Evidence that the variable fluorescence in Chlorella is recombination luminescence

The fluorescence lifetime of oxygen-forming photosynthetic systems as a function of closed traps has been studied by several groups using light and poisons (usually 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)) to fix the closed trap state during the experiment. These measurements have now been carried out using light alone, by means of pump and probe laser pulses and a very efficient fast photomultiplier-digitizing system. It is found that the absolute amplitude of fast fluorescence (mean tau, approx. 0.3 ns) remains constant until over half the traps are filled. The amplitude of the slow fluorescence (tau approximately equal to 1.2 ns) increases with pump energy, and its response is best fit with a lag or finite rise-time of approx. 200 ps. This novel result is consistent with the hypothesis that the slow component of the fluorescence is actually recombination luminescence in the trap. Thus, the full trapping time, i.e., the time to form the P+I- state from an excitation in the O2 photosystem, is relatively slow.