Unit 2
Plant Physiology
Chapter 2.4
Plant HormonesClass 12 Biology – Plant Hormones Notes PDF
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Introduction
Definition
Plant hormones or phytohormones are naturally occurring organic substances produced in very small quantities in plants that regulate growth, development and physiological responses.
In this chapter, the main hormones are auxins, gibberellins and cytokinins. Their effects depend on concentration, tissue type, developmental stage and interaction with other hormones.
1. General Features of Plant Hormones
- Produced naturally in plants in minute quantities.
- May act at the place of synthesis or after transport to another tissue.
- Regulate cell division, elongation, differentiation, dormancy, germination and organ development.
- A single hormone can produce different effects in different tissues.
- Hormones often act together rather than independently.
Fig. 1 – Major Plant Hormones in This Chapter
2. Auxins
DefinitionAuxins are growth-promoting plant hormones associated especially with cell elongation and regulation of apical growth.
Site of Production
Auxins are produced mainly in actively growing regions such as shoot apices, young leaves, developing embryos and seeds.
Physiological Effects
- Cell elongation: promotes elongation of young stem cells.
- Apical dominance: auxin from the shoot apex suppresses growth of lateral buds.
- Root initiation: suitable concentrations promote adventitious and lateral root formation.
- Tropic responses: unequal auxin distribution contributes to phototropism and gravitropism.
- Fruit development: auxin can promote fruit growth and, in some cases, parthenocarpic fruit formation.
- Abscission control: appropriate auxin levels can delay premature leaf and fruit fall.
Fig. 2 – Auxin and Phototropism
Fig. 3 – Apical Dominance
3. Gibberellins
DefinitionGibberellins are growth-promoting hormones particularly important in stem elongation, seed germination and development.
Physiological Effects
- Stimulate elongation of stems and internodes.
- Promote bolting in some rosette plants.
- Help break seed and bud dormancy.
- Stimulate production of hydrolytic enzymes during seed germination.
- Promote growth of some fruits and may induce parthenocarpy.
- Can promote flowering in certain plants under suitable conditions.
Fig. 4 – Gibberellin and Seed Germination
4. Cytokinins
DefinitionCytokinins are plant hormones that promote cytokinesis or cell division and influence shoot formation, nutrient mobilization and senescence.
Physiological Effects
- Stimulate cell division.
- Promote shoot-bud formation in appropriate tissues.
- Reduce apical dominance by encouraging lateral-bud growth.
- Delay leaf senescence and maintain chlorophyll for longer periods.
- Promote nutrient mobilization toward actively growing regions.
- Interact strongly with auxin during organ formation.
Fig. 5 – Auxin–Cytokinin Balance in Organ Formation
5. Comparison of Auxins, Gibberellins and Cytokinins
| Hormone | Major Effects | Typical Association |
|---|---|---|
| Auxins | Cell elongation, apical dominance, rooting, tropisms | Shoot tips, young leaves |
| Gibberellins | Stem elongation, dormancy breaking, germination, bolting | Young tissues, embryos, seeds |
| Cytokinins | Cell division, shoot formation, lateral buds, delayed senescence | Root tips, developing tissues |
6. Quick Revision & Exam Points
Important Questions
- Define plant hormones and state their general characteristics.
- Write the physiological effects of auxins.
- Explain apical dominance.
- How do auxins participate in phototropism?
- Write the physiological effects of gibberellins.
- Explain the role of gibberellins in seed germination.
- Write the physiological effects of cytokinins.
- Differentiate auxins, gibberellins and cytokinins.
- Auxin → cell elongation.
- Auxin → apical dominance.
- Auxin → root initiation.
- Gibberellin → stem elongation.
- Gibberellin → dormancy breaking.
- Gibberellin → seed germination.
- Cytokinin → cell division.
- Cytokinin → lateral bud/shoot growth.
- Cytokinin → delays senescence.
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