Methyl Jasmonate–Nano-Sulfur Interaction Enhances Salinity Tolerance through Coordinated Redox Homeostasis and Proline Metabolism in Mustard

Abstract Background and aims Soil salinity disrupts photosynthetic functions through increased oxidative stress. The interaction between phytohormones and nutrient-based nanomaterials offers a promising strategy for enhancing salt tolerance. This study investigated the individual and interactive effects of MeJA, S and nS in mitigating salinity stress in mustard. Methods Mustard plants were treated with 10 μM MeJA applied individually or in combination with 200 mg S kg⁻¹ soil or 200 mg nS kg⁻¹ soil under 100 mM NaCl. Antioxidant and proline metabolism, stomatal architecture, cell viability and photosynthesis and growth were evaluated. Results Salinity increased H₂O₂ and TBARS levels resulting in reduced photosynthesis and plant growth. The combined MeJA and nS application alleviated salinity stress more prominently than their individual applications by reducing H₂O₂ and TBARS levels, enhancing the activities of antioxidant enzymes and AsA–GSH cycle metabolite pools. This coordinated response preserved stomatal integrity and cell viability and increased net photosynthesis and dry matter accumulation. Furthermore, MeJA+nS enhanced proline accumulation through increased activity of pyrroline-5-carboxylate synthetase (P5CS) and γ-glutamyl kinase (GK) and suppressed proline oxidase (PROX) activity. Conclusion MeJA and nS application together alleviated salinity stress more conspicuously through coordinated regulation of ion homeostasis, antioxidant defense and proline metabolism, maintaining stomatal architecture, photosynthesis and growth. This integrated MeJA plus nS strategy represents a promising nano-enabled approach for maintaining photosynthetic performance and biomass accumulation of mustard under saline conditions.