Bipyridilium herbicides such as paraquat and diquat
are fast-acting foliar-applied herbicides with no soil persistence. These herbicides are reduced by the primary acceptor of photosystem I to a radical dication, which
reacts with molecular oxygen to produce toxic oxygen
species (superoxide, hydroxyl radical, and hydrogen peroxide)
that breakdown cell membranes. Following the
first report of evolved weed resistance to the herbicide
paraquat in 1980, 21 resistant weeds (15 dicotyledonous
and 6 monocotyledonous) have been confirmed.
Weed species that have evolved resistance to paraquat
include mainly fleabanes (Conyza or Erigeron spp.). Two
weed species, capeweed (Arctotheca calendula L.) and
horseweed (Conyza linifolia L.), have evolved resistance
in response to treatments with diquat.
