Many photosystem II inhibiting herbicides still inhibit this process in triazine-resistant plants; i.e. they have no cross resistance with atrazine. Five- to twenty-fold lower concentrations of phenolic type herbicidcs and 5-fold less of the active ingredient of pyridate and half as much ioxynil are required to inhibit thylakoid PS II in atrazine-resistant biotypes than in sensitive biotypes; i.e., they even show “negative cross resistance”. Negative cross resistance may be the major reason that atrazine resistance did not evolve where herbicide mixtures were used, when the mixed herbicide (usually a non-PS II inhibiting acetanilide) also controlled triazine-sensitivc weeds. Mathematical modeling in principle allows quantification of the very low field levels of herbicides possessing negative cross resistance that could be mixed with atrazine that would stop or delay the evolution of resistant populations without affecting the maize crop. There are few available actual dose response curves of atrazine-resistant vs. susceptible weeds at the whole plant level for herbicidcs exerting negative cross resistance. Thus, “real situation” modeling cannot be done. Data acquisition is called for so that the model can be extrapolated from the thylakoid to the field.
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Gressel et al. (1990) studied this question.