Alleviating versus stimulating effects of bicarbonate on the growth of Vallisneria natans under ammonia stress

Bicarbonate plays a crucial role in limiting the growth of submersed aquatic macrophytes in eutrophic lakes, and high ammonia is often toxic to macrophytes. In order to evaluate the combined effect of HCO3− and total ammonia (i.e., the total of NH3 and NH4+) on submersed macrophytes Vallisneria natans, the growth and physiological response of V. natans in the presence of HCO3− and ammonia were studied. The results showed that with the increase of ammonia, morphological parameters of V. natans declined. In contrast, increased HCO3− concentration stimulated the growth of V. natans, especially when the NH4+-N/NO3−-N ratio was 1:7. High ammonia concentration induced excess free amino acids (FAA) accumulation and soluble carbohydrates (SC) depletion in plant tissues. However, the elevated HCO3− promoted the synthesis of SC and rendered the decrease of FAA/SC ratio. The results also suggested that HCO3− could partially alleviate the stress of ammonia, as evidenced by the decrease of FAA/SC ratio and the growth enhancement of V. natans when the ammonia concentration was 0.58 mg L−1. Given the fact that HCO3− is probably the dominant available carbon source in most eutrophic lakes, the ability of V. natans to use HCO3− for SC synthesis may explain the alleviating effect of HCO3− on V. natans under ammonia stress.