Question

Which of the following plant growth regulators is used as herbicide ?

• A. Gibberellin
• B. 2,4-D
• C. Kinetin
• D. Abscisic acid

Hint:

Keep this structural formula or shorthand in mind: 2,4-D = Dicot Destroyer. It kills Dicot weeds selectively. Because it is a synthetic auxin, it mimics normal plant hormones but acts like a poison at high concentrations only for broad-leaved plants, making it a stellar herbicide!

Answer 1

The Correct Option is B

Concept: Synthetic plant growth regulators are extensively utilized in modern agricultural practices to optimize crop yields and manage weeds. Synthetic auxins have a unique property: when applied at exceptionally high or concentrated doses, they selectively target and disrupt the metabolic processes of specific plant groups without harming others. A classic example of such a chemical herbicide (weedicide) is 2,4-D (2,4-Dichlorophenoxyacetic acid).

Step 1: Understanding the action and selectivity of 2,4-D.
The compound 2,4-D is a powerful synthetic auxin. In agricultural fields, it is widely used as a selective herbicide because it specifically targets and eradicates broad-leaved dicotyledonous weeds, while leaving mature monocotyledonous plants (such as cereal crops like wheat, rice, maize, and lawn grasses) completely unaffected. It induces uncontrolled, rapid, and distorted cellular growth in the dicot weeds, leading to epinasty, twisting of stems, disruption of vascular transport phloem tissues, and eventual death of the weed. This makes it an incredibly useful tool for maintaining weed-free lawns and crop fields.

Step 2: Checking the other plant growth regulators for any herbicide properties.
Let us review the options to confirm they lack herbicidal functions:

• (1) Gibberellin: Used primarily to increase the length of grape stalks, elongate and improve the shape of fruits like apples, delay senescence, and speed up the malting process in the brewing industry. It is never used as an herbicide.

• (3) Kinetin: A type of cytokinin that promotes cell division, delays leaf senescence (Richmond-Lang effect), and helps overcome apical dominance. It is used in tissue culture to induce shoot differentiation but has no weed-killing property.

• (4) Abscisic acid (ABA): Functions as an internal plant hormone regulating stress responses and inducing seed dormancy. It is costly to synthesize artificially and is not used as a commercial agricultural spray or herbicide.
Thus, 2,4-D is specifically manufactured and marketed as a potent commercial herbicide.