Silicomolybdate (SiMo) is an electron acceptor that has many characteristics, the ignorance of which makes an interpretation of the results quite troublesome. In photoinhibitíon experiments the photosystem 2 (PS 2) actívity can be best monitored if 1 pM dibromotíiymoquinone (DBMIB) is added after photoinhibitory treatment and SiMo is added in the light. Diuron (DCMU) may complicate interpretation of the results as it is also a competítíve inhibitor of SiMo binding at pH 7.6. The binding niche of SiMo is probably located at the stroma side between the fourth and fifth helix of the Dl and D2 proteins close to Q^- The whole chain actívity was much more affected by the photoinhibitory treatment than the PS 2 actívity itself Uncoupling of electron flow by addition of ammonium chloride accelerated the rate of photoinhibitíon. Photoinhibitory treatment decreased not only the Hill actívity at photon saturation, but also decreased the quantum yield and increased the photon flux density yielding half maximum rate of electron flow (K^). Decrease of quantum yield indicated that the photochemistry of PS 2 was afiected; increase of indicated a coiďormational change of the SiMo binding site. In experiments on PS 2 actívity monitored witíi SiMo, DCMU had no protectíve effect on the damage of the electron transport chain between water and QA.
Photosystem 2 (PS 2) reaction centre can be considered as a water-plastoqninone oxido-reductase. Using four photons it transfers four electrons from two molecules of water to plastoquinone (PQ), producing the molecular oxygen and two molecules of double reduced PQ. PS 2 is the site of the antagonistic action of bicarbonate and formáte on PS 2 electron flow; incubation of isolated chloroplasts with formáte results in full inhibition of electron flow actívity, which can be restored by addition of bicarbonate. This bicarbonate effect is located at the Dl protein and affects the electron flow between the primary quinone and the PQ pool. Bicarbonate is probably involved in stabilizatíon of tíie semireduced secondary quinone Qb, and in the protonation reactions at this site. Under physiological conditions bicarbonate is boimd to thylakoid membranes. Addition of formáte to thylakoids appears to release CO2. The bicarbonate effect is not only observed in isolated chloroplasts, but also in intact organisms as green algae and leaves. Bicarbonate Controls PS 2 electron flow in order to cope with stress conditions leading to, for instance, photoinhibition or to the high rates of photorespiration.