Paraquat is commonly categorized as which type of herbicide?

• A. Phenoxy herbicide
• B. Dipyridyl herbicide
• C. Triazine herbicide
• D. Glyphosate herbicide

Answer: (B)

Paraquat is categorized as a dipyridyl (bipyridyl) herbicide. This chemical family, which includes paraquat, produces its weed-killing effect through a distinct mechanism: it accepts electrons from photosystem I in the chloroplasts and transfers them to oxygen, creating reactive oxygen species. This oxidative burst damages cell membranes and other cellular components, causing rapid, non-selective injury to green plant tissue that is in contact with the foliage. Because it acts mainly on contact and isn’t readily translocated systemically, it quickly desiccates treated parts of the plant.

Context helps distinguish it from other common herbicides: glyphosate is an EPSP synthase inhibitor with systemic action, affecting a broad range of plants after being absorbed and translocated. Phenoxy herbicides (like 2,4-D) are synthetic auxins that trigger uncontrolled growth leading to plant death. Triazines (like atrazine) inhibit photosystem II. The key is recognizing paraquat’s bipyridyl class and its ROS-generating, contact-mode action.