Microplastic contamination in plants: Seed-to-harvest exposure and quantified evidence from medicinal crops

Soil-retained microplastics (MPs) are in direct contact with plant roots. However, their adhesion, penetration, and potential transport in crops at different developmental stages, particularly the contamination of roots following long-term exposure, remain poorly understood. In this study, two widely cultivated medicinal/root crops, Danshen (Salvia miltiorrhiza) and milkvetch (Astragalus membranaceus), were exposed to various types of MPs (0.5-5 μm in size), including polystyrene, polyethylene, polyvinyl chloride, and polylactic acid. Using a novel quantitative approach, we characterized the dynamics of MPs adsorption and penetration from germination to harvest. Results indicate that seed imbibition attracts MPs from surrounding soil, creating hot-spots and disrupting redox homeostasis in seeds. The penetration of MPs into seedling roots is species- and biostructure- dependent, while their distribution in mature roots shows similarities. Specifically, in seedling roots of Danshen, MPs are blocked by the exodermis, whereas in milkvetch, MPs enter the cortex. The periderm of mature roots acts as a double-edged sword: it prevents MPs from entering the vascular system and potential root-shoot transport, while trapping approximately 140,000-150,000 particles per gram of 1 μm MPs after 120 days of exposure at 50 mg/kg. Through the distinctive lens of biological microstructure, our study reveals the mechanisms of MPs-plant interaction throughout the vegetative growth phase, specifically quantifying MPs' contamination levels and their restricted distribution within the outermost barrier tissues. These findings establish a scientific foundation for optimizing cultivation practices and processing methods to reduce MPs in plant-based products.