Which of the following substances is NOT a cytokinin used in plant tissue culture?

Introduction / Context:
Cytokinins promote cell division and shoot formation. Distinguishing cytokinins from auxins prevents hormonal misapplication during in vitro morphogenesis and regeneration protocols.

Given Data / Assumptions:

• BAP, zeatin, kinetin, and TDZ are commonly used cytokinins or cytokinin-like compounds.
• 2,4-D is a synthetic auxin, not a cytokinin.

Concept / Approach:
Identify each compound’s hormone class. 2,4-D is used mainly for callus induction and somatic embryogenesis as an auxin. The others are standard cytokinins for shoot induction and proliferation.

Step-by-Step Solution:

Verification / Alternative check:
Hormone supplier catalogs and tissue culture manuals classify these compounds accordingly.

Why Other Options Are Wrong:

• BAP, zeatin, kinetin, TDZ: all act as cytokinins or cytokinin-like regulators.

Common Pitfalls:
Confusing frequent use of 2,4-D in callus protocols with cytokinin function; assuming all “shoot-promoting” media contain only cytokinins without considering ratios.

Final Answer:
2,4-dichlorophenoxyacetic acid (2,4-D)