Syntrichia caninervis, a typical drought-tolerant moss species found worldwide in various semiarid and arid regions, is the dominant species of soil crust mosses in the Gurbantünggüt Desert, which is a temperate northern desert of Central Asia. It appears able to endure frequent and intensive freeze-thaw cycles in the surface soil and maintain physiological functions active even under extreme low temperatures in winter. However, there have been few studies exploring the adaptive strategies of the species during freeze-thaw processes and interpreting well the phenomena. One of the major goals of this study was to investigate physiological regulatory mechanisms of the species when undergoing freezing-thawing periods and relevant antioxidant enzyme activities. Methods
A series of microclimate and soil conditions were continuously monitored in the sampling sites starting from November 2013. The samples of the moss were collected during the early spring of 2014 from February to April and from three microhabitats including the sites under live shrub, under dead shrub and on an open ground. The moss samples were quickly treated in the field for keeping freshness and then brought to laboratory for measurement and analysis. The traits/characteristics that reflect physiological and biochemical activities were measured for analyzing responses of the species to freeze-thaw processes and for comparing the impacts of different habitats, which included water content, concentrations of proline, soluble sugar, soluble protein and malondialdehyde (MDA), as well as catalase (CAT), peroxidase (POD), superoxide dismutase (SOD) activities. Important findings
The results showed that, during different freeze-thaw periods, microhabitats and their interactions with alternative freeze-thaw processes significantly affect the contents of proline, soluble sugar, soluble protein and MDA in shoots of the species, as well as the enzyme activities of CAT, POD, and SOD. Extreme low temperature and drought caused significantly higher soluble sugar and MDA contents, higher CAT, POD and SOD activities, but lower soluble protein content, in snow covered period (February) and dry period (April) than the snow melting period (March). The existence of live shrub decreased temperature around the bushes during snow melting because of the effect of shade, causing increased contents of soluble sugar and MDA in moss shoots, when compared to those growing in the habitats under dead shrub and on open ground. However, live shrub could provide a moister environment for S. caninervis than dead shrub and exposed ground for the duration of desiccation. As the result, the proline content, soluble sugar content and MDA content of the species under the live shrub canopy were the lowest among the three habitats. Furthermore, the mosses lived on the exposed ground showed the highest antioxidant enzyme activities comparing to those under the dead shrub and living shrub. This may suggest that S. caninervis grown on exposed grounds has developed rather a specific adaptations for a harsher environment condition than that occurred in other two habitats.