Bioaccumulation of Tire Wear Nanoparticles in Spinach (Spinacia oleracea L.) Leaves: Consequences for Metabolic Dysregulation and Growth Inhibition

Tire wear nanoparticles (TWPs) are emerging environmental contaminants with largely unknown effects on crops, necessitating an investigation into their uptake, toxicity, and impact on plant metabolism and metal accumulation. In this study, we characterized TWPs using dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX), revealing their predominantly nanoscale size (average 284.6 nm), irregular morphology, and complex chemical composition, including organometallic phases and trace heavy metals. A controlled pot experiment was conducted to investigate the effects of soil and foliar-applied TWPs at various concentrations (0.05–0.5 mg/kg or mg/L) on spinach ( Spinacia oleracea L.). Fluorescence microscopy confirmed internalization of TWPs through both root and leaf pathways. Low soil concentrations (0.05 mg/kg) transiently enhanced growth and metabolite accumulation, likely reflecting a hormetic response, whereas higher concentrations (≥ 0.25 mg/kg or mg/L) significantly suppressed biomass, leaf area, photosynthetic traits, and growth indices. TWPs induced oxidative stress, as indicated by elevated antioxidant enzyme activities at low doses and their decline at higher concentrations. Metal analysis revealed increased accumulation of Zn, Pb, and Cd in leaves, particularly under higher TWP exposure, highlighting the risk of metal contamination via nanoparticle-mediated transport. Overall, this study demonstrates that TWPs can enter plant tissues via both soil and foliar routes, disrupt morpho-metabolic processes, and facilitate heavy metal accumulation, underscoring their potential to impair crop productivity and pose risks to food safety in contaminated environments.