Aging increases the phytotoxicity of polyethylene and polypropylene to Lactuca Sativa L. compared to original microplastics

As emerging pollutants, microplastics (MPs) pose serious threats to the terrestrial ecosystems, and the long-term presence of aged MPs in soil results in toxic effects on plant growth. However, the phytotoxicity mechanisms of aged MPs remain unclear. To understand the toxic effects of aged MPs and the response mechanism of lettuce plants, we selected polyethylene (PE) and polypropylene (PP) (commonly found in soil), and then studied the effects of the two phytotoxins on the soil-plant system before and after aging of the MPs. We found that aging enhanced the toxicity of the MPs to the plants. Compared with the original MPs-treatment group, aged PE and PP particles reduced plant biomasses by 26.19%-28.44% and 25.58%-26.13%, respectively, potentially due to the effects of aged MPs on the rhizosphere soil, which further inhibited nutrient absorption in lettuce. The metabolic response of lettuce to MPs was also different. Aged PE significantly attenuated malic acid and proline concentrations in lettuce, and the reduction in these two products inhibited photosynthesis, energy metabolism, and cellular homeostasis, thereby aggravating the damage caused by aged PE. Aged PP principally affected the metabolic pathways of phenylalanine, tyrosine and tryptophan, which was postulated to be the reason why aging enhanced the phytotoxicity of PP. This study provides new insights into the assessment of the toxic effects of MPs, as well as the environmental behavior and ecological risks of aged MPs.