Question

Which of the following plant growth regulators promotes internode elongation prior to flowering in cabbage ?

• A. Ethephon
• B. Abscisic acid
• C. Gibberellin
• D. Indole butyric acid

Hint:

To remember the primary function of Gibberellins, associate the word "Gibberellin" directly with "Giant growth" and "Bolting". Whenever you see keywords like "internode elongation in rosette plants", "cabbage", or "bolting prior to flowering", the correct answer is invariably Gibberellin ($\text{GA}_3$).

Answer 1

The Correct Option is C

Concept: Plant Growth Regulators (PGRs) are organic compounds other than nutrients that influence physiological processes in plants at very low concentrations. Gibberellins (GA) form a vital group of such growth-promoting hormones. One of their most characteristic physiological effects is the elongation of internodes in intact plants, especially in rosette plants like cabbage, beet, and many genetic dwarfs. The phenomenon of sudden elongation of internodes followed immediately by flowering is known as

bolting.

Step 1: Understanding the physiological mechanism of bolting.
Rosette plants exhibit a growth habit characterized by a condensed stem with crowded leaves close to the soil level due to negligible internodal growth. Prior to the reproductive phase or under specific environmental cues (like long days or cold treatment), these plants undergo rapid axial elongation of the internodes. Under natural conditions, this is controlled by endogenous gibberellins. When gibberellin is artificially applied to rosette plants like cabbage, it triggers rapid cell division and cell elongation in the sub-apical meristem zone of the stem, prompting dramatic internodal lengthening (bolting) just before the flowering process initiates.

Step 2: Evaluating the other options to understand why they are incorrect.
Let us systematically evaluate the physiological roles of the other listed options:

• (1) Ethephon: Ethephon is an aqueous solution that readily breaks down into ethylene, a gaseous plant growth regulator. Its major functions include promoting fruit ripening, enhancing abscission of leaves and flowers, breaking seed/bud dormancy, and initiating flowering in a few specific crops like pineapple, but it does not cause stem or internode bolting.

• (2) Abscisic acid (ABA): Commonly designated as the stress hormone or growth inhibitor, ABA plays an essential role in lateral bud dormancy, seed development, maturation, and stomatal closure under drought conditions. It acts antagonistically to gibberellins and suppresses growth rather than elongating internodes.

• (4) Indole butyric acid (IBA): This is a naturally occurring auxin. Auxins primarily regulate apical dominance, promote cell division in the cambium, initiate root development in stem cuttings, and prevent premature fruit and leaf drop. They do not induce the characteristic bolting response observed in rosette plants.
Thus, Gibberellin is uniquely responsible for promoting internode elongation prior to flowering in cabbage.