Ion-mediated increase in the hydraulic conductivity of Laurel stems:: role of pits and consequences for the impact of cavitation on water transport

Changes in hydraulic conductivity (K-h) were measured in stems of Laurus nobilis L. during perfusion with KCl, NaCl or sucrose solutions. Ionic solutes induced marked increase of K-h with respect to deionized water but sucrose had no effect. The kinetics of KCl-induced K-h increase was measured together with changes in [K+] of the perfused solution. K-h increases were paralleled by increases in the [K+](out)/[K+](in) ratio. Samples of different lengths or with increasing percentage loss of conductivity (PLC) due to xylem cavitation were tested, with the aim of increasing radial flow through intervessel pits. KCl solutions enhanced the K h of 12-cm-long samples with a concentration-dependent effect up to 100 m M KCl. Delta K-h increased from 3 to 30% in 1.5- and 12-cm-long samples, respectively and remained constant for longer samples. Increasing PLC induced an exponential increase in Delta K-h. PLC measured with KCl solutions was significantly less than that measured with deionized water, suggesting that measurements of PLC can be affected by the composition of the perfused solution. Experiments support the hypothesis that the 'ionic effect' is mediated by physico-chemical changes of pectins of the pit membranes and raise the possibility that plants might alter the ionic composition of the xylem sap to alleviate the hydraulic impact of cavitation.