Environmentally Friendly Applications for Challenging Salinity Stress Using Seaweed Extract, 24-Epibrassinosteroid, and Glutathione to Improve Productivity and Biochemicals in Grapevines cv. Crimson Seedless

Salinity stress, exacerbated by water scarcity, climate change, and irrigation practices, is a growing challenge to agricultural sustainability and crop yield. This study aimed to alleviate salinity stress in ‘Crimson Seedless’ grapevines through the application of biostimulants, including seaweed extract (SW), glutathione (GSH), and 24-Epibrassinosteroid (BR) alone, or in combination during the 2024 and 2025 seasons. Compared with the control, biostimulant applications significantly improved vegetative growth, leaf macro- and micronutrient contents, bud development parameters, wood-ripening parameters, pruning weight, yield components, and biochemical responses. Grapevines treated with SW at 8 L ha−1 + GSH at 0.2 g L−1 + BR at 2 mg L−1 exhibited the most pronounced improvements among the tested biostimulant treatments compared with the control. Compared with the control, this treatment increased the yield per vine, leaf surface area, total chlorophyll content, bud burst, and bud fertility by 63.83%, 36.49%, 37.11%, 31.67%, and 57.14%, respectively. It also increased the leaf petiole mineral nutrient contents, with N, P, K, Ca, Mg, Zn, Mn, and Fe rising by 37.11%, 39.93%, 15.30%, 37.55%, 36.02%, 37.11%, 38.19%, and 37.11%, respectively, while reducing Na and Cl contents by 42.76% and 34.41%, respectively. SW + GSH + BR markedly enhanced the antioxidant defense by increasing superoxide dismutase (SOD), ascorbate peroxidase (APX),, catalase (CAT), peroxidase (POX), and GR activities by 85.06%, 88.06%, 55.35%, 225.84%, and 61.13%, respectively, compared with the control. Conversely, it reduced Malondialdehyde (MDA), polyphenol oxidase (PPO), and proline contents by 33.18%, 69.31%, and 19.16%, respectively. These findings support the potential of combined biostimulant application as a management strategy for improving grapevine performance under salinity stress.