Ultrastructure variations in Sphagnum denticulatum ecotypes in response to desiccation stress matter to conservation.

Global warming and peat bogs drying are having a strong negative effect on the survival of endangered peat mosses. Here, we aimed to identify ultrastructural and physiological trait variation during dehydration and rehydration in the (sub-)meristematic cells of buds among clonally propagated individuals of Sphagnum denticulatum in relation to their ecological origin. We cultivated five clones in common garden conditions (CGCs) to exclude a carryover effect and we subsequently water-stressed (-40 MPa) and rehydrated (7 days) them. For the ultrastructure analysis, over 1280 measurements were recorded for 34 traits. Compared with the control, the treatment led to alterations in organelles that appeared to be ecotype- and genotype-dependent and characteristic for desiccation-sensitive mosses. Also, the recovery of chloroplasts, as measured by the initial and maximal fluorescence yield, were incomplete for all studied plants indicating desiccation sensitivity. Terrestrial genotypes possessed better recovery capability than did aquatic genotypes, suggesting an adaptation of the former to tolerate unpredictable terrestrial conditions in time and space. Genotype-specific requirements of water availability in the original environments should be considered before transplanting gametophytes for peatland restoration programs.

Novel insights on the structure and composition of pseudostomata of Sphagnum.

PREMISE OF THE STUDY: The occurrence of stomata on sporophytes of mosses and hornworts is congruent with a single origin in land plants. Although true stomata are absent in early-divergent mosses, Sphagnum has specialized epidermal cells, pseudostomata, that partially separate but do not open to the inside. This research examined two competing hypotheses that explain the origin of pseudostomata: (1) they are modified stomata, or (2) they evolved from epidermal cells independently from stomata.• METHODS: Capsule anatomy and ultrastructure of pseudostomata were studied using light and electron microscopy, including immunolocalization of pectins.• KEY RESULTS: Cell walls in pseudostomata are thin, two-layered, and rich in pectins, similar to young moss stomata, including the presence of cuticle on exterior walls. Outer and ventral walls have a thick cuticle that suggests that initial separation of ventral walls involves cuticle deposition as in true stomata. Further mechanical separation between ventral walls does not form a pore and occurs as the capsule dries.• CONCLUSIONS: As in moss stomata, pseudostomata wall architecture and behavior facilitate capsule dehydration, shape change, and dehiscence, supporting a common function. The divergent structure and fate of pseudostomata may be explained by the retention of Sphagnum sporophytes within protective leaves until nearly mature. Ultrastructural and immunocytological data suggest that pseudostomata are related to stomata but do not conclusively support either hypothesis. Solving the relationship of early land plants is critical to understanding stomatal evolution. Pseudostomata are structurally and anatomically unique, but their relationship to true stomata remains to be determined.

Sphagnum moss disperses spores with vortex rings.

Sphagnum spores, which have low terminal velocities, are carried by turbulent wind currents to establish colonies many kilometers away. However, spores that are easily kept aloft are also rapidly decelerated in still air; thus, dispersal range depends strongly on release height. Vascular plants grow tall to lift spores into sufficient wind currents for dispersal, but nonvascular plants such as Sphagnum cannot grow sufficiently high. High-speed videos show that exploding capsules of Sphagnum generate vortex rings to efficiently carry spores high enough to be dispersed by turbulent air currents. Spores launched ballistically at similar speeds through still air would travel a few millimeters and not easily reach turbulent air. Vortex rings are used by animals; here, we report vortex rings generated by plants.

Size matters for violent discharge height and settling speed of Sphagnum spores: important attributes for dispersal potential.

BACKGROUND AND AIMS: Initial release height and settling speed of diaspores are biologically controlled components which are key to modelling wind dispersal. Most Sphagnum (peat moss) species have explosive spore liberation. In this study, how capsule and spore sizes affect the height to which spores are propelled were measured, and how spore size and spore number of discharged particles relate to settling speed in the aspherical Sphagnum spores. METHODS: Spore discharge and spore cloud development were filmed in a closed chamber (nine species). Measurements were taken from snapshots at three stages of cloud development. Settling speed of spores (14 species) and clusters were timed in a glass tube. KEY RESULTS: The maximum discharge speed measured was 3.6 m s(-1). Spores reached a maximum height of 20 cm (average: 15 cm) above the capsule. The cloud dimensions at all stages were related positively to capsule size (R(2) = 0.58-0.65). Thus species with large shoots (because they have large capsules) have a dispersal advantage. Half of the spores were released as singles and the rest as clusters (usually two to four spores). Single spores settled at 0.84-1.86 cm s(-1), about 52 % slower than expected for spherical spores with the same diameters. Settling speed displayed a positive curvilinear relationship with spore size, close to predictions by Stokes' law for spherical spores with 68 % of the actual diameters. Light-coloured spores settled slower than dark spores. Settling speed of spore clusters agrees with earlier studies. Effective spore discharge and small, slowly settling spores appear particularly important for species in forested habitats. CONCLUSIONS: The spore discharge heights in Sphagnum are among the greatest for small, wind-dispersed propagules. The discharge heights and the slow settling of spores affect dispersal distances positively and may help to explain the wide distribution of most boreal Sphagnum species.

Freezing cytorrhysis and critical temperature thresholds for photosystem II in the peat moss Sphagnum capillifolium.

Leaflets of Sphagnum capillifolium were exposed to temperatures from -5 degrees C to +60 degrees C under controlled conditions while mounted on a microscope stage. The resultant cytological response to these temperature treatments was successfully monitored using a light and fluorescence microscope. In addition to the observable cytological changes during freezing cytorrhysis and heat exposure on the leaflets, the concomitant critical temperature thresholds for inactivation of photosystem II (PS II) were studied using a micro fibre optic and a chlorophyll fluorometer mounted to the microscope stage. Chlorophyllous cells of S. capillifolium showed extended freezing cytorrhysis immediately after ice nucleation at -1.1 degrees C in the water in which the leaflets were submersed during the measurement. The occurrence of freezing cytorrhysis, which was visually manifested by cell shrinkage, was highly dynamic and was completed within 2 s. A total reduction of the mean projected diameter of the chloroplast containing area during freezing cytorrhysis from 8.9 to 3.8 microm indicates a cell volume reduction of approximately -82%. Simultaneous measurement of chlorophyll fluorescence of PS II was possible even through the frozen water in which the leaf samples were submersed. Freezing cytorrhysis was accompanied by a sudden rise of basic chlorophyll fluorescence. The critical freezing temperature threshold of PS II was identical to the ice nucleation temperature (-1.1 degrees C). This is significantly above the temperature threshold at which frost damage to S. capillifolium leaflets occurs (-16.1 degrees C; LT(50)) which is higher than observed in most higher plants from the European Alps during summer. High temperature thresholds of PS II were 44.5 degrees C which is significantly below the heat tolerance of chlorophyllous cells (49.9 degrees C; LT(50)). It is demonstrated that light and fluorescence microscopic techniques combined with simultaneous chlorophyll fluorescence measurements may act as a useful tool to study heat, low temperature, and ice-encasement effects on the cellular structure and primary photosynthetic processes of intact leaf tissues.