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Phenotypic and Physiological Responses of Rice Seedlings to Co-Exposure of Polystyrene Microplastics and Heavy Metals
Summary
Scientists found that microplastics in soil don't just add their own contamination risk to crops like rice, they can actually change how toxic heavy metals like lead and cadmium affect the plant, making cadmium's harm milder but making lead's harm worse. This matters because farmland is increasingly exposed to both microplastics and heavy metal pollution together, and this study suggests we can't predict crop safety by looking at these contaminants separately, their combined effects are more complex and need further research to understand how they might impact the food we eat.
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.