A thermoluminescence study of photosystem II back electron transfer reactions in rice leaves - effects of salt stress.

The recombination reactions of Photosystem II have been investigated in vivo in rice leaves by using the thermoluminescence (TL) emission technique. Excitation of dark-adapted leaf segments at 0 degrees C with different number of single turn-over flashes induced the appearance of complex TL glow curves. The mathematical analysis of these curves showed the existence of four TL components: B1-band (temperature maximum, t(max), at 24 degrees C, originating from S3QB - recombination), B2-band (tmax at 35 degrees C, from S2QB -), AG-band (tmax at 46 degrees C) and C-band (tmax at 55 degrees C, from TyrD +QA -). Their contributions to the total TL signal were different depending on the number of flashes given. AG-band seems to reflect a special electron transfer from some unknown stroma donor to PS II. Q-band (tmax at 19 degrees C), originating from S2QA - recombination, was recorded after flashing samples incubated in the presence of DCMU. The recombination halftimes (t1/2) at 20 degrees C of S2QA -, S3QB -, S2QB - and TyrD +QA - were, respectively, 0.8 s, 48 s, 74 s and about 1 h. A sharp AG-band (tmax at 50 degrees C and t1/2 of 210 s) could be also observed after illumination of leaves with far-red light and after a dark incubation period of whole plants. Incubation of leaf segments with 0.5 M NaCl abolished the inductions of AG-band by darkness and far-red illumination, significantly decreased Q-band intensity, whereas induced a strong increase in C-band intensity. The possible inhibition of S2/S3 formation and quinone oxidation by saline stress are discussed.