Question
Compare the roles of gibberellin, cytokinin, and abscisic acid (ABA) in plant growth and development. Include their main functions, effects, and whether they are promoters or inhibitors.
Solution — Step by Step
Plant hormones (phytohormones) are chemical messengers that coordinate growth and development. Of the five major classes, three have contrasting roles:
- Gibberellins (GA): Growth promoters — focus on cell elongation and germination
- Cytokinins (CK): Growth promoters — focus on cell division and delay of ageing
- Abscisic Acid (ABA): Growth inhibitor — focus on stress response and dormancy
This promoter vs inhibitor distinction is the single most important concept for board exams.
Site of synthesis: Young leaves, root tips, developing seeds
Key roles:
- Stem elongation: Causes bolting in rosette plants (e.g., cabbage) by increasing internode length
- Seed germination: Breaks seed dormancy and stimulates amylase synthesis in barley seeds (starch → sugar for the embryo)
- Fruit development: Induces parthenocarpy (seedless fruits) — used in grapes to produce larger, seedless berries
- Delays senescence in leaves and citrus peel
Discovery note: First isolated from the fungus Gibberella fujikuroi, which causes “foolish seedling disease” in rice — infected plants grow abnormally tall because of excess GA.
Site of synthesis: Actively dividing tissues, root apex, developing fruits
Key roles:
- Promotes cell division (cytokinesis): Works with auxin — the cytokinin:auxin ratio determines whether a callus differentiates into shoots (high cytokinin) or roots (high auxin)
- Delays leaf senescence: Keeps chlorophyll intact and proteins from being degraded — this is the Richmond-Lang effect (leaves stay green longer when cytokinin is applied)
- Breaks apical dominance in some plants
- Promotes lateral bud growth
Commercial use: Applied to keep cut flowers fresh longer.
Site of synthesis: Chloroplasts, roots (under water stress), ageing leaves
Key roles:
- Stomatal closure: Under water stress, ABA triggers K⁺ efflux from guard cells → guard cells lose turgor → stomata close (reduces water loss)
- Seed dormancy: Maintains dormancy; counteracts GA during germination
- Promotes abscission (leaf and fruit fall) in some species — though ethylene is more important for abscission
- General stress response: ABA accumulates during drought, cold, and salinity stress — the “stress hormone”
Note: The name “abscisic acid” is somewhat misleading because its role in actual abscission is less significant than originally thought. Its primary role is in stress signalling and dormancy.
| Feature | Gibberellin | Cytokinin | Abscisic Acid |
|---|---|---|---|
| Type | Promoter | Promoter | Inhibitor |
| Primary effect | Cell elongation | Cell division | Stomatal closure, dormancy |
| Germination | Promotes | Promotes | Inhibits |
| Senescence | Delays | Delays | Promotes |
| Fruit development | Induces parthenocarpy | Helps | Promotes abscission |
| Stress response | Minimal | Minimal | Major role |
| Synthesised in | Root tips, seeds | Root apex | Chloroplasts, roots |
Why This Works
These three hormones represent a balance system. GA and cytokinins push the plant toward growth and activity; ABA acts as a brake during stress or unfavourable conditions. Plants respond to environmental signals by adjusting the ratio of these hormones.
For example, during drought: ABA increases → stomata close → water loss reduced. When rain returns: ABA decreases, GA rises again, growth resumes. The interplay between promoters and inhibitors lets plants be both responsive and resilient.
Alternative Method
You can remember these using mnemonics:
- Gibberellin = Growth in length (elongation)
- Cytokinin = Cytokinesis (cell division) + “keeps cells young”
- ABA = “A-Bscisic Acid = Anti-growth / Adversity response”
Common Mistake
Students often state that ABA causes abscission. While this was historically believed, current understanding shows ethylene is the primary hormone for abscission, not ABA. ABA’s main roles are stomatal closure and dormancy induction. Writing “ABA causes leaf fall” in NEET or CBSE Class 11 exams will cost marks.