Seasonal changes in the ion-mediated increase of xylem hydraulic conductivity in stems of three evergreens: any functional role?

Changes in hydraulic conductivity (K-h) were measured monthly in stems of Laurus nobilis between July 2005 and June 2006 and in March and June 2006 in stems of Prunus laurocerasus and Phillyrea latifolia. Percent enhancement of K-h (Delta K-h) was recorded as KCl injected into stems compared with deionized water. Stems of L. nobilis perfused with increasing [KCl] saturated the hydraulic effect at 50 mM KCl, while P. laurocerasus and Ph. latifolia showed saturation at 17 and less than 5 mM KCl, respectively. Impressive seasonal changes in Delta K-h were recorded in L. nobilis, from +120% in February to +20% during the growth period. Delta K-h was negatively correlated to minimum air temperatures and was highest during winter frost. Negative correlation also existed between Delta K-h and conduit lengths as detected during transition of plants from activity to winter rest and from winter to spring. In the winter, the thermal effect was prevailing. High cavitation-induced Kh loss coincided with high Delta K-h. Similar but much smaller seasonal effects on Delta K-h were recorded in the other two species where, however, also smaller seasonal changes in conduit lengths were recorded. We conclude that (1) the K+-induced effect on K-h was because of interference of the cation with pectins of intervessel pit membranes (increasingly crossed by the ionic solution in stems with shorter conduits); (2) winter frost might have an effect on Delta K-h and (3) seasonal changes in Delta K-h because of possible change in pectic composition may represent a mechanism to regulate water flows in planta.