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

Far red illumination of photosynthetic material induces a delayed lumi nescence rise, or afterglow, which has been reported in plant leaves, protoplasts or intact chloroplasts and in algal cells, but does not oc cur in isolated thylakoids. The rise kinetics is accelerated by increa sing temperature and we show, by slowly heating a leaf sample after a far-red illumination, that the afterglow emission can be optimally res olved as a sharp thermoluminescence band. Plant material was mainly pe a (Pisum sativum L., cv Kazar and Merveille de Kelvedon) and cucumber (Cucumis sativus L., cv Marketer). Comparisons were done with rape, sp inach, 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 re d illumination, revealed a new thermoluminescence AG band, peaking bet ween 40 degrees C and 50 degrees C. It exhibited the characteristic pr operties of the luminescence afterglow recorded at a constant temperat ure. The AG band was very sensitive to short incubations at both freez ing and moderately warm temperatures. Increasing duration of far red i llumination 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 asc ribed to different behaviors of proton gradients in stroma and in gran a lamellae, respectively. The induction of an afterglow by far red lig ht lasted for several minutes in the dark, at 10 degrees C. Flash sequ ences fired in these conditions confirmed the presence of S-2 and S-3 states stable in the dark, producing luminescence by recombination wit h 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 de monstrates that a metabolic state leading to AG emission may arise spo ntaneously in plant leaves. The strong temperature dependence of the A G emission is discussed in terms of heat-induced conformational change s in the thylakoid membrane. We conclude that thermoluminescence can g ive original information on the photosynthetic mechanisms in whole lea ves, provided that harmful effects of extreme temperatures are avoided .