|其他摘要||As a typical drought-tolerant moss, Syntrichia caninervis Mitt is widely spread across many semi-arid and arid regions worldwide. As one of the dominant species in the moss crusts, Syntrichia caninervis Mitt grows well at different microhabitats of the Gurbantunggut Desert, a cold northern desert in Central Asia. The presence of crusts can roughen the soil surface, while affecting nutrient cycling, soil stability and hydrological processes of soil surface. With its leaves of only a layer of cells, Syntrichia caninervis Mitt is very sensitive to changes of its growing environments. Compared to other desert ecosystems, the Gurbantunggut Desert is extremely hot and drought during summer, being stable snow cover because of frequent and abundant snowin spring and winter. However, how it successfully adapts to the extreme hot and drought environments in summer and the alternative freezing – thawing cycles of soil surface in spring and keeps activity in such dramatically changing environments have not been well explained. This paper is to explore how Syntrichia caninervis Mitt changes in physiological, biochemical and ecological aspects to adapt to different desert microhabitats.
There were significant differences in morphology between different habitats. For pure populations of Syntrichia caninervis Mitt in different habitats, the population biomass and individual biomass are higher in live shrub canopy than dead shrub and bare land. On the contrary, the density of population appeared an opposite trend compared with the biomass: bare land > dead shrub > live shrub. In addition, we found significant differences in the height of plant, the ratio of hair point length to leaf length, the leaf inclination angles, the length of papillae in hair point and coast, the density of the papillae in the leaf and its arrangement mode in different habitats by using the electron and optical microscope. The height of Syntrichia caninervis Mitt showed significantly decreasing trends from live shrub to dead shrub to bare land. And the ratio of hair point length to leaf length, the leaf inclination angles, the length of papillae in hair point and coast in bare land were higher than those under live shrub. The density of the papillae in the leaf and its arrangement mode in different habitats may be affected by both water and light. At the same time, the leaf inclination angels of Syntrichia caninervis Mitt which live in bare land were lower than live in other habitats. The lower leaf inclination angles may provide a greater photosynthesis area for these moss living at bare land.
It is obvious that the chlorophyll fluorescence activity was different among three habitats. Our study showed that Syntrichia caninervis Mitt were different in chlorophyll fluorescence activity in different periods. Syntrichia caninervis Mitt would be inactivated due to dehydration in extreme environment during summer and winter. However, the melt of snow in the early winter and spring could provide water for the bryophytes, and facilitate the recovery of photosynthesis activity. Meanwhile, Syntrichia caninervis Mitt in different microhabitats also showed differences in photosynthesis. Rainfall can be gathered under shrub, and its low-temperature effect under shrub could also reduce the loss of the surface water. These effects of shrub may increase the chances for the bryophytes to restore the activity, and extends the time of photosynthesis to a certain extent. On the contrary, those bryophytes which live in the bare land or in the dead shrub areas are more sensitive to warming events during winter than in the live shrub areas. The activity of Fv/Fm in bryophytes under the shrub canopy appeared significantly higher than that of bare land. But for the value of Y(II), there are no significant differences among the three microhabitats. Removal of shrub would significantly reduce the values of Fv/Fm and Y(II) of Syntrichia caninervis Mitt. Although the shade can increase the Fv/Fm to some extent, no significant effects were found in Y(II).
Osmoregulatory substances were significantly affected by microhabitats. The result showed that the content of soluble sugar was higher in summer and winter, or in middle spring, when it was extremely hot or very drought, than in early winter and early spring when snow were melting. But the changes of soluble proteins were the opposite. The response of proline content of Syntrichia caninervis Mitt was weaker compared with the soluble sugar in low temperature environment, but it would be stronger while the plant was faced with drought. The contents of soluble proteins for all periods in three microhabitats were highest under live shrub, then were under the dead shrub and bare land. However, the contents of proline and soluble sugar were affected by microhabitats, periods and their interactions. The values under live shrub would be high while the environment was severe (such as higher temperature or drought). On the contrary, the contents of them would be an opposite trend when the temperature became low or the water supply was sufficient.
Different habitats significantly affected antioxidant enzyme activity of Syntrichia caninervis Mitt. The extremely low temperature in winter, severe drought in middle spring and extremely higher temperature or drought in summer would significantly enhance the content of malondialdehyde (MDA) of Syntrichia caninervis Mitt than the snowmelt periods in early winter and early spring. Meanwhile, its catalase (CAT) activity, peroxides (POD) activity, superoxide dismutase (SOD) and other antioxidant enzyme activity also showed a similar trend with the content of MDA. Except the snowmelt periods in early spring, the content of MDA was higher under live shrub than under bare land. In other periods, the content of MDA and three kinds of antioxidant enzyme activity were significantly lower under live shrub than under bare land. Removal of shrub would significantly increase the content of MDA of Syntrichia caninervis Mitt. And the activities of three antioxidant enzyme were higher under bare land than under live shrub in all periods. This may indicate that the bryophytes living on bare land may have stronger tolerance to stress than those living under shrub canopy.
In summary, Syntrichia caninervis Mitt could change the density of population, the biomass, and the ratio of hair point length to leaf length, the leaf inclination angles, the length of papillae in hair point and coast, the density of the papillae in the leaf and it arrangement mode to adapt to extreme environments. In physiology, Syntrichia caninervis Mitt could fasten the recovery of its chlorophyll fluorescence activity when snowmelt occurred in early winter and spring. Our study showed that Syntrichia caninervis Mitt could accumulate soluble sugars, proline and antioxidant enzymes in response to extreme temperature or water stress.|