Membrane Stability of the Desert Moss Syntrichia caninervis Mitt. during Desiccation and Rehydration

Syntrichia caninervis Mitt., a typical desert moss, is one of the dominant species in biological soil crusts  (BSCs) in the Gurbantunggut Desert. Recently, it became the focus of scientific research because of its high ecological value. It also has a special significance in physiological ecology research because of the rapid restoration of photosynthesis upon the onset of rehydration.

Numerous ultrastructural observations have confirmed that membranes of vegetative cells of desiccation-tolerant (DT) mosses do not suffer observable damage during desiccation. The desiccation-induced membrane disruption shown in early electron micrographs was suggested to be an artefact of inadequate fixation. Nevertheless, the concept that bryophyte desiccation tolerance is ‘repair-based’ still persists although there is a lack of positive evidence for this hypothesis. Most DT vascular plants and many bryophytes that rely on inducible protection mechanisms must be dried slowly to allow time for these to be put into place. In order to withstand frequent rapid and repeated drying, it appears that for mosses like S. caninervis in drought-prone habitats, it is most probable that their protection is constitutive.

Most studies of bryophyte desiccation tolerance have been on Syntrichia (Tortula) ruralis, and few have mentioned S. caninervis. These two species are believed to be closely related, and any physiological work on one is likely to be relevant to the other. Therefore, changes in membrane structures in S. caninervis were monitored under varying conditions of water stress in order to provide confirmatory electron microscopical and physiological evidence that in S. caninervis, membranes are preserved intact through desiccation/rehydration, and investigate the major ‘constitutive’ protective substances which are hypothesized to play an important role in membrane stabilization.

The result showed that no significant changes in electrical conductivity of the rehydration water were observed either during dehydration or rehydration. Electron micrographs of the fine-structure of leaf cells were obtained through a drying–re-wetting cycle. Major changes in cell ultrastructure were observed over time but there was no evidence of membrane damage during either desiccation or rehydration. Three possible explanations for disorganized or disrupted membranes in the desiccated state are considered: (1) S. caninervis has special morphological and anatomical characteristics (e.g. strong leaf costa which can transport water during the recovery state) that allow it to survive in an hostile, arid environment, and cellular structures that remain intact in the desiccated state permitting membrane integrity to be rapidly regained during rehydration; (2) the moss quickly becomes dormant during dehydration but maintains a certain level of membrane integrity; and (3) during desiccation, rapidly and continuously increasing amounts of both soluble sugar (major ‘constitutive’ protective substances) and free proline contribute to membrane stabilization.

The result has been published on Journal of Bryology, 2012, 34(1): 1–8. The paper is also archived at http://chinesesites.library.ingentaconnect.com/content/maney/jbr/2012/00000034/00000001/art00001.