Exogenous Application of Methionine and Phenylalanine Confers Salinity Tolerance in Tomato by Concerted Regulation of Metabolites and Antioxidants

A pot experiment was carried out to study physiological and yield attributes of tomato by foliar spray of L-methionine (Met) and L-phenylalanine (Phe) under saline stress (0, 4, and 6 dS/m). The tomato plants were sprayed with Met (0.01% and 0.02%), Phe (0.01% and 0.02%), and their combinations (0.01% Met + 0.01% Phe and 0.02% Met + 0.02% Phe) at the vegetative growth stage. The results showed that 0.02% Met concentration significantly improved all the growth and yield parameters like shoot length by 25.7%, plant fresh biomass by 66.89%, fruit weight by 67.26%, dry biomass by 37%, and the number of fruits by 82% under 6 dS/m stress condition. Plants sprayed with 0.02% Met + 0.02% Phe displayed higher carotenoid contents (30%), total carbohydrates (threefold), total free amino acids (53%), and proline contents (82%) compared to only salt-treated plants. Also, the combined application of both amino acids reduced the lipid peroxidation rate in a range of 54–57% and electrolyte leakage by 62% compared to salinity alone. The antioxidative activity of peroxidase and catalase was decreased by foliar application under salinity. These results suggest that foliar application of Met and Phe effectively alleviated the harmful effects of salinity on the growth and yield of tomato plants by enhancing the plasma membrane stability, increased osmolytes contents, and activity of antioxidative enzymes. Thus, the foliar spray of methionine can induce salt resistance and increase tomato yield.