Heat tolerance changes of an intact semi-desert cryptobiotic crust during desiccation dominated by Didymodon species

The thermal stability of photosysthetic apparatus was examined under different (light and desiccation) treatments in the mosses (Didymodon luridus Hornsch, Didymodon nicholsonii Culm) dominated intact semi-desert cryptobiotic crusts. The relatively low temperature tolerance measured in default state (non energised thylakoids at full turgor) in the samples was inadequate to survive the thermal conditions of the original habitat. This was also manifested in the temperature sensitivity of optimal quantum yield (Fv/Fm) measured in dark-adapted state. The temperature dependence of the steady-state level of fluorescence, measured in light-adapted state, indicated a moderate decrease in heat sensitivity of PS II which was further enhanced by moderate water deficit. Moreover, a considerable water loss extremely increased the thermal stability of PS II in both dark and light-adapted states. Temperature dependence of the different quenching parameters suggests that in light-adapted state, the secondary effects of low lumen pH may protect against both temperature stress and water loss: i.e., it is likely that protection against the effects of light, temperature, and water deficit can be partly based on common bases. At very high temperatures the thermal damage of the oxygen-evolving complex is unlikely to be avoided, however, under continuous actinic light, the variable fluorescence (Fm’-Fs=AF) was partially recovered even above the critical temperature values of the Fs-T curves. This indicates the partial restoration of the electron donation of the PS II reaction centres by alternative electron donors. These changes in heat tolerance play a significant role to tolerate the effects of daily changing of the ecological factors in the habitat and it has a pronounced ecological significance and partly explains the survival of the examined crusts even under extreme semi-desert conditions.