Learning Objectives
- Identify major classes of plant hormones.
- Describe their physiological functions in plants.
- Explain mechanisms of hormonal regulation of growth and development.
- Relate phytohormones to horticultural practices and crop productivity.
9.1 Introduction to Phytohormones
Phytohormones are small organic signaling molecules produced by plants that regulate growth, development, and responses to environmental stimuli at very low concentrations.
They are also called:
- Plant hormones
- Plant growth regulators (PGRs)
- Growth factors
The term hormone originates from the Greek word meaning “to set in motion.”
Key Characteristics of Phytohormones
- Active at very low concentrations.
- Produced in one tissue and transported to another (or act locally).
- Bind to specific receptors in target cells.
- Regulate gene expression, enzyme activity, and cellular differentiation.
Applied Perspective:
Phytohormones are central to horticultural productivity, fruit quality, tissue culture, stress tolerance, and post-harvest technology.
9.2 Mechanisms of Hormonal Action
Phytohormones regulate plant growth through:
- Activation or inhibition of enzymes.
- Regulation of gene expression and transcription.
- Control of cell division, elongation, and differentiation.
- Integration of developmental and environmental signals.
9.3 Major Classes of Phytohormones
| Hormone Class |
Main Functions |
Key Horticultural Applications |
| Auxins |
Cell elongation, apical dominance, root formation |
Rooting of cuttings, fruit set, weed control |
| Gibberellins (GA) |
Stem elongation, seed germination, flowering |
Breaking dormancy, increasing fruit size |
| Cytokinins |
Cell division, delay of senescence, lateral bud growth |
Tissue culture, delay leaf aging |
| Ethylene |
Fruit ripening, senescence, abscission |
Post-harvest ripening control |
| Abscisic Acid (ABA) |
Stress response, dormancy, growth inhibition |
Drought tolerance, seed dormancy regulation |
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1. Auxins
Auxins are primarily synthesized in apical meristems and young leaves.
Functions:
- Stimulate cell elongation.
- Maintain apical dominance.
- Promote adventitious root formation.
- Regulate vascular differentiation.
- Influence fruit development and abscission.
Applied Example:
Indole-3-acetic acid (IAA) and synthetic auxins (e.g., 2,4-D) are used in rooting hormones and selective herbicides.
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2. Gibberellins (GA)
Functions:
- Promote stem elongation and bolting.
- Break seed and bud dormancy.
- Stimulate flowering in some species.
- Enhance fruit growth.
Applied Example:
Gibberellins are used to increase berry size in grapes and improve malting in cereals.
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3. Cytokinins
Functions:
- Stimulate cell division.
- Promote lateral bud growth.
- Delay leaf senescence.
- Regulate organ differentiation in tissue culture.
Applied Example:
The auxin-to-cytokinin ratio determines shoot or root formation in plant tissue culture.
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4. Ethylene
Functions:
- Regulates fruit ripening.
- Promotes senescence and abscission.
- Responds to stress and mechanical damage.
Applied Example:
Ethylene management is critical in post-harvest handling of fruits such as bananas and tomatoes.
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5. Abscisic Acid (ABA)
Functions:
- Induces seed and bud dormancy.
- Regulates stomatal closure during drought stress.
- Antagonizes growth-promoting hormones.
Applied Example:
ABA plays a key role in drought tolerance and stress adaptation in crops.
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9.4 Hormonal Interactions (Crosstalk)
Plant growth is regulated by interactions among multiple hormones rather than a single hormone.
- Auxin–cytokinin balance controls organ formation.
- GA–ABA antagonism regulates dormancy and germination.
- Ethylene interacts with auxins and ABA during senescence.
Applied Horticulture:
Manipulating hormonal balance is essential in pruning, grafting, tissue culture, fruit thinning, and stress management.
9.5 Summary
- Phytohormones are central regulators of plant growth and development.
- They act through gene regulation and biochemical pathways.
- Understanding phytohormones is essential for applied horticulture, biotechnology, and crop improvement.