Determination and modification of the redox state of the secondary acceptor of photosystem II in the dark.

The redox state of the secondary electron acceptor B of Photosystem II was studied using flourescence measurements. Preillumination of algae or chloroplasts with a variable number of short saturating flashes followed rapidly by the addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea induces oscillations of the initial level of fluorescence. The phase of these oscillations is characteristic of a given B/B-ratio in the dark-adapted samples. We conclude from our results that about 50% of the secondary electron acceptors are singly reduced in the dark in chlorella cells, but that more than 70% are fully oxidized in the dark adapted chloroplasts. Benzoquinone treatment modifies this distribution in Clorella leading to the same situation as in chloroplasts, i.e. more than 70% of the secondary acceptors are oxidized in the dark. The same ratio is observed if these algae are illuminated and then dark-adapted, unless an artificial donor (hydroxylamine) is added before this illumination. In that case about 50% B- is generated and stabilized in the dark.