CHARACTERIZATION OF THE CHLOROPHYLL THERMOLUMINESCENCE AFTERGLOW IN DARK-ADAPTED OR FAR-RED-ILLUMINATED PLANT-LEAVES

Far red illumination of photosynthetic material induces a delayed luminescence rise, or afterglow, which has been reported in plant leaves, protoplasts or intact chloroplasts and in algal cells, but does not occur in isolated thylakoids. The rise kinetics is accelerated by increasing temperature and we show, by slowly heating a leaf sample after a far-red illumination, that the afterglow emission can be optimally resolved as a sharp thermoluminescence band. Plant material was mainly pea (Pisum sativum L., cv Kazar and Merveille de Kelvedon) and cucumber (Cucumis sativus L., cv Marketer). Comparisons were done with rape, spinach, tobacco, avocado and maize. A 0.2 degrees C(-1)s to 0.5 degrees C(-1)s temperature gradient, started above 0 degrees C after a far red illumination, revealed a new thermoluminescence AG band, peaking between 40 degrees C and 50 degrees C. It exhibited the characteristic properties of the luminescence afterglow recorded at a constant temperature. The AG band was very sensitive to short incubations at both freezing and moderately warm temperatures. Increasing duration of far red illumination caused two kinetically distinct effects on the AG band and on the B band (S-2/S-3 Q(B) over bar recombination), which can be ascribed to different behaviors of proton gradients in stroma and in grana lamellae, respectively. The induction of an afterglow by far red light lasted for several minutes in the dark, at 10 degrees C. Flash sequences fired in these conditions confirmed the presence of S-2 and S-3 states stable in the dark, producing luminescence by recombination with back-transferred electrons. In some plant batches, an AG band could be induced by 2 or 3 flashes in the absence of far red light, which demonstrates that a metabolic state leading to AG emission may arise spontaneously in plant leaves. The strong temperature dependence of the AG emission is discussed in terms of heat-induced conformational changes in the thylakoid membrane. We conclude that thermoluminescence can give original information on the photosynthetic mechanisms in whole leaves, provided that harmful effects of extreme temperatures are avoided.