PARENT SESSION
Posters P1A Proton coupled electron transport and ATPase. Abstracts (172-180)

Origin of thermoluminescence bands in unfrozen Chamydomonas reinhardtii cells. Jean-Marc Ducruet^*,1, Laszlo Kovacs², Gyözö Garab², ¹ Section de Bioénergétique, Gif-sur-Yvette cedex, France² Inst.Plant Biology, Szeged, Hungary

ABSTRACT- Chlamydomonas reinhardtii cells were dark-adapted, then excited at 1C by single turn-over flashes or by far-red light (730 nm), avoiding any transient freezing of the sample. Thermoluminescence emission was immediately recorded (0.5C/s, S. Demeter apparatus). One single xenon flash produced a main band at 20C and a shoulder around 30C. After 2 flashes, the 30C emission increased and appeared as a well-resolved band after 3 flashes or more. Its maximum progressively shifted to 34C. The overall TL emission exhibited maxima after 2 and 6 flashes, as expectable when all Mn-OEC complexes are in S2/S3 luminescent states. However, the intensity ratio of the 34C to the 20C bands showed maxima after the 3-rd and 7-th flashes, indicating that the 34C band was favoured by S3 state. Sequences of 7 flashes produced a well-resolved and prominent 34C band. This band was selectively inhibited by the uncoupler FCCP (0.2 uM), but not by NH4Cl at 10 mM, whilst nigericin (10 uM) required a preincubation of several minutes to act. Antimycin A also selectively suppressed the 34C band with a 2 uM half-inihibitory concentration, leaving the 20C band unchanged. 1 uM Valinomycin, a K+ ionophore, increased the 34C band relatively to the 20C band, but became inhibitory at higher concentrations. Methyl Viologen, a PSI electron acceptor, also increased the 34/20C ratio in the 10 uM range, then decreased it above 100 uM. The period 4 dependency of the 34C band with relative maximum at the 3-rd and 7-th flashes corresponding to the higher amounts of S3, the inhibition by antimycin A, known to block the ferredoxine-plastoquinone-reductase (FQR) chlororespiratory pathway at <5 uM concentrations both suggest that this 34C band is an "afterglow" (AG) emission due to a heat-induced flow of electrons from stroma reductants to the plastoquinones, converting S2/3 QB centers to luminescence emitting S2/3 QB- state. However, far-red light did not induce this 34C band but a sharp band at 27C, which was insensitive to antimycin, and transformed into the broader 20C band by 0.2 uM FCCP. Hence, compared to pea leaves, in which a B band at 20C and an AG band at 45C were induced both by 3 flashes or by far-red light, in C reinhardtii 3 flashes induced an AG band at 34C whereas far-red light induced a band at 27C of unknown origin which depended on the proton gradient.

KEY WORDS: luminescence, photosystem II, cyclic electron flow, chlororespiration