Differential inhibition of phosphoenol-pyruvate carboxylases by 2,4-dichlorophenoxyacetic acid and two newly synthesized herbicides

Abstract

The efrect of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and two new phenoxy compounds on the in vitro activity of plant leaf phosphoenol-pyruvate carboxylases has been examined. The C 4 leaf enzymes (0.66 nkat) were completely inhibited by these chemicals (4 mM). The activity of the sorghum leaf enzyme was partially recovered when either the effector glucose-6-phosphate or the substrate phosphoenol-pyruvate was added to the medium after the reaction had been started in the presence of the inhibitors. Glycine, another enzyme activator, did not reactivate the inhibited enzyme. C 3 plant PEP carboxylases were little affected by the three herbicides. An intermediary inhibition was shown to occur in C 3 Gramineae and etiolated sorghum leaves. Other enzyme activities of carbon and nitrogen metabolism were unaffected. The results allow discrimination between the metabolic type of plants. In this respect, the best response was obtained with the newly synthesized monochlorinated derivative.