Question

A growing shoot of a germinating seedling encounters an underground obstacle. Which one of the following hormones elicits ‘triple response’ to the underground obstacle?

• A. Auxin
• B. Cytokinin
• C. Ethylene
• D. Gibberellins

Answer 1

The Correct Option is C

To understand which hormone elicits the 'triple response' in a germinating seedling encountering an underground obstacle, we need to explore the characteristics of plant hormones.

The 'triple response' is a growth maneuver that helps a seedling navigate around obstacles in the soil. This response is characterized by three main changes in the plant's growth pattern:

1. Reduction in elongation: The elongation of the stem is reduced, which allows the plant to conserve its energy and resources when it faces resistance from the soil.
2. Radial swelling: The stem swells, or becomes thicker, which increases its mechanical strength and helps push through the soil.
3. Horizontal growth: The direction of the stem growth changes from vertical to horizontal, helping the seedling grow around the obstacle.

Among the given options, ethylene is the hormone known for inducing the 'triple response.' Ethylene is a gaseous plant hormone involved in various processes such as fruit ripening, leaf abscission, and responses to mechanical stress.

Let's rule out the other options provided:

• Auxin: Primarily involved in cell elongation and growth. It helps with the directional growth of the plant towards light (phototropism) or against gravity (gravitropism).
• Cytokinin: Promotes cell division and is involved in various growth and differentiation processes but does not induce a 'triple response.'
• Gibberellins: Primarily promote stem elongation, seed germination, and flowering, rather than inducing a restraining response like the 'triple response.'

Thus, the correct answer to the question is Ethylene.