Lantana camara-mediated biogenic magnesium oxide nanoparticles protect PSII function and redox homeostasis under Zn stress in Spirodela polyrhiza

Zinc (Zn) toxicity severely affects aquatic plant productivity by disrupting Mg²⁺ homeostasis, destabilizing photosystem II (PSII), and inducing oxidative stress. This study investigates the protective role of magnesium oxide nanoparticles (MgO NPs), synthesized from Lantana camara , against Zn stress in Spirodela polyrhiza . We hypothesized that MgO NPs supplementation would alleviate Zn-induced phytotoxicity by supporting Mg-dependent physiological processes, stabilizing photosynthetic function, and enhancing antioxidant defence mechanisms. Biogenic MgO NPs (30–70 nm; predominantly cylindrical with slight agglomeration) significantly alleviated Zn-induced phytotoxicity, with maximum efficacy at 10 µg mL⁻¹. Excess Zn²⁺ likely disrupted Mg-dependent physiological processes and Mg-dependent metabolic processes, resulting in reduced chlorophyll biosynthesis, impaired PSII photochemistry (ȹP 0 , ȹE 0 ), decreased active reaction-centre density (RC/CS m ), and enhanced energy dissipation (ȹD 0 , DI 0 /CS m ). Zn stress further promoted electron transport chain over-reduction, leading to excessive reactive oxygen species (ROS) generation and lipid peroxidation. MgO NPs application reduced Zn accumulation and was associated with improved photosynthetic performance and reduced oxidative damage under Zn stress. MgO NPs supplementation improved excitation energy trapping and electron transport fluxes (TR 0 /CS m , ET 0 /CS m ) while reducing excessive energy dissipation. Concurrently, MgO NPs enhanced antioxidant responses, reducing H₂O₂ accumulation and lipid peroxidation under Zn exposure. Overall, green-synthesized MgO NPs effectively alleviated Zn-induced physiological and oxidative stress in S. polyrhiza , with 10 µg mL⁻¹ providing the greatest protection under the tested conditions. The observed improvements in photosynthetic performance, antioxidant responses, and reduced Zn accumulation suggest that MgO NPs mitigate Zn phytotoxicity by maintaining photosynthetic function and limiting oxidative damage in this aquatic macrophyte.