Co-application of Melatonin and Salicylic Acid Counteracts Salt Stress-Induced Damage in Wheat (Triticum aestivum L.) Photosynthetic Machinery

Melatonin (MT) and salicylic acid (SA) have important roles in coping with various abiotic stresses; however, their combined treatment effect under these stressful conditions is unknown. The current study, as a first investigation, was aimed to investigate the effect of their exogenous applications on the photosynthetic machinery in plants grown in salty soils. A pot experiment was conducted to investigate the potential effects of 70 μM MT and/or 75 mg l−1 SA applied to wheat (Triticum aestivum L. cv. Sids 14) plants grown under non-saline or saline conditions (6.0 and 12.0 dS m–1). The experimental layout was completely randomized design with four replications. Exogenously applied MT and/or SA significantly alleviated salt-induced decrease in wheat growth and productivity, and this was closely associated with the improvement of the photosynthetic pigments content, photochemical reactions of photosynthesis, net photosynthetic rate, transpiration rate, stomatal conductance, maximum quantum efficiency of PSII photochemistry, actual photochemical efficiency of PSII, electron transport rate, photochemical quenching coefficient, effective quantum yield of PSII photochemistry, photosynthetic enzymes activity, NADPH content, osmoprotectants accumulation, and grains carbohydrate content along with the reduction of the intercellular CO2 concentration, non-photochemical quenching coefficients, glycolate oxidase activity, and lipid peroxidation, as well as NADP+ and H2O2 content. Notably, the best response was registered with MT and SA combined treatment. Therefore, the co-application of melatonin and salicylic acid can be a better strategy for ameliorating salt toxicity in sustainable agricultural systems.