Changes in leaf hydraulic conductance correlate with leaf vein embolism in Cercis siliquastrum L.

The impact of xylem cavitation and embolism on leaf (K-leaf) and stem (K-stem) hydraulic conductance was measured in current-year shoots of Cercis siliquastrum L. (Judas tree) using the vacuum chamber technique. K-stem decreased at leaf water potentials (Psi(L)) lower than -1.0 MPa, while K-leaf started to decrease only at Psi(L) <-1.5 MPa. Leaf infiltration under vacuum with Phloxine B revealed that minor veins underwent extensive embolism and became non-functional at Psi(L) <-1.5 MPa, thus indicating that leaf vein embolism was closely related to K-leaf changes. Field measurements of leaf conductance to water vapour (g(L)) and Psi(L) showed that stomata closed when Psi(L) decreased below the Psi(L) threshold inducing loss of hydraulic conductance in the leaf. The partitioning of hydraulic resistances within shoots and leaves was measured using the high-pressure flow meter method. The ratio of leaf to shoot hydraulic resistance was about 0.8, suggesting that stem cavitation had a limited impact on whole shoot hydraulic conductance. We suggest that stomatal aperture may be regulated by the cavitation-induced reduction of hydraulic conductance of the soil-to-leaf water pathway which, in turn, strongly depends on the hydraulic architecture of the plant and, in particular, on leaf hydraulics.