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Article

Increasing Productivity and Fruit Quality of ‘Mutsu’ Apple Orchard by Dwarfing Treatments

by
Maria Małachowska
*,
Tomasz Majak
,
Tomasz Krupa
and
Kazimierz Tomala
Department of Pomology and Horticulture Economics, Institute of Horticultural Sciences, Warsaw University of Life Sciences (SGGW-WULS), ul. Nowoursynowska 159 C, 02-787 Warsaw, Poland
*
Author to whom correspondence should be addressed.
Agriculture 2024, 14(10), 1838; https://doi.org/10.3390/agriculture14101838
Submission received: 27 September 2024 / Revised: 15 October 2024 / Accepted: 17 October 2024 / Published: 18 October 2024
(This article belongs to the Special Issue The Impact of Environmental Factors on the Quality of Horticultural Commodities)
Browse Figures
Versions Notes

Abstract

:
The aim of this 2022 study was to evaluate the effect of tree growth-limiting treatments on the tree yield and quality of ‘Mutsu’ apples. The experiment was established on 7-year-old trees on M.9 rootstock in a commercial orchard in Wilga near Warsaw. Growth-limiting treatments included unilateral root pruning, spraying the trees with Regalis Plus 10 WG at various times, and spraying with Flordimex 480 SL. Eight combinations were used, with four replicates of 20 trees per repetition. The measurements included fruit set, length of this year’s shoots, yield per tree, average fruit weight, and the size structure of the yield. The distinctive physiological status of the apples was assessed directly after harvest, directly after 8 months of storage under CA conditions (1.5% CO2, 1.5% O2, 1 °C, >92% RH) and after an additional 7 days of shelf-life. Spraying trees with Regalis Plus 10 WG from the balloon stage onwards, irrespective of the treatment with root pruning, was most effective in both inhibiting long-stem growth and increasing tree yield (by almost two times) by increasing the number of apples per tree. The growth response of long-stemmed apple trees to both unilateral root pruning and Ethephon spray was significantly lower than it was to Regalis Plus 10 WG spray and had relatively little effect on their yield. Regalis Plus 10 WG resulted in a clear reduction in average fruit weight (by about 100 g), which, in the case of the large-fruited cultivar ‘Mutsu’, should be seen as an advantage. Its application from the balloon stage onwards promoted higher apple firmness at harvest and after simulated handling preceded by long-term storage.

1. Introduction

Apple production in Poland is developing dynamically and is the most important sector of the fruit-growing industry. Thanks to modern storage technologies, apples are available on the market year-round. As Europe’s leading apple producer, Poland faces many challenges. Despite their global success, some apple cultivars are not cultivated [1,2] due to their specific climatic requirements. The yield of cultivated apple cultivars, as well as other fruit species grown in temperate climates, is determined by the weather conditions during the peri-bloom period and depends on the occurrence of spring frosts [3,4,5]. According to the Central Statistical Office, the apple harvest in Poland in 2022 is 4.2 million tonnes, while the World Apple and Pear Association (WAPA) has estimated the harvest at 4.495 million tonnes [6]. Despite the increase in apple production, export volumes are not increasing accordingly. The challenges of the Russian embargo continue to affect Polish producers, forcing them to look for new markets and implement advanced agrotechnical practices to ensure high-quality apples that meet global standards [7].
Until recently, Polish producers focused mainly on the ‘Idared’ cultivar for the eastern market. Currently, ‘Gala’, ‘Golden Delicious’, and ‘Red Delicious’ are the leading cultivars. Depending on the genetic characteristics of the cultivar, the problem of adequate growth and the yield of the trees occurs to varying degrees. Bilateral root pruning is performed to maintain the balance between growth and fruiting [8]. However, drought occurring after such treatment can result in both russeting and excessive fruit drop [9,10,11]. Therefore, combining unilateral root pruning with another method of regulating tree growth can alleviate the stress caused by bilateral root pruning [12,13].
Moderate shoot growth and good insolation of the tree crown are critical factors in ensuring high yields, as are proper apple size and coloration. The production of “Extra” Class fruit is facilitated by bioregulators for growth [14,15,16,17]. For this purpose, calcium prohexadione (contained in Regalis Plus 10 WG) is used to prevent the formation of active gibberellin GA1 from inactive gibberellin GA20. In addition, there is increased production of GA4 gibberellin, which reduces fruit russeting [18,19,20].
Intense shoot growth adversely affects flower bud formation and fruit setting after flowering. This is because the strongly growing shoots compete with the fruit for nutrients and contribute to fruit drop [21,22,23,24]. In contrast, calcium prohexadione causes a decrease in auxins and an increase in cytokinins [25,26]. Cytokinins are considered promoters of flower bud set initiation [26,27,28]. An inhibitory effect of calcium prohexadione on ACC (1-aminocyclopropane-1-carboxylic acid) oxidase activity was also observed. The reaction requires 2-oxyglutaric acid and ascorbic acid [29,30]. Perhaps because of the similar structure, calcium prohexadione is incorporated in place of the compounds mentioned above, with the resulting disruption of the production of ethylene—a phytohormone produced in the plant in significant quantities, especially during periods of high stress. Stress can be caused by various factors, including but not limited to adverse weather conditions, such as lack of or excess water in the soil. Intense ethylene production contributes to faster ageing and the formation of a cut-off layer and, consequently, to the dropping of buds and leaves [30,31,32]. To counteract the negative effects of stress and to achieve high-quality yields, the growth of this year’s shoots must be skillfully controlled.
In a high-apple-production environment, obtaining the highest possible percentage of premium quality fruit in the crop is becoming a priority. Such apples are accepted in the world’s most demanding markets. Among the green-skinned apple cultivars, ‘Mutsu’ is the most noteworthy [33]. However, trees of this cultivar require the skillful management of shoot growth, which is significant for the size and regularity of yield. The strong tree growth of this cultivar has a negative effect on flower bud formation and fruit quality [34,35]. Therefore, crown contour pruning and root pruning are recommended, alongside growth regulators, including calcium prohexadione [14,36,37]. This compound was shown to effectively inhibit the growth of this year’s shoots, with no negative effect on either fruit yield and quality or the initiation of tree flowering [38]. For apples’ high quality and storability, their physiological stage at harvest is also important [39,40]. Adequate action in these areas makes it possible to obtain fruit that is well-developed and meets high quality standards, even after long-term storage [41,42,43].
The present study aimed to evaluate the effectiveness of unilateral root pruning, the application of Regalis Plus 10 WG, and the spraying of Ethephon on apple tree growth reduction and to find out the effects of these treatments on tree yield and quality of ‘Mutsu’ apples at harvest, directly after storage, and after simulated marketing.

2. Materials and Methods

The study material consisted of trees and fruit of the ‘Mutsu’ cultivar on a private orchard farm in the municipality of Wilga. The orchard was established in the spring of 2015 with annual trees on M.9 rootstock at a spacing of 3.5 × 1 m on sandy loam soil with a slight acid reaction. Herbicide fallow was maintained in the tree rows and turf in the inter-rows.
Tree protection treatments against disease and pests were carried out based on pheromone trap monitoring and control according to the current requirements of Integrated Plant Protection, using registered plant protection products authorized for use in 2022. A soil analysis for mineral content and pH was also carried out.
Over-crown sprinkling was used to protect the trees from frost. In the spring of 2022, temperature drops below 0 °C at 1 m were recorded several times. Thanks to the use of over-crown sprinkling, no damage to flower buds or flowers was recorded.
The study was carried out in eight treatments of the experiment. Four repetitions were distinguished in each treatment, with 20 trees per repetition. The experiment involved two rows of trees, well apart from each other, to avoid the risk of spray drift on trees in other treatments. In one of these rows, unilateral root pruning was performed using an oblique cut. Regalis Plus 10 WG was applied in two separate treatments, differing in the timing of spraying the trees.
The following treatments were used in the experiment:
  • Control (no growth-limiting treatments)—C;
  • Unilateral root pruning with an oblique knife—RP;
  • Regalis Plus 10 WG applied from the balloon stage (BBCH 60–69)—RB;
  • Regalis Plus 10 WG applied at standard times, according to the label (BBCH 71–73)—RS;
  • Flordimex 480 SL applied 4 times, from the end of flowering, at 2-week intervals, at doses of 250, 200, 150, and 100 mL·ha−1—Ethephon;
  • Unilateral root pruning with an angled knife + Regalis Plus 10 WG applied from the balloon stage (BBCH 60–69)—RP + RB;
  • One-sided root pruning with an angled knife + Regalis Plus 10 WG applied at standard dates, according to the label (BBCH 71–73)—RP + RS;
  • Unilateral root pruning with an angled knife + spraying the trees 4 times with Flordimex 480 SL from the end of flowering, at 2-week intervals, at doses of 250, 200, 150, and 100 mL·ha−1—RP + Ethephon.

2.1. Growth-Restricting Treatments

Root pruning: Unilateral root pruning with an angled knife was carried out on 1 April 2022 at a distance of 50 cm from the trunk and to a depth of 40 cm.
Regalis Plus 10 WG: The first spraying of the trees with Regalis Plus 10 WG was carried out at two different times, depending on the treatment of the experiment. In one treatment, the first application was carried out at the ‘balloon’ stage (BBCH 60–69)—3 May 2022, while in the other treatment, the first spraying of the trees was carried out 2 weeks later, i.e., at the standard date according to the label (BBCH 71–73)—17 May 2022, when this year’s growth reached 4–7 cm in length and had fully developed 4–5 leaves. Another tree spraying in both treatments was performed after 2 weeks.
Flordimex 480 SL: Spraying of trees with Flordimex 480 SL was carried out four times. The first treatment at 250 mL·ha−1 was made when 80% of the flower petals had fallen. Subsequent spraying was carried out at 2-week intervals, using decreasing product doses, on successive dates, of 200, 150, and 100 mL·ha−1. The trees were sprayed under high sun operation in the morning at a temperature of at least 16 °C, using 750 L of water per ha for the treatment.

2.2. Fruit Storage Conditions

To assess the quality of the fruit after storage, one pallet box of fruit (300 kg) was taken each within a given combination of experiments. Directly after harvesting, they were placed in a cold chamber for rapid cooling, and after 7 days, the target atmospheric composition (1.5% CO2 and 1.5% O2) was established. The apples were stored for 8 months at a temperature of 1 °C and a relative humidity above 92%. To reduce the development of apple rot agents (grey mold, wet rot), the fruit-filled chamber was fogged with a biological plant protection product (Polyversum®, at a dose of 150 g·500 m−3; Biopreparaty Ltd., Unetice, Czech Republic). The chamber was fogged for 1 h using a K-22 BIO fogger (Pulsfog®, Überlingen, Germany).

2.3. Indicators Determined in the Orchard

Number of fruitlets per 100 inflorescences: In eight treatments, on 20 trees in each treatment (four repetitions of five trees each), one branch was marked, and inflorescences were counted before full flowering. The selected branches were at a similar height. After the June drop, the number of lingering fruitlets was recorded. These data were used to calculate the number of fruitlets per 100 inflorescences. The shoots used to calculate the number of fruitlets were always selected along the line of the rows, alternating between the north and south sides of the trees.
The average length of this year’s increments: Five representative trees per treatment were each selected in September 2022 after shoot growth had finished. Within each tree, the lengths of 20 randomly taken shoots were measured, with 10 from the lower part of the crown and a further 10 shoots from the upper part of the crown. The shoots used to measure growth were taken along the row lines, with samples collected alternately from the northern and southern sides of the trees. On each side, five shoots were measured from the lower part of the crown and another five from the upper part, ensuring balanced sampling across both sides of the tree. Results are given in centimeters [cm].
Yield per tree: To determine the value of this indicator, fruit was harvested and weighed separately from each of the five trees in the treatment. The results are given in kg·tree−1.
Average fruit weight: Results were obtained by dividing the yield per tree (kg) by the number of apples picked. The results are given in grams [g].
Size structure of apple yield: The percentage of apples in the five size classes in a given treatment was determined after sorting the fruit in a pallet box (300 kg) within a given experimental treatment. A total of eight pallet crates of fruit were calibrated. Calibration was carried out using a Dutch Greefa MSE 2000 (Greefa Machinebouw B.V, Tricht, The Netherlands) sorter equipped with a water-based fruit unloader to prevent bruising and damage to the apples when unloading from the crates.

2.4. Physiological Status of Fruit at Harvest Directly After Storage and After 7 Days of Shelf-Life

The apples were harvested on 29 September 2022. On the same day, their physiological status was assessed in the Department of Horticulture and Horticultural Economics laboratory of the Warsaw University of Life Sciences. All determinations were performed on 40 fruits representing a given treatment (four repetitions of 10 apples per repetition). The evaluation of apple quality after 8 months of storage and after an additional 7 days of storage at room temperature (20 °C). These evaluations were carried out on 40 apples per treatment, again in four repetitions of 10 fruits, with samples taken from the trees in all eight treatments.
Ethylene concentration in the seed chambers directly after fruit harvest (IEC): Measurements were taken using a specialized gas chromatograph (Clarus 690, Perkin Elmerm, Inc., Waltham, MA, USA) after 1 cm3 of air was taken from the apple seed chambers using a syringe. Results are given in µL·L−1 [44].
Starch test directly after fruit picking (SI): After cutting 10 apples in replication, the halves were sprayed with Lugol’s liquid. An applied solution of iodine in potassium iodide (I2 in KI) stained the starch-containing flesh dark blue. The resulting color of the flesh of the assessed fruits was compared with a 10-stage reference table [45].
Background skin color: Measurements of the background color of the apple skin were taken with a Minolta CR-400 colorimeter. This device sends a beam of light and compares it with the light reflected from the surface under examination. The results of the determinations are given in the CIE L*a*b* system, where L* denotes the brightness from 100 to 0, the a* value is for the transition from red to green, and b* is for the transition from yellow to blue.
Flesh firmness (FF): This parameter was determined with an Instron firmness meter (Instron 5542, Instron, Norwood, MA, USA), using an 11 mm diameter stylus for measurements. The skin was removed from the apples on two opposite sides before measurements were taken. During firmness determinations, a pin was driven into the apple flesh to a depth of 8 mm [46]. The results are given in N.
Titratable acidity (TA): This parameter was determined as described by Malachowska and Tomala [44]. For this purpose, a piece of flesh with skin was cut from each of the 10 fruits in the repetition. The juice was squeezed from the samples thus collected, then 10 mL of juice was taken and placed in a volumetric flask and diluted with 100 mL of distilled water. The resulting solution was titrated with 0.1 mol sodium hydroxide (NaOH) until pH = 8.1 using an automated titrator (TitroLine 5000, Xylem Analytics Germany GmbH, Weilheim, Germany). After conversion, the results are given as a percentage of malic acid [% malic acid].
Soluble solids content (SSC): The apple juice prepared for the titratable acidity test was also used to determine SSC content. For this purpose, a few drops of its juice were applied to a digital refractometer (Atago, Palette PR-32, Atago, Co., Ltd., Tokyo, Japan). The results are given in degrees Brix [°Bx].

2.5. Statistical Analysis

The Shapiro–Wilk test was carried out to check whether the results showed a normal distribution. A one-way ANOVA analyzed the results using Tukey’s HSD post hoc test for normal distribution. Differences between average values were reported as significant at p = 0.05 and p = 0.01. Statistical analyses were performed using Statistica 13.3 (Statsoft Inc., Tulsa, OK, USA).
The strength of the relationship between the variables was assessed using Pearson’s linear correlation coefficients.

3. Results

3.1. Indicators Determined in the Orchard

Spraying the trees with Regalis Plus 10 WG, irrespective of both the date of the first treatment and the treatment with prior root pruning, increased the number of fruitlets per 100 inflorescences by more than twofold compared to treatments without Regalis Plus 10 WG (Table 1).
Trees sprayed with Regalis Plus 10 WG from the ‘balloon’ stage (BBCH 60–69) were characterized by approximately four times shorter growth than control trees. This relationship was recorded in both the upper and lower parts of the tree crowns, regardless of the treatment with prior root pruning. In contrast, root pruning ‘solo’ and spraying the trees with Ethephon did little to limit long shoot growth.
The varying number of fruitlets per 100 inflorescences was reflected in the yield of the trees. The highest yield of 37.9 kg per tree was recorded after applying Regalis Plus 10 WG from the ‘balloon’ stage (BBCH 60–69). The average yield per tree in this treatment was two times higher than in the control treatment (Figure 1). Interestingly, Regalis Plus 10 WG applied ‘solo’ increased yield more compared to spraying the trees with this preparation after root pruning. In contrast, root pruning alone or spraying the trees with Ethephon alone resulted in a slight increase in yield relative to the control treatment.
Limiting tree growth with Regalis Plus 10 WG consistently reduced the average weight of the apples. In contrast, in the Ethephon program and the case of unilateral root, pruning both ‘solo’ and in treatment with Ethephon, the apples did not differ from the control fruit regarding the value of the trait in question. In these treatments, the average fruit weight was more than 360 g, while it fluctuated around 300 g in the other four.

3.2. Physiological State of Apples Directly After Harvest

Table 2 contains the results describing the physiological state of the apples directly after harvest. In the experiment, tree growth-limiting treatments had no significant effect on the ethylene concentration in the seed chambers (IEC) or the background color of the fruit skin as expressed by the a* and b* values of the trichromatic color stimulus. In contrast, it was found that apples from control trees had the lowest starch test (SI) value, while those from trees sprayed with the Ethephon preparation had the highest.
There were also significant differences in tree scarification treatment for flesh firmness (FF). Statistically, apples harvested from trees sprayed with Regalis Plus 10 WG from the ‘balloon’ stage onwards (in both root pruning treatments), as well as fruit from trees where root pruning was combined with an Ethephon program, had the highest firmness. The apples harvested from the control trees had the least-firm flesh. The fruit from this treatment also had the lowest soluble solids content. A significantly higher value for this trait was recorded in fruit from trees after root pruning. At the same time, in the other treatments of the experiment, this indicator generally took on the highest values.
As with the previously discussed indicators, tree growth scarification treatments significantly affected apple acidity; in this case, almost all tree growth-limiting treatments significantly reduced the malic acid content of the apples compared to the control sample.

3.3. Quality of Apples Directly After Storage and After Shelf-Life

The data in Table 3 showed that the apple skin’s background color directly after storage did not differ in terms of the L*, a*, and b* values in any tree growth scarification treatments. In contrast, fruit from trees with pruned roots had a higher L* parameter value after shelf-life than trees sprayed with Ethephon. Apples after the Ethephon application were generally characterized by a lower a* value, describing the background color of the skin than apples from the other treatments of the experiment.
In the experiment, there were also statistically proven differences in apple firmness under the influence of tree growth retardation treatments. Directly after storage, it was found that root pruning, spraying the trees with Regalis Plus 10 WG at standard times, as well as the application of the Ethephon program, irrespective of the treatment with root pruning, decreased apple firmness compared to both the control treatment and root pruning in combination with spraying the trees with Regalis Plus 10 WG from the ‘balloon’ stage. On the other hand, after simulated handling, the highest firmness was ensured by applying Regalis Plus 10 WG from the ‘balloon’ stage onwards, regardless of the tree root pruning treatment. In contrast, root pruning alone, as well as the use of the Ethephon program, including treatment with root pruning, resulted in the lowest apple firmness after simulated handling.
Treatments that restricted tree growth also significantly affected SSC and TA, but only directly after apple storage. It was then found that root pruning in treatment with the Ethephon program provided higher soluble solids content in apples than spraying the trees with Regalis Plus 10 WG from the ‘balloon’ stage, regardless of the root pruning treatment. In contrast, the opposite relationship occurred for acidity.

3.4. Values of Correlation Coefficients Between Selected Parameters

Table 4 shown the values of the linear correlation coefficients r between the different parameters determined in the harvest period. Statistical analysis showed a highly significant positive correlation between fruit firmness, tree yield, and SSC but a negative correlation with TA. This means that higher tree yields promoted higher firmness in apples with a higher SSC/TA ratio. In addition, a very strong negative correlation was found between the number of fruitlets per 100 inflorescences and the length of growth at both the top and bottom of the tree crown (r = −0.96 and −0.99, p < 0.01), demonstrating that fruit is a fundamental factor in limiting the growth of tree shoots. These relationships were reflected in the high yield of the trees (r = 0.88, p < 0.01) while yielding fruit of lower weight (r = −0.96, p < 0.01), and such fruit is particularly desirable for the large-fruited cultivar ‘Mutsu’.
Table 5 presents the values of the linear correlation coefficients r between the individual parameters that describe the growth and yield of the trees and the quality of the fruit after storage, as well as between the particular characteristics of the quality determined directly after storage. The number of fruitlets per 100 inflorescences and the yield per tree was positively correlated with TA (r = 0.53, r = 0.58; p < 0.01). This means apples from trees with more abundant yields contained more organic acids. On the other hand, TA was negatively correlated with shoot length at the top and bottom of the crown (r = −0.56 and −0.58, p < 0.01), as well as mean fruit weight (r = −0.50, p < 0.01). This means that larger fruits and those from trees with more intense shoot growth had a lower TA. On the other hand, the relationships between the individual characteristics of apple quality were unclear. In the statistical analysis, a negative correlation was found between SSC and TA (r = −0.59, p < 0.01), which indicates that fruits containing more sugars were characterized by lower acidity. At the same time, a significant positive correlation was found between SSC and the value of b* (r = 0.61, p < 0.01).
Table 6 presents the values of the linear correlation coefficients r between the individual parameters that describe the growth and yield of the trees and the quality of the fruit after simulated handling, as well as between the individual distinguishing characteristics of the quality determined after simulated handling. Directly after harvest, a positive correlation was found with both the number of fruitlets per 100 inflorescences (r = 0.61, p < 0.01) and with the yield per tree (r = 0.63, p < 0.01). However, regarding the average fruit weight and the length of long shoots, a negative relationship was noted with the firmness of apples after simulated handling. These data indicate that moderate growth and high yielding of trees ensure higher firmness of apples. However, with an increase in the weight of apples and the length of this year’s growth, the firmness of the fruit after simulated handling decreases.

3.5. Size Structure of Apple Yield

Treatments limiting tree growth modified the apple size structure of the yield (Figure 2). With the ‘Mutsu’ cultivar, fruits with a diameter of up to 90 mm are the most desirable on the market. Single-sided root pruning in combination with spraying of trees with Regalis Plus 10 WG from the ‘balloon’ phase (BBCH 60–69) provided the highest percentage share in the yield of this expected fruit category. Similar results were obtained by combining tree root pruning with Regalis Plus 10 WG on dates consistent with the label. In both of these combination experiments, the percentage of fruits with a diameter of more than 90 mm was small and amounted to 5 and 8%, respectively. However, after using Regalis Plus 10 WG without root cutting, the percentage of such fruits was twice as high. The structure of apple yield of the trees after root pruning and after application of the Ethephon program was similar to that of the control combination, but the percentage of apples with a diameter greater than 90 mm was alarmingly high and oscillated around 40%.
The dendrogram in Figure 3 showed the degree of similarity between the treatment of tree growth retarding based on the measurements taken in the orchard and considering the results of the evaluation of the physiological state of the fruit directly after harvest. Based on these data, a hierarchical aggregation of eight tree dwarf combinations evaluated in the present experiment was shown. There was a large difference between the cluster formed by the control tree, RP, Ethephon, and RP + Ethephon, and the cluster comprised of the four remaining experiment treatments (RB, RP + RB, RS, and RP + RS). Interestingly, the most similar objects within the first cluster were RP and Ethephon. Therefore, it can be assumed that these treatments can be used interchangeably, depending on the agrotechnical situation in the orchard. In the second cluster formed by all Regalis Plus 10 WG combinations, the most similar objects were RB and RP + RB and RS and RP + RS. It can be assumed that trees and fruits react similarly to both root pruning in combination with Regalis Plus 10 WG and to using Regalis Plus 10 WG alone. The height of the dendrogram, which indicates the order in which the clusters were combined, can be used to select the best treatment to retard tree growth and facilitate the management of their growth for an optimal effect.
An analogous dendrogram considering only the quality characteristics of the shelf-life apples is shown in Figure 4. In this case, there was a big difference between the cluster combining RB and RP + RB and the other cluster combining the six remaining experience combinations. Apples from the RB and RP + RB combinations characterized the most similar quality. This means that in the planned spraying of trees with Regalis Plus 10 WG from the balloon phase, it is unnecessary to prune the roots beforehand. The following two most similar combinations were RP and RP + Ethephon.

4. Discussion

In the coming years, the demand for apples will depend even more on their quality than it does today. The condition to obtain high-quality apples is the proper selection and timely performance of agrotechnical treatments in the orchard. Regarding such treatments, it is important to control the intensity of tree growth. Trees’ moderate growth is conducive to the formation of flower buds and regular fruiting. However, growth that is too strong or weak creates problems with fruit size and yield quality. Undoubtedly, the best method of regulating tree growth is annual/regular fruiting. In the experiment described in this study, a spectacular reduction in shoot growth was observed with a simultaneous increase in the number of apples on trees sprayed with Regalis Plus 10 WG. Such an impact is by all means desirable, especially in the case of the ‘Mutsu’ cultivar, which tends to shed its buds excessively.
It is known that limiting the strength of tree growth modifies the direction of the flow of various components to shoots and fruits. As a result, the fruit is better nourished and has better quality. By reducing the length of this year’s growth, tree growth retardant treatments also increased the penetration of sunlight into the crown [47]. Therefore, they will contribute to better flower buds for the next year and higher tree yields, as well as promote a more economical and rational use of pesticides to protect trees against diseases and pests [48]. In addition, they contribute to improving the color of apples [49,50,51,52].
Ferree and Knee [53] noted the high effectiveness of root pruning in reducing the growth of tree shoots. On the other hand, Wajja-Musukwe et al. [54] believe that root pruning has only a minor effect on tree growth. The results of the experiment described in this study confirm a slight growth reaction of long shoots of apple trees of the ‘Mutsu’ cultivar to unilateral root pruning. It should be noted that the effect of limiting tree growth due to root pruning can be modified under the influence of the timing of this treatment and the weather conditions during the growing season [54].
Other experiments have shown that root pruning, in addition to regulating the growth and abundance of tree flowering, reduced leaf and fruit size while increasing yield [55]. The fineness of apples in the case of the large-fruited cultivar ‘Mutsu’ is a positive effect, also noted in the experiment described in this study. Schupp [56], on the other hand, believes that the yield decreases due to pruning the roots, and the number of flowers in the following year decreases. In our experiment, slightly fewer fruitlets per 100 inflorescences after unilateral root pruning were observed on ‘Mutsu’ trees than in the control combination. However, tree yield after unilateral root pruning was significantly higher than control trees.
Some researchers say limiting tree growth with Regalis Plus 10 WG is more spectacular than it is with cutting the roots. Using the maximum dose of this preparation (2.5 kg·ha−1) per season can reduce shoot growth by 50%, resulting from limited gibberellin synthesis after using calcium prohexadione [57,58]. Greene [59] also observed such a relationship on trees of the ‘Mutsu’ and ‘McIntosh’ cultivars. In this study, Regalis Plus 10 WG increased the number of fruitlets per 100 inflorescences by approximately 2.5 times compared to control trees. This was reflected in the yield of trees, which was about two times higher. Thus, previous observations by Zadravec et al. [60], who reported that Regalis had no significant effect on the tree yield and fruit quality of the ‘Gala’ cultivar, were not confirmed. Therefore, it can be inferred that the application of Pro-Ca at a later stage may be less effective, as vegetative growth is more advanced [61].
All methods of limiting the growth of trees used in the experiment resulted in a significant shortening of long shoots, but only slightly affected the IEC and the background color of apples directly after harvest. On the other hand, retarding the tree growth resulted in a decrease in apple acidity and an increase in SSC. Similar relationships were previously found in the pears of cv. ‘Shinseiki’ [62] and tomatoes [63], while no relationships were found in pears of cv. ‘Pass Crassane’ [64].
In the presented study, using Ethephon caused the most significant decrease in the firmness of apples after the simulated handling period, which repeats the previous observations of Smith et al. [65]. In other studies, the results were inconclusive, which suggests that the efficacy of Ethephon depends on various factors, including the timing of treatment, the course of weather conditions, the degree of fruit ripeness at harvest, and the conditions of their storage [66,67,68,69,70]. For example, the Ethephon used for the late thinning of primordia effectively increased the average weight of the fruit [71], which is particularly important in the cultivation of small-fruit cultivars. According to Koen et al. [72], using Ethephon was the safest and most predictable alternative to labor-intensive manual thinning.
To sum up, all the methods to regulate the growth and fruiting of trees presented in this study influenced the production of high-quality fruit to a greater or lesser extent. When choosing the proper dwarfing method for trees, the fruit grower should rely on his own experience and knowledge about the growth and yield of individual cultivars in the orchard.

5. Conclusions

One-sided spring pruning of the roots with an oblique knife, together with the application of Regalis Plus 10 WG from the balloon phase, is the most effective method of inhibiting the growth of annual shoots and reducing the share of overgrown fruit in the yield. Apples from such trees show a slow rate of firmness decline during storage and simulated handling. Limiting shoot growth and excessive apple growth by pruning the roots or spraying trees with Ethephon gives much worse results compared to using Regalis Plus 10 WG. In addition, trees sprayed with Regalis Plus 10 WG are suppressed for naturally occurring stronger growth of the upper shoots compared to the bottom part of the tree crown.

Author Contributions

Conceptualization, M.M. and K.T.; methodology, M.M., T.M. and K.T.; formal analysis, M.M. and K.T.; investigation, T.M. and K.T.; writing—original draft preparation, M.M., T.M., T.K. and K.T.; writing—review and editing, M.M., T.M., T.K. and K.T.; visualization, M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

All data are present in the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 14 01838 g001
Figure 1. Fruit load in ‘Mutsu’ cultivar trees: (a) control (no growth-limiting treatments)—C; (b) Regalis Plus 10 WG applied from the balloon stage (BBCH 60–69)—RB.
Figure 1. Fruit load in ‘Mutsu’ cultivar trees: (a) control (no growth-limiting treatments)—C; (b) Regalis Plus 10 WG applied from the balloon stage (BBCH 60–69)—RB.
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Figure 2. Size structure of ‘Mutsu’ apples yield in relation to tree growth retardation method (fruits divided into 5 apple size classes). Explanation of abbreviations: C—control; RP—one-sided root pruning (with an oblique knife); RB—Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RS—Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); Ethephon—spraying trees with Flordimex 480 SL; RP + RB—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RP + RS—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); RP + Ethephon—one-sided root pruning (with an oblique knife) + spraying trees with Flordimex 480 SL.
Figure 2. Size structure of ‘Mutsu’ apples yield in relation to tree growth retardation method (fruits divided into 5 apple size classes). Explanation of abbreviations: C—control; RP—one-sided root pruning (with an oblique knife); RB—Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RS—Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); Ethephon—spraying trees with Flordimex 480 SL; RP + RB—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RP + RS—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); RP + Ethephon—one-sided root pruning (with an oblique knife) + spraying trees with Flordimex 480 SL.
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Figure 3. Dendrogram for different tree growth-limiting treatments based on all the results available directly after harvest. Explanation of abbreviations: C—control; RP—one-sided root pruning (with an oblique knife); RB—Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RS—Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); Ethephon—spraying trees with Flordimex 480 SL; RP + RB—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RP + RS—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); RP + Ethephon—one-sided root pruning (with an oblique knife) + spraying trees with Flordimex 480 SL.
Figure 3. Dendrogram for different tree growth-limiting treatments based on all the results available directly after harvest. Explanation of abbreviations: C—control; RP—one-sided root pruning (with an oblique knife); RB—Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RS—Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); Ethephon—spraying trees with Flordimex 480 SL; RP + RB—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RP + RS—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); RP + Ethephon—one-sided root pruning (with an oblique knife) + spraying trees with Flordimex 480 SL.
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Figure 4. Dendrogram for different tree growth-limiting treatments based on apple quality after shelf-life. Explanation of abbreviations: C—control; RP—one-sided root pruning (with an oblique knife); RB—Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RS—Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); Ethephon—spraying trees with Flordimex 480 SL; RP + RB—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RP + RS—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); RP + Ethephon—one-sided root pruning (with an oblique knife) + spraying trees with Flordimex 480 SL.
Figure 4. Dendrogram for different tree growth-limiting treatments based on apple quality after shelf-life. Explanation of abbreviations: C—control; RP—one-sided root pruning (with an oblique knife); RB—Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RS—Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); Ethephon—spraying trees with Flordimex 480 SL; RP + RB—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RP + RS—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); RP + Ethephon—one-sided root pruning (with an oblique knife) + spraying trees with Flordimex 480 SL.
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Table 1. Effect of tree growth retardation method on the number of fruitlets per 100 inflorescences: shoot length, yield, and fruit weight of ‘Mutsu’ cultivar.
Table 1. Effect of tree growth retardation method on the number of fruitlets per 100 inflorescences: shoot length, yield, and fruit weight of ‘Mutsu’ cultivar.
TreatmentNumber of Fruitlets per 100 InflorescencesShoot Length [cm]Yield
[kg·tree−1]		Fruit Weight
[g]
Top of the TreeBottom of the Tree
C47.8 b 169.9 h56.3 g18.3 a393 b
RP44.0 ab59.7 g52.6 f21.5 b367 b
RB126.3 e18.0 b15.0 a37.9 h296 a
RS118.1 cd26.1 d23.9 c32.1 f286 a
Ethephon44.8 ab58.1 f49.0 e25.5 d378 b
RP + RB120.5 d15.6 a14.6 a34.0 g284 a
RP + RS115.0 c22.7 c19.9 b27.9 e284 a
RP + Ethephon42.4 a53.2 e42.7 d23.3 c365 b
1 Tukey’s HSD test; normal distribution (normality was checked using the Shapiro–Wilk test-p ≤ 0.05); means followed by the same letter within a column are not significantly different. Explanation of abbreviations: C—control; RP—one-sided root pruning (with an oblique knife); RB—Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RS—Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); Ethephon—spraying trees with Flordimex 480 SL; RP + RB—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RP + RS—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); RP + Ethephon—one-sided root pruning (with an oblique knife) + Spraying trees with Flordimex 480 SL; n = 4.
Table 2. The effect of tree growth retardation method on the fruit quality of ‘Mutsu’ apple directly after harvest.
Table 2. The effect of tree growth retardation method on the fruit quality of ‘Mutsu’ apple directly after harvest.
TreatmentIEC
[µL·L−1]		SI
[-]		Background ColorFF
[N]		SSC
[°Bx]		TA
[%]
L*a*b*
C0.081 a 16.6 a66.3 ab−15.1 a35.1 a76.9 a11.5 a0.544 b
RP0.041 a8.5 c68.6 b−15.5 a36.2 a77.9 ab12.3 b0.504 ab
RB0.067 a7.6 b65.4 ab−15.1 a35.2 a87.4 c13.0 c0.479 a
RS0.054 a7.4 b67.4 b−15.2 a35.3 a81.8 b12.9 c0.494 a
Ethephon0.049 a9.4 d66.9 ab−15.3 a34.7 a82.3 b12.8 bc0.475 a
RP + RB0.036 a7.4 b65.2 a−15.0 a34.8 a88.7 c12.9 c0.487 a
RP + RS0.064 a7.3 b68.3 b−15.2 a35.4 a81.7 b13.2 c0.491 a
RP + Ethephon0.058 a7.4 b68.2 b−15.5 a35.4 a88.7 c13.1 c0.488 a
1 Tukey’s HSD test; normal distribution (normality was checked using the Shapiro–Wilk test-p ≤ 0.05); means followed by the same letter within a column are not significantly different. Explanation of abbreviations: C—control; RP—one-sided root pruning (with an oblique knife); RB—Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RS—Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); Ethephon—spraying trees with Flordimex 480 SL; RP + RB—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RP + RS—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); RP + Ethephon—one-sided root pruning (with an oblique knife) + spraying trees with Flordimex 480 SL; IEC—internal ethylene content; SI—starch test; L*, a*, and b*—the color of apples in CIE L*a*b* coordinates; FF—flesh firmness; SSC—soluble solids content; TA—titratable acidity; n = 4.
Table 3. The effect of tree growth retardation method on fruit quality of ‘Mutsu’ apples directly after storage and after 7 days of shelf life.
Table 3. The effect of tree growth retardation method on fruit quality of ‘Mutsu’ apples directly after storage and after 7 days of shelf life.
TreatmentsBackground ColorFF
[N]		SSC
[°Bx]		TA
[%]
L*a*b*
Directly after storage
C66.6 a 1−13.3 a35.7 a72.8 b14.8 ab0.403 a
RP69.3 a−13.6 a36.4 a66.4 a14.9 ab0.417 ab
RB64.4 a−13.8 a35.6 a69.1 ab14.3 a0.445 b
RS66.1 a−13.1 a34.6 a66.1 a14.6 ab0.406 a
Ethephon65.5 a−13.0 a34.4 a67.4 a14.5 ab0.415 ab
RP + RB66.2 a−14.3 a36.8 a75.1 b14.4 a0.443 b
RP + RS67.0 a−13.3 a35.3 a68.6 ab14.7 ab0.425 ab
RP + Ethephon66.1 a−13.4 a37.4 a66.8 a15.4 b0.398 a
After 7 days of shelf life
C71.9 ab−10.6 a41.3 b56.3 b15.0 a0.375 a
RP72.9 b−10.7 a41.3 b47.8 a14.5 a0.366 a
RB71.5 ab−11.0 a41.2 b61.7 c14.8 a0.357 a
RS72.2 ab−10.6 a41.3 b53.4 b15.0 a0.375 a
Ethephon69.3 a−10.1 a39.1 a49.5 a14.7 a0.379 a
RP + RB70.4 ab−10.8 a40.7 ab62.0 c15.0 a0.373 a
RP + RS72.4 ab−10.6 a41.4 b54.8 b15.1 a0.376 a
RP + Ethephon71.4 ab−10.4 a40.2 ab48.4 a14.9 a0.374 a
1 Tukey’s HSD test; normal distribution (normality was checked using the Shapiro–Wilk test-p ≤ 0.05); means followed by the same letter within a column are not significantly different. Explanation of abbreviations: C—control; RP—one-sided root pruning (with an oblique knife); RB—Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RS—Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); Ethephon—spraying trees with Flordimex 480 SL; RP + RB—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied from the balloon stage—BBCH 60–69); RP + RS—one-sided root pruning (with an oblique knife) + Regalis Plus 10 WG (applied at standard terms—BBCH 71–73, according to the label); RP + Ethephon—one-sided root pruning (with an oblique knife) + spraying trees with Flordimex 480 SL; L*, a*, and b*—the color of apples in CIE L*a*b* coordinates; FF—flesh firmness; SSC—soluble solids content; TA—titratable acidity; n = 4.
Table 4. Correlation coefficients among various analyzed parameters in the period around the harvest.
Table 4. Correlation coefficients among various analyzed parameters in the period around the harvest.
ABCDEFGHIJKLM
A1.00
B−0.96 **1.00
C−0.99 **0.98 **1.00
D0.88 **−0.92 **−0.89 **1.00
E−0.96 **0.98 **0.95 **−0.87 **1.00
F−0.050.130.07−0.110.141.00
G−0.35 *0.240.35 *−0.040.17−0.37 *1.00
H−0.36 *0.310.37 *−0.46 *0.22−0.010.181.00
I0.28−0.23−0.260.23−0.200.07−0.17−0.57 **1.00
J−0.150.120.11−0.180.12−0.060.030.60 **−0.62 **1.00
K0.38 *−0.55 **−0.46 *0.61 **−0.43 *−0.09−0.08−0.40 *0.25−0.36 *1.00
L0.50 **−0.67 **−0.53 **0.63 **−0.68 **−0.110.180.130.05−0.010.60 **1.00
M−0.290.44 *0.31−0.50 **0.43 *0.34 *−0.39 *0.020.130.05−0.49 **−0.58 **1.00
Explanation of the designations used: A—number of fruitlets/100 inflorescences; B—shoot length in top tree crown; C—shoot length in bottom tree crown; D—yield per tree; E—fruit weight; F—internal ethylene content; G—starch test; H—L* value; I—a* value; J—b* value; K—flesh firmness; L—soluble solids content; M—titratable acidity; L* value, a* value, and b* value—the color of apples in CIE L*a*b* coordinates. *—correlation significant at α = 0.05, **—correlation significant at α = 0.01; n = 30.
Table 5. Correlation coefficients between various analyzed parameters directly after storage.
Table 5. Correlation coefficients between various analyzed parameters directly after storage.
ABCDEFGHIJK
F−0.250.240.21−0.35 *0.201.00
G−0.240.270.29−0.250.20−0.151.00
H−0.110.030.02−0.070.110.46 *−0.67 **1.00
I−0.080.090.050.030.15−0.01−0.090.141.00
J−0.44 *0.36 *0.40 *−0.46 *0.38 *0.36 *−0.120.61 **−0.021.00
K0.53 **−0.56 **−0.58 **0.58 **−0.50 **−0.13−0.31−0.070.10−0.59 **1.00
Explanation of the designations used: A—number of fruitlets/100 inflorescences; B—shoot length in top tree crown; C—shoot length in bottom tree crown; D—yield per tree; E—fruit weight; F—L* value; G—a* value; H—b* value; I—flesh firmness; J—soluble solids content; K—titratable acidity. F–K (consider parameters directly after storage); L* value, a* value, and b* value—the color of apples in CIE L*a*b* coordinates. *—correlation significant at α = 0.05, **—correlation significant at α = 0.01; n = 30.
Table 6. Correlation coefficients between various analyzed parameters as well as after shelf life.
Table 6. Correlation coefficients between various analyzed parameters as well as after shelf life.
ABCDEFGHIJK
F0.050.01−0.06−0.14−0.011.00
G−0.260.260.31−0.230.21−0.091.00
H0.33 *−0.21−0.310.09−0.210.56 **−0.221.00
I0.61 **−0.54 **−0.63 **0.63 **−0.42 *−0.17−0.320.111.00
J0.22−0.19−0.210.05−0.18−0.01−0.06−0.130.191.00
K−0.090.080.09−0.100.050.06−0.08−0.10−0.160.071.00
Explanation of the designations used: A—number of fruitlets/100 inflorescences; B—shoot length in top tree crown; C—shoot length in bottom tree crown; D—yield per tree; E—fruit weight; F—L* value; G—a* value; H—b* value; I—flesh firmness; J—soluble solids content; K—titratable acidity. F–K (consider parameters after shelf life); L* value, a* value, and b* value—the color of apples in CIE L*a*b* coordinates. *—correlation significant at α = 0.05, **—correlation significant at α = 0.01; n = 30.
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MDPI and ACS Style

Małachowska, M.; Majak, T.; Krupa, T.; Tomala, K. Increasing Productivity and Fruit Quality of ‘Mutsu’ Apple Orchard by Dwarfing Treatments. Agriculture 2024, 14, 1838. https://doi.org/10.3390/agriculture14101838

AMA Style

Małachowska M, Majak T, Krupa T, Tomala K. Increasing Productivity and Fruit Quality of ‘Mutsu’ Apple Orchard by Dwarfing Treatments. Agriculture. 2024; 14(10):1838. https://doi.org/10.3390/agriculture14101838

Chicago/Turabian Style

Małachowska, Maria, Tomasz Majak, Tomasz Krupa, and Kazimierz Tomala. 2024. "Increasing Productivity and Fruit Quality of ‘Mutsu’ Apple Orchard by Dwarfing Treatments" Agriculture 14, no. 10: 1838. https://doi.org/10.3390/agriculture14101838

APA Style

Małachowska, M., Majak, T., Krupa, T., & Tomala, K. (2024). Increasing Productivity and Fruit Quality of ‘Mutsu’ Apple Orchard by Dwarfing Treatments. Agriculture, 14(10), 1838. https://doi.org/10.3390/agriculture14101838

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