Microplastic pollution in agriculture: How exposure pathway (Seed, Leaf, Root) dictates phytotoxicity in lettuce (Lactuca sativa L.)

The accumulation of microplastics (MPs) in arable soil poses a significant risk to food safety and crop growth. However, research has often focused on single exposure mechanisms, making it difficult to comprehend the actual environmental effects. This study investigated the effects of polyethylene (PE) microplastics on lettuce (Lactuca sativa L. var. ‘Little Gem Green’) applied via three different pathways (seed, leaf, and root) at different concentrations (0, 50, 250, and 1000 µg/mL). The results demonstrated that the plant exposure technique significantly dictated the phytotoxic outcome. Direct root application, particularly at 1000 µg/mL, was the most detrimental, significantly reducing aerial biomass and height and disrupting the antioxidant defense system. In contrast, seed and leaf administration produced complex responses. Seed treatment (1000 µg/mL) resulted in the shortest plants but the largest leaf areas, indicating an adaptive growth behavior. A possible hormetic effect was observed with 250 µg/mL foliar application, which recorded the maximum plant height. Pathway-dependent differences in photosynthetic pigments, proline accumulation, and antioxidant enzyme activation (e.g., catalase, ascorbate peroxidase) confirmed that plant tolerance and response are highly dependent on the exposure conditions. This comparison of three exposure pathways highlights the variability of plant-MP interactions and stresses the importance of pathway-specific risk assessments for mitigating microplastic pollution in agricultural settings.