Recovery of hydraulic conductivity after the induction of embolisms was studied in woody stems of laurel (Laurus nobilis). Previous experiments confirming the recovery of hydraulic conductivity when xylem pressure potential was less than ؊1 MPa were repeated, and new experiments were done to investigate the changes in solute composition in xylem vessels during refilling. Xylem sap collected by perfusion of excised stem segments showed elevated levels of several ions during refilling. Stem segments were frozen in liquid N 2 to view refilling vessels using cryoscanning electron microscopy. Vessels could be found in all three states of presumed refilling: (a) mostly water with a little air, (b) mostly air with a little water, or (c) water droplets extruding from vessel pits adjacent to living cells. Radiographic probe microanalysis of refilling vessels revealed nondetectable levels of dissolved solutes. Results are discussed in terms of proposed mechanisms of refilling in vessels while surrounding vessels were at a xylem pressure potential of less than ؊1 MPa. We have concluded that none of the existing paradigms explains the results.Over the past two decades, scientists have found substantial evidence that the vulnerability of xylem to cavitation is an important factor in the adaptation of plants to the environment (Tyree and Sperry, 1989;Cochard et al., 1992;Salleo and Lo Gullo, 1993). The cavitation (droughtinduced embolism) of xylem has been detected in stems (Cochard and Tyree, 1990), leaves (Kikuta et al., 1997), and roots (Mencuccini and Comstock, 1997) and has appeared to limit effectively the possible distribution areas of plant species (Cochard et al., 1992). For example, the vulnerability of Holm oak to xylem embolism caused by both drought and freeze stress (Lo Gullo and Salleo, 1993) provides a convincing explanation for the distribution versus elevation and latitude of this species (Pignatti, 1982) in the Mediterranean region.The threshold xylem pressure for cavitation is close to the typical midday xylem pressure of many species in the field (Kikuta et al., 1997). Such a narrow safety margin (Sperry, 1995) is intrinsically dangerous for plant survival under adverse environmental conditions but might be of some advantage in buffering leaf water status (Dixon et al., 1984;Salleo et al., 1997) and in inducing stomatal closure (Sperry, 1995).Debate still exists about the possible mechanisms involved in xylem refilling after cavitation events induced by drought (Tyree and Cochard, 1996) and freezing (Sperry, 1995) stress. The existing paradigm suggests that embolism removal must occur by gas dissolution in the surrounding water. Henry's law states that the solubility of a gas in water is proportional to the partial pressure of the gas species adjacent to the water. Since water in plants is saturated with air at atmospheric pressure, the paradigm requires that the embolism be above atmospheric pressure for the gases to dissolve. Some experiments on angiosperms and gymnosperms fit this paradigm Yang and Tyr...

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Sl'MM.^RYXylfm reco\'CTy from cavitation-induced embolism was studied in 1-yr-old twigs of Laurus nobilis L. Cavitation was induced by applying pre-established pressure differentials (AP,,,) across the pit membranes of xylem conduits. AP,,, were M 3, 1'75 and 2'26 MPa, corresponding to about 50, 77 and 100"t, of tbe measured leaf water potential at the turgor loss point. AP,,, were obtained eitber by increasing xylem tensions or by applying positive pressures from outside, or by a combination ofthe two. The percentage loss of bydraulic conductivity (PLC) did not change, regardless of how the AP,,, were obtained. This confirmed that xylem ca\ itation was nucleated by microbubbles coming fTom outside the vessels. Positn e pressures, htnve\er, amplified (up to 75'^'o) and sped up the .\\'lem refilling (20 min) in comparison with tbat measured in unpressurized twigs (c.SO'^'o in 15 b). Twigs girdled proximally to tbeir pressurized segment 1 min after tbe desired pressure value bad been reacbed, did not recover from embolism. Tbe later tbe twigs were girdled vvith respect to wben they were tested for PLC, the higber was tbeir recovery from embolism, suggesting tbat some messenger was transported in tbe phloem wbicb stimulated xylem refilling. Indol-3-acetic acid (lA.-X) applied to the exposed cortex of both pressurized and unpressurized twigs, induced an almost complete reeo\ery from PLC. We hypotbesize that tbe refilling of capitated xylem might be a result of an auxin-induced increase in the pbloem loading with solutes. Tbis would cause radial transport of solutes to ca\'itated xylem conduits via tbe rays, thus decreasing their osmotic potential and making tbem refill. No positive xylem pressure potentials were measured during x>lem recovery from PLC.

Xylem recovery from embolism was studied in Laurus nobilis L. stems that were induced to cavitate by combining negative xylem pressure potentials ( P X = = = = ----1.1 MPa) with positive air pressures ( P C ) applied using a pressure collar. Xylem refilling was measured by recording the percentage loss of hydraulic conductance (PLC) with respect to the maximum 2 min, 20 min and 15 h after pressure release. Sodium orthovanadate (an inhibitor of many ATP-ases) strongly inhibited xylem refilling while fusicoccin (a stimulator of the plasma membrane H + + + + -ATPase) promoted complete embolism reversal. So, the refilling process was interpreted to result from energy-dependent mechanisms. Stem girdling induced progressively larger inhibition to refilling the nearer to the embolized stem segment phloem was removed. The starch content of wood parenchyma was estimated as percentages of ray and vasicentric cells with high starch content with respect to the total, before and after stem embolism was induced. A closely linear positive relationship was found to exist between recovery from PLC and starch hydrolysis. This, was especially evident in vasicentric cells. A mechanism for xylem refilling based upon starch to sugar conversion and transport into embolized conduits, assisted by phloem pressure-driven radial mass flow is proposed.

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