Leaf growth,senescence and decomposition of Juncus maritimus Lam. in a coastal Mediterranean marsh

In this study, the growth, senescence, leaf loss and nutrient dynamics of Juncus maritimus were followed to examine litter decay in a Mediterranean coastal marsh. Decomposition was studied in dead leaves still attached to the plant and in leaves placed in litterbags (detached leaves/litter) on the sediment surface. The dynamics of fungi, meiofauna and epiphytes associated with detached litter were also followed. No significant differences were observed between decay rates in dead leaves attached to plants (0.0017 day⁻¹) and detached leaves (0.0015 day⁻¹) in litter bags. The percentage of ash-free dry weight lost was inversely proportional to the C:N and C:P ratios in plant detritus during decay, indicating N and P limitation for the decomposer community inhabiting decaying J. maritimus litter and uptake of these nutrients from the environment. Water availability and high temperatures on the sediment surface increased the density of meiofauna and epiphyton and decreased fungal biomass during the first 20 days of the experiment. The density of ciliates and nematodes in decomposing litter was inversely related to the C:N ratio and directly related to the percentage of AFDW lost. On the basis of these observations, it was concluded that meiofauna are primary colonizers of J. maritimus leaf litter.     

Photoperiod buffers responses to salt and temperature during germination of two coastal salt marsh colonizers Juncus acutus and Juncus maritimus

Juncus acutus and J. maritimus are two colonizers of coastal marsh rangeland with worldwide distribution. We tested the effect of salt, temperature, and photoperiod on the germination capacity of seeds of the two Juncus from the Rhône delta (south of France). We measured the first day of germination, the mean time, speed, and rate of germination on seeds subjected to five salinity levels, three Dark–Light temperatures and two D-L photoperiods (12–12 and 10–14). The 10–14 D-L photoperiod, although it corresponds to one of the two main germination periods of J. acutus and J. maritimus in the northern Mediterranean, has never been previously used to study their germination capacity. Analyses showed significant effects of salinity and temperature on the germination parameters tested, and for these two factors we found results comparable to those of previous studies. Salinity slowed down and reduced the germination process while spring temperatures had a positive effect. A surprising result was obtained by changing the D-L photoperiod from 12–12 to 10–14 which then largely buffered the negative effect of salt and high temperatures on germination. This capacity of J. acutus and J. maritimus to withstand salty conditions during the germination phase due to the spring photoperiod could be decisive in the ability of both species to colonize saline environments.     

Temporal dynamics of sediment bacterial communities in monospecific stands of Juncus maritimus and Spartina maritima

In the present study, we used 16S rRNA barcoded pyrosequencing to investigate to what extent monospecific stands of different salt marsh plant species (Juncus maritimus and Spartina maritima), sampling site and temporal variation affect sediment bacterial communities. We also used a bioinformatics tool, PICRUSt, to predict metagenome gene functional content. Our results showed that bacterial community composition from monospecific stands of both plant species varied temporally, but both host plant species maintained compositionally distinct communities of bacteria. Juncus sediment was characterised by higher abundances of Alphaproteobacteria, Myxococcales, Rhodospirillales, NB1–j and Ignavibacteriales, while Spartina sediment was characterised by higher abundances of Anaerolineae, Synechococcophycidae, Desulfobacterales, SHA–20 and Rhodobacterales. The differences in composition and higher taxon abundance between the sediment bacterial communities of stands of both plant species may be expected to affect overall metabolic diversity. In line with this expectation, there were also differences in the predicted enrichment of selected metabolic pathways. In particular, bacterial communities of Juncus sediment were predicted to be enriched for pathways related to the degradation of various (xenobiotic) compounds. Bacterial communities of Spartina sediment in turn were predicted to be enriched for pathways related to the biosynthesis of various bioactive compounds. Our study highlights the differences in composition and predicted functions of sediment‐associated bacterial communities from two different salt marsh plant species. Loss of salt marsh habitat may thus be expected to both adversely affect microbial diversity and ecosystem functioning and have consequences for environmental processes such as nutrient cycling and pollutant remediation.     

Growth patterns of Juncus gerardi clonal populations in a coastal habitat

Juncus gerardi populations demonstrated a logistic growth curve during the colonization stage. Shoot production by vegetative multiplication was virtually continuous from December to June. Experiments suggested that the stabilisation stage of the demographic curve reflected water deficit. Taller, fertile, winter and early spring cohorts could be distinguished from shorter, infertile end of spring and beginning of summer cohorts. Shoot emergence began in March and terminated at the end of June, when water becomes a limiting factor due to a period of water shortage, typical of the thermo-atlantic climate. Spatial extension of populations was due to rhizome growth, which ceased during flowering.Flowering in May temporarily checked growth in shoot height of all emerged cohorts. No cost of reproduction was demonstrated concerning the rate of appearance of new shoots.Although fertile shoots were taller than vegetative shoots, their growth rates were significantly lower from April onwards. The tallest fertile shoots produced the most capsules.Energy allocation to seed production is the only possible means for long-term establishment of new genotypes, and vegetative multiplication appears as the principal source of recruitment of new modules in Juncus gerardi.Resource allocation patterns in this clonal species are discussed in relation to the ecological background in the concerned marshlands and with theoretical proposals derived from models of spatial colonization strategies in clonal plants.Nomenclature: follows Flora Europaea (Tutin et al., 1964ndash;1980).     

Antimicrobial activity of juncusol, a novel 9-10-dihydrophenanthrene from the marsh plant Juncus roemerianus

The 9,10-dihydrophenanthrene phenolic compound juncusol, from the marsh plant $\text{Juncus}$$\text{roemerianus}$, has been shown to be inhibitory to four species of naturally occurring $\text{Bacillus}$ and to two ATCC species $\text{Bacillus}$$\text{subtilis}$ and $\text{Staphylococcus}$$\text{aureus}$. Juncusol may regulate populations of bacillus bacteria in the marsh and has potential as an antimicrobial agent particularly to gram positive microorganisms.     

Leaf senescence in a non-yellowing mutant of Festuca pratensis Huds.

A study was made of linolenic acid-dependent oxidative chlorophyll bleaching (CHLOX) by thylakoid membranes from senescing leaf tissue of a normal cultivar (cv. Rossa) and a non-yellowing mutant genotype (Bf 993) of Festuca pratensis Huds. To overcome the problem of variation in levels of endogenous chlorophyll substrate in membranes from different sources, light-harvesting complex (LHC) was used to supplement thylakoid pigment. It was shown that CHLOX is associated with both Photosystem I and LHC-rich thylakoid subfractions but that purified LHC has negligible associated CHLOX activity and stimulates the rate of bleaching by isolated entire chloroplast membranes. Non-senescent tissue of Bf 993 and Rossa had essentially identical thylakoid CHLOX levels, which subsequently declined during senescence in darkness. The half-life of CHLOX from the mutant was three times greater than that of the normal genotype. In both cultivars, the amount of CHLOX assayed in thylakoids isolated at different times during senescence was more than adequate to support the corresponding in-vivo rate of pigment degradation as calculated from the half-life for chlorophyll. It was concluded that the non-yellowing mutation is not expressed through a lack of CHLOX activity. The role of linolenic acid metabolism in the regulation of thylakoid structure and function during senescence, and as a likely site of the non-yellowing lesion, are discussed.Abbreviations CHLOX linolenic acid-dependent oxidative chlorophyll bleaching activity - CHLPX chlorophyll peroxidase - CPI chlorophyll-protein complex I - LHC light-harvesting complex - LNA linolenic acid - PSI photosystem I - PSII photosystem II - S relative senescence rate - t _1/2 lialf time for degradation     

Ecology of eutrophic waterbodies in a coastal grazing marsh

The ecology of a mainly interconnected, complex system of smallwater-bodies on a coastal grazing marsh in the Greater Thames estuary,north-east of London, England, is described. Sixteen sites were sampled on aregular basis over a two-year period while 37 sites were sampled once in Mayto develop a site classification. Most sites were brackish and alkaline but,following re-wetting after drying out, the pH in some water-bodies decreasedsharply (minimum pH 2.7). All sites had high concentrations of phosphorusbut there were large differences; some, with good growths of submergedaquatic macrophytes, having lower phosphorus than other sites, which weredevoid of macrophytes. Internal loading with phosphorus was important.Nitrate levels were low. Waters were mostly turbid, Secchi depths sometimes falling as low as 2cm, while chlorophyll a was very high, indicating extreme hypertrophy.Diatoms and euglenoids were characteristic members of the phytoplankton;cyanobacterial blooms did not occur. The zooplankton was dominated bycopepods, while Cladocera were much less numerous, largely due to predationby shrimps in waterbodies without macrophytes. The classification of siteswas influenced primarily by water depth, some waterbodies drying out, whilephosphorus and turbidity were also important in differentiating sites. Animal guano is the likely cause of the hypertrophic state but livestockgrazing is an essential management tool for grazing marshes. The dredging ofwaterbodies (with removal of sediments from the site) and the instigation ofa flushing regime, requiring a new system of sluices, should reduce nutrientloading and assist the establishment of submerged macrophytes, providingrefuges for cladocerans from predation.     

Above-ground biomass and species richness in a Mediterranean salt marsh

Abstract. Species composition, above-ground biomass and diversity of herbaceous communities were recorded in 50 sites on the upper salt marsh of the Guadalquivir delta (SW Spain). Through Canonical Correspondence Analysis relationships were shown between plant-species characteristics abundance and environmental variables. Seven community types were distinguished and related to hydro logical regime and physical and chemical soil parameters. The higher species richness was associated with moderate values of above-ground biomass (Biomass at maximum diversity, BMD = 302 g/m²). Salinity was negatively correlated with species richness. There was no significant correlation with total above-ground biomass. Water regime and soil nutrient (Ca, Mg) content were significant predictors of the community above-ground biomass.     

Leaf senescence in rice plants: cloning and characterization of senescence up-regulated genes.

To identify senescence-associated genes (SAGs) in rice leaves, senescence was induced by transferring rice seedlings into darkness. Senescence up-regulated cDNAs were obtained by PCR-based subtractive hybridization. Among 14 SAG clones characterized, 11 were found to be associated with both dark-induced and natural leaf senescence. Three clones were associated only with dark-induced leaf senescence. The possible physiological roles of these SAGs during rice leaf senescence are discussed.     

Productivity and nutrient content of Juncus kraussii in an estuarine marsh in south-western Australia

Juncus kraussii, which is the dominant emergent macrophyte in the marshes of the Blackwood River Estuary, has an estimated net above-ground production there of 0.3 to 1.3 kg dry weight m^-2 yr^-1. A high standing crop of live culms is present throughout the year and dead material generally exceeds live. New culms are produced throughout the year, but especially during the warm season. Growth and senescence of culms also occurs throughout the year although there is evidence for increased growth in the warm season. Concentrations of nitrogen and phosphorus are given on a per g dry weight and per m² basis. Some 60% of the nitrogen and 50% of the phosphorus remain in the dead culm material, the remainder being retranslocated. Sodium and, especially, potassium are readily retranslocated during senescence, but calcium and magnesium are much less mobile.     

Decomposition and nutrient release in halophytes of a Mediterranean salt marsh

This study dealt with the decomposition and nutrient release from the halophytes Atriplex portulacoides, Arthrocnemum macrostachyum, Limoniastrum monopetalum, and Spartina densiflora, the dominant species in the Castro Marim salt marsh, Portugal. Environmental effects on decomposition were also assessed. The study was carried out for one year using the in situ litterbag technique. S. densiflora showed a lower decomposition rate (k = 0.003 day⁻¹) than the other study species (k = 0.005-0.009). Study species showed similar decomposition patterns, that is, the weight loss mostly occurred during the autumn-winter period (study beginning in November). This indicates that temperature in this period did not hamper the decomposition process. The decomposition rate was positively affected by the initial N concentration (r² = 0.87, P < 0.05) and negatively by the C:N ratio (r² = 0.86, P < 0.05) in decomposing materials. At the end of the study, S. densiflora and L. monopetalum, the species with lower initial N concentrations, retained much higher proportion of initial N (89-109%) than the others (5-14%). Also, S. densiflora with the lowest P concentration retained higher proportion of initial P (48%) than the others (5-20%). Release of K and Mg were also slower from S. densiflora and was associated with their initial low concentration in this species. The lowest Mn release was observed from A. macrostachyum and also in relation to the lowest initial concentration. Our study supports the hypothesis that decomposition patterns of marsh species are mostly associated with differences regarding their morphology and chemical composition. Given the higher resistance of S. densiflora to decomposition, its progressive spreading may result in accumulation of organic detritus overtime in invaded salt marshes.     

Phylogenetic diversity of Archaea in sediment samples from a coastal salt marsh.

The Archaea present in salt marsh sediment samples from a tidal creek and from an adjacent area of vegetative marshland, both of which showed active methanogenesis and sulfate reduction, were sampled by using 16S rRNA gene libraries created with Archaea-specific primers. None of the sequences were the same as reference sequences from cultured taxa, although some were closely related to sequences from methanogens previously isolated from marine sediments. A wide range of Euryarchaeota sequences were recovered, but no sequences from Methanococcus, Methanobacterium, or the Crenarchaeota were recovered. Clusters of closely related sequences were common and generally contained sequences from both sites, suggesting that some related organisms were present in both samples. Recovery of sequences closely related to those of methanogens such as Methanococcoides and Methanolobus, which can use substrates other than hydrogen, provides support for published hypotheses that such methanogens are probably important in sulfate-rich sediments and identifies some likely candidates. Sequences closely related to those of methanogens such as Methanoculleus and Methanogenium, which are capable of using hydrogen, were also discovered, in agreement with previous inhibitor and process measurements suggesting that these taxa are present at low levels of activity. More surprisingly, we recovered a variety of sequences closely related to those from different halophilic Archaea and a cluster of divergent sequences specifically related to the marine group II archaeal sequences recently shown by PCR and probing to have a cosmopolitan distribution in marine samples.     

Free amino acid, ammonium and nitrate concentrations in soil solutions of a grazed coastal marsh in relation to plant growth

Soluble free amino acids, ammonium and nitrate ions as sources of nitrogen for plant growth were measured in soils of a coastal marsh grazed by snow geese in Manitoba, Canada. Amounts of nitrogen, primarily ammonium ions, increased in the latter half of the growing season and over winter, but fell to low values early in the growing season. Free amino acid concentrations relative to ammonium concentrations were highest during the period of rapid plant growth in early summer, especially in soils in the intertidal zone, where the median ratio of amino acid nitrogen to ammonium nitrogen was 0·36 and amino acid concentrations exceeded those of ammonium ions in 24% of samples. Amino acid profiles, which were dominated by alanine, proline and glutamic acid, were similar to goose faecal profiles. In a continuous flow hydroponic experiment conducted in the field, growth of the salt‐marsh grass, Puccinellia phryganodes, on glycine was similar to growth on ammonium ions at an equivalent concentration of nitrogen. When supplies of soil inorganic nitrogen are low, amino acids represent a potentially important source of nitrogen for the re‐growth of plants grazed by geese and amino acid uptake may be as high as 57% that of ammonium ions.     

Shrimps and eutrophication in the waterbodies of a coastal grazing marsh

The influence of shrimps on turbidity was examined in laboratory and field experiments. Palaemonetes varians in the laboratory stimulated populations of both rotifers and Anabaena, which was the main cause of increased turbidity. In field experiments, P. varians increased and Neomysis integer decreased turbidity. In feeding trials P. varians took cladocerans and culicids in preference to copepods. It is considered that shrimps are likely to be important predators of cladocerans in shallow, coastal waterbodies and result in phytoplankton blooms, with reduced water transparency. This revised version was published online in July 2006 with corrections to the Cover Date.     

Leaf senescence in a non-yellowing mutant of Festuca pratensis

The lipid compositions of leaves from Festuca pratensis cv. Rossa (yellowing) were compared with those from a non-yellowing mutant, Bf 993. The leaves of Bf 993 contained a higher level of acyl lipids on both a fresh-weight and a dry-weight basis. Diacylgalactosylglycerol, diacylgalabiosylglycerol and phosphatidylinositol were relatively enriched in the Bf 993 mutant while phosphatidylcholine was relatively reduced. There were no differences in the fatty-acid compositions of individual lipids between the two varieties. During senescence, the lipids of cv. Rossa were progressively degraded over an 8-d period. In contrast little lipid degradation was observed in the Bf 993 mutant during the first 4 d. The results support the hypothesis that the slower senescence changes of the Bf 993 mutant may be due, in part, to an altered membrane lipid composition.II=Thomas (1982b)     

Leaf senescence in a non-yellowing mutant of Festuca pratensis

Soluble and thylakoid membrane polypeptides from senescing leaf tissue of Rossa, a normal yellowing Festuca pratensis genotype, were fractionated by sodium dodecyl sulphate polyacrylamide gel electrophoresis and compared with those of the non-yellowing mutant Bf 993. Subunits of ribulose-1,5-bisphosphate carboxylase were the major soluble polypeptides and declined to low levels in senescing leaves of both genotypes. The major thylakoid polypeptides were those associated with the chlorophyllprotein complexes CPI and CPII. The levels of all thylakoid polypeptide species fell during senescence of Rossa leaf tissue but Bf993 lamellae retained CPI, CPII and a number of other hydrophobic low molecular weight polypeptides. The increasing hydrophobicity and decreasing protein complement of Bf 993 thylakoids were reflected in a fall in membrane density from 1.16 to 1.13 g cm^-3 over 8 d of senescence and a decline in the extractability of chlorophyll-containing membranes in the same period. In Bf993 the molar ratio of chlorophyll to hydrophobic membrane protein increased from 92 at day 0 to 296 at day 8. In the same time the ratio for Rossa increased from 88 to 722 and 8 d-senesced Rossa tissue yielded less than 2% of the solvent-soluble protein it contained at day 0 as compared with 24% for the protein of Bf993. These results are discussed in relation to the nature of the non-yellowing lesion.Abbreviations RuBPC ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) - EDTA ethylenediaminetetraacetate - SDS sodium dodecyl sulphate - CP chlorophyll-protein complex