Comparison of bicarbonate effects on them variable chlorophyll a fluorescence of CO2-depleted and non-C02-depleted thylakoids in the presence of diuron

Abstract

Evidence is presented from chlorophyll a fluorescence transient data for two sites of bicarbonate (HCO−3*) action in photosystem II. Both the absence of HCO−3(HCO−3-depleted thylakoids) and a high concentration of HCO−3(60 mMH CO j added to non-depleted thylakoids) accelerate the variable chlorophyll a fluorescence rise in the presence of 10 |i m diuron (DCMU). In non-HCO−3 depleted thylakoids the effect is independent of the order in which HCO−3 and DCMU are added, whereas in HCOJ-depleted thylakoids, the effect is seen only when HCO−3 is added before DCMU. We propose that the effect seen in HCO−3depleted thylakoids is indirectly due to the binding of HCO−3 functionally near the site of DCMU binding, which is also where HCO−3 exerts its major effect on electron transport between the primary quinone QA and the plastoquinone pool. We suggest that the smaller effect seen in non-HCOj-depleted thylakoids is due to the binding of HCOj at a second, lower affinity site. Binding at this site appears to require light, in contrast to the higher affinity site, which is inhibited by light. Bathocuproine, an inhibitor of the H20− to-silicomolybdate partial reaction, is synergistic with HCO−3 in its effect on the variable chlorophyll a fluorescence of non-HCO−3 -depleted thylakoids, and may bind heterotropically with HCOj. Thus, this second site of HCO−3 binding appears to be functionally near the bathocuproine binding site. © 1984, Walter de Gruyter. All rights reserved.