Effects of sodium azide on photosystem II of Chlorella pyrenoidosa

The action of sodium azide on the electron transport chain was investigated by means of oxygen evolution, fluorescence and luminescence measurements. (1) The damping of the oxygen oscillations is progressively reduced with increasing azide concentration in the range of 10(-5) - 10(-1) M. (2) The rate of the dark decay of the S2 and S3 states is considerably slowed. The degree of slowing is dependent on concentration. (3) Luminescence is inhibited by azide both in the presence and absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU). (4) The fluorescence induction curve in strong light is modified in the presence of azide and its shape depends on azide concentration and on incubation time. (5) At a given time after a saturating flash, the fluorescence yield in the presence of azide is much higher than that of the control. It seems to be due to a general fluorescence increase rather than to a slower Q- reduction. (6) We tentatively propose an accelerated reduction of the primary donor P+ in state S2 and S3, by the intermediate donor Z in the presence of azide. Additionally, we have to assume that in the S2 and S3 states, some centers are blocked in an inactive low fluorescent form and that azide decreases their concentration.