Exploring the reagents that control bud break and their effects on metabolites and gene expression during winter dormancy
Every spring, a silent miracle unfolds in vineyards across the world as dormant grapevines awaken from their winter slumber, producing tiny green buds that will eventually become juicy clusters of grapes. This annual event, known as bud break, marks the beginning of the grape growing season. But what precisely controls the timing of this crucial natural process? Behind the scenes, a complex biochemical ballet of hormones, metabolites, and genetic triggers determines when sleeping buds decide to wake up.
For grape growers, achieving uniform and well-timed bud break is more than just a sign of spring—it's an economic necessity. When bud break is delayed or uneven, it can lead to reduced fruit quality and yield, ultimately impacting wine and table grape production.
In regions with warm winters, where natural chilling is insufficient, growers have turned to science for solutions in the form of dormancy-breaking reagents. These chemical agents can influence the vine's internal clock.
Recent scientific advances have begun to unravel the fascinating mechanisms through which compounds like hydrogen cyanamide and abscisic acid influence grape bud break. Through cutting-edge research, scientists are discovering how these reagents rewire metabolic pathways and alter gene expression, providing crucial insights that could help vineyards adapt to changing climate conditions.
To comprehend how bud break reagents work, we must first understand what grapevine dormancy entails. As days shorten and temperatures drop in autumn, grapevines enter a survival state called dormancy—a period of suspended animation that allows them to withstand harsh winter conditions. This isn't a simple pause button on growth but rather an actively maintained state controlled by sophisticated internal mechanisms 1 .
Governed by internal factors within the buds themselves. During this phase, even if environmental conditions become favorable for growth, the buds refuse to break. This internal blockade is maintained by a balance of hormones and inhibitors that prevent growth until the vine has experienced sufficient chilling exposure 9 .
Once the chilling requirement is met, the vine transitions to ecodormancy, where growth is primarily limited by external environmental conditions, particularly temperature 9 .
Carbohydrates are converted into soluble sugars that act as cryoprotectants, lowering the freezing point of tissues 6 .
Antioxidant enzyme systems are activated to manage oxidative stress during the transition from dormancy to growth 6 .
Hormone ratios shift dramatically, with a decline in growth inhibitors and a rise in growth promoters 1 .
Two compounds stand out in the science of bud break regulation: hydrogen cyanamide (HC) and abscisic acid (ABA). These substances represent opposing forces in the dormancy equation—one a powerful promoter of bud break, the other a key inhibitor.
Hydrogen cyanamide, often sold under the trade name Dormex, has become the most widely used bud break agent in the global table grape industry 1 .
Abscisic acid is a hormone now recognized as a central regulator of dormancy maintenance 1 .
HC and ABA work in opposition to regulate bud break timing. HC promotes dormancy release through oxidative signaling and hormonal changes, while ABA maintains dormancy by inhibiting growth and enhancing stress resistance. Their balance determines when buds transition from dormancy to active growth.
To truly understand how these reagents work, let's examine a pivotal research study that delved deep into the molecular mechanisms behind bud break. Scientists from Nanjing Agricultural University conducted a comprehensive investigation using the popular table grape cultivar 'Shine Muscat' to unravel the physiological and molecular responses to HC, ABA, and their combination 1 4 .
Canes collected from six-year-old 'Shine Muscat' grapevines during mid-dormancy period 1 .
Four experimental groups: Control, HC (5%), ABA (100μM), and ABA-HC combination 1 .
Cuttings placed in growth chambers at 25±1°C with 14 hours of light to simulate spring 1 .
The results of the study revealed a fascinating orchestration of biochemical events that accompany dormancy release. By comparing the responses across different treatments, the scientists could pinpoint exactly how HC and ABA influence the waking-up process, and how they interact when applied together.
HC-treated buds showed the most rapid and uniform bud break, while ABA-treated buds demonstrated markedly suppressed bud break 1 4 .
| Treatment | Gibberellins | ABA | IAA |
|---|---|---|---|
| HC | Significant increase | Decrease | Increase |
| ABA | Suppressed | Increase | Variable effect |
| ABA-HC | Moderate increase | Initial increase then decrease | Moderate increase |
| Control | Gradual natural increase | Gradual natural decrease | Gradual natural increase |
HC treatment increased GA and IAA levels while reducing ABA 1 4 .
Sucrose levels peaked significantly in HC-treated buds, while ABA treatment resulted in increased fructose accumulation 1 4 .
| Gene Category | HC Effect | ABA Effect | ABA-HC Effect |
|---|---|---|---|
| PP2C (ABA signaling) | Downregulated | Downregulated | Downregulated |
| GID1 (GA signaling) | Mixed regulation | Variable | Upregulated |
| Antioxidant enzymes | Mild upregulation | Strong upregulation | Moderate upregulation |
| Carbohydrate metabolism | Mild effect | Strong upregulation of aldolase | Moderate effect |
ABA application upregulated genes encoding antioxidant enzymes and carbohydrate metabolism genes 1 4 .
| Reagent/Tool | Specific Function in Research | Application Details |
|---|---|---|
| Hydrogen Cyanamide (Dormex) | Artificial dormancy release agent | Applied as 5% (v/v) solution; suppresses catalase to cause H₂O₂ burst 1 |
| Abscisic Acid (ABA) | Dormancy maintenance hormone | Used at 100 μM concentration to study inhibitory effects 1 |
| Triton X-100 | Surfactant | Added at 0.02% to ensure even coverage of treatments 1 |
| Liquid Chromatography-Tandem Mass Spectrometry | Hormone quantification | Precisely measures endogenous levels of IAA, GA, ABA 1 4 |
| Quantitative RT-PCR | Gene expression analysis | Measures expression levels of dormancy-related genes 1 |
The molecular insights gained from this research extend far beyond academic interest, offering practical solutions for vineyard management and climate adaptation strategies.
For growers in regions with insufficient winter chilling, hydrogen cyanamide has become an indispensable tool for ensuring uniform bud break and consistent yields. The research explaining HC's mechanism validates its use while providing insights that could lead to improved application timing and concentration 1 .
Research into alternative approaches includes the development of ABA analogs that can delay bud break—potentially useful for avoiding spring frost damage—and biostimulants that might enhance natural dormancy release processes 8 .
As winter temperatures warm in many grape-growing regions, insufficient chilling accumulation is becoming an increasing challenge. Understanding the molecular basis of dormancy release may help breeders develop new cultivars with lower chilling requirements 9 .
Future research is likely to build on these findings using advanced technologies. Integrated multi-omics approaches—combining transcriptomics, proteomics, and metabolomics—are already providing unprecedented views of the dormancy release process . For instance, one recent study on 'Shuijing' grapevines using these comprehensive approaches found that HC treatment influenced 26,336 genes and 990 proteins, primarily affecting pathways related to energy metabolism, carbohydrate metabolism, and amino acid metabolism .
The journey from dormant bud to vibrant shoot represents one of nature's most exquisite biological transitions—a process that vineyard scientists are now understanding in remarkable molecular detail. Through meticulous research, we've come to appreciate that dormancy release isn't a simple on/switch but rather a sophisticated reprogramming event that involves hormonal shifts, metabolic reactivation, and genetic changes.
Hydrogen cyanamide and abscisic acid represent two powerful forces in this process—one the starter pistol for growth, the other the gatekeeper of dormancy. Their opposing actions at the molecular level highlight the delicate balance that grapevines maintain between growth and rest, between seizing opportunity and avoiding risk.
The science of bud break reminds us that even the smallest bud contains universe of complexity, and that unlocking its secrets requires both sophisticated tools and humble curiosity. As research continues to decode the molecular language of dormancy, each discovery brings new possibilities for sustainable viticulture—ensuring that regardless of changing climates, the annual miracle of spring in the vineyard will continue for generations to come.