Stimulation versus inhibition: The effect of microplastics on pak choi growth

As an exogenous pollutant, microplastics in soils may affect plant performance. In this study, pot experiments were conducted to evaluate the effects of various concentrations of polyethylene (PE) and polypropylene (PP) with different particle sizes on pak choi (Brassica rapa L. ssp. chinensis) growth. Morphological and physiological indicators of pak choi, and soil properties were measured. The toxic effects of microplastics on pak choi growth varied among the plastic type, concentration, and particle size. The effect of PE on pak choi fresh weight was negligible. However, the inhibition rate of 5% (w/w) PE on the total protein (TP) content reached 84.6% (p < 0.01). PP microplastics, especially those with small particle sizes, significantly reduced the fresh weight, leaf number, and TP content of pak choi. PE and PP had no significant impacts on the photosynthetic pigments in pak choi leaves. Pak choi root activity decreased by 6.7%–14.4% after exposure to PP, suggesting mechanical damage may have occurred. Conversely, Pak choi root activity increased by 13.5%–85.5% after exposure to PE. This may improve pak choi growth, thereby counteracting the negative effects of PE exposure. The activities of superoxide dismutase and peroxidase were lower after exposure to PP with a small particle size than the control. A plausible explanation is that the production of reactive oxygen species under PP stress exceeds pak choi tolerance, thereby inhibiting the expression of antioxidant enzymes. In contrast, pak choi resisted the oxidative stress caused by 5% (w/w) PE by improving the activities of antioxidant enzymes. Furthermore, PP and PE may indirectly inhibit the growth of pak choi by reducing soil aeration. These findings provide basic information for evaluating the phytotoxicity of microplastics in soil-vegetable systems.