Polypropylene Microplastics and Cadmium: Unveiling the Key Impacts of Co-Pollution on Wheat–Soil Systems from Multiple Perspectives

The interaction between microplastics (MPs) and heavy metals and their ecological risks to the soil–plant system has attracted widespread attention. This study explored the effects of polypropylene (PP) alone or combined with cadmium (Cd) pollution on wheat seed germination, plant growth, and the soil environment from multiple perspectives through seed germination experiments and pot experiments. The results of the seed germination experiment showed that the addition of 50 mg L−1 PP could promote the growth of seeds. However, medium and high concentrations of PP had significant inhibitory effects on seeds. For PP + Cd co-pollution, the addition of 50 mg L−1 PP could partially alleviate the stress of Cd alone. However, with the increase in PP concentration, the co-pollution showed stronger toxicity to seeds. Moreover, the synergistic effect of PP and Cd was greater than the antagonistic effect; both of them aggravated the stress on wheat. The results of the pot experiment showed that the soil microenvironment was significantly affected by PP alone or combined with Cd pollution. It was manifested as reducing soil moisture and pH, affecting soil nutrient cycling, and inhibiting the activities of soil enzymes (except for catalase). In addition, the MPs and Cd significantly affected the physiological characteristics of plants. Specifically, the addition of 50 mg L−1 PP alone promoted or had no significant effect on wheat growth. However, with the increase in PP concentration, the biomass and chlorophyll content of plants decreased significantly, while carotenoids, oxidative damage, and antioxidant enzyme activities increased significantly. Moreover, PP + Cd co-pollution led to stronger phytotoxicity. Moreover, PP exposure caused an increase in plant shoot and root Cd concentrations, promoting Cd transport from roots to shoots. Correlation heat maps and RDA analysis revealed that plant Cd concentration was significantly correlated with soil environmental factors and plant physiological indicators. Finally, the results of the linear model (%) of relative importance indicated that pH and MDA content were important soil and plant variables affecting the increase in Cd concentration in plant tissues. This study is of great significance for evaluating the ecological risks of MPs-Cd composite pollution.