Synergistic effects of irregular PET microplastics and cadmium on cucumber seedlings revealed by LA-ICP-MS imaging

Microplastics (MPs) and heavy metals are widespread environmental contaminants. Their co-exposure may pose greater risks to plants than either pollutant alone. Since their toxicity depends on tissue-specific distribution, precise imaging of both in plant tissues is critical for uncovering toxicity mechanisms. In this study, we used LA-ICP-MS imaging to investigate the effects of irregular polyethylene terephthalate (PET) MPs of ca. 200 nm labeled by europium chelate, spherical polystyrene (PS) MPs and cadmium (Cd), on cucumber seedlings. Following hydroponic exposure to 20 mg/L MPs, or in combination with 0.05 or 0.5 mg/L Cd for 5 days, imaging revealed preferential accumulation of MPs at leaf margins, while Cd distributed uniformly along vascular bundles. High-Cd levels promoted microplastic translocation to shoots, with an 87% increase for PS and 47% for PET, whereas co-exposure reduced net Cd accumulation in leaves by 71-76%. Irregular PET exhibited greater tissue accumulation and stronger synergism with Cd than spherical PS. All treatments decreased leaf area and chlorophyll content, with the greatest reductions under co-exposure: leaf area declined by 40% and chlorophyll content by 55%, while root length and stem height remained unchanged. Metabolomics identified persistent glutathione depletion as the primary oxidative stress indicator, accompanied by treatment-specific reprogramming of carbon metabolism, amino acid biosynthesis, and phenylpropanoid pathways. These findings demonstrated that microplastic morphology is a critical determinant of phytotoxicity and microplastic-metal synergism, with direct implications for environmental risk assessment of realistic plastic contaminants in food crop systems.