Leaf xylem embolism, detected acoustically and by cryo‐SEM, corresponds to decreases in leaf hydraulic conductance in four evergreen species

Abstract

Hydraulic conductance of leaves (K(leaf)) typically decreases with increasing water stress. However, the extent to which the decrease in K(leaf) is due to xylem cavitation, conduit deformation or changes in the extra-xylary pathway is unclear. We measured K(leaf) concurrently with ultrasonic acoustic emission (UAE) in dehydrating leaves of two vessel-bearing and two tracheid-bearing species to determine whether declining K(leaf) was associated with an accumulation of cavitation events. In addition, images of leaf internal structure were captured using cryo-scanning electron microscopy, which allowed detection of empty versus full and also deformed conduits. Overall, K(leaf) decreased as leaf water potentials (Psi(L)) became more negative. Values of K(leaf) corresponding to bulk leaf turgor loss points ranged from 13 to 45% of their maximum. Additionally, Psi(L) corresponding to a 50% loss in conductivity and 50% accumulated UAE ranged from -1.5 to -2.4 MPa and from -1.1 to -2.8 MPa, respectively, across species. Decreases in K(leaf) were closely associated with accumulated UAE and the percentage of empty conduits. The mean amplitude of UAEs was tightly correlated with mean conduit diameter (R(2) = 0.94, P = 0.018). These results suggest that water stress-induced decreases in K(leaf) in these species are directly related to xylem embolism.