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