With rapid development of agriculture and urbanization, aquatic systems and aquatic plants in Chinaface a great threat of heavy metal pollution where toxic contaminant cadmium (Cd) concentrations werereported up to 4500 [1]g l？1in some heavy metal polluted waters. Our objectives were to assess the biolog-ical causes of heavy metal Cd in aquatic systems associated with the population decline of an endangeredaquatic species Ceratopteris pteridoides. The effects of cadmium (Cd) toxicity on the photosynthetic perfor-mance, leaf chlorophyll content, antioxidant enzyme activities and nitrogen metabolism in C. pteridoideswere investigated under greenhouse conditions. Seedlings were exposed to various Cd concentrations(0, 5, 10, 20, and 40 [1]M) for 7 days. The accumulation of Cd ions in plant tissues inhibited the rela-tive growth rate of C. pteridoides through net assimilation rate retardation and leaf area ratio reduction.A significant reduction in biomass was observed in C. pteridoides at 20 and 40 [1]M Cd concentrations.Exposure to Cd severely restricted plant net photosynthetic rates. Factors limiting photosynthesis in Cd-treated plants were more related to non-stomatal constraints than to stomatal limitation. Cd reducedchlorophyll content of the treated plants and affected also plant ribulose-1,5-bisphosphate (RuBP) activ-ity and regeneration capacity. Decreased leaf photosynthesis showed that a significant amount of Rubiscowas not active in photosynthesis under Cd stress. The Cd treatments reduced significantly the relativequantum efficiency of PSII, electron transport rate and photochemical efficiency of PSII reaction centers,which corresponded to the reduction in Rubisco activity. Increased Cd concentrations showed similareffects on the superoxide dismutase, catalase, and peroxidase activities in the leaves and roots. Also, Cdretention caused cell membrane damage shown by increased malondialdehyde content. Cd toxicity mayhave interfered with plant nutritional assimilation because there was a deficiency in nitrogen uptakeand translocation, shown by the reduced nitrate reductase and glutamine synthetase activities. We con-clude that physiological disturbances in Cd-stressed plants are congruent with the observed plant growthinhibition. Plant Cd stress inhibited both nitrogen absorption and photosynthesis in C. pteridoides, thusenhancing its mortality risk. An efficient strategy to restore environmental quality for this endangeredfern implies the reduction of Cd pollution in the environment.