Q52. The structures of four plant hormones are shown. Identify the CORRECT
hormone that is responsible for bending of coleoptile of canary grass in
response to unidirectional white light.

Auxin is the plant hormone responsible for the bending of the canary grass coleoptile in response to unidirectional white light, known as phototropism. This classic experiment by Charles Darwin and his son demonstrated auxin’s role in asymmetric growth.​

Question Overview

The query shows structures of four plant hormones labeled (A), (B), (C), and (D). It asks to identify the correct one causing coleoptile bending in canary grass (Phalaris canariensis) under unilateral white light, a key CSIR NET Life Sciences topic on plant growth regulators and tropisms.​

Option Analysis

• (A): Indole-3-acetic acid (IAA), the primary auxin. Features an indole ring fused to a pyrrole with -CH₂COOH side chain. Matches the hormone for phototropism, as light causes auxin redistribution to the shaded side, promoting cell elongation and bending.​

• (B): Likely gibberellin (e.g., GA3). Shows a complex tetracyclic diterpenoid with lactone ring, hydroxyls, and carboxylic acid. Promotes stem elongation but not directional bending via light; involved in internode growth, seed germination.​

• (C): Likely abscisic acid (ABA). Displays a sesquiterpene with cyclohexenone ring, hydroxyls, and carboxylic acid. Inhibits growth, promotes stomatal closure under stress; no role in phototropic bending.​

• (D): Likely cytokinin (e.g., zeatin). Purine adenine derivative with isoprenoid side chain at N6. Stimulates cell division, delays senescence; high auxin:cytokinin ratio favors roots, but irrelevant to light-induced bending.​

Correct Answer: (A) Auxin (IAA). Darwin’s experiments on canary grass coleoptiles showed unilateral light triggers bending only if the tip (auxin source) is exposed; covering it prevents response.​

The bending of coleoptile in canary grass in response to unidirectional white light exemplifies phototropism, a fundamental plant response crucial for CSIR NET aspirants studying plant hormones. Charles Darwin and Francis Darwin pioneered this using Phalaris canariensis seedlings, observing that coleoptiles bend toward light only when the tip senses it.​

Phototropism Mechanism

Unilateral light triggers phototropins in the coleoptile tip, redistributing auxin (IAA) laterally. Higher auxin on the shaded side loosens cell walls via acid growth, causing elongation and bending toward light. Went later isolated auxin from tips, confirming its role.​

Why Not Other Hormones?

Gibberellins elongate stems symmetrically; cytokinins promote division; ABA inhibits growth—none mediate light-directed asymmetry.​

CSIR NET Relevance

This aligns with Unit 5 (Plant Physiology), testing hormone structures and functions. Master Darwin-Went experiments for PYQs.​