Influence of GA₃ and CPPU on the Quality Attributes and Peelability of ‘Wuhe Cuibao’ Grape

Abstract

Gibberellic acid (GA₃) and forchlorfenuron (CPPU) are widely used plant growth regulators for promoting berry enlargement in grapes. To evaluate the effects of GA₃ and CPPU on fruit quality and peelability of the seedless grape cultivar ‘Wuhe Cuibao’, and to determine the optimal concentration combination under the ecological conditions of Jinzhong, Shanxi Province, grape clusters were treated with varying concentrations of GA₃ and CPPU at full bloom and again 14 days later (young fruit stage), with water treatment as the control (CK). After maturation, the fruits were harvested for subsequent analysis of external morphology and internal quality parameters in both fruit clusters and individual berries. Paraffin embedding and sectioning were performed to conduct histological observations of cuticle thickness and cellular morphology in the treated fruits. The results indicate that GA₃ and CPPU treatments significantly enhanced the external quality of ‘Wuhe Cuibao’ grapes by effectively reducing fruit drop during cultivation. With the exception of T3 treatment, all treatments promoted both cluster elongation and berry enlargement. GA₃ treatment alone was more effective than CPPU treatment, and its effects were positively correlated with concentration. The T2 treatment resulted in the greatest increases in fruit length, berry weight, pedicel thickness, and pedicel tensile strength, surpassing the control (CK) by 35.53%, 43.65%, 88.92%, and 104.76%, respectively. The combined application of GA₃ and CPPU showed a synergistic effect, especially in T8, which led to the highest increases in cluster length (21.94%), cluster weight (41.92%), and berry width (13.49%) compared with the control. In addition, all treatments promoted the color transition of berries from green to yellow-green. Histological analysis showed a significant increase in cuticle thickness and in the size of both epidermal and subepidermal cells after treatment. In addition, all treatments increased fruit firmness and peel adherence in a concentration-dependent manner. GA₃ treatment alone produced the greatest increases in both fruit firmness and peel–flesh adherence, while the addition of CPPU treatment alleviated these effects. All treatments improved internal fruit quality by increasing the content of vitamin C, reducing sugars, soluble sugars, starch, and cellulose. GA₃ treatment alone significantly increased the levels of soluble solids, soluble proteins, and total phenols by 5.67%, 1.49%, and 5.38%, respectively, compared to the control (CK). In contrast, CPPU treatment alone significantly reduced the levels of these compounds. Notably, combined GA₃ and CPPU treatment in T5 led to the highest accumulation of vitamin C and reducing sugars, with increases of 3.78% and 62.36%, respectively, compared to the CK. Additionally, all treatments reduced anthocyanin and titratable acid levels, with a synergistic effect observed under combined treatment in lowering titratable acidity. Comprehensive evaluation revealed that the combined application of 50 mg·L⁻¹ GA₃ and 5.0 mg·L⁻¹ CPPU at full bloom and 14 days thereafter resulted in the greatest overall improvement in grape quality, offering theoretical and practical support for the efficient, high-quality cultivation of this cultivar.

1. Introduction

Plant growth regulators (PGRs) are synthetic compounds that modulate plant metabolism and physiological functions. Their diverse functions include breaking seed dormancy, managing canopy architecture, stimulating lateral bud growth, inducing floral differentiation, enhancing fruit set, preventing preharvest drop, modifying fruit quality, increasing yield, and accelerating ripening [1,2]. Over the past decades, PGRs have been widely utilized in table grape cultivation [3]. Among them, gibberellic acid (GA₃) and forchlorfenuron (CPPU) are the two most commonly used PGRs in grape cultivation [1,4]. GA₃ is a natural plant hormone that regulates cell division and expansion and is considered a key agent in grape cultivation for promoting cell elongation and proliferation. It plays a critical role in extending cluster length, modifying internode spacing, enlarging berries, and inducing seedlessness [5,6]. During the early expansion stage of ‘Red Globe’ grapes, GA₃ cluster soaking significantly increases berry size [5]. CPPU is a synthetic cytokinin with high cellular activity that synergizes with endogenous auxins. It significantly promotes parthenocarpy, cell division, fruit set, and berry enlargement in grapes [1,7]. Studies have shown that CPPU enhances fruit set in grapes by increasing the energy status of developing berries [8]. CPPU has also demonstrated significant efficacy in improving fruit quality in kiwifruit, as reported by Nardozza et al. [9]. Notably, the combined application of GA₃ and CPPU exhibits a synergistic effect, resulting in increased cluster weight, larger berry size, and enhanced internal fruit quality [10,11]. Grapes (Vitis vinifera L.) play an important role in the global fruit industry. Owing to their unique growth traits and high nutritional value, grapes have long been classified as one of the six major fruits globally [12]. In recent years, seedless grape cultivars have gained popularity due to their lack of seeds, pleasant eating quality, and ease of processing. These traits have allowed seedless grapes to dominate the fresh fruit and raisin markets, generating significant commercial value [13]. However, many seedless grape cultivars in China face limitations such as small berry size, low yields, poor coloration, extended breeding cycles, and low stress tolerance, which limit their market competitiveness [14]. ‘Wuhe Cuibao’ is an early-maturing, seedless cultivar developed by the Shanxi Academy of Pomology. It belongs to the Vitis vinifera species. It demonstrates high productivity, moderate disease resistance, broad adaptability, elevated soluble solids, and a rich, refreshing rose-like aroma. Its berries feature thin skin and crisp flesh, making this cultivar highly promising for commercial use [15,16]. However, during cultivation, issues such as flower and fruit drop, small berry size, and loose cluster structure significantly reduce shelf life and market appeal. Peelability—the ease with which the skin separates from the flesh—is a key quality attribute of table grapes, meeting consumer preferences for convenience [17]. However, the effects of GA₃ and CPPU on grape peelability remain unclear. Therefore, this study employed ‘Wuhe Cuibao’ as experimental material, applying various concentrations of GA₃ and CPPU. The objective was to investigate fruit texture, morphology, and metabolic changes at maturity—including cluster and berry traits, coloration, firmness, peel–flesh adhesion, cellular structure, nutrient content, phenolic profiles, and sugar composition—to determine the optimal PGR concentrations for enhancing the overall quality and marketability of this cultivar.

2. Materials and Methods

2.1. Experimental Site and Materials

The experiment was carried out from April to July 2024 at a grape production base located in Xiaobai Township, Taigu District, Jinzhong City, Shanxi Province, China (37°27′12″ N, 112°42′55″ E; 803 m above sea level). The experimental site is located in a temperate continental climate. During the grape growing season, the average temperature is 22 °C, with 14 h of daily sunshine and an average precipitation of 16.15 mm. The daily average air quality index and PM2.5 levels consistently indicate excellent air quality.

The experimental material comprised five-year-old ‘Wuhe Cuibao’ grapevines, spaced at 1.0 m × 1.8 m. The vines were trained into a factory-shaped canopy with a double-trunk trellis system, oriented north to south. Cultivation was carried out in loamy soil under rain-shelter conditions with furrow irrigation. The vineyard was well managed, showing normal vine vigor with no symptoms of Botrytis cinerea or pest damage. Vigorous vines with uniform growth and similar flower clusters were selected for the study.

2.2. Chemicals and Reagents

The plant growth regulators (PGRs) used in the trial included gibberellic acid (GA₃, 90% purity) and forchlorfenuron (CPPU, 99% purity), both supplied by Guangzhou Linguo Fertilizer Co., Ltd (Guangzhou, Guangdong, China).

2.3. Experimental Design

The experiment consisted of one control group and eight treatment groups (

Table 1. Combination of GA₃ and CPPU experiment design.

Treatment	Treatment Time
	The First Time (in Full Bloom)	The Second Time (14 d After Flowering)
CK	clear water	clear water
T1	25 mg·L−1 GA3	25 mg·L−1 GA3
T2	50 mg·L−1 GA3	50 mg·L−1 GA3
T3	2.5 mg·L−1 CPPU	2.5 mg·L−1 CPPU
T4	5.0 mg·L−1 CPPU	5.0 mg·L−1 CPPU
T5	25 mg·L−1 GA3 + 2.5 mg·L−1 CPPU	25 mg·L−1 GA3 + 2.5 mg·L−1 CPPU
T6	25 mg·L−1 GA3 + 5.0 mg·L−1 CPPU	25 mg·L−1 GA3 + 5.0 mg·L−1 CPPU
T7	50 mg·L−1 GA3 + 2.5 mg·L−1 CPPU	50 mg·L−1 GA3 + 2.5 mg·L−1 CPPU
T8	50 mg·L−1 GA3 + 5.0 mg·L−1 CPPU	50 mg·L−1 GA3 + 5.0 mg·L−1 CPPU

). Each group comprised three replicates, with five inflorescences selected per replicate, resulting in a total of 135 samples. At full bloom and 14 days after full bloom (early fruit stage), inflorescences or young clusters were dipped in solutions of the designated concentrations for 5 s. Upon fruit maturity, intact clusters were harvested and promptly transported to the laboratory for measuring cluster size and weight. During sampling, 30 berries were randomly selected from each replicate to assess fruit-related traits.

2.4. Measurement and Methods

2.4.1. Measurement of Fruit Appearance Quality

A vernier caliper (Deqing Shengtai Electronic Technology Co., Ltd., Huzhou, Zhejiang, China) was used to measure the maximum transverse and longitudinal diameters (mm) of grape clusters and individual berries, along with pedicel thickness (mm). The fruit shape index (FSI) was calculated as the ratio of longitudinal to transverse berry diameter (FSI = longitudinal diameter/transverse diameter). Berry shapes were categorized based on FSI values into four types: flattened-round (FSI < 1.0), round (1.0 < FSI < 1.1), oval (1.1 < FSI < 1.3), and elongated-oval (1.3 < FSI < 1.6) [18]. Cluster and individual berry weights (g) were measured using an electronic balance (Denver Instrument Co., Ltd., Beijing, China). Fruit firmness (N) was assessed using a digital firmness tester (Aidibao Instrument Co., Ltd., Wenzhou, Zhejiang, China).

2.4.2. Measurement of Fruit Colorimetric Attributes

The brightness (L*), red–green chromaticity (a*), and yellow–blue chromaticity (b*) of the berry equator were measured using a CM-700d colorimeter (Beijing Onkey Technology Co., Ltd., Beijing, China). Based on these parameters, the color saturation (Chroma, C*), hue angle (h°), and color index of red grapes (CIRG) were calculated. The L* value indicates fruit brightness, with higher values corresponding to a lighter skin color. The a* value reflects the red–green axis: positive values represent redder hues, while negative values represent greener hues. The b* value represents the yellow–blue axis: positive values indicate yellower tones, and negative values indicate bluer tones. The C* value indicates color saturation; higher values reflect more vivid coloration. The hue angle (h°) represents the color tone: 0° corresponds to purplish red, 90° to yellow, and 180° to green. CIRG values were classified as follows: <2 = yellow-green; 2–4 = pink; 4–5 = red; 5–6 = dark red; ≥6 = blue-black [19].

$$C*=\sqrt{{\mathrm{a}}^2+{\mathrm{b}}^2};h°=180°+\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{t}\mathrm{a}\mathrm{n}\left({\displaystyle \frac{\mathrm{b}}{\mathrm{a}}}\right)(\mathrm{a}<0,\mathrm{b}>0);\mathrm{C}\mathrm{I}\mathrm{R}\mathrm{G}={\displaystyle \frac{180-h\text{°}}{L^{*}+C^{*}}}$$

2.4.3. Measurement of Pedicel Detachment Force

Grape berries with pedicels approximately 5 mm long were selected for measurement. A digital push–pull force gauge (Wenzhou Weidu Electronics Co., Ltd., Wenzhou, Zhejiang, China) fitted with a clamping device was used. The clamp was adjusted to securely hold the pedicel without slippage. The berry was pulled outward at a constant speed, and the instantaneous force recorded at detachment was defined as the pedicel detachment force (N).

2.4.4. Measurement of Peel–Flesh Adhesion Force

A cylindrical segment 1 cm thick was excised from the central region of the berry. The skin was carefully incised with a scalpel and peeled back by ~5 mm. A digital push–pull force gauge (Wenzhou Weidu Electronics Co., Ltd., Wenzhou, Zhejiang, China) fitted with clamps was used to measure the maximum force needed to detach the peel from the flesh. This value was recorded as the peel–flesh adhesion force (N) [17].

2.4.5. Observation of Fruit Tissue Anatomy

Paraffin sectioning was employed to examine peel thickness and cellular structure in grape berries under a light microscope [20]. Tissue blocks (~5 × 5 × 5 mm), containing both peel and flesh, were excised perpendicular to the skin from the fruit’s equatorial region. The samples were immediately fixed in 70% FAA solution, vacuum-infiltrated for 20 min, and then stored at 4 °C. After 48 h, the samples were dehydrated, cleared, embedded in paraffin, and sectioned using a microtome. The sections were oven-dried, dewaxed, stained with safranin-fast green, sealed with neutral balsam, and air-dried. Microscopic observation and imaging were conducted using a DM 2000 light microscope (Leica Microsystems, Wetzlar, Germany). The thickness of the cuticular layer and the sizes of the epidermal and hypodermal cells were simultaneously quantified using ImageJ software (V1.8.0).

2.4.6. Evaluation of the Intrinsic Quality Parameters of Fruit

The fruit’s soluble solid content was measured using a CSM-1000 digital refractometer (HM Digital, Inc., Seoul, South Korea). Soluble protein content was determined by the Coomassie Brilliant Blue G-250 staining method [21]. Vitamin C content was analyzed using the molybdenum blue colorimetric method [22]. Total phenols, flavonoids, and anthocyanins were quantified using the methanol–hydrochloric acid method [21]. Reducing sugar content was determined by the 3,5-dinitrosalicylic acid (DNS) method. Soluble total sugar, starch, and cellulose contents were measured using the anthrone–sulfuric acid method [22]. A standard alkali titration method was employed to determine titratable acid content [22].

2.4.7. Data Processing and Analysis

The measured indicator data were organized using Microsoft Excel 2020. One-way analysis of variance (ANOVA) was performed using IBM SPSS Statistics 26, and Duncan’s new multiple range test was used to assess statistical significance at the 0.05 level. Entropy weight TOPSIS analysis was conducted via SPSSAU online software (https://spssau.com/index.html, accessed on 16 August 2025). Figures were generated using GraphPad Prism 9.5 and Adobe Photoshop 2020.

3. Results

3.1. Impact of GA₃ and CPPU Application on the Morphological Quality of Clusters and Berries in ‘Wuhe Cuibao’ Grape

As shown in Figure 1, all treatments increased the compactness of the grape berries. However, berries under the T3 treatment exhibited significant size inconsistency, whereas the T4 treatment effectively alleviated this problem. These results suggest that applying CPPU alone at higher concentrations better promotes berry enlargement in ‘Wuhe Cuibao’ without negatively affecting fruit morphology.

Table 2. Influence of GA₃ and CPPU treatments on external morphological traits of grape clusters in ‘Wuhe Cuibao’.

Treatment	Cluster Length (mm)	Cluster Width (mm)	Cluster Weight (g)
CK	16.65 ± 0.21 e	10.70 ± 0.33 cd	331.65 ± 2.03 e
T1	18.51 ± 0.08 bcd	10.80 ± 0.35 bcd	405.76 ± 14.22 c
T2	18.60 ± 0.12 bc	10.85 ± 0.24 bcd	440.27 ± 4.10 b
T3	17.21 ± 0.24 de	10.73 ± 0.38 cd	344.67 ± 4.31 e
T4	17.72 ± 0.55 cde	10.42 ± 0.67 d	371.88 ± 28.38 d
T5	18.70 ± 0.32 bc	11.49 ± 0.48 bc	440.79 ± 14.68 b
T6	19.36 ± 1.17 ab	11.63 ± 0.69 b	455.90 ± 15.07 ab
T7	19.41 ± 0.76 ab	11.40 ± 0.18 bc	456.31 ± 3.09 ab
T8	20.30 ± 1.35 a	12.58 ± 0.08 a	470.67 ± 8.02 a

Note: Different lowercase letters in the same column in the table indicate significant differences between treatments, p < 0.05.

shows that individual application of GA₃ or CPPU significantly promoted inflorescence elongation and increased cluster weight, with effects positively correlated to treatment concentration. Additionally, GA₃ is more effective than CPPU. Combined application of GA₃ and CPPU led to greater increases in cluster length and weight. Under the T8 treatment, cluster length, width, and weight reached their highest values among all treatments, increasing by 21.92%, 17.57%, and 41.92%, respectively, compared to the control (CK). These findings indicate that using both regulators together is more effective in promoting inflorescence elongation in ‘Wuhe Cuibao’. According to

Table 3. Effects of GA₃ and CPPU on morphological and visual attributes of ‘Wuhe Cuibao’ grape berries.

Treatment	Longitudinal Diameter (mm)	Transverse Diameter (mm)	Fruit Shape Index	Single Fruit Weight (g)	Fruit Stalk Coarseness (mm)	Pedicel Detachment Force (N)
CK	16.50 ± 0.14 d	18.45 ± 0.23 e	1.12 ± 0.02 d	3.34 ± 0.08 d	1.23 ± 0.06 g	1.40 ± 0.00 g
T1	16.77 ± 0.14 d	22.68 ± 0.60 b	1.35 ± 0.04 b	4.05 ± 0.04 c	2.10 ± 0.05 b	2.80 ± 0.00 ab
T2	17.75 ± 0.39 bc	25.00 ± 0.46 a	1.41 ± 0.04 a	4.79 ± 0.18 a	2.33 ± 0.07 a	2.87 ± 0.06 a
T3	16.73 ± 0.12 d	18.02 ± 0.30 e	1.08 ± 0.03 d	2.79 ± 0.11 e	1.28 ± 0.10 g	2.07 ± 0.06 e
T4	17.42 ± 0.28 c	18.68 ± 0.24 e	1.07 ± 0.02 d	3.51 ± 0.26 d	1.51 ± 0.11 f	2.30 ± 0.10 d
T5	17.76 ± 0.09 bc	21.01 ± 0.48 d	1.19 ± 0.02 c	4.20 ± 0.20 bc	1.55 ± 0.12 ef	2.33 ± 0.06 d
T6	17.93 ± 0.08 bc	21.45 ± 0.12 d	1.20 ± 0.01 c	4.22 ± 0.14 bc	1.69 ± 0.06 cd	2.40 ± 0.00 cd
T7	18.05 ± 0.07 b	21.60 ± 0.61 cd	1.20 ± 0.03 c	4.12 ± 0.03 c	1.80 ± 0.06 cd	2.50 ± 0.17 c
T8	18.73 ± 0.62 a	22.41 ± 0.55 bc	1.20 ± 0.03 c	4.48 ± 0.07 b	1.92 ± 0.06 c	2.70 ± 0.10 b

Note: Different lowercase letters in the same column in the table indicate significant differences between treatments, p < 0.05.

, applying higher concentrations of GA₃ or CPPU alone improved fruit enlargement. GA₃ significantly increased berry length and individual fruit weight. Under the T2 treatment, berry size increased by 7.58%, 35.50%, and 43.41% compared to the control (CK). In contrast, CPPU treatment had no significant effect on berry elongation and weight; in some cases, it even reduced them. For example, under the T3 treatment, berry length decreased by 2.38%, and weight increased by only 4.07% relative to CK. These results highlight the importance of optimizing both the timing and concentration of plant growth regulators (PGRs) applications in grape production. Co-application of GA₃ and CPPU resulted in greater berry width in ‘Wuhe Cuibao’ compared to other treatments. However, the berry length was shorter than with GA₃ alone, yet still longer than with other treatments, suggesting that CPPU may reduce GA₃’s elongation effect. All treatments significantly affected the fruit shape index (FSI) of ‘Wuhe Cuibao’. Treatments T1 and T2 markedly increased the FSI by 20.54% and 25.89%, producing elongated-oval fruits. In contrast, treatments T3 and T4 decreased the index by 3.70% and 4.67%, resulting in round fruits. Although treatments T5 to T8 raised the index, they did not change the fruit’s external shape, which remained oval. Additionally, all treatments increased pedicel thickness and detachment force, effectively reducing fruit drop during growth.

3.2. Influence of GA₃ and CPPU Application on Skin Color Characteristics of ‘Wuhe Cuibao’ Grape

As shown in

Table 4. Effects of GA₃ and CPPU application on the coloration of ‘Wuhe Cuibao’ grape berries.

Treatment	L*	a*	b*	C*	h°	CIRG
CK	27.41 ± 0.12 d	−3.74 ± 0.05 c	12.56 ± 0.33 de	13.11 ± 0.33 ef	106.58 ± 0.24 d	1.81 ± 0.02 a
T1	27.33 ± 0.53 d	−4.29 ± 0.13 ab	13.53 ± 0.59 bc	14.20 ± 0.60 bcd	107.56 ± 0.23 abcd	1.74 ± 0.02 b
T2	27.13 ± 0.52 d	−4.48 ± 0.07 a	13.90 ± 0.28 bc	14.60 ± 0.25 b	107.85 ± 0.57 abcd	1.73 ± 0.02 bc
T3	30.65 ± 0.43 a	−4.02 ± 0.15 bc	12.81 ± 0.61 d	13.43 ± 0.61 def	107.41 ± 0.67 abcd	1.65 ± 0.01 de
T4	30.15 ± 0.39 ab	−4.10 ± 0.07 bc	12.66 ± 0.12 de	13.31 ± 0.14 ef	107.93 ± 0.17 abc	1.66 ± 0.02 de
T5	29.07 ± 0.18 c	−4.24 ± 0.49 ab	13.85 ± 0.27 b	14.48 ± 0.40 bc	107.00 ± 1.55 cd	1.68 ± 0.05 d
T6	28.97 ± 0.22 c	−4.33 ± 0.06 ab	13.01 ± 0.19 cd	13.71 ± 0.20 cde	108.40 ± 0.12 ab	1.68 ± 0.01 d
T7	29.69 ± 0.76 bc	−4.03 ± 0.02 bc	12.00 ± 0.21 e	12.66 ± 0.20 f	108.56 ± 0.31 a	1.69 ± 0.03 cd
T8	29.43 ± 0.25 bc	−4.57 ± 0.14 a	14.77 ± 0.50 a	15.46 ± 0.50 a	107.21 ± 0.46 bcd	1.62 ± 0.02 e

Note: Different lowercase letters in the same column in the table indicate significant differences between treatments, p < 0.05.

, the L* value (brightness) of ‘Wuhe Cuibao’ fruit decreased slightly under the T1 and T2 treatments—by 0.29% and 1.03% compared to the control (CK). However, the fruit displayed a greener and yellower appearance, with enhanced color purity. CPPU alone increased chroma, shifted fruit hue toward yellow-green compared to the control, and significantly improved surface brightness; the T3 treatment achieved the highest L* value of 30.65—an 11.82% increase over the control (CK). Combined GA₃ and CPPU treatment increased L*, a*, b*, C*, and h° values relative to the control, indicating a brighter surface and a shift toward yellow-green hues. Although C* values were lower than those for the control across treatments, h° provided a more accurate color parameter due to the cultivar’s naturally yellow-green hue. These results suggest that both individual and combined applications of GA₃ and CPPU promote the color shift in ‘Wuhe Cuibao’ grapes from green to yellow-green.

3.3. Effects of GA₃ and CPPU Application on Fruit Texture Traits, Including Firmness and Ease of Peeling, in ‘Wuhe Cuibao’ Grape

Figure 2 shows that increasing concentrations of GA₃ and CPPU treatments enhanced both fruit firmness and peel–flesh adhesion. The parallel increase in these parameters indicates a strong positive correlation. Application of GA₃ alone significantly enhanced both fruit firmness and peel–flesh adhesion. Specifically, the T1 treatment increased fruit firmness and peel–flesh adhesion by 33.61% and 44.44%, respectively, compared to the control. Under the T2 treatment, these two attributes reached their maximum values, with fruit firmness increasing by 40.14% and peel–flesh adhesion by 0.60%. In contrast, CPPU applied alone resulted in only slight, statistically insignificant increases in both traits under the T3 treatment—0.97% for firmness and 2.22% for peel–flesh adhesion compared to the control. Combined GA₃ and CPPU treatments resulted in intermediate firmness and adhesion values—lower than GA₃ alone, but higher than CPPU alone—suggesting that CPPU may moderate GA₃-induced hardening and reduce peelability.

3.4. Cellular Responses of ‘Wuhe Cuibao’ Grape to GA₃ and CPPU Application

Figure 3 shows notable changes in the cell morphology of ‘Wuhe Cuibao’ grapes following GA₃ and CPPU treatments. As shown in

Table 5. Cellular structural responses of ‘Wuhe Cuibao’ grape to GA₃ and CPPU treatments.

Treatment	Cuticle Thickness (μm)	Epidermal Cell Size (μm2)	Hypodermal Cell Size (μm2)
CK	2.42 ± 0.22 e	106.90 ± 7.58 e	145.43 ± 7.02 e
T1	3.77 ± 0.20 cd	414.63 ± 26.91 a	471.06 ± 21.54 a
T2	3.86 ± 0.12 bc	326.63 ± 21.77 bc	323.51 ± 21.22 bc
T3	3.44 ± 0.13 d	426.17 ± 28.47 a	356.70 ± 14.34 b
T4	4.24 ± 0.17 ab	243.60 ± 9.66 d	226.08 ± 3.50 d
T5	3.94 ± 0.26 abc	293.33 ± 19.25 c	249.17 ± 19.71 d
T6	4.28 ± 0.13 a	403.75 ± 2.80 a	317.66 ± 16.59 c
T7	3.80 ± 0.18 cd	339.66 ± 14.77 b	351.90 ± 17.83 bc
T8	3.73 ± 0.25 cd	412.01 ± 17.86 a	346.22 ± 24.39 bc

Note: Different lowercase letters in the same column in the table indicate significant differences between treatments, p < 0.05.

, all treatments caused the fruit cuticle to thicken and led to varying degrees of increase in the sizes of both epidermal and subepidermal cells compared with the control (CK). Under the GA₃-alone treatment, T1 increased cuticle thickness by 55.78% compared to the control, with epidermal and subepidermal cell sizes showing significant increases of 287.87% and 223.91%, respectively—the highest among all treatments. Although T2 resulted in a 2.39% increase in cuticle thickness relative to T1, the increases in epidermal and subepidermal cell sizes were significantly lower than those observed in T1. CPPU applied alone induced a pattern of cellular changes in ‘Wuhe Cuibao’ grapes similar to those under GA₃ treatment: higher CPPU concentrations resulted in thicker cuticles, while the sizes of epidermal and subepidermal cells decreased with increasing concentration. Notably, T4 yielded the smallest increases in epidermal and subepidermal cell sizes among all treatments, at 127.88% and 55.46% above CK, respectively. In the combined GA₃ and CPPU treatment, T6 resulted in the thickest cuticle (4.28 μm), which was 76.86% thicker than that of the control. Among the combined GA₃+CPPU treatments, T8 resulted in the largest epidermal cell size, whereas T7 had the largest subepidermal cell size. Overall, the significant morphological changes in fruit cells under different treatments may influence fruit firmness and ease of peeling.

3.5. Influence of GA₃ and CPPU Application on the Accumulation of Nutritional Compounds in ‘Wuhe Cuibao’ Grape

Figure 4A shows that GA₃ treatment alone significantly increased the soluble solid content (SSC) in ‘Hongyang’ fruits. The T1 treatment reached the highest SSC of 16.84%, representing a 57.12% increase compared to the control (CK), whereas T2 caused only a slight increase of 1.07%. All other treatments decreased SSC, with T5 showing the lowest value of 15.04%, 5.64% less than the control. Figure 4B indicates that CPPU treatment alone reduced soluble protein content in ‘Wuhe Cuibao’ berries by 2.75% and 2.54% under the T3 and T4 treatments, respectively, compared to the control (CK). In contrast, other treatments increased soluble protein, with the combined GA₃ and CPPU treatment significantly enhancing protein levels. The highest soluble protein content was observed under T8 (28.79 mg·g⁻¹), a 4.15% increase over CK, followed closely by T6, with a 4.08% increase. As shown in Figure 4C, all treatments increased vitamin C content in ‘Wuhe Cuibao’. When GA₃ or CPPU was applied individually, T2 showed the smallest increase, rising only 0.98% compared to the control, while the other treatments had similar increases. The combined application of GA₃ and CPPU resulted in higher vitamin C content than individual treatments in all cases except T7. Notably, T5 yielded the highest vitamin C concentration, increasing by 7.10% over the control, equivalent to 1.04 times that of CK.

3.6. Effects of GA₃ and CPPU Application on the Accumulation of Phenolic Compounds in ‘Wuhe Cuibao’ Grape Berries

Figure 5A shows that GA₃ treatment alone significantly increased total phenol content in ‘Wuhe Cuibao’, with T2 treatment producing the highest increase of 5.39% compared to the control (CK). In contrast, CPPU treatment alone significantly decreased the total phenolic content of the fruit, indicating that it inhibits phenolic accumulation. When applied together, T5 and T6 showed lower total phenolic content than the control (CK), while T7 and T8 exceeded CK levels. Notably, T7 had the highest total phenol content, increasing by 4.24% compared to CK, equivalent to 1.04 times that of the control. Figure 5B shows that all treatments except T7 and T8 resulted in lower flavonoid content than CK. Notably, T8 had the highest flavonoid content, increasing by 10.66% compared to CK. This suggests that high concentrations of GA₃ and CPPU can significantly enhance flavonoid content in ‘Wuhe Cuibao’. Additionally, individual application of GA₃ or CPPU at higher concentrations favors flavonoid accumulation. According to Figure 5C, anthocyanin content trends closely mirror those of flavonoids. However, the control had the highest anthocyanin content (0.12 OD·g⁻¹), with T7 second, at 0.03 OD·g⁻¹. These findings suggest that GA₃ and CPPU application may reduce anthocyanin accumulation in ‘Wuhe Cuibao’.

3.7. Impact of GA₃ and CPPU Application on the Accumulation of Flavor Compound Content in ‘Wuhe Cuibao’ Grape Berries

Figure 6A shows that single treatments of GA₃ and CPPU significantly increased the reducing sugar content in ‘Wuhe Cuibao’ grapes, with GA₃ inducing a notably higher increase than CPPU. The enhancement was positively correlated with treatment concentration. Combined GA₃ and CPPU application significantly raised the reducing sugar levels in all treatments, with T5 reaching a peak value of 1.68%, a 62.32% increase, or 1.62 times that of the control (CK). Figure 6B shows that GA₃ alone reduced soluble total sugar content, exhibiting an inverse relationship between GA₃ concentration and sugar accumulation, indicating a negative effect of GA₃ on soluble sugars. Under sole CPPU treatment, soluble total sugar content increased by 22.62% with T3 and 12.60% with T4, indicating that lower CPPU concentrations promote the accumulation of soluble total sugars in ‘Wuhe Cuibao’ seedless fruits. Combined GA₃ and CPPU treatment significantly increased soluble total sugar content, with T7 and T8 (higher GA₃ concentrations) showing greater increases than T5 and T6, highlighting the positive role of elevated GA₃ in sugar accumulation. Figure 6C shows that GA₃ alone significantly increased starch levels, with T2 reaching the highest content (0.27%), 18% above control, demonstrating a positive correlation between GA₃ concentration and starch accumulation. CPPU alone decreased starch content, suggesting a negative effect on starch biosynthesis or accumulation. Combined treatments showed variable effects: lower concentrations (T5, T6) decreased starch content below that of the control, whereas higher concentrations (T7, T8) increased starch, with T8 reaching 0.28%, which is a 23.11% increase compared to the control (CK). Figure 6D shows that cellulose content in ‘Wuhe Cuibao’ seedless fruits increased under all treatments except T5. The T2 treatment produced the highest cellulose level, at 0.12%, a 48.78% increase over the control (CK). These findings indicate that GA₃ and CPPU, whether applied individually or in combination, promote cellulose accumulation in ‘Wuhe Cuibao’ fruits. As shown in Figure 6E, applying CPPU alone had little effect on the fruit’s titratable acidity. In contrast, adding GA₃ reduced titratable acidity, with the T8 treatment exhibiting the strongest effect—a 10.07% decrease compared to the control (CK). These results indicate that GA₃ effectively reduces titratable acidity in grape berries and that its combination with CPPU has a synergistic effect.

3.8. Correlation Analysis of GA₃ and CPPU Application with Fruit Quality Traits in ‘Wuhe Cuibao’ Grape

Figure 7 demonstrates positive correlations among all examined external morphology parameters of ‘Wuhe Cuibao’ berries. Notably, berry width correlates strongly with cluster length, width, and weight. Berry length shows significant positive correlations with fruit shape index, pedicel thickness, pull-off force, firmness, and ease of peel. Regarding fruit color, L* is positively correlated with cluster appearance metrics, berry width, and a*, but negatively correlated with other morphological traits, especially CIRG. Similarly, a* is positively associated with L* and CIRG but negatively correlated with other traits, notably b* and chroma (C*). For internal quality, vitamin C is positively correlated with soluble protein, reducing sugar, and total soluble sugar, but negatively correlated with other traits. Total phenolic content is negatively associated with vitamin C, total soluble sugar, and titratable acidity, while showing positive correlations with soluble solids, protein, starch, and cellulose. Additionally, anthocyanins are positively correlated with flavonoids and titratable acidity. Overall, most external quality traits of ‘Wuhe Cuibao’ berries are positively related to internal nutritional parameters but inversely associated with anthocyanin content and titratable acidity. Most flavor compounds, except for total soluble sugars and titratable acidity, were positively correlated with visual quality indicators. Notably, total soluble sugar negatively affects berry length, fruit weight, shape index, and stem tensile strength, while positively correlating with fruit lightness (L*).

3.9. Integrated Assessment of the Impact of GA₃ and CPPU Application on the Fruit Quality Characteristics of ‘Wuhe Cuibao’ Grape

Table 6. Comprehensive evaluation analysis of different treatments on the quality of ‘Wuhe Cuibao’ grape fruits.

Treatment	Positive Ideal Solution Distance D+	Negative Ideal Solution Distance D−	Relative Proximity C	Sorting of the Results
CK	2.382	0.079	0.032	9
T1	1.114	1.273	0.533	6
T2	0.525	1.864	0.780	5
T3	2.161	0.225	0.094	8
T4	1.695	0.690	0.289	7
T5	0.522	1.869	0.782	4
T6	0.271	2.128	0.887	3
T7	0.260	2.135	0.891	2
T8	0.077	2.381	0.969	1

presents the ranking of relative closeness coefficient (C) for ‘Wuhe Cuibao’ fruit quality as follows: T8 > T7 > T6 > T5 > T2 > T1 > T4 > T3 > CK, indicating that all treatments effectively improved overall fruit quality. Among single treatments, T2 (GA₃) and T4 (CPPU) showed the best results, suggesting that increasing the concentration of either plant growth regulator individually enhances fruit quality. Moreover, combined application of GA₃ and CPPU yielded superior fruit quality compared to single treatments, with improvements positively correlated to concentration.

4. Discussion

4.1. Impact of Various Treatment Applications on the External Fruit Quality Traits of ‘Wuhe Cuibao’ Grape

Gibberellins (GAs) in grape berries are mainly synthesized in seeds; thus, seedless berries, which lack or contain only trace GAs, are significantly smaller than seeded ones [23]. In our study, inflorescences dipped in various GA₃ and CPPU concentrations at anthesis and 14 days later showed increased cluster length, mass, and compactness. This likely results from elevated endogenous gibberellin and other phytohormone levels in ‘Wuhe Cuibao’, promoting berry enlargement. Our results agree with previous studies on ‘Jinyu’ and ‘Sunshine Muscat’, where similar GA₃ and CPPU treatments improved cluster morphology and density [24,25]. Most treatments significantly increased berry size and weight, attributable to the cell enlargement effects of GA₃ and CPPU [26,27]. Furthermore, all treatments enhanced pedicel tensile strength, effectively reducing fruit drop during development. Increases in pedicel girth and rachis rigidity, likely due to increased lignification after GA₃/CPPU treatment, align with previous findings for ‘Suosuo’ grapes [28]. Similarly, Jiang et al. reported that a GA₃/CPPU ratio of 20–33.3 mg·L⁻¹ plus 15 mg·L⁻¹ notably increased mango fruit length and diameter during expansion [1].

Fruit coloration is a key indicator of quality and freshness, greatly affecting the commercial appeal of grapes. All treatments increased the hue angle (h°), shifting ‘Wuhe Cuibao’ berry color from green to yellow-green. Treatments T3 to T8 notably enhanced surface brightness (L*). Except for T6 and T7, most treatments also increased color saturation (C*), improving fruit vibrancy. These results suggest that GA₃ and CPPU enhance visual quality, with effects depending on application timing and concentration. Additionally, all treatments reduced the color index, likely due to decreased anthocyanin accumulation after treatment. Further analysis showed that all treatments enhanced the appearance of ‘Wuhe Cuibao’ fruit, although anthocyanin levels declined. This aligns with the findings of Lin et al. in strawberries, indicating that fruit coloration is not exclusively regulated by anthocyanins [29]. Research on ornamental kale demonstrated that reduced anthocyanin levels, coupled with elevated chlorophyll and carotenoid contents, resulted in a shift from pink to greenish foliage, underscoring the key role of these pigments in color modulation [30]. In addition to pigment changes, fruit color development is influenced by metabolic factors. Elevated sugar levels, in particular, contribute to a brighter appearance [31], in agreement with our observations. These findings suggest that fruit coloration is a multifactorial trait, modulated not only by external conditions such as environment, nutrients, and cultivation practices, but also by internal factors, including anthocyanin, chlorophyll, carotenoid levels, and metabolic activity [32].

Fruit firmness greatly affects texture and is a key indicator of grape flesh quality [33]. Our results showed that all treatments increased fruit firmness in ‘Wuhe Cuibao’, consistent with studies on cultivars like ‘Himrod’ and ‘Jinzao Wuhe’ grape [34]. Peel–flesh separation improves eating convenience and hygiene, contributing to consumer preference [35]. In our experiment, higher treatment concentrations led to increased firmness and stronger peel–flesh adhesion in ‘Wuhe Cuibao’, supporting findings by Yu et al. [36]. GA₃ alone significantly enhanced peel–flesh adhesion; however, co-application with CPPU reduced adhesion compared to GA₃ alone but remained above control levels, possibly due to treatment-induced cellular morphological changes.

4.2. Effects of Various Treatments on the Internal Quality Attributes of ‘Wuhe Cuibao’ Grape Berries

The accumulation of nutritional compounds in grape berries critically influences overall fruit quality. Our results show that GA₃ alone significantly increased soluble solids in ‘Wuhe Cuibao’, whereas combined application with CPPU reduced them, consistent with the findings of Wang Shasha [37] for ‘Shine Muscat’. CPPU alone decreased soluble protein content, but this effect was reversed when combined with GA₃. All treatments significantly increased vitamin C levels, differing from previous reports [38,39], likely due to environmental, vine vigor, and hormonal treatment variations. GA₃ alone increased total phenol content but reduced flavonoid and anthocyanin levels. CPPU alone suppressed all three compounds. However, combined treatments, especially T7 and T8, significantly increased phenolic and flavonoid contents (except anthocyanins), indicating that high GA₃ concentrations promote phenolic accumulation in ‘Wuhe Cuibao’.

Sugar and organic acid levels are major determinants of grape flavor and are commonly used as essential indicators for evaluating fruit taste. Previous studies have reported both increases [40,41] and decreases [42] in sugar levels after GA₃ and CPPU treatments, likely due to differences in cultivar sensitivity, hormone concentration, and application timing. Experimental results showed that all treatments significantly reduced titratable acidity and increased the reducing sugar levels in ‘Wuhe Cuibao’ grape berries. GA₃ treatment alone increased starch and cellulose levels and decreased titratable acidity. CPPU alone enhanced total soluble sugar and cellulose content. When used together, GA₃ and CPPU reduced titratable acidity. However, increasing concentrations led to decreases in reducing sugars and total soluble sugars, while starch and cellulose levels increased. This trend may be due to fruit enlargement and increased photosynthate accumulation caused by plant growth regulators, which raised fruit load and slightly limited nutrient availability, ultimately altering the composition or causing reductions in certain metabolites.

5. Conclusions

‘Wuhe Cuibao’ grape clusters were dipped in GA₃ and CPPU solutions at full bloom and 14 days after anthesis. Treatments were ranked by comprehensive score in descending order: T8 > T7 > T6 > T5 > T2 > T1 > T4 > T3 > CK. This indicates that T8 achieved the highest overall score, particularly in berry size (length and diameter), firmness, color quality, nutritional content, and sugar accumulation. These findings suggest that applying 50 mg·L⁻¹ GA₃ and 5.0 mg·L⁻¹ CPPU at the designated stages produces the greatest improvement in overall fruit quality of ‘Wuhe Cuibao’.