1. Introduction to Plant Growth Regulators (PGRs)
Plant growth regulators, also known as phytohormones, are naturally occurring organic compounds that control and coordinate plant growth, development, and responses to environmental stimuli. Think of them as the plant's internal communication system, similar to how hormones work in animals, but with some important differences.
The study of plant growth regulators began in the late 19th century when Charles Darwin observed that grass seedlings bend toward light. This discovery led to the identification of auxin, the first plant hormone. Today, we recognize five major classes of plant growth regulators, each with distinct but often overlapping functions.
2. The Five Major Classes of Plant Growth Regulators
2.1 Auxins
Chemical Nature and Discovery
Auxins are a group of compounds with indole acetic acid (IAA) being the most common naturally occurring form. The name "auxin" comes from the Greek word "auxein," meaning "to grow." These hormones were first discovered through experiments showing that the growing tip of a plant shoot produces a substance that promotes cell elongation.
Physiological Roles of Auxins:
1. Apical Dominance: Auxin produced by the shoot apex inhibits the growth of lateral buds. This is why when you pinch off the growing tip of a plant, side shoots begin to develop. The mechanism works through auxin transport down the stem, where high concentrations suppress lateral bud growth.
2. Cell Elongation: Auxin promotes cell wall loosening by activating enzymes that break bonds in the cell wall, allowing cells to expand. This process is crucial during stem elongation and root growth.
3. Tropisms: Auxin redistributes in response to environmental stimuli, causing differential growth. In phototropism, auxin accumulates on the shaded side of the stem, causing those cells to elongate more and bend the plant toward light.
4. Root Initiation: Auxin stimulates the formation of adventitious roots from stems and leaves, which is why it's commonly used in plant propagation.
Agricultural Applications of Auxins:
Vegetative Propagation: Auxins are extensively used in nurseries to promote rooting in cuttings of ornamental plants, fruit trees, and forest species. Different concentrations are used for different plant types.
Fruit Development: Synthetic auxins can induce parthenocarpy (fruit development without fertilization) in crops like tomatoes and peppers, ensuring fruit set even under poor pollination conditions.
Herbicides: Some synthetic auxins, like 2,4-D, act as selective herbicides by causing uncontrolled growth in broadleaf weeds while leaving grasses relatively unaffected.
2.2 Gibberellins
Chemical Nature and Discovery
Gibberellins are a large family of compounds (over 130 known forms) first discovered in Japan while studying a fungal disease called "bakanae" in rice. The infected plants grew unusually tall due to gibberellins produced by the fungus Gibberella fujikuroi.
Physiological Roles of Gibberellins:
1. Stem Elongation: Gibberellins promote cell division and elongation in stems, particularly in the internodes. They're especially important in genetically dwarf plants, where gibberellin application can restore normal height.
2. Seed Germination: Gibberellins break seed dormancy by promoting the synthesis of enzymes like α-amylase, which breaks down starch reserves in the seed to provide energy for germination.
3. Flowering: In many plants, especially those requiring long days or cold treatment to flower, gibberellins can substitute for these environmental requirements and induce flowering.
4. Fruit Development: Gibberellins promote fruit growth by stimulating cell division and expansion, leading to larger fruit size.
Gibberellin Action in Seed Germination
Water Absorption → GA Synthesis → Enzyme Production → Starch Breakdown → Energy ReleaseAgricultural Applications of Gibberellins:
Grape Production: Gibberellin application in grapes increases berry size and can induce seedless fruit development. This is commercially important in table grape production.
Malting Industry: Gibberellins are used to promote uniform germination in barley for brewing, ensuring consistent enzyme production for the malting process.
Sugar Production: In sugarcane, gibberellins increase internode length, resulting in taller plants and higher sugar yields per hectare.
2.3 Cytokinins
Chemical Nature and Discovery
Cytokinins are purine derivatives, with zeatin being the most common naturally occurring form. They were discovered during research on tissue culture, where scientists found that coconut milk contained factors that promoted cell division.
Physiological Roles of Cytokinins:
1. Cell Division: As their name suggests, cytokinins promote cytokinesis (cell division) and are essential for maintaining actively dividing tissues like shoot and root meristems.
2. Shoot Development: Cytokinins promote the development of lateral buds, working in opposition to auxin's apical dominance effect. The ratio of auxin to cytokinin determines whether a tissue develops roots or shoots.
3. Leaf Senescence Delay: Cytokinins can delay leaf aging by maintaining protein synthesis and chlorophyll content. This is why cut flowers treated with cytokinins stay fresh longer.
4. Nutrient Mobilization: Cytokinins act as signals that attract nutrients to actively growing tissues, ensuring that developing organs receive adequate resources.
High auxin : low cytokinin = Root development
Low auxin : high cytokinin = Shoot development
Balanced ratio = Callus (undifferentiated tissue) formation
Agricultural Applications of Cytokinins:
Tissue Culture: Cytokinins are essential in micropropagation, where they promote shoot multiplication and help maintain actively dividing cell cultures.
Fruit Storage: Cytokinin treatments can extend the shelf life of leafy vegetables and cut flowers by delaying senescence and maintaining nutritional quality.
Breaking Bud Dormancy: In fruit tree production, cytokinins can be used to promote lateral branching and overcome apical dominance, creating more fruiting sites.
2.4 Abscisic Acid (ABA)
Chemical Nature and Discovery
Abscisic acid is a sesquiterpene compound originally thought to be primarily involved in leaf abscission (hence the name). However, research revealed its major role is in stress responses and dormancy regulation.
Physiological Roles of Abscisic Acid:
1. Stomatal Regulation: ABA is the primary hormone controlling stomatal closure during water stress. When plants detect drought, ABA levels increase rapidly, causing guard cells to close stomata and conserve water.
2. Seed Dormancy: ABA prevents premature germination by maintaining seed dormancy until environmental conditions are favorable. It inhibits the action of gibberellins and other growth-promoting hormones.
3. Stress Tolerance: ABA coordinates plant responses to various abiotic stresses including drought, salt, and cold by regulating gene expression and metabolic processes.
4. Growth Inhibition: Under stress conditions, ABA inhibits growth to conserve resources and redirect energy toward stress tolerance mechanisms.
Agricultural Applications of Abscisic Acid:
Transplant Success: ABA applications can help reduce transplant shock by promoting water conservation and stress tolerance in newly transplanted seedlings.
Fruit Ripening: ABA plays a role in initiating fruit ripening processes and can be used to coordinate harvest timing in certain crops.
Drought Management: Research is ongoing into using ABA or ABA analogs to improve crop drought tolerance, though this is still largely experimental.
2.5 Ethylene
Chemical Nature and Discovery
Ethylene is unique among plant hormones as it's a gaseous compound (C₂H₄). It was first discovered when gas lamps caused nearby plants to show abnormal growth, later traced to ethylene in the gas.
Physiological Roles of Ethylene:
1. Fruit Ripening: Ethylene is the primary hormone controlling fruit ripening in climacteric fruits (those that continue to ripen after harvest). It coordinates color changes, softening, and flavor development.
2. Senescence: Ethylene accelerates the aging process in leaves and flowers, triggering abscission and programmed cell death.
3. Response to Stress: Physical damage, pathogen attack, or environmental stress can trigger ethylene production, leading to defensive responses.
4. Sex Determination: In some plants like cucumber, ethylene influences whether flowers develop as male or female.
Agricultural Applications of Ethylene:
Fruit Ripening Control: Ethylene is used commercially to ripen fruits like bananas and tomatoes after harvest, allowing for transportation of unripe, more durable fruit.
Flower Induction: In pineapple production, ethylene or ethylene-releasing compounds are used to synchronize flowering across the entire field.
Abscission Induction: Ethylene can be used to promote fruit drop for mechanical harvesting in crops like cherries and nuts.
3. Photoperiodism
3.1 Understanding Photoperiodism
Photoperiodism is the ability of plants to measure day length (photoperiod) and use this information to time important developmental events, particularly flowering. This remarkable adaptation allows plants to coordinate their reproductive cycles with seasonal changes, ensuring that flowering, fruiting, and seed dispersal occur at optimal times.
The discovery of photoperiodism came from experiments with tobacco plants in the early 1900s. Scientists Wightman Garner and Harry Allard noticed that a mutant tobacco variety called 'Maryland Mammoth' would only flower when days became shorter, regardless of the plant's age or size.
3.2 Types of Photoperiodic Responses
Plants can be classified into different categories based on their photoperiodic requirements for flowering:
Short-Day Plants (Long-Night Plants)
These plants require a photoperiod shorter than a critical length to induce flowering. More accurately, they need a continuous dark period longer than a critical duration. Short-day plants typically flower in late summer or fall when nights become longer.
• Chrysanthemums (critical photoperiod: ~13.5 hours)
• Soybeans (critical photoperiod: varies with variety, ~12-14 hours)
• Rice (critical photoperiod: varies with variety)
• Poinsettias (critical photoperiod: ~12.5 hours)
Long-Day Plants (Short-Night Plants)
These plants require a photoperiod longer than a critical length to flower. They need the dark period to be shorter than a critical duration. Long-day plants typically flower in late spring or early summer when days are getting longer.
• Wheat (critical photoperiod: varies with variety, ~12-16 hours)
• Barley (critical photoperiod: varies with variety)
• Spinach (critical photoperiod: ~13 hours)
• Lettuce (critical photoperiod: ~12 hours)
Day-Neutral Plants
These plants flower regardless of photoperiod, as long as other environmental conditions are suitable. Their flowering is controlled by factors other than day length, such as temperature, plant age, or nutritional status.
• Tomatoes
• Corn (maize)
• Cucumbers
• Beans
3.3 The Mechanism of Photoperiodic Response
The photoperiodic response involves a complex molecular mechanism that has been extensively studied, particularly in model plants like Arabidopsis. Understanding this mechanism is crucial for appreciating how plants can be such precise timekeepers.
The Circadian Clock
At the heart of photoperiodism lies the plant's circadian clock - an internal timing system that maintains approximately 24-hour rhythms even in constant conditions. This biological clock consists of interconnected feedback loops involving clock genes and their protein products.
Photoperiodic Signal Transduction
Light Detection → Circadian Clock → Hormone Signals → Gene Expression → FloweringPhotoreceptors
Plants use several types of photoreceptors to detect light quality, quantity, and duration:
Phytochromes: These are the primary photoreceptors for photoperiodism. They exist in two interconvertible forms - Pr (red light-absorbing) and Pfr (far-red light-absorbing). The ratio of these forms changes throughout the day and provides information about day length.
Cryptochromes: These blue light receptors also contribute to circadian clock regulation and photoperiodic responses.
The Flowering Hormone Concept
Research has identified a protein called FLOWERING LOCUS T (FT) that acts as the long-sought "flowering hormone" or "florigen." This protein is produced in leaves in response to appropriate photoperiodic conditions and travels to the shoot apex where it triggers flowering.
3.4 Agricultural Applications of Photoperiodism
Understanding photoperiodism has tremendous practical importance in agriculture, horticulture, and plant breeding. This knowledge allows farmers and growers to manipulate flowering time and optimize crop production.
Controlled Environment Agriculture
In greenhouses and growth chambers, artificial lighting can be used to manipulate photoperiods and control flowering time. For example, chrysanthemum growers use blackout curtains and supplemental lighting to produce flowers year-round for the cut flower market.
Crop Scheduling
Understanding the photoperiodic requirements of crops allows farmers to schedule plantings to ensure flowering and harvest occur at optimal times. This is particularly important for crops grown in different latitudes where natural photoperiods vary.
Plant Breeding
Breeders work to develop crop varieties with appropriate photoperiodic responses for specific growing regions. For example, soybean varieties are bred with specific photoperiod requirements to ensure they flower at the right time when grown at different latitudes.
4. Vernalisation
4.1 Understanding Vernalisation
Vernalisation (also spelled vernalization) is the process by which plants acquire the competence to flower through exposure to prolonged cold temperatures. The term comes from the Latin word "vernus," meaning "of spring," reflecting the fact that many plants require winter's cold temperatures before they can flower in spring.
This phenomenon was first systematically studied in the 1920s by Russian scientist T.D. Lysenko, who worked with winter wheat varieties that required cold treatment to flower properly. While some of Lysenko's later work became controversial for political rather than scientific reasons, his initial observations about vernalisation were scientifically sound and remain important today.
4.2 Types of Vernalisation Requirements
Obligate Vernalisation
Some plants have an absolute requirement for cold treatment and will not flower regardless of other conditions unless they receive adequate vernalisation. These plants typically show no flowering response to long days or other flowering stimuli until the cold requirement is satisfied.
• Winter wheat varieties
• Some biennial plants like carrots and beets
• Many fruit trees including apples and cherries
• Tulips and other spring bulbs
Quantitative Vernalisation
Other plants show a quantitative response to vernalisation - they may eventually flower without cold treatment, but cold exposure accelerates flowering or increases the flowering response. The more cold treatment they receive (up to a point), the stronger their flowering response.
• Some wheat and barley varieties
• Certain Arabidopsis ecotypes
• Some garden plants like pansies
4.3 Mechanism of Vernalisation
The molecular mechanism of vernalisation has been extensively studied, particularly in Arabidopsis and cereals. The process involves epigenetic changes - modifications to gene expression that don't involve changes to the DNA sequence itself but are maintained through cell divisions.
The Vernalisation Response in Arabidopsis
In Arabidopsis, the key player is a gene called FLOWERING LOCUS C (FLC), which acts as a floral repressor. Before vernalisation, FLC is highly expressed and prevents flowering. During cold treatment, chromatin modifications gradually reduce FLC expression, and this repressed state is maintained even after plants return to warm conditions.
Vernalisation Mechanism
Cold Exposure → Chromatin Changes → FLC Repression → Flowering CompetenceTemperature and Duration Requirements
Different plants have varying requirements for vernalisation:
Temperature Range: Most plants require temperatures between 0-10°C (32-50°F) for effective vernalisation. The optimal temperature is often around 4-6°C (39-43°F).
Duration: The cold treatment period can range from a few weeks to several months, depending on the species and variety. Winter wheat typically requires 6-8 weeks of cold treatment.
Plant Stage: Some plants can be vernalised as seeds (seed vernalisation), while others require vernalisation of the growing plant. The stage at which vernalisation is most effective varies among species.
4.4 Agricultural Applications of Vernalisation
Understanding vernalisation has important practical applications in agriculture and horticulture, allowing farmers and growers to manipulate flowering time and improve crop production.
Crop Production Strategies
Winter vs. Spring Varieties: Many cereal crops have both winter and spring varieties. Winter varieties require vernalisation and are planted in fall, while spring varieties have reduced or no vernalisation requirements and can be planted in spring.
Controlled Vernalisation: Seeds or young plants can be given artificial cold treatment to satisfy vernalisation requirements. This allows greater flexibility in planting times and can be used to produce flowers or crops out of season.
Breeding Programs: Plant breeders work to develop varieties with appropriate vernalisation requirements for specific climates and cropping systems. This includes developing varieties with reduced vernalisation requirements for warmer climates.
Climate Change Implications
Understanding vernalisation is becoming increasingly important as climate change affects winter temperatures. Warmer winters may not provide adequate chilling for some crops, potentially affecting agricultural productivity in certain regions.
5. Interactions Between Growth Regulators and Environmental Responses
Plant growth regulators don't work in isolation. The responses to photoperiodism and vernalisation involve complex interactions between multiple hormones, creating an integrated system that allows plants to respond appropriately to environmental cues.
5.1 Hormonal Integration in Flowering
The transition from vegetative to reproductive growth involves coordination between several plant growth regulators:
Gibberellins and Flowering
Gibberellins play crucial roles in the flowering process, particularly in plants that require vernalisation or specific photoperiods. In many long-day plants, gibberellins can substitute for the long-day requirement, suggesting they're part of the natural flowering pathway.
Cytokinins and Flowering
Cytokinins influence flowering both positively and negatively, depending on the plant species and environmental conditions. They can promote flowering in some species while delaying it in others, often by affecting the sensitivity to photoperiodic signals.
Other Hormones
Auxins generally inhibit flowering, maintaining vegetative growth. However, they're essential for flower development once flowering is initiated. Ethylene can either promote or inhibit flowering depending on the species and concentration.
5.2 Practical Applications in Crop Management
| Application | Hormone/Process | Mechanism | Commercial Use |
|---|---|---|---|
| Flower Induction | Gibberellins | Substitute for environmental requirements | Greenhouse production, off-season flowering |
| Uniform Flowering | Ethylene | Synchronize flowering time | Pineapple production, ornamental crops |
| Flower Delay | ABA, Auxin | Maintain vegetative growth | Extending harvest period |
| Photoperiod Manipulation | Light control | Artificial day/night cycles | Year-round flower production |
| Vernalisation Control | Temperature treatment | Satisfy cold requirements | Bulb forcing, winter crop production |
6. Case Studies in Commercial Applications
6.1 Case Study: Strawberry Production
Modern strawberry production demonstrates the sophisticated application of plant growth regulator knowledge and environmental control:
Day-Neutral Varieties: Breeders have developed strawberry varieties that are day-neutral for flowering, allowing continuous production throughout the growing season rather than just one flush in spring.
Chilling Requirements: Strawberry plants require a certain amount of chilling (vernalisation-like process) for optimal flowering. Producers use controlled-temperature storage to ensure plants receive adequate chilling.
Hormone Applications: Gibberellins are sometimes used to promote runner production for propagation, while cytokinin applications can improve fruit quality and delay senescence during storage.
6.2 Case Study: Wheat Production Across Latitudes
Wheat cultivation demonstrates how understanding photoperiodism and vernalisation enables global crop production:
• High-latitude varieties: Strong vernalisation requirements, less photoperiod sensitivity
• Mid-latitude varieties: Moderate vernalisation needs, some photoperiod sensitivity
• Low-latitude varieties: Minimal vernalisation requirements, often photoperiod insensitive
This genetic diversity allows wheat to be grown from near the Arctic Circle to tropical regions, with each variety adapted to its specific environmental conditions.
6.3 Case Study: Ornamental Plant Production
The ornamental plant industry relies heavily on manipulating plant growth regulators and environmental conditions:
Poinsettia Production: Requires precise photoperiod control (14+ hours darkness daily for 6-8 weeks) to develop colored bracts for Christmas sales.
Easter Lily Forcing: Bulbs are given controlled vernalisation, then forced with specific temperature and photoperiod regimes to bloom exactly for Easter, regardless of the calendar date.
Growth Retardants: Synthetic growth regulators that inhibit gibberellin biosynthesis are used to produce compact, marketable potted plants.
7. Future Directions and Biotechnology
7.1 Genetic Engineering of Hormone Responses
Modern biotechnology allows for precise modification of plant hormone responses:
Modified Flowering Time: Genes controlling photoperiodic and vernalisation responses can be modified to create crops that flower at desired times regardless of environmental conditions.
Stress Tolerance: Engineering enhanced ABA responses or modified auxin transport can improve crop tolerance to drought, salt, or other environmental stresses.
Improved Yields: Modifying cytokinin or gibberellin responses can potentially increase fruit size, seed number, or overall plant productivity.
7.2 Precision Agriculture Applications
Technology is enabling more precise application of plant growth regulators:
7.3 Climate Change Adaptations
Understanding plant growth regulators and environmental responses is crucial for developing climate-resilient crops:
Temperature Tolerance: Developing varieties with modified vernalisation requirements for changing winter conditions.
Photoperiod Adaptation: Creating crops that can maintain productivity as they're moved to different latitudes due to changing climate zones.
Stress Resilience: Enhanced understanding of ABA and other stress hormones enables development of crops better adapted to increasing environmental variability.
8. Study Questions and Key Concepts Review
Essential Concepts to Master:
1. Hormone Interactions: Understand that plant responses result from the balance and interaction of multiple hormones, not single hormone effects.
2. Environmental Integration: Plants use hormone systems to integrate multiple environmental signals (light, temperature, water status) into coordinated developmental responses.
3. Agricultural Applications: Each hormone class has specific commercial applications, and understanding the underlying biology enables more effective use.
4. Photoperiod vs. Vernalisation: These are distinct environmental responses - photoperiodism measures day length while vernalisation requires cold temperature exposure.
5. Practical Problem-Solving: Apply hormone knowledge to solve real agricultural challenges like timing flower production or improving crop stress tolerance.
Practice Questions:
1. A greenhouse grower wants to produce chrysanthemums for sale in June. What environmental manipulations would be necessary, and why?
2. Explain why a winter wheat variety grown in a warm climate might not flower properly, and suggest possible solutions.
3. A plant cutting treated with auxin develops roots but no shoots. What additional treatment might promote shoot development?
4. Compare and contrast the mechanisms by which plants measure day length versus cold exposure.
5. Describe how climate change might affect vernalisation-dependent crops and potential adaptation strategies.
9. Conclusion
Plant growth regulators represent one of the most fascinating and practically important aspects of plant biology. These molecular messengers coordinate complex developmental programs, enabling plants to respond appropriately to environmental cues and optimize their growth and reproduction.
The study of photoperiodism and vernalisation reveals the sophisticated timing mechanisms that plants have evolved to synchronize their development with seasonal changes. Understanding these processes has revolutionized agriculture, enabling year-round production of crops and ornamental plants, expanding the geographical range of cultivation, and improving crop yields and quality.
As we face challenges from climate change and increasing global food demands, knowledge of plant growth regulators and environmental responses becomes even more critical. This understanding forms the foundation for developing climate-resilient crops, optimizing agricultural practices, and creating sustainable food production systems.
10. Glossary of Key Terms
| Term | Definition |
|---|---|
| Apical Dominance | The inhibition of lateral bud growth by the shoot apex, mediated primarily by auxin |
| Bolting | Rapid stem elongation, often associated with flowering in rosette plants |
| Circadian Clock | Internal biological timing mechanism that maintains approximately 24-hour rhythms |
| Critical Photoperiod | The day length threshold that determines whether a photoperiodic plant will flower |
| Epigenetic | Changes in gene expression that don't involve DNA sequence changes but are heritable |
| Florigen | The mobile flowering signal, now identified as the FT protein |
| Parthenocarpy | Fruit development without fertilization, often induced by auxin treatment |
| Phytochrome | Red/far-red light receptor crucial for photoperiodic responses |
| Senescence | The aging process in plants, involving programmed breakdown of cellular components |
| Tropism | Directional growth response to environmental stimuli, often mediated by auxin |
This study guide provides a comprehensive foundation in plant growth regulators, photoperiodism, and vernalisation for undergraduate students. Regular review of these concepts and their applications will support success in plant biology, agriculture, and related fields.
