The application of abscisic acid (ABA) in grape cultivation is one of the important achievements of modern fruit tree physiology and molecular biology research. As a key endogenous plant hormone, ABA not only deeply participates in the ripening initiation process of non-climacteric fruits (such as grapes, strawberries, and citrus), but is also a core technical means to regulate fruit quality and promote coloring through exogenous application. From the perspective of production practice, the application of abscisic acid runs through almost every key stage of grape fruit quality formation; from the perspective of theoretical research, it is also a core hub connecting environmental signals, gene expression, and the accumulation of secondary metabolites.
This article will combine the latest domestic and international research results to elaborate on the comprehensive application and far-reaching impact of abscisic acid on grapes from five dimensions: color improvement, intrinsic quality enhancement, ripening regulation, flavor compound regulation, and molecular regulatory mechanisms.
Color Improvement
The Core Application Effect of ABA Color is one of the most important commercial quality indicators of grape fruit, directly determining its market value and consumer acceptance. For red and black varieties, the quality of color depends on the types and content of anthocyanins accumulated in the peel. The role of abscisic acid in this process is the most direct and significant.
1. Significantly Increases Anthocyanin Content
Numerous studies have shown that exogenous spraying or dipping with ABA around the veraison stage of grapes can significantly increase the total anthocyanin content in the grape skin. For example, in the Chinese table grape varieties ‘Yuanxiangfei’ and ‘Lanman Hongyan’, dipping the fruit in 250 mg/L ABA at the early veraison stage resulted in a highly significant increase in the total anthocyanin content in the skin at harvest compared to the control group (P<0.001). In international varieties, for the subtropical table grapes ‘Benitaka’ and ‘Rubi’, which suffer from poor coloring, the combined application of S-ABA (a commercial formulation of abscisic acid) and ACC (1-aminocyclopropane-1-carboxylic acid) 7 days after veraison not only significantly increased the anthocyanin concentration but also greatly improved the fruit color index (CIRG), resulting in a uniform deep red color.
2. Affects the Composition of Anthocyanins
ABA not only affects the total amount of anthocyanins but also alters their composition, thus influencing the hue of the color. A study comparing the anthocyanin profiles of ‘Benitaka’ grapes and its dark bud mutation ‘Brasil’ with those of the ABA-treated group revealed the following:
The dark color of the ‘Brasil’ bud mutation primarily stems from the accumulation of highly modified anthocyanins such as peonidin-3-O-monoglucoside and malvidin-3-O-monoglucoside.
The increased coloring in the ABA-treated group was mainly attributed to a significant increase in cyanidin-3-O-monoglucoside and peonidin-3-O-monoglucoside.
This finding indicates that while ABA treatment cannot completely mimic all the metabolic characteristics of natural bud mutations, it can still achieve the desired color enhancement effect by directionally regulating the synthesis of specific anthocyanin branches.
3. Promoting Chlorophyll Degradation
The fading of the peel is a prerequisite for coloring. ABA treatment, while promoting anthocyanin synthesis, also accelerates the degradation of chlorophyll in the peel. Studies have shown that ABA treatment significantly reduces the content of carotenoids, chlorophyll a, and chlorophyll b in the skin of ‘Yuanjinxiang’ grapes, causing the background color (yellow or white) to fade and providing a clean “canvas” for the red color development of anthocyanins.
Enhancing Internal Quality: From Sugar Accumulation to Berry Enlargement
Besides improving appearance, abscisic acid also has a positive impact on the internal physiological quality of grapes, which is crucial for both table grapes and wine grapes.
1. Promoting Sugar Accumulation and Organic Acid Metabolism
ABA can significantly promote the accumulation of soluble solids (TSS, mainly sugars) in the fruit and accelerate the degradation of organic acids, thereby optimizing the sugar-acid ratio and improving flavor and taste.
Increasing Sugar Content: In ‘Yuanxiangfei’ and ‘Yuanjinxiang’ grapes, the soluble solids content of the fruit significantly increased after ABA treatment. In the wine grape ‘Cabernet Franc,’ the soluble solids content even increased by about 5% at harvest.
Reducing Acidity: Application in ‘Xiahei’ grapes shows that ABA treatment effectively reduces the titratable acid content of the fruit, making it sweeter and smoother.
2.Increasing Berry Weight and Size
Although ABA’s primary function is not fruit enlargement, multiple experimental data show that exogenous ABA treatment can significantly increase yield.
Data Support: Under 250 mg/L ABA treatment, the berry weight of ‘Yuanxiangfei’ and ‘Yuanjinxiang’ grapes increased significantly by 16.0% and 16.5%, respectively; the diameter and berry weight of ‘Sunshine Rose’ grapes also increased by 8.2% and 21.7%, respectively. This may be related to ABA promoting the transport and unloading of photosynthetic products into the fruit.
3.Maintaining Post-Harvest Storage Quality
For table grapes with extremely high commercial requirements, post-harvest preservation is crucial. Encouragingly, ABA treatment, while promoting coloring, did not sacrifice storage quality. Studies have shown that ABA-treated ‘Benitaka’ and ‘Rubi’ grapes, after being refrigerated at 1.0°C for 45 days, did not exhibit reduced post-harvest quality indicators such as fruit firmness and stem freshness, nor did they affect the vine’s regrowth vigor the following year. This indicates that the rational use of ABA is safe and efficient.
Promoting Ripening and Regulating Flavor Compounds
1. Inducing Synchronized Ripening
ABA is considered the initiation signal for non-climacteric fruit ripening. Exogenous application of ABA can effectively induce earlier ripening of grapes and concentrate the ripening period, facilitating mechanized harvesting and unified marketing. Its mechanism of action lies in the fact that exogenous ABA triggers the biosynthesis and signal transduction of endogenous ABA in the fruit, forming a positive feedback loop, thereby accelerating the initiation of the entire ripening process. Studies have found that ABA treatment can significantly upregulate the expression of key ABA synthesis genes—members of the 9-cis-epoxycarotenoid dioxygenase (NCED) family (such as VvNCED1 and VvNCED3), thereby increasing endogenous ABA content and promoting the ripening process.
2. Improved Flavor and Functional Substances
Promoting Resveratrol Accumulation: Resveratrol is an important antioxidant and disease-fighting active substance in grapes, beneficial to human health. Studies have found that exogenous ABA treatment can increase the resveratrol content of ‘Cabernet Franc’ grapes by approximately 33% at harvest. Transcriptomic analysis also confirmed that ABA treatment significantly enriched the “stilbene biosynthesis” pathway, which is the pathway for resveratrol synthesis.
Eliminating Green Flavor: In ‘Cabernet Sauvignon’ wine grapes, higher concentrations (100-400 mg/L) of ABA treatment can significantly reduce the concentration of methoxypyrazines, which cause unpleasant “green flavor” in wine, thereby improving the aroma quality of the wine.
Molecular Mechanism: How Does ABA Regulate All of This?
All of the above macroscopic effects are ultimately attributed to a series of intricate molecular biological events within the cell. With the development of transcriptomics and metabolomics, the network of ABA regulating grape ripening has gradually become clear.
1. Activation of Anthocyanin Synthesis Structural Genes
ABA treatment directly upregulates the expression of several key enzyme genes in the anthocyanin biosynthesis pathway. These genes include:
Early synthesis genes (EBGs): such as phenylalanine ammonia-lyase gene (VvPAL) and cinnamic acid-4-hydroxylase gene (VvC4H).
Late synthesis genes (LBGs): such as chalcone isomerase gene (VvCHI), dihydroflavonol-4-reductase gene (VvDFR), and flavonoid glycosyltransferase gene (VvUFGT).
UFGT, in particular, is considered the rate-limiting enzyme in anthocyanin synthesis, responsible for converting unstable anthocyanins into stable anthocyanins. Strong ABA induction of this gene is a direct cause of fruit coloring.
2. Regulation of the MYB Transcription Factor Family
MYB transcription factors are the “master switch” regulating the anthocyanin synthesis pathway. ABA influences the activity of these switches through a complex regulatory network:
Upregulating activators: ABA significantly upregulates the expression of R2R3-MYB activators such as VvMYBA1, VvMYBA2, and VvMYB24. These activators form MBW protein complexes with bHLH and WD40 proteins, binding to the promoters of structural genes and initiating transcription.
Inhibiting repressors: Interestingly, ABA also inhibits the expression of a transcription repressor—VvMYBC2-L1. By simultaneously “accelerating” and “releasing” the brakes, ABA achieves highly efficient promotion of anthocyanin synthesis.
3.Cross-talk among hormones
Grape ripening is not determined by ABA alone, but rather by the integration of multiple hormonal signals.
Promoting endogenous ABA synthesis: Exogenous ABA promotes the accumulation of endogenous ABA by upregulating the VvNCEDs gene, creating a signal amplification effect. Conversely, if negative regulators (such as the E3 ubiquitin ligase VlPUB38) degrade key enzymes in the ABA synthesis pathway (such as abscisic acid oxidase VlAAO), ripening will be inhibited.
Interactions with ethylene and auxin: Studies have found that ABA treatment can also significantly affect the abundance of related genes in the ethylene and auxin signaling pathways, indicating that ABA ultimately activates the fruit ripening burst by coordinating the balance of multiple hormones.
Application Strategies and Prospects
In summary, the application of abscisic acid in grapes is a comprehensive regulatory process involving “appearance-internal-safety.” To achieve the best results, the following points should be noted in production:
Application timing: The generally accepted optimal application time is the early veraison stage (when about 5% of the fruit begins to soften and change color). At this time, the peel cells are most sensitive to ABA signals.
Suitable concentration: The concentration varies depending on the variety and purpose. For color enhancement in table grapes, a concentration of 200-400 mg/L is generally recommended. Too low a concentration will not have a significant effect, while too high a concentration may lead to excessive softening of the berries or leaf drop, among other side effects.
Combination therapy: The future trend is the combined use of ABA with other regulators (such as jasmonic acid esters and ACC) to reduce the dosage of single agents and achieve better color uniformity and higher quality.
In summary, abscisic acid (ABA) comprehensively improves the overall quality of grapes by activating complex gene networks, resulting in improved color, sugar accumulation, reduced acidity, larger berries, and optimized flavor. It not only solves the industry’s pain point of difficult grape coloring under adverse conditions such as low light and high temperature, but also provides a scientific and efficient technical means for producing high-quality, flavorful grapes. With further analysis of its molecular mechanisms, we will be able to more precisely control this “key to maturity” in the future, driving the grape industry towards higher-quality development.
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