Assessment of the Phytotoxicity of Metal Oxide Nanoparticles on Two Crop Plants, Maize ( Zea mays L.) and Rice ( Oryza sativa L.)

In this study, the phytotoxicity of seven metal oxide nanoparticles(NPs)—titanium dioxide (nTiO 2 ), silicon dioxide (nSiO 2 ), cerium dioxide (nCeO 2 ), magnetite (nFe 3 O 4 ), aluminum oxide (nAl 2 O 3 ), zinc oxide (nZnO) and copper oxide (nCuO)—was assessed on two agriculturally significant crop plants (maize and rice). The results showed that seed germination was not affected by any of the seven metal oxide NPs. However, at the concentration of 2000 mg·L −1 , the root elongation was significantly inhibited by nCuO (95.73% for maize and 97.28% for rice), nZnO (50.45% for maize and 66.75% for rice). On the contrary, minor phytotoxicity of nAl 2 O 3 was only observed in maize, and no obvious toxic effects were found in the other four metal oxide NPs. By further study we found that the phytotoxic effects of nZnO, nAl 2 O 3 and nCuO (25 to 2000 mg·L − 1 ) were concentration dependent, and were not caused by the corresponding Cu 2+ , Zn 2+ and Al 3+ ions (0.11 mg·L − 1 , 1.27 mg·L − 1 and 0.74 mg·L − 1 , respectively). Furthermore, ZnO NPs (<50 nm) showed greater toxicity than ZnO microparticles(MPs)(<5 μm) to root elongation of both maize and rice. Overall, this study provided valuable information for the application of engineered NPs in agriculture and the assessment of the potential environmental risks.