Refilling of embolized vessels in young stems of laurel. Do we need a new paradigm?

Recovery of hydraulic conductivity after the induction of embolisms was stu died in woody stems of laurel (Laurus nobilis). Previous experiments confir ming the recovery of hydraulic conductivity when xylem pressure potential w as less than -1 MPa were repeated, and new experiments were done to investi gate the changes in solute composition in xylem vessels during refilling. X ylem sap collected by perfusion of excised stem segments showed elevated le vels of several ions during refilling. Stem segments were frozen in liquid N-2 to view refilling vessels using cryoscanning electron microscopy. Vesse ls could be found in all three states of presumed refilling: (a) mostly wat er with a little air, (b) mostly air with a little water, or (c) water drop lets extruding from vessel pits adjacent to living cells. Radiographic prob e microanalysis of refilling vessels revealed nondetectable levels of disso lved solutes. Results are discussed in terms of proposed mechanisms of refi lling in vessels while surrounding vessels were at a xylem pressure potenti al of less than -1 MPa. We have concluded that none of the existing paradig ms explains the results.