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Uptake and Accumulation of Engineered Nanomaterials by Agricultural Crops and Associated Risks in the Environment and Food Safety

Engineered nanomaterials (ENMs) are being discharged into the environment and to agricultural fields, with unknown impacts on crop species. This study focused on the bioaccumulation of engineered nanomaterials into crops and the associated impact on plant growth and plant uptake of secondary contaminant. Investigations into the interactions between nanomaterials and agricultural plants will provide a more developed understanding of nanomaterials implications in the environment; in addition, evaluations of the risks associated with plant-nanomaterials interactions will provide guidelines for safe use of nanomaterials in agriculture. In the screening study on phytotoxicity, carbon-based nanoparticles (NPs) including C60, MWCNTs, NH2-MWCNTs and COOH-MWCNTs, were significantly less phytotoxic to the seedlings of three crop species, compared to metal-based NPs (Ag, CuO, TiO2, ZnO, CeO2, SiO2, Al2O3). While CuO and ZnO NPs presented the highest growth inhibition, the toxicity of NPs was not distinguished from that of corresponding bulk particles (BPs) and ion controls. However, dissolved ions was only partially responsible for phytotoxicity of NPs. Thus, SiO2 and TiO2 NPs with negligible dissolution became the ideal materials to examine the particle-specific impact of NPs on plants. As a typical example of metal oxide-based NPs, TiO2 nanoparticles was further examined in rice (Oryza sativa L.) plants. Through a chronic TiO2 NP exposure with rice plants at 5 mg/L and 50 mg/L, TiO2 NPs were found to penetrate into the plant roots and result in Ti accumulation in aboveground tissues at a significantly higher level compared to when rice were exposed to TiO2 BPs. Over a 9-week exposure to TiO2 nanoparticles, the Ti concentration in rice plants decreased with substantial biomass increase and reached 9.2 mg Ti/kg and 650 mg Ti/kg per dry weight in leaves and roots, respectively. Meanwhile, plant growth performance was not affected until the 4th week when TiO2 NP-treated rice plants started to demonstrate enhanced vegetative growth, including increased total biomass, root biomass, and transpiration rates. At the end of first week, H2O2 overproduction with activated ascorbate peroxidase (APX) and catalase (CAT) activities was also observed; however, oxidatively induced DNA base damage was not observed. These results suggest that the long term effect of TiO2 nanoparticles exposure on plant growth could not be foreseen through tests in seedling stages. While metal-based NPs resulted in metal accumulation, posing direct food safety risks, carbon-based NPs were more concerned with their interactions with co-existing contaminants. The bioaccumulation and translocation of the pharmaceutical residue carbamazepine at 100 µg/L in collard greens (Brassica oleracea) was evaluated upon concurrent exposure to pristine or carboxyl-functionalized multiwall carbon nanotubes (pCNTs or cCNTs) at 50 mg/L under hydroponic exposure (28 d) and at 0.5 mg/g in soil-grown conditions (42 d). Growth inhibition of B. oleracea was dependent on carbamazepine concentrations in hydroponics. The 50 mg/L pCNTs alone had no impact on plant growth and cCNTs alone in hydroponics increased total biomass by 25%. The pharmaceutical and CNTs had no impact on the growth of soil-grown plants. Without the presence of CNTs, both hydroponic and soil-grown B. oleracea substantially accumulated carbamazepine and carbamazepine demonstrated exceptional translocation potential. The co-exposure of carbon materials (pCNTs, cCNTs and activated carbon) significantly suppressed carbamazepine accumulation in both hydroponics and soil. In general, the adsorption capacity of carbon materials correlated with the suppression of carbamazepine uptake under hydroponic and soil exposure. The results also suggested that functionalization of CNTs enhanced carbamazepine translocation potential in soil-grown B. oleracea and significantly affected nanomaterial\co-contaminant interactions comparing to its pristine analog. These findings show that the presence of CNTs in agricultural system may significantly affect the bioavailability and translocation pattern of coexistent contaminant.
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