THERMOLUMINESCENCE STUDIES ON THE FACULTATIVE CRASSULACEAN-ACID-METABOLISM PLANT MESEMBRYANTHEMUM-CRYSTALLINUM L

Thermoluminescence (TL) signals were measured from leaves of the facul tative CAM (crassulacean acid metabolism) plant Mesembryanthermum crys tallinum L.. Following the induction of CAM by salt treatment, a TL ba nd at 46 degrees C was induced, which was charged by a single-turnover flash. The intensity of the 46 degrees C-band depends on the number o f excitation flashes and oscillates with a period of four. A similar b and was induced in Cg plants by far-red illumination. Under CAM condit ions, the intensity of the 46 degrees C-band underlies a diurnal rhyth m. The maximal intensity of the 46 degrees C-band is observed in the m orning after onset of the light and in the evening. At around 12 a.m. it is suppressed. The intensity of the 46 degrees C-band relates to di urnal changes in the ratio of dihydroxy acetone phosphate/3-phosphogly cerate (DHAP/PGA) which is an indicator of the energy status of the ch loroplast. During high-intensity illumination, the 46 degrees C-band d isappears, but it is restored in the dark. We propose that the 46 degr ees C-band is an indicator of the metabolic state of the leaf, origina ting from photosystem II centres initially in the S-2(S-3)Q(B) oxidati on state, in which the electron acceptor QB becomes reduced either by reverse electron flow or reduction of the plastoquinone pool via an NA D(P)H plastoquinone oxidoreductase. We present evidence that the redox state of the electron-transport chain is different under conditions o f CAM compared to C-3 metabolism and that changes induced by CAM can b e monitored by measuring the amplitude of the 46 degrees C-band after flash excitation.