Question

Which of the following chemicals cannot be used as a weedicide?
A. Dalapon
B. Metachlor
C. $2$, $4$-Dichlorophenoxy acetic acid
D. Colchicine

Answer 1

Hint: The Weedicides are used to kill the unwanted growth of weeds around crops but this chemical is not used as a weedicide but can be used as a mitotic inhibitor to induce polyploidy. Polyploidy occurs due to inability of the chromosomes to separate leading to a condition having more than one set of homologous chromosomes.

Complete answer: Weedicides are chemicals that are sprayed over the fields to get rid of the weeds. They do not affect the main crop but destroy the weeds that grow around it. Hence, they are also known as herbicides.
Dalapon is an example of an organochlorine herbicide that is highly soluble in water and relatively volatile. It is used to kill the annual and perennial grasses which grow around the main crop.
Therefore, option (a) is not correct.
Metolachlor is an organic compound that is a derivative of the aniline family, used as a herbicide. It kills weeds by inhibiting the synthesis of long-chain fatty acids in it. It is generally used on corn, soybean, potatoes, etc.
Therefore, option (b) is also not correct.
$2$ ,\[4\] -Dichlorophenoxy acetic acid is an auxin type herbicide, which is used to control broad-leaved weeds. It starts multiple divisions in the cells of the tissue carrying water and their uncontrollable growth leads to the death of the plant.
Therefore, option (c) is also incorrect.
Colchicine is a chemical which induces polyploidy in plants. It prevents the formation of microtubules in the cells undergoing division. This leads to an uneven division of the chromosomes. This is generally a mitotic inhibitor.
So, the correct answer is “Option D”.

Note:
Colchicine is a mitotic inhibitor and not a weedicide. Therefore out of the following list of options Colchicine is not a weedicide and the rest of them Dalapon, Metachlor, and $2$ ,\[4\] -Dichlorophenoxy acetic acid are used as weedicides to destroy unwanted plants growing around the main crop with a different mechanism of action.
Colchicine is used as a polyploidy inducer inhibiting the normal mitotic process to initiate development of new plant species. Addition of colchicine prevents the formation of microtubules that are responsible for proper division of chromosomes and hence polyploidy is induced.