VARIABLE THERMAL EMISSION AND CHLOROPHYLL FLUORESCENCE IN PHOTOSYSTEM-II PARTICLES

In photosynthetic systems, the absorbed light energy is used to genera te electron transport or it is lost in the form of fluorescence and th ermal emission. While fluorescence can be readily measured, the detect ion of thermal deactivation processes can be achieved by the photoacou stic technique. In that case, the pressure wave generated by the therm al deactivations in a sample irradiated with modulated light is detect ed by a sensitive microphone. The relationships between the yield of f luorescence and thermal emissions measured simultaneously were analyze d by using a spinach photosystem II (PSII)-enriched preparation. It is shown that the quenching of fluorescence due to the photochemical act ivity of the preparations (photochemical quenching) increases in propo rtion to the fraction of thermal deactivations that is not immediately released as heat but is stored in photochemical intermediates (energy -storage yield) as the intensity of the photoacoustic modulated measur ing beam (35 Hz) is decreased. Maximal levels of fluorescence and ther mal emissions were both decreased in similar proportions upon photored uction of pheophytin (Pheo), the primary acceptor of PSII. The variabl e components of fluorescence and thermal emissions were strongly decre ased upon depletion of Mn from the Mn complex that catalyzes water oxi dation and were recovered proportionally during reconstitution with Mn 2+ at various Mn2+/reaction center ratios. Finally, depletion of Mn fr om the Mn complex together with the Fe of the Q(A)-Fe-Q(B) complex tha t is composed of the secondary quinone acceptors of PSII resulted in a n increased initial level of fluorescence F(o) and in the loss of the variable components of fluorescence and thermal emissions. The initial F(o) and the variable components could be partially recovered by reco nstitution of both donor and acceptor sides with Mn2+, Co2+, HCO3- and plastoquinone. It is concluded that the photochemical fluorescence qu enching is correlated with a simultaneous ''quenching'' of a variable component of thermal emission. It is proposed that the measured compon ent of variable thermal emission is related to the decay of the pair [ P680+ Pbeo-]. The suggestion is also made that a bicarbonate-induced p rotonation of reduced Q(A) or Q(B) or conformational change in the PSI I complex, or both, adds an additional entropic factor to the variable thermal emission component.