Size-dependent effects of polystyrene micro- and nanoplastics on the quality of rice grains and the metabolism mechanism.

Micro- and nanoplastics (MNPs) exhibited size-dependent impacts on crop physiological processes and metabolic functions, ultimately threatening crop quality. The size-specific response patterns warrant close attention to enable a more accurate assessment of their environmental and human health risks. In this study, both soil (10 mg/kg and 100 mg/kg) and hydroponic (1 μg/L and 10 μg/L) exposure systems were employed to investigate the effects of six spherical polystyrene (PS) MNPs particle diameters (80, 100, 200, 500, 800, and 1000 nm) on the grain quality of rice (Oryza sativa L.), and to further elucidate the underlying metabolic mechanisms. The results indicated that PS MNPs were absorbed through rice roots and transported upward, with concentrations in rice leaves ranging from 297 to 701 μg/g. When exposure to PS MNPs with diameters ≤100 nm (PS ≤ 100 nm), the reactive oxygen species (ROS) levels in the plants increased by 25 %, and glutenin content in the grain decreased by up to 29 %. Metabolomic analysis revealed that glycolysis/gluconeogenesis and pentose and glucuronate interconversion in rice leaves were closely associated with the particle diameters of PS MNPs. Compared to larger particles, exposure to PS ≤ 100 nm downregulated carbohydrate metabolism and decreased the accumulation of sugars (starch, soluble sugars), thereby weakening the plant's defense response. This study demonstrated that PS ≤ 100 nm triggered weaker defense response in rice, leading to greater biotoxicity, thereby deserving particular attention for governing the MNPs risk and crop safety assessment.