Plant Hormones

• Plant hormones (phytohormones) — auxin, gibberellins, cytokinins, abscisic acid, ethylene.
• Auxin — cell elongation; phototropism and geotropism.
• Gibberellins — stem elongation, seed germination.
• Cytokinins — cell division, delay senescence.
• Abscisic acid (ABA) — stress hormone; stomatal closure; dormancy.

1. Auxin — synthesised at shoot tip; uneven distribution causes bending toward light (phototropism).
2. Geotropism — roots grow down (positive geotropism); shoots up (negative).
3. Gibberellins — increase stem length; break seed dormancy.
4. Cytokinins — promote lateral bud growth; used in tissue culture.
5. Abscisic acid — inhibits growth; closes stomata in drought.
6. Ethylene — gaseous; fruit ripening (banana ripening with ripe fruit).
7. Tropic movements — growth-dependent; permanent bending.
8. Nastic movements — non-directional (e.g. touch-me-not folding — turgor change, not hormone main).
9. Commercial use — auxin rooting powder; ethylene for uniform ripening.
10. NCERT experiment — auxin diffuses away from light side → faster growth on dark side → bend.

Phototropism in shoot — role of auxin

Step 1 — Light from one side; auxin moves to shaded side.
Step 2 — Higher auxin on shaded side → more cell elongation there.
Step 3 — Shoot bends toward light source.
Step 4 — Root tip auxin distribution different — governs geotropism.
Conclusion: auxin uneven distribution causes tropic curvature.

• Applying animal hormone names to plants (insulin not in plants).
• Misconception: plants have no coordination (phytohormones coordinate growth).
• Confusing ethylene with auxin for bending (ethylene = ripening).
• Thinking ABA promotes growth (it inhibits).
• Mixing tropic (directional growth) with nastic (turgor).

• Which hormone causes phototropism?
• Function of abscisic acid?
• How does ethylene affect fruits?
• Name one plant hormone that promotes cell division.

Open the Practice tab for graded questions on Plant Hormones.