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

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.