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.
Variation in Sulfide Tolerance of Photosystem II in Phylogenetically Diverse Cyanobacteria from Sulfidic Habitats
