CORRELATION BETWEEN DELAYED LIGHT EMISSION AND FLUORESCENCE OF CHLOROPHYLL a IN SYSTEM II PARTICLES DERIVED FROM SPINACH CHLOROPLASTS

Abstract

Abstract— The induction transient of delayed light of chlorophyll a, excited by repetitive flashes (0.5 ms in duration) and emitted 0.1 ‐ 1.2 ms after the flashes, was measured in system II particles derived from spinach chloroplasts. An uncoupler, gramicidin S, was always added to the particles in order to eliminate the influence of the phosphorylation system on the delayed light and to isolate a direct relationship between the delayed light emission and the primary photochemical reaction, except for the experiments described in the next paragraph.The yield of delayed light emission from the system II particles was found to be about three–times higher than that of chloroplasts on a chlorophyll content basis. System I particles, on the other hand, emitted much weaker delayed light than chloroplasts.Upon intermittent illumination, induction of delayed light in system II particles showed a decrease from the initial rise level to the steady‐state level. The initial rise level was the maximum. The fluorescence induction, on the other hand, exhibited an increase from the initial rise level to the maximum steady‐state level. The induction of both delayed light emission and fluorescence arrived at their final steady‐state levels after the same period of illumination.Induction of delayed light emission was measured under various conditions that changed the oxidation‐reduction state of the primary electron acceptor, X, of photoreaction II: by adding an electron acceptor and an inhibitor of electron transport, and by changing the light intensity. The state of A'was monitored by measuring the fluorescence yield. The yield of delayed light emission excited by each flash was found to depend on the amount of oxidized form of X present before the flash.To examine the role of the primary electron donor Y of photoreaction II in delayed light emission, effects of electron donors of photoreaction II such as Mn2+, hydroquinone and p‐phenylenediamine were investigated. These agents were found to markedly decrease the yield of delayed light emission without altering the pattern of its induction. They had little effect on the induction of fluorescence.These findings are interpreted by a mechanism in which transformation of the reaction center from the form (X‐Y+) into (X Y) produces a singlet excitation of chlorophyll a that is the source of millisecond delayed light emission. This reaction is probably non–physiological and must be very slow if compared to the transformation of (X‐Y+) into (X‐Y).Since the form (X‐Y+) is produced when the excitation is transferred to the reaction center in the form (XY), it is expected in this scheme that the yield of delayed light emission should depend on the amount of the form (X Y) present before the excitation flashes. Electron donors stimulate transformation of the reaction center from (X‐Y+) into (X‐Y). Since this reaction competes with the process of delayed light emission, electron donors are expected to suppress delayed light emission.