Foliar Uptake and Distribution of Metallic Oxide Nanoparticles in Maize (Zea mays L.) Leaf

The foliar uptake of Fe₃O₄, Cr₂O₃, CuO, and ZnO nanoparticles (NPs) by maize (Zea mays L.) was studied in a lab-scale experiment. The significant increase of Fe concentrations in leaves exposed to Fe₃O₄ was observed in both stomatal closing and stomatal opening treatments, suggesting the presence of a nonstomatal uptake. In parallel treatments with equal doses of Fe₃O₄ (∼200 nm), Cr₂O₃ (∼300 nm), CuO (∼30 nm), and ZnO (∼40 nm) (20–200 μg), the retention percentage of Fe in the leaves (21.0–69.0%) was higher than that of Cr, Cu, and Zn (0.5–14.0%). The steric hindrance effect seems more important for NPs of >200 nm, while hydrophobic surface and negative charge promote the foliar uptake of NPs smaller than 200 nm. The accumulation of NPs in the cuticle was observed through dark-field hyperspectral microscopy. Cr₂O₃, Fe₃O₄, and CuO NPs were difficult to penetrate the cuticle. In comparison, ZnO further migrated and distributed within the extracellular space of epidermal and mesophyll cells of the exposed leaf, possibly due to its comparatively higher solubility and hydrophilicity. The findings highlight the potential of the nonstomatal uptake, which might be a critical route for metallic oxide NPs to enter the food chain.