Limits to xylem refilling under negative pressure in Laurus nobilis and Acer negundo

The ability of juvenile Laurus nobilis and Acer negundo plants to refill embolized xylem vessels was tested under conditions of soil drought when xylem sap pressure was substantially negative, thus violating the expected condition that pressure must rise to near atmospheric for refilling. Intact potted plants were dried to a stem water potential ( Y W ) corresponding with approximately 80% loss of hydraulic conductivity (PLC) in shoots. Then plants were re-watered and kept at a less negative target Y W for 1–48 h. The Y W was measured continuously with stem psychrometers. Rewatered L. nobilis held at the target Y W for 1 h showed no evidence for refilling unless Y W was within a few tenths of a MPa of zero. In contrast, re-watered L. nobilis held for 24 and 48 h at water potentials well below zero showed a significant reduction in PLC. The recovery was highly variable, being complete in some stem segments, and scarcely evident in others. Embolism repair was accompanied by a significant but moderate decrease in the osmotic potential ( Yp ) of the bulk xylem sap ( Yp = 67 kPa in recovering plants versus 31 kPa in controls). In contrast, embolized and re-watered A. negundo plants held for 24 h at target Y W of 0·9 and 0·3 MPa showed no embolism reversal. The mechanism allowing L. nobilis plants to refill under negative pressure is unknown, but does not appear to operate in A. negundo , and is slower to act for droughtinduced embolism than when embolism was artificially induced by air injection as previously shown for L. nobilis . Key-words : embolism repair; osmotic potential; pit membrane osmosis; refilling mechanism; xylem cavitation; xylem parenchyma; xylem transport.