Interaction mechanisms between α-Fe2O3, γ-Fe2O3 and Fe3O4 nanoparticles and Citrus maxima seedlings.

The interactions between α-Fe2O3, γ-Fe2O3, and Fe3O4 nanoparticles (NPs) and Citrus maxima seedlings were examined so as to better understand possible particle applications as an Fe source for crop plants. NPs toxicity to the exposed plant was investigated as well. The α- and γ-Fe2O3 NPs were accumulated by plant root cells through diapirism and endocytosis, respectively, but translocation to the shoots was negligible. Analysis of malondialdehyde (MDA), soluble protein content, and antioxidant enzyme activity revealed that Fe deficiency induced strong oxidative stress in Citrus maxima seedlings, which followed an order of Fe deficiency>Fe3+>α-Fe2O3, γ-Fe2O3 NPs>Fe3O4 NPs. However, the chlorophyll leaf content of plants exposed to α-Fe2O3, γ-Fe2O3, Fe3O4 NPs and Fe3+ were significantly reduced by 31.1%, 14.8%, 18.8% and 22.0%, respectively, relative to the control. Furthermore, RT-PCR analysis revealed no up-regulation of AHA and Nramp3 genes in Citrus maxima roots; however, the relative FRO2 gene expression upon exposure to iron oxide NPs was 1.4-2.8-fold higher than the control. Ferric reductase activity was consistently enhanced upon iron oxide NPs exposure. These findings advance understanding of the interaction mechanisms between metal oxide NPs and plants, and provide important knowledge need for the possible application of these materials in agriculture.