INVITRO MODE OF ACTION OF N-PHENYLIMIDE PHOTOBLEACHING HERBICIDES

The effect of N-Phenylimide photobleaching herbicides on in vitro synthesis of protoporphyrin was examined. The N-phenylimide photobleaching herbicide S-23142 [N-(4-chloro-2-fluoro-5-propargyloxyphenyl)-3,4,5,6-tetrahydrophthalimide] inhibited the Mg-protoporphyrin IX synthesis of intact plastids by 50% at 10-10 M. Protoporphyrin IX synthesizing activity was solubilized from plastid membranes by n-dodecyl β-d-maltoside, and the solubilized activity was inhibited by several photobleaching herbicides. In vitro binding of N-phenylimide photobleaching herbicide, S-23121 [N-[4-chloro-2-fluoro-5-[(1-methyl-2-propynyl)oxyl]phenyl]-3,4,5,6-tetrahydrophthalimide], to a solubilized plastid fraction was examined. A good correlation between protoporphyrin IX synthesizing activity and [14C]S-23121 specific binding was observed. Binding site affinity (Kd) was 8.9 - 9.8 nM. The binding was displaced by a diphenylether photobleaching herbicide, acifluorfenethyl, and another N-phenylimide, S-23142, but not by a photosystem II electron transport inhibitor, DCMU. Bound [14C]S-23121 was dissociated by an excess of cold S-23121, which indicated the reversible binding of S-23121. © 1991.