42. The herbicide, dichlorophenyldi-methylurea (DCMU), is an inhibitor of
(1) Shikimate pathway for biosynthesis of aromatic amino acids.
(2) Electron transport from P680 to P700.
(3) Branch in amino acid pathway.
(4) Electron transport from P700 to ferrodoxin

The herbicide DCMU (dichlorophenyldimethylurea) is an inhibitor of (2) electron transport from P680 to P700.

DCMU specifically blocks the transfer of electrons from Photosystem II (PSII), where P680 acts as the reaction center, to Photosystem I (PSI), where P700 is the reaction center. This inhibition occurs by preventing electron movement at the plastoquinone (QB) site, stopping electrons from moving from QA to QB, thereby blocking their subsequent flow to PSI and onward reduction of NADP+.​

DCMU Herbicide: Blocking Electron Flow Between Photosystems

Introduction

Key phrase: DCMU herbicide photosynthesis electron transport inhibition PSII P680 P700 QB site

DCMU is a classic research herbicide and agricultural chemical that targets the photosynthetic electron transport chain at a precise step. By preventing electron transfer from PSII to PSI, it disrupts the production of ATP and NADPH and hinders plant growth.​

Explanation of Each Option

• (1) Shikimate pathway for biosynthesis of aromatic amino acids

  • Incorrect. DCMU does not affect amino acid biosynthesis; its action is specific to the photosynthetic electron transport chain.​

• (2) Electron transport from P680 to P700

  • Correct. DCMU inhibits electron transport between PSII (P680) and PSI (P700) by blocking electron passage at the QB site in PSII, interrupting linear photosynthesis.​

• (3) Branch in amino acid pathway

  • Incorrect. DCMU does not influence amino acid synthesis pathways; it specifically targets the electron chain in photosynthesis.​

• (4) Electron transport from P700 to ferrodoxin

  • Incorrect. DCMU does not affect electron transport within PSI (from P700 to ferredoxin); its effect is limited to PSII and the following quinones.​

Key Facts: DCMU and Photosynthetic Electron Transport

• Inhibits electron flow at PSII QB site, halting movement toward PSI (P700).

• Blocks ATP and NADPH formation, disrupting energy conversion in plants.

• Does not impact PSI directly or amino acid biosynthesis pathways.​