Thermal stability changes of photosynthesis during osmotic and salt stress in wheat varieties cultivated in Central Europe and Mediterranean North Africa.

The thermal stability of photosynthetic apparatus under osmotic/salt stress was examined in wheat cultivars grown under different climatic conditions. The thermostability of nonstressed plants did not differ significantly from each other and it was not improved by osmotic treatment in the absence of light. In contrast, the salt stress resulted in better thermostability. This was also manifested in the temperature dependence of maximal quantum yield of PSII photochemistry. The temperature dependence of steady-state fluorescence and other photosynthetic parameters indicated a moderate reduction in thermal sensitivity of photosynthesis in well-watered plants which was further enhanced by osmotic, but even more by salt treatment. It seems likely that the osmotic stress-induced thermal stability increase of PSII occurs only in energized thylakoids. The temperature dependence of quantum yield of regulated energy dissipation seems to suggest that the secondary effects of lumen pH might have a role in the protective mechanisms concerning these stresses, but salt stress can also affect thermal stability in other ways as well.