Author
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Rusness Jr, Donald |
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Huwe, Janice |
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LAMOUREUX, GERALD - RETIRED ARS |
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Submitted to: Pesticide Biochemistry and Physiology
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/5/1998 Publication Date: N/A Citation: N/A Interpretive Summary: The new herbicide quinclorac was shown to be very effective in the control of leafy spurge, a noxious perennial weed which has invaded the Great Plains. Quinclorac and 13 ester derivatives were monitored in leafy spurge soil systems in order to compare their relative effectiveness against this weed. All of the quinclorac esters were less phytotoxic than quinclorac when applied to the foliage of leafy spurge. However, intermediate molecular weight esters were superior to quinclorac at 24 weeks when applied to the soil. Translocation and metabolism studies in the plant showed that little movement of the esters downward from the foliage or upward from the roots occurred relative to that of quinclorac. Significant ester metabolism occurred in both tissues, but little quinclorac production resulted. However, metabolism of the esters to quinclorac did occur in soil and nutrient solution. The release of quinclorac from the esters by soil microorganisms did not proceed by simple hydrolysis of the ester bond; instead, quinclorac was produced through a series of hydroxylations and sequential oxidations on the ester side chain. Key intermediates in this pathway were identified. The rate of quinclorac release varied dependent upon the number of carbons in the ester chain. These reaction sequences increased the herbicide efficacy by acting as a mechanism for the slow time-release of quinclorac in soil. It is possible that a combination of quinclorac and the appropriate odd/even ester applied to the soil might further increase the efficacy. Efficacy of other acid herbicides in other weeds also might be enhanced by soil treatment as the alkyl esters, dependent upon the esters ability to be sequentially oxidized. Technical Abstract: Quinclorac and 13 synthesized esters were monitored for uptake, translocation, metabolism, and toxicity in foliar- and root-treated leafy spurge. Foliar application resulted in rapid uptake and metabolism of the esters in the treated leaves; quinclorac was a minor product of foliar metabolism, and radiolabel was not translocated from the leaves. Toxicity symptoms were evident; however, the plants recovered with time and no mortality occurred. Root treatment resulted in rapid uptake of esters, but much radiolabel (ester, quinclorac, and/or metabolites) effluxed from the roots. Translocation to foliar tissues occurred with the low MW esters but rapidly declined with increasing n-alkyl chain length. Ester metabolism occurred in the roots, but little quinclorac was detected. Micro-organisms associated with the roots in both nutrient solution and soil metabolized the esters to quinclorac, and this metabolism occurred through a series of omega- and beta-oxidations and not by ester hydrolysis. This metabolism appeared to be different from that which actually occurred in roots. The intermediate metabolites produced by the micro-organisms were identified by MS- and NMR-analyses. The metabolism to quinclorac in soil and subsequent foliar toxicity and mortality to leafy spurge increased with time and was dependent on ester chain length. The nC7, nC5, and nC8 esters were metabolized to quinclorac at relative rates of 16, 3.2, and 1, respectively; the nC6 and nC8 esters were metabolized to the quinclorac ester of 4-hydroxybutyric acid as a major intermediate in soil before conversion to quinclorac. Foliar toxicity from the nC5-nC8, isoC8, nC16, and 1,3-propanediol ester soil treatments was superior to quinclorac after 24 wk. |
