OXYGEN EVOLUTION BY PHOTOSYSTEM-II - THE CONTRIBUTION OF BACKWARD TRANSITIONS TO THE ANOMALOUS BEHAVIOR OF DOUBLE-HITS REVEALED BY A NEW ANALYSIS METHOD

Backward transitions in the analysis of oxygen production under flashi ng light were introduced by Packham et al., 1988, Photosynth. Res. 15: 221-232. In order to take backward transitions into account, a new me thod of analysis is presented: the 'eigenvalue method'. This method is based on the recurrence relation of oxygen production with four coeff icients (also known as the four 'sigma' coefficients). It shows less s usceptibility to round-off errors than other methods and permits the c omputation of double-hits directly from the coefficients, which was no t possible before. With it we discovered that the inconsistent behavio ur of double-hits observed previously under low flash intensities or l ow flash frequencies was mainly due to the inclusion of the backward t ransitions into the double-hit probability. In these conditions backwa rd transitions seemed to be due either to the combination of an S-stat e deactivation and a miss, or to two S-state deactivations and a singl e-hit. In the presence of 3-(3,4-Dichlorophenyl)-1, 1-dimethylurea (DC MU), the previous methods of 'sigma' analysis failed. In contrast, the new method resolved all four S-state probabilities; thus it has the f urther advantage of being more 'robust' (robustness being defined as t he ability to yield a meaningful answer under difficult conditions).