These NCERT Notes on the Functions of Auxin, Gibberellin, and Cytokinin provide a clear and concise explanation of the major plant growth regulators, their functions, and their importance in plant development. The notes are ideal for Class 9 and Class 11 Biology students, as well as NEET, CUET, and other competitive exam aspirants following the NCERT syllabus. Written in a simple, exam-oriented style, they help students understand key concepts quickly and revise them effectively before examinations.
Auxin, Gibberellin and Cytokinin
Plant growth and development are controlled by special chemical substances called Plant Hormones (Phytohormones). These hormones are produced in very small amounts but have a major effect on growth, development, reproduction and responses to environmental conditions.
Among the important plant hormones, Auxin, Gibberellin and Cytokinin play a major role.
What are Plant Hormones?
Plant hormones are chemical messengers produced in plants that regulate:
- Growth
- Cell division
- Cell elongation
- Flowering
- Fruit development
- Dormancy
- Aging of plants
1. Auxin – Functions
What is Auxin?
Auxin is a growth-promoting plant hormone mainly produced in:
- Shoot apex
- Young leaves
- Developing seeds
Example:
- Indole Acetic Acid (IAA)
Auxin mainly controls cell elongation.
Functions of Auxin
1. Promotes Cell Elongation
- Auxin increases the length of cells.
- Responsible for growth of stems.
Example:
- Young shoots grow upward due to cell elongation.
2. Controls Phototropism
- Plant bends toward light.
- Auxin accumulates on the shaded side.
- Cells elongate more on that side.
Example:
- Stem bending toward sunlight.
3. Controls Geotropism
- Roots and shoots respond to gravity.
- Auxin distribution helps directional growth.
4. Initiates Root Formation
- Stimulates development of roots.
- Used in vegetative propagation.
Example:
- Stem cuttings develop roots.
5. Prevents Early Leaf and Fruit Drop
- Delays abscission (falling of leaves and fruits).
6. Promotes Fruit Development
- Helps fruit growth after fertilization.
7. Can Produce Seedless Fruits (Parthenocarpy)
Examples:
- Tomato
- Cucumber
2. Gibberellin – Functions
What is Gibberellin?
Gibberellins are growth hormones mainly responsible for:
- Stem elongation
- Seed germination
- Flowering
Example:
- Gibberellic Acid (GA₃)
Functions of Gibberellin
1. Stimulates Stem Elongation
- Increases internode length.
- Makes plants taller.
Example:
- Rapid shoot growth.
2. Breaks Seed Dormancy
- Activates inactive seeds.
- Initiates germination.
3. Promotes Seed Germination
- Stimulates enzyme production.
- Stored food becomes available.
4. Induces Bolting
Bolting:
- Rapid elongation of stem before flowering.
Examples:
- Cabbage
- Beet
5. Promotes Flowering
- Helps flowering in some long-day plants.
6. Increases Fruit Size
Examples:
- Grapes
- Apples
7. Delays Senescence in Some Cases
- Keeps tissues active for longer.
3. Cytokinin – Functions
What is Cytokinin?
Cytokinins are plant hormones responsible mainly for:
- Cell division
- Growth regulation
- Delaying aging
Produced mainly in:
- Root tips
- Developing fruits
- Seeds
Example:
- Zeatin
Functions of Cytokinin
1. Promotes Cell Division
- Main function of cytokinin.
- Helps formation of new cells.
2. Promotes Growth of Lateral Buds
- Reduces apical dominance.
- Encourages branching.
3. Delays Senescence (Anti-aging Effect)
- Leaves remain green for longer.
4. Promotes Nutrient Mobilization
- Directs nutrients toward growing tissues.
5. Works Together with Auxin
- Auxin + Cytokinin regulate tissue development.
6. Helps in Tissue Culture
- Used for shoot formation in laboratories.
Comparison Table – Auxin vs Gibberellin vs Cytokinin
| Feature | Auxin | Gibberellin | Cytokinin |
|---|---|---|---|
| Main Function | Cell elongation | Stem elongation | Cell division |
| Produced In | Shoot tip | Young tissues | Root tip |
| Root Formation | Yes | No | Indirect |
| Seed Germination | No | Yes | No |
| Delays Aging | Limited | Sometimes | Strong |
| Branching | Low | Moderate | High |
NCERT Key Points (Quick Revision)
- Auxin → Cell elongation and tropic movements
- Gibberellin → Stem elongation and seed germination
- Cytokinin → Cell division and delayed aging
- Hormones work together to regulate growth
- Very small amounts can produce major effects
Practice Questions with Answers
Q1. Which hormone promotes cell elongation?
Answer: Auxin
Q2. Which hormone breaks seed dormancy?
Answer: Gibberellin
Q3. Which hormone delays leaf aging?
Answer: Cytokinin
Q4. Where is cytokinin mainly produced?
Answer: Root tips
Q5. What is parthenocarpy?
Answer: Development of fruit without fertilization.
Fill in the Blanks
- Auxin mainly promotes __________.
Answer: Cell elongation - Gibberellin helps in __________ germination.
Answer: Seed - Cytokinin promotes __________ division.
Answer: Cell - Root tips produce __________.
Answer: Cytokinin - Stem elongation is mainly caused by __________.
Answer: Gibberellin
MCQ Questions (With Answers)
1. Which hormone promotes phototropism?
A. Cytokinin
B. Auxin
C. Ethylene
D. ABA
Answer: B
2. Gibberellin mainly promotes:
A. Root cutting
B. Stem elongation
C. Dormancy
D. Leaf fall
Answer: B
3. Cytokinin is known for:
A. Cell division
B. Fruit fall
C. Dormancy
D. Root death
Answer: A
4. Which hormone helps parthenocarpy?
A. Auxin
B. Ethylene
C. ABA
D. Cytokinin
Answer: A
5. Bolting is caused by:
A. Cytokinin
B. Gibberellin
C. Auxin
D. Ethylene
Answer: B
6. Zeatin is a:
A. Auxin
B. Cytokinin
C. ABA
D. Gibberellin
Answer: B
7. Which hormone delays senescence?
A. Cytokinin
B. Auxin
C. ABA
D. Ethylene
Answer: A
8. Auxin is mainly synthesized in:
A. Root tip
B. Shoot apex
C. Stem base
D. Flower
Answer: B
9. Seed dormancy is broken by:
A. Gibberellin
B. Auxin
C. Ethylene
D. Cytokinin
Answer: A
10. Cytokinin encourages:
A. Branching
B. Leaf fall
C. Dormancy
D. Root drying
Answer: A
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