Synergistic alleviation of salinity stress in tomato: unraveling the physiological, biochemical, and antioxidant mechanisms modulated by manganese nanoparticles and chitosan

Salinity stress severely limits tomato (Solanum lycopersicum L.) productivity by inducing oxidative damage, reducing photosynthesis, and inhibiting growth. This study investigated the individual and combined effects of manganese oxide nanoparticles (Mn2O3-NPs) and chitosan in mitigating salinity stress. A controlled factorial experiment was conducted with three salinity levels (0, 40, and 80 mM NaCl) and foliar applications of Mn2O3-NPs (10 mg L−1) and chitosan (150 mg L−1), applied separately and in combination. Salinity significantly decreased biomass and photosynthetic pigments while increasing lipid peroxidation and oxidative stress. Under severe salinity (80 mM NaCl), the combined treatment substantially enhanced plant performance, doubling shoot fresh weight and increasing root fresh weight by 55.5% compared to stressed controls. Chlorophyll a, chlorophyll b, total chlorophyll, and carotenoids increased by 25.8%, 29.4%, 35.8%, and 91.1%, respectively. Oxidative damage markers, including malondialdehyde and aldehydes, were markedly reduced. Antioxidant responses were significantly elevated, with guaiacol peroxidase and polyphenol oxidase activities increasing by up to 300% and 104%, respectively. Additionally, phenolics, flavonoids, DPPH activity, and soluble proteins improved. These findings demonstrate a synergistic nano–biopolymer effect that enhances physiological resilience and stress tolerance, offering a promising strategy for sustainable tomato production in saline environments.