A Sustainable Approach Based on Sheep Wool Mulch and Soil Conditioner for Prunus domestica (Stanley Variety) Trees Aimed at Increasing Fruit Quality and Productivity in Drought Conditions

Abstract

In the context of extreme climate change, experts in fruit production face a significant challenge in developing new strategies aimed at increasing the productivity of fruit tree crops. In order to investigate the changes in various horticultural indices (production, tree growth, and development) as well as the quality of plum fruits, sheep’s wool mulch, a cornstarch-based soil conditioner, and a combination of the two were applied in a Stanley plum orchard. In parallel, an experimental control variation was used. The results showed that the methods used had a substantial impact on fruit yield, size, and weight, with the best results obtained when mulching with sheep’s wool and soil conditioner. Plum fruits from mulching with sheep wool + soil conditioner exhibited the greatest total phenol concentration (1.30 ± 0.09 mg GAE/g dw), followed by the reference sample at 1.16 ± 0.09 mg GAE/g dw. The antioxidant capacity assessed using the three different methods provided favorable results for the experimental variant, sheep wool + soil conditioner. The results indicate that using the three experimental versions increased the fruit yield with 27% (sheep’s wool mulch) and with, 37% (sheep wool + soil conditioner) on average compared to that of the control group, while also improving the fruit quality. The fruit weight increased with 17.26% (cornstarch-based soil conditioner) and with 48.90% (sheep wool + soil conditioner) compared to that of the control, and the fruit size increased with 5% in two experiments (sheep’s wool mulch and a cornstarch-based soil conditioner) with 19% (sheep wool + soil conditioner), compared to the control group.

1. Introduction

Using sheep wool mulch, an ecological soil management method, and a soil condi-tioner are options to increase soil water reserves and, therefore, production and frits quali-ty yet extensive research on plum cultivation is needed.

Drought is a major factor affecting agricultural production in the context of climate change [1], which, in turn, affects food security on a worldwide scale. Crop and fruit production is severely hampered by little rainfall, particularly during the growing season. These issues are made worse if the drought conditions persist for an extended length of time. The lack of rainfall during the growing season, the drop in water level at the roots, the rise in temperatures, and the total amount of evapotranspiration, which results in severe droughts, are all examples of climate change (Figure 1). This results in deep underground boreholes, poor growth, and low-yielding fruit plants due to the drying up of water courses and a decrease in the overall amount of useful water. All these have a major impact on the economy and the environment [2,3,4,5,6,7].

During a protracted drought, crop damage can be lessened using collected rainwater for supplemental irrigation or by preserving rainwater in the soil through a variety of techniques [8], including mulching and using soil conditioners. The time between irrigations can be shortened with the use of proper conservation techniques. By using these techniques, crop survival and productivity can be guaranteed.

The trees in an orchard are planted in a picketing scheme that allows the planting row, the space next to the planting row, and the space in between the planting rows—referred to as the lane—to be identified [9,10].

Weeds are pulled and soil conditioners are applied to the planting row and the area next to it in order to preserve the water content of the soil. In order to reduce competition between weeds and fruit trees for water in orchards and ensure the production of high-quality fruit, proper weed management is essential [11,12].

The development of tree roots, fruit yield and quality, soil fertility, and tree growth are all negatively impacted by conventional soil tillage for weed control [13,14,15,16]. However, because tree roots have a lower density than weed roots, managing weeds in the tree row area presents a significant challenge for orchardists as weeds can strongly compete with fruit trees for water and nutrients [17].

Mulching with both organic and inorganic materials is one of the alternative weed control techniques that can be used to retain water in the soil. Mulching is primarily used to retain soil moisture by inhibiting weed growth and preventing surface water evaporation. Mulching also helps control temperature fluctuations; enhances the physical, chemical, and biological properties of the soil; and, lastly, increases orchard biodiversity [18,19,20]. Figure 2 lists these additional benefits of mulching. To achieve steady and reliable yields, tree crops require the benefits of mulching [21].

Natural, biodegradable mulch materials are a promising alternative to synthetic mulch materials. These materials include a variety of waste products from the cotton industry as well as commonly discarded fibers from the textile industry, such as jute and wool mulch. They provide new possibilities for crops. Sheep wool is also included in the category of organic mulch; it is used to keep the soil moist and fight weeds in plant rows (shrubs, protected crops, etc.) [22,23]. Since there has been a decline in the demand for textiles recently, sheep’s wool is usually wasted after shearing [24].

Wool is an excellent mulch material because of its high nitrogen content and good moisture-retention properties, as demonstrated by numerous trials [24]. Wool is an excellent mulch material because it is high in nutrients (especially nitrogen) and has a good moisture-retention capacity, according to several trials [24]. Although there is information available on the advantages of mulching with other organic materials, we were unable to locate any information regarding the use of sheep wool as a mulching material to produce fruit of a higher quality in the literature [21]. Balawejder et al. (2023) looked into the effects of adding sheep wool to coconut fiber substrates for soilless raspberry cultivation. Fruits grown on sheep wool-enriched coconut substrates had the largest increase in polyphenol content—37.7% more than the control sample—when compared to the substrate made entirely of coconut fiber (100% coconut fiber substrate). Additionally, a unique profile of bioactive compounds was observed in these fruits, indicating a significant impact from the added ingredients [22].

One of the best ways to keep water in the soil is to apply superabsorbent polymers, which are also known as soil conditioners. These have been shown to be effective tools for improving soil water holding capacity, decreasing cumulative evaporation and infiltration rates [24]. They can help plants have better access to nutrients, particularly in situations where there is a shortage of water or in soil that cannot hold much water [25].

So far, a wide range of natural polymers, such as cellulose, starch, guar gum, chitosan, and gelatin, have been used to produce superabsorbers [20]. Hydrogels based on biopolymers offer the benefits of low toxicity, biodegradability, and biocompatibility. Chiorescu’s research intends to test two types of hydrogels—TerraCottem (which contains zeolite) and Zeba (made from natural corn starch)—with the goal of determining their effectiveness and influence on the vegetative state of aromatic plants like Artemisia absinthium L., Artemisia lancea L., and Artemisia lavandulaefolia L. High initial swelling rates in hydrogels lead to a high capacity for water retention, which in turn promotes plant germination and growth. Zeba can release moisture when the plants ask it to, and it can absorb more than 400 times its original weight in water. Additionally, Zeba binds and releases water-soluble nutrients, extending the time that fertilizers are in the root zone and fostering the development of a favorable microenvironment. Zeba breaks down and is consumed by soil microorganisms over time, leaving no trace [26].

There are studies in which such superabsorbers have been used to improve the amount of water in the soil with consequences on fruit quality, especially in tree crops in arid areas. Khalid S. Alshallash et al. studied the effect of hydrogel addition (as a soil conditioner) on the growth and yield of mango trees cultivar cv. Shelly. They demonstrated that different amounts of hydrogel agent can improve the yield and fruit quality of mango trees cv. Shelly under arid and semi-arid conditions in a dose-dependent manner [27].

The aim of the current was to investigate the effects of applying a cornstarch-based soil conditioner, sheep wool mulch, and the combination of the two techniques on plum species’ productivity, physicochemical characteristics, antioxidant capacity, and fruit composition.

2. Materials and Methods

2.1. General Experimental Conditions

The trials were carried out in 2023 in an intensive culture system within a plum tree (Prunus domestica L.) orchard that was established in the spring of 2013. The orchard covers an area of 1 ha and is located in Bihor County of Romania’s Berechiu area (latitude: 46° 57′ 55.464′ N, longitude: 21° 44′ 8.249′ E). The orchard’s climate is constant due to its lowland location, which has uniform forest cover and altitude. These experimental conditions can be applied to other orchards with comparable soil and climate characteristics.

With a density of 833 trees/hectare, the plum trees are planted with a 4 m spacing between rows and a 3 m spacing between trees per row. The base variety is Stanley, and the pollinator varieties are Anna Spath and Centenar.

The orchard’s soil type is a wet phreatic chernozem, characterized by a medium clay profile and an undisturbed texture with uniform granulometric fractions across all horizons. The Phreatic Zone has an effect on the soil. Up to a depth of 68 cm, the humus content is low (below 2%) in the lower horizons and medium (3.1%) in the bioaccumulation horizon. The soil’s reaction has a neutral to weakly alkaline pH, ranging from 7.2 to 8.6. The medium values of apparent density and total porosity indicate its favorable physical properties. With values above 1% at the surface and reaching 13.1% at the base of the profile, carbonates are visible. It is a medium-textured soil that is rich in nutrients, particularly potassium and mobile phosphorus [28].

Over the past three years, the weather conditions in the orchard have been as follows: mean monthly temperatures have been nearly in line with multiannual monthly averages, with mild springs and autumns; however, summers have been excessively hot, with peak temperatures of 36–39 °C for periods of two to three weeks; additionally, rainfall from June to September has been insufficient, primarily during the tree growing season (Figure 3).

No irrigation or chemical fertilizers were applied to the orchard from the time it was planted until the completion of the experiment. Depending on the stage of development of the tree, different maintenance tasks were performed in the orchard. For every species, the crown developed during the first four years following planting. Recurrent tilling and disc work on tree intervals were used to maintain the soil, and manual weed control was carried out on each row of trees. Fruiting cuts, also known as pruning, phytosanitary treatments, and fertilization with organic fertilizers incorporated into the soil were executed during the vegetative rest period, which started in the fifth year after planting when the trees started to bear fruit and reached commercial production. Throughout the vegetation period, weed control was conducted on each row of trees using a feeler tiller in conjunction with manual mowing. Soil maintenance was accomplished by repeatedly working with the tiller and disc on the tree intervals.

2.2. Experimental Design

Stanley variety plum trees were used for the experiments. Sheep wool was used as the mulch, and a polymer based on corn starch in the form of granules (UPL Ben-elux B.V. Holland) was selected as the soil conditioner. The purpose of a soil conditioner is to improve crop performance and the overall properties of the soil by increasing water and nutrient efficiency. It is water-absorbing and biodegradable; when it comes into contact with water, it forms a gel that repeatedly rehydrates and dehydrates over the course of a season, releasing the moisture it has absorbed in accordance with the needs of the plants. It can absorb 400 times its weight in water, reaching 90% of its potential absorption in just one hour. It releases water when plants experience moisture stress; this process lasts for 12 to 15 days, and the plants remain active in the soil for five months [29].

Because sheep wool is easily accessible, it was decided to mulch in the direction of the row. Because of the current economic reorientations and market requirements that impose specific quality standards, local sheep breeders are unable to profit from this product. Sheep wool presents a complex structure made up of 30–40% proteins, particularly keratins, which, as they break down, release nitrogen into the soil; 1–2% lipids; and minerals like potassium, sodium, calcium, magnesium, and iron [30].

On 15 April 2023, the experimental variants (Variants 3 and 4) were installed by adding the conditioner to the soil. These were followed by sheep wool mulching (Variants 2 and 4), with Variant 1 serving as the control and being linear on 5 trees/variant of plum trees × 3 repetitions (Figure 4). The conditioner was applied by deep loosening the soil to a depth of 25 cm using a modified scarifier equipped with a bucket to administer 100 g of conditioner granules per tree. Next, 20 cm of sheep wool mulch was spread on each side of the row, with a width of 80 cm.

2.3. Production and Fruit Quality Parameters

2.3.1. Fruit Production

The fruit on the 15 trees of each variant was completely harvested and weighed with an electronic scale, and the fruit yield was expressed as the average of the amount of fruit on the 15 trees in kg fruit/tree, per hectare. According to size, fruits were sorted by hand, at the same time of harvesting, into two categories according to market norms: Quality I, which consisted of fruits with a diameter of 28–35 mm, tails, and no defects of uniform color and variety-specific characteristics, and Quality II, which included fruits with a minimum acceptable defect of no more than 25% of the fruit surface and a diameter of 25–30 mm. A sample of ten fruits/tree was collected and immediately transported to the laboratory for an evaluation of fruit quality characteristics [31].

2.3.2. Fruit Quality Indices

From each variant, five sample trees were chosen at random, and 25 plum fruits of a similar size and color were selected from each sample tree’s various locations. Three iterations of each determination were made. The fruit quality indices listed below were calculated—with the electronic caliper, the height (h), transversal diameter (d), and vertical diameter (D) were measured. The measurements were then averaged, and the fruit’s size was determined in millimeters using Formula (1):

(1)$\mathrm{S}\mathrm{i}\mathrm{z}\mathrm{e} \mathrm{o}\mathrm{f} \mathrm{f}\mathrm{r}\mathrm{u}\mathrm{i}\mathrm{t} (\mathrm{m}\mathrm{m})={\displaystyle \frac{\mathrm{D}+\mathrm{d}+\mathrm{h}}{3}}$

A sample of fifty fruits/tree was collected from each variant, of average size and different positions and weighed on electronic scales, and the average weight (g) of a fruit was determined [32].

By making an alkaline solution with cupric salt and reducing it over heat in the presence of reducing sugars, the iodometric technique measures the amount of sugar in fruits. The cuprous oxide generated during the reaction is indirectly titrated using a sodium thiosulfate solution [33,34].

Once a single drop of juice from each fruit was removed and combined, the total soluble solid (TSS) content was determined using a refractometer (ABBE, model 2VAJ, Optika SRL, Ponteranica, Italy) at 22 °C. A percentage was used to express the results [34,35].

Fruit maturity is generally determined by a number of factors, one of which is titratable acidity. By neutralizing the fruit juice samples with a strong base at a set pH, the titratable concentration of hydrogen ions was determined. Malic acid percentage was used to express titratable acidity [34,36].

2.3.3. Determination of Bioactive Compounds and Antioxidant Capacity of Plum Extracts

The plum fruits were dried at a temperature of 40 °C in order to extract the bioactive compounds. They were then immersed for a few days in a 70% ethyl alcohol solution (at a weight/volume ratio of 1:10). Following the removal of the alcohol using a rotary evaporator, the remaining extract was subjected to the tests listed below.

The Folin–Ciocalteu method, with some modifications, was used to determine the total phenol content (TPh) of plum fruits. In brief, the aqueous plum extract (100 µL) was mixed with 1700 µL of distilled water, 200 µL of freshly prepared Folin–Ciocalteu reagent (diluted 1:10, v/v), and 1000 µL of 7.5% Na₂CO₃ solution. The mixture was then incubated in the dark at room temperature for 2 h. The absorbance was measured at a wavelength of 765 nm (Shimatzu miniUV-Vis, Kyoto, Japan). The results are reported as milligrams of gallic acid equivalents (GAE) per 1 g of dry weight (dw), with gallic acid as the standard [37].

The colorimetric method of aluminum chloride was used to determine the total flavonoid content (FLAV). A 1 mL solution of plum extract in water, 4 mL of distilled water, and 0.3 mL of a 5% NaNO₂ solution were mixed together in a 10 mL volumetric flask and left to react for 5 min. Subsequently, 0.3 mL of a 10% solution of AlCl₃ was introduced into the mixture, which was then thoroughly mixed before a 6 min pause. A final volume of 10 mL was achieved by combining 2 mL of a 1 M NaOH solution with distilled water. After a 15 min interval, the absorbance at a wavelength of 510 nm was measured using a spectrophotometer (Shimatzu miniUV-Vis, Kyoto, Japan). The results were quantified in mg Quercetin equivalents (QE)/g dw [37].

The antioxidant capacity of plum extract was assessed using three spectrophotometric assays: DPPH (2.2, diphenyl-picryl-hydrazyl), FRAP (ferric reducing antioxidant power), and CUPRAC (cupric reducing antioxidant capacity) [37].

The radical scavenging capacity of the plum extract was evaluated using the stable DPPH radical, following the method described by Brand Williams et al. in 1995, with slight modifications [38].

A 100 µL sample of plum extract was combined with 2800 µL of 80 µM DPPH solution and kept in a dark room at room temperature. The absorbance of the samples was measured after a period of 30 min using a Shimadzu mini-UV-VIS spectrophotometer, at a wavelength of 517 nm. The results were quantified as µmol of Trolox equivalent (TE)/g dw [37].

The FRAP assay tested the antioxidant capacity of the plum extract by measuring its ability to convert Fe³⁺ from the tripyridyltriazine Fe(TPTZ)3+ complex to the blue-colored Fe(TPTZ)3+ complex in an acidic environment. The plum extract (100 µL) was mixed with 500 µL of FRAP working solution and 2 mL of distilled water. The mixture was left to react for 1 h in a dark place. The results were quantified in terms of µmol TE/g dw [37].

The CUPRAC method involves mixing a solution of copper (II) chloride, an alcoholic solution of neocuproine (2.9-dimethyl-1, 10-phenanthroline), an aqueous buffer of ammonium acetate at pH 7, and 0.5 mL of plum extract. The final volume is adjusted to 4.1 mL with water. The absorbance of the samples was measured at a wavelength of 450 nm after 30 min. The results were quantified in µmol TE/g dw [39].

2.4. Measurements and Determinations of Tree Growth

2.4.1. Trunk Cross-Sectional Area (TCSA)

For each tree, its diameter was measured 30 cm above the grafting site to calculate the trunk cross-sectional area (TCSA), which was then expressed as TCSA = girth2/4π [40]. For each of the five trees from the experimental variants, the average area of the trunk section was measured in centimeters [40,41].

2.4.2. The Number and Length of Annual Grown Branches

The average number of annual grown branches was calculated by counting all of the annual branches that developed on each of the five trees in each variant. The annual branch growth length was determined by measuring the length of every annual branch on each of the five trees in each variant and averaging them.

2.4.3. Chlorophyll Determination

Chlorophyll determination was performed with the SPAD 502PLUS Digital Chlorophyll Meter. The determinations were made monthly, on the 10th of the month, by measuring, on the same 6 leaves from the crown of the tree, 3 leaves from one side of the crown and 3 leaves from the opposite side of the crown (2 leaves from the basal part of the crown, 2 leaves from the middle of the crown, and 2 leaves from the top of the crown) from 5 different trees, after which the readings were averaged [42].

2.5. Statistical Analysis

Standard deviation (SD) or mean ± SD was used to present the data. The data were analyzed using GraphPadPrism (version 8.01) software (GraphPad Software, Inc., La Jolla, CA, USA) and a one-way analysis of variance [43]. Tukey’s multiple comparison test was employed with a significance level set at p < 0.05 to find statistically significant changes. With the PAST program—version 4.09—correlation and principal component analysis (PCA) were carried out.

3. Results

3.1. The Production of Fruits

The harvesting of the Stanley variety fruit took place on 26 August 2023. Table 1 displays the fruit production achieved for the plum cultivar along with the fruit percentage for each quality category. All experimental methods had a significant (p < 0.05) positive influence on fruit production, according to data analysis. Additionally, the technique used had a big impact on output; mulching with sheep wool and soil conditioner produced the best results (V4). When it comes to production quality, the Stanley variety’s mulching version with sheep wool and soil conditioner (V4) improved Quality I production more than the control version by 4.42%.

The highest increase in production compared to the control was recorded in V4 (37.35%), followed by V2 (27.49%) and V3 (12.87%), an aspect shown in Figure 5.

3.2. Physical and Chemical Parameters of Fruit Quality

The size and weight of the fruits varied significantly depending on the method applied (Table 2). Significantly better results, both compared to the control and compared to the other variants (p < 0.05), were also obtained with V4, which combines the two procedures (mulching with sheep wool + soil conditioner).

With a weight difference of 48.90%, V4 had the largest difference in fruit weight from the control, followed by V2 (30.78%) and V3 (17.26%).

Table 3 displays the findings of the chemical parameters that were observed (total sugar, total soluble solids, total acidity, and TSS/TA ratio). The total sugar content for the Stanley variety ranges from 11.58 ± 0.02% at V2, which is marginally lower (p > 0.05) than V1, to 15.97 ± 0.03% at V3, which is significantly higher (p < 0.05) than V1, V2, and V4. The variant V1 control has a total soluble solid content of 18.60 ± 0.32%, which is significantly lower than variants V2, V3, and V4. The variant with soil conditioner administration has a total soluble solid content of 21.50 ± 0.2%, which is significantly higher than the other methods.

Fruit maturity is determined by a number of factors, one of which is titratable acidity. One of the most crucial elements in determining the fruit’s performance and flavor over time is its level of ripeness. Plums’ overall acidity varies, with V1, V3, and V4 having 0.61 ± 0.1%, which is substantially less than V2’s 0.64 ± 0.1%.

The fruit TSS/TA ratio is a significant factor in taste, feel, and texture. The ratio of sugar to acid determines the distinctive flavor of many fruits. Additionally, the TSS/TA ratio is a commercial and sensory ripeness indicator. The TSS/TA ratio values for the control sample ranged from 31.11 ± 5.70 to 34.87 ± 0.17 for the V3 samples, with the V2 and V4 samples following suit. The TSS/TA value for Stanley plum fruits is in line with the findings of Bozhkova, 2014, which show that the same Bulgarian variety produced a value of 31.49 [44].

3.3. Bioactive Compounds and Antioxidant Capacity

The total polyphenol content of plum fruits exposed to sheep’s wool, a soil conditioner application, and a combination of the two methods is displayed in Figure 6A. The control sample (V1) had the lowest total phenol content (1.16 ± 0.09 mg GAE/g dw), while the plum fruits from variant V4 had the highest (1.30 ± 0.09 mg GAE/g dw). Conversely, applying conditioner (V3) or sheep’s wool (V2) as separate procedures resulted in a notable reduction in the overall polyphenol content when compared to sample V4 and the control. The flavonoid content of the control samples (V1) was higher than that of the combined methods (V4) and the conditioner application (V3), which demonstrated a significant 15% decrease in flavonoid content (Figure 6B). Similar to the results of the polyphenol content, the treatment with sheep’s wool (V2) produced the fruits with the lowest content of flavonoids (Figure 6).

Three distinct approaches were used to measure antioxidant capacity. It has been observed that V4 exhibits noticeably better results than V1, V2, and V3 using all three methods. Sample V4, which received both mulching and soil conditioner application, exhibited the highest antioxidant capacity among the three applied methods (Figure 7A–C). The ability to reduce iron and copper ions in the V4 sample was measured at 7.65 ± 0.14 µmol TE/g dw and 37.85 ± 0.04 µmol TE/g dw, respectively. Regarding antioxidant capacity, the samples treated with conditioner (V4) and sheep wool (V2) showed lower values than the control group (V1).

The techniques used had no effect on the trunk cross-sectional area, the quantity of annual branches, or their length (Table 4). Significant variations are observed in all experimental variants when compared to the control and amongst the variants, as indicated by the SPAD index values. At V4, the highest values (63.66 ± 0.45) were found.

3.4. Correlation Analysis

Pearson’s correlation analysis is commonly used to determine the degree of correlation between the two parameters. The Pearson correlation coefficient has a range of values from −1 to 1, where 1, 0, and −1 represent a perfect match, no correlation, and a perfect negative correlation, respectively. A strong, positive relationship between two variables is indicated when their correlation value is 0.7 [45].

A highly significant positive correlation was observed between the total phenol content (TPh) and all antioxidant capacities examined (Figure 8). As a result, the FRAP and CUPRAC had a 0.98 correlation, and the DPPH had a 0.91 Pearson correlation. Furthermore, there was a significant positive correlation (r = 0.85) between TPh and total flavonoid. Figure 8 depicts a highly significant positive correlation between TCSA, branch length and number, and chlorophyll content and also with fruit yield, weight, and size.

3.5. Principal Component Analysis

Figure 9 illustrates the quality indicators, antioxidant capacity, and bioactive compounds of the Stanley variety that were submitted for various applications of sheep wool mulching and corn starch-based polymers based on PCA. The total of PC1 and PC2 for the application of corn starch, sheep wool mulching, and their combination was 89.03, of which PC1 and PC2, respectively, explained 50.21% and 38.82% of the total variation. A visible separation was produced by the V1, V2, V3, and V4 applications based on PC1, and a visible separation was produced by the V4 and V2 samples based on PC2. The loading plots showed that fruit production, branch length and number, and PC1 loadings were all highly significant. Strong second principal component loadings were observed for the antioxidant capacity (FRAP, CUPRAC, and DPPH) and total phenol content, which may be significant determinants of plum fruit’s response to the application of mulching with sheep wool or starch-based polymers.

4. Discussion

Water scarcity is a global issue for agronomy. A number of factors have contributed to the recent depletion in water supplies, the main one being poor management. Cropland expansion in response to water scarcity impacts horticulture crops worldwide and may even impact climate change. In commercial orchards, fruit yield is the ultimate goal. However, dryness can impede tree development, which consequently lowers the quantity and quality of fruit produced. Therefore, finding a solution to this issue is crucial [46].

Three approaches to soil water accumulation for plum fruit crops were tested in this study: applying a soil conditioner (V3), mulching with sheep wool (V2), and combining the two approaches (V4). Their function is to augment the water intake in situations where precipitation and irrigation are unable to supply the required amount. By measuring certain parameters, the effects on fruit production, quality, and tree development were assessed. For the Stanley plum variety, all applied techniques generally had positive effects on the parameters under investigation.

The techniques used had a big impact on the amount of fruit produced. The highest percentage increase in production over the control was recorded at V4 (37.35 V4%), with V2 (27.49%) and V3 (12.87%) following closely behind. The greatest impact on the increase in fruit production was achieved by combining the two methods of mulching sheep wool with a soil conditioner. Variant V4—sheep wool mulching + soil conditioner—was the most advantageous, followed by Variant V2—sheep wool mulching—and Variant V3—soil conditioner—in terms of how the trials affected the size and weight of the fruits.

Total acidity and soluble dry matter, the two evaluated chemical parameters, have values that fall within the usual ranges for the Stanley cultivar. The V3 administration soil conditioner variant has the highest recorded values in terms of total sugar (15.97%). Due to the physical properties of the soil, during the summer months, it becomes compacted and loses water, resulting in large holes in the soil layer that range in size from 0 to 40 cm. As the water content at the root level drops, the fruit’s sugar content naturally increases. Yang et al. conducted a study in 2023 to investigate the effects of five different mulching techniques [47]. The fruit’s sugar content rises as a result of an osmotic regulation mechanism caused by mulch application. Additionally, their findings show that applying a reflective membrane to improve lighting not only increases photosynthetic efficiency but also controls sucrose metabolism and enzyme activities involved in the synthesis of energy substances. Some of the most essential metrics for evaluating fruit quality include soluble solids, total sugar concentration, and titratable acidity. Fruit acidity is a good predictor of organoleptic qualities that impact the way fruit flavor is perceived. Organic acid complexes (citric, malic, tartaric, etc.) are responsible for the sour flavors in fruits, whereas soluble solid content determines a fruit’s sweet taste. Fruit quality and market value can be determined by measuring its soluble solids concentration and qualitative titratable acidity indicators [48]. The proportion of sugar to acid in the fruit gives it its distinct flavor and aroma.

Plums are a great source of various health-promoting nutrients and compounds, including several phenolic compounds, which are thought to be natural antioxidants. Additionally, they contain a lot of organic acids, such as citric and malic acid. Plums have more antioxidant activity than other common fruits [49,50]. Polyphenols are a large class of phytochemicals that are used by plants for many biological functions, such as adapting to various biotic and abiotic stresses [50,51]. Drought stress frequently affects plant metabolism and physiological functions, which leads to a variety of changes in the plants’ biochemical makeup [51]. Numerous studies have shown that fruits’ polyphenol content can increase in response to a water deficit as a stress-adaptive response [52]. Environmental conditions are the most significant factor influencing the content of bioactive compounds with antioxidant activity. Plant defense mechanisms such as the concentration of polyphenols can be enhanced by hydrostatic stress. When compared to the V2 and V3 samples in our study, the control variant (V1) had the highest content of total phenols and flavonoids. A plausible explanation for this could be that the application of soil conditioner and sheep wool improved water retention, preventing the plant from utilizing polyphenolic compounds as a defense mechanism. However, it is interesting to note that adding soil conditioner (variant V4) along with sheep wool increased fruit quality, as measured by plum physicochemical quality indicators and polyphenol content and antioxidant capacity. This finding suggests that the combined application of soil conditioner and sheep’s wool has a synergistic effect that results in high-quality fruit production even under challenging circumstances as documented in 2023.

Although the development of the trees was positively impacted by the use of the three experimental methods, the data analysis revealed no statistically significant differences in trunk cross-sectional area, the number of annual branches, or branch length when compared to the control. Conversely, the SPAD index values demonstrate notable variations across all experimental variants, with exceptionally high values in V4 (63.66 ± 0.45) indicating excellent mineral substance supply to the plant, which promotes good photosynthesis and yields higher fruits with noticeable weight and size changes. The value of the SPAD index is used in agricultural research as an indicator of the supply of mineral substances to plants. The higher the value of this index, the higher the chlorophyll concentration and the plant’s ability to carry out its physiological functions as well as possible [53].

Sheep wool, according to Balawejder, M., released available nitrogen when added to the 30% SW coconut substrate used in raspberry crops. This directly resulted in a high level of chlorophyll content (SPAD 46.81). Raspberry fruits grown on a coconut substrate supplemented with 30% sheep wool showed a notable increase in polyphenol content [22].

Romanian researchers tested the efficacy of using sheep wool as mulch on a culture of roses for jam. The results of the experiment indicated that plant growth was highest in rows mulched with wool, indicating that the mulch improved the pH and humus content of the soil. During the second blooming wave, varieties that had been mulched with wool produced more flowers. Additionally, the mulching technique had an impact on the biochemical composition of the petals; plants growing in wool-mulched rows, for example, had higher anthocyanin concentrations [54].

Applying sheep wool mulch has the drawback of encouraging rodent (field mouse) reproduction. For this reason, it is advised to apply raticide substances once a year, at the end of the tree’s vegetation cycle, before the cold season arrives, and before the leaves fall.

Thus far, as we are aware, fruit tree crops have not been the subject of any research on these techniques. The results of this study indicate that these techniques are superior to plum varieties in the climate of the studied area. Even though the V4 sheep wool mulch combination with the soil conditioner variant yields higher values in terms of fruit production, weight, and size, the values obtained with the V2 sheep mulch variant show that it is a viable solution to be used in fruit plantations. This is because the material is free because it meets the commercial quality requirements for this product, and the V2 sheep mulch variant requires less capital.

5. Conclusions

In order to highlight the effects on plum tree development as well as fruit quality under environmental conditions in 2023 in an orchard in northwest Romania, we used two distinct agricultural practices in our study: mulching with whole sheep and soil conditioner (based on various water retention mechanisms) and a combination between them. Based on the research findings, it appears that the implementation of these agricultural practices had no bearing on tree growth measurements, most likely due to the brief monitoring period. In comparison, the application of sheep’s wool and soil conditioner (V4 sample) resulted in favorable outcomes in terms of fruit production, weight, and size, as well as antioxidant capacity and the content of bioactive compounds. Our study’s findings confirmed that the most beneficial option during the drought was to combine soil conditioner and sheep’s wool for their synergistic effect, specifically for the Stanley variety of Prunus domestica.

Thus, in the context of climate change, which leads to an increase in water deficit, finding alternative solutions for retaining soil moisture and, thus, reducing the stress of water deficit are urgent objectives, especially in the case of horticultural and agricultural crops. For the purposes of this project, we believe that using byproducts such as sheep’s wool is a sustainable way to repurpose these wastes and turn them into potentially useful green materials that can be employed to reduce the effects of drought.

Author Contributions

Conceptualization, M.A.G., C.G.D. and C.A.R.; Data Curation, I.M.B., C.G.D., C.A.R. and S.I.V.; Formal Analysis, C.G.D., D.G., C.A.R., S.I.V. and M.B.P.; Investigation, M.A.G., I.M.B., D.G., C.A.R. and M.B.P.; Methodology, M.A.G., C.G.D., C.A.R. and S.I.V.; Project Administration, M.A.G.; Resources, M.A.G.; Visualization, M.B.P.; Writing—Original Draft Preparation, M.A.G. and D.G.; Writing—Review and Editing, I.M.B., C.G.D., D.G. and M.B.P. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. The influence of climate changes on fruit crops.

Figure 2. The main characteristics of mulched and unmulched soil.

Figure 3. Average (2021–2023) monthly precipitation and temperature. The capital letters represent the months of the year (J—January; F—February; M—March; A—April; M—May; I—June; I—July; A—August; S—September; O—October; N—November; and D—December).

Figure 4. Experimental conditions: V1, control; V2, mulching with sheep wool; V3, soil conditioner based on corn starch; V4, mulching with sheep wool + soil conditioner.

Figure 5. The variations in the three variants’ production in relation to the control variant. V2, sheep wool mulching; V3, soil conditioner made of corn starch; and V4, sheep wool mulching combined with soil conditioner.

Figure 6. Total phenol content (A) and total flavonoid content (B) of plum fruits with different treatments of mulching soil. The results represent the means ± SD (n = 3). Different lowercase letters represent the difference between samples with significance at the p [less than] 0.05 level. V1, control; V2, mulching with sheep wool; V3, soil conditioner based on corn starch; V4, mulching with sheep wool + soil conditioner. GAE—equivalent gallic acid, QE—equivalent quercetin.

Figure 7. Antioxidant capacity of plum fruits determined by the DPPH (A), FRAP (B), and CUPRAC assays (C). The results represent the means ± SD (n = 3). Different lowercase letters represent the differences between samples, with significance at the p [less than] 0.05 level. V1, control; V2, mulching with sheep wool; V3, soil conditioner based on corn starch; V4, mulching with sheep wool + soil conditioner. DPPH—2,2-Diphenyl-1-picryl-hydrazyl-hydrate, FRAP—ferric reducing antioxidant power, CUPRAC—cupric reducing antioxidant capacity, and TE—equivalent Trolox.

Figure 8. Pearson’s correlation analysis of total polyphenols, antioxidant capacity, and quality indicators of Stanley plums after application of sheep wool mulching, corn starch-based polymers, and a combination of them. The significant correlation is marked in bold. TPh—total phenol content, FLAV—total flavonoid content, DPPH—2,2-Diphenyl-1-picryl-hydrazyl-hydrate, FRAP—ferric reducing antioxidant power, CUPRAC—cupric reducing antioxidant capacity, TSS—total soluble solid, TA—titratable acidity, and TCSA—trunk cross-sectional area.

Figure 9. Principal component analysis of total polyphenols, antioxidant capacity, and quality indicators of Stanley plums after the application of sheep wool mulching (V2), corn starch-based polymers (V3), a combination between them (V4), and the control (V1). TPh—total phenol content, FLAV—total flavonoid content, DPPH—2,2-Diphenyl-1-picryl-hydrazyl-hydrate, FRAP—ferric reducing antioxidant power, CUPRAC—cupric reducing antioxidant capacity, TSS—total soluble solid, TA—titratable acidity, and TCSA—trunk cross-sectional area.

Table 1

Fruit production in Stanley cultivar.

Samples	Fruit Productionkg/Tree	Fruit ProductionTons/ha	Quality I%	Quality II%
Control	41.49 ± 1.13 d	34.56 ± 0.94 d	91.07	8.93
Mulching with sheep wool	52.90 ± 1.74 b	44.06 ± 1.45 b	94.65	5.35
Soil conditioner based on corn starch	46.83 ± 1.68 c	39.01 ± 1.40 c	94.08	5.92
Mulching with sheep wool + soil conditioner	56.99 ± 1.48 a	47.47 ± 1.23 a	95.49	4.51

The results represent the means ± SD (n = 15). Different lowercase letters indicate significant differences between samples (p < 0.05).

Table 2

Fruit quality indices of Stanley cultivar in relation to the method applied.

Samples	Size (mm)	Weight (g)
Control	38.96 ± 0.27 d	32.48 ± 1.15 d
Mulching with sheep wool	44.20 ± 0.61 b	42.95 ± 0.84 b
Soil conditioner based on corn starch	40.93 ± 0.17 c	38.51 ± 0.42 c
Mulching with sheep wool + soil conditioner	46.25 ± 0.90 a	48.90 ± 0.54 a

The results represent the means ± SD (n = 25). Different lowercase letters indicate significant differences between samples (p < 0.05).

Table 3

Total sugar, soluble solids content, titratable acidity, and TSS/TA ratio.

Samples	Total Sugar(%)	TSS(° Brix)	TA(% Acid Malic)	TSS/TA
Control	11.64 ± 0.06 c	18.60 ± 0.32 d	0.61 ± 0.10 b	31.11 ± 0.70 c
Mulching with sheep wool	11.58 ± 0.02 c	21.05 ± 0.04 b	0.64 ± 0.10 a	32.90 ± 0.45 b
Soil conditioner based on corn starch	15.97 ± 0.03 a	21.50 ± 0.20 a	0.61 ± 0.10 b	34.87 ± 0.17 a
Mulching with sheep wool + soil conditioner	14.65 ± 0.03 b	20.20 ± 0.25 c	0.61 ± 0.10 b	32.76 ± 0.51 b

TSS—total soluble solid; TA—titratable acidity. The results represent the means ± SD (n = 25). Different lowercase letters indicate significant differences between samples (p < 0.05).

Table 4

Results of tree growth determinations for the Stanley cultivar.

Samples	TCSA(cm2)	The Number of Annual Grown Branches	The Length of the Annual Branch Growths(cm)	ChlorophyllsSPAD
Control	177.54 ± 31.13 a	79.60 ± 16.49 a	64.26 ± 5.11 a	54.27 ± 0.45 d
Mulching with sheep wool	217.43 ± 37.58 a	104.40 ± 17.80 a	82.47 ± 3.64 a	58.48 ± 0.45 b
Soil conditioner based on corn starch	184.74 ± 52.32 a	99.60 ± 21.69 a	69.36 ± 6.66 a	56.60 ± 0.51 c
Mulching with sheep wool + soil conditioner	230.77 ± 27.98 a	106.80 ± 27.18 a	84.19 ± 3.05 a	63.66 ± 0.45 a

TCSA—trunk cross-sectional area. The results represent the means ± SD (n = 15). Different lowercase letters indicate significant differences between samples (p < 0.05).

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