Effects of ZnO nanoparticles on the toxicity of cadmium to duckweed Lemna minor

Release of nanoparticles into the aquatic environment will inevitably influence the behavior and toxicities of other existing pollutants. In the present study, 10 mg/L of nano-ZnO (diameter 20-30 nm) was used to evaluate its impacts on cadmium(Cd) toxicity on duckweed Lemnaminor based on IC50 values and four biological parameters including percent inhibition of growth rate (I-r), ratio of chlorophyll/pheophytin (D665/D665a), antioxidant enzymes, and H+-ATPase. Results of the 96-h IC50 values of Cd with or without nano-ZnO indicate no additional toxicological effects of nano-ZnO to plants. Further examinations using two Cd concentrations (0.1 and 1 mg/L) showed that nano-ZnO did not influence the inhibitory effect of 0.1 mg/L Cd, but significantly (P < 0.05) reduced the stress of 1 mg/L Cd to the duckweed. The index D665/D665a reflected that the toxic effect of 1 mg/L Cd was significantly (P < 0.05) suppressed by nano-ZnO. H+-ATPase was also sensitive to reveal the protective effects of nano-ZnO on the duckweed under Cd exposure. However, the responses of the antioxidant enzymes SOD and CAT failed to reflect the effects of nano-ZnO on Cd toxicity. Hysteretic addition of nano-ZnO for 24 h showed that the protective effects of nano-ZnO were weakened. Our results suggest that the adsorption of Cd to nano-ZnO may result in lower Cd uptake by L. minor, thus reducing its toxicity. (C) 2019 Published by Elsevier B.V.