Characterization of the photosystem II centers inactive in plastoquinone reduction by fluorescence induction.

In order to characterize the photosystem II (PS II) centers which are inactive in plastoquinone reduction, the initial variable fluorescence rise from the non-variable fluorescence level Fo to an intermediate plateau level Fi has been studied. We find that the initial fluorescence rise is a monophasic exponential function of time. Its rate constant is similar to the initial rate of the fastest phase (α-phase) of the fluorescence induction curve from DCMU-poisoned chloroplasts. In addition, the initial fluorescence rise and the α-phase have the following common properties: their rate constants vary linearly with excitation light intensity and their fluorescence yields are lowered by removal of Mg(++) from the suspension medium. We suggest that the inactive PS II centers, which give rise to the fluorescence rise from Fo to Fi, belong to the α-type PS II centers. However, since these inactive centers do not display sigmoidicity in fluorescence, they thus do not allow energy transfer between PS II units like PS IIα.