Background/Question/Methods
There is increasing evidence that diel variation in plant water status is associated with daily cycles of decline and recovery of leaf hydraulic conductance (Kleaf) in many species. However, the extent to which the observed fluctuations in Kleaf are related to reversible xylem embolism, reversible partial conduit collapse, or changes in the extra-xylary portion of the hydraulic pathway is uncertain. Ultrasonic acoustic emission (UAE) has been used previously to detect xylem embolism in leaves and other plant organs. Our goals for the present study were to measure Kleaf along with UAE, in leaves as they dried to determine the extent to which declining Kleaf was associated with accumulation of UAE events, and to assess the similarity of hydraulic vulnerability curves obtained using different methods.  In addition, cryo-scanning electron microscopy was used to confirm that UAE events were due to xylem embolism.
Results/Conclusions
Hydraulic conductance measured using a timed leaf-rehydration method decreased as water potentials became more negative in leaves of all sample species (Pinus ponderosa, Pinus nigra, Castanopsis chrysophylla and Pieris japonica). Leaf turgor loss points were similar for all species (ΨTLP = -2.2 to -2.7 MPa) and corresponded to minimum measured Kleaf in both of the Pinus species but not in the two broadleaf species (30-50% Kleaf max at ΨTLP). Additionally, leaf water potentials corresponding to a 50% loss in conductivity ranged from -1.5 to -2.4 MPa across species. Decreases in Kleaf were closely associated with accumulated UAE, the amplitude of emissions (dB) and the percentage of empty vessels/tracheids as observed by cryo-SEM. These results suggest that decreases in Kleaf in these species are directly related to xylem embolism.