What Are Tropisms?

Plants respond to stimuli in their environment using tropisms, which involve differential cell growth and elongation:

• Phototropism: response to light.
• Geotropism (Gravitropism): response to gravity.

Both are controlled by the plant hormone IAA (indoleacetic acid), a type of auxin. Other examples of tropisms include hydrotropism (water) and chemotropism (chemical gradient).

Plants respond to directional stimuli by regulating growth.

Step-by-Step Process:

1. Light is detected by photoreceptors, usually on the shoot tip.
2. IAA is produced in the tip of the shoot and diffuses down the shoot.
3. IAA accumulates on the shaded side of the shoot.
4. IAA stimulates cell elongation on the shaded side by increasing the stretchiness of cell walls.
5. The shaded side grows faster, causing the shoot to bend towards the light.

Roots and shoots respond differently to gravity.

Shoots:

• IAA accumulates on the lower side due to gravity.
• IAA promotes cell elongation: shoot bends upwards (negative geotropism).

Roots:

• IAA accumulates on the lower side.
• High concentrations of IAA Inhibits cell elongation in roots.
• Upper side grows faster: root bends downwards (positive geotropism).

These classic experiments show how IAA mediates phototropic responses in plants:​

1. Darwin’s Experiment (1880) – Identifying the Site of Light Perception

Aim: To determine which part of the plant detects light leading to phototropism.

Method:

• Observed young shoots of grass seedlings.
• Applied opaque caps to the tips of some shoots (C), while others had their bases covered (E).​
• Also used multiple control experiments: tip not removed (A), tip removed (B) and clear cap on the tip (D). These demonstrate the normal phototropic response and that the tip is responsible.
  

Observations:

• Shoots with opaque covered tips did not bend towards light.
• Those with covered bases still exhibited phototropic bending.
  

Conclusion:

• The tip of the shoot is responsible for detecting light, initiating a signal that causes bending in the lower part.

2. Boysen-Jensen’s Experiment (1913) – Demonstrating a Diffusible Signal

Aim: To investigate whether the signal from the tip to the growth region is a diffusible substance.

Method:

• Removed the tip of the shoot and replaced it with:

  • A block of gelatin (permeable to chemicals).
  • A sheet of mica (impermeable to chemicals).

Then place the tip back onto the shoot.

Observations:

• Shoots with gelatin between the tip and the stump still bent towards light.
• Those with mica did not exhibit phototropic bending.
  

Conclusion:

• A chemical signal (later identified as IAA) diffuses from the tip to the growth region to induce bending.​

3. Frits Went’s Experiment (1928) – Isolating and Demonstrating the Effect of IAA

Aim: To isolate the growth-promoting chemical from the shoot tip and demonstrate its effect

Method:

• Removed shoot tips and placed them on agar blocks to allow diffusion of the chemical into the agar.
• Placed these agar blocks:
  
  • Centrally on decapitated shoots.
  • Off-center on decapitated shoots.

Observations:

• Shoots with centrally placed agar blocks grew straight.
• Those with off-center blocks bent away from the side with the block.​
  

Conclusion:

• The chemical (auxin/IAA) diffuses into the agar and, when applied asymmetrically, accumulates on one side, causing differential growth leading to bending.