74. 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:

• The ‘triple response’ in dark-grown seedlings exposed to ethylene consists of three characteristic morphological changes:

  • Decrease in epicotyl (hypocotyl) elongation (shortening of the stem)

  • Thickening of the shoot (radial swelling)

  • Horizontal growth of the epicotyl (increased lateral expansion, causing the hypocotyl to grow horizontally rather than vertically)

• Rapid unfolding and expansion of leaves is not part of the triple response. In fact, leaves remain closed or unexpanded in dark-grown seedlings.

• This triple response helps seedlings push through the soil without damage before reaching light.

Option-wise:

• (1) Decrease in epicotyl elongation — part of triple response.

• (2) Rapid unfolding and expansion of leaves — NOT part of triple response.

• (3) Thickening of shoot — part of triple response.

• (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.