HEAT TOLERANCE OF INTACT SEMI-DESERT CRYPTOBIOTIC CRUSTS DOMINATED BY MOSSES

The heat sensitivity of plants is closely connected to the thermal stability of PSII. It is more or less clear that the heat sensitivity of the photosynthetic apparatus, and the thermal stability of PSII, can change rapidly. It is also recognized in higher plants that the heat tolerance of PS II is influenced by other stress factors like light and water deficit. In desert and semi-desert conditions parallel with the increase of irradiation and leaf temperature the water content decreases very rapidly in these poikilohydric crusts. To achieve an acceptable dry matter production, an efficient photosynthetic functioning is necessary even under such unfavorable conditions: the combined effects of the three stress factors need to be tolerated at the same time. According to the above mentioned facts, the thermal stability of photosysthetic apparatus was examined under different (light and water deficit) treatments in the mosses ( Didymodon luridus Hornsch , Didymodon nicholsonii Culm , Grimmia capillata De Not and Crossidium squamiferum Juratzka) dominated intact semi-desert cryptobiotic crusts. Before the measurements the samples were rehydrated and transferred to a growth chamber for two days. Desiccation treatments were carried out by