Polyethylene Microplastics Exert Dose-Dependent Effects on the Growth, Physiology, and Rhizosphere Microbiome of Persicaria capitata

Microplastic pollution has become a major environmental challenge for terrestrial ecosystems; however, the mechanisms by which it affects the growth of medicinal plants and their rhizosphere microecology remain unclear. This study selected the important medicinal herb P. capitata as the research subject. A pot experiment was conducted to investigate the effects of different concentrations (0%, 1%, 4%, and 8% w/w) of polyethylene microplastics (PE-MPs) on its growth performance, physiological responses, and rhizosphere bacterial communities. The main findings are as follows: (1) PE-MPs exhibited a hormetic effect on the biomass of P. capitata. A low concentration (1%) slightly promoted total biomass (44.78 g) compared to the control (40.94 g), whereas higher concentrations caused significant inhibition. Total biomass decreased by 28.5% and 53.5% under the 4% and 8% treatments, respectively, indicating that the transition from stimulation to inhibition occurred between the 1% and 4% PE-MPs. (2) Chlorophyll a was more sensitive to stress, with its content significantly increasing under the 1% treatment but declining at higher concentrations. (3) Regarding the antioxidant system, POD activity was significantly inhibited at low and medium concentrations, while MDA content increased significantly only under the 8% treatment (by 72%). SOD and CAT showed no significant changes, indicating that POD and MDA were more sensitive indicators of oxidative damage. (4) PE-MPs significantly reduced rhizosphere bacterial community richness (Chao1 index), and the loss of microbial diversity was highly coupled with the decline in plant biomass and the exacerbation of oxidative damage. In conclusion, this study confirms a threshold effect in the toxicity of PE-MPs to P. capitata, with the transition from hormetic stimulation to toxic inhibition occurring between 1% and 4% (w/w). Furthermore, rhizosphere microecological imbalance is identified as a key indirect mechanism underlying phytotoxicity. These findings provide a new theoretical basis for understanding the potential impacts of microplastic pollution on medicinal plants and agroecosystems.