Salicylic Acid Alleviates Aluminum Toxicity in Tomato Seedlings (Lycopersicum esculentum Mill.) through Activation of Antioxidant Defense System and Proline Biosynthesis

The aim of this work was to evaluate the alleviating efficacy of salicylic acid (100, 250 and 500 μM) against the toxic effects of aluminum (Al) on two tomato cultivars (GOWRI and SIRI) differing in their sensitivity to Al stress. Al treatment (500 μM) caused 40% - 80% drop in plant growth, relative water content (RWC) and cell viability and a reduction of 1 - 2.5 fold and 0.5 - 2 fold in glutathione and proline content respectively, when compared to their control plants grown in Al free medium. Al treatment also resulted in 2 - 5 fold raise in malondialdehyde (MDA) levels, 2 - 3 fold higher Al uptake and 55% - 80% more electrolyte leakage and caused severe DNA damage. Al stress enhanced (1 - 2 fold) the activities of superoxide dismutase (SOD) and peroxidase (POD), but decreased catalase (CAT) activity over their respective controls. Exogenously-applied Salicylic acid (SA) significantly (p < 0.05) and dose-dependently alleviated Al-induced toxicity in tomato seedlings as marked by much improved plant growth, retention of higher RWC, cell viability, glutathione and proline content. SA also caused 0.6 - 1.5 fold reduction in Al uptake, 50% - 80% less electrolyte leakage, 40% - 80% drop in lipid peroxidation and considerable protection against DNA damage. Also, supplementation of SA could considerably reverse the Al-induced changes in the activities of SOD, POD and CAT. Together, our findings demonstrate that, SA is an efficient growth regulator with diversified roles that contribute to its potential alleviating effect against Al induced toxicity and SIRI is a relatively Al-resistant cultivar compared to GOWRI.