Response of the Growth of Vallisneria natans Under High Zn 2+ Stress and Different NH 4 + /NO 3 − Ratios

In order to evaluate the combined effects of Zn 2+ and NH 4 + /NO 3 – stress, a cross experiment composed of different concentrations of Zn 2+ (0, 2, 4, 8 mg L −1 ) and ratios of NH 4 + and NO 3 – (1:6, 3:4, and 5:2) was conducted with Vallisneria natans under eutrophication ( N  = 4.667 mg L −1 ). Chlorophyll, protein, malondialdehyde (MDA), superoxide dismutase (SOD) activity, and ascorbate peroxidase (APX) activity of V. natans leaves were measured after cultivation for 7 days under stress of NH 4 + /NO 3 – and Zn 2+ . Variance analysis indicated that chlorophyll continuously decreased significantly with increasing Zn 2+ concentration and NH 4 + /NO 3 – ratio, with the highest value at 0 mg L −1 Zn 2+ and NH 4 + /NO 3 – , 1:6. However, MDA, soluble protein, SOD activity, and APX activity increased first and decreased subsequently, showing the greatest values at 2 mg L −1 Zn 2+ and NH 4 + /NO 3 – , 1:6. Physiological responses of V. natans under high Zn 2+ stress were obvious. A Zn 2+ concentration of <2 mg L −1 could promote the growth of V. natans . Physiological functions of V. natans were partly affected under high NH 4 + /NO 3 – . However, it could promote the growth of V . natans at NH 4 + /NO 3 – <3:4 and a total nitrogen concentration of 4.667 mg L −1 . Meanwhile, this study also showed that Zn 2+ and NH 4 + /NO 3 – have a synergistic effect on V. natans .