Climatic modifiers of the response to nitrogen deposition in peat-forming Sphagnum mosses: a meta-analysis

Peatlands in the northern hemisphere have accumulated more atmospheric carbon (C) during the Holocene than any other terrestrial ecosystem, making peatlands long-term C sinks of global importance. Projected increases in nitrogen (N) deposition and temperature make future accumulation rates uncertain. Here, we assessed the impact of N deposition on peatland C sequestration potential by investigating the effects of experimental N addition on Sphagnum moss. We employed meta-regressions to the results of 107 field experiments, accounting for sampling dependence in the data. We found that high N loading (comprising N application rate, experiment duration, background N deposition) depressed Sphagnum production relative to untreated controls. The interactive effects of presence of competitive vascular plants and high tissue N concentrations indicated intensified biotic interactions and altered nutrient stochiometry as mechanisms underlying the detrimental N effects. Importantly, a higher summer temperature (mean for July) and increased annual precipitation intensified the negative effects of N. The temperature effect was comparable to an experimental application of almost 4 g?N m(-2) yr(-1) for each 1°C increase. Our results indicate that current rates of N deposition in a warmer environment will strongly inhibit C sequestration by Sphagnum-dominated vegetation.     

Growth of two peat-forming mosses in subarctic mires: species interactions and effects of simulated climate change

In patches of co‐occurring species in natural plant communities, there is a finely poised balance between species in the ways in which they respond to prevailing moisture and temperature regimes. However, environmental change scenarios, in which temperature, moisture and ultraviolet‐B radiation are suggested to increase, may favour one of the species. The imbalance is likely to occur at the levels of interactions between patches of the different species and at the shoot level when neighbouring shoots belong to different species. We increased temperature and UV‐B in a two‐way factorial experiment and increased water supply independently in two subarctic mire communities dominated by the mosses Sphagnum fuscum and Dicranum elongatum. The effects of simulated increase in UV‐B were studied using two separate radiation systems, i.e. a “square wave” system and a “modulated” system.
When precipitation was enhanced, both species showed an increase in growth but this was not sustained beyond 5 mm per day. S. fuscum showed a 50% greater response to enhanced precipitation than did D. elongatum, as would be expected from their habitat preferences. Under ambient temperature, S. fuscum grew 67% faster than D. elongatum and this relative difference in response was maintained after one year under a temperature enhancement. The response by species over the winter period was moderated by their neighbours. S. fuscum growth was enhanced when it grew next to D. elongatum whereas D. elongatum grew better with neighbours of its own species. Increased temperature and UV‐B radiation did not affect the interaction between the species.
Although a balance was maintained between the two species over the short duration of the experiment, potential was shown for an imbalance to occur over longer periods and particularly if winter warming and precipitation are greater than those in summer.
During the peak growing season 20% increased UV‐B over ambient had a negative effect on S. fuscum under increased temperature but there were no overall seasonal effects on either species, irrespective of method of UV supplementation.     

Response of photosynthetic carbon gain to ecosystem retrogression of vascular plants and mosses in the boreal forest

In the long-term absence of rejuvenating disturbances, forest succession frequently proceeds from a maximal biomass phase to a retrogressive phase characterized by reduced nutrient availability [notably nitrogen (N) and phosphorus (P)] and net primary productivity. Few studies have considered how retrogression induces changes in ecophysiological responses associated with photosynthetic carbon (C) gain, and only for trees. We tested the hypothesis that retrogression would negatively impact photosynthetic C gain of four contrasting species, and that this impact would be greater for vascular plants (i.e., trees and shrubs) than for non-vascular plants (i.e., mosses). We used a 5,000-year-old chronosequence of forested islands in Sweden, where retrogression occurs in the long-term absence of lightning-ignited wildfires. Despite fundamental differences in plant form and ecological niche among species, vascular plants and mosses showed similar ecophysiological responses to retrogression. The most common effects of retrogression were reductions in photosynthesis and respiration per unit foliar N, increases in foliar N, δ(13)C and δ(15)N, and decreases in specific leaf areas. In contrast, photosynthesis per unit mass or area generally did not change along the chronosequence, but did vary many-fold between vascular plants and mosses. The consistent increases in foliar N without corresponding increases in mass- or area-based photosynthesis suggest that other factor(s), such as P co-limitation, light conditions or water availability, may co-regulate C gain in retrogressive boreal forests. Against our predictions, traits of mosses associated with C and N were generally highly responsive to retrogression, which has implications for how mosses influence ecosystem processes in boreal forests.     

The relation of root systems to shoot systems in vascular plants

The roots and shoots of vascular plants may be positionally and developmentally related in various ways. However, botanical teaching and research are strongly influenced by the paradigmatic annual dicotyledon, whose bipolar embryo develops into a plant with root and shoot meeting only at the hypocotyl. In 1930 Goebel criticized this example as a general model for plants, proposing instead the opposed concepts “allorhizy” (referring to plants whose root and shoot are related as above) and “homorhizy”(referring to plants without a bipolar embryo, all of whose roots are shoot-borne, e.g., pteridophytes). Goebel’s approach permeates the extensive German morphological literature, but has been virtually ignored in English-language literature. The allorhizy/homorhizy dichotomy has proved heuristic. However, it suggests a correlation between embryo type and mature morphology that does not always hold. Furthermore, it does not take into account the root-borne shoots typical of many plant species. Finally, Goebel’s presentation of the terms (which he does not explicitly define) creates ambiguity as to whether they designate structural concepts or the attributes of evolutionary groups. The alternative proposed here is a structural analysis of the possible topological relationships among root and shoot systems. Each structural class is then considered with regard to embryo types, potential for clonal growth and other ecological correlates, and phylogenetic distribution. This approach provides both a test of Goebel’s concepts and a basis for further comparative study of wholeplant form.     

Modelling the development and arrangement of the primary vascular structure in plants

Background and Aims

The process of vascular development in plants results in the formation of a specific array of bundles that run throughout the plant in a characteristic spatial arrangement. Although much is known about the genes involved in the specification of procambium, phloem and xylem, the dynamic processes and interactions that define the development of the radial arrangement of such tissues remain elusive.

Methods

This study presents a spatially explicit reaction–diffusion model defining a set of logical and functional rules to simulate the differentiation of procambium, phloem and xylem and their spatial patterns, starting from a homogeneous group of undifferentiated cells.

Key Results

Simulation results showed that the model is capable of reproducing most vascular patterns observed in plants, from primitive and simple structures made up of a single strand of vascular bundles (protostele), to more complex and evolved structures, with separated vascular bundles arranged in an ordered pattern within the plant section (e.g. eustele).

Conclusions

The results presented demonstrate, as a proof of concept, that a common genetic–molecular machinery can be the basis of different spatial patterns of plant vascular development. Moreover, the model has the potential to become a useful tool to test different hypotheses of genetic and molecular interactions involved in the specification of vascular tissues.     

Studying plant development in mosses: the transgenic route

The current status of transgenic studies in mosses is reviewed with particular attention being given to the mosses Physcomitrella patens and Ceratodon purpureus. This paper reviews the advantages of using mosses as models for higher plants in the study of plant development, and includes developmental processes, already partially characterized at the genetic level by mutant analysis, for which transgenic studies may be applicable. The P. patens transformation process is being studied in this laboratory and details are given for a class of transformants which contain extrachromosomal plasmid DNA. Publications which present the nucleic acid and/or protein sequence for nuclear, chloroplast and mitochondrial genes are reviewed. Areas of research in which transgenic studies promise to complement existing cell biological and physiological approaches are discussed. These include the measurement of calcium levels in mutant and wild-type transformants expressing the apoaequorin gene and a role for phytochrome gene expression in the establishment of polarity.     

Growth and reproduction of Antarctic vascular plants in response to warming and UV radiation reductions in the field

Along the west coast of the Antarctic Peninsula springtime ozone depletion events can lead to a two-fold increase in biologically effective UV-B radiation (UV-B_BE) and summer air temperatures have risen ≈1.5°C during the past 50 years. We manipulated levels of UV radiation and temperature around Colobanthus quitensis (a cushion-forming plant, Caryophyllaceae) and Deschampsia antarctica (a tussock grass) along the Peninsula near Palmer Station for two field seasons. Ambient levels of UV were manipulated by placing filters that either transmitted UV (filter control), absorbed UV-B (reducing diurnal levels of UV-B_BE by about 82%), or absorbed both UV-B and UV-A (reducing UV-B_BE and UV-A_BE by about 88 and 78%, respectively) on frames over naturally growing plants from November to March. Half the filters of each material completely surrounded the frames and raised diurnal and diel air temperatures around plants by an average of 2.3°C and 1.3°C, respectively. Reducing UV or warming had no effect on leaf concentrations of soluble UV-B absorbing compounds, UV-B absorbing surface waxes or chlorophylls. Warming had few effects on growth of either species over the first season. However, over the second field season warming improved growth of C. quitensis, leading to a 50% increase in leaf production (P < 0.10), a 26% increase in shoot production, and a 6% increase in foliar cover. In contrast, warming reduced growth of D. antarctica, leading to a 20% decline in leaf length, a 17% decline in leaf production (P < 0.10), and a 5% decline in foliar cover. Warming improved sexual reproduction in both species, primarily through faster development of reproductive structures and greater production of heavier seeds. Over the second field season, the percentage of reproductive structures that had reached the most developed (seed) stage in C. quitensis and D. antarctica was 20% and 15% higher, respectively, under warming. Capsules of C. quitensis produced 45% more seeds under warming and these seeds were 11% heavier. Growth of D. antarctica was improved when UV was reduced and these effects appeared to be cumulative over field seasons. Over the second season, tillers produced 55% more leaves and these leaves were 32% longer when UV-B was reduced. Tillers produced 137% more leaves that were 67% longer when both UV-B and UV-A were reduced. The effects of UV reduction were not as pronounced on C. quitensis, although over the second season cushions tended to be 17% larger and produce 21% more branches when UV-B was reduced, and tended to be 27% larger and produce 38% more branches when both UV-B and UV-A were reduced (P < 0.10). Few interactions were found between UV reduction and warming, although in the absence of warming, reducing UV led to slower development of reproductive structures in both species. The effects of warming and UV reduction were species specific and were often cumulative over the two field seasons, emphasizing the importance of long-term field manipulations in predicting the impacts of climate change. Received: 4 August 1998 / Accepted: 1 December 1998     

Genome-edited plants in the field

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Interactions of polyamines and nitrogen nutrition in plants

Biogenic amines occupy an important position among the many nitrogenous plant compounds. Polyamines are part of the overall metabolism of nitrogenous compounds, yet they do not seem to function in the 'normal' nitrogen nutrition. Rather, these widespread polycations (e. g. putrescine, spermidine and spermine) are involved in the regulation of growth and stress, probably by binding to negatively charged macromolecules. In addition, some diamines and polyamines are metabolized to yield 'secondary 'metabolites such as nicotine and other alkaloids. Previous studies have indicated that the ratio of nitrate to ammonium nutrition affects polyamine biosynthesis and content in intact plants. Thus, an increase in putrescine accumulation was found under conditions of excess ammonium ions, relative to nitrate. Modifications of nitrogen sources in the culture medium of tobacco cell suspensions (depletion of ammonium nitrate, or potassium nitrate, or both) resulted in marked changes in the content of cellular free polyamines. Considerable changes in the content of specific polyamines were also found with exposure to specific inhibitors of polyamine biosynthesis (difluoromethyl ornithine, difluoromethyl arginine, cyclohexylamine, methylglyoxal-bis-guanylhydrazone). However, a combination of nitrogen depletion of the medium and some inhibitors resulted in a very marked over-production of spermidine and spermine. The significance of these findings is discussed in relation to the assumption that polyamines act as a metabolic buffer, and maintain cellular pH under conditions where ammonium assimilation produces an excess of protons.     

Orestovia and the origin of vascular plants

The enigmatic Orestovia from the Lower Devonian of Siberia has a vascular cylinder of spiral and annular tracheids, external and internal cuticles in the cortex, and stomata. Abundant spores are occasionally preserved between the cortex cuticles. The 'reticulate structures' are thyriothecia of a hemisphaeriacean fungus. Spores of several types are found sticking to the cuticle. Orestovia can be conceived as a transitional form between the alga-like Protosalvinia and the sporangiate vascular plants. The origin of land plants is discussed.     

Interactions of chromium with microorganisms and plants

Chromium is a highly toxic non-essential metal for microorganisms and plants. Due to its widespread industrial use, chromium (Cr) has become a serious pollutant in diverse environmental settings. The hexavalent form of the metal, Cr(VI), is considered a more toxic species than the relatively innocuous and less mobile Cr(III) form. The presence of Cr in the environment has selected microbial and plant variants able to tolerate high levels of Cr compounds. The diverse Cr-resistance mechanisms displayed by microorganisms, and probably by plants, include biosorption, diminished accumulation, precipitation, reduction of Cr(VI) to Cr(III), and chromate efflux. Some of these systems have been proposed as potential biotechnological tools for the bioremediation of Cr pollution. In this review we summarize the interactions of bacteria, algae, fungi and plants with Cr and its compounds.     

Species composition and ecologo-phytocenotic spectra of vascular plants, mosses, and lichens on gold-mining sites in the taiga zone of the lower Amur River basin, the Russian Far East

Two-hundred-three species of vascular plants, 50 species of bryophyte and 41 species of lichens have been found on dredging and hydraulic gold-mining sites in the taiga zone of the lower Amur River basin. The main features of the anthropogenic relief and vegetation have been described. Dredging and hydraulic dumps are supposed to have the biggest similarity in the species composition and set of ecologo-phytocenotic elements, while the biggest difference is observed between dredging and hydraulic dumps on the one hand and pockets and settling pits, on the other. According to the data obtained in different regions, the species diversity of vascular plants growing on dredging and hydraulic gold-mining sites is quite rich.     

Metals in some dominant vascular plants, mosses, lichens, algae, and the biological soil crust in various types of terrestrial tundra, SW Spitsbergen, Norway

Arctic environments are commonly considered to be relatively pristine because of minimal local human activity. However, these areas receive air pollution from lower latitude regions. Our goal was to determine concentrations of metals (Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, and Zn) in dominant species of vascular plants, mosses, lichens, algae, and in the biological soil crust (BSC), and topsoil (0–3 cm) from various types of tundra in the southwestern part of Spitsbergen, Norway. Results indicate that mosses are more efficient bioaccumulators of Cd, Co, Cr, Cu, Fe, Mn, and Zn than lichens. The highest levels of Co, Cr, Cu, Fe, Hg, Mn, Ni, and Pb were found in the BSC, and the moss species Racomitrium lanuginosum, Sanionia uncinata, and Straminergon stramineum from the polygonal tundra, initial cyanobacteria-moss wet tundra, snow bed cyanobacteria-moss tundra, and flow water moss tundra alimented by melting ice or snow. The observed higher concentrations of Cu and lower concentrations of Hg in mosses, lichens, and vascular plants compared with values observed 20 years earlier were apparently associated with changes in the atmospheric deposition of contaminants over Spitsbergen due to changes in the long-distance transport of anthropogenic emissions from industrialized areas. Prasiola crispa and Salix polaris may be useful bioindicators of Cd and Zn, and the BSC, R. lanuginosum, S. uncinata, and S. stramineum as bioindicators of Co, Cr, Cu, Fe, Hg, Mn, Ni, and Pb. These results may be extrapolated across other areas of Spitsbergen with similar climates.