Investigation of the Effect of a New Type of Copper–Sucrose Complex Compound on the Yield and Quality Parameters of Winter Wheat (Triticum aestivum L.)

Abstract

We conducted experiments on winter wheat grown in copper-deficient soil, where soil tests revealed a more pronounced deficiency in the deeper layers. As climate change reduces precipitation, plants increasingly rely on nutrients from these deeper layers. A copper–sucrose complex—previously unused in agriculture—was applied as a foliar spray during the tillering and flowering stages. Across the three-year average, significant increases were observed starting from the 1 kg ha⁻¹ copper dose in yield, from 0.3 kg ha⁻¹ in crude protein content, and from 0.5 kg ha⁻¹ in wet gluten content compared to the untreated control. For all three parameters, the highest values were achieved with the 2 kg ha⁻¹ dose. Yield increased by 1.03 t ha⁻¹, crude protein by 0.9%, and wet gluten by 2.3% relative to the control. In 2019, high humidity and favorable temperatures during flowering led to fungal infections in control plots, with DON toxin concentrations exceeding the regulatory safety threshold. Following copper–sucrose complex application, DON levels dropped below this threshold, demonstrating a measurable protective effect.

1. Introduction

1.1. The Importance of Copper in Agricultural Production

Global population growth has remained steady in the 21st century, with projections estimating it will reach 9.8 billion by 2050. Ensuring an adequate supply of food—both in quantity and quality—is a growing concern for producers. Meeting the nutritional needs of this expanding population presents increasing challenges for global agriculture. Among cereals, winter wheat is one of the most important food staples and is especially vital in developing countries [1,2]. Hungary’s soil and climatic conditions are favorable for winter wheat production, which currently covers nearly 1 million hectares. However, intensive crop cultivation, urbanization, and the reduced use of by-products from animal husbandry (such as straw-based manure) have significantly decreased soil nutrient levels while degrading soil structure and water balance [3,4]. Environmental factors also strongly influence nutrient uptake in plants [5]. To maintain crop security, growers are increasingly recognizing the need to not only supplement the three major macroelements (nitrogen, phosphorus, potassium) but also to address deficiencies in essential microelements [6].

The essential roles of macro- and microelements in plant nutrition have been well documented by Marschner (2012) [7]. In wheat cultivation, copper plays a pivotal role as a microelement. Different crop species have varying optimal copper requirements. Plants primarily absorb copper from the soil [8]. Deficiency symptoms have been observed in 15–20% of Hungary’s soils, indicating a widespread need for supplementation. Polish studies have similarly reported a steady decline in soil copper levels [9]. Copper deficiency is especially prevalent in soils where the element is strongly bound and thus unavailable to plants [10]. This includes peat-rich soils, highly calcareous soils, and those with a high clay content [11,12]. Even when soil copper levels are adequate, plants may still exhibit deficiency symptoms due to the poor internal transport of the nutrient [13]. In such cases, copper accumulates in the roots, limiting its movement to the shoots and interfering with the absorption of other nutrients [14,15,16].

Hungary’s main cultivated crop is winter wheat, grown on nearly 1 million hectares. One of the essential conditions for producing high-yield and high-quality wheat is an adequate supply of the micronutrient copper [17]. Based on this knowledge, the aim of our research was to complete the following tasks:

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Develop a copper-complex compound that is less toxic;

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Ensure that the complex does not have excessively high stability;

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Assess how the anionic component of the applied copper salt negatively affects the plant by causing foliar burn;

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Use a ligand that is beneficial for plant nutrition;

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Create a compound that is suitable for foliar application;

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Enhance wheat yield and quality through the application of the complex;

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Provide fungicidal effects to reduce the fungal contamination of the crop.

With these criteria in mind, sucrose was selected as the ligand. The development and formulation of the appropriate complex compound was carried out at the University of Szeged in collaboration with Kálmán Burgert.

Copper supplementation as a microelement is becoming increasingly important and can be administered either through the soil or via foliar application. However, soil application is often limited by the higher quantities required and the associated costs [18]. An additional complication is that different crop species have varying nutritional needs, which also shift throughout their development. To match these changing requirements, foliar application (spraying) offers a more adaptable solution [13,19]. The use of copper salts for foliar treatment has declined due to their potential to scorch plant tissues—an effect largely influenced by the chemical form, properties, and anion concentration of the compound [20]. To reduce toxicity while maintaining efficacy, copper is now primarily applied in the form of complex compounds. Copper uptake depends heavily on the structure, size, and stability of the ligand in the complex compound [21].

1.2. The Role of Copper in Plants

The essential role of copper in plants was first recognized by Arnon and Stout (1939) [22]. They noted that an optimal copper supply is important not only for plant health but also for the animals and humans who consume those plants [23]. Charkiewicz (2024) provides a detailed overview of copper’s beneficial and adverse effects [24]. For humans, the recommended daily copper intake is approximately 1.5 mg, with 0.8 mg/day considered to be the minimum needed to regulate and maintain copper levels in the body [25,26]. The average copper content in plants ranges from 2 to 50 mg kg⁻¹, depending largely on plant species, environmental conditions, soil composition, light availability, and developmental stage [27]. While the reported range of 2 to 50 mg kg⁻¹ refers to copper concentration by dry weight, the typical human dietary intake remains well within safe limits, as only a small fraction of plant biomass is consumed and the body regulates copper levels efficiently.

Microelements like copper play key roles in reducing oxidative stress, supporting lignification, regulating enzyme activity, and maintaining cellular and metabolic functions [28,29,30]. Copper exists in multiple oxidation states (Cu⁺, Cu²⁺) within the plant and is involved in regulating cellular redox states, oxidative phosphorylation, photosynthetic electron transport, and iron ion mobilization [31,32,33]. Higher copper levels may reduce iron content. Copper is also involved in mitochondrial respiration, nitrogen metabolism, cell wall synthesis and remodeling, and hormonal signaling [7,34].

In nitrogen fertilization studies on acidic loamy sand in Australia, copper and zinc were found to be critical for maintaining the plant’s nitrogen balance [35]. Application of copper increased the number of ears in early wheat development, while pre-flowering treatments improved nitrogen uptake.

Copper plays a crucial role in pollen formation, fertilization, and increasing yield [34,36,37]. An insufficient copper supply in the plant causes the distortion and whitening of young leaves. A secondary effect of copper deficiency is reduced water uptake efficiency, which can leave plants more susceptible to drought stress. To avoid this, we can obtain information by examining the copper content in both the soil and the vegetation [4].

However, excessive copper doses are toxic and have an inhibitory effect on plant development. An excessive copper supply disrupts the nutrient balance in the plants, which can also lead to a decrease in crop yield. Copper toxicity has a detrimental effect on plant development and can cause irreversible damage [38]. It impairs the activity of chlorophyll and photosynthesis, damages lipids, proteins, and DNA, and can also lead to cell death [39,40]. Copper inhibits protein function and enzyme activity by reacting with sulfhydryl groups in proteins [41]. This irreversible enzyme inhibition is the basis for its effectiveness as a fungicide and bactericide. However, concerns over environmental accumulation and non-target toxicity have led many researchers and regulatory agencies to advocate for more cautious or reduced use [42,43,44]. Copper ions irreversibly inhibit fungal growth by reacting with the sulfhydryl groups of protein molecules [45]. According to Liu et al. (2020) [46], copper treatment applied at flowering resulted in a reduced amount of deoxynivalenol (DON) mycotoxin. Regarding the effect of copper ions on Fusarium species, Ibarra-Laclette (2022) [47] reported that, at the molecular level, RNA-sequencing analysis suggested that the observed growth inhibition and changes in colony morphology are due to the reduced ergosterol biosynthesis caused by the accumulation of free cytosolic copper ions.

1.3. Effects of Copper on the Quantitative and Qualitative Parameters of Winter Wheat

Cereal crops are important sources of energy, carbohydrates, proteins, and vitamins for the human diet. They also play a key role in replacing animal-derived food products by providing plant-based protein alternatives [48,49].

To reduce costs and effectively address macro- and microelement deficiencies, foliar application has proven to be the most efficient method. Foliar application of nitrogen-based fertilizers and micronutrients such as copper (Cu), manganese (Mn), and zinc (Zn) has been shown to increase both yield and nitrogen uptake [50].

Favorable outcomes were also reported with the use of copper–biochar composites [51]. Foliar fertilization with copper-tetramine-sulfate increased both yield and protein content in winter wheat grown on copper-deficient soils [20]. Other researchers have reported enhanced protein content in wheat following foliar copper treatments [52], and Polish studies have documented an increase in glutenin content [53].

2. Materials and Methods

2.1. Meteorological Conditions During the Study Period

The meteorological conditions of the period that largely determines the development of winter wheat, i.e., the months of March–June, and especially the most relevant, May, were significantly different in the individual years of the experiment (

Table 1. Meteorological conditions during the study period.

Meteorological Element	Year	March	April	May	June	March–June
Average temperature (°C)	2019	8.6	12.1	12.7	23.0	14.0
	2020	6.9	11.9	14.0	19.0	12.9
	2022	5.5	9.7	17.6	21.9	13.6
	1991–2020	6.0	11.4	15.9	19.6	13.2
Average relative humidity (%)	2019	58	57	75	68	64
	2020	60	49	65	74	62
	2022	54	63	66	67	63
	1991–2020	71	64	68	68	68
Precipitation sum (mm)	2019	15	17	134	60	226
	2020	40	3	40	92	175
	2022	12	19	60	118	208
	1991–2020	38	36	61	68	202

). The spring of 2019 was rainier and more humid than average, and May of 2019 was even cooler. In contrast, the precipitation sums, but even more so the relative humidity, of the spring and May of 2020 and 2022 caused dry conditions in the most critical period for wheat. In terms of temperature, the conditions of May of 2022 contributed the most to this drought.

2.2. Soil Analysis

Foliar fertilization trials were conducted over three years—2019, 2020, and 2022—in Győrszentiván (Northwest Hungary), on chernozem soil. To assess baseline conditions, soil composition was analyzed at three depths: 0–30 cm, 30–60 cm, and 60–90 cm. The results of these analyses are presented in

Table 2. Soil test results from Győrszentiván (NW Hungary), by year and depth.

Year	Depth (cm)	pH (KCl)	KA *	CaCO3 (w/w%)	Humus (w/w%)	P2O5 (mg kg−1)	K2O (mg kg−1)	Na (mg kg−1)	Zn (mg kg−1)	Cu (mg kg−1)
2019	0–30	7.42	36	11.16	2.1	128	146	38	1.82	0.68
	30–60	7.41	36	9.4	1.4	87	124	34	1.64	0.41
	60–90	7.24	38	6.8	1.2	63	89	37	0.76	0.29
2020	0–30	7.47	37	10.6	2.4	144	218	38	2.82	0.73
	30–60	7.38	36	9.8	2.1	122	196	41	1.94	0.64
	60–90	7.32	37	7.4	1.7	87	135	48	1.27	0.31
2022	0–30	7.42	36	11.16	2.1	128	146	38	1.82	0.68
	30–60	7.41	36	9.4	1.4	87	124	34	1.64	0.41
	60–90	7.24	39	6.8	1.2	63	89	37	0.76	0.29

* KA (Arany’s binding number) is a Hungarian soil classification index that reflects soil texture and plasticity, similar in concept to the liquid limit.

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Table 2 shows that copper concentrations decreased consistently with depth in each year, confirming copper deficiency in the subsoil layers. Other soil characteristics, such as humus content, phosphorus (P₂O₅), potassium (K₂O), sodium (Na), and pH (KCl), were also measured to provide a comprehensive nutrient profile for the experimental site.

The acidity (pH [KCl]) was determined using the potentiometric method from a 1:2.5 (m + V) suspension in 1 mol/dm³ KCl solution. Soil texture was assessed via Arany’s binding number (KA). Lime content (CaCO₃ w/w%) was measured using the gas volumetric method, while humus content was determined spectrophotometrically following wet oxidation with concentrated sulfuric acid and potassium dichromate.

From the plant-available nutrients, phosphorus, potassium, and sodium were extracted using the Ammonium Lactate method (Egnér–Riehm–Domingo). Copper and zinc were extracted with a KCl–EDTA solution (0.05 mol/dm³ EDTA and 0.1 mol/dm³ KCl). Potassium and sodium concentrations were determined using flame photometry; phosphorus was measured with an FIA analyzer; and copper and zinc were measured using ICP-AES.

The 2022 trials were conducted on the same plots used in 2019, and the soil test results were consistent with those obtained in the initial year.

2.3. Cultivation Technology

Table 3. Cultivation technology.

Parameters	Year 2019	Year 2020	Year 2022
Preceding crop	Sunflower	Sunflower	Sunflower
Soil cultivation	Chisel plowing (up to 25 cm depth) and disk harrowing	Chisel plowing (up to 25 cm depth) and disk harrowing	Chisel plowing (up to 25 cm depth) and disk harrowing
Fertilizer applied to the soil (kg ha−1)	N = 120	N = 120	N = 120
	P = 80	P = 80	P = 80
	K = 100	K = 100	K = 100
Winter wheat variety	early-maturing Montecarlo	early-maturing Montecarlo	early-maturing Obiwan
Sowing rate (seeds/m2)	500	500	500
Inter-row spacing (cm)	12	12	12
Herbicide used	Moderator	Moderator	Moderator

summarizes the copper fertilization experiments on winter wheat in Győrszentiván (years: 2019, 2020, 2022) including details about the soil tillage system and the equipment used.

After harvesting the previous crop, the soil was immediately tilled. Fertilizers used include ammonium-nitrate, ammonium-phosphate, and potassium-chloride. No fungicide was applied and no irrigation was used. In all three years, fertilizer was applied at the end of September and sown in the first week of October. No fertilizer was applied to the vegetation after sowing. A moderate amount of herbicide applied per 10 ha: Ergon 350 g + Galistop 5 L + Kabuki 0.75 L.

2.4. Foliar Treatment Trials

Over a three-year period, foliar fertilization trials were conducted on copper-deficient soil using a copper–sucrose complex compound. The experiments followed a randomized block design with four replicates on 10 m² plots, and treatments were applied during the tillering and flowering phenological phases. The copper doses applied were 0, 0.1, 0.3, 0.5, 1.0, and 2.0 kg ha⁻¹.

After harvest, yield, crude protein content, and wet gluten content were measured. Crude protein and gluten levels were analyzed using the Infratec™ 1241 Grain Analyzer (FOSS Analytical A/S, Hillerød, Denmark) in the near-infrared (NIR) range. Measurements were conducted in transmission mode across wavelengths between 570 and 1050 nm.

2.5. Fungicide Tests

The fungicidal effect of the copper–sucrose complex compound, as used in the foliar treatment trials, was tested against the plant pathogen Fusarium graminearum using the agar diffusion method. The tested concentrations reflected those used in the field and corresponded to copper levels of 0, 0.17, 0.50, 0.83, 1.67, 3.33, and 6.67 g L⁻¹. Copper oxychloride served as the reference fungicide (Figure 1) [17].

At a concentration equivalent to at least 1 kg ha⁻¹ of copper in the field, the copper–sucrose foliar fertilizer exhibited a statistically significant fungicidal effect (p < 0.01). Although its efficacy was slightly lower than that of commercially available copper oxychloride, the compound still demonstrated clear antifungal activity.

2.6. Deoxinivalenol (DON) Test

DON toxin was detected using the RIDA^® QUICK Deoxynivalenol (DON) RQS ECO (Art. Nr. R5911) immunochromatographic test following aqueous extraction. The analysis method, applicable for infection levels between 0.25 and 7.5 mg kg⁻¹, was carried out as follows:

A 5 g ground wheat sample was combined with 25 cm³ of distilled water in a capped plastic vial. The mixture was shaken manually for 30 s and then allowed to settle for approximately 3 min. It was subsequently centrifuged for 1 min at 2000× g (relative centrifugal force). From the resulting supernatant, 100 µL of the clear upper layer was mixed with 500 µL of running solution. Then, 100 µL of this dilution was applied to the sample application area of the test strip. After a 3 min incubation period, the results were evaluated using the RIDA^®SMART APP software (version number: 4.3.4.).

2.7. Statistical Evaluation

Statistical analysis was conducted using RStudio (version 2023.9.1), with the agricolae, car, and stats packages. Prior to analysis of variance (ANOVA), tests for normality and homogeneity of variances were performed. ANOVA was conducted at significance levels of α = 0.05 and 0.01. Differences between means were evaluated using Tukey’s test (α = 0.05).

ANOVA was performed separately for each year and also in a combined analysis across years. Because no consistent year-to-year trend was observed, the year factor was treated as a random variable. In the combined analysis, following the approach of Berzsenyi (2016), the interaction term (year × treatment) was used as the error term for the F-test assessing treatment effects [54].

3. Results

3.1. Effect of Foliar Fertilization on Yield Components

The yield, crude protein content, and wet gluten content of winter wheat were evaluated following foliar fertilization with a copper–sucrose compound. The research was conducted over three years using small-plot experiments with four replications. The average results from the three trial years are summarized in

Table 4. Effect of copper–sucrose foliar treatments on winter wheat yield, crude protein, and wet gluten content in 2019, in 2020, in 2022, and in the average of the three experimental years.

Treatment (kg ha−1)	Yield (t ha−1)					Crude Protein (%)					Wet Gluten (%)
	2019	2020	2022	Avg		2019	2020	2022	Avg		2019	2020	2022	Avg
Control	7.28	8.08	6.33	7.23 ± 1.34	c	13.0	14.0	12.3	13.1 ± 1.2	e	29.3	31.0	30.1	30.1 ± 1.3	d
0.1 (kg ha−1) Cu	7.20	7.98	6.98	7.38 ± 0.84	c	13.2	14.3	12.4	13.3 ± 1.3	de	29.4	31.0	30.6	30.3 ± 1.3	d
0.3 (kg ha−1) Cu	7.38	8.28	7.15	7.60 ± 0.95	bc	13.4	14.3	12.5	13.4 ± 1.3	cd	29.7	31.3	30.9	30.6 ± 1.4	d
0.5 (kg ha−1) Cu	7.40	8.35	7.13	7.63 ± 1.19	bc	13.6	14.8	12.6	13.6 ± 1.5	bc	30.1	32.1	31.6	31.3 ± 1.6	c
1.0 (kg ha−1) Cu	7.95	8.55	7.30	7.93 ± 1.11	ab	13.9	15.0	12.9	13.9 ± 1.5	ab	30.3	33.1	32.3	31.9 ± 2.1	b
2.0 (kg ha−1) Cu	8.23	8.90	7.65	8.26 ± 1.19	a	14.1	14.9	12.9	14.0 ± 1.4	a	30.3	33.8	33.0	32.4 ± 2.5	a

Average data are expressed as mean ± 95% CI (n = 12). Letters indicate significant differences between treatment means (p < 0.05).

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Preliminary analyses were conducted for all three measured variables across each experimental year. Analysis of variance (ANOVA) indicated that treatment effects were statistically significant in every year for all traits measured:

• Yield: F(5,15) = 3.91, p = 0.018 (2019); F(5,15) = 3.85, p = 0.019 (2020); F(5,15) = 3.32, p = 0.032 (2022)
• Crude protein content: F(5,15) = 11.4, p < 0.01; F(5,15) = 13.7, p < 0.01; F(5,15) = 3.65, p = 0.023
• Wet gluten content: F(5,15) = 3.74, p = 0.021; F(5,15) = 35.4, p < 0.01; F(5,15) = 28.8, p < 0.01

The primary objective was to identify a treatment with consistent effects across varying environmental conditions. To this end, a combined statistical analysis was conducted using data from all three years. Following the approach of Berzsenyi (2016) [54], the three-year ANOVA showed significant treatment effects for all traits examined:

• Yield: F(5,10) = 14.9, p < 0.01
• Crude protein content: F(5,10) = 23.3, p < 0.01
• Wet gluten content: F(5,10) = 12.8, p < 0.01 [54]

Given the significance of treatment effects, pairwise differences between means were evaluated using Tukey’s test (α = 0.05). The minimum significant difference (MSD) values were 0.51 t ha⁻¹ for yield, 0.31% for crude protein, and 0.51% for wet gluten content.

In the combined evaluation of treatments over the three-year trial, the highest yield was achieved with the 2 kg ha⁻¹ copper treatment, reaching 8.26 t ha⁻¹—an increase of 1.03 t ha⁻¹ compared to the control (Figure 2). This yield was significantly higher than all other treatments, except for the 1 kg ha⁻¹ dose. In that case, the yield increase of 0.33 t ha⁻¹ was not statistically significant at the 95% confidence level.

The highest crude protein content, 14.1%, was also recorded in the 2 kg ha⁻¹ treatment (Figure 3). This represented a 0.91% increase over the control and a 0.36% increase over the 0.5 kg ha⁻¹ treatment. The differences in crude protein content were statistically significant at the 95% confidence level across all treatments, with the exception of the 1 kg ha⁻¹ dose, where the increase was only 0.07%.

For wet gluten content, the highest measured value—32.4%—was likewise obtained from the 2 kg ha⁻¹ copper treatment (Figure 4). This value was significantly higher than those of all other treatments, with a 2.25% increase compared to the control and a 0.53% increase over the 1 kg ha⁻¹ treatment.

3.2. Results of Deoxynivalenol (DON) Test

Given the known fungicidal properties of copper compounds, we considered it important to examine the infection rate of winter wheat grains in our experiment. Although DON values from small-plot trials may not fully reflect field conditions due to the risk of cross-infection, they still offer useful insight into the potential efficacy of the copper–sucrose complex.

The 2020 and 2022 growing seasons were marked by drought during flowering and the subsequent stages. As a result, DON levels in the harvested wheat samples were naturally low, making it impossible to evaluate the antifungal effects of copper treatments in those years.

In contrast, the 2019 season in Hungary featured cooler and more humid conditions during wheat heading and flowering. At the experimental site, relative humidity in May remained between 80 and 95% for 14 consecutive days. These conditions justified a focused evaluation of DON levels to assess the potential antifungal impact of the copper treatment.

The small-plot experiments were surrounded by large-scale wheat fields. Due to the randomized block design of the trials, measured DON values showed considerable variability (Figure 5), likely influenced by external contamination from neighboring plots. The figure also indicates a decreasing trend in DON concentrations with higher copper treatment doses; however, this trend was not statistically significant at the 95% confidence level.

4. Discussion

The alkaline pH of the experimental soil restricts the uptake of cationic micronutrients, including copper ions. Furthermore, elevated phosphorus levels contribute to the reduced availability of essential micronutrients through the formation of various insoluble phosphorus compounds.

The experimental results demonstrated that the copper–sucrose complex used as a foliar fertilizer had a beneficial effect on the yield (Figure 2) and quality (Figure 3 and Figure 4) parameters of winter wheat grown in copper-deficient soils. The positive impact of copper–sucrose on yield and quality is linked to its role in biochemical processes. Copper-containing enzymes play a significant role in plant biochemical processes. As cofactors, they are crucial for proteins such as cytochrome oxidase, polyphenol oxidase, ascorbate oxidase, polyamine oxidase, Cu-Zn-superoxide dismutase, etc., which influence vital processes like photosynthesis, protein metabolism, and carbohydrate distribution.

The higher yield can be attributed to the relatively strong root activity and enhanced photosynthesis induced by copper. Sucrose is transported across plant cell membranes via sucrose transporters. These transporters are essential for processes such as overall plant growth. As a highly polar compound, sucrose requires proteins for effective membrane transport. Sucrose and its derivative, trehalose-6-phosphate, also serve as signaling molecules that regulate gene expression either directly or through cross-talk with other signaling pathways [55]. The cleavage of the enzyme sucrose synthase yields products that serve as key substrates in various metabolic pathways, including energy production, the biosynthesis of primary metabolites and the formation of complex carbohydrates [56].

It has been proven that a higher copper content is associated with increased yield and improved quality (Figure 1, Figure 2 and Figure 3). According to soil test data, while the upper soil layer contains a satisfactory amount of copper, the lower layers are deficient. In such cases, homogenization of the upper and lower layers is recommended, which can be facilitated by using deep tillage (subsoiling) [57].

Several researchers have reported on the yield- and quality-enhancing effects of copper treatments [52,53,58]. Svecnjak et al. (2013) demonstrated a strong correlation between yield and copper content [59]. Stepien and Wojtkowiak (2016) found that the combined application of copper with other micronutrients significantly increased the content of low-molecular-weight glutenins in wheat [53]. In some cases, the yield results of copper treatments show a wide range of variability. Both positive and negative effects of copper foliar fertilization have been reported [9]. Possible reasons include the form of copper compounds (e.g., effects of anions and ligands), the timing of application, temperature, varying soil composition, and the phenological stage of the treatment. Arshad et al. (2011) [60] emphasized the importance of soil structure. Their studies on saline-alkaline soil showed that copper and zinc supplementation had a positive effect on wheat yield.

In experiments conducted during the tillering stage, copper applied as a foliar fertilizer increased grain yield [52,61]. Using copper-tetramine-sulfate with different ligands, with treatments applied during the tillering and flowering stages, demonstrated increased yield, crude protein, and gluten content [8].

Cheng et al. (2024) [57] investigated the effect of nighttime warming and copper application in copper-deficient soils. They observed an increase in enzyme activity, total biomass, and wheat yield following copper supplementation. The copper concentration in the grains was close to the threshold value found in food products (10 mg kg⁻¹).

Moreira et al. (2019) reported higher yields in winter wheat following copper supplementation on soils rich in organic matter [62]. Korzeniowska and Stanisławska-Glubiak (2011) found that foliar copper sulfate treatments increased yield, although copper concentrations in the seeds decreased [9]. Similar findings were observed by Jankowski et al. (2016) [63], with increased yield accompanied by a decrease in seed copper concentration—likely a dilution effect resulting from greater biomass production. Flynn et al. (1987) likewise found that foliar copper application improved yield and flour quality [58]. According to Kumar et al. (2009), a soil copper concentration of 1.5 mg kg⁻¹ provides optimal conditions for wheat growth and yield [64].

Excess copper levels can cause morphological changes in the development of plant organs. For instance, Nazir et al. (2019) experimentally demonstrated that stress caused by excessive copper exposure could be reduced with H₂O₂ treatment [65,66]. When applied in low concentrations, hydrogen peroxide improved the growth, photosynthesis, and metabolic status of plants, enhancing their tolerance and helping them reduce copper stress. Alhammad et al. (2023) [67] found that hydrogen-peroxide (H₂O₂) is produced in plants in response to biotic or abiotic stress in order to mitigate oxygen-induced cellular toxicity. A high copper content in the soil reduced photosynthesis, leaf area, chlorophyll content, grain yield, and yield-related traits in wheat compared to the control treatment. Foliar application of H₂O₂ to wheat plants grown under soil stress conditions resulted in an increased grain yield relative to the control.

Based on several years of research, it has been observed that due to the impacts of climate change affecting our region, nutrient uptake in crops is increasingly occurring from the deeper soil layers, which retain more moisture.

As a result, greater emphasis was placed on the analysis of subsurface soil layers. Data from these investigations revealed that while the nutrient supply in the topsoil remains adequate, deficiencies are present in the deeper layers.

In light of these findings, a copper–sucrose complex compound was applied. This formulation contains a beneficial ligand with moderate stability, which enhances its suitability for plant nutrition.

In the case of winter wheat cultivated on copper-deficient soils, foliar applications of the copper–sucrose complex were carried out at two phenological stages: tillering and flowering. The treatments not only led to an increase in grain yield, but also resulted in statistically significant improvements in crude protein content and gluten levels.

Our results suggest that the environmental conditions in 2019 may have contributed to increased Fusarium infection, with elevated DON levels and the potential for increased fumonisin contamination. Environmental factors such as rainfall, temperature, and relative humidity are key drivers of Fusarium head blight (FHB). Wheat is most vulnerable to FHB infection during anthesis, although susceptibility can extend through to the soft dough stage [68,69].

The fungicidal properties of copper compounds are well-documented. The examined copper–sucrose complex demonstrated strong antifungal activity against Fusarium graminearum, leading to a substantial reduction in deoxynivalenol (DON) levels, thereby contributing to improved food safety in wheat-based products.

Ibarra-Laclette et al. (2022) [47] observed via microscopic analysis that in the control group, healthy hyphae exhibited smooth surfaces, a tubular morphology, and the formation of characteristic fusiform, clavicle-shaped macroconidia. In contrast, treatment with copper nanoparticles induced multiple morphological alterations in both hyphae and macroconidia. Structural deformation was evident in both fungal elements, indicating that copper had a disruptive effect on their development.

Morphological changes and oxidative stress play a crucial role in antimicrobial and antifungal activity. Due to their small size, nanoparticles can readily interact with cell membranes and penetrate into cells, leading to increased membrane permeability and impaired respiration. Nanoparticles may damage deoxyribonucleic acid (DNA), proteins, and cellular membranes, as well as interfere with the nutrient uptake of bacterial cells [70,71].

5. Conclusions

In this study, a copper–sucrose complex, not previously used for copper supplementation, was applied as a foliar fertilizer to winter wheat grown on copper-deficient soil.

The copper–sucrose complex was applied to the foliage at the tillering and flowering growth stages.

On average across the three years, yield showed a significant increase starting from the 1 kg ha⁻¹ dose, the crude protein content from 0.3 kg ha⁻¹, and the gluten content from 0.5 kg ha⁻¹, compared to the untreated control. In all three parameters, the highest values were achieved with the 2 kg ha⁻¹ treatment, which resulted in a 1.03 t ha⁻¹ increase in yield, a 0.9% increase in crude protein, and a 2.3% increase in wet gluten content.

In 2019, elevated humidity and favorable temperatures during flowering led to DON contamination levels in the control plots that exceeded accepted food safety limits. Application of the copper–sucrose complex resulted in a measurable reduction in DON levels, bringing them below those limits. These findings suggest that copper–sucrose complexes may offer a promising alternative or complement to traditional fungicidal strategies, particularly in years with high disease pressure.

An important finding of the research is that nutrient uptake in winter wheat predominantly occurs from the lower soil layers, whose copper content requires greater attention.