Exposure to cadmium causes declines in growth and photosynthesis in the endangered aquatic fern (Ceratopteris pteridoides)

Abstract

With rapid development of agriculture and urbanization, aquatic systems and aquatic plants in China face a great threat of heavy metal pollution where toxic contaminant cadmium (Cd) concentrations were reported up to 4500μgl−1 in some heavy metal polluted waters. Our objectives were to assess the biological causes of heavy metal Cd in aquatic systems associated with the population decline of an endangered aquatic species Ceratopteris pteridoides. The effects of cadmium (Cd) toxicity on the photosynthetic performance, leaf chlorophyll content, antioxidant enzyme activities and nitrogen metabolism in C. pteridoides were investigated under greenhouse conditions. Seedlings were exposed to various Cd concentrations (0, 5, 10, 20, and 40μM) for 7 days. The accumulation of Cd ions in plant tissues inhibited the relative 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 40μM 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 reduced chlorophyll content of the treated plants and affected also plant ribulose-1,5-bisphosphate (RuBP) activity and regeneration capacity. Decreased leaf photosynthesis showed that a significant amount of Rubisco was not active in photosynthesis under Cd stress. The Cd treatments reduced significantly the relative quantum 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 similar effects on the superoxide dismutase, catalase, and peroxidase activities in the leaves and roots. Also, Cd retention caused cell membrane damage shown by increased malondialdehyde content. Cd toxicity may have interfered with plant nutritional assimilation because there was a deficiency in nitrogen uptake and translocation, shown by the reduced nitrate reductase and glutamine synthetase activities. We conclude that physiological disturbances in Cd-stressed plants are congruent with the observed plant growth inhibition. Plant Cd stress inhibited both nitrogen absorption and photosynthesis in C. pteridoides, thus enhancing its mortality risk. An efficient strategy to restore environmental quality for this endangered fern implies the reduction of Cd pollution in the environment.