44. The one of the most widely used herbicides methyl viologens (paraquat) interfere photosynthesis of higher plants. They are responsible for
(1) Evolution of more oxygen
(2) Dissipation of proton gradient across thylakoid membrane
(3) Inhibition of flow of electron from PS II to PSI
(4) transfer of electrons from PS I (ferroredoxin) to molecular oxygen to form ROS.

The correct answer is (4): transfer of electrons from PSI (ferredoxin) to molecular oxygen to form ROS.

Methyl viologens (paraquat) are herbicides that act by accepting electrons from reduced ferredoxin, the terminal electron carrier of Photosystem I, and transferring them directly to molecular oxygen. This leads to the formation of highly reactive oxygen species (ROS) such as superoxide and hydrogen peroxide, causing oxidative cellular damage, chloroplast dysfunction, and rapid plant death.​

Paraquat (Methyl Viologen): Mechanism and Photosynthetic Impact

Introduction

Key phrase: paraquat methyl viologen photosynthesis inhibition PSI ROS electron transfer ferredoxin

Methyl viologens are potent, fast-acting herbicides. Their primary mechanism in higher plants is to hijack the flow of electrons at the end of photosystem I, transferring them from ferredoxin to oxygen and triggering the formation of destructive reactive oxygen species.​

Explanation of Each Option

• (1) Evolution of more oxygen
  Incorrect. Paraquat does not increase oxygen evolution; it causes ROS formation by transferring electrons from PSI to oxygen rather than splitting water at PSII.​

• (2) Dissipation of proton gradient across thylakoid membrane
  Incorrect. Paraquat’s main effect is not on proton gradient dissipation, but on electron flow and ROS generation.​

• (3) Inhibition of flow of electron from PSII to PSI
  Incorrect. Paraquat acts after electrons reach PSI, specifically at the step of transfer from PSI (ferredoxin) to O2, not between the two photosystems.​

• (4) Transfer of electrons from PSI (ferredoxin) to molecular oxygen to form ROS
  Correct. Paraquat diverts electrons from ferredoxin, resulting in direct reduction of oxygen and mass generation of ROS, which destroy plant cell components and rapidly inhibit photosynthesis.​

Key Facts: Paraquat Inhibition of Photosynthesis

• Acts on Photosystem I by redirecting electrons normally bound for NADP+ to oxygen.

• Triggers accumulation of superoxide radicals and other ROS, causing chlorophyll and cell damage.​

• Results in rapid chlorosis, necrosis, and plant death; highly toxic herbicide for effective weed control.​