Zn₃(PO₄)₂ shell effects on Zn uptake and cellular distribution of root applied ZnO NPs

Touted benefits of nanoparticle-based fertilizers include enhancing crop nutrition by fortifying fruits or grains with nutrient metals and reducing environmental impacts of fertilizer use. However, the properties of the nanoparticles (NPs) and application routes required to achieve these benefits are not yet established. This study examined how a Zn-phosphate shell on ZnO NPs (ZnO_Ph NPs) affected root uptake, cellular distribution, transformation, and translocation of Zn in pepper plants (Capsicum annuum), and compared the efficacy of root- to foliarly-applied NPs. Pepper plants roots were exposed to ZnO NPs (26 ± 8 nm), ZnO_Ph NPs (48 ± 12 nm), or ionic Zn. After 6 weeks, 30-37% of root-applied Zn was absorbed, with 6.0-7.2% (2.4-2.9 μg) reaching the fruits. ZnO_Ph NPs resulted in lower total Zn uptake, but higher mobility into the root vasculature and stem epidermis, likely due to P-Zn co-delivery modulating translocation mechanisms. Foliar application of these NPs led to lower Zn uptake (2.4% for ZnO_Ph NPs; 0.5% for ZnO NPs) compared to root application. However, a greater proportion of the Zn that was taken up for foliar-applied ZnO_Ph NP translocated to the fruits (27%) compared to root application (10%). Root and foliar applications also led to contrasting Zn speciation in the stem vasculature. Foliar-applied Zn formed only carboxyl and phytate-like complexes, while root-applied Zn also formed Zn-S-R complexes, indicating distinct Zn transport and storage responses, possibly explaining the higher relative mobility to the fruits when foliar-applied. These findings demonstrate that Zn uptake efficiency and speciation depend on both application method and nanoparticle formulation. They also suggest that multi-nutrient NPs can fortify foods, potentially offering a new strategy for improving plant nutrition.