Gravitropism & Phototropism: Plants on the Move

 

What are Tropisms?

• Tropisms are directional growth responses in plants where parts of a plant grow towards or away from a stimulus.

 

Type of Tropism	Stimulus	Direction of Growth	Negative response	Positive response
Phototropism	Light	Towards (positive) or away (negative) from light.	Grows away from the light, i.e roots	Grows towards from the light, i.e shoots
Gravitropism	Gravity	Roots grow towards gravity (positive); shoots grow away (negative).	Grows away from gravity i.e shoots	Grows towards from the gravity, i.e roots

 

Analogy:
Think of tropisms as plants "choosing" to grow where conditions are best for survival, like seeking sunlight or growing deep roots for stability.

 

Phototropism: Growing Towards the Light 

• What Happens?
  Shoots of plants grow towards light (positive phototropism) to maximize photosynthesis.

• How It Works:

  1. Light hits the shoot unevenly.
  2. Auxin (a plant growth hormone) accumulates on the shaded side.
  3. Auxin causes cells on the shaded side to elongate.
  4. The shoot bends towards the light.

Key Role of Auxin
Auxin is produced in the shoot tip and diffuses downward. It stimulates cell elongation on the shaded side.

 

Gravitropism: Responding to Gravity 

• What Happens?

  • Roots grow downwards (positive gravitropism) to anchor the plant and absorb water/minerals.
  • Shoots grow upwards (negative gravitropism) to access light for photosynthesis.

• How It Works:

  1. Gravity causes auxin to accumulate on the lower side of roots and shoots.
  2. In roots, high auxin concentration inhibits cell growth, causing the upper side to grow faster, and the root bends downward.
  3. In shoots, high auxin concentration stimulates cell growth, causing the lower side to elongate and the shoot to bend upward.

 

Auxin’s Role in Tropisms

Action	Effect in Shoots	Effect in Roots
High auxin concentration	Stimulates cell elongation.	Inhibits cell elongation.
Unequal auxin distribution	Causes bending towards light or away from gravity.	Causes bending towards gravity.

Quick Fact: Auxin is made in the shoot tip and spreads unevenly in response to light or gravity.

 

Investigating Tropisms: Experiments

1. Phototropism Experiment:

   • Place potted plants near a unidirectional light source.
   • Observe that shoots bend towards the light.

2. Gravitropism Experiment:

   • Place germinating seeds horizontally.
   • Observe that roots grow downward, and shoots grow upward.

Explanation:
Both experiments demonstrate the uneven distribution of auxin, causing differential growth.