Similarities and differences in physiological adaptation to cadmium interactions with nitrogen levels between two aquatic iris life forms in urban wetlands

Plants play a key role in the ecological restoration of urban wetlands. Previous studies have shown that heavy-metal accumulation capacities and adaptation strategies of wetland plants may be related to their life forms. In this study, pot experiments were conducted to investigate the effects of nitrogen (N) on the adaptation strategies of two evergreen and deciduous aquatic iris life forms under cadmium (Cd) stress. Our results showed that Cd stress decreased the gas exchange parameters and biomass in both evergreen and deciduous irises. However, the interactions between N and Cd reversed this effect. Specifically, for deciduous irises, the shoot mass (SM) and root-to-shoot ratio (S/R) increased with higher N concentrations, whereas in evergreen irises, these parameters initially increased and then decreased as N levels increased, suggesting that the two life forms have different efficiencies in utilizing N. Additionally, under the combined stress of N and Cd, evergreen irises exhibited higher malondialdehyde (MDA) content and antioxidant enzyme activity than deciduous irises, whereas deciduous irises had higher chlorophyll content and aboveground biomass. These findings suggest that evergreen and deciduous irises employ distinct adaptive strategies to Cd toxicity; evergreen irises mitigate oxidative stress through enhanced reactive oxygen species (ROS) scavenging, whereas deciduous irises dilute Cd toxicity by increasing biomass. These results provide valuable insight into the use of different aquatic iris life forms for heavy-metal pollution remediation in wetlands.