Effects of Copper Oxide Nanoparticles on the Growth of Rice ( Oryza Sativa L.) Seedlings and the Relevant Physiological Responses

Rice ( Oryza sativa L.), a major staple food for billions of people, was assessed for its phytotoxicity of copper oxide nanoparticle (CuO NPs, size < 50 nm). Under hydroponic condition, seven days of exposure to 62.5, 125, and 250 mg/L CuO NPs significantly suppressed the growth rate of rice seedlings compared to both the control and the treatment of supernatant from 250 mg/L CuO NP suspensions. In addition, physiological indexes associated with antioxidants, including membrane damage and antioxidant enzyme activity, were also detected. Treatment with 250 mg/L CuO NPs significantly increased malondialdehyde (MDA) content and electrical conductivity of rice shoots by 83.4% and 67.0%, respectively. The activity of both catalase and superoxide dismutase decreased in rice leaves treated with CuO NPs at the concentration of 250 mg/L, while the activity of the superoxide dismutase significantly increased by 1.66 times in rice roots exposed to 125 mg/L CuO NPs. The chlorophyll, including chlorophyll a and chlorophyll b , and carotenoid content in rice leaves decreased with CuO NP exposure. Finally, to explain potential molecular mechanisms of chlorophyll variations, the expression of four related genes, namely, Magnesium chelatase D subunit, Chlorophyll synthase , Magnesium-protoporphyrin IX methyltransferase , and Chlorophyllide a oxygenase, were quantified by qRT-PCR. Overall, CuO NPs, especially at 250 mg/L concentration, could affect the growth and development of rice seedlings, probably through oxidative damage and disturbance of chlorophyll and carotenoid synthesis.