Heat-induced reversible changes in Photosystem 1 absorption cross-section of pea chloroplasts and sub-chloroplast preparations : Evidence from excitation fluorescence spectra

Heat stress-induced effects of photosystem I: an overview of structural and functional responses

Temperature is one of the main factors controlling the formation, development, and functional performance of the photosynthetic apparatus in all photoautotrophs (green plants, algae, and cyanobacteria) on Earth. The projected climate change scenarios predict increases in air temperature across Earth's biomes ranging from moderate (3-4 °C) to extreme (6-8 °C) by the year 2100 (IPCC in Climate change 2007: The physical science basis: summery for policymakers, IPCC WG1 Fourth Assessment Report 2007; Climate change 2014: Mitigation of Climate Change, IPCC WG3 Fifth Assessment Report 2014). In some areas, especially of the Northern hemisphere, even more extreme warm seasonal temperatures may occur, which possibly will cause significant negative effects on the development, growth, and yield of important agricultural crops. It is well documented that high temperatures can cause direct damages of the photosynthetic apparatus and photosystem II (PSII) is generally considered to be the primary target of heat-induced inactivation of photosynthesis. However, since photosystem I (PSI) is considered to determine the global amount of enthalpy in living systems (Nelson in Biochim Biophys Acta 1807:856-863, 2011; Photosynth Res 116:145-151, 2013), the effects of elevated temperatures on PSI might be of vital importance for regulating the photosynthetic response of all photoautotrophs in the changing environment. In this review, we summarize the experimental data that demonstrate the critical impact of heat-induced alterations on the structure, composition, and functional performance of PSI and their significant implications on photosynthesis under future climate change scenarios.

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

A time-resolved study of the effects of heat stress (23 to 50°C) on Fo level of chlorophyll fluorescence of leaves having different antenna content has been performed in order to elucidate the causes of heat induced increase of Fo in vivo. The multi-exponential deconvolution of the decays after a picosecond flash at Fo 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°C). In intermittent light greened pea leaves, a fourth long lifetime component (4 ns at 23°C) is needed. The comparison of the three types of leaves at 23°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, Ti, included several changes. Utilizing for PS II unit an extended 'Reversible Radical Pair' model, having three compartments, to interpret the variations of Ti and Ai induced by temperature increases, it can be inferred that heat determines: (i) an irreversible disconnection of a monor antenna complex which is not the LHC IIb complex, this effect is induced by temperatures higher than 40°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°C and are partially reversible.