The Influence of Light, Oxygen and Temperature on the Herbicidal Properties of Paraquat.

Paraquat (1,lYdimethyl-4,4-dipyridylium dichloride) is an effective herbicide and (lesiccant which is being used for the rapid top kill of certain woo(ly plants. The rapidity of kill suggests that the mo(le of action of paraquat is tlifferent fr'9m the slowver acting hormone-type herbicides. Light has been shown to enhance the herbicidal effectiveniess of certain dipyridvlium cations (4). Consequently, it has been conclude(l that the herbici(lal action of the dipyridylium comlpoundls is related to photosynthesis wvhich results in the reductioni of the dlipyridylium cation to a fr-ee radical (3). Calvin (2) reported that free ra(licals are usually form-ledl (luring photosynthesis. Furthermiiore, free radical forimlation is sometimes foundI in compounds containinig quadrivalent nitrogen (6) and more specifically in clipyridyl quaternary salts (5). Brian (1) founid that light and chlorophyll are essential for free radical formation and that 0. is required for toxic symptomiis to occur in plants. The final effect of paraquat is consideredl to be a (lisruptioni -f cell membranes by peroxide radicals formed through the interaction of the paraquat free radlical. wvater and 0.,.