Plant Hormones and Growth
~7 min read
- Five major: Auxin (apical dominance), Gibberellin (stem elongation), Cytokinin (cell division), Abscisic acid (growth inhibitor), Ethylene (fruit ripening).
- Not a plant hormone: Thyroxine — that's an animal hormone.
- Practical: Cytokinin promotes cell division; abscisic acid INHIBITS growth (stress hormone).
Plant hormones (phytohormones) coordinate plant growth in response to light, gravity, and stress. CDS/OTA asks about hormone-function pairs.
The Five Major Plant Hormones
| Hormone | Chemical type | Main effect |
|---|---|---|
| Auxin (IAA) | Indole compound | Cell elongation; apical dominance; phototropism (bends shoot toward light) |
| Gibberellin (GA) | Terpene | Stem elongation; breaks seed dormancy; bolting |
| Cytokinin | Adenine derivative | Promotes cell division; delays leaf senescence |
| Abscisic acid (ABA) | Terpene | INHIBITS growth; closes stomata in drought; induces dormancy |
| Ethylene | Gas (CH₂=CH₂) | Ripens fruits; ageing; abscission (leaf fall) |
Chemistry of Plant Hormones
- Plant hormones are small, simple molecules of diverse chemical composition — not large proteins like animal hormones.
- They include indole compounds (auxin), adenine derivatives (cytokinin), carotenoids and terpenes (gibberellin, ABA).
- Thyroxine is an animal hormone (from thyroid) — it is NOT a plant hormone.
- Plant hormones often work in opposing pairs (e.g. cytokinin promotes growth; ABA inhibits it).
Practical Applications
- Auxins (NAA, IBA): rooting powder for cuttings; herbicide (2,4-D kills broadleaf weeds).
- Gibberellins: increase fruit size in seedless grapes; bolting of vegetables.
- Cytokinins: used in tissue culture to multiply plant shoots.
- Ethylene: commercially used to ripen bananas, mangoes uniformly.
- ABA: spray crops with ABA to close stomata and reduce water loss in drought.
CDS/OTA PYQ Examples
Q: Plant growth regulators are small simple molecules of diverse chemical composition. They are:
(a) carbohydrates, fats and proteins (b) indole compounds, adenine derivatives, carotenoids and terpenes (c) fatty acids, glucose and vitamins (d) vitamin C, D and glucose
Answer: (b) Indole compounds, adenine derivatives, carotenoids and terpenes. [CDS-I 2017]
Q: Which plant hormone would you suggest to increase cell division?
(a) Abscisic acid (b) Gibberellins (c) Cytokinin (d) Auxin
Answer: (c) Cytokinin — promotes cell division. [CDS-I 2021]
Q: Which hormone inhibits growth activity in plants?
(a) Auxins (b) Cytokinins (c) Abscisic acid (d) Gibberellins
Answer: (c) Abscisic acid — the 'stress hormone' that closes stomata and induces dormancy. [CDS-II 2024]
Q: Which one is NOT a plant hormone?
(a) Gibberellins (b) Abscisic acid (c) Auxins (d) Thyroxine
Answer: (d) Thyroxine — that's an animal thyroid hormone. [CDS-I 2025]
Drill Plant Hormones and Growth for CDS/OTA
CDS/OTA-pattern items on Plant Hormones and Growth with answer keys and explanations.
Start Free Mock TestFrequently Asked Questions
Why do plants bend toward light?
Auxin redistributes to the shaded side of the shoot, where it stimulates more cell elongation. The shaded side grows longer than the lit side, bending the shoot toward light (positive phototropism).
How does ripe fruit make other fruit ripen?
A ripening fruit releases ethylene gas, which signals nearby fruit to ripen too. This is why one bad apple ruins the bunch — and why you put unripe avocados near a banana to speed them up.
What is apical dominance?
Auxin from the shoot apex suppresses growth of side buds. Removing the tip (pinching) releases the side buds and produces a bushier plant — a common gardening technique.