Question

Given below are two statements:
Statement (I) : Dichlorophenyl dimethylurea (DCMU), also known as Diuron - used as herbicide, is an inhibitor of light reactions of photosynthesis.
Statement (II) : DCMU acts by accepting electrons from early receptors of PS I.
In light of above statements, choose the most appropriate answer from the given options.

• A. Both Statement (I) and Statement (II) are correct
• B. Both Statement (I) and Statement (II) are incorrect
• C. Statement (I) is correct and Statement (II) is incorrect
• D. Statement (I) is incorrect and Statement (II) is correct

Hint:

Remember the different target sites of photosynthesis inhibitors:
- DCMU (Diuron) $\rightarrow$ Blocks the $Q_B$ site of PS II (stops electron flow to plastoquinone).
- Paraquat (Methyl Viologen) $\rightarrow$ Accepts electrons from PS I (intercepts electrons at ferredoxin and generates toxic free radicals).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Photosynthesis is driven by light reactions occurring in the thylakoid membranes of chloroplasts.
These reactions involve two photosystems: Photosystem II (PS II) and Photosystem I (PS I), linked by an electron transport chain.
Specific chemical compounds can selectively bind to and inhibit different steps of this electron transport pathway.

Step 2: Detailed Explanation:
Let us evaluate both statements individually:
Statement (I) is correct because Dichlorophenyl dimethylurea (DCMU), commercially sold as Diuron, is a widely used herbicide.
It is a classic inhibitor that blocks the light-dependent reactions of photosynthesis by disrupting electron transport.
Statement (II) is incorrect because of its described site of action.
DCMU does not inhibit Photosystem I (PS I).
Instead, it binds specifically to the plastoquinone-binding ($Q_B$) site on the D1 protein of the reaction center of Photosystem II (PS II).
By occupying this site, DCMU physically blocks the transfer of electrons from the primary electron acceptor plastoquinone ($Q_A$) to the secondary plastoquinone ($Q_B$).
This halts the electron flow between PS II and PS I, stopping ATP and NADPH synthesis.
Therefore, Statement (I) is correct, and Statement (II) is incorrect.

Step 3: Final Answer:
Statement (I) is correct and Statement (II) is incorrect.
This corresponds to Option (C).