Salinity acclimation ameliorates salt stress in tomato (Solanum lycopersicum L.) seedlings by triggering a cascade of physiological processes in the leaves

Salinity stress remains a major global impediment to agricultural productivity. Recent studies have shown that pre-exposure to mild stress primes plants against subsequent stress events, this is termed acclimation. However, it is not well-known if cultivar differences influence the salinity acclimation process. Moreover, the key regulatory physiological processes underlying salinity acclimation in tomato remain unclear. Hence the present study was aimed at investigating these phenomena. Tomato seedlings of acclimating (Rodede) and non-acclimating (NA, Heinz) cultivars were pre-treated with 0 (NPT) and 10 mM NaCl (PT) for 7 days and then subjected to 150 mM NaCl for 14 days under hydroponic conditions. Shoot dry weight and mean growth rate were remarkably improved only in acclimating cultivar Rodede while no significant differences were induced by PT in NA cultivar Heinz. These were coincidental with considerable reductions in leaf Na(+ )concentration in PT Rodede, but were nonetheless 2-folds higher than in Heinz. Therefore, the much better performance in acclimating cultivar points to an efficient sequestration of Na+ in the vacuole, thereby preventing cytosolic Na+ toxicity. This was further supported by hyperaccumulation of proline and activation of antioxidant enzymes in PT plants. Consequently, PT acclimating plants accumulated less H2O2 in leaves and suffered less membrane damage. Unlike in leaves, despite remarkable reductions in root Na+ concentration by PT in NA cultivar, lack of accompanying improvements in growth suggests that in tomatoes, key physiological processes underlying the acclimation process were operating in leaves.