X-rays shed light on enhancing zinc uptake in pepper plants

Zinc (Zn) is essential for both plants and humans, supporting growth and overall health. In agriculture, zinc deficiency in soil significantly impacts crop quality, often requiring application of zinc fertilizers. However, traditional soil application methods have low zinc uptake efficiency, leading to potential soil and groundwater pollution [1].

To address these issues, researchers are exploring the application of zinc-based nanoparticles (NPs) directly to plant leaves (i.e., foliar application) for controlled, sustained zinc uptake, which could potentially mitigate the adverse effects of soil fertilization. Despite the potential of this approach, the mechanisms underlying NP foliar uptake, translocation within plants, and their persistence were not fully understood. In particular, the influence of NP surface modifications on these processes required further investigation [2]. 

An international team of researchers studied the bioavailability of different zinc species following foliar application of zinc nanoparticles (Zn-NPs) to pepper plants. They also explored how the Zn-NPs interact with the leaves and their pathways of uptake and translocation from leaves to stem. Pepper plants were exposed to two types of NPs: bare zinc oxide (ZnO) NPs and ZnO NPs coated with a zinc-phosphate (Zn₃(PO₄)₂) shell (ZnO_Ph NPs). Leaf and stem samples were collected two hours and one week after foliar application to track the evolution of zinc internalization and translocation in the plant cells.

To preserve element distribution and speciation, cross-sections of hydrated plant tissue samples were embedded in optimal cutting temperature (OCT) resin and then flash frozen using liquid nitrogen (Figure 1). Vertical cross-sections (20 µm thick) were analyzed using micro X-ray fluorescence (μ-XRF) maps and micro X-ray absorption near-edge structure (μ-XANES) spectroscopy measurements at ID21.

Fig. 1: Experimental setup for leaf sampling and cross-sectioning. Pepper plant grown in sand matrix (a), punched exposed leaf (b), flash-freezing of the punched sample in OCT resin (c), cryo-sectioning on microtome (d) and leaf cross-section (e).

The results revealed clear differences in how zinc accumulated and transformed in pepper plants treated with ZnO NPs compared to those treated with ZnO_Ph NPs, affecting both the leaves and stems. ZnO NP treatment resulted in high zinc accumulation in the lower epidermis cell walls and cytosol of leaves (Figure 2a), as well as in the stem epidermis one week after exposure.

In contrast, plants treated with ZnO_Ph NPs showed zinc accumulation mainly in leaf and stem vasculature, suggesting phloem loading as the primary uptake mechanism for this treatment. These findings indicate that the surface chemistry of the nanoparticles may cause differences in how zinc is distributed, stored, or transformed within the leaves of pepper plants.
 

Fig. 2: In planta transformation in pepper plant leaf after one week of exposure to ZnO NPs. µ-XRF mapping of exposed leaf (a) and principal component (PC) analysis of the 2nd derivative of XANES done on selected POIs in selected leaf compartment (b).

μ-XANES spectra obtained from various points of interest (POIs) on each sample revealed distinct transformations of both ZnO-based NPs within different cellular compartments and over time. Within two hours of exposure, zinc from ZnO_Ph NPs was associated to thiol groups, indicating binding to metalloproteins and phloem loading [3], and suggesting higher mobility compared to zinc from ZnO NPs, which was primarily associated to cell walls (i.e., Zn-carboxyl binding).

After one week of exposure to ZnO NPs, zinc speciation varied depending on the compartment analyzed (Figure 2b). Zinc in the upper and lower epidermis remained bound to carboxyl groups, while in the palisade mesophyll, it was associated with thiol groups, and in the spongy mesophyll, partly with phosphates.

These differences in cellular zinc internalization were attributed to the presence or absence of phosphate on the NPs, influencing zinc translocation to other plant organs, likely due to modulation of plant active transport by these elements.

This study shows that the Zn-phosphate shell notably boosts zinc foliar uptake, accelerating phloem loading compared to uncoated ZnO NPs. This discovery offers avenues for enhancing nutrient delivery in plants, thus bolstering crop resilience. These results highlight the importance of surface coatings in nanoparticle design to fine-tune plant–metal interactions and optimize nutrient bioavailability. Further research into the application of phosphate-containing NPs and their effects on plant nutrient uptake mechanisms is essential to fully harness their potential benefits in agriculture.

Principal publication and authors
Effect of a Zinc Phosphate Shell on the Uptake and Translocation of Foliarly Applied ZnO Nanoparticles in Pepper Plants (Capsicum annuum), S. Rodrigues (a), A. Avellan (b,d), G.D. Bland (c), M.C.R. Miranda (d), C. Larue (e), M. Wagner (b,e), D.A. Moreno-Bayona (e), H. Castillo-Michel (f), G.V. Lowry (c), S.M. Morais (a) Environ. Sci. Technol. 58, 7 (2024); https://doi.10.1021/acs.est.3c08723
(a) Centre for Environmental and Marine Studies (CESAM), Department of Environment and Planning, Universidade de Aveiro, Aveiro (Portugal)
(b) Géosciences Environnement-Toulouse (GET), CNRS, UMR 5563 CNRS, UT3, IRD, CNES, OMP, Toulouse (France)
(c) Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania (USA)
(d) Centre for Environmental and Marine Studies (CESAM), Department of Chemistry, Universidade de Aveiro, Aveiro (Portugal)
(e) Centre de Recherche sur la Biodiversité et l’Environnement (CRBE), Université de Toulouse, CNRS, IRD, Toulouse INP, Toulouse (France)
(f) ESRF

References 
[1] N.C.T. Martins et al., ACS Appl. Nano Mater. 3, 2134-2148 (2020).
[2] A. Avellan et al., Environ. Sci. Technol. 55, 13417-13431 (2021).
[3] M. Hamzah Saleem et al., Frontiers Plant Sci. 13, 1033092 (2022).
 
 

About the beamline: ID21

ID21 is an X-ray micro-spectroscopy beamline dedicated to micro X-ray fluorescence (μXRF) and micro X-ray absorption near-edge structure (μXANES) spectroscopy. The scanning X-ray microscope can be operated in an energy range from 2-11 keV, thus giving access to the K-edges from phosphorus to zinc, and to the L- and M-edges of some heavier elements. As well as being used to analyze ancient and artistic materials, ID21 has been a pioneer in the use of X-ray microspectroscopy applied to environmental and earth sciences. The range of materials analyzed at the beamline spans a broad spectrum of research areas, from nano-engineered materials to soils and plants. In the field of environmental and earth sciences, a primary goal is to track the distribution (localization) and transformation (speciation) of these materials in the environment. Therefore, ID21 can provide detailed information on the submicron scale and microspatial distribution of nanoparticle nutrients and pollutants, as well as how these elements transform (including potential bioavailability or toxicity) within the environment.