Foliar zinc oxide nanoparticles for cereal biofortification: dose–response and quality

Zinc (Zn) deficiency remains a major nutritional constraint in populations that rely heavily on cereal-based diets, while the low phytoavailability of Zn in many agricultural soils limits the effectiveness of conventional fertilization. This review therefore evaluates whether foliar-applied zinc oxide nanoparticles (ZnO NPs) can provide a more efficient strategy for cereal biofortification while maintaining crop safety and grain quality. Specifically, we critically synthesize evidence on foliar ZnO NP uptake, translocation, biotransformation, dose-response behavior, effects on yield and grain quality, and potential ecotoxicological risks in cereal systems. The reviewed literature indicates that foliar ZnO NPs can increase grain Zn concentration, improve selected yield components, and reduce the phytic acid:Zn molar ratio when applied within an appropriate dose window. These benefits appear to arise from a combination of gradual Zn release, improved leaf retention, vascular transport of Zn species, and stress-mitigation effects. However, responses are strongly dependent on crop species, cultivar, particle properties, spray concentration, exposure duration, and environmental conditions. Excessive doses may cause oxidative stress, disrupt soil and aquatic organisms after off-target movement, and create uncertainty regarding long-term fate. By integrating findings from agronomy, plant physiology, materials science, and toxicology, this review identifies foliar ZnO NPs as a promising but dose-sensitive tool for cereal Zn biofortification and highlights the need for standardized field trials, clear application-rate reporting, and risk assessment before broad agricultural deployment.