The effects of arsenate,selenite and distilled water on the physiology and fine structure of Hylocomium splendens

Abstract

The physiological response of Hylocomium splendens (Hedw) Br. Eur. to incubation in distilled water and solutions of phosphate, arsenate, selenite and selenate was investigated. Potassium efflux was found useful for comparing toxicity but was unreliable for short-term studies due to an initial rapid loss of potassium within ten minutes of immersion in any solution. This potassium loss could, under noninhibitory conditions, be regained by moss shoots. Selenite prevented re-accumulation of potassium lost in the first few minutes of incubation whilst arsenate promoted potassium loss throughout incubation. Photosynthetic oxygen evolution was progressively reduced under conditions of increasing arsenic uptake. Thus, photosynthetic rates were a more sensitive measure of anion toxicity than potassium efflux.

Electron microscopic observations revealed that incubation in distilled water had little effect on immature leaves of H. splendens whilst arsenate incubation induced loss of cytoplasmic electron density and swelling of chloroplast lamellae in immature tissue. Mature leaves, incubated in both arsenate and distilled water, showed changes in celluar morphology compared to non incubated tissue. These changes were not reflected in potassium efflux data. The results indicate that shortterm submersion is disruptive to the fine structure of moss shoots. This is discussed in terms of the design and interpretation of experiments involving toxic ions.     

Photosynthesis, chlorophyll fluorescence and spectral reflectance in Sphagnum moss at varying water contents

Moss samples from the Fluxnet-Canada western peatland flux station in the Boreal Region of Alberta were measured in the laboratory to obtain the net photosynthesis rate and chlorophyll fluorescence of the moss under controlled environmental conditions, including the regulation of moss water content, simultaneously with measurements of moss spectral reflectance. One objective was to test whether the photochemical reflectance index (PRI) detected changes in moss photosynthetic light-use efficiency that were consistent with short-term (minutes to hours) changes in xanthophyll cycle pigments and associated changes in non-photochemical quenching (NPQ), as recorded by chlorophyll fluorescence. The rate of net photosynthesis was strongly inhibited by water content at values exceeding approximately 9 (fresh weight/dry weight) and declined as the water content fell below values of approximately 8. Chlorophyll fluorescence measurements of maximum photosystem II efficiency generally remained high until the water content was reduced from the maximum of about 20 to values of approximately 10–11, and then declined with further reductions in moss water content. A significant linear decline in NPQ was observed as moss water content was reduced from maximum to low water content values. There was a strong negative correlation between changes in NPQ and PRI. These data suggest that PRI measurements are a good proxy for short-term shifts in photosynthetic activity in Sphagnum moss. A second objective was to test how accurately the water band index (WBI, ratio of reflectance at 900 and 970 nm) recorded changes in moss water content during controlled laboratory studies. Strong linear relationships occurred between changes in moss water content and the WBI, although the slopes of the linear relationships were significantly different among sample replicates. Therefore, WBI appeared to be a useful tool to determine sample-specific water content without destructive measurements.     

Water Stress and Protein Synthesis: V. Protein Synthesis, Protein Stability, and Membrane Permeability in a Drought-sensitive and a Drought-tolerant Moss

The effects have been studied of water stress and desiccation on protein synthesis in the drought-tolerant moss Tortula ruralis and the drought-sensitive moss Hygrohypnum luridum. At any particular level of steady state water stress, the inhibition of protein synthesis was greater in H. luridum than in T. ruralis. Water stress-induced changes in the pattern of protein synthesis, as determined by the double label ratio technique, were minor in T. ruralis, but major in H. luridum. Proteins of both mosses were found to be stable during desiccation and subsequent rehydration. Changes in membrane permeability, as indicated by the leakage of amino acid, were observed during rehydration of desiccated moss and were dependent on the rate of desiccation. The leakage was small and reversible in T. ruralis but large and irreversible in H. luridum. Although H. luridum failed to recover from complete desiccation (80% loss in fresh weight), it was able to recover fully from steady state stress under conditions where a maximum loss of 55% in fresh weight was recorded.     

Seasonal shift in factors controlling net ecosystem production in a high Arctic terrestrial ecosystem

We examined factors controlling temporal changes in net ecosystem production (NEP) in a high Arctic polar semi-desert ecosystem in the snow-free season. We examined the relationships between NEP and biotic and abiotic factors in a dominant plant community (Salix polaris–moss) in the Norwegian high Arctic. Just after snowmelt in early July, the ecosystem released CO₂ into the atmosphere. A few days after snowmelt, however, the ecosystem became a CO₂ sink as the leaves of S. polaris developed. Diurnal changes in NEP mirrored changes in light incidence (photosynthetic photon flux density, PPFD) in summer. NEP was significantly correlated with PPFD when S. polaris had fully developed leaves, i.e., high photosynthetic activity. In autumn, NEP values decreased as S. polaris underwent senescence. During this time, CO₂ was sometimes released into the atmosphere. In wet conditions, moss made a larger contribution to NEP. In fact, the water content of the moss regulated NEP during autumn. Our results indicate that the main factors controlling NEP in summer are coverage and growth of S. polaris, PPFD, and precipitation. In autumn, the main factor controlling NEP is moss water content.     

Desiccation sensitivity and tolerance in the moss <Emphasis Type="Italic">Physcomitrella patens</Emphasis>: assessing limits and damage

The moss Physcomitrella patens is becoming the model of choice for functional genomic studies at the cellular level. Studies report that Physcomitrella survives moderate osmotic and salt stress, and that desiccation tolerance can be induced by exogenous ABA. Our goal was to quantify the extent of dehydration tolerance in wild type moss and to examine the nature of cellular damage caused by desiccation. We exposed Physcomitrella to humidities that generate water potentials from −4 (97% RH) to −273 MPa (13% RH) and monitored water loss until equilibrium. Water contents were measured on a dry matter basis to determine the extent of dehydration because fresh weights (FW) were found to be variable and, therefore, unreliable. We measured electrolyte leakage from rehydrating moss, assessed overall regrowth, and imaged cells to evaluate their response to drying and rehydration. Physcomitrella did not routinely survive water potentials <−13 MPa. Upon rehydration, moss dried to water contents >0.4 g g dm⁻¹ maintained levels of leakage similar to those of hydrated controls. Moss dried to lower water contents leaked extensively, suggesting that plasma membranes were damaged. Moss protonemal cells were shrunken and their walls twisted, even at −13 MPa. Moss cells rehydrated after drying to −273 MPa failed to re-expand completely, again indicating membrane damage. ABA treatment elicited tolerance of desiccation to at least −273 MPa and limited membrane damage. Results of this work will form the basis for ongoing studies on the functional genomics of desiccation tolerance at the cellular level.     

Rapid photosynthetic recovery of a snow-covered feather moss and Peltigera lichen during sub-Arctic midwinter warming

Background: Arctic lichens and mosses are covered by snow for more than half the year and are generally considered as being dormant for most of this period. However, enhanced frequency of winter warming events due to climate change can cause increased disturbance of their protective subnivean environment.

Aim: To further understand cryptogamic responses to midwinter warming we compared the ecophysiological performance of one lichen and one moss species during a simulated warming event.

Methods: We measured photosynthesis and dark respiration in samples of the moss Hylocomium splendens and the lichen Peltigera aphthosa removed from under snow, and on natural refreezing after the warming event, which was simulated by using infrared heaters suspended above the ground.

Results: The moss exposed to light at +5 °C immediately after removal from their subnivean environment and from warmed plots showed positive net gas exchange within 332 s; the lichen required 1238 s. Photosynthesis and nitrogen fixation rates were equal to that, or higher than, during the preceding growing season. Upon refreezing after the event, moss photosynthesis declined considerably.

Conclusions: The moss, and to a lesser extent the lichen, may contribute to subnivean midwinter ecosystem respiration, and both are opportunistic, and can take advantage of warmer winter phases for photosynthesis and growth. This ought to be taken into account in vegetation change projections of cryptogam-rich ecosystems.     

Carbon dioxide exchange of two ecodemes of Schistidium antarctici in Continental Antarctica

Summary Schistidium antarctici is the commonest of five bryophytes known in the Windmill Islands area of Wilkes Land, Greater Antarctica. In moist habitats it forms closed carpets, but in dry sites it develops a short cushion growth form. Carbon dioxide exchange of both a mesic (Sm) and a xeric growth form (Sx) was investigated by means of an IRGA system in the field near Casey Station under natural light and simulated ambient or controlled temperature conditions in the plant chamber. The chlorophyll content in Sm was three times higher than in Sx. The light compensation point of Sm was lower than in Sx. The data for photosynthesis and dark respiration were computed by means of non-linear and linear regression analysis. Sm was more productive and had a wider temperature range of positive net photosynthesis than Sx under similar conditions. Dark respiration per gram of the whole moss sample was identical in both ecodemes. A decline of the photosynthesis curves at quantum flux densities above 500 mol m^-2 s^-1 PAR indicated a photoinhibitory effect in Sm. Sx was even more sensitive to high irradiance levels. Photoinhibition was not apparent in laboratory measurements under artificial light. According to our field measurements the photoinhibitory effect increases with increasing temperature. Moisture loss was avoided during the experiments by water supply from the bottom and frequently spraying the moss samples with water. In the natural habitat the desiccating effect of solar radiation is important, as it quickly causes photosynthesis to cease. The moss will dry out sooner in a xeric habitat than in one which is continuously moist. Consequently, the mesic Schistidium might particularly be subjected to photoinhibition by bright sunshine.Cordially dedicated to Professor Dr. O.H. Volk, Würzburg, on the event of his 85th birthday     

Surface reflectance properties of Antarctic moss and their relationship to plant species, pigment composition and photosynthetic function

In this study the variations in surface reflectance properties and pigment concentrations of Antarctic moss over species, sites, microtopography and with water content were investigated. It was found that species had significantly different surface reflectance properties, particularly in the region of the red edge (approximately 700 nm), but this did not correlate strongly with pigment concentrations. Surface reflectance of moss also varied in the visible region and in the characteristics of the red edge over different sites. Reflectance parameters, such as the photochemical reflectance index (PRI) and cold hard band were useful discriminators of site, microtopographic position and water content. The PRI was correlated both with the concentrations of active xanthophyll‐cycle pigments and the photosynthetic light use efficiency, F_v/F_m, measured using chlorophyll fluorescence. Water content of moss strongly influenced the amplitude and position of the red‐edge as well as the PRI, and may be responsible for observed differences in reflectance properties for different species and sites. All moss showed sustained high levels of photoprotective xanthophyll pigments, especially at exposed sites, indicating moss is experiencing continual high levels of photochemical stress.     

Activity pattern of the moss<Emphasis Type="Italic"> Hennediella heimii</Emphasis> (Hedw.) Zand. in the Dry Valleys,Southern Victoria Land,Antarctica during the mid-austral summer

The activity pattern of the moss Hennediella heimii (Hedw.) Zand. was monitored over a period of 18 days during the austral summer season 2000/2001 at the Canada Flush in Taylor Valley, continental Antarctica. Provided with melt water from the massive Canada Glacier, the moss showed a constant potential photosynthetic activity during the entire measurement period. Permanently hydrated, the moss faced high light levels at surprisingly low moss temperatures, which is commonly supposed a deleterious situation for plants. The electron transport rate response of the moss to photosynthetic photon flux densities was linear at all temperatures and did not show a sign of saturation or photoinhibition. H. heimii seems to be well adapted to its environment and tolerates the ambient conditions without apparent harm. This might be due to the fact that mosses can acclimatise to high light conditions by building up highly effective non-photochemical quenching systems.     

Precipitation frequency alters peatland ecosystem structure and CO2 exchange: Contrasting effects on moss,sedge, and shrub communities

Climate projections forecast a redistribution of seasonal precipitation for much of the globe into fewer, larger events spaced between longer dry periods, with negligible changes in seasonal rainfall totals. This intensification of the rainfall regime is expected to alter near‐surface water availability, which will affect plant performance and carbon uptake. This could be especially important in peatland systems, where large stores of carbon are tightly coupled to water surpluses limiting decomposition. Here, we examined the role of precipitation frequency on vegetation growth and carbon dioxide (CO₂) balances for communities dominated by a Sphagnum moss, a sedge, and an ericaceous shrub in a cool temperate poor fen. Field plots and laboratory monoliths received one of three rainfall frequency treatments, ranging from one event every three days to one event every 14 days, while total rain delivered in a two‐week cycle and the entire season to each treatment remained the same. Separating incident rain into fewer but larger events increased vascular cover in all peatland communities: vascular plant cover increased 6× in the moss‐dominated plots, nearly doubled in the sedge plots, and tripled in the shrub plots in Low‐Frequency relative to High‐Frequency treatments. Gross ecosystem productivity was lowest in moss communities receiving low‐frequency rain, but higher in sedge and shrub communities under the same conditions. Net ecosystem exchange followed this pattern: fewer events with longer dry periods increased CO₂ flux to the atmosphere from the moss while vascular plant‐dominated communities became more of a sink for CO₂. Results of this study suggest that changes to rainfall frequency already occurring and predicted to continue will lead to increased vascular plant cover in peatlands and will impact their carbon‐sink function.     

Radical Formation and Accumulation In Vivo,In Desiccation Tolerant and Intolerant Mosses

Water loss in a desiccation-sensitive moss resulted in destruction of chlorophyll, loss of carotenoids and increased lipid peroxidation, indicating the presence of damaging forms of activated oxygen. These effects were exaggerated when the plants were desiccated at high light intensities. During water-deprivation there was a build up of a free radical, detected in vivo, with a close correlation between molecular damage and radical accumulation. In contrast, in a desiccation-tolerant moss there was almost no indication of molecular (oxidative) damage. However a stable radical similar in type and concentration to that found in the desiccation-sensitive species accumulated, particularly under high irradiances. The stable radical appears to be one of the end-products of a process initiated by environmental stress, desiccation and high irradiance: its association with molecular damage depending on the degree to which the species is tolerant of desiccation. Identification of the radical in intact tissue from EPR and ENDOR studies, suggests that this is not a short-lived proxy-radical but instead is relatively stable and carbon-centred.     

Daily malate fluctuations in Sedum dasyphyllum L.

Abstract

Sedum dasyphyllum L. leaves of well-watered plants kept outdoors and under controlled conditions show diurnal malic acid fluctuations. In well-watered plants growing outdoors the malate accumulation undergoes seasonal variations and seems to be inhibited by short photoperiod and/or by low temperature. The seasonal variations of CAM activity correspond to seasonal variations of mesophyll succulence. Water stress markedly depressed CAM activity. In fact, plants of S. dasyphyllum show, under controlled conditions, a decrease of malate accumulation as relative water content decreases. Recovery from water stress is fairly slow. Water potential quickly increases during rewatering and exceeds the original value after few days, suggesting a consumption of osmotic compounds during the water stress period.     

Stress responses and metal exchange kinetics following transplant of the aquatic moss Fontinalis antipyretica

1. We studied stress responses and metal exchange (uptake and loss) kinetics in the aquatic moss Fontinalis antipyretica Hedw. following transplant (in plastic mesh bags) to clean and metal-contaminated river sites and under laboratory conditions. 2. The stress response (estimated on the basis of chlorophyll:phaeophytin ratio) was more pronounced, and the moss took longer to recover, following transplant to heavily contaminated sites. 3. Metal uptake over the 28 day exposure period was predicted well by a two-compartment kinetic model: uptake velocity was initially high and gradually declined, with metal concentration in the moss showing a tendency to reach an equilibrium with metal concentration in the water. Mean uptake rate, time to reach equilibrium and metal concentration in moss at equilibrium all increased with increasing metal concentration in the water. 4. Bioconcentration constants calculated on the basis of our data rank the metals studied in the order Zn < Ni < Co < Cu < Pb < Cd. 5. Following retransplant of mosses to a clean site, metal loss was not predicted well by a passive exchange mode of the above type; instead, two phases (rapid and slow) were apparent. Loss rates during both phases, and proportion of metal load lost during the rapid phase, were proportional to the concentration of metal in the moss at the start of the recuperation trial. 6. We present nomograms to allow estimation of severity of contamination on the basis of metal concentration in moss and duration of transplant.     

Hydraulic Properties of a Mangrove Avicennia Germinans as Affected by NaCl

Water transport was assessed in seedlings of the mangrove Avicennia germinans L. grown at 171 and 684 mol m^–3 NaCl. Leaf specific conductivity declined by 25 % at high salinity. This was related to low specific conductivity, because Huber values remained similar. Leaves of A. germinans featured low internal conductance to water transport. This was lowered further under high salinity. Water transport constrains imposed by whole shoot and leaf blade at high salinity were balanced by stomatal regulation of water loss, which possibly maintain stem water potentials above embolisms levels.     

Photoinhibition as a control on photosynthesis and production of Sphagnum mosses

The effect of high light intensity on photosynthesis and growth of Sphagnum moss species from Alaskan arctic tundra was studied under field and laboratory conditions. Field experiments consisted of experimental shading of mosses at sites normally exposed to full ambient irradiance, and removal of the vascular plant canopy from above mosses in tundra water track habitats. Moss growth was then monitored in the experimental plots and in adjacent control areas for 50 days from late June to early August 1988. In shaded plots total moss growth was 2–3 times higher than that measured in control plots, while significant reductions in moss growth were found in canopy removal plots. The possibility that photoinhibition of photosynthesis might occur under high-light conditions and affect growth was studied under controlled laboratory conditions with mosses collected from the arctic study site, as well as from a temperate location in the Sierra Nevada, California. After 2 days of high-light treatment (800 mol photons m^–2 s^–1) in a controlled environmental chamber, moss photosynthetic capacity was significantly lowered in both arctic and temperate samples, and did not recover during the 14-day experimental period. The observed decrease in photosynthetic capacity was correlated (r ²=0.735, P<0.001) with a decrease in the ratio of variable to maximum chlorophyll fluorescence (F _v/F _m) in arctic and temperate mosses. This relationship indicates photoinhibition of photosynthesis in both arctic and temperate mosses at even moderately high light intensities. It is suggested that susceptibility to photoinhibition and failure to photoacclimate to higher light intensities in Sphagnum spp. may be related to low tissue nitrogen levels in these exclusively ombrotrophic plants. Photoinhibition of photosynthesis leading to lowered annual carbon gain in Sphagnum mosses may be an important factor affecting CO₂ flux at the ecosystem level, given the abundance of these plants in Alaskan tussock tundra.     

Seasonal vartadoosln the nutrtent content of carpets of Acrocladium cuspidatum (Hedw.)Lindb.

Abstract

1. Seasonal variations in the percentaae of potassium, sodium, calcium and phosphorus in the current season's growth segment of Acrocladium cuspidatum at three different habitats are presented.

2. The results show that the nutrient content of the moss carpet varies with time of year and also between habitats. For this reason comparisons between the nutrient content of moss carpets from different habitats are unlikely to be valid unless made at the same time of year or during comparable stages in the annual growth cycle of the species concerned.

3. In conditions where the moss becomes inundated by natural waters having a high concentration of calcium ions, the moss accumulates calcium to abnormally high levels.

4. Comparison of the nutrient content of the moss on open downland and under scrub shows a greater accumulation of potassium in the moss carpet of the scrub habitat. The monthly differences between the potassium contents from the two sites are positively correlated with the monthly rainfall as are also the actual percentages of potassium present in the moss carpet beneath the scrub.     

Environmental factors influencing decomposition rates in two Antarctic moss communities

Summary Rates of disappearance of dead material of Polytrichum alpestre and Chorisodontium aciphyllum from a moss turf community and of Drepanocladus uncinatus, Calliergon sarmentosum and Cephaloziella varians from a moss carpet community, measured using litter bags over 2 years, were 1.5% year^-1. Decomposition potential, estimated using loss in tensile strength of cotton strips inserted into the different bryophytes on the two sites, was also low. Ranking the five plant species in order of decomposition potential, from highest to lowest, gave D. uncinatus, C. aciphyllum, C. sarmentosum, P. alpestre and C. varians. The time taken for the tensile strength of the cotton strips at depths of 1–3 and 4–6 cm beneath the surface to decline by 50% varied from 1–2 years under the first two species to 3–4 years beneath the last two species. The main causes of these slow rates were low temperatures, short active season and low pH. Differences in decomposition between species, sites and with depth were related to temperature, nutrient status, water content and anaerobic conditions. Variation in anaerobic conditions beneath D. uncinatus, C. sarmentosum and C. varians in the moss carpet resulted in wide variation of decomposition rate beneath these species and with depth beneath C. varians. The peat in the moss turf was aerobic and experienced higher temperatures, but the average decomposition rate was no higher than in the moss carpet, because the peat was of a poorer quality and had a lower pH.     

Macrophytes of the River Wear: 1966, 1976, 1986

Summary

A survey was made during 1986 of the longitudinal distribution of macrophytes in the River Wear (N-E. England) in order to make comparisons with surveys in 1966 and 1976. The presence or absence of species recognizable with the naked eye was recorded in 1986 for 74 of the possible 211 0.5-km lengths of river. In contrast to the marked differences between 1966 and 1976, changes since 1976 were minor; many were probably merely a reflection of recent flow events. The most clear-cut difference was the loss of most of the Elodea canadensis population. The only other change in records which may reflect a longer-term environmental change was a marked increase in the moss Amblystegium riparium, a species characteristically associated with organic pollution.     

Germination and seed water status of four grasses on moss-dominated biological soil crusts from arid lands

Biological soil crusts dominated by drought-tolerant mosses are commonly found through arid and semiarid steppe communities of the northern Great Basin of North America. We conducted growth chamber experiments to investigate the effects of these crusts on the germination of four grasses: Festuca idahoensis, Festuca ovina, Elymus wawawaiensis and Bromus tectorum. For each of these species, we recorded germination time courses on bare soil and two types of biological soil crusts; one composed predominantly of the tall moss Tortula ruralis and the other dominated by the short moss Bryum argenteum. On the short-moss crust, the final germination percentage was about half of that on bare soil. Also, the mean germination time was 4 days longer on short-mosses than on bare soil. In contrast to the short-moss crust, the tall-moss crust did not reduce the final germination percentage but increased the mean germination time. Similar results were observed in the four grasses studied. To investigate the mechanism by which moss crusts affected germination, we analyzed the water status of seeds on bare soil and moss crusts. Six days after seeding, the water content of seeds on bare soil was approximately twice that of seeds on tall- or short-moss crust. Analysis of the time course of changes in seed weight and water potential in Bromus tectorum revealed that overtime seeds on tall mosses reached higher water content than those on short mosses. The increase in the water content of seeds on tall mosses occurred as the seeds gradually fell through the moss canopy. Taken together, our results indicate that biological soil crusts with distinct structural characteristics can have different effects on seed germination. Furthermore, this study revealed that a biological soil crust dominated by short mosses had a negative effect on seed water status and significantly reduced seed germination.     

Responses of CO2 Exchange and Primary Production of the Ecosystem Components to Environmental Changes in a Mountain Peatland

The complexity of natural ecological systems presents challenges for predicting the impact of global environmental changes on ecosystem structure and function. Grouping of plants into functional types, that is, groups of species sharing traits that govern their mechanisms of response to environmental perturbations, reduce the complexity of species diversity to a few key plant types for better understanding of ecosystem responses. Chambers were used to measure CO₂ exchange in grass and moss growing together in a mountain peatland in southern Germany to assess variations in their response to environmental changes and how they influence ecosystem CO₂ exchange. Parameter fits and comparison for net ecosystem exchange (NEE) in two ecosystem components were conducted using an empirical hyperbolic light response model. Annual green biomass production was 320 and 210 g dwt m ⁻², whereas mean maximum NEE was –10.0 and –5.0 μmol m ⁻² s ⁻¹ for grass and moss, respectively. Grass exhibited higher light use efficiency (α) and maximum gross primary production [(β+γ)₂₀₀₀]. Leaf area index explained 93% of light use and 83% of overall production by the grass. Peat temperature at 10-cm depth explained more than 80% of the fluctuations in ecosystem respiration (R _eco). Compared to grass, moss NEE was more sensitive to ground water level (GWL) draw-down and hence could be more vulnerable to changes in precipitation that result in GWL decline and may be potentially replaced by grass and other vegetation that are less sensitive. Author’s Contribution  Werner Borken conceived the study. Ai Nishiwaki, Margerete Wartinger, G. Lischeid and Zaman Hussain conducted measurements. Jan Muhr helped with the methodologies and result discussion. Dennis O. Otieno designed and conducted measurements and wrote the paper.