Question

Inhibitors of photosynthesis at photosystem I.

• A. Sulfonylureas
• B. Benzoic acids
• C. Diphenyl ethers
• D. Bipyridyls

Hint:

Recall which herbicide group diverts electrons directly at photosystem I.

Answer 1

The Correct Option is D

Step 1: Understanding the Question.
The question asks which herbicide group blocks photosynthesis specifically at photosystem I rather than at photosystem II.

Step 2: Key Concept.
Bipyridyl herbicides, such as paraquat and diquat, act by intercepting electrons at photosystem I. They accept an electron meant for the normal electron transport chain and then get reoxidised by oxygen, forming reactive oxygen species that tear apart cell membranes. This is the classic photosystem I diverting mechanism.

Step 3: Ruling Out Other Options.
Sulfonylureas work by blocking the enzyme acetolactate synthase, which stops branched chain amino acid production, unrelated to the photosynthetic electron chain. Benzoic acids, such as dicamba, act as synthetic auxins that disturb plant growth hormone balance. Diphenyl ethers, such as oxyfluorfen, block protoporphyrinogen oxidase, an enzyme in chlorophyll synthesis, causing light dependent membrane damage, but this is a different site of action from photosystem I itself.

Step 4: Final Answer.
Only bipyridyls act directly at photosystem I, so option 4 is correct.