Combined effects of light reduction and ammonia nitrogen enrichment on the submerged macrophyte Vallisneria natans

The decline of submerged plants resulting from low light and high ammonia nitrogen (ammonia-N) has become a serious problem worldwide. In the present study, three levels of ammonia-N concentrations (0, 3 and 6mgL–1) and four levels of light intensity (control, 15, 2.5 and 0.75% underwater light) were designed to investigate the combined effects of low light and high ammonia-N stress on the submerged plant Vallisneria natans. The effects of low light and ammonia-N were examined by measuring the relative growth rate (RGR), chlorophyll content and superoxide dismutase (SOD) and peroxidase (POD) activity in response to the stressors. The decline in RGR and increase in SOD and POD activity in high ammonia-N water were more significant than under low light conditions, indicating that the stress imposed on submerged plants due to ammonia-N enrichment is stronger. Moreover, the combination of ammonia-N enrichment and low light had a greater effect on submerged plants. This study indicates that V. natans were tolerant to ammonia-N concentrations <6mgL–1. Moreover, low light intensity (0.75% underwater light) amplified the toxic effects of ammonia-N, reducing ammonia-N tolerance from <6 to <3mgL–1.