The Hill reaction is significantly and reversibly inhibited in chloroplasts o...

gen evolution; (Synechocystis) Abst ract. In this communication, evidence is presented from the kinetics of QA decay (where QA is the first plastoquinone electron acceptor of photosystem II) and oxygen evolution for the requirement of bicarbonate in the electron transport in a cyanobacterium Synechocystis (Pasteur Culture Collection 6803). A large slowing down of QA oxidation, measured from the variable chlorophyll a fluorescence after saturating actinic flashes, was observed in the thylakoids of Synechocystis 6803 depleted of bicarbonate in the presence of 25 mM formate. Qualitatively similar results were obtained with DCMU-treated thylakoids. This shows that bicarbonate depletion inhibits electron transport on the acceptor side of photosystem II between QA and the plastoquinone (PQ) pool in cyanobacteria. Addition of 2.5 mM HCO3 fully reversed the inhibition of electron flow caused by bicarbonate depletion. Two exponen-tial phases of QT ~ decay, a fast one and a slow one, were observed with halftimes of approx. 400#s (fast) and 26ms (slow) at pH 6.5. At pH 7.5, these phases were approx. 330#s (fast) and 21 ms (slow), respectively. The amplitude, but not the halftime, of the fast component decreased by about 70 % (pH 6.5) or 50 % (pH 7.5); this was accompanied by a concomittan