HETEROGENEITY AND PHOTOINHIBITION OF PHOTOSYSTEM-II STUDIED WITH THERMOLUMINESCENCE

Thermoluminescence (TL) signals were recorded from grana stacks, margi ns, and stroma lamellae from fractionated, dark-adapted thylakoid memb ranes of spinach (Spinacia oleracea L.) in the absence and in the pres ence of 2,6-dichlorphenylindophenol (DCMU). In the absence of DCMU, th e Tt signal from grana fractions consisted of a homogenous B-band, whi ch originates from recombination of the semi-quinone Q(B)(-) with the S-2 state of the water-splitting complex and reflects active photosyst em II (PSII). In the presence of DCMU, the B-band was replaced by the Q-band, which originates from an S(2)Q(A)(-) recombination. Margin fra ctions mainly showed two TL-bands, the B- and C-bands, at approximatel y 50 degrees C in the absence of DCMU, and Q- and C-bands in the prese nce of DCMU. The C-band is ascribed to a Tyr(D)(+)-Q(A)(-)-recombinati on. In the absence of DCMU, the fractions of stromal lamellae mainly g ave rise to a TL emission at 42 degrees C. The intensity of this band was independent of the number of excitation flashes and was shifted to higher temperatures (52 degrees C) after the addition of DCMU. Based on these observations, this band was considered to be a C-band. After photoinhibitory light treatment of uncoupled thylakoid membranes, the TL intensities of the B- and Q-bands decreased, whereas the intensity at 45 degrees C (C-band) slightly increased. It is proposed that the 4 2 to 52 degrees C band that was observed in marginal and stromal lamel lae and in photoinhibited thylakoid membranes reflects inactive PSII c enters that are assumed to be equivalent to inactive PSII Q(B)(-) nonr educing centers.