Fast and Slow Phases of Luminescence in Chlorella

The decay of luminescence of Chlorella in repetitive steady‐state flashing light displays two categories of phase, broadly characterized as ‘fast’ and ‘slow’. They are strongly contrasted in amplitude and lifetime; the transition from fast to slow occurs a few milliseconds after the flash. The slow phases observed in the presence of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) are clearly of the deactivation type. i.e. the luminescence intensity is quantitatively correlated with the rate of relaxation of the System II reaction center to its photoactive state. The significance of the deactivation type is a reverse flow of the light‐produced ⊕ and ⊖ charges through the luminescence‐producing recombination path. This mechanism is probably not limited to the DCMU‐poisoned systems. The light regime (flash duration, flashing period, induction effects) modifies specifically the amplitude of the slow phases; this effect is strikingly not dependent on the presence of DCMU. Although the light‐driven pH gradient is the likely explanation, it is argued that its action bears more directly on the exciton and/or emission yield of luminescence rather than on the rate of recombination itself. Most of the fast‐phase components are ascribed under normal conditions to stabilization steps involving the donor side of the System II reaction centers. However, when the reaction center turnover is much reduced (DCMU) or completely abolished (DCMU + NH2OH, after preillumination), a fast luminescence phase is still visible. This phase is barely affected by the light regime (notably the flash duration). It must be attributed to an anomalous residual photochemical turnover of the reaction centers. Another type of fast photochemical turnover has been characterized with NH2OH‐pretreated cells. A two‐quantum functioning of some sort seems required to account for the anomalous photochemical turnover. The importance of the luminescence loss during the fast phases and its possible connection with the ‘misses’ of the O2‐evolving system are discussed.