Edaphically distinct habitats shape the crown architecture of <Emphasis Type="Italic">Lychnophora ericoides</Emphasis> Mart. (Asteraceae) on tropical mountaintops

Different architectural arrangements may represent contrasting morphological solutions to different environmental pressures. This work aims to elucidate whether the crown architecture of Lychnophora ericoides (Asteraceae) modifies in response to harsh soil conditions (nutrient poor and heavy metal rich) and how its crown architecture affects its reproduction. One hundred and sixty L. ericoides individuals were randomly sampled from eight populations, four on quartzite and four on iron canga rocky complexes in the Iron Quadrangle, southeastern Brazil. We performed soil analyses to characterize edaphic differences and used eight morphometric parameters to describe the crown architecture of the plants. We calculated the population density and reproductive potential to verify the relationship between habitat, architecture, and fitness. Canga soils were more nutrient rich than quartzite soils and plants were architecturally distinct in each habitat. Plants established on canga soils were shorter, had a thinner main branch, and a smaller leaf than those established on quartzite soils. Moreover, plants on canga soils had a larger crown diameter and a greater number of branches and inflorescences. There was no difference in population density but the reproductive potential varied among populations and habitats. The crown architecture of L. ericoides closely relates to reproductive potential and may favor the reproduction of more architectonically complex plants, regardless of habitat.     

Defence reactions in the apoplastic proteome of oilseed rape (<Emphasis Type="Italic">Brassica napus</Emphasis> var. <Emphasis Type="Italic">napus</Emphasis>) attenuate <Emphasis Type="Italic">Verticillium longisporum</Emphasis>growth but not disease symptoms

Background

Verticillium longisporum is one of the most important pathogens of Brassicaceae that remains strictly in the xylem during most stages of its development. It has been suggested that disease symptoms are associated with clogging of xylem vessels. The aim of our study was to investigate extracellular defence reactions induced by V. longisporum in the xylem sap and leaf apoplast of Brassica napus var. napus in relation to the development of disease symptoms, photosynthesis and nutrient status.

Results

V. longisporum (strain VL43) did not overcome the hypocotyl barrier until 3 weeks after infection although the plants showed massive stunting of the stem and mild leaf chlorosis. During this initial infection phase photosynthetic carbon assimilation, transpiration rate and nutrient elements in leaves were not affected in VL43-infected compared to non-infected plants. Proteome analysis of the leaf apoplast revealed 170 spots after 2-D-protein separation, of which 12 were significantly enhanced in response to VL43-infection. LS-MS/MS analysis and data base searches revealed matches of VL43-responsive proteins to an endochitinase, a peroxidase, a PR-4 protein and a β-1,3-glucanase. In xylem sap three up-regulated proteins were found of which two were identified as PR-4 and β-1,3-glucanase. Xylem sap of infected plants inhibited the growth of V. longisporum.

Conclusion

V. longisporum infection did not result in drought stress or nutrient limitations. Stunting and mild chlorosis were, therefore, not consequences of insufficient water and nutrient supply due to VL43-caused xylem obstruction. A distinct array of extracellular PR-proteins was activated that might have limited Verticillium spreading above the hypocotyl. In silico analysis suggested that ethylene was involved in up-regulating VL43-responsive proteins.

     

Trophic plasticity among spring vs. cave populations of <Emphasis Type="Italic">Gammarus minus:</Emphasis> examining functional niches using stable isotopes and C/N ratios

In some environments, species may exhibit trophic plasticity, which allows them to extend beyond their assigned functional group. For Gammarus minus, a freshwater amphipod classified as a shredder or detritivore, cave populations have been observed consuming heterotrophs as well as shredding leaves, and therefore may be exhibiting trophic plasticity. To test this possibility, we examined the C and N stable isotope and C/N ratios for cave and spring populations of G. minus. A 15-day feeding experiment using leaves and G. minus from a spring population established that the diet-tissue discrimination factor was 3.2 ‰ for δ¹⁵N. Cave G. minus were 8 ‰ higher in δ¹⁵N relative to cave leaves, indicating they did not derive nitrogen from leaves, whereas field collected spring populations were 2–3 ‰ higher than spring leaves, indicating that they did. Cave G. minus were 2.6 ‰ higher in δ¹⁵N than the cave isopod, Caecidotea holsingeri. Relative to spring populations, Organ Cave G. minus were ¹⁵N enriched by 6 ‰, suggesting they occupied a different trophic level, or incorporated an isotopically distinct N source. While stable isotopes cannot tell what the cave G. minus are eating, the isotopes certainly show that G. minus are not eating leaves and are trophically distinct form the surface populations. Differences in C/N ratios were observed, but reflect the size of the G. minus examined and not feeding group or habitat. The isotope data strongly support the hypothesis that cave populations of G. minus have become generalist or omnivorous by including animal protein in their diet.     

Variations in the Carbon and Nitrogen Isotope Composition of the Crabs <Emphasis Type="Italic">Chionoecetes opilio</Emphasis> (Fabricius, 1788) and <Emphasis Type="Italic">Hyas coarctatus</Emphasis> Leach, 1816 (Crustacea: Decapoda) from the Chukchi Sea

Some regions of the Chukchi Sea shelf are characterized by an unusually high abundance of the benthos. The carbon and nitrogen isotope composition (δ¹³C and δ¹⁵N) of two crab species, Chionoecetes opilio and Hyas coarctatus, which occupy the upper trophic levels in bottom communities in the southern regions influenced by the Anadyr Water (AW) and in the regions exposed to the influence of the Siberian Coastal Current (SCC), was analyzed in order to compare between the trophic characteristics of their benthic communities. It has been found that these two species have similar spatial trends of stable isotope signature variations. As the effect of the productive AW on bottom communities decreases, a substantial depletion of δ¹³C with a simultaneous growth of δ¹⁵N values are observed in crab tissues. The trophic niches of C. opilio and H. coarctatus, which are represented by “isotope” niches as standard ellipse areas in the δ¹³C/δ¹⁵N biplot, significantly overlap in communities of the southern part of the shelf and become completely separated in those of the regions influenced by the SCC. The separation of the isotope niches of C. opilio and H. coarctatus and the simultaneous increase in δ¹⁵N values in both species by more than 3‰ indicate partitioning of the trophic resources between them and elongation of the food chains of both these carnivorous generalists by almost one trophic level in the communities that exist under the influence of less productive waters.     

Ecophysiological responses of native invasive woody <Emphasis Type="Italic">Juniperus virginiana</Emphasis> L. to resource availability and stand characteristics in the semiarid grasslands of the Nebraska Sandhills

Vegetation in grasslands is changing at an unprecedented rate. In the Nebraska Sandhills, this shift is attributed in part to encroachment of the woody species Juniperus virginiana. We investigated changes in resource availability and their feedback on seasonal trends in photosynthetic characteristics of J. virginiana trees scattered in open grasslands vs. a dense 57-year-old stand. Dense stand exhibited lower volumetric soil water content, NH₄ ⁺, NO₃ ^–, and δ¹³C, as well as foliage δ¹³C, δ¹⁵N, and N content, compared to grasslands. Water potential was higher in trees in grasslands compared to dense stand. J. virginiana in dense stand exhibited similar trends to trees in grasslands for net photosynthetic rate (P _N), stomatal conductance, transpiration, maximum photochemical efficiency of PSII, maximum carboxylation velocity, and maximum rate of electron transport. P _N peaked early summer and declined in the fall, with trees in open grasslands lagging behind those in dense stand. Plasticity of this species may place it at a competitive advantage in the Sandhills, further altering grasslands vegetation and ecosystem processes.     

The ectomycorrhizal status of a tropical black bolete, <Emphasis Type="Italic">Phlebopus portentosus</Emphasis>, assessed using mycorrhizal synthesis and isotopic analysis

Phlebopus portentosus is one of the most popular wild edible mushrooms in Thailand and can produce sporocarps in the culture without a host plant. However, it is still unclear whether Phlebopus portentosus is a saprotrophic, parasitic, or ectomycorrhizal (ECM) fungus. In this study, Phlebopus portentosus sporocarps were collected from northern Thailand and identified based on morphological and molecular characteristics. We combined mycorrhizal synthesis and stable isotopic analysis to investigate the trophic status of this fungus. In a greenhouse experiment, ECM-like structures were observed in Pinus kesiya at 1 year after inoculation with fungal mycelium, and the association of Phlebopus portentosus and other plant species showed superficial growth over the root surface. Fungus-colonized root tips were described morphologically and colonization confirmed by molecular methods. In stable isotope measurements, the δ¹³C and δ¹⁵N of natural samples of Phlebopus portentosus differed from saprotrophic fungi. Based on the isotopic patterns of Phlebopus portentosus and its ability to form ECM-like structures in greenhouse experiments, we conclude that Phlebopus portentosus could be an ECM fungus.     

Invasive species alter ontogenetic shifts in the trophic ecology of Lake Sturgeon (<Emphasis Type="Italic">Acipenser fulvescens</Emphasis>) in the Niagara River and Lake Ontario

Species invasions can alter food web structure and change ecosystem-level functioning, but it is often unclear how these invasions may affect the life history of native species. The Lake Sturgeon (Acipenser fulvescens), a large long-lived native fish species in the Great Lakes, has increased in abundance in the lower Niagara River and nearby Lake Ontario during a period of invasive species-induced ecosystem change precipitated most recently by Dreissenid mussels (Driessena polymorpha and Driessena bugensis) and Round Goby (Neogobius melanostomus). Material taken from cross-sections of archived pectoral spines from Niagara River Lake Sturgeon captured in 1998–2000 and 2010–2012 were analyzed for stable isotopes across discrete growth zones to provide an ontogenetic assessment of diet, and diet analysis of Lake Sturgeon captured in 2014 was conducted to assess the contribution of invasive prey. Round Goby was the most important Lake Sturgeon prey item (86% by weight) in 2014, which corroborated results of δ¹⁵N and δ¹³C. Lake Sturgeon captured after the invasion of Round Goby exhibited ontogenetic changes in δ¹⁵N that differed from pre-Round Goby patterns, though this effect was weaker for δ¹³C. Values of δ¹⁵N from spine growth zones indicated non-linear increases in trophic position with age and increased rate of δ¹⁵N enrichment after the Round Goby invasion. We conclude that Round Goby establishment in western Lake Ontario changed the feeding ecology of Lake Sturgeon, which may have a positive effect on population growth for this native species.     

Early <Emphasis Type="Italic">Acacia</Emphasis> invasion in a sandy ecosystem enables shading mediated by soil,leaf nitrogen and facilitation

Australian species of the genus Acacia are amongst the most invasive trees. As nitrogen fixers, they are able to invade oligotrophic ecosystems and alter ecosystem functioning to their benefit. We aimed to answer three questions: How does early Acacia invasion influence nitrogen and light in a sandy savanna? How does early Acacia invasion impact biodiversity? Does early invasion alter ecosystem functioning towards the dominance of Acacia? We analyzed (using generalized linear mixed models and richness estimators) paired plots focused on plants of Acacia mangium (Fabaceae) and plants of Marcetia taxifolia (Melastomataceae) by taking hemispherical photos and sampling plants, leaves and soil for measurements of light, richness, leaf nitrogen, leaf δ15N, soil nitrogen and soil coarse sand. The results suggest that early Acacia invasion alters the control of soil and of leaf nitrogen and increases shading, enabling a much wider range of light variation. The δ15N results suggest that the nitrogen taken up by Acacia is transferred to neighboring plants and influences the light environment, suggesting facilitation. The enrichment of plant species observed during early Acacia invasion is consistent with the wider range of light variation, but the forecasted leaf nitrogen conditions during the established phase of Acacia invasion might cause loss of light-demanding species because of increased shading. If early Acacia invasion turns into an established phase with highly increased shading, Acacia seedlings might be favored and ecosystem functioning might change towards its dominance.     

Regional-scale patterns of δ<Superscript>13</Superscript>C and δ<Superscript>15</Superscript>N associated with multiple ecosystem functions along an aridity gradient in grassland ecosystems

Background and aims

Nitrogen (N) deficiency and drought are two key limiting factors for rice production worldwide, but the relationship of drought stress with N homeostasis in rice is rarely advanced. The aim of this study was to dissect the physiological effects of drought stress on rice growth that coupled unbalanced N metabolism.

Results

Water-deficient stress (WD) limited stomatal aperture function and activity of Rubisco carboxylase to photosynthesis. The rate of total electron transport (J_t) and the electron to carboxylation (J_c) were considerably decreased, whereas the proportion of e^? flow to photorespiration was stimulated by WD, especially at 1600 μmol m^?2 s^?1 PPFD. Concurrently, the expressions of glycolate oxidase genes (GOX1, GOX5) and glycine decarboxylase complex (GDCH, GDCP and GDCT) were significantly induced in leaves of WD treatment, which led to the accumulation of reactive oxygen species in leaves. With the photosynthetic change, nitrate uptake and reduction were suppressed. Moreover, the enhanced photorespiration generated excess NH₃ accumulation in leaves and stimulated the expressions of GS1;1, GS1;2 and GS2, which were tightly coupled with the expressions of PEPC1 and PEPC2 under WD stress.

Conclusions

Our results suggest that the inhibited nitrate reduction associated with diminished electron transport rate, and the photorespiration-associated accumulation of hydrogen peroxide and NH₃ were critical in the drought-induced rice growth inhibition.

     

Hypoxia within macrophyte vegetation limits the use of methane-derived carbon by larval chironomids in a shallow temperate eutrophic lake

Methane-derived carbon (MDC) can subsidize lake food webs. However, the trophic transfer of MDC to consumers within macrophyte vegetation is largely unknown. We investigated the seasonality of δ¹³C in larval chironomids within Nelumbo nucifera (Gaertn.) and Trapa natans var. Japonica (Nakai) vegetation in the shallow, eutrophic Lake Izunuma in Japan. Over the past several years, N. nucifera has rapidly expanded across more than 80% of the lake surface. Prior to the expansion of N. nucifera (2007–2008), a previous study reported extremely low larval δ¹³C levels with peak sediment methane concentrations in August or September. After the expansion of N. nucifera (2014–2015), we observed extreme hypoxia as low as or lower than 1 mg l^?1 among the macrophyte coverage during June and August. During August and September, no larvae could be found among N. nucifera, and larvae in T. natans showed relatively high δ¹³C levels (>???40‰). In contrast, larvae were markedly ¹³C–depleted (down to ??60‰) during October and November. The renewed supply of oxygen to the lake bottom may stimulate MOB activity, leading to an increase in larval assimilation of MDC. Our results suggest that macrophyte vegetation can affect the seasonality of MDC transfer to benthic consumers under hypoxic conditions in summer.     

Genetic engineering of polyamine metabolism changes <Emphasis Type="Italic">Medicago truncatula</Emphasis> responses to water deficit

In the current scenario of climate change and increasing water scarcity there is an increased need to combine research efforts for the development of abiotic stress resistant crops, specifically plants able to support water deficit (WD). Polyamines (PAs) have been described as being involved in the regulation of many physiological processes and a variety of stress responses in plants. Arginine decarboxylase (ADC) is considered a key enzyme of the polyamine (PA) biosynthetic pathway. In this study, a T₂ transgenic homozygous line of Medicago truncatula expressing the oat Adc under the control of CaMV 35S was obtained and was shown to have higher leaf accumulation of putrescine, spermidine and norspermidine compared to wild type plants. The photosynthetic parameters, leaf internal CO₂ concentration (Ci), net CO₂ assimilation rate (A), transpiration (E) and stomatal conductance (gs) of transformed and untransformed lines during WD and water deficit recovery experiments were measured by IRGA (infrared gas analyzer) and compared over time. Two light intensities were used, growth light intensity (391 μmol m^?2 s^?1) and saturating light intensity (1044 μmol m^?2 s^?1). Independently of the light intensity, and under WD, the transgenic line stood out with increased Ci, A, E and gs; suggesting a possible benefit of the augmented PAs under such disturbing environmental conditions. We showed that the constitutive expression of the oat Adc gene improve the physiological responses to WD and that WD recovered transgenic plants had higher seed yield, suggesting a possible benefit of PA metabolism manipulation in legumes.     

Competition between native Antarctic vascular plants and invasive <Emphasis Type="Italic">Poa annua</Emphasis> changes with temperature and soil nitrogen availability

Over the last decades human have introduced non-native organisms to Antarctica, including the grass species Poa annua. This non-native grass under constant growth temperatures has been shown negatively affect the growth of the only two native Antarctic vascular plants, Deschampsia antarctica and Colobanthus quitensis, under constant growth temperatures. However, whether there are changes in the interaction between these species under warmer conditions is an important question. In cold ecosystems, soil nutrient status directly affects plant responses to increases in temperature and Antarctic soils are highly variable in nutrient supply. Thus, in this study we experimentally assessed the interaction between the non-native Poa with the two native Antarctic vascular plant species at two different temperatures and levels of nutrient availability. Individual mats of the study species were collected in King George Island, and then transported to Concepcion where we conducted competition experiments. In the first experiment we used soil similar to that of Antarctica and plants in competition were grown at two temperatures: 5°/2° and 11°/5 °C (day/night temperature). In a second experiment plants were grown in these two temperature regimes, but we varied nitrogen (N) availability by irrigating plants with Hoagland solutions that contained 8000 or 300 µM of N. Overall, Poa exerted a competitive effect on Deschampsia but only at the higher temperature and higher N availability. At 5°/11 °C the competitive response of Deschampsia to Poa was of similar magnitude to the competitive effect of P. Deschampsia, and the competitive effect was greater with at low N. The competitive effect of Poa was similar to the competitive response of Colobanthus to Poa at both temperatures and N levels. Thus, at low temperatures and N soil content the native Antarctic species might withstand Poa invasion, but this might change with climate warming.     

The fatty-acid composition and nutrition of deep-sea holothurians from the Sea of Okhotsk

The results of a comparative study of the fatty-acid composition in eight species of holothurians that were collected in the Sea of Okhotsk in the area of the Kuril Islands (depths of 90–560 m) are presented. It is shown that interspecific differences in the fatty-acid compositions of the holothurians were consistent with the isotope composition (δ¹³C and δ¹⁵N) and the structural features of the tentacles and the lifestyle of holothurians, as indicators of trophic resources used by these holothurians. According to the results of the cluster analysis, the holothurians were divided into three groups. The first group included suspension-feeding dendrochirotides Eupentacta pusilla and Pseudocnus fallax with a high content of the fatty acid 20:5(n-3), which is a marker of diatoms, and with the δ¹⁵N values that are typical of consumers of suspended organic matter. The second group consisted of the dendrochirotides Psolus chitonoides and Psolidium sp. with a much lower content of 20:5 (n-3) and higher contents of 20:4 (n-6) and 22:6 (n-3), as well as high values of δ¹⁵N, which are typical of surface deposit-feeders. The third group consisted of surface and subsurface depositfeeders, Chiridota sp., Molpadia orientalis, Pseudostichopus mollis, and Synallactes nozawai. Very high contents of 20: 4 (n-6) and 21: 4 (n-7) were typical of the third group and the highest values of δ¹⁵N, indicating feeding on repeatedly recycled organic matter.     

Deep rooting and global change facilitate spread of invasive grass

Abiotic global change factors, such as rising atmospheric CO₂, and biotic factors, such as exotic plant invasion, interact to alter the function of terrestrial ecosystems. An invasive lineage of the common reed, Phragmites australis, was introduced to North America over a century ago, but the belowground mechanisms underlying Phragmites invasion and persistence in natural systems remain poorly studied. For instance, Phragmites has a nitrogen (N) demand higher than native plant communities in many of the ecosystems it invades, but the source of the additional N is not clear. We exposed introduced Phragmites and native plant assemblages, containing Spartina patens and Schoenoplectus americanus, to factorial treatments of CO₂ (ambient or +300 ppm), N (0 or 25 g m^?2 year^?1), and hydroperiod (4 levels), and focused our analysis on changes in root productivity as a function of depth and evaluated the effects of introduced Phragmites on soil organic matter mineralization. We report that non-native invasive Phragmites exhibited a deeper rooting profile than native marsh species under all experimental treatments, and also enhanced soil organic matter decomposition. Moreover, exposure to elevated atmospheric CO₂ induced a sharp increase in deep root production in the invasive plant. We propose that niche separation accomplished through deeper rooting profiles circumvents nutrient competition where native species have relatively shallow root depth distributions; deep roots provide access to nutrient-rich porewater; and deep roots further increase nutrient availability by enhancing soil organic matter decomposition. We expect that rising CO₂ will magnify these effects in deep-rooting invasive plants that compete using a tree-like strategy against native herbaceous plants, promoting establishment and invasion through niche separation.     

Polar night ecology of a pelagic predator,the chaetognath <Emphasis Type="Italic">Parasagitta elegans</Emphasis>

The annual routines and seasonal ecology of herbivorous zooplankton species are relatively well known due to their tight coupling with their pulsed food source, the primary production. For higher trophic levels of plankton, these seasonal interactions are less well understood. Here, we study the mid-winter feeding of chaetognaths in high-Arctic fjord ecosystems. Chaetognaths are planktivorous predators which comprise high biomass in high-latitude seas. We investigated the common species Parasagitta elegans around the Svalbard archipelago (78–81°N) during the winters of 2012 and 2013. Our samples consisted of individuals (body lengths 9–55 mm) from three fjords, which were examined for gut contents (n = 903), stable isotopes, fatty acid composition, and maturity status (n = 352). About a quarter of the individuals contained gut contents, mainly lipid droplets and chitinous debris, whilst only 4 % contained identifiable prey, chiefly the copepods Calanus spp. and Metridia longa. The δ¹⁵N content of P. elegans, and its average trophic level of 2.9, confirmed its carnivorous position and its fatty acid profile [in particular its high levels of 20:1(n-9) and 22:1(n-11)] confirmed carnivory on Calanus. Observations of undeveloped gonads in many of the larger P. elegans, and the absence of small individuals <10 mm, suggested that reproduction had not started this early in the year. Its average feeding rate across fjords and years was 0.12 prey ind.^?1 day^?1, which is low compared to estimates of spring and summer feeding in high-latitude environments. Our findings suggest reduced feeding activity during winter and that predation by P. elegans had little impact on the mortality of copepods.     

Opposite effects of litter and hemiparasites on a dominant grass under different water regimes and competition levels

Direct and indirect biotic interactions may affect plant growth and development, but the magnitude of these effects may vary depending on environmental conditions. In grassland ecosystems, competition is a strong structuring force. Nonetheless, if hemiparasitic plant species are introduced the competition intensity caused by the dominant species may be affected. However, the outcome of these interactions may change between wet or dry periods. In order to study this, we performed a pot experiment with different densities of the dominant species Schedonorus arundinaceus (1, 2 or 4 individuals) under constantly moist or intermittently dry conditions. The different Schenodorus densities were crossed with presence or absence of hemiparasites (either Rhinanthus minor or R. alectorolophus). Additionally, pots remained with bare ground or received a grass litter layer (400 g m^?2). We expected that indirect litter effects on vegetation (here Schedonorus or Rhinanthus) vary depending on soil moisture. We measured Schedonorus and Rhinanthus aboveground biomass and C stable isotope signature (δ¹³C) as response variables. Overall, Schedonorus attained similar biomass under moist conditions with Rhinanthus as in pots under dry conditions without Rhinanthus. Presence of Rhinanthus also increased δ¹³C in moist pots, indicating hemiparasite-induced water stress. Litter presence increased Schedonorus biomass and reduced δ¹³C, indicating improved water availability. Plants under dry conditions with litter showed similar biomass as under wet conditions without litter. Hemiparasites and litter had opposite effects: hemiparasites reduced Schedonorus biomass while litter presence facilitated grass growth. Contrary to our expectations, litter did not compensate Schedonorus biomass when Rhinanthus was present.     

Involvement of reactive oxygen species and Ca<Superscript>2+</Superscript> in the differential responses to low-boron in rapeseed genotypes

Background and aims

Invasive weeds may exert negative impact on other plant species and soil processes. The observed negative impact of an invasive weed species may be driven by allelopathy or nutrient availability.

Methodology

Sorghum halepense is one of the worst invasive weeds in crop fields. We quantified the species richness in the S. halepense-invaded communities and communities not yet invaded by the weed. Sorghum soil and no-sorghum soil were analysed for total phenolics, microbial activity, available nitrogen (N) and organic carbon. Manipulative experiments were carried out to understand the interference potential of S. halepense. Soil was amended with root or shoot leachate of S. halepense, and its impact on plant growth and soil properties was studied.

Results

Out of four S. halepense-sites, lower plant species richness was observed in one site compared to uninvaded sites. S. halepense-invaded soil had higher levels of total phenolics and lower levels of available N. Higher inhibition in the root growth of Brassica juncea or Bidens pilosa was observed in root leachate-amended soil than shoot leachate-amended soil. Shoot leachate-amended soil had higher levels of total phenolics and available N than root leachate-amended soils. Significant reduction in the available N was observed in soil amended with root leachate. Significant decline in the total phenolics over a period of time was observed in soil amended with root leachate or shoot leachate of S. halepense. Higher CO₂ release was observed 24 h after amending soil with root leachate or shoot leachate of S. halepense.

Conclusions

Sorghum halepense interference potential in its soil is likely due to lower levels of available N. Greater reduction in root dry weight of assay species in root leachate amended soil compared to shoot leachate amended soil was likely due to lower levels of available N in root leachate-amended soil. Relative interference potential of both root and shoot leachates or extracts should be evaluated in allelopathy bioassays and further experiments should be designed to distinguish the role of allelochemicals and nutrient availability in plant growth inhibition.

     

Distinct diversity patterns of <Emphasis Type="Italic">Planctomycetes</Emphasis> associated with the freshwater macrophyte <Emphasis Type="Italic">Nuphar lutea</Emphasis> (L.) Smith

Members of the phylum Planctomycetes were originally described as freshwater bacteria. Most recent studies, however, address planctomycete diversity in other environments colonized by these microorganisms, including marine and terrestrial ecosystems. This study was initiated in order to revisit the specific patterns of planctomycete diversity in freshwater habitats using cultivation-independent approaches. The specific focus was made on planctomycetes associated with Nuphar lutea (L.) Smith, an emergent macrophyte with floating leaves, which is widespread in the Holarctic. As revealed by Illumina pair-end sequencing of 16S rRNA gene fragments, the bacterial assemblages colonizing floating leaf blades of waterlilies sampled from two different boreal lakes displayed similar composition but were distinct from the planktonic bacterial communities. 16S rRNA gene fragments from the Planctomycetes comprised 0.1–1 and 1–2.2% of total 16S rRNA gene reads retrieved from water samples and plant leaves, respectively. Planktonic planctomycetes were mostly affiliated with the class Planctomycetaceae (77–97%), while members of the Phycisphaerae were less abundant (3–22%). The relative proportion of the latter group, however, increased by 13–45% on leaves of N. lutea. The Phycisphaera-related group WD2101, Pirellula-like planctomycetes, as well as Gemmata, Zavarzinella and Planctopirus species were the most abundant groups of planctomycetes associated with plant leaves, which may suggest their involvement in the degradation of plant-derived organic matter.     

Changes in growth and composition of the marine microalgae <Emphasis Type="Italic">Phaeodactylum tricornutum</Emphasis> and <Emphasis Type="Italic">Nannochloropsis salina</Emphasis> in response to changing sodium bicarbonate concentrations

Oils, carbohydrates, and fats generated by microalgae are being refined in an effort to produce biofuels. The research presented here examines two marine microalgae, Nannochloropsis salina (green alga) and Phaeodactylum tricornutum (diatom), when grown with 0 (no addition), 0.5, 1.0, 2.0, and 5.0 g L^?1 NaHCO₃ added to an f/2 medium during the growth phase (GP) and a nutrient induced (nitrate limitation) lipid formation phase (LP). We hypothesize that the addition of NaHCO₃ is a sustainable and practical strategy to increase cellular density and concentrations of lipids in microalgae as well as the rate of lipid accumulation. In N. salina, final cell densities were significantly (p?<?0.05) higher in the NaHCO₃-treated cells than the control while in P. tricornutum the cell densities were higher with >[NaHCO₃] during the GP. During the LP, cell densities were generally higher in the NaHCO₃-treated cells compared with controls. F _V/F _M (efficiency of photosystem II) patterns paralleled those for cell density with generally higher values with higher concentrations of NaHCO₃ and significantly different values between controls and 5.0 g L^?1 NaHCO₃ at the end of the GP (p?<?0.05). F _V/F _M was variable between treatments in P. tricornutum (0.3–0.65) but less so in N. salina for (0.5–0.7) regardless of [NaHCO₃]. The lipid index (measured with Nile red), used as a proxy for triacylglycerides (TAGs), was 10.2?±?6.5 and 4.4?±?2.9 (fluorescence units/OD cells ×1000) for N. salina and P. tricornutum, respectively, at the end of the GP. At the end of the LP, the lipid index was eight and four times higher than during the GP in the corresponding 5.0 g L^?1 NaHCO₃ treatments, revealing that N. salina was accumulating more lipid than P. tricornutum. Dry weights essentially doubled during LP compared with GP for N. salina; this was not the case for P. tricornutum. In general, the percentage of ash in dry weights was significantly higher in the LP relative to the corresponding GP treatments for P. tricornutum; this was not the case for N. salina. During the LP, there was also less soluble protein in N. salina compared to GP; differences were not significant in cells growing with 2.0 or 5.0 g L^?1 NaHCO₃. In P. tricornutum, faster growing cells had more soluble protein during the GP and LP; differences between treatments were significant. P. tricornutum generally accumulated significantly more crude protein than N. salina at higher [NaHCO₃]; there was three times more crude protein in the highest NaHCO₃ (5.0 g L^?1) treatment compared with the controls. C:N ratios (mol:mol) were similar across treatments during GP: 7.03?±?0.12 and 10.16?±?0.41 for N. salina and P. tricornutum, respectively. Further, C:N ratios increased with increasing [NaHCO₃] during LP. Species-specific fatty acid methyl ester (FAMEs) profiles were observed. While C16:0 was lower in P. tricornutum compared to N. salina, the diatom produced more C16:1 and C14 but not C18:3. Monounsaturated fatty acids (MUFA) significantly increased in N. salina in the LP compared to GP and in response to increasing [NaHCO₃] (t tests; p?<?0.05). Saturated fatty acids (SFA) responded similarly but to a lesser degree. There were more polyunsaturated fatty acids (PUFA) in N. salina than MUFAs or SFAs. In P. tricornutum, there were generally more SFAs, MUFAs and PUFAs in P. tricornutum during LP than GP in the corresponding NaHCO₃ treatments. These findings reveal the importance of considering NaHCO₃ as a supplemental carbon source in the culturing marine phytoplankton in large-scale production for biofuels.     

Shrub Expansion of <Emphasis Type="Italic">Alnus viridis</Emphasis> Drives Former Montane Grassland into Nitrogen Saturation

The N₂-fixing shrub Alnus viridis is currently encroaching on montane grasslands in the Alps as a result of reduced land management and complete abandonment. Alnus introduces large amounts of nitrogen (N) into these formerly N-poor grasslands and restricts the succession to montane forests. We studied pools and fluxes of N and the associated C pools in pastures (controls) and adjacent Alnus shrublands at two elevations (1650 versus 1950 m a.s.l.) in three valleys in the Swiss central Alps. The total N and C pools stored in 50-year-old Alnus shrubland did not exceed those in adjacent pastures with a total of approximately 610 g N m^?2 in phytomass plus soil (down to 30 cm) at both elevations. In Alnus stands, reduced soil N pools balanced the gain in phytomass N pools, a likely result of a faster turnover of soil N. The soil solution under Alnus was continuously enriched with nitrate, with a total N leaching of 0.79 g N m^?2 season^?1 (June–October) under 50-year-old stands at both elevations and the highest flux of 1.76 g N m^?2 season^?1 in 25-year-old shrubland at low elevation, clearly indicating an excess of available N in Alnus shrubland. In contrast, N leaching across all pastures was close to zero (0.08 g N m^?2) throughout the season. At the catchment scale, streamlet water showed increased nitrate concentrations with typical flushing peaks in spring and autumn, provided more than one fifth of the catchment area was covered by Alnus shrubs. We conclude that the expansion of Alnus rapidly converts centuries-old, N-poor grassland into N saturated shrubland, irrespective of elevation, and it reduces the C storage potential of the landscape because the Alnus dominance constrains re-establishment of a natural montane forest.