Heat stress induces in leaves an increase of the minimum level of chlorophyll fluorescence, F-0: A time-resolved analysis

A time-resolved study of the effects of heat stress (23 to 50 degrees C) on F-o level of chlorophyll fluorescence of leaves having different antenna content has been performed in order to elucidate the causes of heat induced increase of F-o in vivo. The multi-exponential deconvolution of the decays after a picosecond flash at F-o have shown that the best fit in both wild-type and the mutant chlorina F2 of barley leaves is obtained with three components in the temperature range utilized (100, 400 and 1200 ps at 23 degrees C). In intermittent light greened pea leaves, a fourth long lifetime component (4 ns at 23 degrees C) is needed. The comparison of the three types of leaves at 23 degrees C shows that the content of the LHCII b complex does not affect the lifetimes of the two main components (100 and 400 ps) and affects their preexponential factors. This result suggests that in the PS II unit the exciton transfer from LHC IIb to the rest of the antenna is irreversible. The effects of heat stress on individual lifetime components, T-i, included several changes. Utilizing for PS II unit an extended 'Reversible Radical Pair' model, having three compartments, to interpret the variations of T-i and A(i) induced by temperature increases, it can be inferred that heat determines: (i) an irreversible disconnection of a minor antenna complex which is not the LHC IIb complex, this effect is induced by temperatures higher than 40 degrees C; (ii) a decrease of the quantum efficiency of Photosystem II photochemistry which is due to several effects: a decrease of the rate of charge separation, an increase of P+I- recombination rate constant and a decrease of the stabilization of charges. These effects on Photosystem II photochemistry start to occur above 30 degrees C and are partially reversible.