THE RISE IN CHLOROPHYLL a FLUORESCENCE YIELD AND DECAY IN DELAYED L I G H T EMISSION IN TRIS-WASHED CHLOROPLASTS IN TH E 6-100 #s TIME RANGE AFTER AN EXCITATION FLASH

Parallel measurements of the rise m chlorophyll a fluorescence yield and delayed hght emission decay, after a 10 ns saturating excitation flash, have been made in trls(hydroxymethyl)ammomethane-washed chloroplasts Various electron donor systems (Mn2+; ascorbate; reduced phenylenediamlne and benzidme) were used m conjuction with different preilluminatlon regimes to alter [P +-680], the oxidized form of the Photosystem I! reaction center chlorophyll a. Condmons gwmg rise to high [,o+-680] resulted m only a small rise in fluorescence yield, an inhibmon of a 6 #s delayed hght component, and an enhancement of a 60 #s component of delayed hght emission. These results confirm the hypothesis that P+-680 acts as a quencher of fluorescence and that delayed hght emission in the microsecond tlme range is due to the back reaction of P+-680 and Q. (Q is the first "stable" electron acceptor of Photosystem II.) Two preillumlnahon flashes are reqmred before the full effect of Trls washing is observed m the delayed light emission decay and fluorescence yield rise; this suggests that a capacity to hold two charges exzsts between the Trls 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 vta Z.