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Question

Difficulty: EasyHormonal Coordination in Plants and Animals

During positive phototropism, a plant shoot curves toward a unilateral light source. Which of the following best describes the physiological distribution and effect of auxin responsible for this curvature?

  1. Auxin accumulates on the shaded side of the shoot, stimulating greater cell elongation on that sideAnswer
  2. B
    Auxin accumulates on the illuminated side of the shoot, stimulating faster cell division on the lit side
  3. C
    Auxin is completely broken down on the shaded side, causing cell expansion to stop on that side
  4. D
    Auxin remains uniformly distributed throughout the stem, while light directly accelerates cell growth on the lit side

Answer

Auxin accumulates on the shaded side of the shoot, stimulating greater cell elongation on that side
Unilateral light perception leads to the lateral transport of auxin away from the light, resulting in a higher concentration of auxin on the shaded side of the shoot. In plant stems, higher auxin levels stimulate cell elongation. The resulting higher rate of cell elongation on the shaded side causes the stem to bend toward the light source.

Step-by-Step Solution

1
Identify the signal stimulus and hormone involved in phototropism
Unilateral light triggers phototropism mediated by the plant growth regulator auxin (indole-3-acetic acid).
Shoot tips synthesize auxin, which moves downwards and responds to directional light stimuli.
2
Determine the pattern of hormone redistribution across the stem tip
Auxin moves laterally from the illuminated side to the shaded side of the shoot tip.
Photoreceptors in the tip detect light direction and induce lateral auxin transport toward the dark side.
3
Analyze the cellular effect of unequal hormone concentration
Cells on the shaded side undergo greater elongation than cells on the illuminated side, producing differential growth.
In shoots, higher auxin concentrations promote cell elongation, forcing the stem to bend toward the light source.

Key Concept

Hormonal Regulation of Phototropism in Plants
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Topics

Hormonal Coordination in Plants and Animals

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