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) photochemistry in sorghum leaves exposed to salinity (0-100 mM NaCl) and/or high temperature stress (30-50°C). No differences were detected in the steadystate 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 Q(B)-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.