Heat-induced multiple effects on PSII in wheat plants

The effects of heat stress on the various functional aspects of photosystem II (PSII) were investigated by analysis of in vivo chlorophyll fluorescence in wheat (Tritium aestivum L.) leaves exposed in the dark to a wide range of elevated temperatures (25-45 degrees C) for 10 min. The results revealed that the effects of heat stress on PSII were characterised by two distinct domains of temperatures: moderately elevated temperatures (30-37.5 degrees C) and severely elevated temperatures (higher than 37.5 degrees C). In moderately elevated temperatures, no changes in the maximal efficiency of PSII photochemistry (F-v/F-m) and photochemical quenching (q(P)) were observed. The decrease in the quantum yield of PSII electron transport (Phi(PSII)) and the efficiency of excitation energy capture by open PSII reaction centers (F-v'/F-m') was reversible and was due to a significant increase in non-photochemical quenching (q(N)). In severely elevated temperatures, the further decrease in Phi(PSII) and F-v'/F-m' was irreversible and was associated with a decrease in F-v/F-m, which was a result of the decrease in the oxygen-evolving complex activity and of an inhibition of electron transport at the acceptor side of PSII. Our results suggest that heat stress displayed multiple effects on PSII. The following sequential events leading to an inhibition of PSII electron transport in the descending order of sensitivity to heat stress can be proposed: excitation energy capture by open PSII reaction centers --> the oxygen-evolving complex of PSII --> the acceptor side of PSII.