Fig. 4.

Time courses of the X-ray fluorescence at pO₂ of 0.2 bar (Left) and 13 bar (Right) as induced by the first 8 of a train of 10 laser flashes applied to dark-adapted PSII samples (excitation energy of 6,552.5 eV). Approximately 500 (0.2 bar) and 1,500 (13 bar) transients were averaged and are displayed at a resolution of 50 μs per data point. The 13 bar transients were obtained by averaging measurements for pO₂ values ranging from 11 to 16 bar. Data were normalized such that the amplitude of the response to the 1st flash was similar for the 0.2 and the 13 bar datasets. Smooth lines represent simulations by a biexponential function using a halftime of 1.2 ms to account for Mn reduction (positively directed changes) and variable halftimes for Mn oxidation (negatively directed signals). For selected transients the contributions from Mn oxidation (open arrows) and Mn reduction (filled arrows) are indicated.