Effects of NaCl addition to the growing medium on plant hydraulics and water relations of tomato

This study reports experimental evidence for the role of ion-mediated changes of xylem hydraulic conductivity in the functional response of Solanum lycopersicum L. cv Naomi to moderate salinity levels. Measurements were performed in fully developed 12 week-old plants grown in half-strength Hoagland solution (Controls, C) or in the same solution added with 35 mM NaCl (NaCl-plants). NaCl-plants produced a significantly less but heavier leaves and fruits but had similar gas exchange rates compared to C-plants. Moreover, NaCl-plants showed higher vessel multiple fraction (FVM) than C-plants. Xylem sap potassium and sodium concentrations were significantly higher in NaCl-plants than in C plants. When stems were perfused with 10 mM NaCl or KCl, the hydraulic conductance of NaCl plants was nearly 1.5 times higher than in controls. Accordingly, stem hydraulic conductance measured in planta was higher in NaCl- than in C-plants. Our data suggest that tomato plants grown under moderate salinity up-regulate xylem sap [Na+] and [K+], as well as sensitivity of xylem hydraulics to sap ionic content, thus increasing water transport capacity.