41. Dichlorophenyl dimethyl urea       (DCMU) inhibits photosynthesis at which step?
(1) phe–àQ_A                          (2) Q_A–àQ_B
(3) Cyt bf6–àPC                       (4) Q_B–àCytbf₆

DCMU inhibits photosynthesis at the step (2) QA → QB.

DCMU acts by binding to the QB site of Photosystem II and prevents the electron transfer from the primary quinone electron acceptor QA to the secondary quinone electron acceptor QB. This blockage stops the flow of electrons from PSII to plastoquinone and halts the linear photosynthetic electron transport chain.​

DCMU Inhibition in Photosynthesis: Mechanism and Step

Introduction

Key phrase: DCMU inhibition photosynthesis QA QB electron transport PSII mechanism herbicide

DCMU (diuron) is a selective herbicide and a classic research inhibitor in plant physiology, blocking Photosystem II electron transfer at the QA to QB step. By doing so, it disrupts ATP and NADPH production and impairs plant growth.​

Explanation of Each Option

• (1) Phe → QA

  • Incorrect. DCMU does not prevent electron transfer from pheophytin (Phe) to QA; electrons are still transferred to QA normally in the presence of DCMU.​

• (2) QA → QB

  • Correct. DCMU binds specifically at the QB site of PSII, blocking QA from transferring electrons to QB. This effectively inhibits electron flow from water to plastoquinone and terminates oxygen evolution and linear photosynthesis.​

• (3) Cyt bf6 → PC

  • Incorrect. This step is downstream of the PSII complex and is not the target of DCMU. DCMU does not block the transfer from cytochrome b6f to plastocyanin (PC).​

• (4) QB → Cyt bf6

  • Incorrect. DCMU prevents QB formation, so this pathway cannot operate, but the actual inhibition occurs at the preceding QA → QB transfer.​

Key Facts: DCMU Action in Chloroplasts

• Inhibits the transfer of electrons from QA to QB within PSII.

• Reduces oxygen evolution and photosynthetic productivity.

• Widely used in physiological studies to explore electron transport and photosynthesis.​