Propachlor metabolism in soybean plants, excised soybean tissues, and soil

Abstract

Propachlor was rapidly metabolized to a homoglutathione conjugate in the roots and foliage of soybean plants. No other competing reactions were observed. The homoglutathione conjugate was rapidly metabolized to the cysteine conjugate which was slowly converted to a variety of other metabolites, four of which were present up to 72 days after treatment. Those four metabolites were the malonylcysteine, malonylcysteine S-oxide, 3-sulfinyllactic acid, and the O-malonylglucoside conjugates of propachlor. A cysteine S-oxide conjugate was also observed as a transient metabolite. Fast atom bombardment mass spectrometry was used to characterize the polar metabolites. Some metabolites were isolated and identified from both soybean plants and peanut cell suspension cultures. The terminal metabolites in soybean plants were concentrated in the roots and foliage. Less than 1% of the metabolites were present in the beans and pods of the mature plants. In a sandy loam soil, propachlor and the cysteine conjugate of propachlor were both metabolized in approximately the same ratio to the bound residue fraction and three major metabolites: N-isopropyloxanilic acid, 2-sulfo-N-isopropylacetanilide, and 2-(sulfinylmethylenecarboxy)-N-isopropylacetanilide. The methyl sulfoxide and methyl sulfone analogs of propachlor were characterized as minor soil metabolites. The major metabolites present in soybean plants grown in soil treated with propachlor were those produced in the soil and taken up by the plants. It was speculated that propachlor metabolism in the soil involved microbial activity and conjugation with glutathione or cysteine. It was concluded that homoglutathione conjugates are metabolized in soybean in a manner similar to the metabolism of other glutathione conjugates in species such as peanut.