Question

Which one of the following hormones can be used as herbicide to kill dicot weeds?

• A. GA3
• B. NAA
• C. BAP
• D. 2,4-D

Hint:

2,4-D was one of the components of "Agent Orange."
It is the most famous example of a hormone being used as a pesticide.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks to identify a plant growth regulator that, when applied at high concentrations, acts as a selective herbicide to eliminate broad-leaved (dicot) weeds.
Detailed Explanation:

• 2,4-D (2,4-Dichlorophenoxyacetic acid):
This is a synthetic Auxin. While auxins in low doses promote growth, synthetic auxins like 2,4-D in high doses cause rapid, uncoordinated, and lethal growth in certain plants.

• Selectivity:
2,4-D is a selective herbicide. Dicotyledonous plants (broad-leaf weeds) are highly sensitive to it, whereas monocotyledonous plants (like cereal crops, grasses, and maize) are relatively resistant.
This allows farmers to spray a field of wheat or lawn to kill weeds without harming the crop.

• Mechanism:
It causes excessive cell elongation and division, leading to twisted stems and leaves, eventually resulting in the collapse of the plant's vascular system.

Step 2: Final Answer:
2,4-D is the synthetic auxin used widely as a selective dicot herbicide.