PhotosynthesisResearch 46: 219-225, 1995. ~) 1995 Kluwer AcademicPublishers. Printed in the Netherlands. Regular paper Effects of bicarbonate and formate on the donor side of Photosystem 2 V. V. Klimov, S. I. Allakhverdiev, S. V. Baranov & Ya. M. Feyziev 1 Institute of Soil Science and Photosynthesis, Russian Academy of Sciences, Pushchino, Moscow region, 142292, Russian Federation; lpermanent address: Institute of Botany, Azerbaijan Academy of Science, 370602, Baku, Patamdartskoe shosse 40, Azerbaijan Received 16 March 1995;accepted in revisedform25 May 1995 Key words: Photosystem 2, bicarbonate, formate, donor side, manganese Abstract Sodium formate at concentration of 5-20 #M suppresses electron flow on the donor side of Photosystem 2 (PS 2) in pea subchloroplast membranes (DT-20) which is revealed by inhibition of photoinduced changes of chlorophyll fluorescence yield related to photoreduction of QA and pheophytin (the primary and intermediary electron acceptors) and oxygen evolution and the increase of absorbance changes related to photooxidation of P680, the primary electron donor, under continuous illumination. These activities are also inhibited upon partial depletion of bicarbonate in the medium and restored by the addition of 0.1-10 mM NaHCO3. At concentrations higher than 20/zM formate induces the known bicarbonate effect on the acceptor side of PS 2 which dominates at millimolar concentrations of the agent. In Tris-treated (Mn-depleted) DT-20 the restoration of electron flow with 0.2 #M MnCI2 (4 Mn atoms per one PS 2 reaction center) in the medium depleted of bicarbonate is efficient only after the addition of 5 mM NaHCO3. The restoration in the presence of NaHCO3 is accompanied by an increased functional binding of Mn 2+ to PS 2 membranes which is confirmed by experiments on removal of added Mn 2+ by either sedimentation or the addition of EDTA. Pre-illumination increases the Mn binding in the presence of bicarbonate. The data show that the bicarbonate effect on the donor side of PS 2 is related to a relatively low-affinity bound pool of bicarbonate. It is suggested that bicarbonate takes part in the formation of the Mn-cluster capable of water oxidation as an obligatory ligand or through modification of the binding site(s) of Mn. Abbreviations: CCCP - carbonyl cyanide-m-chlorophenylhydrazone; DCMU - 3-(3,4-dichlorphenyl)-l,1- dimethylurea; DPC - diphenylcarbazide; EDTA - ethylenediaminetetraacetic acid; SiMo - silicomolibdate Introduction The bicarbonate ion (HCO 3) is essential in operation of Photosystem 2 (PS 2) reactions (Warburg and Krip- pahl 1958; Stemler and Govindjee 1973; Wydrzynski and Govindjee 1975). There is at least two pools of bicarbonate with different affinity of its binding to PS 2, and evidently only the high-affinity one (1 molecule per 400-600 Chl molecules or per one PS 2 reaction cen- ter) controls PS 2 activity while the lower-affinity pool (close to Chl concentration) can be removed without essential consequence for PS 2 (Stemler 1977; Stemler and Murphy 1983). Interpretation of the stimulating effect of bicarbonate on PS 2 activities (the 'bicarbon- ate effect') remains controversial. In the early 1970s the high-affinity pool had been ascribed to the donor side ofPS 2 (Stemler and Govindjee 1973) and a model including bicarbonate as a mediator for photosynthet- ic water oxidation had been suggested (Metzner 1978; Stemler 1980) which however was in contradiction with the results of isotopic experiments (Stemler and Radmer 1975; Radmer and Ollinger 1980). Wydrzyn- ski and Govindjee (1975) showed that the major bicar- bonate effect is located between the primary electron acceptor, QA, and the plastoquinone pool and this idea was supported by a number of data (Mende and Wiess-