Site of Action of Oxyfluorfen

Abstract

The effects of oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene] were studied on electron transport and phosphorylation in isolated spinach (Spinacia oleraceaL.) chloroplasts and on the response of green bean (Phaseolus vulgarisL. ‘Spartan Arrow’) to applications of oxyfluorfen alone or in combination with other herbicides. Coupled non-cyclic electron transfer and phosphorylation through photosystems I and II (H2O å methyl viologen) were inhibited about 30% and 55%, respectively, by 10−4M oxyfluorfen. Photosystem II-linked phosphorylation with dimethylbenzoquinone (2,5-dimethyl-p-benzoquinone) as the electron acceptor (H2O å DMQ) was completely inhibited by 10−4M oxyfluorfen. Photosystem II electron transport with dimethylbenzoquinone as the electron acceptor was inhibited 60% by 10−4M oxyfluorfen, whereas photosystem I electron transport with 2,6-dichlorophenol indophenol as electron donor (DCIPH2å MV) was not susceptible to oxyfluorfen inhibition. Photosystem I-linked phosphorylation and the accompanying electron transport supported by durohydroquinone electron donation (DQH2å MV) were inhibited about 50% by 10−4M oxyfluorfen, whereas cyclic phosphorylation was not inhibited at that concentration. Increased conductivity of a solution that contained leaf discs taken from green beans treated with various combinations of foliar-applied herbicides was a measure of membrane damage caused by the herbicides, and revealed that oxyfluorfen has a different site of action than do photosynthesis inhibitor and bipyridilium herbicides. Oxyfluorfen plus dinoseb (2-sec-butyl-4,6-dinitrophenol) injury to green beans was additive, but the two herbicides did not have the same site of action. Oxyfluorfen did not appear to inhibit electron transport in chloroplasts at herbicidal rates, nor was it dependent on electron transport for activation.