Uptake and distribution of microplastics of different particle sizes in maize (Zea mays) seedling roots

Microplastics (MPs) pollution may be harmful to terrestrial ecosystems and is receiving increasing attention. A microcosm study on the uptake of MPs in maize (Zea mays) seedling roots exposed to small polystyrene (PS) beads (0.2, 0.5 and 1.0 μm) and large PS beads (2.0 and 5.0 μm) at 50 mg L^-1 for 7 d was performed. Additionally, the absorption ability of different parts of the roots was also investigated after 10 d of exposure with 0.2 μm PS beads. The results showed that root and shoot biomass remained unchanged under different particle sizes of PS beads. The small PS beads markedly increased the accumulation and distribution of PS beads in roots more than large ones. Confocal laser scanning micrographs confirmed that strong fluorescence signals from small PS beads (0.2 μm) were seen in all tissues, as compared with the control. Large PS beads (2.0 μm) were mainly distributed in the xylem, and no PS beads were detected in any root tissues when treated with 5.0 μm PS beads. More PS beads were absorbed by the root maturation zone than by the root tip zone. Fluorescence intensity values of PS bead accumulations measured across the tissues further confirmed these results. As seen in scanning electron microscopy images, small PS beads assembled on the cell wall of the xylem, while large PS beads (2.0 μm) were scattered on the cell walls of root xylem. The present study revealed the effects of different PS bead sizes on accumulation and distribution in maize roots, as well as the absorption ability of different positions of the roots. Moreover, fluorescence intensity could be a useful method to evaluate the uptake and distribution of MPs accurately.