Phenotypic and Physiological Responses of Rice Seedlings to Co-Exposure of Polystyrene Microplastics and Heavy Metals

The co-occurrence of microplastics (MPs) and heavy metals in agricultural ecosystems poses emerging threats, yet their combined ecotoxicological effects on crop plants remain poorly understood. To address this gap, a hydroponic exposure experiment was conducted to evaluate the individual and combined effects of 50 mg·L^-1 polystyrene (PS) microplastics, lead (Pb, 35 mg·L^-1), and cadmium (Cd, 20 mg·L^-1) on the phenotypic growth, biomass accumulation, and peroxidase (POD) activity of rice seedlings. The results indicated that: (1) Individual polystyrene microplastics (PS-MPs) treatment did not induce morphological inhibition; rather, it exhibited a growth-promoting trend, with a significant increase in fresh weight and a non-significant increasing trend in dry weight compared to the control. (2) The phenotypic impact of PS-MPs on the toxicity of heavy metals was element-specific. In the PS + Cd co-exposure system, microplastics significantly alleviated Cd-induced inhibition of fresh weight relative to the single Cd treatment, although this recovery effect was not observed in dry weight. Conversely, in the PS + Pb system, microplastics aggravated the phenotypic toxicity of Pb, with biomass showing a numerical decrease relative to the single Pb treatment, though the difference did not reach statistical significance. (3) The plant antioxidant system exhibited organ-specific responses to the combined stresses. Under PS + Pb co-exposure, root POD activity was significantly up-regulated while shoot POD activity was notably suppressed, revealing an asynchrony in physiological responses between roots and shoots. In contrast, the decrease in root POD activity under the PS + Cd system was consistent with phenotypic recovery in fresh weight. In conclusion, PS-MPs can significantly alter the phenotypic and physiological responses of rice seedlings to heavy metals, with the direction of modulation being element-specific. While the underlying mechanisms require further elucidation, this study provides a phenotypic and physiological basis for assessing the early ecological risks associated with the co-exposure of microplastics and heavy metals.