Patterns of desiccation tolerance and recovery in bryophytes

The study of desiccation tolerance in bryophytes avoids thecomplications of higher-plant vascular systems and complex leaf structures, butremains a multifaceted problem. Some of the pertinent questions have at leastpartial analogues in seed biology – events during a drying-rewettingcyclewith processes in seed maturation and germination, and the gradual loss ofviability on prolonged desiccation, and the relation of this to intensity ofdesiccation and temperature, with parallel questions in seed storage. Pastresearch on bryophyte desiccation tolerance is briefly reviewed. Evidence ispresented from chlorophyll-fluorescence measurements and experiments withmetabolic inhibitors that recovery of photosynthesis in bryophytes followingdesiccation depends mainly on rapid reactivation of pre-existing structures andinvolves only limited de novo protein synthesis. Followinginitial recovery, protein synthesis is demonstrably essential to themaintenanceof photosynthetic function in the light, but the rate of maintenance turnoverinthe dark appears to be slow. Factors leading to long-term desiccation damagearediverse; indications are that desiccation tolerant species often survive bestinthe range –100 to –200 MPa.     

Photosynthesis on the edge: photoinhibition,desiccation and freezing tolerance of Antarctic bryophytes

Photosynthesis Research - In Antarctica, multiple stresses (low temperatures, drought and excessive irradiance) hamper photosynthesis even in summer. We hypothesize that controlled inactivation of...     

Effects of desiccation on the toxicant sensitivity of rotifers

Non-native aquatic macrophytes have invaded different types of ecosystems all over the world. The exotic submersed macrophyte Hydrilla verticillata recently invaded the Paraná basin, Brazil, being recorded by the first time in the natural habitats of this river in 2005. We investigated the effects of this species on ostracod assemblages and compared the abundance, richness, and Shannon–Wiener diversity of ostracod assemblages that colonize the invading species with those that colonize Egeria najas, a native submersed species with similar architecture and physical complexity. Fragments of these two species were left for 28 days in tanks to root and grow and then they were transferred to a floodplain lake where they remained in pairs (one plant of each species; N = 7) during 30 days for colonization by ostracods. A detrented correspondence analysis was used to summarize ostracod assemblage composition. Although there were no significant differences in ostracod abundance, richness and Shannon diversity when analyzed separately, cumulative curves, which permit to eliminate effects of abundance on richness, indicated a significantly higher number of ostracod species on H. verticillata. Assemblage composition was significantly different between both plant species, as shown by the first DCA axis. Our results show that H. verticillata might provide favorable habitats for native ostracod assemblages.     

Responses to desiccation stress in bryophytes and an important role of dithiothreitol-insensitive non-photochemical quenching against photoinhibition in dehydrated states

The effects of air drying and hypertonic treatments in the dark on seven bryophytes, which had grown under different water environments, were studied. All the desiccation-tolerant species tested lost most of their PSII photochemical activity when photosynthetic electron transport was inhibited by air drying, while, in all the sensitive species, the PSII photochemical activity remained at a high level even when photosynthesis was totally inhibited. The PSI reaction center remained active under drying conditions in both sensitive and tolerant species, but the activity became non-detectable in the light only in tolerant species due to deactivation of the cyclic electron flow around PSI and of the back reaction in PSI. Light-induced non-photochemical quenching (NPQ) was found to be induced not only by the xanthophyll cycle but also by a DeltapH-induced, dithiothreitol-insensitive mechanism in both the desiccation-tolerant and -intolerant bryophytes. Both mechanisms are thought to have an important role in protecting desiccation-tolerant species from photoinhibition under drying conditions. Fluorescence emission spectra at 77K showed that dehydration-induced quenching of PSII fluorescence was observed only in tolerant species and was due to neither state 1-state 2 transition nor detachment of light-harvesting chlorophyll protein complexes from PSII core complexes.The presence of dehydration-induced quenching of PSI fluorescence was also suggested.     

Outdoor Studies on the Effects of Solar UV-B on bryophytes: Overview and Methodology

In this review all recent field studies on the effects of UV-B radiation on bryophytes are discussed. In most of the studies fluorescent UV-B tubes are used to expose the vegetation to enhanced levels of UV-B radiation to simulate stratospheric ozone depletion. Other studies use screens to filter the UV-B part of the solar spectrum, thereby comparing ambient levels of UV-B with reduced UV-B levels, or analyse effects of natural variations in UV-B arising from stratospheric ozone depletion. Nearly all studies show that mosses are well adapted to ambient levels of UV-B radiation since UV-B hardly affects growth parameters. In contrast with outdoor studies on higher plants, soluble UV-B absorbing compounds in bryophytes are typically not induced by enhanced levels of UV-B radiation. A few studies have demonstrated that UV-B radiation can influence plant morphology, photosynthetic capacity, photosynthetic pigments or levels of DNA damage. However, there is only a limited number of outdoor studies presented in the literature. More additional, especially long-term, experiments are needed to provide better data for statistical meta-analyses. A mini UV-B supplementation system is described, especially designed to study effects of UV-B radiation at remote field locations under harsh conditions, and which is therefore suited to perform long-term studies in the Arctic or Antarctic. The first results are presented from a long-term UV-B supplementation experiment at Signy Island in the Maritime Antarctic.     

连续性干出对孔石莼生长和生化组成的影响

以孔石莼(Ulva pertusa)为试验材料,研究了连续性干出对其生长和生化组成的影响.结果表明:与对照组(0 h)相比,短时间的连续性干出促进孔石莼生长,表现在特定生长率( SGR)升高、叶绿素a含量上升;随着连续性干出时间延长,孔石莼SGR、叶绿素a、蛋白质含量下降,叶绿素b、可溶性糖含量上升;叶绿素a/b比值变化和可溶性糖含量变化有利于抵抗干出胁迫.     

Effects of temperature and desiccation on ex situ conservation of nongreen fern spores

? Premise of the study: Fern spores are unicellular and haploid, making them a potential model system to study factors that regulate lifespan and mechanisms of aging. Aging rates of nongreen spores were measured to compare longevity characteristics among diverse fern species and test for orthodox response to storage temperature and moisture. ? Methods: Aging of spores from 10 fern species was quantified by changes in germination and growth parameters. Storage temperature ranged from ambient room to -196°C (liquid nitrogen); spores were dried to ambient relative humidity (RH) or using silica gel. ? Key results: Survival of spores varied under ambient storage conditions, with one species dying within a year and two species having greater than 50% survival after 3 years. Few changes in germination or growth were observed in spores stored at either -80°C or -196°C over the same 3-yr study period. Spores stored at -25°C aged anomalously quickly, especially those dried to ambient RH or subjected to repeated freeze-thaw cycles. ? Conclusions: Spore longevity is comparable to orthodox seed longevity under ambient storage conditions, with wide variation among species and shelflife extended by drying or cooling. However, faster aging during freezer storage may indicate a similar syndrome of damage experienced by seeds categorized as "intermediate". The damage is avoided by storage at -80°C or liquid nitrogen temperatures, making cryoconservation an effective and broadly applicable tool to extend fern spore longevity. The study demonstrates that spore banks are a feasible approach for ex situ conservation of this important plant group.     

Effects of starvation and desiccation on energy metabolism in desert and mesic Drosophila

Energy availability can limit the ability of organisms to survive under stressful conditions. In Drosophila, laboratory experiments have revealed that energy storage patterns differ between populations selected for desiccation and starvation. This suggests that flies may use different sources of energy when exposed to these stresses, but the actual substrates used have not been examined. We measured lipid, carbohydrate, and protein content in 16 Drosophila species from arid and mesic habitats. In five species, we measured the rate at which each substrate was metabolized under starvation or desiccation stress. Rates of lipid and protein metabolism were similar during starvation and desiccation, but carbohydrate metabolism was several-fold higher during desiccation. Thus, total energy consumption was lower in starved flies than desiccated ones. Cactophilic Drosophila did not have greater initial amounts of reserves than mesic species, but may have lower metabolic rates that contribute to stress resistance.     

间断性低氧对大鼠腓肠肌介电性能的影响

目的：在40Hz～110MHz频率范围观察间断性低氧暴露4周大鼠离体腓肠肌细胞介电性能的改变。方法：采用低压氧舱建立模拟低氧模型,雄性SD大鼠随机分为间断低氧组和正常对照组。利用Agilent 4294A阻抗分析仪测量了离体大鼠腓肠肌的交流阻抗,通过频域介电谱、Cole—Cole图、介电损耗因子频谱、电导率虚部频谱和介电损耗角正切频谱的数据分析,观察间断性低氧暴露对大鼠离体腓肠肌细胞介电性能的影响。结果：间断性低氧暴露4周大鼠腓肠肌的介电常数（εL,εh）降低,介电增量△ε减小,绝缘性降低;低频电导率κL升高,高频电导率κh降低,电导率增量△κ降低;特征频率（f1,f2）增加;介电损失峰值ε”peak、电导率虚部峰值κ”peak和损耗角正切峰值婶谳均降低。结论：间断性低氧暴露致骨骼肌细胞介电性能降低,但其特征频率增加。     

Effects of continuous and repeated dehydration on carbon fixation by bryophytes from the maritime Antarctic

 The effects of dehydration and rehydration on carbon exchange in 14 bryophytes from the maritime Antarctic were investigated using an infra-red gas analysis system. Continuous long-term (1–12 months) and repeated (1–6 one-month cycles) desiccation responses were investigated under controlled conditions. Loss of photosynthetic rate increased with length of dehydration period in all species, although some desiccation tolerance was observed even in those bryophytes from the most hydric habitats. Percentage retention of photosynthetic rate increased from hydric to xeric species, but this pattern was not repeated in terms of absolute rates of carbon fixation due to the high initial rates in the hydric species. Repeated cycles caused a greater loss of photosynthetic rate than continuous dehydration in hydric species, but the opposite situation occurred in mesic and xeric mosses. The latter groups were possibly better able to utilise the short periods of rehydration during cycles. In most bryophytes an increase in the percentage loss of photosynthetic rate following dehydration-rehydration occurred from spring to summer to autumn samples. This pattern was clearest in the hydric species and reduced in the xeric species. These variations were largely due to changes in the initial rates of photosynthesis during the growing season. It is suggested that this increased photosynthetic capacity is stress-sensitive, and is lost during either desiccation or winter freezing; the base photosynthetic capacity, being stress-tolerant, survives either of these events. The results obtained support the hypothesis that water availability is of importance in determining the distribution of bryophytes in the Antarctic. However, only the broad scale of variation in plant communities could be explained by these observations; other factors must be important in determining the finer scale of species distribution and community composition. The results are applicable to attempts to model the productivity of Antarctic bryophytes from known or predicted environmental data. Received: 15 April 1996 / Accepted: 28 September 1996     

Effects of desiccation and rehydration on nitrogen fixation by epiphylls in a tropical rainforest

Nitrogen fixation rates by epiphyllous microorganisms are affected by desiccation. Rates from leaf samples which had been dried for 12 h were 0.66 ng N/10 cm²/h. In contrast, rates from leaves which had been kept continuously wet were 18.69 ng N/10 cm²/h. Leaf samples which had been rehydrated for 2 and 4 h showed intermediate fixation rates (4.26 and 9.76 ng N/10 cm²/h, respectively). Epiphyllous bryophytes maintain moist conditions on the leaf surface and thus create a microenvironment suitable for prolonged fixation by the microorganisms.     

Effects of bryophytes and lichens on seedling emergence of alvar plants: evidence from greenhouse experiments

Emergence of seedlings of four alvar grassland species ( Arenaria serpyllifolia , Festuca ovina , Filipendula vulgaris and Veronica spicata ) in bryophyte and lichen carpets was analysed in a series of greenhouse experiments. The aspects investigated were: the influence of thickness of moss mats, both in dry and moist conditions, the effects of thick Cladonia spp. clumps, and of living vs dead moss shoots and lichen podetia. Overall, Festuca seedlings emerged best whereas the small-seeded species, Arenaria and Veronica , had the lowest emergence. Moisture had a significant effect only on the emergence of Festuca seedlings, which emerged better in the dry treatment than in the moist. A thick moss cover negatively affected seedling emergence of Arenaria and Veronica but did not affect the emergence of Festuca . Filipendula showed lower seedling emergence in both thick and thin moss than on bare soil only in the dry treatment, whereas in the moist treatment emergence did not differ among the three substrates. Arenaria seedlings emerged less in thick and thin moss than on bare soil in the dry treatment, whereas in the moist treatment emergence in the thin moss was not different from bare soil. Thus, in relatively dry environments even a thin moss cover may inhibit rather than facilitate seedling emergence. The lichen clumps inhibited only the emergence of the forbs. Both living moss shoots and lichen podetia inhibited emergence of Veronica seedlings but did not affect Festuca . In contrast, emergence in the presence of dead mosses and lichens did not differ from emergence in their absence for both species. Hence, inhibition of seedling emergence by bryophytes and lichens of at least some vascular plant species may be mediated by some biotic factor. However, the effect of differences in substrate properties on germination cannot be excluded     

Contrasting effects of ammonium enrichment on fen bryophytes

Abstract

In the second half of the 20th century, frequency and diversity of brown mosses strongly declined in Dutch fens while species of Sphagnum and Polytrichum increased markedly. We hypothesized that high ammonium (NH₄⁺) concentrations in Dutch precipitation have promoted these species changes. To test this hypothesis, we examined the effects of varying concentrations (1–1000 μM) of NH₄⁺ on a brown moss (Calliergonella cuspidata), two species of Sphagnum (S. contortum and S. squarrosum) and Polytrichum commune in a hydroponic experiment. A second goal was to identify possible interspecific differences in the mechanisms responsible for NH₄⁺ detoxification. Dry matter production and tissue cation concentrations indicated decreasing sensitivity to NH₄⁺ in the order C. cuspidata>S. contortum>S. squarrosum and P. commune. In S. squarrosum and P. commune, the highest NH₄⁺ treatments induced strong increases in tissue nitrogen (N) and amino acid concentrations. Sphagnum contortum showed a similar, but less marked response. In contrast, C. cuspidata showed a negative relationship between external NH₄⁺ concentrations and both tissue N and amino acid concentrations (except arginine). Our findings support the hypothesis that the present rates of NH₄⁺ deposition in The Netherlands are detrimental to brown mosses, but not to fast-growing Sphagnum and Polytrichum species. The efficiency of the NH₄⁺ assimilation apparatus, producing N-rich amino acids, probably plays a key role in determining sensitivity to increased NH₄⁺ deposition.     

The influence of phosphorus enrichment on lotic bryophytes

• 1 Phosphorus enrichments of 5 weeks' duration were performed in two woodland streams in eastern Tennessee, U.S.A. to determine their effect on the dominant stream bryophyte, Porella pinnata.
• 2 In a second-order reach of Walker Branch, which had an N:P ratio (by atoms) of 3.5:1 in the stream water, the P:C ratio of Porella was not significantly affected by enrichment, but the P:N ratio did increase significantly. In Sludge Creek, which had an N:P ratio of 21.6:1 in the stream water, both P:C and P:N ratios of Porella increased significantly following phosphorus addition. Increased phosphorus ratios may have resulted from either assimilation or adsorption.
• 3 Absolute phosphorus concentrations in Porella tissue were significantly greater prior to enrichment in the control reach of Walker Branch, confounding the effect of phosphorus enrichment. In Sludge Creek, absolute phosphorus content in Porella tissue increased significantly in the treated reach and showed no significant difference in the control reach.
• 4 Although mean primary production increased by approximately 15% following enrichment, the increase was not significantly different from that prior to enrichment.
• 5 Epiphyte structure and abundance were not significantly influenced by enrichment in either stream. It is suggested that grazing pressure by snails may have masked any potential epiphyte response to enrichment.