Kinetics of reduction of the oxidized primary electron donor of photosystem II in spinach chloroplasts and in Chlorella cells in the microsecond and nanosecond time ranges following flash excitation.

Absorption changes (deltaA) at 820 nm, following laser flash excitation of spinach chloroplasts and Chlorella cells, were studied in order to obtain information on the reduction time of the photooxidized primary donor of Photosystem II at physiological temperatures. In the microsecond time range the difference spectrum of deltaA between 750 and 900 nm represents a peak at 820 nm, attributable to a radical-cation of chlorophyll a. In untreated dark-adapted material the signal can be attributed solely to P+-700; it decays in a polyphasic manner with half-times of 17 microseconds, 210 microseconds and over 1 ms. The oxidized primary donor of Photosystem II (P+II) is not detected with a time resolution of 3 microseconds. After treatment with 3--10 mM hydroxylamine, which inhibits the donor side of Photosystem II, P+II is observed and decays biphasically (a major phase with t1/2=20--40 microseconds, and a minor phase with t1/2 congruent to 200 microseconds), probably by reduction by an accessory electron donor. In the nanosecond range, which was made accessible by a new fast-response flash photometer operating at 820 nm, it was found the P+II is reduced with a half-time of 25--45 ns in untreated dark-adapted chloroplasts. It is assumed that the normal secondary electron donor is responsible for this fast reduction.