Effects of three kinds of growth regulating substances on fruit coloring and quality in Suxin No. 1 grape

【Objective】The extremely early-maturing table grape cultivar Suxin 1 is characterized by a short growth and development cycle and early ripening. Endowed with the ability to enter the market ahead of competitors, it is granted for distinct market advantages. However, it is frequently confronted with challenges during fruit development, such as a short coloring period, incomplete color transition, uneven and pale skin pigmentation, and low single-berry weight. These drawbacks severely impaire its external commercial quality, and its economic value and market competitiveness are ultimately diminished. 2,4- Epibrassinolide (EBR), methyl jasmonate (MeJA), and melatonin (MT) are three types of plant growth regulators (PGRs) known to regulate fruit quality. They have been demonstrated to promote fruit coloring, increase sugar content, and improve overall quality in various fruit tree species. Unfortunately, the optimal application concentrations and efficacy of these PGRs for the extremely earlymaturing Suxin 1 grape remain unclear. In this study, EBR, MeJA, and MT were applied at different concentration gradients during the color transition stage to systematically assess their impacts on the external quality, dynamic color changes, pigment accumulation, and internal quality of Suxin 1 grapes. The objective was to identify treatment protocols that could significantly improve fruit coloring and quality, thereby providing a theoretical basis and technical guidance for the standardized production and commercial quality enhancement of this cultivar.【Methods】Field experiments were carried out in 2024 at Tontunhe Farm of the 12th Division of Xinjiang Production and Construction Corps. The test materials were 5- year- old Suxin 1 grapevines with uniform growth vigor and without pest or disease symptoms. Cluster dipping treatments were implemented at the initial stage of berry coloration (30 days after flowering), with the immersion duration set at 5 seconds. A total of 13 treatments were designed, including one water control (CK) and four concentration gradients for EBR, MeJA, and MT respectively: EBR: B1 (1.0 mg·L-1 ), B2 (0.7 mg·L-1 ), B3 (0.4 mg·L-1 ), and B4 (0.1 mg·L-1 ); MeJA: M1 (120 mg·L-1 ), M2 (90 mg·L-1 ), M3 (60 mg·L-1 ), and M4 (30 mg·L-1 ); MT: T1 (200 μmol·L-1 ), T2 (150 μmol·L-1 ), T3 (100 μmol·L-1 ), and T4 (50 μmol·L-1 ). Each treatment was assigned to a plot containing 5 grapevines, and a total of 30 clusters per plot were selected for treatment application. Sampling was performed every 7 days starting from the treatment date until 51 days after flowering, with a total of three sampling events conducted. External quality parameters: Single-berry mass, longitudinal diameter, transverse diameter, and berry shape index (calculated as longitudinal diameter/transverse diameter) were determined. Color parameters: L* (lightness), a* (red-green value), and b* (yellow-blue value) were measured using an NR310 colorimeter. The color index of red grapes (CIRG) was calculated according to the formula: (180−h° )/(L* +c* ). Skin pigment contents: Total anthocyanin content was determined by the pH differential method; proanthocyanidin content was determined by the vanillin- hydrochloric acid method; and chlorophyll and carotenoid contents were determined by the ethanol extraction method. Internal quality parameters: Soluble solid content (SSC) and total acid content (TA) were measured with a digital sugar-acid meter, and the SSC/TA ratio was subsequently calculated. Principal component analysis (PCA) was used for dimensionality reduction to comprehensively evaluate the effects of different treatments. Dose-response models were employed to verify the reliability of the optimal application concentration.【Results】(1) External quality: Single- berry mass of mature Suxin 1 grapes significantly increased by PGR treatments. The most pronounced effects were observed in the 0.4 mg · L- 1 EBR (B3) and 60 mg ·L-1 MeJA (M3) treatments, with single-berry mass reaching 8.43 g and 7.70 g, respectively. Compared with the control (CK), these values represented statistically significant increases of 54.11% and 40.77%. The fruit shape index was consistently maintained at approximately 1.40 across all treatments, which met the commercial standards for high- quality table grapes. (2) Fruit coloring and pigment accumulation: The B3 treatment exhibited optimal performance in both fruit coloring and pigment accumulation. Anthocyanin content in the skin was 43.24% higher than that in CK, and the color index of red grapes (CIRG) reached 5.39, a value significantly higher than those of other treatments, indicating improved coloring uniformity as well as promoted fruit development. The highest proanthocyanidin content was obtained in the 90 mg ·L-1 MeJA (M2) treatment, which was significantly greater than that in all other groups. (3) Internal quality: The highest soluble solid content (SSC, 19.08%) and SSC/TA ratio (30.94) were achieved in the M2 treatment, indicating a well-balanced sweet-acid ratio favorable for consumer taste. (4) Comprehensive evaluation: Principal component analysis (PCA) showed that the first two principal components accounted for a cumulative variance contribution rate of 44.81%. The main contributing factors were identified as single-berry mass, longitudinal diameter, transverse diameter, SSC/TA ratio, anthocyanin content, CIRG index, and SSC. Based on the PCA results, the comprehensive ranking of treatments was determined as: B3＞M3＞B4＞M4＞T4＞T3＞T2＞T1＞M1＞ B1＞M2＞CK＞B2.【Conclusion】Through comprehensive evaluation of mature fruit quality indices, combined with principal component analysis (PCA) and dose-response model validation, the 0.4 mg·L-1 EBR (B3) treatment was found to effectively promote anthocyanin biosynthesis in the skin of Suxin 1 grapes. Consequently, the coloring intensity and uniformity of the fruit were significantly enhanced. Meanwhile, this treatment also increased the single-berry mass thereby achieving synergistic improvements in both external commercial quality and flavor-related internal traits. The cluster dipping method (5-second immersion) proposed in this study was characterized as simple, cost-effective, and easy to implement. It was deemed suitable for production practices targeting harvest period regulation and quality improvement of extremely early-maturing grapes. Although MeJA and MT also exhibited positive regulatory effects on specific fruit quality traits at certain concentrations, their overall efficacy was proven to be inferior to that of the 0.4 mg·L-1 EBR treatment. This study established a reliable regulatory strategy for the standardized cultivation and high-quality production of Suxin 1 grapes, thus providing important application value for enhancing the market competitiveness of this cultivar.