A one microsecond component of chlorophyll luminescence suggesting a primary acceptor of system II of photosynthesis...

The kinetics of the luminescence of chlorophyll a in Chlorella vulgaris were studied in the time range from 0.2 mus to 20 mus after a short saturating flash (t 1/2 = 25 ns) under various pretreatment including anaerobiosis, flashes, continuous illumination and various additions. A 1 mus luminescence component probably originating from System II was found of which the relative amplitude was maximum under anaerobic conditions for reaction centers in the state SPQ- before the flash, about one third for centers in the state S+PQ- or SPQ before the flash, and about one tenth for centers in the state S+PQ before the flash. S is the secondary donor complex with zero change; S+ is the secondary donor complex with 1 to 3 positive charges; P, the primary donor, is the photoactive chlorophyll a, P-680, of reaction center 2; Q- is the reduced acceptor of System II, Q. Under aerobic conditions, where an endogenous quencher presumably was active, the luminescence was reduced by a factor two. The 1 mus decay of the luminescence is probably caused by the disappearance of P+ formed in the laser flash according to the reaction ZP+ leads to Z+ in which Z is the molecule which donates an electron to P+ and which is part of S. After addition of hydroxylamine, the 1 mus luminescence component changed with the incubation time exponentially (tau = 27 s) into a 30 mus component; during the same time, the variable fluorescence yield, measured 9 mus after the laser flash, decreased by a factor 2 with the same time constant. Hereafter in a second much slower phase the fluorescence yield decreased as an exponential function of the incubation time to about the dark value; meanwhile the 30 mus luminescence increased about 50% with the same time constant (tau = 7 min). Heat treatment abolished both luminescence components. The 1 mus luminescence component saturated at about the same energy as the System II fluorescence yield 60 mus after the laser flash and as the slower luminescence components. From the observation that the amplutide is maximum if the laser flash is given when the fluorescence yield is high after prolonged anaerobic conditions (state SQ-), we conclude that the 1 mus luminescence is probably caused by the reaction PWQ- + hv leads to P*WQ- leads to P+W-Q- leads to P*WQ- leads to PWQ- + hv in which W is an acceptor different from Q. The presence of S+ reduced the luminescence amplitude to about one third. Two models are discussed, one with W as an intermediate between P and Q and another, which gives the best interpretation, with W on a side path.