THE ORIGIN OF 40-50-DEGREES-C THERMOLUMINESCENCE BANDS IN PHOTOSYSTEM-II

We have used thermoluminescence (TL) and EPR measurements of Photosystem II (PS II) from spinach in order to identify charge pairs responsible for TL bands in the region of 40-50 degrees C including the 'C-band' (peak V) and the TL band from PS II depleted of calcium. In intact PS II membrane preparations, in the presence of DCMU, a TL band at 50 degrees C is induced following illumination at 77 K. This band decays, at 30 degrees C, with a half-time of 10 min. This decay corresponds to the disappearance of the EPR signal arising from Q(A)(-) and an accelerated decay of the organic free radical Tyr D+. It is concluded that recombination of this charge pair is probably responsible for the thermoluminescence emission. In PS II preparations that have been depleted of calcium using a salt/EGTA wash followed by rebinding of the extrinsic polypeptides, a TL band is produced at around 45-50 degrees C following 198 K illumination. In such samples a stable S-2 state of the water-splitting complex is present, giving rise to a modified form of the EPR multiline signal. During incubation at 30 degrees C in the dark this signal decays with a half-time around 20-25 min. This decay is not accelerated by the presence of Q(A)(-) induced by low-temperature illumination of the sample. In contrast, low-temperature illumination does result in an acceleration in the decay of Tyr D+, indicating that Tyr D+/Q(A)(-) recombination is again the dominant origin of thermoluminescence. In PS II depleted of calcium by incubation at pH 4.0, the possibility that TL emission temperature is determined by a change in the mid-point redox potential of Q(A) (Krieger, A. and Weis, E. (1992) Photosynthetica 27, 89-98) was investigated by comparing TL from equivalent samples of control and Ca2+-depleted PS II. It was shown that the emission temperature of the high temperature TL band induced by illumination at 77 K did not differ significantly between control and treated samples, suggesting that, under the conditions used, the potential of Q(A) does not change significantly.