Paraquat: The Marijuana Herbicide

Photosystem I (PSI) is a key component of the photosynthetic electron
transport system in plants. It transfers electrons from chlorophyll to an
electron acceptor in a reaction called photooxidation. Subsequent redox
reactions culminate in the reduction of NADP⁺ to generate NADPH,
which is required for CO₂ fixation by the Calvin–Benson cycle. Paraquat
is an herbicide that prevents reduction of NADP⁺ by accepting electrons
from intermediate reductants in PSI. It also generates reactive oxygen
species that are toxic to cells, thus killing the plant. Paraquat was used
extensively in the 1980s as an herbicide in North and South America,
primarily to destroy illegal marijuana crops by aerial spraying. In some
cases, the herbicide-sprayed marijuana crops were harvested and sold
in the United States, which led to a public outcry over fears that paraquat
was causing human health problems as a result of people smoking
paraquat-contaminated plant matter.