Ecophysiological and Transcriptomic Responses of Mangrove Seedlings to Environmentally Relevant Cu–Pb–Cd Combined-Metal Stress

Mangrove wetlands are important coastal blue-carbon ecosystems that are increasingly exposed to combined-metal stress by copper (Cu), lead (Pb), and cadmium (Cd). However, how mangrove seedlings coordinate metal exclusion, redox regulation, and photosynthetic adjustment under environmentally relevant metal mixtures remains insufficiently resolved. In this study, five dominant mangrove species, Bruguiera gymnorrhiza, Rhizophora stylosa, Kandelia obovata, Aegiceras corniculatum, and Avicennia marina, were exposed to a gradient of Cu–Pb–Cd combined stress under hydroponic and soil-culture systems for 30 days. We measured tissue metal accumulation, growth, leaf hydrogen peroxide (H2O2), chlorophyll content and fluorescence, and examined root and leaf transcriptomic patterns. Roots generally accumulated more Cu, Pb, and Cd than leaves, while seedlings grown in soil accumulated less metal than those in solution. At higher mixture concentrations, A. corniculatum and K. obovata showed greater visible injury, H2O2 accumulation, and reductions in chlorophyll and PSII-related fluorescence parameters. B. gymnorrhiza and A. marina showed comparatively smaller changes. Root- and leaf-associated transcriptomic patterns suggested possible involvement of transport-related and photosynthesis-related pathways, although these associations do not establish causal mechanisms. These findings indicate that mangrove tolerance to Cu–Pb–Cd combined-metal stress is jointly regulated by root metal exclusion, H2O2-mediated redox changes, and photoprotective adjustment, thereby clarifying plant-level mechanisms of abiotic stress tolerance under combined metal exposure. The results identify B. gymnorrhiza and A. marina as preliminary candidates for further evaluation under field conditions.