🌱 Plant Hormones & Tropisms (Higher Tier)

1⃣ Phototropism (Growth towards light)

• Definition: Shoots grow towards light.

• Hormone involved: Auxin (produced in the shoot tip).

• How it works:

  1. Auxin is produced at the tip of the shoot.

  2. Light causes auxin to concentrate on the darker side of the shoot.

  3. Auxin spreads down the shoot → cells on the darker side grow faster.

  4. Shoot bends towards the light.

• Key experiments:

  • Removing shoot tips → no growth towards light → tips produce auxin.

  • Covering tips with foil → no growth → tips are light-sensitive.

  • Covering lower shoot → shoots still bend → lower shoot not light-sensitive.

2⃣ Gravitropism / Geotropism (Growth in response to gravity)

• Definition: Roots grow towards gravity.

• Hormone involved: Auxin (produced in roots).

• How it works:

  1. Gravity causes auxin to concentrate on the lower side of the root.

  2. In roots, auxin inhibits cell growth.

  3. Lower side grows slower → root bends downwards.

3⃣ Other Plant Hormones (Higher Tier)

• Gibberellins: Trigger seed germination.

• Ethene (ethylene): Controls cell division and fruit ripening.

💡 Key idea: Auxin can promote growth in shoots but inhibit growth in roots, allowing plants to respond to both light and gravity.