Charge accumulation at the reducing side of system 2 of photosynthesis

A study was made of the reactions between the primary and secondary electron acceptors of Photosystem 2 by measurements of the increase of chlorophyll fluorescence induced in darkness by dithionite or by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). The experiments were done either with chloroplasts to which hydroxylamine or carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP) was added, or with chloroplasts treated with tris(hydroxymethyl)aminomethane (Tris) to which phenylenediamine and ascorbate were added as donor system. Under these conditions the fluorescence increase induced by dithionite or DCMU added after illumination with short light flashes was dependent on the flash number with a periodicity of two; it was large after an uneven number of flashes, and small after a long darktime or after an even number of flashes. The results are interpreted in terms of a model which involves a hypothetical electron carrier situated between Q and plastoquinone; this electron carrier is thought to equilibrate with plastoquinone in a two-electron transfer reaction; the results obtained with DCMU are explained by assuming that its midpoint potential is lowered by this inhibitor.