Effect of a Zinc Phosphate Shell on the Uptake and Translocation of Foliarly Applied ZnO Nanoparticles in Pepper Plants (Capsicum annuum)

Abstract

Here, isotopically labeled ⁶⁸ZnO NPs (ZnO NPs) and ⁶⁸ZnO NPs with a thin ⁶⁸Zn₃(PO₄)₂ shell (ZnO_Ph NPs) were foliarly applied (40 μg Zn) to pepper plants (Capsicum annuum) to determine the effect of surface chemistry of ZnO NPs on the Zn uptake and systemic translocation to plant organs over 6 weeks. Despite similar dissolution of both Zn-based NPs after 3 weeks, the Zn₃(PO₄)₂ shell on ZnO_Ph NPs (48 ± 12 nm; -18.1 ± 0.6 mV) enabled a leaf uptake of 2.31 ± 0.34 μg of Zn, which is 2.7 times higher than the 0.86 ± 0.18 μg of Zn observed for ZnO NPs (26 ± 8 nm; 14.6 ± 0.4 mV). Further, ZnO_Ph NPs led to higher Zn mobility and phloem loading, while Zn from ZnO NPs was stored in the epidermal tissues, possibly through cell wall immobilization as a storage strategy. These differences led to higher translocation of Zn from the ZnO_Ph NPs within all plant compartments. ZnO_Ph NPs were also more persistent as NPs in the exposed leaf and in the plant stem over time. As a result, the treatment of ZnO_Ph NPs induced significantly higher Zn transport to the fruit than ZnO NPs. As determined by spICP-TOFMS, Zn in the fruit was not in the NP form. These results suggest that the Zn₃(PO₄)₂ shell on ZnO NPs can help promote the transport of Zn to pepper fruits when foliarly applied. This work provides insight into the role of Zn₃(PO₄)₂ on the surface of ZnO NPs in foliar uptake and in planta biodistribution for improving Zn delivery to edible plant parts and ultimately improving the Zn content in food for human consumption.

Keywords: Capsicum annuum; Zn cellular distribution; Zn speciation; micro X-ray absorption near-edge structure; micro X-ray fluorescence; micronutrient foliar delivery; nanoparticle persistence; phloem loading; spICP-TOFMS.

Figure 1

⁶⁸Zn mass in each plant organ. The ⁶⁸Zn in the nonexposed controls was subtracted from all treatments. Error bars represent the weighted standard deviation of the samples from four replicate plants. Statistically significant differences (p < 0.05) of the means of total ⁶⁸Zn masses between treatments are indicated by different letters (on top of each bar chart). Numbers and asterisks inside each bar represent statistical significance for the stem and the fruit, respectively.

Figure 2

Particle size distribution of NPs containing ⁶⁸Zn measured 1 week after exposure by spICP-TOFMS in exposed leaves (a) and stems (b). Weighted average of the particle sizes detected by spICP-TOFMS in the exposed leaves (after washing) and in stems at 1, 4, and 6 weeks after foliar exposure (c) of pepper plants exposed to ZnO NPs (purple), ZnO_Ph NPs (pink), and in control plants (DIW; yellow). The DIW control contained naturally occurring ⁶⁸Zn particles. Error bars represent the weighted standard deviation of the samples from three replicate plants. Statistically significant differences (p < 0.05) between treatments in exposed leaves are indicated by asterisks and in stem by different letters.

Figure 3

Elemental μ-XRF maps of the seventh leaf of the pepper plant exposed to ZnO_Ph NPs and ZnO NPs: 2 h after exposure (top row); 1 week after exposure (middle row), and on the pepper plant stem near the seventh leaf node 1 week after exposure (bottom row). The fluorescence signals of Zn, K, and Ca are represented in red, blue, and green, respectively.

Figure 4

PCA plots of the XANES spectra done on selected POIs in selected leaf compartments for leaves exposed to ZnO NPs (2 h and 1 week after exposure) or ZnO_Ph NPs (2 h after exposure), and stems of plants exposed to ZnO NPs and ZnO_Ph NPs (1 week after exposure). Linear combination fitting of the groups of these POIs can be found in the Supporting Information (Table S9) (leaf and stem scheme created with BioRender.com).