- Dark-grown seedlings display ‘triple response’ when exposed to ethylene. Which one of the fallowing is NOT a part of ‘triple response’?
(1) Decrease in epicotyl elongation.
(2) Rapid unfolding and expansion of leaves.
(3) Thickening of shoot.
(4) Horizontal growth of epicotyl.
Correct answer: (2) Rapid unfolding and expansion of leaves
Explanation:
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The ‘triple response’ in dark-grown seedlings exposed to ethylene consists of three characteristic morphological changes:
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Decrease in epicotyl (hypocotyl) elongation (shortening of the stem)
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Thickening of the shoot (radial swelling)
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Horizontal growth of the epicotyl (increased lateral expansion, causing the hypocotyl to grow horizontally rather than vertically)
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Rapid unfolding and expansion of leaves is not part of the triple response. In fact, leaves remain closed or unexpanded in dark-grown seedlings.
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This triple response helps seedlings push through the soil without damage before reaching light.
Option-wise:
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(1) Decrease in epicotyl elongation — part of triple response.
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(2) Rapid unfolding and expansion of leaves — NOT part of triple response.
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(3) Thickening of shoot — part of triple response.
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(4) Horizontal growth of epicotyl — part of triple response.
The ethylene-induced triple response consists of inhibited stem elongation, hypocotyl thickening, and horizontal growth in etiolated seedlings, facilitating soil emergence. Rapid leaf expansion does not occur as part of this response.
