Xylem sap cation concentration and salt-tolerance in tomato

Recent studies have shown that xylem hydraulic conductance (Kh) increases with increasing xylem sap cation concentration (ionic effect). This study reports experimental evidence for the potential role of cation-mediated increase of xylem hydraulic conductance on salt tolerance in Solanum lycopersicum L. cv Naomi. Measurements were performed in 12 week-old plants grown in half-strength Hoagland hydroponics solution (Controls) or in the same solution enriched with 35 mM NaCl (NaCl plants). Salt-treated plants showed lower numbers of leaves and fruits per plant but the same rate of gas exchange with respect to Controls although leaf water potential was more negative in the former than in the latter. Xylem sap concentration of potassium (and of sodium) was significantly higher in NaCl plants than in Controls. This finding suggests that increase in external Na+ concentration favoured potassium uptake. Shoots from NaCl plants when perfused with a commercial mineral water enriched with 25 mM NaCl or 25 mM KCl showed an increase of Kh by more than 60% than that measured in shoots from Controls, with no difference between the effects of the two cations. At the plant level, salinity induced a substantial increase in leaf thickness and succulence due to the increased volume of leaf parenchyma cells. This was likely to increase the amount of Na+ segregated in vacuoles with the final consequence of better tolerance of salinity. In other words, Na+ had beneficial effects on xylem efficiency whilst its negative effects were buffered by higher leaf cell retention.