45. A researcher wanted to study light reaction during photosynthesis by blocking photosynthetic electron flow using the herbicide, di-chloro-phenyl-dimethyl-urea (DCMU) and paraquat. The researcher listed the following observations:
    A. Both DCMU and paraquat block the electron flow in Photosystem II .
    B. Both DCMU and paraquat block the electron flow in Photosystem I
    C. DCMU blocks electron flow in Photsystem I while paraquat blocks in Photosystem II
    D. DCMU blocks electron flow in Photosystem II while paraquat blocks in Photosystem I
    Which of the following combinations of the above statements is INCORRECT?

(1) A, B and C                   (2) A, B and D
(3) A, C and D                  (4) B, C and D

The INCORRECT combination is (4) B, C and D.

• A. Both DCMU and paraquat block the electron flow in Photosystem II.

  • Incorrect for paraquat. DCMU blocks electron flow in PSII (between QA and QB), but paraquat does not; paraquat acts primarily on PSI.​

• B. Both DCMU and paraquat block the electron flow in Photosystem I.

  • Incorrect. DCMU inhibits PSII, while paraquat inhibits PSI by accepting electrons from ferredoxin and passing them to O₂, generating reactive oxygen species (ROS).​​

• C. DCMU blocks electron flow in Photosystem I while paraquat blocks in Photosystem II.

  • Incorrect. DCMU blocks PSII, not PSI, and paraquat is specific to PSI, not PSII.​​

• D. DCMU blocks electron flow in Photosystem II while paraquat blocks in Photosystem I.

  • Correct. This statement accurately describes their respective sites of action: DCMU at PSII and paraquat at PSI.​​

DCMU and Paraquat: Sites of Action and Misconceptions

Introduction

Key phrase: DCMU paraquat herbicide photosynthesis inhibitor PSII PSI electron transport site of action

DCMU and paraquat are experimental and commercial herbicides, each blocking different steps of photosynthetic electron transfer—DCMU at PSII and paraquat at PSI. Confusion about their targets leads to incorrectly attributed sites of action.​​

Explanation of Each Statement

• A. DCMU inhibits PSII; paraquat does not.

• B. DCMU does not block PSI; paraquat does block PSI.

• C. Both points are incorrect; DCMU targets PSII, paraquat targets PSI.

• D. Correct, accurately reflects their sites of inhibition.

Key Facts: Herbicide Action in Photosynthetic Electron Flow

• DCMU blocks electron transfer at QA→QB in PSII.​

• Paraquat blocks electron flow at ferredoxin in PSI, redirecting electrons to oxygen and causing ROS formation.​​