The effect of herbicides on components of the PS II reaction centre measured by EPR

Abstract

Incubation of PS II membranes with herbicides results in changes in EPR signals arising from reaction centre components. Dinoseb, a phenoiic herbicide which binds to the reaction centre polypeptide, changes the width and form of the EPR signal arising from photoreduced Q.iFe. o-Phenanthroline slightly broadens the QAFe signal. These effects are attributed to changes in the interaction between the semiquinone and the iron. DCMU, which binds to the 32 kDa protein, has virtually no effect on the width of the QAFe signal but does give rise to an increase in its amplitude. This could result from a change in redox state of an interacting component. Herbicide effects can also be seen when QAFe is chemically reduced and these seen to be reflected by changes in splitting and amplitude of the split pheophytin-signal. Dinoseb also resuhs in the loss of 'Signal II dark', the conversion of reduced Hugh-potential cytochrome b559 to its oxidized low-potential form and the presence of transiently photooxidized carotenoid after a flash at 25°C; these effects indicate that dinoseb may also act as an ADRY reagent.

EPR measurements on the effects of bicarbonate and triazine resistance on the acceptor side of Photosystem II

“…Depletion of CO2 had no effect on either of these signals (not shown); the amplitudes of the signals were similar to those reported in this kind of PS II preparation [7]. When samples were reduced with sodium dithionite in darkness before freezing, EPR spectra from QA -Fe'+ were obtained ( fig.2) which were similar to those photoinduced in fig.1.…”

“…In all samples cytochrome bss9 and the Signal II present in the dark were monitored before and after 77 K illumination using EPR conditions as previously reported [7]. Depletion of CO2 had no effect on either of these signals (not shown); the amplitudes of the signals were similar to those reported in this kind of PS II preparation [7].…”

“…Both resonance forms are affected by herbicides which block electron transfer between the primary and secondary quinone acceptors [5-71. Pheo-) is almost identical to the analogous signal previously observed in purple bacteria [9]. In PS II the split Pheo-signal is also affected by pH [5] and the presence of herbicides [7].…”

“…Oxygen-evolving PS II-enriched thylakoid fragments were prepared from spinach, or from triazine-resistant and -susceptible rape seed (Brassicu napus L.) according to [ZO] with modifications as in [7]. The thylakoid fragments used were prepared freshly, or had been stored at high concentrations (5-10 mg -ml-') at -80°C in darkness.…”

“…The splitting of the Pheo-signal was =40 G in control and reconstituted samples, but was found to be decreased to =31 G in COz-depleted samples. Earlier results show that when the QA -Fe2+ signal at g = 1.82 increases compared to that at g = 1.90 (at low pH [5] or in the presence of dinoseb [7]) the width of the split Pheo-signal decreases. The minimal width reported was 33 G in the presence of dinoseb, where a large g = 1.82 but no g = 1.90 signal was detectable.…”