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Question

Difficulty: MediumHormonal Coordination in Plants and Animals

When a seedling coleoptile is exposed to unilateral light, it bends towards the light source. Which of the following physiological processes accounts for this phototropic curvature?

  1. Lateral migration of auxin to the shaded side, causing greater cell elongation on that sideAnswer
  2. B
    Accumulation of auxin on the illuminated side, causing accelerated cell division on the lit side
  3. C
    Direct destruction of auxin on the shaded side by darkness, slowing growth on the unlit side
  4. D
    Uniform distribution of auxin throughout the tip with increased cell division on the shaded side

Answer

Lateral migration of auxin to the shaded side, causing greater cell elongation on that side
Under directional lighting, auxin is transported laterally to the shaded side of the shoot tip. This asymmetrical accumulation causes cells on the shaded side to elongate faster than those on the illuminated side, causing the shoot to bend towards the light source.

Step-by-Step Solution

1
Analyze the perception of unilateral light by the coleoptile tip.
Unilateral light is detected by photoreceptors at the tip of the coleoptile.
The shoot tip acts as the primary site of light perception during phototropism.
2
Trace the lateral movement and redistribution of auxin.
Auxin (indole-3-acetic acid) translocates laterally from the light-exposed side toward the shaded side.
Photoreceptors induce an asymmetrical lateral transport of auxin away from the illuminated region.
3
Determine the cellular effect of unequal auxin distribution in stem tissues.
The higher concentration of auxin on the shaded side stimulates greater cell elongation compared to the illuminated side, causing the stem to curve toward the light.
In shoot tissues, higher concentrations of auxin promote cell wall loosening and cell elongation.

Key Concept

Phototropism and Auxin Redistribution
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Topics

Hormonal Coordination in Plants and Animals

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