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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 .

Growth and physiological responses of submerged plantVallisneria natansto water column ammonia nitrogen and sediment copper