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Article

Effects of Pneumatic Defoliation on Fruit Quality and Skin Coloration in ‘Fuji’ Apples

by
Nay Myo Win
1,
Jingi Yoo
1,
Van Giap Do
1,
Sangjin Yang
1,
Soon-Il Kwon
1,
Hun-Joong Kweon
1,
Seonae Kim
1,
Youngsuk Lee
1,
In-Kyu Kang
2 and
Juhyeon Park
1,2,*
1
Apple Research Center, National Institute of Horticultural and Herbal Science, RDA, Daegu 43100, Republic of Korea
2
Department of Horticultural Science, Kyungpook National University, Daegu 41566, Republic of Korea
*
Author to whom correspondence should be addressed.
Agriculture 2024, 14(9), 1582; https://doi.org/10.3390/agriculture14091582
Submission received: 23 July 2024 / Revised: 29 August 2024 / Accepted: 9 September 2024 / Published: 11 September 2024
(This article belongs to the Special Issue Analysis of Agricultural Food Physicochemical and Sensory Properties)
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Abstract

:
Fruit skin color and physical quality are important for customer acceptability and market value. Therefore, this study aimed to evaluate the effect of pneumatic defoliation on the fruit quality, coloration, and anthocyanin content of ‘Fuji’ apples. Apple trees were subjected to no defoliation (control) and defoliation at low (0.6 bar) and high (0.9 bar) air pressure 20 days before harvest at 1 km/h of tractor speed. High-defoliation treatment increased the leaf damage rate but did not significantly affect the defoliation rate compared to low-defoliation treatment. Additionally, photosynthetically active radiation and solar irradiance inside the tree canopies were highest in the high-defoliation group, followed by the low-defoliation and control groups. With the exception of higher firmness in the high-defoliation treatment, pneumatic defoliation treatments had little effect on fruit size and weight, titratable acidity, soluble solids content, the starch pattern index, and the sunburn incidence of fruit. Compared with that of the control group, both defoliation treatments significantly increased the a* and C values and decreased the ho values of the fruit color. Moreover, both defoliation treatments significantly increased anthocyanin content and upregulated the anthocyanin biosynthesis genes (MdPAL, MdCHS, MdCHI, MdF3H, MdANS, MdANS, MdUFGT) and the transcription factor (MdMYB10). A Pearson′s correlation analysis also showed that anthocyanin production was strongly correlated with each of the anthocyanin biosynthesis genes, especially in the pneumatic defoliation treatments. Conclusively, the results show that pneumatic defoliation at low pressure bars could be an effective strategy for improving the red coloration of ‘Fuji’ apples.

1. Introduction

Anthocyanin is a class of flavonoids that is commonly found in fruits, leaves, and flowers as red, blue, and purple colors [1]. Anthocyanins are responsible for the red pigments in the skin and flesh of mature apples [2,3]. Importantly, skin color is a key determinant of consumer appeal and the economic value of apples [4]. Additionally, anthocyanins possess several health benefits for humans, and the consumption of anthocyanin-rich foods can reduce the risk of cardiovascular diseases, diabetes, and cancer [5]. Therefore, a high anthocyanin content contributes both to fruit quality and improving the market value of apples [4,6].
Notably, genetic factors [7,8] and other factors, such as environmental conditions and management practices, influence anthocyanin biosynthesis and accumulation in various apple cultivars [9,10,11]. Sunlight is a key factor that stimulates anthocyanin biosynthesis and accumulation in apples [12]. Light exposure induces anthocyanin production by activating the transcription factor genes, which then upregulate the expression of genes involved in the anthocyanin biosynthesis pathway, resulting in increased red coloration in fruits [13,14]. Giap et al. [15] reported that light exposure specifically increased the expression of major genes involved in the anthocyanin biosynthesis pathway (MdPAL, MdCHS, MdCHI, MdF3H, MdANS, MdANS, MdUFGT) and transcription factor gene (MdMYB10) in apples. Additionally, a positive correlation between light exposure and anthocyanin accumulation has been reported [16,17]. Therefore, orchard management strategies aimed at improving sunlight irradiation are important for attaining optimum apple coloration.
Defoliation prior to harvest is a widely conducted practice on apple trees aimed at improving the skin coloration of apples [18,19]. Generally, trees can be partially defoliated for various purposes, including to increase sunlight penetration and air circulation inside the canopies, enhance fruit coloration, reduce pests and diseases, and improve spray penetration. However, excessive defoliation can negatively affect tree nutrient levels, resulting in reduced fruit yield and reduced plant carbohydrate and sugar accumulation [20,21]. In previous decades, fruit cluster leaves have been defoliated using hand-stripping and chemical defoliant spraying approaches in many apple orchards [22,23,24,25]. However, defoliation using the hand-stripping approach is time-consuming and labor-intensive. Additionally, the application of some chemical defoliants at high concentrations can be harmful to humans and requires special safety regulations. Recently, mechanical defoliation strategies have been applied to apple trees to reduce time intensiveness and production costs and to improve environmental safety. One such mechanical approach is pneumatic defoliation, removing leaves and parts of leaves by blowing a high-frequency pulsating air-stream into the trees. It is generally applied prior to harvest time. Win et al. [26] reported that pneumatic defoliation increased intense light exposure in the trees, thereby enhancing anthocyanin pigments in the skin of ‘Picnic’ apples. However, Lavely [27] reported that the efficacy of pneumatic defoliation varied depending on the apple cultivar, tree type, timing of application, tractor driving speed, and air pressure. For example, early application of pneumatic defoliation could negatively affect fruit size and quality, while late application could result in less coloration [27]. Andergassen and Pichler [28] reported that pneumatic defoliation reduced fruit size but less affected ripening indicators in ‘Pink Lady’ apples. However, pneumatic defoliation reduced sugar accumulation in ‘Nicoter’ apples, although it improved the anthocyanin content in fruit [29]. Additionally, a combination of summer pruning and pneumatic defoliation did not increase fruit coloration compared to that of summer pruning or pneumatic defoliation alone [29]. Moreover, a close distance application of pneumatic defoliation can induce fruit drop and physical damage in some apple cultivars [27]. Therefore, a tree-adaptable application method must be determined for each apple cultivar and environmental condition.
‘Fuji’ (Malus domestica Borkh.) is the most widely cultivated apple cultivar globally. In Korea, this cultivar accounts for 67% of the total apple cultivation area [30]. Additionally, ‘Fuji’ is a late-season apple cultivar, and it is generally harvested at the end of October or early November [22]. Leaf abscission in apples is highly associated with the environmental conditions, and low temperatures induce leaf and fruit abscission [31]. Differences in the leaf morphological characteristics of each apple cultivar can also influence the efficiency of pneumatic defoliation. However, no information exists on the effects of pneumatic defoliation on the leaf defoliation rate, fruit color, and quality of the ‘Fuji’ apple. Therefore, this study aimed to investigate the effect of pneumatic defoliation on the quality characteristics and coloration of ‘Fuji’ apples. Moreover, we examined the anthocyanin content and the expression of anthocyanin biosynthesis genes in apple skin.

2. Materials and Methods

2.1. Experimental Field and Apple Trees

This study was conducted in the experimental orchard at the Apple Research Center in Sobo-myeon, Gunwi-gun, Daegu, Republic of Korea (36.28′ N, 128.47′ E). Seven-year-old ‘Fuji’ apple trees growing in the same soil and environmental conditions with a spacing of 1.5 m × 3.0 m were used for this study. The study was set up in a completely randomized design with three replicates and five individual apple trees per replicate. In total, 45 apple trees grafted onto M.9 rootstocks were selected and trained into a spindle–slender shape approximately 160 cm in width (80 cm from each side) and 2.55 m in height. The apple trees fully bloomed on 25 April 2022, and the crop levels of each tree were adjusted to seven fruits per trunk cross-sectional area after one month of full bloom. The experimental field was irrigated with a drip irrigation system and managed using an integrated pest management system. Daily temperatures and precipitation in the experimental field were recorded from treatment application until harvest (Figure 1).

2.2. Treatments

Pneumatic defoliation was performed using a Red-pulse Duo pneumatic defoliation device (Fruit Tec Co., Markdorf, Germany) attached to the tractor. The operation height of the air pressure area of the pneumatic defoliation device was 1.0 m, and the operation depth of the air-stream ranged from 400 mm to 600 mm. The air-stream pressure can be adjusted from 0.6 bars to 0.9 bars during application. In this study, apple trees were subjected to non-defoliation (control) and pneumatic defoliation at low (0.6 bar) and high (0.9 bar) air pressure 20 days before harvest (DBH; 6 October 2022). According to the results of the previous report [26], the tractor driving speed was reduced to 1 km/h to improve the leaf defoliation rate in the present study. Alternatively, the air pressure for low-defoliation treatment was reduced to 0.6 bars to decrease the leaf damage rate in the tree. As described above, fifteen apple trees (three replicates with five trees) were used for each treatment. During operation, the defoliation treatment was first applied to the lower canopy and then to the upper canopy from both sides of the apple tree, following the method of Win et al. [26]. The pneumatic defoliation device and its application to apple trees are shown in Figure 2A. During the treatment operation, the tractor was driven at a speed of 1 km/h by a skilled driver, and the pneumatic defoliation treatment was applied approximately 10 cm away from the apple canopy. The fruits were harvested on 26 October 2022.

2.3. Assessments of Leaf Defoliation and Damaged Rates

The defoliation rate of apple trees was assessed before and after pneumatic defoliation. Twelve branches per tree (four individual branches from four different sides at the lower, middle, and upper canopies of the trees) were marked as model branches. Specifically, we counted the total number of apple leaves from each model branch before and after the treatment. Leaves that defoliated along with the petioles from the tree branches after defoliation treatment were specified as defoliated leaves, and the defoliation rate was calculated using the following formula described by Win et al. [26]: defoliation rate (%) = (number of defoliated leaves after treatment/total number of leaves before treatment) × 100. Apple leaves that did not defoliate but were damaged by the air pressure streams of the pneumatic defoliation treatment were specified as damaged leaves, and the rate of damaged leaves was calculated using the following formula described by Win et al. [26]: leaf damage rate (%) = (number of damaged leaves after treatment/total number of leaves before treatment) × 100. Photographs of the defoliated and damaged leaves after pneumatic defoliation are shown in Figure 2B–D.

2.4. Assessments of Sunlight Penetration inside the Canopy

Photosynthetically active radiation for sunlight (PAR SUN) and solar irradiance were measured inside the canopy of apple trees following the method described by Win et al. [26]. Both PAR and solar irradiance were measured at four different locations (the east, west, south, and north sides) in each tree at approximately 1.0 m plant height level from the ground, and the measurements were conducted at midday (between 12:00 and 13:00 p.m. of full sunlight conditions with no clouds) on a sunny day (15 October 2022) using a light sensor meter (3415FX, Spectrum Technologies Inc., Aurora, IL, USA). For both measurements of PAR and solar irradiance, the light sensor meter was calibrated under direct sunlight, and the calibration was made after each measurement of each apple tree. Fifteen apple trees per treatment were used for measurements of sunlight penetration.

2.5. Assessments of Fruit Quality Characteristics

In this study, the number of sunburned fruits was counted and calculated using the following formula described by Moon et al. [32]: fruit sunburn rate (%) = (number of sunburned fruits after treatment/total number of fruits) × 100. Thereafter, 10 fruits were randomly chosen per tree (a total of 150 fruits per treatment) to assess fruit quality characteristics, including fruit size, weight, skin color, starch pattern index (SPI), flesh firmness, titratable acidity (TA), and soluble solids content (SSC). Fruit size (length and diameter) was measured using a digital caliper (CD-15APX; Kawasaki, Japan). Fruit weight was measured using a digital weight scale (AND Co., Daejeon, Republic of Korea). Fruit color values (L*, a*, and b*) were measured at three locations around the equatorial regions of each fruit using a chroma meter (CR-400; Konica Minolta, Tokyo, Japan). The chroma (C) and hue angle (ho) values were calculated based on the L*, a*, and b* values using the formula described by Giap et al. [15]. Flesh firmness was measured on three equatorial locations of each fruit using a firmness tester equipped with an 11 mm plunger (FT-327, TR Co., Folić, Italy). Thereafter, the fruits were cut into horizontal slices and digested in I2-KI solution, and starch degradation was scored from 1 to 8 following the Cornell starch index method [33]. Juice was extracted from the fruits for the measurements of SSC and TA. SSC was measured in the juice using a digital refractometer (PR-201; Atago, Tokyo, Japan). TA was measured by titrating the juice with 1N NaOH following the malic acid reduction method [34]. Fruit skin tissues were collected around the equatorial regions of each fruit for analysis of anthocyanin content and gene expression analysis.

2.6. Extraction and Determination of Anthocyanin Contents

Anthocyanin was extracted and measured using the pH differential method [35,36]. Apple peels were extracted with 80% acetone, and the extracts were filtered through a filter paper. Thereafter, the filtrates were evaporated using a rotary evaporator (N-100; Rikakikai, Japan), and the volume of the samples was adjusted to 10 mL using distilled water. For anthocyanin determination, the samples were mixed with potassium chloride buffer (pH1.0) and sodium acetate buffer (pH4.5). After incubating for 15 min at 20 °C, the absorbance was read at 700 and 510 nm using a spectrophotometer (UV-1800, Shimadzu, Japan). Absorbance was calculated using the following equation: Absorbance = (A510 − A700)pH1.0 − (A510 − A700)pH4.5. Finally, the anthocyanin content was calculated based on the cyaniding-3-galactoside using the following formula: Anthocyanin = (Absorbance × MW × 1000)/(ε × C), where molecular weight (MW) is 449.2, molar mass (ε) is 26,900, and C is the concentration of the buffer solution. Three biological replicates were used for anthocyanin determination.

2.7. RNA Extraction and Gene Expression Analysis

To elucidate the mechanism of pneumatic defoliation on anthocyanin biosynthesis, we examined the expression of eight genes, including MdPAL, MdCHS, MdCHI, MdDNS, MdF3H, MdANS, MdUFGT, and MdMYB10 [15]. Total RNA was extracted from apple peels using the cetyltrimethyl ammonium bromide (CTAB) method [37]. RNA quality was measured using a UV spectrometer and 0.8% agarose gel. Thereafter, RNA was reverse-transcribed to generate cDNA using the PrimeScriptTM cDNA synthesis kit (Takara, Kusatsu, Japan). Quantitative real-time PCR (qTR-PCR) was performed on LightCycler 48 II (Roche Diagnostics, Mannheim, Germany) and specific primers. Notably, the expression of the genes was normalized to that of the reference gene (MdP0000336547) [38]. Three biological replicates were used for the gene expression analysis. Primers and PCR conditions are listed in Supplementary Table S1.

2.8. Statistical Analysis

All data were gathered using SPSS statistical software (version 26; IBM SPSS Inc., Armonk, NY, USA). A Student’s t-test was used to compare the mean differences in the data of leaf defoliation and damaged rates between low- and high-defoliation treatment groups at the significant level of p < 0.05. For remaining data (PAR, solar irradiance, fruit quality, anthocyanin contents, and anthocyanin gene expression), a Tukey’s HSD test was used to analyze the mean differences within no defoliation (control) and low- and high-defoliation treatment groups at the significant levels of p < 0.05. A Pearson’s correlation analysis was performed at significant levels of p < 0.05 and p < 0.01 [34].

3. Results

3.1. Leaf Defoliation and Damaged Rates

Figure 3 shows photographs of the apple tree after the pneumatic defoliation treatments. Both pneumatic defoliation treatments visually reduced the leaves on the trees compared with those in the non-defoliated group (Figure 3).
High pneumatic defoliation removed 21.4% of apple leaves, whereas low pneumatic defoliation removed only 16.8% of leaves on the trees (Figure 4A). However, the two pneumatic defoliation treatments had no significant difference in defoliation rates (Figure 4A). Compared with that in the low-defoliation group (5%), the leaf damage rate was significantly higher in the high-defoliation group (9.3%) (Figure 4B).

3.2. Photosynthetically Active Radiation (PAR) and Solar Irradiance

Pneumatic defoliation significantly increased PAR and solar irradiance inside the tree canopy (Figure 5A,B). Compared with that of the non-defoliated group, the PAR was 1.5- and 2.0-fold higher in the low- and high-defoliation groups, respectively (Figure 5A). Similarly, solar irradiance was 2.0- and 3.0-times higher in the low- and high-defoliation groups, respectively, than that in the non-defoliated group (Figure 5B). Notably, the PAR and solar irradiance were significantly higher in the high-defoliation group than in the low-defoliation group (Figure 5A,B).

3.3. Fruit Quality Characteristics

Pre-harvest pneumatic defoliation treatments had little effect on fruit quality characteristics of ‘Fuji’ apples at harvest (Table 1). For fruit development, pneumatic defoliation treatments did not significantly affect fruit size, including length, diameter, and shape (length–diameter ratio) and fruit weight at harvest (Table 1). Flesh firmness is an important index for fruit maturity, and the apples in the high-defoliation group had significantly firmer flesh at harvest than those in the low- and non-defoliated groups (Table 1). However, the indicators of fruit taste, such as SSC and TA, were not affected by defoliation treatments. The SSC/TA ratio was higher in the low-defoliation group compared to that of the non-defoliation group. The indicator of starch degradation (SPI score) in fruit was not affected by pneumatic defoliation. Additionally, pneumatic defoliation did not significantly increase the incidence of sunburned fruits (Table 1).

3.4. Apple Skin Color

Apples in the low- and high-pneumatic-defoliation groups showed more intense skin coloration than those in the non-defoliated group (Figure 6).
Compared with the non-defoliation group, the a* value was higher in the low- and high-defoliation groups (Table 2). Lower L* and b* values were observed in the high-defoliation group than in the non-defoliated and low-defoliation groups. Although the low-defoliation group showed higher L* and b* values than the high-defoliation group, the values were lower than those in the non-defoliated group (Table 2). Additionally, higher C values and lower ho values were observed in the low- and high-defoliation groups than in the non-defoliated group. However, no significant differences in the C, ho, and a* values existed between the low- and high-defoliation groups (Table 2).

3.5. Anthocyanin Contents and Expression of Anthocyanin Biosynthesis Genes

The anthocyanin content was significantly higher in the low- and high-defoliation groups than in the non-defoliated group (Figure 7). However, the anthocyanin content did not significantly differ between the low- and high-defoliation groups (Figure 7).
Gene expression analysis showed that both defoliation treatments significantly upregulated the expression of anthocyanin biosynthesis genes, including MdPAL, MdCHS, MdCHI, MdF3H, MdDFR, MdANS, MdUFGT, and MdMYB10, compared with that in the non-defoliated group (Figure 8A–H). Although MdDFR mRNA expression was significantly downregulated in the low-defoliation group, no significant differences existed in the expression levels of the other anthocyanin biosynthesis genes between the low- and high-pneumatic-defoliation groups (Figure 8A–H).

3.6. Pearson’s Correlation Coefficient Analysis

A Pearson’s correlation coefficient analysis was performed to further evaluate the relationship between the expression of anthocyanin biosynthesis genes and anthocyanin content in each defoliation treatment (Figure 9). The anthocyanin biosynthesis genes were positively correlated with the anthocyanin content in all defoliation treatments. A strong relationship was observed between the anthocyanin content and MdMYB10 expression, and that result was prevalent in the defoliation treatments. With the exception of MdCHS and MdF3H, the MdMYB10 expression was strongly correlated with other anthocyanin biosynthesis genes (MdPAL, MdCHI, MdDFR, MdANS, and MdUFGT) in both pneumatic defoliation treatments. Finally, the overall result showed that the correlations were more significant in the pneumatic defoliation treatments compared to that of the control (Figure 9).

4. Discussion

Defoliation is a strategic practice to optimize apple color, improve fruit quality, and reduce pests and diseases in trees. Additionally, mechanical defoliation can reduce production costs and improve environmental benefits [27]. Andergassen et al. [29] found that the impacts of pneumatic defoliation varied among apple cultivars, and the defoliation of plant leaves in more than 40% of trees negatively affected the fruit weight and yield of apples. In a previous report, low (0.7) and high (0.9) air pressure bars of pneumatic defoliations removed 9.3% and 16.6% of the plant leaves in the ‘Picnic’ apple tree at a tractor driving speed of 2 km/h at 3 weeks before harvest [26]. In the present study, low (0.6) and high (0.9) air pressure bars of pneumatic defoliations removed 16.8% and 21.4% of the plant leaves at a tractor driving speed of 1 km/h at 20 days before harvest. Additionally, no significant differences existed in the leaf damage rate between the low- and high-defoliation treatment groups [26]. However, high pneumatic defoliation did not significantly enhance defoliation rates but increased leaf damage rates compared with low pneumatic defoliation in the present study. A damaged leaf involves the incomplete removal of a whole leaf along with the petiole, which remains in the tree. Unlike with ‘Picnic’ apple [26], differences in leaf defoliation and damage rates were caused by two main reasons. Firstly, the increased defoliation rate occurred because the tree had more time to defoliate the plant leaves when the tractor driving speed was reduced to 1 km/h. Secondly, differences in apple cultivars and leaf morphological characteristics (leaf area, size, thickness, etc.) could be another reason for the varying results among studies. Additionally, leaf damage can make plants more susceptible to pathogen infection, and the significant reduction in leaf damage rate by low defoliation can reduce the risks of pathogen infection in the plants. Overall, these results indicate that strong air pressure did not completely remove apple leaves but only partially removed parts of the leaves, resulting in an increased leaf damage rate.
Compared with that of the non-defoliated group, low- and high-defoliation treatments significantly increase PAR and solar irradiance inside the tree canopies. PAR and solar irradiance are generally measured to evaluate the amount and quality of sunlight reaching trees and different parts of the tree canopy [39,40]. A previous report indicated that apple trees receiving low light intensity inside the tree canopy produced poorly colored fruits [29]. The increased PAR and solar irradiance indicated that pneumatic defoliation removed a large quantity of plant leaves, which allowed sunlight exposure and solar radiations to penetrate inside the trees. Consistent with findings in ‘Picnic’ apples [26], defoliation-induced removal of plant leaves improved sunlight penetration inside the trees, which induced pigment accumulation in the fruits. Additionally, Lavely [27] reported that some physical damages appeared on the skin of ‘Minneiska’ and ‘Honeycrisp’ apple fruits after pneumatic defoliation. Notably, physical damage to the fruits might be due to the application of pneumatic defoliation extremely close to the apple canopy or contact with the rotating parts of the machine during the treatment operation. In the present study, the pneumatic defoliation treatments applied at a specific distance (10 cm from the tree canopy) caused neither fruit dropping nor physical damage to the fruits, which was consistent with previous findings [26].
Fruit enlargement is important in regard to increasing fruit yield and profit. Matsumoto et al. [23] reported that early defoliation of ‘Fuji’ apples 1.5 months prior to harvest produced small fruits. Additionally, Han et al. [41] observed that the degree and timing of defoliation influenced the growth of ‘Sinano Sweet’ apple fruits and that early and severe defoliation reduced fruit yield. Andergassen and Picher [22] reported that pneumatic defoliation slightly reduced fruit size in ‘Pink Lady’ apples, which might be due to the reduction in leaf area and a large quantity of plant leaves during the fruit cell division phase. However, Lee et al. [22] reported that defoliation at 30 DBH did not reduce the apple fruit weight or size of ‘Fuji’ apples. Additionally, Win et al. [26] reported that defoliation at 3 WBH did not affect fruit growth and development in ‘Picnic’ apples. Overall, the effects of defoliation on fruit weight and size can vary depending on the apple cultivar and time of application, and early or severe defoliation can result in the production of small fruits. In this study, defoliation treatments did not affect the size and weight of ‘Fuji’ apple fruits at harvest.
Fruit quality parameters, including flesh firmness, SSC, TA, SSC/TA, and SPI, are important marketability indicators. In the present study, pneumatic defoliation treatments did not significantly affect SSC, TA, and SPI scores. However, apples in the high-defoliation group had firmer flesh, an indicator of fruit hardness, than those in the low-defoliation and non-defoliation groups. Sagong et al. [42] reported that defoliation decreased SSC and TA but increased fruit maturity and the SPI score in ‘Fuji’ apples. Han et al. [41] reported that defoliation timing is important for plant carbohydrate metabolism and that early defoliation reduced carbohydrate contents in ‘Sinano Sweet’ apples. Contrary to previous findings, the higher SSC/TA ratio observed in the low-defoliation group in the present study indicates that pneumatic defoliation did not reduce sugar accumulation in the fruits. Similarly, Win et al. [26] reported that pneumatic defoliation at 3 WBH did not reduce fruit quality attributes in ‘Picnic’ apples. Additionally, maturity indicators such as starch degradation and flesh firmness were not affected by pneumatic defoliation [28,29]. According to previous studies, defoliation rate and application timing may influence apple fruit size and quality, and defoliation treatments applied near harvest time may minimize the reduction in fruit quality. Therefore, the application of pneumatic defoliation treatments close to harvest time may have prevented any negative effect on the size and quality of ‘Fuji’ apple fruits.
Fruit sunburn is a physiological disorder caused by excessive sunlight exposure and high temperatures, and sunburned fruits are low in price and less marketable [43]. Sunburn generally occurs when the fruit surface is exposed to prolonged sunlight or sustained high temperatures for a long period of time [44]. Therefore, the removal of plant parts or excessive defoliation under high-temperature conditions can increase the incidence of sunburning in apples. Importantly, the lower temperature threshold observed in the experimental field from the defoliation treatment time until harvest might be a reason for the lack of sunburn incidence in the fruits.
Notably, the ripening phase of apple fruit is important for pigment accumulation and is characterized by a progressive increase in anthocyanin accumulation [45]. Many previous studies indicate that cyanidin-3-galactoside is the predominant monomeric anthocyanin in apple skin [45,46,47]. In the present study, pneumatic defoliation treatments significantly increased anthocyanin accumulation in apple skin. Moreover, the higher a* and C values and lower ho values observed in the defoliated trees confirmed that the indicators of red coloration increased with defoliation treatments. Similarly, a previous study showed increased skin color values in ‘Fuji’ apples following defoliation [22]. Increased red coloration and anthocyanin contents after pneumatic defoliation treatments was also reported in the ‘Nicotar’ and ‘Rosy Glow’ apples [29]. Additionally, both pneumatic defoliation treatments significantly upregulated anthocyanin biosynthesis genes (MdPAL, MdCHS, MdCHI, MdF3H, MdDFR, MdANS, MdUFGT) and the transcriptional factor (MdMYB10).
Generally, anthocyanin synthesis is controlled by structural genes [48], and its content increases with increasing expression of related genes [49]. Espley et al. [50] reported that the increased red coloration in apples is due to the activity of the MdMYB10 transcription factor, and higher expression of the MdMYB10 gene resulted in more intense red coloration in apple skin. Meng et al. [51] reported that MdMYB10 is a transcription factor in apples and that its expression is influenced by light exposure. Similarly, Giap et al. [15] reported that light induces the expression of anthocyanin biosynthesis genes in apples, resulting in anthocyanin accumulation. Therefore, the anthocyanin biosynthesis in mature fruit is a light-dependent process and sufficient sunlight exposure is necessary to initiate anthocyanin production [47]. Due to the shading of the tree canopy, apple fruits receive a limited amount of light intensity and radiation. Light penetration inside tree canopies was increased after pneumatic defoliation, and each apple fruit received increased amounts of light intensity and solar radiations [29]. Consequently, the increased acceptability of sunlight and radiations in fruit activates the transcription factor gene (MdMYB10); therefore, light-induced upregulation of MdMYB10 further stimulates anthocyanin synthesis and promotes the expression of anthocyanin biosynthesis genes (MdPAL, MdCHS, MdCHI, MdF3H, MdDFR, MdANS, MdUFGT), leading to increased anthocyanin production in the apple skins [14,51]. The Pearson’s correlation analysis showed that anthocyanin content was positively correlated with anthocyanin biosynthesis genes, and the MdMYB10 gene was strongly correlated with anthocyanin biosynthesis genes, which means anthocyanin production is regulated by anthocyanin biosynthesis and transcription factor genes. The overall correlation results also indicated that anthocyanin production was more significant in the pneumatic defoliation treatments. Similar to this study, the significant expression of anthocyanin biosynthesis genes and anthocyanin contents after pneumatic defoliation were reported in the ‘Picnic’ apples [26]. Additionally, ‘Fuji’ is a late-harvesting apple cultivar; therefore, the application of pneumatic defoliation at 20 DBH may be no longer enough to induce anthocyanin accumulation in apples from high-defoliation treatment compared to those from low-defoliation treatment in the present study.
Overall, the pneumatic defoliations improved fruit skin color and anthocyanin production in apple fruits, thereby removing shading parts and plant leaves in trees, increasing light availability to the fruits and activating expression of anthocyanin biosynthesis genes. The results also suggest that the application of pneumatic defoliation at 20 DBH at a 1 km/h tractor driving speed is less affected by the reduction in fruit quality indicators and sunburn incidence. Hence, improving the fruit color of apples using pneumatic defoliations may require specific implementing strategies such as avoiding application during high temperature conditions and a too-early application time. Furthermore, this study was conducted in a short time duration, which may limit the potential variability of the results, and continuous studies of pneumatic defoliation should be considered to ensure the stable and efficient production of high-quality apples across different growing seasons and environmental conditions.

5. Conclusions

High-defoliation treatment increased the leaf damage rate but did not affect the defoliation rate. Although defoliation did not negatively affect fruit quality characteristics, including fruit weight, size, SSC, TA, SPI, and sunburn incidence, fruits in the high-defoliation group had firmer flesh. Additionally, both defoliation treatments improved light penetration inside tree canopies, which enhanced anthocyanin accumulation and the red color of apple skins. Moreover, higher expression of anthocyanin biosynthesis genes confirmed that pneumatic defoliation enhances red coloration in apples. Conclusively, the results suggest that pneumatic defoliation could be an effective approach for enhancing apple coloration and that defoliating at low air pressure (0.6 bars with 1 km/h) is adequate for improving the coloration of ‘Fuji’ apple fruits. Overall, this study would help growers improve the fruit coloration in ‘Fuji’ apples while also preventing reductions in fruit quality through pneumatic defoliation. Additionally, the long-term effects of pneumatic defoliation on plant health, fruit development, and carbohydrate metabolism require further study.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/agriculture14091582/s1. Table S1: Primer sequences for anthocyanin biosynthesis genes and PCR conditions.

Author Contributions

Conceptualization, N.M.W., S.Y. and J.P.; methodology, N.M.W., S.Y. and J.P.; software, N.M.W. and J.P.; validation, N.M.W., S.Y., S.-I.K. and J.P.; formal analysis, N.M.W. and V.G.D.; investigation, N.M.W. and V.G.D.; resources, N.M.W.; data curation, N.M.W. and J.P.; writing—original draft preparation, N.M.W.; writing—review and editing, N.M.W. and J.P.; visualization, N.M.W., J.Y., V.G.D., S.Y., S.-I.K., H.-J.K.; S.K., Y.L., I.-K.K. and J.P.; supervision, N.M.W., S.Y. and J.P.; project administration, S.Y., S.-I.K., H.-J.K. and J.P.; funding acquisition, J.P. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by the RDA fellowship program of the National Institute of Horticultural and Herbal Science, Rural Development Administration, Republic of Korea. This work was carried out with the support of the “Cooperative Research Program for Agriculture Science and Technology Development (Project No. RS-2021-RD009831)” of the Rural Development Administration, Republic of Korea.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The data are included in the manuscript and Supplementary File.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Daily temperature and precipitation at the experimental field located in Sobo-myeon, Gunwi-gun, Daegu, Republic of Korea. Yellow and blue dotted lines indicate the daily maximum and minimum temperatures, respectively, while the red bars indicate the daily precipitation at the experiment field. Pneumatic defoliation treatments were applied on 6 October 2022, and apples were harvested on 26 October 2022.
Figure 1. Daily temperature and precipitation at the experimental field located in Sobo-myeon, Gunwi-gun, Daegu, Republic of Korea. Yellow and blue dotted lines indicate the daily maximum and minimum temperatures, respectively, while the red bars indicate the daily precipitation at the experiment field. Pneumatic defoliation treatments were applied on 6 October 2022, and apples were harvested on 26 October 2022.
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Figure 2. The ‘Fuji’ apple tree canopies and application of pneumatic defoliation using a Red-pulse pneumatic device attached to a tractor (A). Pneumatic defoliation treatments were applied at 10 cm away from the tree canopies at a tractor driving speed of 1 km/h at 20 days before harvest. (B–D) shows samples of branches and leaves of ‘Fuji’ apples trees after pneumatic defoliation treatments; yellow circles indicate damaged leaves and black circles indicate defoliated leaves from the stems of the trees (B). Black circles indicate samples of defoliated leaves from the stem (C) and yellow circles indicate samples of damaged leaf from the stem (D) after pneumatic defoliation treatments.
Figure 2. The ‘Fuji’ apple tree canopies and application of pneumatic defoliation using a Red-pulse pneumatic device attached to a tractor (A). Pneumatic defoliation treatments were applied at 10 cm away from the tree canopies at a tractor driving speed of 1 km/h at 20 days before harvest. (B–D) shows samples of branches and leaves of ‘Fuji’ apples trees after pneumatic defoliation treatments; yellow circles indicate damaged leaves and black circles indicate defoliated leaves from the stems of the trees (B). Black circles indicate samples of defoliated leaves from the stem (C) and yellow circles indicate samples of damaged leaf from the stem (D) after pneumatic defoliation treatments.
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Figure 3. ‘Fuji’ apple trees after application of non-defoliation (control) (A) and low (0.6) (B) and high (0.9) (C) air pressure bars of pneumatic defoliation at a tractor driving speed of 1 km/h at 20 days before harvest.
Figure 3. ‘Fuji’ apple trees after application of non-defoliation (control) (A) and low (0.6) (B) and high (0.9) (C) air pressure bars of pneumatic defoliation at a tractor driving speed of 1 km/h at 20 days before harvest.
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Figure 4. Leaf defoliation (A) and damage (B) rates of ‘Fuji’ apple trees after application of low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest. Different letters represent significant differences between two treatment groups using a t-test (p < 0.05). Data are shown as mean ± standard error (n = 15). n.a.: Not applicable.
Figure 4. Leaf defoliation (A) and damage (B) rates of ‘Fuji’ apple trees after application of low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest. Different letters represent significant differences between two treatment groups using a t-test (p < 0.05). Data are shown as mean ± standard error (n = 15). n.a.: Not applicable.
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Figure 5. Photosynthetically active radiation (PAR) (A) and solar irradiance (B) inside the canopies of ‘Fuji’ apple trees after application of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliations at a tractor driving speed of 1 km/h at 20 days before harvest. Different letters represent significant differences within treatments using a Tukey’s HSD test (p < 0.05). Data are shown as mean ± standard error (n = 15).
Figure 5. Photosynthetically active radiation (PAR) (A) and solar irradiance (B) inside the canopies of ‘Fuji’ apple trees after application of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliations at a tractor driving speed of 1 km/h at 20 days before harvest. Different letters represent significant differences within treatments using a Tukey’s HSD test (p < 0.05). Data are shown as mean ± standard error (n = 15).
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Figure 6. ‘Fuji’ apple fruits at harvest after application of no defoliation (control) (A) and low (0.6) (B) and high (0.9) (C) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest.
Figure 6. ‘Fuji’ apple fruits at harvest after application of no defoliation (control) (A) and low (0.6) (B) and high (0.9) (C) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest.
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Figure 7. Anthocyanin content of ‘Fuji’ apples after application of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest. Different letters represent significant differences within treatments using a Tukey’s HSD test (p < 0.05). Data are shown as mean ± standard error (n = 3).
Figure 7. Anthocyanin content of ‘Fuji’ apples after application of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest. Different letters represent significant differences within treatments using a Tukey’s HSD test (p < 0.05). Data are shown as mean ± standard error (n = 3).
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Figure 8. Relative expression levels of anthocyanin biosynthesis genes, including MdPAL (A), MdCHS (B), MdCHI (C), MdF3H (D), MdDFR (E), MdANS (F), MdUFGT (G), and MdMYB10 (H), in ‘Fuji’ apples after of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest. Different letters represent significant differences within treatments using a Tukey’s HSD test (p < 0.05). Data are shown as mean ± standard error (n = 3).
Figure 8. Relative expression levels of anthocyanin biosynthesis genes, including MdPAL (A), MdCHS (B), MdCHI (C), MdF3H (D), MdDFR (E), MdANS (F), MdUFGT (G), and MdMYB10 (H), in ‘Fuji’ apples after of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest. Different letters represent significant differences within treatments using a Tukey’s HSD test (p < 0.05). Data are shown as mean ± standard error (n = 3).
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Figure 9. Pearson’s correlation coefficient analysis (r) of anthocyanin content and anthocyanin biosynthesis gene expression of ‘Fuji’ apples after application of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments. Red indicates positive correlation and blue indicates negative correlation at the significant levels of * p < 0.05 and ** p < 0.01.
Figure 9. Pearson’s correlation coefficient analysis (r) of anthocyanin content and anthocyanin biosynthesis gene expression of ‘Fuji’ apples after application of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments. Red indicates positive correlation and blue indicates negative correlation at the significant levels of * p < 0.05 and ** p < 0.01.
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Table 1. Fruit quality characteristics, including fruit weight and size, flesh firmness, soluble solids content (SSC), titratable acidity (TA), SSC/TA ratio, starch pattern index (SPI), and the incidence of fruit sunburn rate in ‘Fuji’ apples after application of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest.
Table 1. Fruit quality characteristics, including fruit weight and size, flesh firmness, soluble solids content (SSC), titratable acidity (TA), SSC/TA ratio, starch pattern index (SPI), and the incidence of fruit sunburn rate in ‘Fuji’ apples after application of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest.
Defoliation
Treatments		Fruit Weight
(g)		Fruit Size (mm)
Length (L)Diameter (D)L/D Ratio
No defoliation297.65 ± 8.02 z a y76.22 ± 0.92 a90.54 ± 0.68 a0.85 ± 0.00 a
Low defoliation302.60 ± 5.09 a77.77 ± 0.83 a91.36 ± 0.35 a0.85 ± 0.00 a
High defoliation299.42 ± 4.28 a76.58 ± 0.24 a89.83 ± 0.42 a0.85 ± 0.00 a
Flesh Firmness
(N)		SSC
(%)		TA
(%)		SSC/TA
ratio		SPI
(1–8)		Sunburn
(%)
No defoliation56.39 ± 0.32 b14.29 ± 0.13 a0.38 ± 0.01 a37.01 ± 0.73 b7.20 ± 0.34 a2.45 ± 0.36 a
Low defoliation55.82 ± 0.62 b14.30 ± 0.17 a0.36 ± 0.02 a39.30 ± 0.92 a7.50 ± 0.26 a2.80 ± 0.22 a
High defoliation58.17 ± 0.74 a14.07 ± 0.18 a0.36 ± 0.01 a38.81 ± 0.90 ab7.05 ± 0.19 a3.05 ± 0.41 a
z Data are shown as mean ± standard error (n = 15). y Different letters represent significant differences using Tukey’s HSD test (p < 0.05).
Table 2. Fruit skin color values (L*, a*, b*, C, and ho) in ‘Fuji’ apples after application of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest.
Table 2. Fruit skin color values (L*, a*, b*, C, and ho) in ‘Fuji’ apples after application of no defoliation (control) and low (0.6) and high (0.9) air pressure bars of pneumatic defoliation treatments at a tractor driving speed of 1 km/h at 20 days before harvest.
Defoliation TreatmentsFruit Skin Color Values
L*a*b*Cho
No defoliation 44.18 ± 0.20 z a y19.22 ± 0.68 b15.21 ± 0.14 a24.52 ± 0.54 b38.45 ± 1.04 a
Low defoliation41.25 ± 0.70 b22.60 ± 0.19 a13.50 ± 0.32 b26.36 ± 0.23 a29.42 ± 1.66 b
High defoliation39.12 ± 0.54 c23.25 ± 1.04 a12.55 ± 0.41 c26.92 ± 0.78 a30.02 ± 0.86 b
z Data are shown as mean ± standard error (n = 15). y Different letters represent significant differences using a Tukey’s HSD test (p < 0.05).
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Win, N.M.; Yoo, J.; Do, V.G.; Yang, S.; Kwon, S.-I.; Kweon, H.-J.; Kim, S.; Lee, Y.; Kang, I.-K.; Park, J. Effects of Pneumatic Defoliation on Fruit Quality and Skin Coloration in ‘Fuji’ Apples. Agriculture 2024, 14, 1582. https://doi.org/10.3390/agriculture14091582

AMA Style

Win NM, Yoo J, Do VG, Yang S, Kwon S-I, Kweon H-J, Kim S, Lee Y, Kang I-K, Park J. Effects of Pneumatic Defoliation on Fruit Quality and Skin Coloration in ‘Fuji’ Apples. Agriculture. 2024; 14(9):1582. https://doi.org/10.3390/agriculture14091582

Chicago/Turabian Style

Win, Nay Myo, Jingi Yoo, Van Giap Do, Sangjin Yang, Soon-Il Kwon, Hun-Joong Kweon, Seonae Kim, Youngsuk Lee, In-Kyu Kang, and Juhyeon Park. 2024. "Effects of Pneumatic Defoliation on Fruit Quality and Skin Coloration in ‘Fuji’ Apples" Agriculture 14, no. 9: 1582. https://doi.org/10.3390/agriculture14091582

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