Turnover kinetics of photosystem I measured by the electrochromic effect in Chlorella

The rise kinetics of the absorption changes induced at 515 nm and 480 nm by a flash were studied using two types of xenon flashes of different durations. The 'slow' rise of the absorption change (t 1/2 = 15--20 microseconds) observed by Cox and Delosme (1978 C.R. Acad. Sci. (Paris) Sér. D 282, 775--778) and Joliot was found to be due to double hits occurring in the reaction centers of System I during the flash. The turnover kinetics of the reaction centers of System I after a short flash were studied by a double flash method. They are in agreement with a second order reaction between P+-700 and its electron donor.