Short-lived delayed luminescence of photosynthesizing organisms II. The ratio between delayed and prompt fluorescence as studied by the modulation method

The ratio between the intensities of delayed and prompt fluorescence was studied for different photosynthetic objects under different conditions by a modulation method. The method is based on excitation of luminescing objects by light, modulated harmonically, and on a combined study of phase shifts and demodulation coefficients of the luminescence as related to excitation light. The presence of intense delayed emissions was revealed in purple bacteria, Ectothiorhodospira shaposhinokovii, Rhodospirillum rubrum and Rhodopseudomonas sphaeroides, in the micro- and nanosecond range. Under conditions of saturating light, their proportion was several percent of the total emission.The most striking phenomenon was observed under reducing conditions (addition of 1 · 10^?2 M Na₂S₂O₄ to whole-cell suspensions of purple bacteria) where the intensity of the delayed emissions grew dramatically and became comparable to that of prompt fluorescence.The data obtained indicate that, at room temperature, reversal of some early stages of charge separation in bacterial reaction centres may proceed largely via the channel that includes generation of the reaction-centre bacteriochlorophyll in the excited singlet state, followed by excitation-energy migration to antenna bacteriochlorophyll.The relation of these phenomena to the efficiency of solar energy utilization in photosynthetic apparatus is discussed.