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

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.