leaf), g(L) and psi leaf than those growing in sites with higher water availability. On the contrary, LMA was similar in the two groups. The K(leaf) values recorded in sites with two different levels of soil water availability were constantly different from each other regardless of the amount of precipitation recorded over 20 days before measurements. Moreover, K(leaf) was correlated with g(L) values. Our data suggest that down-regulation of K(leaf) is a component of adaptation of plants to drought-prone habitats. Low K(leaf) implies reduced gas exchange which may, in turn, influence the climatic conditions on a local/regional scale. It is concluded that leaf hydraulics and its changes in response to resource availability should receive greater attention in studies aimed at modelling biosphere-atmosphere interactions.