Plant Growth Regulators · NCERT Biology

The 'triple response' in seedlings (shortening of the axis, thickening of the axis, and horizontal growth) is a characteristic effect of:

  1. Auxin
  2. Cytokinin
  3. Ethylene
  4. Gibberellin
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Correct answer: Ethylene

Ethylene induces a specific growth pattern in dicot seedlings known as the triple response. This includes inhibition of stem elongation, stimulation of radial swelling, and horizontal growth/apical hook formation.

Difficulty: Medium Question 9 of 20

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