Sites of action of photosynthetic inhibitor herbicides; Experiments with trypsinated chloroplasts

The effect of four photosynthetic inhibitor herbicides, bromacil, ioxynil, metribuzin and monuron, on chloroplast electron transport was investigated. All four compounds completely inhibited electron flow with tripotassium hexacyanoferrate as oxidant, but the inhibition caused by bromacil, metribuzin and monuron was almost totally reversed by trypsin treatment. With ioxynil, only a partial degree of reversability was shown. With a molybdosilicate as oxidant, electron transport was not completely inhibited by any of the herbicides. Whereas the partial inhibition was reversed by tryptic digestion in the presence of bromacil, metribuzin and monuron, there was virtually no reversal in the presence of ioxynil. The results suggest a common site of action for all four herbicides and an additional site for ioxynil nearer to photosystem II.