Physiological basis for antagonism induced by mixtures of quizalofop-ethyl and bromoxynil in maize (Zea mays)

In this study, the physiological basis for antagonism induced by mixtures of quizalofop-ethyl and bromoxynil was investigated in maize seedlings. In sequential applications, antagonism was observed when bromoxynil was applied before quizalofop-ethyl or in a mixture with quizalofop-ethyl, but was minimal when bromoxynil was applied afterwards. The degree of antagonism differed with application rates of bromoxynil and with the timing of the treatment. When test herbicides were applied locally to the second leaf, the inhibition of photosystem II (PS-II) in the herbicide-treated leaf was higher with the mixture than with bromoxynil or quizalofop-ethyl alone. Subsequent growth of the untreated third leaf inhibited by quizalofop-ethyl alone then recovered, depending on the dose of bromoxynil. There was no evidence that bromoxynil affected absorption of quizalofop-ethyl. In local applications at different positions on the second leaf, antagonism was only observed when quizalofop-ethyl was applied to the distal part of the leaf and bromoxynil applied to the proximal part. The antagonism of bromoxynil + quizalofop-ethyl did not occur at the level of acetyl CoA carboxylase and Hill reaction, as revealed by in vitro assays. These results suggest that bromoxynil inhibits the phloem transport of quizalofop-ethyl and thus antagonises its whole-plant activity in maize.