Mechanism of zinc oxide nanoparticle entry into wheat seedling leaves

Nanoparticles (NPs) are increasingly used as agrochemical components through foliar spraying such as foliage fertilizers or pesticides. However, the understanding of the mechanisms of nanoparticle absorption and translocation from the leaf surface is limited. In this study, ZnO NPs (30 nm) labeled with fluorescein isothiocyanate (FITC) were foliar applied to wheat leaf tissues to investigate the process of attachment and absorption. Using laser confocal microscopy, we observed that FITC–ZnO NPs cross the leaf epidermis through the stomata and accumulate first in the apoplast, followed by subsequent transport to mesophyll cells. The Zn concentrations in wheat leaf apoplast and cytoplasm decreased by 33.2% and 8.3% with stomatal aperture diameter reduction, respectively; the apoplastic Zn concentration is influenced more by stomatal aperture than the cytoplasmic Zn level. Scanning electron microscopy with energy-dispersive X-ray analysis was used to map Zn in the wheat leaves and data suggest a different Zn distribution for ZnO NPs and ZnSO4. Zn ions in ZnO NP-treated samples are heterogeneously distributed in comparison with those in ZnSO4-treated samples. The results indicate that the main route to cross the wheat leaf epidermis for ZnO NPs is via the stomata; then these nanoparticles accumulate and release Zn ions in the apoplast, and the released Zn ions and ZnO NPs are absorbed by mesophyll cells. Our findings demonstrate how ZnO NPs cross the wheat leaf epidermis, distribute within mesophyll tissues, and enter into plant cells, and this information is useful for the development of sustainable nano-enabled platforms for nanoscale micronutrient delivery.