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Difficulty: MediumGrowth, Development, and Metamorphosis

When a growing dicotyledonous seedling stem is exposed to light coming from a single direction (unilateral illumination), the stem curves towards the light source. Which physiological mechanism explains this directional growth response?

  1. Auxin migrates away from the illuminated side to the shaded side, causing cells on the shaded side to elongate faster.Answer
  2. B
    Auxin accumulates on the illuminated side, stimulating rapid cell division on the side facing the light.
  3. C
    Light breaks down auxin on the shaded side, inhibiting cell enlargement on that side.
  4. D
    Auxin distribution remains equal on both sides, but light directly increases the rate of cell wall synthesis on the lit side.

Answer

Auxin migrates away from the illuminated side to the shaded side, causing cells on the shaded side to elongate faster.
In stem tips, unilateral light induces the lateral transport of auxin to the shaded side. The higher concentration of auxin on the shaded side causes cells in that region to elongate more rapidly than those on the illuminated side, leading to differential growth that bends the stem toward the light.

Step-by-Step Solution

1
Identify the growth hormone responsible for shoot elongation in response to light stimuli.
Auxin (indole-3-acetic acid) regulates cell elongation in plant shoot tips.
Shoot phototropism is driven by asymmetric distribution of plant growth substances.
2
Analyze how unilateral light alters the spatial distribution of auxin in the shoot apex.
Unilateral light causes auxin to move laterally from the lit side to the shaded side.
Phototropin photoreceptors detect light direction and trigger lateral auxin translocation.
3
Determine the effect of higher auxin concentration on the shaded side cells.
Higher auxin concentration accelerates cell elongation on the shaded side relative to the illuminated side.
Differential elongation rates cause the stem to curve toward the light source.

Key Concept

Phototropism and Auxin Redistribution in Plant Growth
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Topics

Growth, Development, and Metamorphosis

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