CONSEQUENCES ON NITROGEN-METABOLISM IN SOYBEAN (GLYCINE-MAX L) AS A RESULT OF IMAZETHAPYR ACTION ON ACETOHYDROXY ACID SYNTHASE

The extractable activity of acetohydroxy acid synthase (AHAS; EC 4.1.3 .18) was found to be severely decreased in soybean (Glycine max L.) sh oots for a period of 120 h after treatment with imazethapyr. Inhibitio n did not take place when a mixture of valine, leucine, and isoleucine (VLI mixture) was added to the herbicide treatment. As a consequence of this inhibition valine, leucine, and isoleucine decreased and most of the other amino acids increased. Also a decrease in total soluble p roteins was found with respect to the untreated controls. This did not exceed 25% and was prevented by the addition of the VLI mixture to th e herbicide treatment. The increases in most of the free amino acids s eem to be attributable to a change in the protein turnover rate of pre -existing proteins rather than to new amino acid synthesis. This behav ior has been hypothesized to be a result of the demand for ammonia in connection with the decrease in branched-chain amino acids deriving fr om AHAS inhibition. The reduction of nitrite reductase (NiR; EC 1.7.7. 1), glutamine synthetase (GS; EC 6.3.1.2), and glutamate synthase (GOG AT; EC 1.4.1.14) activities in imazethapyr-treated samples and the rec overy in the imazethapyr plus VLI mixture treated samples of GS and GO GAT activities after a 24 h period following NiR recovery support this hypothesis.