Toxic effects of microplastics in plants depend more by their surface functional groups than just accumulation contents

Differentially charged microplastics (MPs) engendered by plastic aging (e.g., plastic film) widely existed in the agricul-tural ecosystem, yet minimal was known about the toxic effects of MPs on plants and their absorption and accumula-tion characteristics. Root absorption largely determined the migration and accumulation risks of MPs in the soil-crop food chain. Here, five types of MPs exposure experiments of leaf lettuce were implemented to simulate root absorption by hydroponics. MPs exposure caused different degrees of growth inhibition, root lignification, root cell apoptosis, and oxidative stress responses; accelerated chlorophyll decomposition and hampered normal electron transfer within the PSII photosystem. Moreover, the uptake of essential elements by roots was inhibited to varying degrees due to the pore blockage in the cell wall and the hetero-aggregation of opposite charges after MPs exposure. MPs exposure ob-servably up-regulated the organic metabolic pathways in roots, thus affecting MPs mobility and absorption through the electrostatic and hydrophobic interactions between the root exudations and MPs. Importantly, MPs penetrated the root extracellular cortex into the stele and were transported to the shoots by transpiration through xylem vessels based on confocal laser scanning microscopy and scanning electron microscopy images. Quantitative analysis of MPs indicated that their toxic effects on plants were determined to a greater extent by the types of surface functional groups than just their accumulation contents, that is, MPs were confirmed edible risks through crop food chain transfer, but bioaccumulation varied by surface functional groups.

Automated summary

Differentially charged microplastics (MPs) generated from plastic aging can be found widely in the agricultural ecosystem. However, little is known about the toxic effects of MPs on plants and their absorption and accumulation characteristics. This study investigated the effects of MPs exposure on leaf lettuce through five different experiments, simulating root absorption in hydroponics. The results showed that MPs exposure caused growth inhibition, root lignification, root cell apoptosis, oxidative stress responses, chlorophyll decomposition, and hindered electron transfer within the PSII photosystem. Additionally, MPs exposure inhibited the uptake of essential elements by roots due to pore blockage and hetero-aggregation of opposite charges. The study also revealed that MPs could penetrate the root cortex and be transported to shoots through transpiration in xylem vessels. The toxic effects of MPs on plants were found to be determined by the types of surface functional groups rather than just their accumulation contents. Overall, this study emphasizes the importance of understanding the toxic effects and uptake mechanisms of MPs in agricultural ecosystems.