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Article

Pneumatic Defoliation Enhances Fruit Skin Color and Anthocyanin Pigments in ‘Picnic’ Apples

by
Nay Myo Win
1,
Youngsuk Lee
1,
Seonae Kim
1,
Van Giap Do
1,
Young Sik Cho
1,
In-Kyu Kang
2,
Sangjin Yang
1 and
Juhyeon Park
1,*
1
Apple Research Institute, National Institute of Horticultural and Herbal Science, RDA, Gunwi 39000, Republic of Korea
2
Department of Horticultural Science, Kyungpook National University, Daegu 41566, Republic of Korea
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(8), 2078; https://doi.org/10.3390/agronomy13082078
Submission received: 12 July 2023 / Revised: 1 August 2023 / Accepted: 4 August 2023 / Published: 7 August 2023
(This article belongs to the Special Issue The Effects of Modern Agricultural Management Practices on the Growth, Yield and Fruit Quality of Horticultural and Fruit Plants)
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Versions Notes

Abstract

:
Apple skin color is essential for market value. Therefore, this study evaluated the efficiency of pneumatic defoliation (PD) on the enhancement of red skin color and anthocyanin pigments in ‘Picnic’ apples. Three weeks before harvesting, apple trees were treated with PD with low- (0.7 bar) and high- (0.9 bar) air pressure at a tractor driving speed of 2 km/h. Undefoliated trees served as controls. Higher leaf defoliation and leaf damage rates were observed in the high-PD treatment than those in the low-PD treatment. Photosynthetically active radiation inside the canopies was the highest in trees subjected to high-pressure PD than in those that underwent low-pressure PD and the controls. At harvest, the fruit color (a*) value, red-colored area, and anthocyanin content in the fruit skin were the highest in the high-PD treatment, intermediate in the low-PD treatment, and the lowest in the control treatments. Additionally, a higher expression of anthocyanin biosynthetic genes was observed in both defoliation treatments, especially under high PD. However, except for higher flesh firmness under low PD, the L* and b* values and fruit quality indices (fruit weight, starch pattern index, titratable acidity, soluble solids content, and sunburn occurrence) were not significantly affected by either PD treatment. In conclusion, PD can be used to enhance skin coloration and anthocyanin pigments in apples; further PD with high air pressure achieved optimum red skin coloration in ‘Picnic’ apples.

1. Introduction

The quality of apples is generally determined by their external and internal attributes. External quality attributes include fruit size, shape, color, and lack of defects, while internal quality attributes include texture, taste, acidity, sweetness, aroma, and nutritional value [1]. Fruit color is an important external quality attribute that directly influences consumer acceptance and marketability [2]. Consumers prefer better-colored apples, either in different cultivars or within a particular cultivar [3]. Additionally, red apples with good flavor are often better in terms of economic value and demand than non-red apples [4,5]. Therefore, enhancing the development of fruit skin color is important for growers.
The fruit skin color of apples is determined by various pigments; anthocyanin is a major pigment that drives the development of red coloration in apples [6,7]. Enhancement of anthocyanin content in apple fruit skins is influenced by many factors, including light, temperature, crop loads, tree nutrition, tree form, rootstock, and other environmental conditions [8,9,10]. Many studies have reported that anthocyanin biosynthesis in apple skin is light-dependent, and that sunlight exposure increases anthocyanin content as well as the transcript levels of anthocyanin synthesis-related structural (MdPAL, MdCHS, MdCHI, MdF3H, MdDFR, MdANS, and MdUFGT) and transcription factor (MdMYBs) genes in apple skin [7,11,12,13,14]. Thus, light interception is the most important factor for enhancing red coloration and anthocyanin pigments in apple skin.
Therefore, most growers remove the fruit cluster leaves and water sprouts from trees by manual (hand) defoliation before harvesting to increase light exposure to the fruit and to enhance the coloration of apples [15,16]. Reflective film mulching under tree canopies has also been used to improve the fruit skin color of apples [17,18]. Additionally, chemical defoliants have been used in foliar applications for leaf thinning and fruit cluster leaf defoliation in apple trees [19,20,21,22]. However, manual defoliation and reflective film mulching are time-consuming and labor-intensive; further, some chemical defoliants are expensive and pose environmental safety issues. Therefore, pneumatic defoliation (PD) is an alternative method for improving fruit coloration in apples. Few studies have examined leaf thinning in apple trees using PD, but the effects can vary depending on the cultivar, tree canopy, time of application, and air pressure used [23]. Therefore, evaluating tree-adaptable PD practices for specific apple cultivars is essential.
‘Picnic’ (‘Fuji’ × ‘Sansa’) is a mid-season apple cultivar with a light red skin color and the optimum harvest time in late September [24]. This apple cultivar is becoming increasingly popular among growers and consumers owing to its favorable sweetness and quality [25,26]. Therefore, we selected the ‘Picnic’ apple cultivar as a model for this study and hypothesized that pneumatic defoliation could enhance red coloration and anthocyanin contents in the fruit skin of ‘Picnic’ apples. First, the thinning efficiency of PD at different pressure bars was evaluated for apple trees. Additionally, the efficiency of PD in improving apple fruit quality, color, and anthocyanin content in the apple skin was determined at harvest. Moreover, the expression of anthocyanin synthesis-related genes in the skin tissues of ‘Picnic’ apples was continuously analyzed in this study.

2. Materials and Methods

2.1. Plant Materials and Field Trial

This study was conducted as an experimental field trial at the Apple Research Institute in Gunwi, Gyeongsangbuk-do, Republic of Korea. Seven-year-old ‘Picnic’ apple trees with a 3.0 m × 1.5 m spacing, planted in the same trial and soil conditions, were used. The trees were grafted to M.9 rootstock and trained into a slender-spindle-shape (average plant height of 2.3 m and width of 70 cm). The crop load levels of the apple trees were set to eight fruits per cm2 of trunk cross-sectional area. The trial was managed using an integrated pest management system and irrigation with drip pipe lines. The average daily air temperature (°C), precipitation (mm), and relative humidity (%) of the experimental field trial from defoliation to harvest are shown in Figure 1.

2.2. Defoliation Treatments

Three defoliation treatment groups were used. In the first group, apple trees were treated with pneumatic defoliation (PD) at low air pressure (0.7 bar). In the second group, apple trees were treated with PD at high air pressure (0.9 bar). The third group served as a control (no defoliation). A pneumatic defoliation device (REDPulse Duo, Fruit Tec, Markdorf, Germany) was used for PD. The operating height of the PD was 1.0 m height; therefore, the treatment was first applied to the lower part of the tree canopy and then to the upper part of the canopy (Figure 2). During the application of PD treatments, the tractor was driven approximately 10 cm apart from the tree canopies at a driving speed of 2 km/h by a skilled driver. The treatments were performed three weeks before the estimated harvest (2 September 2022). The estimated harvest date for ‘Picnic’ apples was predicted using previous reports by Kwon et al. [24] and Yoo et al. [27].

2.3. Assessments of Defoliation Rate, Leaf Damage Rate, and Sunlight Availability in the Tree

For each treatment, a total of fifteen apple trees, including five individual trees from three blocks, were randomly selected for this study. All assessments were performed using the selected apple trees. In each tree, a total of eighteen individual branches (nine branches from the top, middle, and bottom parts of each side of the tree canopy) were selected as model branches and used to assess the leaf defoliation and leaf damage rates. The total number of leaves per branch was counted at two times before (1 September 2022) and immediately after (2 September 2022) PD. The defoliation and leaf damage rates were calculated as the number of defoliated or damaged leaves per 100 apple leaves and presented as percentages (%) [21,22]. The fruit drops and skin damage caused by PD were also assessed. Sunlight availability inside the tree canopy was measured as the amount of photosynthetically active radiation (PAR) at midday on a sunny day (8 September 2022) at a height of approximately 1 m in two horizontal zones of each canopy using a light sensor (3415FX, Spectrum Technologies Inc., Aurora, IL, USA), as described by Yoo et al. [28].

2.4. Assessments of Fruit Quality Attributes

The ‘Picnic’ apple fruit samples were harvested on 23 September 2022. For each treatment, a total of one-hundred-and-fifty fruits (ten fruits per tree) were randomly harvested and used for assessing the fruit quality attributes at harvest.
The development of red-colored (red-blushed) areas on the fruit skin surface was measured as described by Serra et al. [9]. Fruit skin color (L*, a*, b*) values were measured around the equatorial region of the fruit using a chroma meter (CR-400, Konica Minolta, Tokyo, Japan) following the methods described by Lee et al. [20] and Kim et al. [29]. The data were described as L* (lightness), a* (red-green sensation), and b* (yellow-blue sensation). Sunburn disorders were assessed as described by Yoo et al. [28]. Flesh firmness was determined at three equatorial locations on each fruit using a firmness tester (FT-327, TR Co., Forlì, Italy) equipped with an 11-mm probe. For starch pattern index (SPI) measurement, the cut fruit slices were dipped in an iodine solution and scored using a score chart (1–8 score), as described by Yoo et al. [27]. Juice samples were then extracted from the fruits to assess titratable acidity (TA) and soluble solids content (SSC). For TA, the juice samples were titrated with 0.1 N NaOH using the malic acid reduction method until reaching pH 8.1 [30]. For SSC, juice samples were analyzed using a refractometer (PR-201; Atago, Tokyo, Japan). During assessment, fruit skin tissue samples (from red-pink area of the fruit skins) were removed using a sharp knife, frozen in liquid nitrogen, and stored at −80 °C for further analyzing the anthocyanin content.

2.5. Determination of Anthocyanin Contents

Extraction was performed as described by Fawbush et al. [31]. Apple skin tissue (5 g) was extracted with 80% acetone using a blender, and the sample extracts were filtered through filter paper. Next, the filtrate was evaporated at 45 °C using a rotary evaporator (N-1000, Rikakikai, Tokyo, Japan), and the sample volumes were brought to 10 mL using distilled water. The anthocyanin content was determined using the pH differential method [32]. The sample extracts were separately mixed with potassium chloride buffer (pH1.0) and sodium acetate buffer (pH4.5), and the absorbance was read at 515 and 700 nm using a spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan). The sample absorbance was calculated as follows: absorbance = (A515 − A700)pH1.0 − (A515 − A700)pH4.5. The anthocyanin content was calculated as follows: anthocyanin = (absorbance × MW × 1000)/(ε × C), where the molecular weight (MW) was 449.2, the molar absorptivity of cyanidin-3-galactoside (ε) was 26,900, and C was the concentration of the buffer solution. The anthocyanin content was expressed as the concentration in mg equivalents of cyanidin-3-galactoside per g of fresh weight.

2.6. RNA Extraction and Quantitative Real-Time PCR

Total RNA was isolated from apple skin tissues using the CTAB method [33]. A TURBO DNA-free kit (Invitrogen, Carlsbad, CA, USA) was used to remove the genomic DNA contamination in the RNA samples. The concentration and quality of RNA were determined using a UV spectrophotometer, and visual verification was performed using a 0.8% agarose gel. Total RNA was synthesized into cDNA using the PrimeScriptTM 1st strand cDNA synthesis kit (Takara, Kusatsu, Japan). Gene expression analysis was performed using qRT-PCR system (LightCycler 48 II, Roche Diagnostics, Mannheim, Germany). The primers were selected from a previous report by Do et al. [13] and the gene transcript levels were normalized to those of a reference gene (MdP0000336547) [34]. The analysis was performed in triplicate. The primer sequences and PCR conditions are described in detail in Supplementary Table S1.

2.7. Statistical Analysis

The experimental trial was arranged in a completely randomized block design (CRBD) with three replicates per treatment block. Each treatment block comprised five apple trees; therefore, a total of forty-five apple trees were used in the three treatment groups. Data were subjected to analysis of variance, and mean comparisons were performed using Tukey’s honest significant difference (HSD) test (p < 0.05). All analyses were performed using SPSS software (Version 25; IBM SPSS Corp., Armonk, NY, USA). Data are expressed as means with standard error.

3. Results

3.1. Defoliation Rate, Leaf Damage Rate, and Sunlight Availability in Trees

An increased defoliation rate was observed in the high-PD group (16.6%) compared to that in the low-PD group (9.3%) (Figure 3A). Specifically, the defoliation rate was 7.3% higher under high PD than under low PD. A higher leaf damage rate, although not statistically significant, was also observed in the high-PD treatment than in the low-PD treatment (Figure 3B). No fruit drop or skin damage was observed with either PD treatment (data not shown).
Sunlight availability inside the tree canopies was measured based on the amount of photosynthetically active radiation (PAR). After defoliation, the PAR inside the tree canopy was significantly increased in both low- and high-defoliated apple trees compared with that in the control trees (Figure 4). The PAR was the highest in the high-PD treatment, intermediate in the low-PD treatment, and the lowest in the control trees (Figure 4).

3.2. Fruit Quality Attributes

Flesh firmness was significantly higher in fruit samples from the low-PD treatment than from the controls (Table 1). However, the flesh firmness of the high-PD treatment group did not significantly differ from that in the control and low-PD treatments. Further, SSC, TA, SPI, and sunburn occurrence were not affected by the PD treatments (Table 1).

3.3. Fruit Skin Color

The fruit samples from the high-PD treatment showed the highest red skin coloration at harvest (Figure 5).
Additionally, the development of red-colored areas on the fruit skin was 20.5% and 9.4% higher in the high or low-PD treatments, respectively, than in the control (Table 2). Between the two defoliation treatments, the red-colored area in fruits from the high PD treatment was significantly (11.1%) larger than that in fruits from the low-PD treatment. Further, the fruit skin color (a*) value was the highest in the high-PD treatment, followed by that in low-PD, whereas the control group showed the lowest value. However, the L* and b* values of the fruit skin were the lowest in the high-PD treatment and the highest in the control samples (Table 2).

3.4. Anthocyanin Contents and Expression of Anthocyanin Synthesis-Related Genes

Both PD treatments significantly enhanced the anthocyanin content at harvest (Figure 6). Briefly, the anthocyanin content in apple skin was the highest in the high-PD treatment, intermediate in the low-PD treatment, and the lowest in the control treatment (Figure 6).
Compared to the control, both PD treatments enhanced the expression of anthocyanin synthesis-related structural (MdPAL, MdCHS, MdCHI, MdF3H, MdDFR, MdANS, and MdUFGT) and transcription factor (MdMYB10) genes in the apple fruit skin at harvest; their expression was particularly high in the high-PD treatment (Figure 7). Except for MdPAL expression, compared with that in the low-PD treatment, the expression of all anthocyanin synthesis-related genes was significantly higher in the high-PD treatment than in the control and low-PD treatments. Except for MdCHI and MdF3H expression, a higher expression of anthocyanin synthesis-related genes was also observed in the low-PD treatment than in the control (Figure 7).

4. Discussion

Leaves are important for plant photosynthesis and are the main source of plant carbohydrates [35,36]. However, Iqbal et al. [37] reported that defoliation could positively impact the biochemical characteristics, yield, nutritional value, and harvest indices of plants. However, these effects can vary depending on the crop and timing of treatment application. In apples, defoliation is generally performed prior to harvest for improving fruit color appearance [15]. In this study, approximately 9.3% and 16.6% of apple leaves were defoliated using the low- and high-PD treatments, respectively. A slightly higher leaf damage rate was also observed in the high-PD treatment because of the high air pressure used. However, no physical damage to the fruit surface or fruit dropping were observed after PD (data not shown). Lavely [38] reported that fruit damage and fruit dropping were slightly observed in ‘Honeycrisp’ apples upon PD, which might be attributed to the close distance between the machine and trees during the treatment operation. The PAR inside the tree canopies improved after the PD treatments, whereas non-defoliated control trees had lower PAR. Anten and Ackerly [39] reported that partial defoliation in plum trees improved light penetration into the canopy. Hence, increased PAR in the interior parts of tree canopies in defoliated trees improved light availability to the fruits, resulting in improved skin coloration.
In deciduous plants, the photosynthetic products synthesized in the leaves are usually sent to fruits during the fruit maturation period [40]. Iqbal et al. [37] reported that defoliation influenced fruit growth and nutritional content. Hence, excessive defoliation in trees could reduce fruit enlargement and sugar accumulation. Defoliation at 30 days prior to harvest was reported to reduce the fruit weight of ‘Hongro’ apples [22] but had no effect on ‘Fuji’ apples [21]. In contrast, early defoliation (approximately 1.5 and 2 months before harvest) in ‘Fuji’ apple trees decreased the apple fruit weight and produced small-sized fruits at harvest [16]. In this study, we applied PD treatments as close as possible to the harvest time; this might be one of the reasons why the fruit size and weight did not noticeably reduce at harvest. Therefore, the effect of defoliation on fruit enlargement highly depends on the cultivar and timing of the treatment applied.
Matsumoto et al. [16] found that defoliation in ‘Fuji’ apple trees did not affect flesh firmness but reduced the SSC at harvest. However, the application of chemical defoliants improved flesh firmness in ‘Fuji’ [21] but decreased it in ‘Hongro’ apples [22]. They additionally reported that SSC was reduced in ‘Hongro’ but was not affected in ‘Fuji’ apples upon using chemical defoliants. The accumulation of soluble solids and sugar content in fruits is also associated with crop load levels and source–sink relationships in trees [37,41]. In our study, neither PD treatment reduced SSC compared to that in the control. Further, PD treatments also improved flesh firmness, especially with low PD, compared with that in the control.
No difference in SPI was observed between the PD treatments. Andergassen and Pichler [23] reported that apple-ripening indicators and starch breakdown were not affected after PD. In kiwis, fruit maturation can be delayed by different application times of defoliation [42]. Both SPI and flesh firmness are important maturity indicators, and fruits with higher values of these have a better potential for market value [1]. In this study, fruit sunburn did not remarkably occur in any of the defoliation treatments. Lavely [38] reported that PD should be performed nearly at the harvest to avoid sunburn in fruits. However, sunburn occurrence is highly dependent on cultivar differences, sunlight, and temperature [43]. The non-significant occurrence of fruit sunburn in the treated apple trees in this study might also be due to the application of PD treatments near harvest. Therefore, the effect of PD on apple quality may vary depending on the timing of application, type of defoliation, and cultivar used. Moreover, defoliation is not reported to affect the TA of apples [16,21,22], as observed in the present study.
Matsumoto et al. [16] reported that conventional defoliation enhanced the red-color index and a* value in the fruit skin of ‘Fuji’ apples at harvest. However, the L* and b* values of the fruit skin were decreased by defoliation. The increase in a* value and decrease in L* and b* values were also reported after the chemical defoliation of ‘Fuji’ apple skins [20,21]. The development of red color in apple skin is mainly attributed to anthocyanin pigments, whereas cyanidin-3-galactoside is the pigment most responsible for red coloration [6,44]. In our study, the anthocyanin content (determined as the amount of cyanidin-3-galactoside) and expression levels of genes related to its synthesis (MdPAL, MdCHS, MdCHI, MdF3H, MdDFR, MdANS, MdUFGT, and MdMYB10) were enhanced in apple skin after PD. Many studies have reported that these genes are mainly involved in anthocyanin biosynthesis metabolism, sunlight induced anthocyanin accumulation, and the expressions of anthocyanin biosynthetic genes [7,12,13,45]. Recently, Andergassen and Pichler [23] reported that PD treatments enhanced the red-colored area in apple fruit skin at harvest by over 33%, but this could vary depending on the apple cultivar used. Additionally, they reported that an increase in air pressure resulted in the highest red coloration of fruit skin. Similar to previous studies, the enhancement of red-colored area, a* value, anthocyanin content, and expression of anthocyanin biosynthetic genes in apple fruit skins via PD was also observed in our study; PD removed the shading leaves and parts of trees, increased the light exposure inside the tree canopies, allowed greater light availability to the fruits, and stimulated anthocyanin synthesis in the fruit skin, resulting in increased red coloration of the apple skin at harvest. Moreover, the results were more significant for PD with high air pressure in our study, similar to the results of Andergassen and Pichler [23].

5. Conclusions

In conclusion, both PD treatments removed apple leaves and increased light availability inside the tree canopies. In particular, high PD resulted in higher defoliation rates and greater light intensities than at low-PD rates. The development of red coloration, anthocyanin content, and the expression of anthocyanin synthesis-related genes in apple skin were greatly enhanced by both PD treatments, especially in the defoliation treatment with high air pressure. However, except for increased flesh firmness under low PD, the L* and b* values, as well as the fruit quality parameters (fruit weight, TA, SPI, and SSC), were not significantly improved by either PD treatment at harvest. Our results thus suggest that PD can be used for enhancing apple red skin coloration in ‘Picnic’ apples, and that optimal results can be achieved via PD with high air pressure. Further studies are necessary for increasing the efficiency of PD and its effect on the changes of physiology and nutrient metabolism in fruits and plants.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy13082078/s1, Table S1: Primer sequences of the genes and PCR conditions used in this study.

Author Contributions

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

Funding

This work was carried out with the support of ‘Cooperative Research Program for Agriculture Science and Technology Development (Project No. RS-2021-RD009831)’ Rural Development Administration, Republic of Korea. This work was supported by 2023 the RDA fellowship program of the National Institute of Horticultural and Herbal Science, Rural Development Administration, Republic of Korea.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Daily air temperature (A), daily precipitation (B), and daily relative humidity (C) of the experimental field trial during pneumatic defoliation treatments. The black-colored arrow indicates the application date of defoliation treatments (2 September 2022), and the red-colored arrow indicates the harvest date (23 September 2022) of ‘Picnic’ apples.
Figure 1. Daily air temperature (A), daily precipitation (B), and daily relative humidity (C) of the experimental field trial during pneumatic defoliation treatments. The black-colored arrow indicates the application date of defoliation treatments (2 September 2022), and the red-colored arrow indicates the harvest date (23 September 2022) of ‘Picnic’ apples.
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Figure 2. Tree canopy and application of pneumatic defoliation treatment (A), application of pneumatic defoliation treatment in the experimental field trial (B), apple tree branches at one day after pneumatic defoliation treatment with high air pressure (0.9 bar) (C), and apple fruit samples at one day before the harvest date from defoliated trees with high air pressure (D). The tractor was driven at a driving speed of 2 km/h at a distance of 10 cm away from the tree canopy during treatment application.
Figure 2. Tree canopy and application of pneumatic defoliation treatment (A), application of pneumatic defoliation treatment in the experimental field trial (B), apple tree branches at one day after pneumatic defoliation treatment with high air pressure (0.9 bar) (C), and apple fruit samples at one day before the harvest date from defoliated trees with high air pressure (D). The tractor was driven at a driving speed of 2 km/h at a distance of 10 cm away from the tree canopy during treatment application.
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Figure 3. Defoliation rate (A) and leaf damage rate (B) of ‘Picnic’ apple trees after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before the harvest. Data are shown as the mean ± standard error (n = 15). Different letters denote significant differences based on a Tukey’s HSD test (p < 0.05). n.a.: not applicable.
Figure 3. Defoliation rate (A) and leaf damage rate (B) of ‘Picnic’ apple trees after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before the harvest. Data are shown as the mean ± standard error (n = 15). Different letters denote significant differences based on a Tukey’s HSD test (p < 0.05). n.a.: not applicable.
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Figure 4. Photosynthetically active radiation (PAR) inside the canopies of ‘Picnic’ apple trees after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before the harvest. Data are shown as the mean ± standard error (n = 15). Different letters denote the significant differences based on a Tukey’s HSD test (p < 0.05).
Figure 4. Photosynthetically active radiation (PAR) inside the canopies of ‘Picnic’ apple trees after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before the harvest. Data are shown as the mean ± standard error (n = 15). Different letters denote the significant differences based on a Tukey’s HSD test (p < 0.05).
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Figure 5. ‘Picnic’ apple fruit samples at harvest after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before harvest.
Figure 5. ‘Picnic’ apple fruit samples at harvest after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before harvest.
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Figure 6. Anthocyanin contents in the skin tissues of the ‘Picnic’ apples at harvest after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before harvest. Data are shown as the mean ± standard error (n = 3). Different letters denote the significant differences based on a Tukey’s HSD test (p < 0.05).
Figure 6. Anthocyanin contents in the skin tissues of the ‘Picnic’ apples at harvest after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before harvest. Data are shown as the mean ± standard error (n = 3). Different letters denote the significant differences based on a Tukey’s HSD test (p < 0.05).
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Figure 7. Relative expression analysis of anthocyanin synthesis-related structural (MdPAL, MdCHS, MdCHI, MdF3H, MdDFR, MdANS, MdUFGT) and transcription factor (MdMYB10) genes in the skin tissues of ‘Picnic’ apples after control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before the harvest. Data are shown as the mean ± standard error (n = 3). Different letters denote significant differences based on a Tukey’s HSD test (p < 0.05).
Figure 7. Relative expression analysis of anthocyanin synthesis-related structural (MdPAL, MdCHS, MdCHI, MdF3H, MdDFR, MdANS, MdUFGT) and transcription factor (MdMYB10) genes in the skin tissues of ‘Picnic’ apples after control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before the harvest. Data are shown as the mean ± standard error (n = 3). Different letters denote significant differences based on a Tukey’s HSD test (p < 0.05).
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Table 1. Fruit weight, flesh firmness, soluble solids content (SSC), titratable acidity (TA), starch pattern index (SPI), and sunburn disorders of ‘Picnic’ apples after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before harvest.
Table 1. Fruit weight, flesh firmness, soluble solids content (SSC), titratable acidity (TA), starch pattern index (SPI), and sunburn disorders of ‘Picnic’ apples after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before harvest.
Defoliation
Treatments		Fruit Weight (g)Flesh Firmness (N)SSC (%)TA (%)SPI (1–8)Sunburn (%)
No defoliation (control)199.60 ± 3.66 z a y71.61 ± 1.50 b13.94 ± 0.18 a0.44 ± 0.01 a5.60 ± 0.48 a1.00 ± 0.26 a
Low pneumatic defoliation (0.7 bar)205.68 ± 3.54 a75.32 ± 0.64 a14.30 ± 0.14 a0.43 ± 0.01 a5.80 ± 0.12 a1.50 ± 0.34 a
High pneumatic defoliation (0.9 bar)198.19 ± 2.86 a74.30 ± 0.92 ab14.27 ± 0.18 a0.41 ± 0.01 a5.60 ± 0.10 a1.70 ± 0.49 a
z Data are shown as mean ± standard error (n = 15). y Different letters denote significant differences based on a Tukey’s HSD test (p < 0.05).
Table 2. The development of red-colored (red-blushed) area, and fruit skin color (L*, a*, and b*) values of ‘Picnic’ apples after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before harvest.
Table 2. The development of red-colored (red-blushed) area, and fruit skin color (L*, a*, and b*) values of ‘Picnic’ apples after the control (no defoliation), low- (0.7 bar), and high- (0.9 bar) air pressure pneumatic defoliation treatments at three weeks before harvest.
Defoliation TreatmentsRed-Colored Area (%)L*a*b*
No defoliation (control)57.78 ± 1.69 z c y47.99 ± 1.54 a22.49 ± 1.28 c15.98 ± 0.44 a
Low pneumatic defoliation (0.7 bar)67.22 ± 2.65 b42.98 ± 0.44 b24.99 ± 0.79 b14.24 ± 0.79 b
High pneumatic defoliation (0.9 bar)78.33 ± 1.86 a39.72 ± 0.91 c28.56 ± 0.34 a13.09 ± 0.47 c
z Data are shown as mean ± standard error (n = 15). y Different letters denote significant differences based on a Tukey’s HSD test (p < 0.05).
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MDPI and ACS Style

Win, N.M.; Lee, Y.; Kim, S.; Do, V.G.; Cho, Y.S.; Kang, I.-K.; Yang, S.; Park, J. Pneumatic Defoliation Enhances Fruit Skin Color and Anthocyanin Pigments in ‘Picnic’ Apples. Agronomy 2023, 13, 2078. https://doi.org/10.3390/agronomy13082078

AMA Style

Win NM, Lee Y, Kim S, Do VG, Cho YS, Kang I-K, Yang S, Park J. Pneumatic Defoliation Enhances Fruit Skin Color and Anthocyanin Pigments in ‘Picnic’ Apples. Agronomy. 2023; 13(8):2078. https://doi.org/10.3390/agronomy13082078

Chicago/Turabian Style

Win, Nay Myo, Youngsuk Lee, Seonae Kim, Van Giap Do, Young Sik Cho, In-Kyu Kang, Sangjin Yang, and Juhyeon Park. 2023. "Pneumatic Defoliation Enhances Fruit Skin Color and Anthocyanin Pigments in ‘Picnic’ Apples" Agronomy 13, no. 8: 2078. https://doi.org/10.3390/agronomy13082078

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