The Toxic Mechanism Of CuO Nanoparticles To Rice (Oryza Sativa L.) Roots As Affected By Dissolved Organic Matter

Engineered nanoparticles (NPs) can inevitably be released to the environment and pose risks to organisms during manufacture, transport, usage, and disposition. To date, many studies have investigated the phytotoxicity of various NPs by measuring some growth-related parameters, such as germination rate, root elongation, and biomass production. However, the underlying mechanism of nanotoxicity to plants is not yet fully understood. Dissolved organic matter (DOM), which is ubiquitous in waters and soils, could interact with the released NPs and consequently alter their behavior and toxicity. Whereas, the impact of DOM on NPs phytotoxicity needs further investigation. In this study, the typical plant rice (Oryza sativa L.) was chosen as test organism. The aim of this work was to explore the impact of CuO NPs on rice roots and the toxic mechanism affected by humic acid (HA, as a model DOM). The main results were presented as follows:CuO NPs were easily agglomerated with hydrodynamic diameters above600nm, positively charged with low stability in ultrapure water at neutral pH.100mg/L CuO NPs released approximately as high as0.13mg/L Cu2+in ultrapure water. The addition of HA inhibited the aggregation of CuO NPs with smaller hydrodynamic diameters. In the presence of HA, the zeta potentials of CuO NPs were reversed to be negative with higher absolute values. The stability and dissolution of CuO NPs suspension were both enhanced in the presence of HA.CuO NPs treatment resulted in inhibition of root elongation (EC50:8.9mg/L), destruction of tissues and cells in root tips, loss of plasma membrane integrity and cell viability in rice roots. The addition of HA significantly alleviated the toxic effects of CuO NPs to root elongation, root tips structure, and cell viability, and reduced the Cu contents in root/shoot of rice, which is due to the enhanced steric hindrance and electrostatic repulsion between CuO NPs and root cells that inhibits the uptake of CuO NPs by rice and mitigates the toxicity of CuO NPs to the root cells.In contrast to CuO NPs, CuO bulk particles (BPs) and corresponding Cu+had no obvious effect on root elongation and cell viability, demonstrating that the toxicity of CuO NPs to rice roots was attributed to the NPs rather than the dissolved Cu2+. Further study showed that CuO NPs caused significant generation of reactive oxygen species (ROS) and malondialdehyde (MDA), decrease of mitochondrial membrane potential (△ψm), as well as programmed cell death (PCD) in rice roots, indicating that the toxicity of CuO NPs to the roots could be attributed to ROS-mediated membrane injury, mitochondrial dysfunction and PCD. The presence of HA significantly inhibited the CuO NPs-induced ROS generation and oxidative stress. Therefore, HA can also act as an antioxidant by scavenging ROS, which would be another possible mechanism of HA-alleviated phytotoxicity of CuO NPs to the rice roots.