The origin of 40–50°C thermoluminescence bands in Photosystem II

Abstract

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°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°C is induced following illumination at 77 K. This band decays, at 30°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°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°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 Ca 2+-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.