Quantitative analysis of fluorescence induction curves in isolated spinach chloroplasts

Based on the current reaction scheme for the photoreduction of the primary plastoquinone of Photosystem II (PS II) referred to as Q A , and its reoxidation by the plastoquinone pool it was analyzed to what extent the redox state of Q A dominates the fast chlorophyll a fluorescence rise elicited by continuous illumination of isolated class-II chloroplasts in the absence of exogenous electron acceptors. The present study shows that the fluorescence rise kinetics as a function of actinic light (0.2-1 mW/cm 2 ) and of different blockage degrees of Q A reoxidation by the PQ pool can be quantitatively described by the assumption that the Q A redox state is the only factor determining the variable fluorescence yield. In order to account for the nonlinearity between the variable fluorescence F v (r) and Q A (r) appropriate assumptions have to be introduced for excitation energy migration. It was found that the existence of α- and β-units with excitation energy transfer only between the a-entities (P = 0.5) provides a satisfactory fit of the experimental data. Implications for the practical use of fluorometric methods are briefly discussed.