Characterization of the photosystem II inactivation of heat-stressed barley leaves as monitored by the various parameters of chlorophyll a fluorescence and delayed fluorescence

Abstract

The effects of elevated temperatures on the photochemical activities of photosystem II (PS II) in situ have been studied for barley leaves using a combination of chlorophyll a fluorescence, delayed fluorescence (DF) in the microsecond range and oxygen evolution rate (OE). Attention is given to the succession of PS II inactivation on donor and acceptor sides and its correlation with reversible and/or irreversible depression of OE. The partial decrease of oxygen evolution at 37.5°C (by 23% compared with the optimal value at 35°C) is found to be mostly reversible and to correlate with a significant increase of the DF signal. The increase of DF indicates that this moderate level of heat stress preferentially induces PS II donor-side inactivation. A nearly irreversible decline of OE occurs at 42.5–45°C. The corresponding decrease of DF signal, by more than 70% if compared with the DF maximum at 40°C, can be explained by the structural changes within PS II which prevent the recombination of separated charges (P680 + Q A − ). Further analysis of chlorophyll a fluorescence supports an idea that this level of heat stress induces significant inactivation of PS II photochemistry on both donor and acceptor sides.