Application of γ-aminobutyric acid alleviates salinity-mediated growth decline and oxidative damage by increasing proline and antioxidant functioning in olive (Olea europaea L.)

Salinity stress is a major environmental constraint limiting crop productivity by inducing osmotic imbalance, ion toxicity, and oxidative damage. This study investigated the potential of exogenous γ-aminobutyric acid (GABA) to enhance plant tolerance under saline conditions. Plants were subjected to three salinity levels (0, 100, 150 mM NaCl) with or without foliar application of GABA (0, 1, 2 mM). Salinity significantly reduced growth parameters, including plant height, leaf area, and biomass accumulation, while increasing oxidative stress markers such as malondialdehyde (MDA), hydrogen peroxide (H₂O₂), and superoxide anions (O₂•^-). However, GABA application, particularly at 1 mM, substantially mitigated these negative effects. GABA-treated plants exhibited higher relative water content (RWC), membrane stability index (MSI), and accumulation of osmolytes (proline, soluble sugars), reflecting improved water status and metabolic adjustment. Antioxidant defense was enhanced, with increased activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), ascorbate peroxidase (APX), and glutathione reductase (GR), alongside elevated levels of ascorbate (AsA), glutathione (GSH), and favorable redox ratios (AsA/DHA, GSH/GSSG). GABA also improved nutrient uptake by increasing macronutrient (N, P, K, Ca) and micronutrient (Fe, Mn, Zn, Cu) levels while reducing Na⁺ accumulation and Na⁺/K⁺ ratios. Multivariate analyses (heatmap, PCA) revealed that GABA-treated plants under moderate salinity closely clustered with non-stressed controls, highlighting its protective role. Overall, GABA enhances salinity tolerance by modulating osmotic balance, ion homeostasis, and antioxidant defense mechanisms, supporting its use as a promising agrochemical for improving plant resilience under salt stress.