Tire wear nanoparticles induce systemic metabolic disruptions and heavy metal accumulation in spinach

Tire wear nanoparticles (TWPs) pose an emerging environmental concern in agricultural systems. In this study, TWPs were characterized using dynamic light scattering, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy/energy-dispersive X-ray spectroscopy, revealing predominantly submicron particles (average hydrodynamic diameter 284.6 nm), irregular morphology, and complex composition. The ICP analysis confirmed the presence of Zn, Cu, Cr, Pb, Ni, Cd, and As in the TWP matrix. A controlled pot experiment evaluated the effects of soil and foliar-applied TWPs at concentrations of 0.05–0.5 g kg−1 (soil) or g L−1 (foliar) on spinach (Spinacia oleracea L.). Fluorescence microscopy showed internalization of TWPs via both root and leaf pathways. Low soil concentration (0.05 g kg−1) transiently enhanced growth and metabolite accumulation, whereas higher concentrations (≥0.25 g kg−1 or g L−1) significantly reduced biomass, leaf area, photosynthetic traits, and growth indices. Antioxidant responses showed enzyme-specific modulation, with marked induction of peroxidase and catalase at higher TWP levels and variable superoxide dismutase responses depending on the exposure pathway. Metal analysis revealed concentration-dependent accumulation of Zn and non-essential metals, including Pb and Cd, in leaves, particularly at elevated TWP levels. These findings indicate that TWPs can enter plant tissues through both soil and foliar routes, alter morpho-metabolic processes, and promote metal accumulation, posing potential risks to food safety.