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

The physiological significance of ion-mediated enhancement of xylem hydraulic conductivity (Kh) 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 Kh. In the present study, ion-mediated changes in Kh were measured in twigs of five woody species during perfusion with 25 mM KCl 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 KCl to the three reference fluids caused Kh to increase by about 20%. The KCl-mediated increase of Kh was even larger (up to 100%) in embolized twigs. The presence of Ca2+ in the reference solution decreased, but not suppressed, the KCl-mediated enhancement of Kh in fully hydrated twigs of three species, but not in the other two species tested. Ca2+ did not affect the Kh response to KCl 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 Kh may play a role in planta partially to compensate for cavitation-induced loss of xylem hydraulic conductivity.