Abstract: In this paper, submersed plant Hydrilla verticillata was cultivated in a 1/10 Hoagland culture medium added with different concentration NH₄⁺-N (0, 0.5, 2.0, 4.0 mg·L^-1) and 10 mg·L^-1 cadmium to study the effects of NH₄⁺-N on H. verticillata growth and its antioxidant en-zyme system under Cd stress, and approach the degradation mechanism of submersed macrophyte in eutrophic and polluted water bodies. The results showed that under 10 mg·L^-1 Cd stress, the chlorophyll content of H. verticillata decreased obviously, and SOD and POD activities increased first but declined then. NH₄⁺-N accelerated the stress effect of Cd. Under 0.5 h joint stress of Cd and NH₄⁺-N, SOD activity increased significantly, while such an increase needed 3 hours under single Cd stress. Relative to chlorophyll and protein, antioxidant enzyme system was an early sensi-tive index, and SOD was more sensitive than POD. After two days exposure to Cd plus NH₄⁺-N, H. verticillata was mainly stressed by Cd, and NH₄⁺-N stress could be neglected.

Key words: Global consequences, Prevention and control strategies, Ecosystem, Biodiversity, Risk analysis