Question

Given below are two statements:
Statement I: Methyl viologen, also known as Paraquat, used as herbicide inhibits light reactions of photosynthesis.
Statement II: Paraquat blocks electron flow at plastoquinone acceptors of PS II by competing for binding sites of plastoquinone.
In light of the above statements, choose the most appropriate answer from the options given below: [Question ID = 706][Question Description = 206_34_PLB_AUG22_Q106]

• A. Both Statement I and Statement II are correct [Option ID = 2821]
• B. Both Statement I and Statement II are incorrect [Option ID = 2822]
• C. Statement I is correct but Statement II is incorrect [Option ID = 2823]
• D. Statement I is incorrect but Statement II is correct [Option ID = 2824]

Hint:

Mnemonic: DCMU acts at PSII (D comes before P, II comes after I). Paraquat acts at PSI (P comes later, I comes first).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Herbicides often target specific steps in the photosynthetic electron transport chain. Paraquat (Methyl Viologen) and Diuron (DCMU) are the two most common examples, but they act at different sites within the thylakoid membrane.

Step 2: Detailed Explanation:
Statement I Analysis: Methyl viologen (Paraquat) is indeed a powerful herbicide that acts by intercepting electrons from the photosynthetic light reactions. This prevents the formation of NADPH and leads to the generation of toxic reactive oxygen species (ROS). This statement is correct.
Statement II Analysis: This statement is incorrect. The mechanism described—blocking electron flow at the plastoquinone acceptors of PS II by competing for the \(Q_B\) binding site—is actually the mechanism of action for DCMU (Diuron).
Mechanism of Paraquat: Paraquat acts at the level of Photosystem I (PSI). It has a very negative redox potential and competes with Ferredoxin (Fd) for electrons coming from the FeS centers of PSI. Once reduced to a Paraquat radical, it reacts with molecular oxygen to produce superoxide (\(O_2^{\cdot -}\)), which causes rapid lipid peroxidation and membrane destruction in the plant cell.

Step 3: Final Answer:
Statement I is correct because Paraquat inhibits light reactions; Statement II is incorrect because Paraquat acts at PSI, not PSII.