Inhibition of Photosynthetic Electron Transport by Ioxynil

Abstract

THE herbicide ioxynil (4-hydroxy-3,5-diiodo benzonitrile)1 has been found to inhibit the photoreduction of ferricyanide by chloroplasts from a range of plants2 and also NADP photoreduction, the accompanying photophosphorylation and carbon dioxide fixation by the broad bean (Vicia faba) chloroplasts3. Recently, Paton and Smith4 further reported that ioxynil inhibited the photoreduction of endogenous plastoquinone in broad bean chloroplasts, even in the presence of ascorbate and indophenol. It was previously found5 that although o-phenanthroline and the herbicide DCMU inhibit the photoreduction of endogenous plastoquinone they do not inhibit the enhanced photoreduction in the presence of ascorbate and indophenol. Furthermore, ascorbate and indophenol reduce plastoquinone in darkness6, in a reaction which has been shown to be non-enzymatic7. No other inhibitors previously investigated have inhibited the dark reduction and we therefore decided to investigate the effects of ioxynil on plastoquinone reduction in more detail.