Role of the oxidized secondary acceptor QB of Photosystem II in the delayed 'afterglow' chlorophyll luminescence

Leaf discs of dark-adapted tobacco plants were excited by 2 flashes and kept in darkness at 20 degrees C for various time periods, then thermoluminescence emission was recorded without freezing the sample. The B band at 30 degrees C decreased with a half-time t(1/2)similar to 1 min and the AG band at 45 degrees C with a t(1/2)similar to 5 min. This corresponds to the decay kinetics of S-2/3 in PS II centres in the state S-2/3 Q(B)(-) (B band) or S-2/3 Q(B). Assuming that the 45 degrees C band is an 'afterglow' emission originating from those centres with an oxidized Q(B) on which an electron is back-transferred from stroma reductants through a pathway induced by warming, the theoretical ratio of the B and AG band was compared to that measured experimentally. After 2 or 3 flashes producing mainly S-3, the intensity of AG band encompassed several fold that of the B band, because recombining S-3 recreated S-2 Q(B) AG-emitting centres. In order to confirm that the AG band is governed by the heat-induced activation of a dark Q(B)-reducing pathway rather than by PS II charge recombination, the AG emission was characterized in triazine-resistant Chenopodium album weed biotypes. In these mutants where the Q(B) pocket is altered, the B band is strongly downshifted to 18 degrees C, compared to 32 degrees C in the wild type, whereas the AG band is only downshifted by 3 or 4 degrees C, demonstrating that S-2/3 Q(B)(-) is not the limiting step of the AG emission.