Combined exposure to copper and polypropylene microplastics alters metal partitioning, chlorophyll homeostasis and phenylpropanoid metabolism in Nymphaea ‘Black Beauty’

Background Microplastics can modify metal behavior in aquatic environments, but how polypropylene (PP) microplastics influence copper (Cu) partitioning and plant stress responses remains poorly understood. This study investigated the physiological, elemental, transcriptomic, and metabolomic responses of Nymphaea ‘Black Beauty’ to Cu, PP, and combined Cu-PP exposure. Results This study examined physiological, elemental, transcriptomic, and metabolomic responses of Nymphaea ‘Black Beauty’ after 28 days of exposure to Cu, PP, and combined Cu-PP stress. Combined exposure caused the greatest chlorophyll loss, with total chlorophyll declining to approximately 63.9% of the control level. PP markedly reshaped Cu partitioning: Cu alone was mainly retained in roots, whereas Cu–PP exposure reduced root Cu accumulation but increased leaf Cu content and root-to-leaf translocation. Lignin responses were organ-specific, with enhanced root lignification but reduced leaf lignin accumulation. Multi-omics analyses showed that combined Cu-PP exposure induced a distinct stress state involving chlorophyll/porphyrin metabolism, light-harvesting complexes, phenylpropanoid and flavonoid biosynthesis, redox regulation, hormone signaling, and transport pathways. Conclusions PP microplastics reshaped Cu distribution and shifted the primary injury site from roots to leaves, intensifying chloroplast disturbance and defense reprogramming. These findings highlight the need to consider organ-specific metal exposure and multi-omics responses when assessing microplastic–metal co-contamination risks in aquatic plants.