Abstract:
Hydraulic conductance of leaves (K[subscript leaf]) typically decreases
with increasing water stress and recent studies have proposed
different mechanisms responsible for decreasing
K[subscript leaf]. We measured K[subscript leaf] concurrently with ultrasonic acoustic
emissions (UAEs) in dehydrating leaves of several
species to determine whether declining K[subscript leaf] was associated
with xylem embolism. In addition, we performed experiments
in which the surface tension of water in the leaf
xylem was reduced by using a surfactant solution. Finally,
we compared the hydraulic vulnerability of entire leaves
with the leaf lamina in three species.
Leaf hydraulic vulnerability based on rehydration kinetics
and UAE was very similar, except in Quercus garryana.
However, water potentials corresponding to the initial
decline in K[subscript leaf] and the onset of UAE in Q. garryana were
similar. In all species tested, reducing the surface tension of
water caused K[subscript leaf] to decline at less negative water potentials
compared with leaves supplied with water. Microscopy
revealed that as the fraction of embolized xylem increased,
K[subscript leaf] declined sharply in Q. garryana. Measurements on leaf
discs revealed that reductions in lamina hydraulic conductance
with dehydration were not as great as those observed
in intact leaves, suggesting that embolism was the primary
mechanism for reductions in K[subscript leaf] during dehydration.
