Ion-mediated enhancement of xylem hydraulic conductivity is not always suppressed by the presence of Ca2+ in the sap

The physiological significance of ion-mediated enhancement of xylem hydraulic conductivity (K-h) in planta has recently been questioned. The phenomenon has been suggested to be an artefact caused by the use of deionized water as a reference fluid during measurements of the impact of different ions on K-h. In the present study, ion-mediated changes in K-h were measured in twigs of five woody species during perfusion with 25 mM KCI compared with different reference fluids like deionized water, a commercial mineral water containing different ions (including 0.5 mM Ca2+), and a 1 mM CaCl2 solution. Both fully hydrated twigs and twigs with about 50% loss of hydraulic conductivity due to cavitation-induced embolism were tested. Adding 25 mM KCI to the three reference fluids caused K-h to increase by about 20%. The KCI-mediated increase of K-h was even larger (up to 100%) in embolized twigs. The presence of Ca2+ in the reference solution decreased, but not suppressed, the KCI-mediated enhancement of K-h in fully hydrated twigs of three species, but not in the other two species tested. Ca2+ did not affect the K-h response to KCI in embolized twigs. These data suggest that the recently reported suppression of the 'ionic effect' by the presence of calcium in the xylem sap is not a general phenomenon and that ion-mediated changes of K-h may play a role in planta partially to compensate for cavitation-induced loss of xylem hydraulic conductivity.