Exogenously Applied γ-Aminobutyric Acid Improves Salt Tolerance in Alfalfa by Modulating Ion Homeostasis, Proline Accumulation and Antioxidant Defence System

Salt stress is one of the major environmental stresses that limit plant growth and productivity. Gamma-aminobutyric acid (GABA), a non-proteinogenic amino acid, modulates plant responses and improves tolerance against abiotic stresses. This study was conducted to evaluate the effect of exogenously applied GABA, as seed priming, foliar spray and root dipping, in improving tolerance against salinity stress in alfalfa (Medicago sativa L.). Two alfalfa genotypes viz. OMA-285 (salt tolerant) and OMA-84 (salt sensitive) were used in this study. Salinity significantly reduced the plant growth, tissue K+ contents, carbon assimilation and the efficiency of photosystem II in both tested alfalfa genotypes whereas an increase in leaf GABA contents, leaf Na+ and Cl- contents, the activities of antioxidant enzymes, leaf free proline and total soluble phenolics was recorded under salt stress. However, GABA application caused a further increase in the tissue GABA level that restricted the uptake of Na+ and Cl-, favored K+ uptake and improved the activities of antioxidant enzymes, leaf proline and total soluble phenolics. These GABA-modulated changes helped alfalfa plants sustain carbon assimilation and growth under salt stress. The GABA application through seed priming was the most effective that was followed by root dipping and foliage spray. GABA application was effective in improving salt tolerance in alfalfa. In this regard, seed priming was the most effective application method. GABA application increased its endogenous level and modulated the regulation of ion homeostasis, activation of the antioxidant defense system, and accumulation of leaf proline and total soluble phenolics. These GABA-induced changes helped alfalfa plants to continue carbon assimilation and sustain growth under salt stress.