Fe₃O₄ nanoparticles enhance saline-alkali tolerance in melon by modulating hormonal signaling and organic acid metabolism

• Nano-Fe₃O₄ enhances melon growth and alleviates saline-alkali stress. • Nano-Fe₃O₄ enhances antioxidant activation and ion homeostasis in melon. • Nano-Fe₃O₄ promotes ABA, JA, and organic acid synthesis in melon. Saline-alkali stress is a major abiotic factor limiting the productivity of melon ( Cucumis melo L.). Although nano-biotechnology shows promising potential for enhancing crop tolerance, the mechanisms through which nano-Fe₃O₄ affects melons under saline-alkali stress remain unclear. This study investigated the effectiveness of foliar-applied nano-Fe₃O₄ in improving tolerance to saline-alkali stress in melon seedlings. Treatment with 100 mg/L of nano-Fe₃O₄ significantly promoted seedling growth, increased biomass, and enhanced root system architecture, while notably reducing oxidative damage and membrane injury under saline-alkali stress. Mechanistic analysis revealed that nano-Fe₃O₄ enhanced stress tolerance through three key pathways: (1) it activated the antioxidant defense system and optimized ion homeostasis; (2) it modulated hormone signaling by elevating endogenous levels of abscisic acid (ABA) and jasmonic acid (JA), supported by the upregulation of their biosynthesis genes; (3) it promoted organic acid (OA) metabolism, resulting in increased accumulation of malate and citrate, along with the upregulation of genes involved in their synthesis. Overall, nano-Fe₃O₄ systematically enhanced melon tolerance to saline-alkali stress by co-regulating hormone signaling, antioxidant defense, ion homeostasis, and OA metabolism. The findings of this study may provide a theoretical basis for the application of nano-Fe₃O₄ as a novel biostimulant in saline-alkali agriculture.