The Metabolic Fate of (4-Chloro-2-Methylphenoxy)Acetic Acid in Higher Plants

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Abstract
The metabolic fate of the auxin herbicide (4-chloro-2-methylphenoxy)acetic acid (MCPA) has been determined in a number of species using a vacuum infiltration technique. In all cases MCPA became hydroxylated to form (4-chloro-2-hydroxymethylphenoxy)acetic acid, which accumulated largely as a glycosidic conjugate. The nature of the oxidized metabolite from oat (Avena sativa L.) was verified by GC/MS. In all cases at least one diethyl ether-soluble conjugate of MCPA was formed; these are suggested to be amino acid conjugates. Several minor aglycones were also formed. Important species variations in both the rate and quantitative nature of metabolism were observed. Pretreatment of potato (Solarium tuberosum L.) tuber slices with unlabelled MCPA and other auxins increased the capacity for hydroxylation, but particularly induced the formation of an MCPA-glycoside. This was never a major metabolite under normal circumstances. The rate of hydroxylation was also enhanced by ageing in MnCl2. Although the ether-soluble conjugates of MCPA were stable metabolites, exogenously applied conjugate isolated from carrot (Daucus carota L.) was readily cleaved in four species. Free MCPA and the products normally derived from it were identified. MCPA Metabolism Hydroxylation Conjugation