Combined Exposure to Polyethylene Microplastics and Copper Affects Growth and Antioxidant Responses in Rice Seedlings

The co-existence of polyethylene microplastics (PE-MPs) and heavy metals in aquatic ecosystems poses emerging threats to crop systems, yet their combined phytotoxic effects remain insufficiently understood. In this study, hydroponic rice (Oryza sativa) seedlings were exposed to PE-MPs (50 mg/L) and copper (Cu, 20 mg/L) individually and in combination. The results showed that PE-MPs alone had no significant impact on shoot or root elongation, while Cu exposure slightly reduced root length (from 6.2 cm in the control to 5.8 cm) without affecting shoot growth (~37 cm). Combined PE+Cu treatment resulted in intermediate biomass values, suggesting that microplastics partially mitigated but did not eliminate Cu toxicity. Antioxidant responses displayed organ specificity: shoot peroxidase (POD) activity dropped sharply from >10,000 U/g in the control to ~1200 U/g under Cu exposure, while root POD activity decreased from >11,000 U/g in the control to ~1500 U/g under combined exposure. Cu accumulation was markedly elevated under co-exposure, reaching ~450 mg/kg, about 25% higher than Cu alone and more than 12 times greater than control. These findings demonstrate that PE-MPs can enhance Cu bioavailability and uptake, thereby intensifying oxidative stress in roots while altering shoot defense responses. The study highlights the ecological risks of microplastic–metal co-contamination in agricultural systems and underscores the need for further investigation into long-term impacts on crop productivity and food safety.