THERMOLUMINESCENCE MEASUREMENTS ON CHLORIDE-DEPLETED AND CALCIUM-DEPLETED PHOTOSYSTEM-II

Photosystem II (PSII) in which O-2 evolution was inhibited by depletio n of either chloride or calcium ions was studied by thermoluminescence (TL) and luminescence (L) measurements in the presence and absence of 3-(3',4'-dichlorophenyl)-1,1-dimethyl urea (DCMU). Cl--depleted PSII gives rise to TL and L signals which are similar to those in untreated controls i.e., DCMU shifts the TL band from 30 degrees C to 8 degrees C and suppresses the L component with t(1/2) = 10-15 s, In Ca2+-deple ted PSII a TL-band at around 50 degrees C and a slow luminescence deca y (t(1/2) = 60 s) is observed, Under these conditions, DCMU does not l ead to a downshift of the peak temperature of the TL-band nor does it accelerate the decay kinetics of the luminescence. This indicates that in Ca2+-depleted PSII the Q(A)/Q(B) electron transfer is inhibited pr ior to the addition of DCMU while in Cl--depleted PSII Q(A)/Q(B) elect ron transfer seems unaffected. These results are consistent with previ ous fluorescence measurements which showed that the midpoint potential of the redox couple Q(A)/Q(A)(-) is unchanged in Cl--depleted PSII co mpared to the control while in Ca2+-depleted PSII it is shifted toward s a more positive value [A. Krieger, A.W. Rutherford, Biochim. Biophys . Acta, 1319 (1997) 91-98]. In the literature there are several confli cting reports concerning the TL in Ca2+ and Cl--depleted material, so we attempted to understand the origin of some of these discrepancies. We find that in the absence of cryoprotectants, excitation of TL at lo w temperatures leads to an upshift of TL-bands in Cl--depleted PSII, b oth in the presence and absence of DCMU, while the peak temperature of TL-bands in control and Ca2+-depleted PSII are downshifted. When TL i s excited at 20 degrees C or at low temperature in the presence of a c ryoprotectant then there was no shift of the peak temperature of TL-ba nds. These unexpected results suggest that the formation of the charge pair triggers modifications in its environment and that the exact nat ure of these modifications differs depending on the temperature of exc itation. It seems that once these modifications have occurred at a giv en temperature they remain 'locked in' being unaffected by subsequent temperature changes until charge recombination has occurred. (C) 1998 Elsevier Science B.V.