Monovalent Anion Interaction on the Electron Flow of Photosystem II in Soyabean Cells

Abstract

Steady state of electron transport measurements from water to DMQ (2, 5 - dimethyl - P- benzoquinone) show a maximum bicarbonate effect (about four fold stimulation) following the addition of 5 mM HCO"3 to the previously 25 mM (formate or nitrate) treated cells at pH 6.7. However, addition of 5 mM bicarbonate to the non-anion treated cells (the cells were exposed for 150 min to nitrogen gas only) produced about 70% restoration compared to the anion-treated cells. Increasing the incubation time up to 3 hours decreased the restoration to 43%. Thus, exposing the cells to N2 gas in the absence of the inhibitory anions caused an irreversible damage of photosystem II reflected by the linear electron transport from H2O to DMQ. Studying the kinetics of QA" reoxidation (QA is the primary quinone electron acceptor of photosystem II) of the anion treated cells shows that nitrite and formate can substitute each other in replacing bicarbonate between QA" and QB (QB is the secondary quinone electron acceptor of photosystem II) and to the plasto-quinone pool. Analysis of chlorophyll a fluorescence decays of non-anion treated and HCO'3 restored cells demonstrates a remarkable irreversible inhibition of the electron flow between QA' and QB at the electron acceptor side of photo-system II. Thus, the anion interaction on the electron transport is suggested to be located at the level of the two electron gate of photosystem II in the intact soybean cells.