Effective uptake of submicrometre plastics by crop plants via a crack-entry mode

Abstract

Most microplastics are emitted, either directly or via the degradation of plastics, to the terrestrial environment and accumulate in large amounts in soils, representing a potential threat to terrestrial ecosystems. It is very important to evaluate the uptake of microplastics by crop plants because of the ubiquity of microplastics in wastewaters often used for agricultural irrigation worldwide. Here, we analyse the uptake of different microplastics by crop plants (wheat (Triticum aestivum) and lettuce (Lactuca sativa)) from treated wastewater in hydroponic cultures and in sand matrices or a sandy soil. Our results provide evidence in support of submicrometre- and micrometre-sized polystyrene and polymethylmethacrylate particles penetrating the stele of both species using the crack-entry mode at sites of lateral root emergence. This crack-entry pathway and features of the polymeric particles lead to the efficient uptake of submicrometre plastic. The plastic particles were subsequently transported from the roots to the shoots. Higher transpiration rates enhanced the uptake of plastic particles, showing that the transpirational pull was the main driving force of their movement. Our findings shed light on the modes of plastic particle interaction with plants and have implications for crops grown in fields contaminated with wastewater treatment discharges or sewage sludges. The presence of microplastics in wastewaters used for irrigation highlights the urgency of analysing the possible uptake of microplastics by crop plants. This study shows that submicrometre and micrometre plastic particles from treated wastewater enter the steles of crop plants via a crack entry at sites of lateral root emergence.