Thermoluminescence and fluorescence study of changes in Photosystem II photochemistry in desiccating barley leaves.

An effect of desiccation (a decrease of relative water content from 97% to 10% within 35 h) on Photosystem II was studied in barley leaf segments (Hordeum vulgare L. cv. Akcent) using chlorophyll a fluorescence and thermoluminescence (TL). The O-J-I-P fluorescence induction curve revealed a decrease of F(P) and a slight shift of the J step to a shorter time with no change in its height. The analysis of the fluorescence decline after a saturating light flash revealed an increased portion of slow exponential components with increasing desiccation. The TL bands obtained after excitation by continuous light were situated at about -27 degrees C (Z(v) band - recombination of P680(+)Q(A) (-)), -14 degrees C (A band - S(3)Q(A) (-)), +12 degrees C (B band - S(2/3)Q(B) (-)) and +45 degrees C (C band - TyrD(+)Q(A) (-)). The bands related to the S-states of oxygen evolving complex (A and B) were reduced by desiccation and shifted to higher and lower temperatures, respectively. In accordance with this, the band observed at about +27 degrees C (S(2)Q(B) (-)) after excitation by 1 flash fired at -10 degrees C and band at about +20 degrees C (S(2/3)Q(B) (-)) after 2 flashes decreased with increasing water deficit and shifted to lower temperatures. A new band around 5 degrees C appeared in both regimes of TL excitation for a relative water content of under 42% and was attributed to the Q band (S(2)Q(A) (-)). It is suggested that under desiccation, an inhibition of the formation of S(2)- and S(3)-states in OEC occurred simultaneously with a lowering of electron transport on the acceptor side of PS II. The temperature down-shift of the TL bands obtained after the flash excitation was induced at the initial phases of water stress, indicating a decrease of the activation energy for the S(2/3)Q(B) (-)recombination.

Low and high temperature dependence of minimum F0 and maximum FM chlorophyll fluorescence in vivo

The aim of this paper is to give a global insight into the behaviour of F0 and FM in a wide temperature range from -100 degreesC to 75 degreesC. We show that the F0 increases upon linear freezing, similarly to the widely published increase of the F0 upon linear heating. In contrast to this the FM decreases upon linear heating in the whole temperature range from -100 degreesC to 75 degreesC. A comparison of low and high temperature induced increase of the F0 is presented. Copyright 1998 Elsevier Science B.V.