Question

Identify the synthetic inhibitors used in crop plants
(A) Maleic Hydrazide
(B) Chlormequat Chloride
(C) Paclobutrazol
(D) Triidobenzoic acid
Choose the correct answer from the options given below:

• A. (A) and (C) only
• B. (A) and (D) only
• C. (B), (C) and (D) only
• D. (A), (B) and (C) only

Hint:

Synthetic growth retardants like Chlormequat and Paclobutrazol function by blocking different enzymatic steps in the gibberellin biosynthesis pathway, producing compact, lodging-resistant plants.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Plant growth regulators include synthetic chemical compounds that inhibit or retard plant growth.
These are classified into growth inhibitors (which halt metabolic and developmental processes) and growth retardants (which reduce stem elongation by inhibiting gibberellin biosynthesis without altering other developmental features).

Step 2: Detailed Explanation:
Let us analyze the physiological roles of the given compounds:
(A) Maleic Hydrazide:
This synthetic compound is a classic growth inhibitor.
It prevents cell division in apical meristems by inhibiting nucleic acid synthesis and is used commercially to prevent sprouting in onions and potatoes during storage.
Thus, (A) is an inhibitor.
(B) Chlormequat Chloride (CCC Cycocel):
This synthetic compound is a major gibberellin biosynthesis inhibitor.
It blocks the cyclization of geranylgeranyl pyrophosphate to copalyl pyrophosphate, reducing stem elongation and producing shorter, sturdier plants.
Thus, (B) is an inhibitor.
(C) Paclobutrazol:
This triazole-type plant growth regulator is a highly effective gibberellin biosynthesis inhibitor.
It blocks the oxidation of ent-kaurene to ent-kaurenoic acid, reducing vegetative growth and promoting flowering in fruit crops like mango.
Thus, (C) is an inhibitor.
(D) Triiodobenzoic acid (TIBA):
TIBA is classified specifically as an auxin transport inhibitor rather than a direct growth inhibitor or retardant.
It blocks the polar transport of auxin, altering plant branching patterns but not serving as a general growth inhibitor.
Therefore, Maleic Hydrazide, Chlormequat Chloride, and Paclobutrazol are the primary synthetic growth inhibitors and retardants.

Step 3: Final Answer:
The synthetic inhibitors are (A), (B), and (C) only.
This corresponds to Option (D).