HACKE, UWE G.* and JOHN S. SPERRY. Department of Biology, University of Utah, 257 S. 1400 E., Salt Lake City, Utah 84112. - The threshold for reversal of xylem embolism in Laurus nobilis.
Xylem cavitation occurs in plants as a result of drought stress and
freeze-thaw cycles. In woody plants, recovery from cavitation and
embolism has been shown to be associated with root pressure and the
production of new vessels/tracheids. According to our current
understanding of bubble dissolution, xylem pressure (Px) has to rise
close to atmospheric values or above for refilling to occur. However,
vessels in Laurus nobilis have been reported to refill under
certain experimental conditions even when Px was presumably very
negative (e.g., Px = -1 MPa). In order to test whether a new paradigm
for xylem refilling is necessary, we tested the ability of
Laurus plants to refill vessels after a controlled drought
cycle while continuously monitoring Px - a critical measurement that
was not done in previous studies. Whole plants were dried in pots, and
a natural "vulnerability curve" was measured, showing the
percentage loss in hydraulic conductivity (PLC) caused by cavitation
as a function of the xylem pressure. A subset of intact plants was
dried to Px = -3 MPa, which was associated with a PLC of c. 75%. These
plants were then rewatered to various Px, and the PLC was measured. If
Laurus is able to refill its embolized vessels under negative
Px, then the PLC should decrease upon rewatering, and any embolism
induced during the drying cycle should be readily reversed. However,
there was no embolism reversal until Px rose above -0.2 MPa. Only at
Px>-0.2 MPa embolism was rapidly reversed, and refilling became
apparent. Assuming the osmotic potential of the xylem sap was between
-0.1 and -0.2 MPa, results are consistent with our current
understanding of bubble dissolution. Results suggest that refilling of
embolized vessels requires near-atmospheric xylem pressure - a
condition that may only occur when transpiration is minimal and soil
is saturated.
Key words: cavitation, Laurus, Xylem embolism, xylem refilling