Thermoluminescence measurements on chloride-depleted and calcium-depleted photosystem II

Photosystem II (PSII) in which O2 evolution was inhibited by depletion 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 degreesC to 8 degreesC and suppresses the L component with t1/2=10-15 s. In Ca2+-depleted PSII a TL-band at around 50 degreesC and a slow luminescence decay (t1/2=60 s) is observed. Under these conditions, DCMU does not lead 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 QA/QB electron transfer is inhibited prior to the addition of DCMU while in Cl--depleted PSII QA/QB electron transfer seems unaffected. These results are consistent with previous fluorescence measurements which showed that the midpoint potential of the redox couple QA/QA- is unchanged in Cl--depleted PSII compared to the control while in Ca2+-depleted PSII it is shifted towards a more positive value [A. Krieger, A.W. Rutherford, Biochim. Biophys. Acta, 1319 (1997) 91-98]. In the literature there are several conflicting 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 low temperatures leads to an upshift of TL-bands in Cl--depleted PSII, both in the presence and absence of DCMU, while the peak temperature of TL-bands in control and Ca2+-depleted PSII are downshifted. When TL is excited at 20 degreesC or at low temperature in the presence of a cryoprotectant then there was no shift of the peak temperature of TL-bands. These unexpected results suggest that the formation of the charge pair triggers modifications in its environment and that the exact nature of these modifications differs depending on the temperature of excitation. It seems that once these modifications have occurred at a given temperature they remain 'locked in' being unaffected by subsequent temperature changes until charge recombination has occurred. Copyright 1998 Elsevier Science B.V.