Thermostability of photosystem II is increased in salt-stressed sorghum

Modulated chlorophyll fluorescence and rapid fluorescence induction kinetics were used to evaluate the functions of photosystem II (PSII) photochemsitry in sorghum leaves exposed to salinity (0–100 mM NaCl) and/or high temperature stress (30–50°C). No differences were detected in the steady- state fluorescence parameters and rapid fluorescence induction kinetics in salt-stressed leaves, indicating that PSII was highly resistant to salinity stress alone. However, salinity stress modified the responses of PSII to high temperature. When the temperature was above 45°C, the thermostability of PSII was strongly enhanced in salt-stressed leaves, which was reflected in a smaller decrease in maximum efficiency of PSII photochemistry, coefficients of photochemical and non-photochemical quenching, and efficiency of excitation capture by open PSII reaction centres, and in a smaller increase in the proportion of the QB-non-reducing PSII centres in salt-stressed leaves than in control leaves. This increased thermostability in salt-stressed leaves exposed to high temperature seemed to be independent of the imposed salt concentration since there were no significant variations in the above fluorescence parameters among the salt-stressed plants treated with different salt concentrations. The results are discussed in terms of the physiological significance of such increased resistance of PSII to high temperature.