The mechanism of the ozone-induced changes in thermoluminescence glow curves of barley leaves

Abstract

The changes in thermoluminescence (TL) signals induced by short-term ozone exposure of leaves are characterized by a down-shift of the peak-temperature of the TLB-band and an increase of a TL band at 55°C. We investigated the relationship of these changes to photosystem 2 (PS2) photochemistry. The changes were not only detectable in the presence of ozone, but also after irradiation of dark-adapted leaves and after aging of irradiated detached leaf segments. The opposite effect on TL, an up-shift of the peak-temperature of the B-band and the decrease of the intensity of the band at 55°C were found after infiltration of leaves with nigericin, antimycin A, and diphenyleneiodonium chloride (DPI). Propyl gallate down-shifted the peak-temperature of the B-band. 2,5-dimethyl-1,4-benzoquinone up-shifted the peak-temperature of the B-band and decreased the intensity of the 55°C band. The intensity of the 55°C band did not change significantly in the presence of oxygen in comparison to that in nitrogen atmosphere. It decreased with time of dark adaptation (50% intensity was observed after 3 h of dark adaptation at room temperature), however, it was reactivated to its initial value (at 5 min of dark adaptation) after 1 single-turnover flash. The 55°C band was not significantly changed in the presence of DCMU. Thus the ozone-induced band at 55°C is assigned to charge recombination in PS2. Changes in the electron transport chain at the acceptor side of PS2, probably related to the cyclic electron transport around photosystem 1 and/or chlororespiration, could play an important role in the increase of the 55°C band and the down-shift of the B-band. The changes at the acceptor side indicated by TL can be an ex pression of a physiological regulatory mechanism functional under stress conditions.