Function of plastoquinone in heat stress reactions of plants

The effect of high temperature treatment (40 °C, 3 h, illumination at 100 μmol m^− 2 s^− 1) on the photosynthetic electron flow in barley seedlings of different age was investigated. Thermoinduced inhibition of the liner electron flow due to partial impairment of the water oxidizing complex (WOC) and the increase in the extent of Q_A⁻ reoxidation by Tyr_z^ox in thylakoids isolated from 4-day-old leaves was shown by measurements of oxygen evolution using benzoquinone or potassium ferricyanide as electron acceptors, as well as by following Q_A⁻ reoxidation kinetics in the absence and presence of exogenous electron acceptors, DCBQ and DMBQ. Using HPLC analysis, an increase in the oxidation of the photoactive plastoquinone pool in young leaves under heating was shown. In older, 11-day-old leaves, heat treatment limited both photosynthetic electron flow and oxygen evolution. The same effects of heat shock on oxygen evolution caused an inhibition of electron flow on the donor side of PSII only. However, a rise in the proportion of PSII with Q_A⁻ reoxidized through recombination with the S₂/S₃ state of the WOC was observed. The addition of exogenous electron acceptors (DCBQ and DMBQ) and a donor (DPC) showed that the thermoinduced decrease in the electron transport rate was caused by an impediment of electron flow from Q_A⁻ to acceptor pool. The decrease in size of the photoactive PQ-pool and a change in the proportions of oxidized and reduced PQ in older leaves under heat treatment were shown. It was suggested that a thermoinduced change of the redox state of the PQ-pool and a redistribution of plastoquinone molecules between photoactive and non-photoactive pools are the mechanisms which reflect and regulate the response of the photosynthetic apparatus under heat stress conditions.