Paraquat blocks photosynthesis by accepting electrons from photosystem I (PSI) and preventing electron transport to nicotinamide-adenine dinucleotide phosphate (NADPH) and become a reduced monocation radial that reacts with oxygen yielding superoxide anion.
Paraquat, with its distinctive mode of action, is one of few chemical options that can be used to prevent and mitigate problems with weeds that have become resistant to the very widely used non-selective herbicide glyphosate.