Combined effects of micro/nanoplastics and ZnO nanoparticles on lactuca sativa seedlings under varied lighting

Micro/nanoplastics (MNPs) can serve as vectors for engineered nanoparticles (ENPs), thereby eliciting mixture toxicological responses in target species. However, our understanding of the combined environmental impacts of lighting conditions and co-exposure to MNPs and ENPs on crop plants remains limited. Herein, we investigated the individual and joint effects of polystyrene microplastics (PMs; 1 μm) or nanoplastics (PNs; 50-100 nm) and ZnO nanoparticles (ZnO NPs) on seed germination and early seedling growth of lettuce (Lactuca sativa) under the dark condition or the light-dark cycle condition. The results showed that the joint toxic effects of PMs/PNs and ZnO NPs on seed germination were antagonistic, regardless of lighting conditions. Furthermore, the coexistence of PMs/PNs attenuated the inhibitory effect of ZnO NPs on the total photosynthetic pigment content in seedling shoots. Under the light-dark cycle condition, the translocation factor values of zinc from seedling roots to shoots when exposed to PMs or PNs with ZnO NPs exceeded 1, indicating that zinc in the form of ZnO NPs easily migrated from seedling roots to shoots. When ZnO NPs were combined with either PMs or PNs under varying light conditions, a consistent alleviation of oxidative stress was observed in the seedlings compared to individual particle treatments. This finding may represent a key cellular mechanism underlying the antagonistic effects between PMs/PNs and ZnO NPs. Additionally, the expression of Cu/Zn-superoxide dismutase gene in seedlings was differentially modulated in response to both individual and combined exposures to PMs/PNs and ZnO NPs. Collectively, this study provides comprehensive insights into the cellular and molecular responses of agricultural crops under complex environmental conditions.