Question:
Inhibitors of photosynthesis at photosystem II
Inhibitors of photosynthesis at photosystem II
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PSII inhibitors like ureas (diuron, isoproturon) and triazines (atrazine) cause characteristic interveinal chlorosis and necrosis in older leaves because the blocked photosynthetic apparatus produces highly toxic singlet oxygen under light.
Updated On: Jul 24, 2026
- Thiocarbamates
- Ureas
- Glycines
- Benzoic acids
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The Correct Option is B
Solution and Explanation
Step 1: Understanding the Concept:
Herbicides that inhibit photosynthesis at Photosystem II (PSII) act by binding to the \(Q_B\)-binding niche on the D1 protein of the photosynthetic reaction center in the thylakoid membrane. This blocks the transfer of electrons from \(Q_A\) to \(Q_B\), stopping light-driven CO\(_2\) fixation.
Step 2: Detailed Explanation:
Let us analyze the modes of action of the listed chemical groups:
Ureas (Substituted Ureas): This group includes common herbicides such as diuron, monuron, linuron, and isoproturon.
Substituted ureas are classic inhibitors of Photosystem II.
They bind to the D1 protein, blocking electron transport and causing a backup of energy that leads to the generation of highly reactive singlet oxygen and free radicals.
These radicals cause lipid peroxidation of the thylakoid membranes, resulting in chlorophyll bleaching and leaf necrosis. Thus, this group consists of PSII inhibitors.
Thiocarbamates: This group (including EPTC and butylate) acts primarily by inhibiting lipid synthesis, particularly the synthesis of very long-chain fatty acids, disrupting cuticular wax formation. They are not photosynthesis inhibitors.
Glycines: This group is represented by glyphosate, which inhibits the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) in the shikimate pathway, blocking the synthesis of aromatic amino acids.
Benzoic acids: This group (such as dicamba) acts as synthetic auxins, causing uncontrolled growth, epinasty, and vascular tissue collapse.
Thus, substituted ureas are the inhibitors of Photosystem II among the choices.
Step 2: Final Answer:
The correct option is (B), representing Ureas.
Herbicides that inhibit photosynthesis at Photosystem II (PSII) act by binding to the \(Q_B\)-binding niche on the D1 protein of the photosynthetic reaction center in the thylakoid membrane. This blocks the transfer of electrons from \(Q_A\) to \(Q_B\), stopping light-driven CO\(_2\) fixation.
Step 2: Detailed Explanation:
Let us analyze the modes of action of the listed chemical groups:
Ureas (Substituted Ureas): This group includes common herbicides such as diuron, monuron, linuron, and isoproturon.
Substituted ureas are classic inhibitors of Photosystem II.
They bind to the D1 protein, blocking electron transport and causing a backup of energy that leads to the generation of highly reactive singlet oxygen and free radicals.
These radicals cause lipid peroxidation of the thylakoid membranes, resulting in chlorophyll bleaching and leaf necrosis. Thus, this group consists of PSII inhibitors.
Thiocarbamates: This group (including EPTC and butylate) acts primarily by inhibiting lipid synthesis, particularly the synthesis of very long-chain fatty acids, disrupting cuticular wax formation. They are not photosynthesis inhibitors.
Glycines: This group is represented by glyphosate, which inhibits the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) in the shikimate pathway, blocking the synthesis of aromatic amino acids.
Benzoic acids: This group (such as dicamba) acts as synthetic auxins, causing uncontrolled growth, epinasty, and vascular tissue collapse.
Thus, substituted ureas are the inhibitors of Photosystem II among the choices.
Step 2: Final Answer:
The correct option is (B), representing Ureas.
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