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Difficulty: HardHormonal Coordination in Plants and Animals

An experimental setup involves exposing an intact oat coleoptile tip to unilateral light from the right. A thin, impermeable sheet of mica is inserted vertically halfway through the apex on the right (illuminated) side, leaving the left (shaded) side open for lateral transport. Which of the following best describes the resulting phototropic curvature of the shoot?

  1. The shoot bends toward the light because auxins migrate laterally to the unblocked shaded side, stimulating elongation on that side.Answer
  2. B
    The shoot remains completely straight because the mica insert blocks auxin accumulation on the illuminated side.
  3. C
    The shoot bends away from the light because auxins are concentrated entirely on the illuminated side above the mica barrier.
  4. D
    The shoot stops elongating because light stimulates auxin production only on the illuminated side of the tip.

Answer

The shoot bends toward the light because auxins migrate laterally to the unblocked shaded side, stimulating elongation on that side.
Unilateral light causes auxins to migrate laterally from the illuminated side to the shaded side of the tip. Placing a mica barrier on the illuminated side leaves the shaded side's transport pathway intact. Consequently, auxins accumulate on the shaded side, causing cells there to elongate faster than those on the illuminated side, bending the shoot toward the light source.

Step-by-Step Solution

1
Analyze the effect of unilateral illumination on auxin translocation in shoot tips.
Unilateral light causes auxins to move laterally from the illuminated side to the shaded side of the tip.
Auxins are light-sensitive in their transport pathways and accumulate in higher concentrations on the shaded side.
2
Evaluate the placement of the physical barrier (mica sheet).
The mica sheet on the illuminated side does not prevent auxin from migrating across to or moving down the shaded side.
The pathway for lateral migration to the shaded side and subsequent basipetal transport along the shaded side remains uninhibited.
3
Determine the growth response resulting from differential auxin distribution.
Higher auxin concentration on the shaded side causes greater cell elongation there, bending the tip toward the light source.
Auxins promote cell wall loosening and expansion in shoot cells proportionally to their concentration.

Key Concept

Lateral auxin translocation and asymmetric cell elongation in phototropism
Estimated Time:1m 30s
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Topics

Hormonal Coordination in Plants and Animals

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