Herbicide metabolism is the primary mechanism of selectivity where the desirable plant is not injured because it has rendered the herbicide less toxic and the undesirable weed dies because the herbicide stays in its active form. Herbicide or xenobiotic metabolism proceeds in plants through a three phase process. The first phase introduces reactive groups which then can be acted on by Phase II reactions. Phase II reactions usually increase the size and polarity of the molecule by adding either glucose or glutathione. Under Phase III reactions which are unique to plants because they cannot excrete metabolites, the plant either stores the xenobiotic metabolite in the vacuole or incorporates it into cell wall materials. Metabolism-based selectivity is also important in herbicide resistance and can be altered through the addition of synergists or antagonists. 


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