Responses of Vallisneria natans (Lour.) Hara to the combined effects of Mn and pH.

Aquatic plants play a vital role in maintaining the health and stability of ecosystems and in ecological restoration of contaminated water bodies. Herein, a 21-day-long laboratory-scale experiment was designed to explore the growth and physiological responses of Vallisneria natans (Lour.) Hara (V. natans) to the combined effects of manganese (Mn, 5, 20, and 80 mg L^-1) and pH (pH 4.0, 5.5, and 7.0). Our results showed the combined toxicity intensity was closely related to Mn concentration and the toxicity exhibited by Mn gradually strengthened with the decrease of pH level. High concentration of Mn stress significantly reduced plants leaf area, final leaf number, photosynthetic pigment content, RGR (relative growth rate) and biomass accumulation, but significantly increased the contents of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂). At the same time, V. natans plants can resist the adverse stress by activating the antioxidant defense system, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and polyphenol oxidase (PPO) activities. Besides, V. natans tended to adjust the biomass allocation strategy and transferred more energy to the subsurface and the ramets and stolons parts under the combined stress. This experiment also showed that the increasing pH within a certain range could largely improve the removal rate of Mn (at highest by 84.28%). This may indicate the V. natans plant species can act as a promising tool for the Mn phytoremediation in aquatic environments which needs to be further explored by longer cycle field studies.