Phytotoxicological Effects of Bulk-NiO and NiO Nanoparticles on Lesser and Giant Duckweeds as Model Macrophytes: Changes in the Plants Physiological Responses

Background: Although the production and usage of nanoparticles and their entrance to the ecosystem have increased in last decades, information about their negative impacts on organisms is scarce. We concentrated on the comparison of the toxicological effects of different concentrations of bulk-nickel oxide and nickel oxide nanoparticles (NiO-NPs) on lesser (Lemna minor L.) and giant (Spirodela polyrhiza (L.) Schleid.) duckweeds as two model macrophytes. Methods: The morphology of nickel oxide nanoparticles was studied by scanning electron microscopy (SEM) and transition electron microscopy (TEM). The plant's growth, photosynthetic pigments content, the contents of total phenols, flavonoids and MDA, and the activities of some antioxidant enzymes were investigated as indices to assess the toxicological effects of the NPs on two plant species. Results: The content of photosynthetic pigments in both of the plant species was significantly reduced by high concentration of NiO-NP. Increasing the concentration of the contaminants in the plant species was led to the remarkable enhancement of total phenol and flavonoid and MDA contents. Moreover, increasing the activity of the plant's antioxidant enzymes could reflect high reactive oxygen species (ROS) production after the plant's treatments with the contaminants. Conclusion: The negative effects of the NiO-NPs, especially in high concentrations, on L. minor and S. polyrhiza were more than those of Bulk-NiO.