Integrated Physiological, Transcriptomic, and Metabolomic Analyses Reveal Coordinated Responses of the Invasive Plant Pistia stratiotes to Copper Oxide Nanoparticles

To characterize the responses of Pistia stratiotes to copper oxide nanoparticles (CuO NPs), plants were exposed to CuO NPs at nominal Cu concentrations of 0 (CK), 0.1 (T1), 1.0 (T2), 5.0 (T3), and 15.0 (T4) mg L−1. Cu accumulation and translocation, growth, photosynthetic performance, reactive oxygen species (ROS), antioxidant responses, and molecular profiles were assessed. CuO NP exposure increased root Cu accumulation and reduced Cu translocation to leaves. Relative to CK, the bioconcentration factor (BCF) increased by 301.93% at T1 but decreased by 61.65% and 73.69% at T3 and T4, respectively. At T1, leaf chlorophyll and carotenoid contents increased, while photosynthetic performance and most chlorophyll fluorescence parameters remained largely unchanged. Nevertheless, root dry weight, relative growth rate, and stress tolerance index declined. At T3 and T4, net photosynthetic rate (Pn) decreased by 42.62–60.88% and 52.21–60.86%, respectively, accompanied by increased superoxide (O2·−) levels. Antioxidant enzyme activities also responded to exposure, while glutathione (GSH) and cysteine (Cys) contents increased at higher concentrations. Transcriptomic and metabolomic analyses of leaves after 14 d were conducted for CK, T1, and T3. The CK-vs.-T1 and CK-vs.-T3 comparisons identified 588 and 1088 differentially expressed genes (DEGs) and 166 and 352 differentially accumulated metabolites (DAMs), respectively. Comparison of transcriptomic and metabolomic results suggested associations with pectin-related cell wall processes, carotenoid biosynthesis, and hormone-related metabolism, including changes in the expression of NCED1 and other related genes, as well as in ABA-associated metabolites. Overall, the results reveal contrasting physiological responses to low and higher CuO NPs expose concentrations and identify cell wall-, carotenoid-, and hormone-related pathways for further functional investigation.