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Difficulty: MediumHormonal Coordination in Plants and Animals

An experiment was set up to observe the growth response of a young plant shoot exposed to unilateral illumination from the left side. Which of the following physiological processes accounts for the bending of the shoot toward the light source?

  1. Auxins migrate laterally from the illuminated side to the shaded side, causing greater cell elongation on the shaded side.Answer
  2. B
    Auxins accumulate preferentially on the illuminated side, stimulating accelerated cell division on the side facing the light.
  3. C
    Gibberellins are destroyed on the shaded side, preventing cell expansion away from the light source.
  4. D
    Abscisic acid accumulates on the illuminated side to actively pull the stem cells toward the light direction.

Answer

Auxins migrate laterally from the illuminated side to the shaded side, causing greater cell elongation on the shaded side.
In positive phototropism of plant shoots, light falling from one side causes auxins produced at the apex to translocate laterally to the shaded side. The increased concentration of auxin on the shaded side causes those cells to elongate faster than the cells on the illuminated side, causing the shoot to curve toward the light source.

Step-by-Step Solution

1
Identify the primary plant hormone responsible for tropic responses in stem tips.
Auxins (specifically indole-3-acetic acid, IAA) control stem elongation and phototropism.
Auxins regulate cell wall extensibility and cell elongation in response to directional environmental stimuli.
2
Determine the distribution of auxins under unilateral lighting conditions.
Auxins move laterally away from the illuminated side toward the shaded side of the shoot tip.
Light triggers the lateral translocation of auxin transport proteins, directing auxin to the shaded region.
3
Analyze the differential growth effect resulting from this auxin distribution.
Cells on the shaded side elongate faster than cells on the illuminated side, producing an asymmetrical growth curvature toward light.
Higher auxin concentration in stems promotes elongation; asymmetrical elongation causes the shoot tip to bend toward the light.

Key Concept

Phototropism and Auxin Redistribution
Estimated Time:1m 0s
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Topics

Hormonal Coordination in Plants and Animals

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