Responses of morphology, antioxidant systems, ascorbate–glutathione cycle, and photosynthesis in Pistia stratiotes to cadmium pollution in aquatic environments

Growth responses and tolerance of Pistia stratiotes to cadmium (Cd) pollution are not well characterized. We examined P. stratiotes responses to Cd exposure (0, 10, 25, 50, 75, and 100 μM) for 18 days in a hydroponic system. After 6 days, 75–100 μM Cd resulted in typical stress symptoms and reductions in gas exchange; Pn, carbon use efficiency, and Gs were significantly decreased by 20.23–80.65%. By day 12, 50–100 μM Cd decreased root activity and chlorophyll content (+44.44–70.00%) and stimulated the ascorbate–glutathione cycle; ascorbate and glutathione reductase were significantly decreased by 40.38–171.23%. By day 18, all Cd treatments reduced superoxide dismutase activity (−13.40–80.58%) but increased peroxidase (155.73–672.95%), ascorbate peroxidase (+63.73–113.19%), and glutathione reductase activities (+53.14–87.08%), indicating a shift in antioxidant defense. Prolonged exposure to supra-threshold Cd concentrations inactivated part of the photosynthetic reaction centers (Vj increased by 8.89–22.22%) and impaired electron transport (PIabs decreased by 36.26–76.92%); however, P. stratiotes partially compensated via enhanced carotenoid synthesis (+40.33%–44.29%) and thermal energy dissipation (DI0/CS0 increased by up to 140.62%). Overall, P. stratiotes maintained core physiological functions under Cd stress, supporting its potential for phytoremediation in Cd-contaminated aquatic environments.