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The effect of glyphosate on aromatic amino acid metabolism in purple nutsedge sprouted tubers and shoots was investigated. Glyphosate at 33.5 mM caused inhibition of bud elongation, increased total free amino acid concentration, and caused rapid accumulation of shikimic acid in sprouted tubers. However, only one aromatic amino acid, tryptophan, decreased quickly to 22% of control 3 d after treatment (DAT) and remained low afterwards. This suggests that the inhibition of bud elongation is due to the rapid accumulation of shikimic acid and the repression of tryptophan synthesis. Foliar application of glyphosate at 14.5 mM to purple nutsedge shoots resulted in the rapid accumulation of glyphosate which was rapidly converted to its metabolite, aminomethylphosphoric acid. Free amino acids in leaves were also increased by glyphosate 3 DAT. The reduction in soluble protein 5 DAT and increased acid protease activity 3 DAT suggests that the late accumulation of free amino acids partially resulted from protein hydrolysis. Shikimic acid accumulated in glyphosate-treated leaves 5 DAT, but the concentration of the three aromatic amino acids was not reduced. This suggests that glyphosate toxicity in purple nutsedge shoots was associated with the rapid accumulation of glyphosate, followed by large accumulation of shikimic acid. Aromatic amino acids deficiency was apparently not a factor in toxicity.
Weed Technology © 2001 Weed Science Society of America