Foliar Application of Humic-Stabilized Nanoferrihydrite Resulted in an Increase in the Content of Iron in Wheat Leaves
Foliar Application of Humic-Stabilized Nanoferrihydrite Resulted in an Increase in the Content of Iron in Wheat Leaves. Zimbovskaya M.M., Polyakov A.Y., Volkov D.S., Kulikova N.A., Lebedev V.A., Pankratov D.A., Konstantinov A.I., Parfenova A.M., Zhilkibaev O.T., Perminova I.V. //Agronomy. 2020. V.10. №12. P.1891.
The objective of this study was to synthesize iron (hydr)oxide nanoparticles (IONPs) stabilized by humic substances, and to estimate the feasibility of their use for foliar application on iron deficient plants. The IONPs were synthesized by rapid hydrolysis of iron(III) nitrate in a solution of potassium humate. The iron speciation and nanoparticle morphologies were characterized using X-ray diffraction, transmission electron microscopy, and Mossbauer spectroscopy. The obtained sample of IONPs was applied at concentrations of 1- and 10-mM Fe, and 0.2% urea was used as an adjuvant. Wheat plants (Triticum aestivum L. cv. L15) were used for the iron uptake test. For both of the concentrations tested, spraying the nanoparticles resulted in a 70–75% higher iron content in wheat leaves compared to ferric ammonium salt of ethylenediaminetetraacetic acid (Fe-EDTA). The synergistic effect of humic substances acting as a surfactant seemed to promote an increase in the iron uptake of the ferrihydrite nanoparticles compared to the aqueous Fe-EDTA solution used in this study. We concluded that humic-stabilized IONPs are much better suited to foliar application as compared to soil amendment when applied as a source of iron for plants. This is because humic substances act as a capping agent for nanoparticles and the surfactants enhance iron penetration into the leaf.
Fe deficiency is a frequent issue in numerous crops, particularly in calcareous soils. This is because of the poor bioavailability of Fe, which produces insoluble Fe(III) oxides and oxohydroxides in soil. At the same time, iron (hydr)oxide nanoparticles (IONPs) measuring < 5–20 nm are the most frequently used as iron-containing nanofertilizers . Bare IONPs are prone to aggregation due to their high surface energy; this tendency is more pronounced near the point of zero charge (PZC), which usually ranges between pH 7 and 9 for synthetic iron oxides, and at elevated salt concentrations. Among the different ligands used for nanoparticle stabilization, humic substances (HS) deserve particular attention as they are eco-friendly natural polyelectrolytes. HS stabilize iron (hydr)oxides in soils, and they are widely used for stabilization of synthetic IONPs . Substantial efforts have been undertaken in the last decade to combine the benefits of HS and IONPs in designing eco-friendly nanofertilizers  that meet the demands of sustainable chemistry. Despite the progress made in this area, and well documented increases in iron uptake by plants, the efficacy of HS-based iron nanofertilizers remains much lower compared to that of iron chelates. The main reason is aggregation of HS-stabilized IONPs on the root surface upon soil application as well as interaction with soil particles.
Foliar application of micronutrients, including their nano-forms is an advanced fertilization technology that has great potential for the production of nutritious and safe foods. It also provides a more efficient route for the application of nanoparticles, whose bioavailability is much higher when applied via leaf sprays as compared to soil amendments, as has been shown in two recent reviews. Comparative investigations of nanoparticle delivery to plants by foliar versus soil applications have indicated that leaf spraying has significant advantages for nanoscale nutrient uptake. This effect was consistent for different crops including brown rice, wheat grain, soybean (Glycine max (L.) Merr.), and black-eyed peas.
Numerous reports have shown the benefits of foliar application for both soluble forms of micronutrients and for nanoparticles. HS were used for improving iron uptake upon foliar application of Fe-chelates. Moreover, complexes of Fe-HS and Fe-lignosulfonate were successfully tested as foliar iron fertilizers. However, so far, we have not found reports on the foliar application of HS-stabilized IONPs. The combination of surface-active HS and IONPs is expected to be particularly beneficial for the foliar spray, which requires improved wettability of a moderately hydrophobic leaf surface and strong adhesion of the fertilizing component. According to a recent study, the combined particle-and-surfactant systems exhibited (i) a decreasing contact angle along with increasing nanoparticle concentration, and (ii) a much lower wetting angle than that of the only-surfactant system at the same surfactant concentration (in the whole range of nanoparticle concentrations studied). Mixtures of surface-active nanoparticles and surfactants also stabilize the liquid–liquid interface in emulsions applied in the oil industry and commercial formulations including agrochemical suspoemulsions. Therefore, humic-stabilized IONPs might be suited for the droplet design of foliar spraying.
The objective of this study was to synthesize humic-stabilized IONPs and to estimate iron uptake as compared to ferric ammonium salt of ethylenediaminetetraacetic acid (Fe-EDTA) upon foliar application on wheat plants (Triticum aestivum L. cv. L15).
In this study, ultrasmall ferrihydrite nanoparticles (with a mean size of 2.5 ± 0.3 nm) stabilized by potassium humate (HS) were synthesized and used in the form of a spray as an iron source for foliar application on wheat plants (Triticum aestivum L.) grown under iron-deficient conditions. This resulted in a 70–75% higher iron content in washed and dried wheat shoots compared to a coordination complex of ferric ions and ethylenediaminetetraacetic acid (Fe-EDTA). The higher uptake of iron from the ferrihydrite (FeH) stabilized with HS was related to the enhanced wettability of the wheat leaves. The obtained results are promising with regard to developing humic-stabilized nanofertilizers. Additional benefits are provided by the eco-friendly properties and plentiful resources of humic substances.