This study was conducted to investigate the effects of foliar application of γ-aminobutyric acid (GABA) and salinity stress on morphological traits, biomass, physiological characteristics, photosynthetic pigments, secondary metabolite compounds, oxidative and enzymatic responses, as well as ionic balance and signaling molecules in pot marigold (Calendula officinalis L.) under greenhouse conditions at the University of Ilam during 2025–2026. The experiment was arranged as a factorial based on a completely randomized design (CRD) with three replications. Treatments included GABA at concentrations of 0, 5, and 10 mM, and salinity levels of 0, 50, 100, and 150 mM NaCl. The results showed that salinity stress significantly reduced growth indices including plant height, root length and volume, leaf area, flower number, capitulum diameter, and fresh and dry weights of shoots and roots. Also, physiological indices, including photosynthesis rate, transpiration, relative water content (RWC), and water use efficiency (WUE) were negatively affected by salinity. In contrast, the application of GABA, especially at a concentration of 10 mM, significantly improved these traits under stress conditions and increased the yield of treated plants compared to stressed controls. In terms of pigments and secondary metabolites, salinity reduced chlorophyll a, chlorophyll b, total chlorophyll, total carotenoids, protein content, total phenols, and α-cadinol, while GABA treatment reduced the severity of these decreases and helped maintain higher levels of these compounds. In this regard, oxidative stress indicators including hydrogen peroxide (H₂O₂), malondialdehyde (MDA) and the activities of lipoxygenase (LOX) and phospholipase D (PLD) enzymes increased under salt stress, but the application of GABA reduced these oxidative damage indicators. Also, the activities of antioxidant enzymes including catalase (CAT), peroxidase (POD), superoxide dismutase (SOD), polyphenol oxidase (PPO) and phenylalanine ammonia lyase (PAL) increased in GABA-treated plants, indicating the strengthening of the plant defense system. In addition, GABA treatment increased the accumulation of endogenous GABA, salicylic acid and proline, which helps to improve stress tolerance; so that the increase in proline is probably related to the metabolic connection between the GABA and proline biosynthetic pathways, because these pathways have common precursors. Finally, GABA application improved the K⁺/Na⁺ ratio by reducing sodium absorption and increasing potassium accumulation, thereby helping to maintain protein content and ionic balance in leaves. Overall, foliar application of GABA, particularly at 10 mM, effectively alleviated the detrimental effects of salinity stress on morphology, biomass, physiology, pigments, and biochemical traits of pot marigold, thereby improving plant growth and performance under saline conditions. These findings suggest that GABA, as an effective signaling molecule, can be utilized in the management of salinity stress in ornamental-medicinal plants.
