Enhanced thermotolerance of photosystem II in salt-adapted plants of the halophyte Artemisia anethifolia

Thermotolerance of photosystem II (PSII) in leaves of salt-adapted Artemisia anethifolia L. plants (100-400 mM NaCl) was evaluated after exposure to heat stress (30-45degreesC) for 30 min. After exposure to 30degreesC, salt adaptation had no effects on the maximal efficiency of PSII photochemistry (F-v/F-m), the efficiency of excitation capture by open PSII centers (F-v'/F-m'), or the actual PSII efficiency (Phi(PSII)). After pretreatment at 40degreesC, there was a striking difference in the responses of F-v/F-m, F-v'/F-m' and Phi(PSII) to heat stress in non-salt-adapted and salt-adapted leaves. Leaves from salt-adapted plants maintained significantly higher values of F-v/F-m, F-v'/F-m' and Phi(PSII) than those from non-salt-adapted leaves. The differences in Fv/Fm, Fv'/Fm' and Phi(PSII) between non-salt-adapted and salt-adapted plants persisted for at least 12 h following heat stress. These results clearly show that thermotolerance of PSII was enhanced in salt-adapted plants. This enhanced thermotolerance was associated with an improvement in thermotolerance of the PSII reaction centers, the oxygen-evolving complexes and the light-harvesting complex. In addition, we observed that after exposure to 42.5degreesC for 30 min, non-salt-adapted plants showed a significant decrease in CO, assimilation rate while in salt-adapted plants CO2, assimilation rate was either maintained or even increased to some extent. Given that photosynthesis is considered to be the physiological process most sensitive to high-temperature damage and that PSII appears to be the most heat-sensitive part of the photosynthetic apparatus, enhanced thermotolerance of PSII may be of significance for A. anethifolia, a halophyte plant, which grows in the high-salinity regions in the north of China, where the air temperature in the summer is often as high as 45degreesC.