The size of the plastoquinone pool on the reducing side of photosystem 2 in the cyanobacterium Synechococcus sp. was estimated by measuring the area over the fluorescence induction curve in the presence of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone relative to the area in the presence of 3-(3,4-dichlorophenyl)-l,l-dimethylurea. Plastoquinone was found mostly in the reduced state in freshly harvested cells but was oxidized by aeration of the cells in the dark. The pool in the starved cells usually consisted of five or six plastoquinone molecules with a maximum of eight per photosystem 2 reaction center. Addition of glucose or fructose to the starved cells completely reduced the plastoquinone pool under anaerobic conditions or in the presence of KCN. The quinone reduced by brief illumination was rapidly and completely oxidized in the dark. The dark oxidation proceeded at a rate comparable to that of respiratory O2 uptake in the cyanobacterium and was strongly inhibited by KCN. It is concluded that a major population of the plastoquinone molecules present in the cells functions as the acceptor pool of photosystem 2 and that the pool is entirely shared by respiratory electron transport in the cyanobacterium.
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Aoki et al. (1983) studied this question.