Enhanced tolerance of photosynthesis against high temperature damage in salt-adapted halophyte Atriplex centralasiatica plants

Thermotolerance of photosynthesis in salt-adapted Atriplex centralasiatica plants (100-400 mm NaCl) was evaluated in this study after detached leaves and whole plants were exposed to high temperature stress (30-48 degreesC) either in the dark or under high light (1200 mol m(-2) s(-1)). In parallel with the decrease in stomatal conductance, intercellular CO2 concentration and CO2 assimilation rate decreased significantly with increasing salt concentration. There was no change in the maximal efficiency of PSII photochemistry (F-v/F-m) with increasing salt concentration, suggesting that there was no damage to PSII in salt-adapted plants. On the other hand, there was a striking difference in the response of PSII and CO2 assimilation capacity to heat stress in non-salt-adapted and salt-adapted leaves. Leaves from salt-adapted plants maintained significantly higher F-v/F-m values than those from non-salt-adapted leaves at temperatures higher than 42 degreesC. The F-v/F-m differences between non-salt-adapted and salt-adapted plants persisted for at least 24 h following heat stress. Leaves from salt-adapted plants also maintained a higher net CO2 assimilation rate than those in non-salt-adapted plants at temperatures higher than 42 degreesC. This increased thermotolerance was independent of the degree of salinity since no significant changes in F-v/F-m and net CO2 assimilation rate were observed among the plants treated with different concentrations of NaCl. The increased thermotolerance of PSII induced by salinity was still evident when heat treatments were carried out under high light. Given that photosynthesis is considered to be the physiological process most sensitive to high temperature damage, increased thermotolerance of photosynthesis may be of significance since A. centralasiatica, a typical halophyte, grows in the high salinity regions in the north of China, where the temperature in the summer is often as high as 45 degreesC.