POLYPHASIC CHLOROPHYLL a FLUORESCENCE TRANSIENT IN PLANTS AND CYANOBACTERIA*

Abstract— The variable chlorophyll (Chl) a fluorescence yield is known to be related to the photochemical activity of photosystem II (PSII) of oxygen‐evolving organisms. The kinetics of the fluorescence rise from the minimum yield, F 0 , to the maximum yield, F m , is a monitor of the accumulation of net reduced primary bound plastoquinone (Q A ) with time in all the PSII centers. Using a shutter‐less system (Plant Efficiency Analyzer, Hansatech, UK), which allows data accumulation over several orders of magnitude of time (40 μs to 120 s), we have measured on a logarithmic time scale, for the first time, the complete polyphasic fluorescence rise for a variety of oxygenic plants and cyanobacteria at different light intensities. With increasing light intensity, the fluorescence rise is changed from a typical O‐I‐P characteristic to curves with two intermediate levels J and I, both of which show saturation at high light intensity but different intensity dependence. Under physiological conditions, Chl a fluorescence transients of all the organisms examined follow the sequence of O‐J‐I‐P. The characteristics of the kinetics with respect to light intensity and temperature suggest that the O‐J phase is the photochemical phase, leading to the reduction of Q A to Q A ‐ . The intermediate level I is suggested to be related to a heterogeneity in the filling up of the plastoquinone pool. The P is reached when all the plastoquinone (PQ) molecules are reduced to PQH 2 . The addition of 3‐(3–4‐dichlorophenyl)‐1,1‐dimethylurea leads to a transformation of the O‐J‐I‐P rise into an O‐J rise. The kinetics of O‐J‐I‐P observed here was found to be similar to that of O‐I 1 ‐I 2 ‐P, reported by Neubauer and Schreiber ( Z. Naturforsch. 42c , 1246–1254, 1987). The biochemical significance of the fluorescence steps O‐J‐I‐P with respect to the filling up of the plastoquinone pool by PSII reactions is discussed.