The rise in chlorophyll a fluorescence yield and decay in delayed light emission in Tris-washed chloroplasts in the 6–100 μs time range after an excitation flash

Abstract

Parallel measurements of the rise in chlorophyll a fluorescence yield and delayed light emission decay, after a 10 ns saturating excitation flash, have been made in tris(hydroxymethyl)aminomethane-washed chloroplasts. Various electron donor systems (Mn 2+; ascorbate; reduced phenylenediamine and benzidine) were used in conjuction with different preillumination regimes to alter [ P +-680], the oxidized form of the Photosystem II reaction center chlorophyll a. Conditions giving rise to high [ P +-680] resulted in only a small rise in fluorescence yield, an inhibition of a 6 μs delayed light component, and an enhancement of a 60 μs component of delayed light emission. These results confirm the hypothesis that P +-680 acts as a quencher of fluorescence and that delayed light emission in the microsecond time range is due to the back reaction of P +-680 and Q −. (Q is the first “stable” electron acceptor of Photosystem II.) Two preillumination flashes are required before the full effect of Tris washing is observed in the delayed light emission decay and fluorescence yield rise; this suggests that a capacity to hold two charges exists between the Tris block and P +-680. Tris washing has no direct effect on the movement of electrons from Z (the first electron donor to P +-680) to P +-680. Finally, Mn 2+ donates electrons to P +-680 via Z.