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 (ΔK 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 m M KCl, while P. laurocerasus and Ph. latifolia showed saturation at 17 and less than 5 m M KCl, respectively. Impressive seasonal changes in ΔK h were recorded in L. nobilis , from +120% in February to +20% during the growth period. ΔK h was negatively correlated to minimum air temperatures and was highest during winter frost. Negative correlation also existed between ΔK 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 K h loss coincided with high ΔK h . Similar but much smaller seasonal effects on ΔK 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 ΔK h and (3) seasonal changes in ΔK h because of possible change in pectic composition may represent a mechanism to regulate water flows in planta.