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Difficulty: EasyHormonal Coordination in Plants and Animals

A plant seedling shoot exposed to light from one side bends towards the light source. Which of the following best explains how auxin causes this tropic curvature?

  1. A
    Auxin accumulates on the illuminated side, stimulating faster cell division on that side.
  2. Auxin migrates to the shaded side, stimulating cell elongation on the shaded side.Answer
  3. C
    Auxin is completely broken down by light on the shaded side, stopping cell growth there.
  4. D
    Auxin remains uniformly distributed throughout the stem while light inhibits shaded cell elongation.

Answer

Auxin migrates to the shaded side, stimulating cell elongation on the shaded side.
Under unilateral light exposure, auxin moves laterally from the illuminated side to the shaded side of the shoot tip. The resulting higher auxin concentration on the shaded side promotes differential cell elongation, causing the stem to curve toward the light source.

Step-by-Step Solution

1
Identify the stimulus and response in phototropism
Unilateral light causes positive phototropism (bending of shoot towards light).
The shoot responds to directional light by adjusting cell elongation.
2
Determine the effect of directional light on auxin movement
Auxin produced at the shoot tip moves laterally from the illuminated side to the shaded side.
Light triggers asymmetric lateral transport of auxin.
3
Relate auxin concentration to growth in shoots
The higher concentration of auxin on the shaded side speeds up cell elongation relative to the light side, resulting in bending towards the light source.
In shoots, higher auxin concentrations within physiological limits stimulate cell elongation.

Key Concept

Phototropism and Auxin Distribution
Estimated Time:45s
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Topics

Hormonal Coordination in Plants and Animals

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