Hydrophobic microplastics with a relatively large surface area can act as carriers for pollutants and exert a series of indirect effects on crop plants. This study investigated the toxic effects of small polystyrene (SPS, 100-1000 nm) and large polystyrene (LPS, >10,000 nm) microplastics, on lettuce under di-butyl phthalate (DBP) stress. The results indicated that single SPS, LPS, and DBP treatments significantly decreased lettuce biomass, and induced oxidative stress and damaged lettuce leaves and roots. According to Gaussian analysis, SPS or LPS could interact with DBP through van der Waals force, thereby reducing lettuce biomass and DBP enrichment in roots and leaves under combined treatments, increasing antioxidant enzyme activities and exacerbating oxidative stress and subcellular damage, compared to single DBP treatments. Observation using scanning electron microscopy demonstrated that polystyrene (PS) adhered to the root surfaces, which, in turn, caused physical blockage of the root pores. Cell membrane and wall damage was observed during PS and/or DBP exposures, as identified by transmission electron microscopy. Molecular docking illustrated that DBP and monobutyl phthalate could interact with superoxide dismutase residues through hydrogen bonding, π-π stacking, alkyl conjugation, and van der Waals forces. Interestingly, there were no statistical differences between the phytotoxicity of nano- and microplastics to lettuce. These findings showed that PS aggravated DBP-induced phytotoxicity.
Keywords: Combined pollution; Oxidative damage; Polystyrene particles; Toxic mechanism; di-butyl phthalate.
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