Plant Hormones and Tropisms

PLANT HORMONES

Plants exhibit bending or movement in response to stimuli like gravity or light.
These responses, known as tropisms, are generally slower compared to animals.
Plants grow either towards a stimulus (positive response) or away from it (negative response).

Plants use a growth hormone called auxin to control the direction of root or stem growth in response to stimuli.
Auxin is primarily produced in the tips of growing stems and roots.
It diffuses to other areas, moving from a high concentration in the shoot tips to areas of lower concentration.
Auxin promotes cell elongation (enlargement) in the region behind the tip, known as the meristem.
Higher auxin concentrations lead to faster elongation and growth.
Plant growth stimulated by auxin occurs through the enlargement of existing cells, not through cell division to create new cells.

Phototropism explained: When light comes mostly from one direction, auxin from the tip moves to the shaded side, causing cells there to elongate faster than those on the lit side.
Unequal growth: This leads to the shoot bending and growing towards the light.
If light is evenly distributed around the tip, auxin spreads evenly, and the shoot grows straight without bending.

Auxin is synthesized in the tips of the shoots.
In the presence of directional light, auxin diffuses away from the light towards the shaded side.
This uneven distribution of auxin results in uneven growth.
Auxin increases cell growth in the meristem, causing faster elongation on the shaded side compared to the side exposed to light.

Occurs in roots.
Higher auxin concentrations result in a lower rate of cell elongation.
Auxin accumulation on the lower side of the root inhibits cell elongation.
The lower side grows slower than the upper side, causing the root to bend downwards.
Advantages: This leads to enhanced access to water and minerals, as well as improved anchorage in the soil.

Occurs in shoots.
When a plant is placed horizontally, auxin diffuses to the lower region of the shoot causing it to bend upwards.
Advantages:
- Increased access to light and, consequently, increased photosynthesis.
- More access to CO_2, improving the rate of photosynthesis.
- Greater access to O_2, enhancing the rate of respiration.
- Improved access to pollinators.
- Movement away from potential consumers.

Definition: A clinostat is a device that slowly rotates plants to negate directional stimuli, such as light and gravity.
Purpose: Used to study plant growth without external directional influences and serves as a control experiment

Setup A (Control):
- Seedlings are placed in a petri dish with moist cotton wool, positioned on its side in complete darkness.
- Each seedling's radicle (developing root) and plumule (developing shoot) point in different directions.
Setup B (Clinostat):
- Similar setup to A but placed on a clinostat, rotating to negate gravitropism.
Observations after 2+ Days:
- Setup A:
  - All radicles grew downwards (positive gravitropic response).
  - All plumules grew upwards (negative gravitropic response).
- Setup B:
  - Radicles and plumules continued to grow in whichever direction they were initially placed.
  - The effect of gravitropism was cancelled out by the rotating clinostat.

Experiments demonstrate that auxins can pass through a semipermeable material to cause phototropism.
If the tip is cut off or blocked by an opaque object, auxins cannot diffuse, thus inhibiting the expected growth response.