Growth and physiological responses of submerged plant Vallisneria natans to water column ammonia nitrogen and sediment copper

Background. The decline of submerged plant populations due to high heavy metal (e.g., Cu) levels in sediments and ammonia nitrogen (ammonia-N) accumulation in the freshwater column has become a significant global problem. Previous studies have evaluated the effect of ammonia-N on submerged macrophytes, but few have focused on the influence of sediment Cu on submerged macrophytes and their combined effects. Methods. In this paper, we selected three levels of ammonia-N (0, 3, and 6 mg L(-1)) and sediment Cu (25.75 ± 6.02 as the control, 125.75 ± 6.02, and 225.75 ± 6.02 mg kg(-1)), to investigate the influence of sediment Cu and ammonia-N on submerged Vallisneria natans. We measured the relative growth rate (RGR), above- and below- ground biomass, chlorophyll, non-protein thiol (NP-SH), and free proline. Results and Discussion. The below-ground biomass of V. natans decreased with increasing Cu sediment levels, suggesting that excessive sediment Cu can result in significant damage to the root of V. natans. Similarly, the above-ground biomass significantly decreased with increasing ammonia-N concentrations, indicating that excessive water ammonia-N can cause significant toxicity to the leaf of V. natans. In addition, high ammonia-N levels place a greater stress on submerged plants than sediment Cu, which is indicated by the decline of RGR and chlorophyll, and the increase of (NP-SH) and free proline. Furthermore, high sediment Cu causes ammonia-N to impose greater injury on submerged plants, and higher sediment Cu levels (Cu ≥ 125.75 mg kg(-1)) led to the tolerant values of ammonia-N for V. natans decreasing from 6 to 3 mg L(-1). This study suggests that high sediment Cu restricts the growth of plants and intensifies ammonia-N damage to V. natans.