Diquat is a quaternary ammonium salt built from a complex fused bipyridyl ring system. It is most frequently sold and applied commercially as the highly soluble diquat dibromide salt. When exposed to natural sunlight, the diquat molecule easily enters the chloroplasts of the target plant.
It rapidly intercepts energized electrons directly from photosystem I during the natural photosynthesis process. This chemical interference creates a highly stable radical cation within the living plant cells. This radical cation immediately transfers its extra electron to surrounding atmospheric molecular oxygen.
This specific reaction produces superoxide and other incredibly damaging reactive oxygen chemical species. These strong chemical oxidants attack and completely rupture the plant cell membranes within a few hours. This massive cellular damage causes rapid wilting and severe browning of the green plant foliage.
Diquat only damages the specific green plant tissue that it directly contacts upon initial application. It does not move easily through the plant stems down into the underground root system. This limited physical movement makes it highly effective for controlling submerged and floating aquatic weeds.
Students often confuse contact herbicides like diquat with systemic agricultural herbicides like chemical glyphosate. Systemic herbicides travel throughout the entire plant, while diquat strictly kills the directly sprayed parts.