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Difficulty: MediumGrowth, Development, and Metamorphosis

A growing plant shoot tip is exposed to continuous unilateral light coming from the right side. Which of the following correctly describes the movement of auxins and the resulting physiological response of the shoot?

  1. Auxins migrate to the shaded left side, promoting greater cell elongation on the shaded side and causing the shoot to bend toward the right.Answer
  2. B
    Auxins accumulate on the illuminated right side, promoting faster cell elongation on that side and causing the shoot to bend away from the light.
  3. C
    Auxins are concentrated evenly on both sides, but light directly stimulates accelerated cell division exclusively on the illuminated right side.
  4. D
    Auxins are broken down on the shaded left side, causing cells on the illuminated side to grow longer and bend the shoot toward the left.

Answer

Auxins migrate to the shaded side, where high concentration stimulates cell elongation, causing the stem to curve toward the light source.
When a shoot tip receives light from one direction, auxins diffuse laterally from the illuminated side to the shaded side. In plant stems, higher auxin concentration stimulates cell elongation. Because cells on the shaded side grow longer than those on the lit side, the stem curves toward the light source (positive phototropism).

Step-by-Step Solution

1
Identify the stimulus and primary plant hormone involved in phototropism
Unilateral light acts as the stimulus, and auxins (indole-3-acetic acid) regulate the growth response.
Auxins are synthesized at the shoot apex and are light-sensitive in their distribution.
2
Determine the direction of auxin lateral movement
Auxin moves laterally away from the illuminated side toward the shaded side of the shoot.
Photoreceptors at the tip direct the transport of auxin to the darker region.
3
Relate auxin concentration to cellular response in shoots
Higher auxin concentration on the shaded side causes greater cell elongation compared to the illuminated side.
In shoots, higher auxin levels promote cell wall loosening and differential elongation, producing bending toward the light source.

Key Concept

Phototropism and Auxin Redistribution
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Topics

Growth, Development, and Metamorphosis

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