Translocation and Metabolism of MAA-¹⁴C in Johnsongrass and Cotton

Radioautographs showed that root uptake of 14 C-methanearsonic acid (MAA- 14 C) by johnsongrass ( Sorghum halepense (L.) Pers.) from nutrient solution was rapid and translocation into all portions of the plant occurred within 4 hr after treatment. Basipetal and acropetal translocation of foliar and stem-applied MAA- 14 C occurred in both johnsongrass and cotton ( Gossypium hirsutum L., var. Rex smooth leaf), indicating that movement of the herbicide probably occurs in both phloem and xylem tissue. However, acropetal movement of MAA in both plants occurred at a higher rate than basipetal movement, and basipetal movement in cotton was not as rapid as in johnsongrass. Methanol extracts of MAA- 14 C johnsongrass were chromatographed. Radioactive spots with R f values different from the MAA- 14 C standard were observed. Use of location reagents on the chromatographs suggested that MAA- 14 C may complex with either a sugar or organic acid or both. Amino acid fractions were prepared from methanol extracts of both plants. An MAA metabolite, with a positive reaction to ninhydrin, was located on chromatographs spotted with the amino acid fraction from johnsongrass but was not present in the cotton fraction. A comparison of R f values suggests that the metabolite may be a combination of the MAA molecule with histidine or one of its analogues. Spectrophotometric determination showed a build-up of amino acids in MAA-treated johnsongrass but not in cotton. The increased levels of amino acids in johnsongrass suggest that the MAA metabolite may be blocking a protein synthesis pathway, or some other unknown biosynthetic pathway.