Uptake and accumulation of microplastics in a cereal plant wheat

Abstract

Microplastics pollution is becoming a global environmental concern, and growing evidence has demonstrated the accumulation and distribution of microplastics in terrestrial ecosystems. Once entering into soil, microplastics can change the physical, chemical and biological properties of soil, and then affect the growth of plants. Currently, most attentions have focused on the toxic effects of microplastics on terrestrial plants, only very limited report showed the uptake of microplastics by higher plants under hydroponic culture conditions. The nutrient solution is useful in understanding the mechanism of microplastics uptake, however, it does not account for the importance of affecting factors in the real environment (e.g., the presence of soil organic matter) and therefore do not represent the actual uptake of microplastics in the real-world. Here, we aim to determine whether wheat plants growing in a sand matrix are able to take up 0.2 μm polystyrene (PS) microbeads and translocate these particles from roots to shoots. Wheat was chosen as a representative of cereal crops because it is one of the main staple foods worldwide. A simple and rapid approach for the imaging of fluorescently labelled PS microbeads within plant tissues by confocal laser scanning microscope (CLSM) was used to investigate the uptake, accumulation, translocation and distribution of microspheres in the wheat plant. Two different fluorescent dyes were encapsulated into the PS microbeads matrix and they were used to detect the localization of PS beads in the root and the green tissue respectively. The presence of PS microbeads in plant tissue was then verified using scanning electron microscopy (SEM). Confocal images revealed that the PS luminescence signals were mainly located in the vascular system and on the cell walls of the cortex tissue of the wheat seedling roots after exposure in sand matrix with a concentration of 0.5 g kg−1 of PS beads for 21 d, indicated that the beads passed through the intercellular space via the apoplastic transport system. Microbeads clusters were observed in the intercellular space of epidermal tissues and the steles by SEM. Once inside the central cylinder, the 0.2 μm PS beads were transferred from the roots to the stems and leaves via the vascular system. Here, for the first time, we provide evidence of the adherence, uptake, accumulation, and translocation of submicrometer (0.2 μm) PS within the cereal plant in real sand matrix. Our findings provide a methodology and scientific basis for study of the accumulation mechanism of microplastics in soil-crop systems and their potential risk in food chain transfer.