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

Microplastics can modify metal behavior in aquatic environments, but how polypropylene (PP) microplastics influence organ-level copper (Cu) accumulation and coordinated 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. After 28 days, combined Cu–PP exposure produced the greatest chlorophyll loss, with total chlorophyll declining to approximately 63.9% of the control level. Cu accumulated predominantly in roots under both Cu-containing treatments. Compared with Cu exposure alone, combined exposure substantially decreased root Cu accumulation but slightly increased leaf Cu concentration, and a higher leaf-to-root Cu concentration ratio than Cu exposure alone. Lignin responses were organ specific: root lignin was highest under combined exposure, whereas leaf lignin was highest under CU and CP, which did not differ significantly; both were significantly higher than PP and CK. Transcriptomic and metabolomic analyses showed that combined exposure generated a distinct response pattern involving chlorophyll and porphyrin metabolism, light-harvesting complexes, phenylpropanoid and flavonoid pathways, hormone signaling, transport, energy metabolism, and redox-related regulation. Combined Cu–PP exposure was associated with a distinct physiological and multi-omics response characterized by pronounced pigment loss, altered organ-level Cu accumulation, and contrasting root and leaf lignin patterns. 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.