Transformation Of Copper Oxide Nanoparticles In The System Of Paddy Soil And Rice (Oryza Sativa L.)

Due to the extensive application of metal-based nanoparticles (MNPs), MNPs are inevitably released into the environment during the process of production, use, and disposal of MNP-related products, which may pose a potential threat to the environment and ecosystem. The environmental behavior and fate of MNPs may be influenced by the changing environmental condition. MNPs in the agricultural soil as a primary sink can directly interact with the soil particles and microorganisms, so that the speciation and bioavailability of MNPs will be changed. Moreover, the released MNPs in the environment may be absorbed and accumulated by plants, and further contaminate food and have unexpected risks for human health. Thus, the study on the transformation of MNPs in the system of soil and plant can provide a scientific evidence for the evaluation on the safety and quality of agricultural products and the environmental and ecological risks of MNPs. In this dissertation, copper oxide nanoparticles (CuO NPs) were used as one of typical MNPs. The impact of different environmental factors on the environmental behavior of CuO NPs was studied. The bioavailability and speciation of CuO NPs in the paddy soil was also investigated. Furthermore, the uptake and translocation pathway of CuO NPs in the rice plants was revealed. Then, we focused on the absorption, accumulation, and transformation of CuO NPs in the rice plant during the whole life recycle. In addition, the response mechanism of proteome in the plant stressed by CuO NPs was explored. The original conclusions of this work are summarized as follows:(1) The effect of environmental factors on the aggregation, sedimentation, and dissolution of CuO NPs has been revealed, which mechanism can be connected with the interaction force between two particles. The surface charge of NPs largely depended on the environmental pH. With pH away from the zero point of charge, the increasing energy barriers inhibited the aggregation and sedimentation of CuO NPs. The dissolution of CuO NPs was enhanced with the decrease of pH. The electrostatic repulsion was reduced with the increase of ionic strength and cationic valence which weakened the aggregation and sedimentation of CuO NPs. The increasing ionic strength promoted the dissolution of CuO NPs, but compared with Na+, Ca+ induced less Cu2+released from CuO NPs. Natural organic matter (NOM) such as humic acid, citrate, and L-cysteine affected the environmental behavior of CuO NPs by altering the electrostatic interaction and steric hindrance or forming the chemical bond of Cu-S. All the NOM especially citrate enhanced the dissolution of CuO NPs.(2) We found that the bioavailability and speciation of Cu was altered by the different flooding condition. The basic physic-chemical properties were impacted by the addition of CuO NPs, which was more significant than that of CuO BPs; however, in the maturation stage, no significant difference between CuO NPs and BPs treatment could be found.The pH and the Eh were increased with the increasing concentration of CuO NPs in the paddy soil, whereas the electrical conductivity (EC) showed an opposite trend. It is noteworthy that the bioavailability of CuO NPs was dramatically reduced with the growth of rice plant but was substantially increased in the maturation stage with alternate flooding-drying process. Meanwhile, CuO and Cu associated with humic acid in the soil were gradually transformed to the speciation of CU2S and Cu associated with geothitite.(3) The uptake, translocation, and transformation of CuO NPs in the paint has been demonstrated. CuO NPs moved into the root epidermis, exodermis, and cortex, and they ultimately reached the endodermis but could not easily pass the Casparian strip; however, the formation of lateral roots provided a potential pathway for MNPs to enter the stele. Moreover, bulk-XANES (X-ray absorption near edge structure) data showed that CuO NPs were transported from the roots to the leaves, and that Cu (II) combined with cysteine, citrate, and phosphate ligands and was even reduced to Cu (I). In addition, CuO NPs and Cu-citrate were observed in the root cells and the intercellular space.(4) The accumulation and transformation of CuO NPs during the whole life recycle of rcie plant has been clarified, which showed a negative effect of CuO NPs on the safety and quality of agricultural products. The long-term exposure to CuO has led to a dramatic decrease in the biomass and yield of rice, but CuO NPs had an acute toxicity to the plant while CuO BPs showed a chronic one. Most of Cu content was accumulated in the root, and CuO NPs induced the accumulation of Cu element in the rice grain, particularly in the aleurone layer and seed coat. Moreover, CuO was found in the root, leaf, and even chaff but not in the rice grain.(5) The expression of rice proteome was regulated as a response to the stress from CuO NPs. Much more differentially expressed proteins especially the down-regulated could be found in the treatment with CuO NPs than that with Cu2+ and CuO BPs. Most of the differential proteins induced by CuO NPs participated in the cellular process and the metabolic process, and involved in the function of binding and catalytic activity, which located in the cell and organelle. Furthermore, the plant was induced to express more glutathione S-transferase and oxidoreductase against the oxidative stress caused by CuO NPs. These distinct proteins constituted protein-protein interaction networks which had a relatively small number of nodes in the network and the physical connectivity. A portion of them were up-regulated which enhanced the resistance of rice to CuO NPs.