Variable thermal emission and chlorophyll fluorescence in photosystem II particles

In photosynthetic systems, the absorbed light energy is used to generate electron transport or it is lost in the form of fluorescence and thermal emission. While fluorescence can be readily measured, the detection of thermal deactivation processes can be achieved by the photoacoustic technique. In that case, the pressure wave generated by the thermal deactivations in a sample irradiated with modulated light is detected by a sensitive microphone. The relationships between the yield of fluorescence and thermal emissions measured simultaneously were analyzed by using a spinach photosystem II (PSII)-enriched preparation. It is shown that the quenching of fluorescence due to the photochemical activity of the preparations (photochemical quenching) increases in proportion 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 measuring beam (35 Hz) is decreased. Maximal levels of fluorescence and thermal emissions were both decreased in similar proportions upon photoreduction of pheophytin (Pheo), the primary acceptor of PSII. The variable components of fluorescence and thermal emissions were strongly decreased upon depletion of Mn from the Mn complex that catalyzes water oxidation and were recovered proportionally during reconstitution with Mn²⁺ at various Mn²⁺/reaction center ratios. Finally, depletion of Mn from the Mn complex together with the Fe of the Q(A)-Fe-Q(B) complex that is composed of the secondary quinone acceptors of PSII resulted in an 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 reconstitution of both donor and acceptor sides with Mn²⁺, Co²⁺, HCO₃/^- and plastoquinone. It is concluded that the photochemical fluorescence quenching is correlated with a simultaneous 'quenching' of a variable component of thermal emission. It is proposed that the measured component of variable thermal emission is related to the decay of the pair [P680⁺ Pheo^-]. The suggestion is also made that a bicarbonate-induced protonation of reduced Q(A) or Q(B) or conformational change in the PSII complex, or both, adds an additional entropic factor to the variable thermal emission component.