Effects of Zinc Oxide and Zinc–Silica-Based Nanofertilizers with Yeasts on Selected Components of Soybean in the Central European Agronomic Region: A Short-Term Study

Abstract

The action-to-reaction dynamics of next-generation nanofertilizers (NFs) towards field crops are currently being addressed in precision and sustainable agriculture. Therefore, our aim was to evaluate the effects of foliar application of ZnO nanoparticles (ZnO-NPs) or their combination with hybrid nanoporous biosilica mixed with yeast (ZnSi-bio) for soybean plants’ selected production and physiological indices in comparison to an NF-free control. The experiment was conducted at eco-friendly concentrations in Veľký Krtíš, Slovakia, a location within the Central European agronomical region. The ZnSi-bio variant had an improved number of pods, seed count, and yield, while the ZnO-NPs variant had an enhanced seed bulk density compared to the NF-free control, which had a greater effect on thousand-seed weight (TSW). Significant differences were found in the final quality components of soybeans with respect to phosphorus content without ZnO-NP biofortification. In the case of the ZnSi-bio variant, soybeans were biofortified with zinc. Both leaf-applied NFs markedly improved nutritional and energetic values for soybeans. NFs continued to positively affect seasonal physiology, such as the stomatal conductance (Ig) and crop water stress index (CWSI), compared to the control. The results suggest that the ZnO-NPs, especially when combined with hybrid biosilica and yeast, open new avenues for interdisciplinary research in agro-food science.

1. Introduction

At present, a range of nanotechnological disciplines and smart solutions are being incorporated into agricultural practices. This includes electronic bio/nanosensors, functional nano-agrochemicals, nanoproducts for plant protection, and nanofertilizers (NFs), among other innovations [1]. Many of these nanodomains are associated with silica- or metal-based nanosystems that are up to 100 nm in size [2]. Due to their variable crystallinity, diverse morphology with nanoporous-specific reactive surfaces [3], and plant essentiality, they exhibit various physicochemical and biological effects, such as hypothetically improved photosynthesis, abiotic stress tolerance, productivity, and microbial community structure [4,5]. When these nanoporous-silica hybrid biomaterials are combined with different types of microorganisms and micronutrient-based NPs, they offer a novel approach to NF technology. The synergistic effects of these hybrid formulations, such as those combining zinc oxide nanoparticles (ZnO-NPs) with biosilica and yeast, may enhance nutrient delivery and plant uptake. This combination may not only improve plant growth and stress resistance but also significantly boost the nutritional profile and physiological parameters, leading to increased yield and quality. Biosilica may act as an adjuvant and desiccant while also slowly releasing Si ions that improve plant health [6]. ZnO-NPs provide the Zn nutrient essential for many plant enzymes, reduce the damage to plant leaf organs from UV irradiation, and provide antimicrobial protection [7]. ZnO-NP foliar application has been beneficial in diverse plant species. Here, NPs could be absorbed, taken up through leaf stomata, and gradually distributed, transformed, or accumulated in leaves [8], stems [9], and whole plants via the vascular system. ZnO-NPs were also used for Zn biofortification in seeds, e.g., in wheat [10]. Foliar spray applications of yeast, depending on the species, dosage, and environmental conditions, have improved plant production, protection, stress resistance, and the potential nutritional value of the produce. However, they have also led to negative effects, such as pathogen growth and development or changes in microbial–leaf balance [11]. Although the use of nanoporous bio-silica was recently researched for soil applications [12], the combination may provide synergistic effects and reduce the potential negative effects compared to the foliar application of just the single component of the hybrid NF.

The effects of ZnO-NPs or nanoporous biogenic silica particles have been observed at low concentrations in sunflower [13], foxtail millet [14], and lentil [15], but their impact on production, physiological, or nutritional values in soybean within the Central European agronomic region remains unknown.

From a global-nutritional perspective, soybeans are a significant model crop that is also used in experiments with ZnO-NPs [16,17], but there is a lack of academic studies on the positive effects of hybrid silica-based NFs. Soybean (Glycine max), a species within the Fabaceae family, is notable for its protein-rich seeds, which are extensively used in food production, animal feed—due to their energy value—and biofuel manufacturing [18,19].

Currently, studies on the effects of NPs have predominantly focused on toxicological responses at high concentrations in laboratory or greenhouse settings [20,21], with less emphasis on their application in real-world agricultural environments. The influence of bio-silica with yeasts on the absorption of Zn and soybean health has not been evaluated yet. This study addresses this gap by investigating the foliar application of ZnO-NPs combined with a nanoporous hybrid bio-SiO₂ material and yeast (ZnSi-bio), in comparison with ZnO-NPs alone and a NF-free control. This research evaluates the impact of these treatments on soybean production, nutritional components, and potential zinc biofortification under real-world conditions in the Central European agronomic region. The hypothesis proposes more efficient utilization and an active role for selected nanofertilizers in foliar applications with a dispersed aqueous medium as a control. The changes resulting from NFs and aqueous medium-spray deposition could be observed through the overall response of the production process. Key indicators include increased yield, changes in yield-contributing components, and shifts in final harvest quality. Physiologically, a decrease in crop water stress index and an increase in stomatal conductance values are expected. The hypothesis also seeks to confirm that ZnSi-bio has a greater synergistic effect than ZnO-NPs in improving seed quality, yield, and physiological response.

2. Materials and Methods

2.1. Utilized Nanoparticles, Nanoporous Biosilica, and Food-Grade Yeast

The ZnO-NPs for spray application to soybean leaves were obtained from Sigma-Aldrich (Saint Louis, MO, USA). Nanoporous biosilica was sourced from Radegast Brewery (Plzeňský Prazdroj, Nošovice, Czech Republic), which is used for beer filtration, and commercially purchased baker’s yeast was used for dispersion.

2.2. Soybean Variety Used

The “Korus” soybean variety was employed to examine its effects on NPs. This variety is characterized by its intensive initial growth (00+), medium plant height, and resistance to lodging and pests, including Sclerotinia. It presents a high protein content of up to 42% at 12% moisture, coupled with excellent yield, making it ideal for tofu production [22].

2.3. Description of Natural Conditions, Weather, and Climate at the Experimental Site

The experimental site is located near Pôtor village with coordinates 48.238290° N, 19.413773° E in the Veľký Krtíš, in the southern part of central Slovakia, at an altitude of 204 m above sea level. The soil type is classified as Fluvisol, and the locality is situated near the alluvium of the “Stará rieka” stream [23]. Generally, the site belongs to a warm, very dry basin with a Central European continental climate. Meteorological data from Meteoblue [24] show recorded variability in precipitation (mm) (Figure 1a) and monthly temperature (°C) (Figure 1b) during the 2023 vegetation season.

2.4. Agronomic Practices Establishing the Soybean Experiment

The experiment was conducted using a conventional soil-management system, with winter rye as a forecrop. The autumn soil preparation for 2022 was carried out in October, involving deep tillage to a depth of up to 30 cm. The soil procedure in early spring was carried out in two phases, with leveling of the land and a pre-sow seedbed preparation. Here, a fertilizer was integrated into the soil at a dosage of 80 kg·ha⁻¹, specifically consisting of 50 kg of Physio Mescal G18 fertilizer (containing 18% P₂O₅, 6% SO₃, 65% CaO, and 5% MgO) with 30 kg of ammonium sulfate. Soybean sowing was conducted in April using a seed drill (Väderstad Rapid RDA 600, Väderstad, Sweden) with an inter-row spacing of 12.5 cm and a sowing depth of 4 cm.

The first preemergence herbicide application was carried out using Escort^® Nový (containing 16.7 g·L⁻¹ imazamox and 1250 g·L⁻¹ pendimethalin) at a rate of 3 L·ha⁻¹ (BASF SE, Ludwigshafen, Germany). The second application used Pulsar^® 40 (containing 40 g·L⁻¹ imazamox) at a rate of 1 L·ha⁻¹ (BASF Agrochemical Products de PuertoRico, Manati, PR, USA), along with the NP application. During the soybean vegetation, all treatments, including the NP-free control, were treated with the foliar fertilizer VermiVital^® (VermiVital s.r.o., Záhorce, Slovakia) at a concentration of 5 L per hectare. The fertilizer composition was as follows: 10.5–13% N, 0.64% S, 0.33% Mg, 0.33% Mn, 0.25% Cu, 0.58% B, and 0.33% Zn.

The experiment was conducted following the methodology of Duflo and Banerjee [25] on an area of approximately 1.8 ha with three treatments, including a control, each with dimensions of 180 × 24 m, resulting in individual treatment plots of 4320 m². The foliar application variants for soybean consisted of (i) ZnO-NPs; (ii) ZnO-NPs combined with silicon nanoporous recrystallized diatomite and food-grade yeast (ZnSi-bio); and (iii) NF-free control. Each variant was applied in three replications. For the ZnO-NPs, the concentration was 10 mg·L⁻¹ for both variants, nanoporous biosilica corresponded to 15 mg·L⁻¹, and, for yeasts, the concentration was 7.5 g·L⁻¹. The NP dispersion was applied early in the morning under windless conditions, ensuring that the soybean leaves were completely wet 45 days after sowing. Prior to leaf deposition, each colloidal dispersion was subjected to ultrasonication for 15 min.

2.5. Evaluation of Selected Seasonal Physiological Indices of Soybean

For the seasonal physiology determination, indexes such as the stomatal conductance index (Ig) and the crop water stress index (CWSI) were selected and measured using an EasIR-4 thermal camera (Bibus AG, Fehraltorf, Switzerland) following the methodology described by Jones et al. [26].

In this context, various thermal-image characteristics are related, such as leaf temperature parameters (T_leaf), which include dry (T_dry) and wet (T_wet) leaf surface temperatures, as well as atmospheric moisture. The stomatal conductance index (Ig) and the crop water stress index (CWSI) were calculated according to the following equations:

Ig = (T_dry − T_leaf)/(T_leaf − T_wet),

(1)

CWSI = (T_leaf − T_wet)/(T_dry − T_wet)

(2)

Temperature images were captured using the thermal camera at a diagonal angle relative to the soybean leaves, from a distance of 2 m, at a height of 1.5 m, and with a resolution of 20.6° × 15.5° in auto-focus mode. To ensure consistency and accuracy, all measurements were performed on the same day on equivalent plants with fully developed leaves at a similar developmental stage (Figure 2). Measurements were carried out between 11 a.m. and 1 p.m.

2.6. Analysis of Soybean Production

When the soybean seeds reached full maturity and the required moisture, they were harvested with a combine Claas Lexion 670 (CLAAS GmbH & Co. KGaA, Harsewinkel, Germany). After that, the quantitative parameters were assessed for each variant, where the number of pods was manually counted. The number of seeds per plant was evaluated using a Numirex seed count device (MEZOS spol. s.r.o., Hradec Králové, Czech Republic); weight of thousand-seed (TSW) and bulk density were measured according to STN standard 461011 [27] which involves filling a standard container with grain, leveling the surface, and weighing the grain using the Kern PCB3500-2 laboratory scale (KERN & Sohn GmbH, Balingen, Germany). The soybean yield was calculated in tons per hectare (t·ha⁻¹).

2.7. Analysis of Selected Nutritional Parameters and Energy Value of Soybean Seeds

The mature soybean seeds were evaluated for nutritional parameters. Initially, they were dried at 105 °C in a drying oven. The gross energy (GE), expressed in MJ·kg⁻¹, was assessed using a bomb calorimeter Parr 6400 Calorimeter (Parr Instrument Company, Moline, IL, USA) with benzoic acid as the standard. Procedures and calculations for other energy parameters, specifically digestible energy (DG), metabolizable energy (ME), net energy (NE) for pig growth, and protein solubility in KOH, were conducted according to the STN standard 2300-7/94 [28]. The standardized method 945.39 was used for the ether extracts.

The seed samples weighing 0.15–0.30 g were digested in a mixture of HNO₃ and HClO₄ (2:1, v/v). For soybean zinc analysis, flame atomic absorption spectrometry (F-AAS) was used (Perkin-Elmer Model 1100, Waltham, MA, USA) according to the method described by Losak et al. [29], while phosphorus content was determined colorimetrically using a spectrophotometer at a wavelength of 660 nm.

2.8. Statistical Analysis

The obtained results were statistically evaluated using one-way analysis of variance (ANOVA) at a significance level of 95% with Tukey’s Honestly Significant Difference (HSD) post-hoc test (TIBCO Statistica™ 14.0.0).

3. Results and Discussion

During the initial NFs foliar application in the growing season 2023 at the Veľký Krtíš location, as was expected, no negative stress reactions were observed in soybeans at our eco-friendly concentration. Inappropriate NFs concentration and timing of application often lead to a reduction in individual plant numbers, surface leaf destruction, the appearance and spread of pathogens [1,30], or genetic modifications in soil applications [31]. Negative growth effects in crops are generally associated with excessive concentrations of NFs, such as 2000 mg·L⁻¹ in habanero peppers [32]. In our study, however, the adaptability and beneficial effect of NFs were demonstrated through improvements in several quantitative parameters (

Table 1. Analysis of selected quantitative parameters and yield of soybean.

Parameter	Control Variant (No NF Application)	ZnO-NPs (Foliar Applied Variant)	ZnSi-bio (Foliar Applied Variant)
Quantitative parameters
Number of pods per plant	17.32 ± 2.47 a	17.08 ± 3.20 a	17.93 ± 5.26 a
Number of seeds per plant	38.9 ± 6.06 a	39.8 ± 7.38 a	41.52 ± 12.56 a
Weight of thousand-seeds (g)	189.75 ± 5.73 a	175.83 ± 1.83 b	187.5 ± 7.05 ab
Bulk density of seeds (kg·m−3)	693.06 ± 2.22 a	701.73 ± 0.99 b	678.86 ± 2.88 c
Yield (t·ha−1)	3.80 ± 0.58 a	3.57 ± 0.68 a	4.07 ± 1.24 a

The significance: different letters indicate significant differences between variants according to the Tukey HSD test (p < 0.05).

).

The ZnO-NP application had the most pronounced effect on the bulk density of soybean seeds, while other yield-forming factors did not show statistically significant differences. This result can be attributed to a lower number of pods per plant yet a relatively larger number of smaller seeds, leading to a reduced seed size and a lower thousand-seed weight (Table 1). This presents an interesting agronomic outcome, as it suggests a higher potential for seed quality despite reduced seed size. The functional relationship between soybean production and the dynamics of Zn absorption, uptake, or deficiency can manifest in either optimal growth or, conversely, the onset of chlorosis, which directly reduces seed production and indirectly affects overall yield [33]. In this case, the physicochemical nature of ZnO-NPs provided sufficient and gradual Zn release, offering a more targeted effect than conventional fertilizers [34]. The statistically non-significant negative results of the number of pods per plant and weight of thousand seeds associated with ZnO-NPs application can be explained by the higher number of inflorescences and the pollen-favorable development as reproductive organs [35,36]. These improved the bulk density of seeds rather than their weight and yield (Table 1). A similar stimulatory effect, with an increase in the number of pods per plant and a slight increase in soybean biomass achieved through soil application, was also reported by Priester et al. [37], while Yusefi-Tanha et al. [38] observed comparable effects on yield.

During the interval of inflorescence formation and fruit ripening, there exists an important functional dependence on other non-ideal environmental factors, which leads to thermal stress and bioenergetically limits soil nutrient uptake and utilization [39].

Regarding production parameters, the ZnSi-bio application produced the highest yield, albeit at a statistically non-significant level (Table 1). Silica, compared to zinc, is less essential for plants; however, it enhanced soybean yield in nanoformulations with oligochitosan [40]. The combination of ZnO-NP with nanoporous biosilica and food yeasts has confirmed the active role of next-generation nanofertilizers in integrating micronutrients, nanoporous hybrid materials, and microorganisms to enhance agronomical functional properties through foliar spray application [41].

Although the cultivar we used was genetically treated for higher average yields [22], our results highlight that all three variants, including the control, exceeded FAO standards for maximum yield, i.e., more than 3.5 t·ha⁻¹ [42]. Here, the ZnSi-bio variant led to a yield that surpassed the threshold of 4 tons per hectare (Table 1), posting an excellent output in the context of the moderate climate of Central European agronomy.

Generating sufficient energy is the most economically demanding nutritional component in plant-based feed for pigs [43]. In general, soybean seeds contain 7.5–10% moisture, 3–10% sugars (including sucrose, stachyose, raffinose, and polysaccharides), 16.3–21% fats (such as palmitic, stearic, oleic, linoleic, and linolenic acids), 34–40% proteins (primarily lysine and methionine), 4–5.24% mineral nutrients, and approximately 8% fiber (in the form of cellulose, hemicellulose, and lignin) [44,45]. The ratio of synthesized nutritional components in seeds, e.g., soybeans, enables the development of grain-energy productivity with feeding strategies based on their bioenergetic value for individual animals with ideal marked prices [46].

Compared to the individual soybean variants, the higher energy-caloric content is associated with increased concentrations of digestible proteins, essential amino acids, fats (especially oil and fatty acids), soluble carbohydrates, and oligosaccharides, as well as soluble fiber (pectin), and a reduction in anti-nutritional factors, such as trypsin inhibitors and phytic acid [18,19]. Our findings reveal that both the ZnO-NPs and ZnSi-bio variants are more effective than the control across all observed energy parameters at statistically significant levels (

Table 2. Comparison of selected energy parameters of soybeans after NF application.

Parameter	Control Variant (No NF Application)	ZnO-NPs (Foliar Applied Variant)	ZnSi-bio (Foliar Applied Variant)
Energy parameters of soybean
Gross energy (MJ·kg−1)	21.269 ± 0.010 a	21.382 ± 0.002 b	21.323 ± 0.010 c
Digestible Energy (MJ·kg−1)	17.368 ± 0.015 a	17.552 ± 0.072 b	17.500 ± 0.080 b
Metabolizable energy (MJ·kg−1)	16.674 ± 0.015 a	16.907 ± 0.030 b	16.820 ± 0.020 c
Net energy (MJ·kg−1)	11.944 ± 0.004 a	12.145 ± 0.001 b	12.030 ± 0.002 c

The significance: different letters indicate significant differences between variants according to the Tukey HSD test (p < 0.05).

).

It is most likely that zinc plays a direct role as a cofactor in carbon and nitrogen metabolism, such as enhancing carbohydrate formation, facilitating atmospheric N-fixation during amino acid synthesis, glycerol utilization, and the biosynthesis of growth hormones and nucleic acids, thereby intensifying their bioenergetic value [19,47].

The results for the energy parameters are directly related to the obtained ether extract (

Table 3. Comparison of final qualitative-nutritional parameters of soybeans with potential zinc biofortification after spray deposition of ZnO-NPs and ZnSi-bio nanofertilizers.

Parameter	Control Variant (No Application)	ZnO-NPs (Foliar Applied Variant)	ZnSi-bio (Foliar Applied Variant)
Qualitative-nutritional parameters of soybean
Ether extract (%)	19.3 ± 0.15 a	19.7 ± 0.20 b	19.6 ± 0.10 b
Protein solubility in KOH (%)	92.8 ± 0.20 a	92.6 ± 0.10 a	93.6 ± 0.30 b
Phosphorus (mg·kg−1)	5899.0 ± 31.8 a	5906.3 ± 30.4 a	5764.0 ± 17.4 b
Zinc (mg·kg−1)	53.93 ± 0.40 a	52.57 ± 0.15 b	55.77 ± 0.25 c

The significance: different letters indicate significant differences between variants according to the Tukey HSD test (p < 0.05).

), which generally correlates with the quantitative oil content in soybeans and impacts the total energy value. However, this only partially correlates with the complex relationship of the protein digestibility profile (Table 3), where the ZnSi-bio variant was indicatively more effective than the control and ZnO variants. In this context, it could be interpreted as a basic relationship between more intense oil synthesis and protein production, which was also demonstrated to be dependent on soil characteristics [48].

In the case of Zn-positive biofortification affecting the final quality of soybean seeds, no increased concentrations were observed (Table 3) in the case of the ZnO-NPs variant. Zinc showed a stimulatory biofortification effect on soybean seeds in the ZnSi-bio variant.

However, the Zn levels do not exceed the average concentrations found in soybean seeds [49]. It is likely that Zn was redistributed throughout the plant, similarly to what has been observed in tomatoes [50], and could have had a physiological–metabolic effect in the soybeans as well.

This is partially supported by p values, where higher P content can reduce the final quality of the product because, after digestion, it binds to divalent and trivalent macronutrients in the form of phytic acid [51]. In our case, the highest values were observed in the control and, surprisingly, in the ZnO-NPs variant, while the lowest value corresponded to the ZnSi-bio variant, likely due to the relative positive physiological effects of nanoporous biosilica or applied food yeasts.

A concentration-adequate, bioavailable, and well-distributed Zn form plays significant roles in plants, including as a growth regulator, enzyme activator, protein synthesis facilitator, environmental stress-tolerance enhancer, root developer, pollinator attractant, and seed producer, as well as in maintaining membrane integrity, stability, and chlorophyll activation [34].

In our case, the foliar application of nano-domain NFs showed a positive response in overall physiological indices (

Table 4. Comparison of physiological indices in soybeans after NFs-foliar deposition on soybean leaves.

Parameter	Control Variant (No Application)	ZnO-NPs (Foliar Applied Variant)	ZnSi-Bio (Foliar Applied Variant)
Physiology components
Stomatal Conductance Index (Ig)	1.59 ± 0.26 a	2.60 ± 0.63 b	2.63 ± 0.87 b
Crop Water Stress Index (CWSI)	0.39 ± 0.04 a	0.29 ± 0.05 b	0.29 ± 0.06 b

The significance: different letters indicate significant differences between variants according to the Tukey HSD test (p < 0.05).

) at a high significance level, particularly in Ig and CWSI (Table 4). However, the physiological dynamics throughout the growing season (Figure 3) reveal statistically significant changes in both applied variants, specifically during the vegetative growth stage V3 (third trifoliolate) and reproductive growth stage R6 (full seed development).

In this context, our applied NFs can be considered a relative functional tool for mitigating climate changes, as the thermal requirements for soybeans aligned with a relative seasonal optimum at around 2500 °C (Figure 1a) with an interactive effect of CO₂ on plant growth [52]. However, the water management supply (Figure 1b) was non-ideal during the growing season [53], with the NF-treated variants showing better physiological adaptation in our agronomical region (Table 4, Figure 3).

The study conducted by Sedghi et al. [54] found that ZnO-NPs can mitigate the adverse effects of drought, thereby helping soybeans to better manage thermal stress and increase their drought tolerance [55]. Tripathi et al. [56] observed that SiO₂ accumulation facilitates the opening of stomata, leading to increased transpiration and CO₂ uptake. The SiO₂ improves the utilization of available water to achieve higher net assimilation.

Principally, ZnO-NPs in soybean spray applications may act as a Zn²⁺ micronutrient or photosynthesis-encouraging agent. Despite soybeans being classified as a plant with a critical photoperiod of about 13 h of daylight, they initiate flowering below this photoperiod and reach optimal photosynthetic efficiency at temperatures of up to 30 °C in certain developmental stages [39]. In this case, theoretically, the energy provided by the transformation of solar irradiation through the NP’s photocatalytic activity could enhance photosynthesis, similar to the effect observed with TiO₂-NPs in spinach [57,58]. It is similar in terms of size and crystallography to the ZnO-NPs we applied. The authors Hong et al. [57] and Hong et al. [58] evidenced that, in addition to enhancing photosynthesis, this approach also increased enzymatic activity in catalases and peroxidases, as well as oxygen release. Furthermore, exposure of ZnO-NPs to sunlight radiation leads to gradual photocorrosion [59], with the Zn ions released adsorbing into plant tissues and subsequently complexing with oxalates, citrates, phytates, or hydrated Zn²⁺ ions, thereby positively influencing plant’s metabolism [8].

The ZnSi-bio variant utilizes the synergistic positive physiological effect of silica in soybeans [56] with ZnO-NPs and yeasts. In general, crystalline SiO₂ particles, either alone or combined with other oxides, serve as significant photocatalytic materials widely used in industry and the environment [60]. The porous structure of the biosilica used may have improved the rate at which Zn ions were released to the leaf surface and may have mitigated the potential negative effects yeast can have on plants [6].

In our previous short-term empirical outcomes with other plants, ZnO-NPs positive physiological responses were observed typically one month after the second foliar application of NFs at a statistically significant level in sunflower [13] and lentil [15], as well as in extending critical metabolic–energetic processes during the vegetation period in foxtail millet [14]. Results of this agronomic study demonstrated that a single foliar application of nanofertilizers (NFs) was sufficient for soybeans, potentially lowering the financial costs associated with fertilizer use.

4. Conclusions

Our study in the Central Europe agronomical region found interesting results in the foliar application of environmentally friendly concentrations of NFs on soybeans. Although the production parameters were generally inconclusive, the ZnSi-bio variant showed a beneficial effect on the number of pods, seed count, and yield, while the ZnO-NPs variant excelled in terms of seed bulk density compared to the NF-free control. In terms of seed weight, the NF-free control variant showed a relatively greater effect, so the hypothesis that foliar application of NFs would lead to a more effective yield was only partially confirmed.

Our work focused on optimizing the final harvesting quality of soybean seeds, and our results have demonstrated improved outcomes at a statistically significant level for ether extract content and bulk density of seeds but without Zn biofortification with ZnO-NP application. However, in the case of the ZnSi-bio variant, zinc exhibited a stimulatory biofortification effect on the quality of soybeans. This was most likely due to the synergistic effect of ZnO-NPs with bio-silica and food yeast sprayed onto soybean leaves once per season for agronomic and economic benefit.

Both foliar NFs showed statistically significant increases in the highest nutritional-energetic values of soybean beans. Additionally, the seasonally intensified physiology was observed in stomatal conductance (Ig) and the crop water stress index (CWSI) compared to the control variant, fully confirming the hypothesis.