COOPERATION OF CHARGES IN PHOTOSYNTHETIC O2 EVOLUTION‐II. DAMPING OF FLASH YIELD OSCILLATION, DEACTIVATION*

Abstract

Abstract— A quantitative analysis is made of a linear 4‐step model for photosynthetic O2 evolution in which each photochemical trapping center or an associated enzyme cycles through 5 oxidation states (S0, S1, S2, S3, S4). The overall reaction is: S0→ S4, S4→ S0+ O2, where kd= rate of dark reaction. Based on data obtained with isolated chloroplasts, four aspects were considered: (1) The two perturbations which damp the oscillation of the O2 flash yield in a flash sequence given after a dark period‐(a) a failure rate (α) of the trapping centers in the photochemical conversion (‘misses’) and (b) double effective excitations in a fraction (8) of the centers which are in the S0 and S1 states (‘double hits’). Best fit with the experimental data is obtained for α= 0.1, β=0.05. (2) The kinetics and the mechanism of deactivation—the loss of + charges (O2 precursors) in the dark. The momentary distribution of the four oxidation states (S0‐‐S3) in a sample can be computed from the O2 yields of four consecutive flashes with corrections for a and β. The time course of the various states in the dark following specific preilluminations reveals that deactivation proceeds in single equivalent steps: S3→S2, S2,→S1. S1, itself stable in the dark, is the end product of deactivation. The ground state S0 cannot be formed by deactivation in the dark but is only formed during illumination. (3) The various ratios [S0]/[S1] which can be observed in a sample after deactivation following different preilluminations with flashes or continuous light. (4) The transients of the O2 evolution rate in weak continuous light as observed after deactivation with or without flash preillumination. In all instances satisfactory agreement is found between the observations and the predictions of the model.