XYLEM RECOVERY FROM CAVITATION-INDUCED EMBOLISM IN YOUNG PLANTS OF LAURUS-NOBILIS - A POSSIBLE MECHANISM

Xylem recovery from cavitation-induced embolism was studied in 1-yr-ol d twigs of Laurus nobilis L. Cavitation was induced by applying pre-es tablished pressure differentials (Delta P-o-i) across the pit membrane s of xylem conduits. Delta P-o-i were 1.13, 1.75 and 2.26 MPa, corresp onding to about 50, 77 and 100% of the measured leaf water potential a t the turgor loss point. Delta P-o-i were obtained either by increasin g xylem tensions or by applying positive pressures from outside, or by a combination of the two. The percentage loss of hydraulic conductivi ty (PLC) did not change, regardless of how the Delta P-o-i were obtain ed. This confirmed that xylem cavitation was nucleated by microbubbles coming from outside the vessels. Positive pressures, however, amplifi ed (up to 75%) and sped up the xylem refilling (20 min) in comparison with that measured in unpressurized twigs (c.50% in 15 h). Twigs girdl ed proximally to their pressurized segment 1 min after the desired pre ssure value had been reached, did not recover from embolism. The later the twigs were girdled with respect to when they were tested for PLC, the higher was their recovery from embolism, suggesting that some mes senger was transported in the phloem which stimulated xylem refilling. Indol-3-acetic acid (IAA) applied to the exposed cortex of both press urized and unpressurized twigs, induced an almost complete recovery fr om PLC. We hypothesize that the refilling of cavitated xylem might be a result of an auxin-induced increase in the phloem loading with solut es. This would cause radial transport of solutes to cavitated xylem co nduits via the rays, thus decreasing their osmotic potential and makin g them refill. No positive xylem pressure potentials were measured dur ing xylem recovery from PLC.