ABSTRACUChlorsulfuron, an inhibitor of acetolactate synthase (EC 4.13.18) (TB Ray 1984 Plant Physiol 75: 827-831), markedly inhibited the growth of Lemma minor at concentrations of 10' molar and above, but had no inhibitory effects on growth at 10' molar. At growth inhibitory concentrations, chlorsulfuron caused a pronounced increase in total free amino acid levels within 24 hours. Valine, leucine, and isoleucine, however, became smaller percentages of the total free amino acid pool as the concentration of chlorsulfuron was increased. At concentrations of chlorsulfuron of 10'8 molar and above, a new amino acid was accumulated in the free pool. This amino acid was identified as a-amino-n-butyrate by chemical ionization and electron impact gas chromatography-mass spectrometry. The amount of a-amino-n-butyrate increased from undetectable levels in untreated plants, to as high as 840 nanomoles per gram fresh weight (2.44% of the total free pool) in plants treated with 10 molar chlorsulfuron for 24 hours. The accumulation of this amino acid was completely inhibited by methionine sulfoximine. Chlorsulfuron did not inhibit the methionine sulfoximine induced accumulations of valine, leucine, and isoleucine, supporting the idea that the accumulation of the branched-chain amino acids in methionine sulfoximine treated plants is the result of protein turnover rather than enhanced synthesis. Protein turnover may be primarily responsible for the failure to achieve complete depletion of valine, leucine, and isoleucine even at concentrations of chlorsulfuron some 104 times greater than that required to inhibit growth. Tracer studies with "N demonstrate that chlorsulfuron inhibits the incorporation of '"N into valine, leucine, and isoleucine. The a-amino-nbutyrate accumulated in the presence of chlorsulfuron and ["NjR.+ was heavily labeled with "N at early time points and appeared to be derived by transamination from a rapidly labeled amino acid such as glutamate or alanine. We propose that chlorsulfuron inhibition of acetolactate synthase may lead to accumulation of 2-oxobutyrate in the isolencine branch of the pathway, and transamination of 2-oxobutyrate to a-aminon-butyrate by a constitutive transaminase utilizing either glutamate or alanine as a-amino-N donors.