Mode of action of paraquat in leaves of paraquat‐resistant Conyza canadensis (L.) Cronq

ABSTRACT The mode of action of paraquat (1,1‐dimethyl‐4,4‐bipyridinium) and the mechanism of resistance to it were studied in leaves of atrazine/paraquat co‐resistant (R) and susceptible (S) biotypes of horseweed ( Conyza canadensis ) collected from Hungarian vineyards. The application of 0·5 mol m −3 paraquat by spraying onto the surface of the leaves of intact plants in the light rapidly led to typical symptoms of paraquat action in the initial period in both biotypes, i.e. inhibition of CO 2 fixation, suppression of variable chlorophyll fluorescence (F v ), decrease of oxygen evolution and stimulation of ethane production. The inhibitory effect of paraquat in the S plants was irreversible, whereas it was transient in the R plants and those plants recovered gradually afterwards. The R plants recovered from the inhibitory effect of paraquat only in the light, and an increase in light intensity was found to have a pronounced effect on the recovery of F v . The mechanism of resistance to paraquat in C. canadensis is discussed.