Effects of nitric oxide on zinc tolerance of the submerged macrophyte Hydrilla verticillata

Heavy metal pollution has become an increasingly recognized threat to the aquatic environment. In response to heavy metal stresses, various terrestrial plants produce the hormone nitric oxide (NO), but minimal information is available about the role of NO in aquatic macrophytes under high zinc (Zn) stress. In this study, we measured physiochemical parameters in the leaves of Hydrilla verticillata exposed to Zn2+ (10 mg l−1) alone or combined with NO (supplied as sodium nitroprusside, SNP) for 1 d and 4 d. Excess Zn disturbed nutrient uptake and altered the redox status of the aquatic plants. NO application partially reversed negative impacts of Zn stress on the contents of photosynthetic pigments, malondialdehyde, nutrient concentration, ascorbic acid, and the activity of antioxidant enzymes. However, the metabolites of SNP did not have similar effects. After treatments for 4 d, plants were further cultured in a solution containing Zn2+ (10 mg l−1) for 10 d. NO application partially reversed Zn-induced negative effects in parameters such as nutrient uptake, antioxidant enzymatic activities, and biomass, as compared to treatments with Zn alone. These results indicate that NO supply could mitigate Zn stress in H. verticillata, as a defense mechanism of the plant against Zn toxicity.