Response of the Growth of Vallisneria natans Under High Zn2+ Stress and Different NH4+/NO3− Ratios

Abstract

In order to evaluate the combined effects of Zn2+ and NH4+/NO3– stress, a cross experiment composed of different concentrations of Zn2+ (0, 2, 4, 8 mg L−1) and ratios of NH4+ and NO3– (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 NH4+/NO3– and Zn2+. Variance analysis indicated that chlorophyll continuously decreased significantly with increasing Zn2+ concentration and NH4+/NO3– ratio, with the highest value at 0 mg L−1 Zn2+ and NH4+/NO3–, 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 Zn2+ and NH4+/NO3–, 1:6. Physiological responses of V. natans under high Zn2+ stress were obvious. A Zn2+ concentration of <2 mg L−1 could promote the growth of V. natans. Physiological functions of V. natans were partly affected under high NH4+/NO3–. However, it could promote the growth of V. natans at NH4+/NO3– <3:4 and a total nitrogen concentration of 4.667 mg L−1. Meanwhile, this study also showed that Zn2+ and NH4+/NO3– have a synergistic effect on V. natans.