Two types of inactive photosystem II centers in green alga Chlorella grownunder conditions of nitrogen deficiency

Growth of the alga Chlorella pyrenoidosa in a nitrogen-free medium results in a reversible inactivation of PS II, which is manifested in a decrease in the yield of variable fluorescence F-V of chlorophyll and disappearance of a very slow (half-rise time of about 100 s) component of the fluorescence induction curve measured in the presence of diuron. The process of PS II re activation after the addition of KNO3 to cells grown in the absence of nitr ogen, as monitored by the recovery of the original yield of variable fluore scence, consisted of two phases. The first phase of reactivation was indepe ndent of light; during this phase, the maximum yield of chlorophyll fluores cence F-M slowly increased, whereas the initial fluorescence level F-0 rema ined constant. As a result, the F-V/F-M ratio increased from 0.35 to 0.5 af ter 10-h dark incubation of cells in the presence of nitrogen; this was acc ompanied by the appearance of the slow component of the fluorescence induct ion curve. Subsequent illumination of a culture in which the F-V yield had been partly restored in the dark resulted in a rapid decrease in the F-0 fl uorescence yield, within 15 min, which was accompanied by a corresponding i ncrease in the F-V/F-M ratio from 0.5 to 0.75 and by enhancement of the fas t (half-rise of about 100 ms) component of the fluorescence induction curve . It is supposed that the slow and fast components of chlorophyll fluoresce nce induction in Chlorella reflect the presence of two types of PS II react ion centers that significantly differ in the efficiency of formation of sta ble reduced (in the presence of diuron) primary quinone acceptor Q(A). The data suggest that inactivation of two types of PS II reaction centers durin g nitrogen-deficient growth of Chlorella occurs through different mechanism s, which results in the appearance of inactive PS II centers with an elevat ed F-0 and reduced F-M yield.