Physiological and metabolomic responses of pepper (Capsicum annuum L.) to polyethylene microplastics and cadmium stress

Combined contamination of microplastics (MPs) and heavy metals in agricultural soils is an emerging environmental concern, yet crop responses to long-term co-exposure remain insufficiently characterized. This study aimed to determine how polyethylene microplastics (PE-MPs) (0.1%, 1%, 5%, and 10% w/w) influence cadmium (Cd) behavior and plant performance under a fixed Cd background in a soil-crop system. We conducted an outdoor pot experiment using pepper ( Capsicum annuum L.) grown in Cd-contaminated soil (5 mg kg −1 ) and quantified Cd accumulation and translocation, growth traits, oxidative-stress indicators, and organ metabolomic profiles at the fruiting stage. PE-MPs significantly reduced Cd accumulation across organs (root > stem > leaf > fruit), with the strongest inhibition in roots; at the highest PE-MPs levels, Cd in the edible fruit declined to below the food-safety threshold. Growth responses were concentration-dependent: low PE-MPs levels were associated with higher biomass relative to the Cd-only treatment, whereas high levels reduced biomass and intensified oxidative stress, as indicated by elevated antioxidant enzyme activities. Untargeted metabolomics revealed dose-dependent and organ-specific metabolic perturbations. Roots showed suppression of energy-demanding secondary metabolism alongside enrichment of lipid, cutin-related pathways, whereas fruits exhibited broader reprogramming with enrichment of ABC transporters, glutathione metabolism, and nitrogen-related pathways, indicating altered detoxification- and redox-related metabolism rather than evidence of organ-differentiated adaptive strategies. Overall, these results demonstrate that under Cd-background conditions, PE-MPs can modify Cd uptake and internal allocation and are associated with organ-specific metabolic perturbations in pepper.