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The coexistence of microplastics (MPs) and heavy metals (Cd, Pb) in agricultural soils has become a global environmental and ecological risk. In this study, a pot experiment was conducted to investigate the effects of different concentrations of polyethylene (PE) microplastics and combined Cd/Pb contamination on the growth and development, heavy metal accumulation, and antioxidant system of tobacco (Nicotiana tabacum L. cv. Yunyan 87). The results showed that low-dose PE and low concentrations of heavy metals had minor impacts on tobacco growth and the antioxidant system; in contrast, high-dose PE and elevated heavy metal treatments markedly induced increases in malondialdehyde content (MDA) and enhanced the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). Under co-contaminated conditions, the addition of low-dose PE reduced the translocation capacity of heavy metals, alleviated heavy metal-induced oxidative stress responses, and promoted tobacco growth. Conversely, high-dose PE promoted the translocation of Cd into tobacco plants and increased Cd contents in tobacco leaves, leading to marked decreases in soluble protein and soluble sugar contents, and causing severe reductions in plant height, number of functional leaves, and biomass. Structural equation modeling (SEM) analysis revealed that the direct effect of PE on tobacco growth was not significant; instead, it primarily acted as a regulatory factor, exerting either promotional or inhibitory effects on tobacco growth at different doses. The impact of Cd/Pb on tobacco growth appeared to involve two potential pathways. On the one hand, Cd/Pb induced direct toxicity through their accumulation within tobacco tissues. On the other hand, they exerted indirect regulation primarily by modulating the activities of the tobacco antioxidant system.
Guan et al. (Fri,) studied this question.