Natural organic matter-induced alleviation of the phytotoxicity to rice (Oryza sativa L.) caused by copper oxide nanoparticles.

Natural organic matter (NOM) can interact with engineered nanoparticles (NPs) in the environment and modify their behavior and toxicity to organisms. In the present study, the phytotoxicity of copper oxide (CuO) NPs to rice seedlings in the presence of humic acid as a model NOM was investigated. The results showed that CuO NPs induced the inhibition of root elongation, aberrations in root morphology and ultrastructure, and losses of cell viability and membrane integrity. The adverse effects partly resulted from the generation of reactive oxygen species caused by CuO NPs, which led to lipid peroxidation, mitochondrial dysfunction, and programmed cell death in rice seedlings. However, all the phytotoxicity was alleviated with the addition of humic acid because humic acid coatings on nanoparticle surfaces enhanced electrostatic and steric repulsion between the CuO NPs and the plant cell wall/membrane, reducing contact between NPs and plant and CuO NP-induced oxidative damage to plant cells. The present study's results shed light on the mechanism underlying NP phytotoxicity and highlight the influence of NOM on the bioavailability and toxicity of NPs.