Application of ZnO Nanoparticles Encapsulated in Mesoporous Silica on the Abaxial Side of a Solanum lycopersicum Leaf Enhances Zn Uptake and Translocation via the Phloem

Abstract

Foliar application of nutrient nanoparticles (NPs) is a promising strategy for improving fertilization efficiency in agriculture. Phloem translocation of NPs from leaves is required for efficient fertilization but is currently considered to be feasible only for NPs smaller than a cell wall pore size exclusion limit of <20 nm. Using mass spectrometry imaging, we provide here the first direct evidence for phloem localization and translocation of a larger (∼70 nm) fertilizer NP comprised of ZnO encapsulated in mesoporous SiO₂ (ZnO@MSN) following foliar deposition. The Si content in the phloem tissue of the petiole connected to the dosed leaf was ∼10 times higher than in the xylem tissue, and ∼100 times higher than the phloem tissue of an untreated tomato plant petiole. Direct evidence of NPs in individual phloem cells has only previously been shown for smaller NPs introduced invasively in the plant. Furthermore, we show that uptake and translocation of the NPs can be enhanced by their application on the abaxial (lower) side of the leaf. Applying ZnO@MSN to the abaxial side of a single leaf resulted in a 56% higher uptake of Zn as well as higher translocation to the younger (upper) leaves and to the roots, than dosing the adaxial (top) side of a leaf. The higher abaxial uptake of NPs is in alignment with the higher stomatal density and lower density of mesophyll tissues on that side and has not been demonstrated before.

Keywords: Nanofertilizer; ZnO nanoparticles; foliar application; mesoporous silica nanocarriers; phloem translocation.

Figure 1

Schematic of a leaf cross-section of a dicotyledon illustrating the structural differences between abaxial and adaxial sides, adapted from Husted et al. Possible nanoparticle translocation pathways are shown by the purple lines: A, apoplastic pathway; B, symplastic pathway.

Figure 2

Characterizations of ZnO@MSN. High-resolution TEM images and EDS analysis of (a) synthesized ZnO@MSN and (b) bare ZnO NPs. EDS analysis was performed for both the core and the shell of ZnO@MSN NPs. Size distribution of (c) ZnO and (d) SiO₂ shell of ZnO@MSN measured by spICP-MS at the initial time and after 5 days of contact in a 10 mM KNO₃ and 10 mM citrate buffer at pH 5 (averaged size distribution of triplicate samples). The dissolution experiment was conducted in triplicate, and only the mean sizes are shown.

Figure 3

Zn distribution in different plant parts (dosed leaf, upper leaf, lower leaf, stem, and root) 2 and 5 days after foliar application of ZnO@MSN to the adaxial or abaxial side of a leaf. For each treatment, the same amount of total Zn, 40 μg, was dosed by drop deposition to the second-youngest (i.e., second-highest) leaf. The total amount of Zn measured for each treatment can be found in Table S2. Error bars are standard deviations of the mean (n = 3). The raw data used to calculate the percentage of Zn are presented in Figure S2.

Figure 4

ESEM images on the stomatal entry of ZnO@SiO₂. Representative large area scan (a) and zoomed-in scan (b) of SEM image taken on the abaxial surface of a leaf, 6 h after ZnO@MSN treatment and elemental mapping of (c) Si and (d) Zn in the same region of image b, using EDS analysis.

Figure 5

Total ion, Si, and Zn distribution in vascular bundle of the dosed leaf plant petiole. TOF-SIMS images of total ion and Si distribution in (a) phloem and (b) xylem tissues. The TOF-SIMS images show the count after 0, 150, and 300 s of sputtering time, as well as a combined image showing the total count over all sputter cycles. Ion counts are shown at the bottom of the ion maps. The scale bars shown in the TOF-SIMS images in a and b are 100 μm. (c) The regions for TOF-SIMS analysis were marked with a white dotted box in a microscope image (10× magnification), of the same sample. The vascular bundles are indicated with white oval outlines to aid the eye. (d) LA-ICP-MS image of Zn near the phloem and xylem tissues of the petiole connected to a leaf dosed with ZnO@MSN. (e) The area of the LA-ICP-MS scan with the phloem (blue) and xylem (yellow) regions. (f) Overlay of the LA-ICP-MS Zn signal (in red) with microscopy image e, the area of the sample that was analyzed by LA-ICP-MS in d is marked with a red box in (f).

Figure 6

Particle size distribution of ZnO core and SiO₂ shell in different parts of the plant as measured by spICP-MS in different parts of the tomato, 5 days after dosing ZnO@MSN. ZnO size distributions for ZnO@MSN dosed to (a) the adaxial side of the leaf and (b) the abaxial side. SiO₂ size distributions for ZnO@MSN dosed to (c) the adaxial side of the leaf and (d) the abaxial side. The sizes of ZnO were shown assuming spherical NPs, whereas the sizes of SiO₂ were calculated assuming that the SiO₂ particle is a hollow shell structure. spICP-MS measurements were conducted in triplicates, and only the mean sizes are shown.