The role of salinity and sodicity in the dieback of Acacia xanthophloea in Ngorongoro Caldera, Tanzania

Dieback of Acacia xanthophloea (Benth.) has opened up the once densely forested Lerai area in Ngorongoro Caldera, Tanzania. Soil samples were taken from profiles in the Ngorongoro Conservation Area and Lake Manyara National Park at sites of dieback and at sites with healthy A. xanthophloea trees. Dieback sites had significantly greater electrical conductivity (EC), water‐soluble Na⁺, K⁺, Cl^?, SO and sodium adsorption ratios (SAR) than healthy sites. The following mean values were recorded: EC (179 versus 70 mS m^?1; P < 0.001, Student's t‐test, n = 8 and 10, respectively; 40–60 cm); Na⁺ (99 versus 30 mmol_c kg^?1, P < 0.001, n = 7 and 8 respectively); K⁺ (11 versus 3 mmol_c kg^?1, P < 0.05); Cl^? (36 versus 7 mmol_c kg^?1, P < 0.01); SO (31 versus 5 mmol_c kg^?1, P < 0.01); and SAR (28 versus 8 mmol l^?1/2, P < 0.01). Water‐soluble Na⁺, Cl^? and SO concentrations in the Lerai profiles have probably resulted in toxicity and osmotic stress which contributed to dieback. Accumulation of salts may have occurred because of reduced flow of freshwater through Lerai and/or flow of water from Lake Magadi into Lerai. Forest recovery may be possible if salinity is reduced. Management strategies for reducing salinity have been implemented and included re‐establishing streams that flow through Lerai. Exclusion of elephants (Loxodonta africana) from Lerai is another management strategy presently under consideration.     

DFT study of geometrical and vibrational features of a 3′,5′-deoxydisugar-monophosphate (dDSMP) DNA model in the presence of counterions and solvent

The B3LYP/6–31++G* theoretical level was used to study the influence of various hexahydrated monovalent (Li⁺, Na⁺, K⁺) and divalent (Mg²⁺) metal counterions in interaction with the charged PO₂^? group, on the geometrical and vibrational characteristics of the DNA fragments of 3′,5′-dDSMP, represented by four conformers (g⁺g⁺, g⁺t, g^?g^? and g^?t). All complexes were optimized through two solvation models [the explicit model (6H₂O) and the hybrid model (6H₂O/Continuum)]. The results obtained established that, in the hybrid model, counterions (Li⁺, Na⁺, K⁺, Mg²⁺) always remain in the bisector plane of the O1–P–O2 angle. When these counterions are explicitly hydrated, the smallest counterions (Li⁺, Na⁺) deviate from the bisector plane, while the largest counterions (K⁺ and Mg²⁺) always remain in the same plane. On the other hand, the present calculations reveal that the g⁺g⁺ conformer is the most stable in the presence of monovalent counterions, while conformers g⁺t and g^?t are the most stable in the presence of the divalent counterion Mg²⁺. Finally, the hybrid solvation model seems to be in better agreement with the available crystallographic and spectroscopic (Raman) experiments than the explicit model. Indeed, the six conformational torsions of the C4′-C3′-O3′-PO^?₂-O5′-C5′-C4′ segment of all complexes of the g^?g^? conformer in 6H₂O/Continuum remain similar to the available experimental data of A- and B-DNA forms. The calculated wavenumbers of the g⁺g⁺ conformer in the presence of the monovalent counterion and of g^?t conformer in presence of the divalent counterion in the hybrid model are in good agreement with the Raman experimental data of A- and B-DNA forms. In addition, the maximum deviation between the calculated wavenumbers in the 6H₂O/Continuum for the g⁺g⁺ conformer and experimental value measured in an aqueous solution of the DMP-Na⁺ complex, is <1.07% for the PO₂^? (asymmetric and symmetric) stretching modes and <2.03% for the O5′-C5′ and O3′-C3′ stretching modes.

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Graphical abstract dDSMP-(OO)^? Mg²⁺/6W/Continuum

     

Syntheses and structural analyses of chiral rhenium containing amines of the formula (η-C5H5)Re(NO)(PPh3)((CH2)nNRR′) (n = 0, 1)

The reaction of the racemic chiral methyl complex (η⁵-C₅H₅)Re(NO)(PPh₃)(CH₃) (1) with CF₃SO₃H and then NH₂CH₂C₆H₅ gives [(η⁵-C₅H₅)Re(NO)(PPh₃)(NH₂CH₂C₆H₅)]⁺ ([4a-H]⁺; 73%), and deprotonation with t-BuOK affords the amido complex (η⁵-C₅H₅)Re(NO)(PPh₃)(NHCH₂C₆H₅) (76%). Reactions of 1 with Ph₃C⁺ X⁻ and then primary or secondary amines give [(η⁵-C₅H₅)Re(NO)(PPh₃)(CH₂NHRR′)]⁺ X⁻ ([6-H]⁺ X⁻; R/R′/X = a, H/NH₂CH₂C₆H₅/BF₄; a′, H/NH₂CH₂C₆H₅/PF₆; b, H/NH₂CH₂(CH₂)₂CH₃/PF₆; c, H/(S)-NH₂CH(CH₃)C₆H₅/BF₄); d, CH₂CH₃/CH₂CH₃/PF₆; e, CH₂(CH₂)₂CH₃/CH₂(CH₂)₂CH₃/PF₆; f, CH₂C₆H₅/CH₂C₆H₅/PF₆; g, -CH₂(CH₂)₂CH₂-/PF₆; h, -CH₂(CH₂)₃CH₂-/PF₆; i, CH₃/CH₂CH₂OH/PF₆ (62-99%). Deprotonations with t-BuOK afford the amines (η⁵-C₅H₅)Re(NO)(PPh₃)(CH₂NRR′) (6a-i; 99-40%), which are more stable and isolated in analytically pure form when R ≠ H. Enantiopure 1 is used to prepare (R_ReS_C)-[6c-H]⁺, (R_ReS_C)-6c, (S)-[6g-H]⁺, and (S)-6g. The crystal structures of [4a-H]⁺, a previously prepared NH₂CH₂Si(CH₃)₃ analog, [6a′,d,f,h-H]⁺, (R_ReS_C)-6c, and 6f are determined and analyzed in detail, particularly with respect to cation/anion hydrogen bonding and conformation. In contrast to analogous rhenium containing phosphines, 6a-i show poor activities in reactions that are catalyzed by organic amines.     

Nitrogen metabolism by heterotrophic bacterial assemblages in Antarctic coastal waters

Field studies to examine the in situ assimilation and production of ammonium (NH₄ ⁺) by bacterial assemblages were conducted in the northern Gerlache Strait region of the Antarctic Peninsula. Short term incubations of surface waters containing ¹⁵N-NH₄ ⁺ as a tracer showed the bacterial population taking up 0.041–0.128 g-atoms Nl^–1d^–1, which was 8–25% of total NH₄ ⁺ uptake rates. The large bacterial uptake of NH₄ ⁺ occurred even at low bacterial abundance during a rich phytoplankton bloom. Estimates of bacterial production using ³H-leucine and -adenine were l.0gCl^–1 d^–1 before the bloom and 16.2 g Cl^–1 d^–1 at the bloom peak. After converting bacterial carbon production to an estimate of nitrogen demand, NH₄ ⁺ was found to supply 35–60% of bacterial nitrogen requirements. Bacterial nitrogen demand was also supported by dissolved organic nitrogen, generally in the form of amino acids. It was estimated, however, that 20–50% of the total amino acids taken up were mineralized to NH₄ ⁺. Bacterial production of NH₄ ⁺ was occurring simultaneously to its uptake and contributed 27–55% of total regenerated NH₄ ⁺ in surface waters. Using a variety of ¹⁵N-labelled amino acids it was found that the bacteria metabolized each amino acid differently. With their large mineralization of amino acids and their relatively low sinking rates, bacteria appear to be responsible for a large portion of organic matter recycling in the upper surface waters of the coastal Antarctic ecosystem.     

Cutaneous ion exchange,and renal and extrarenal partitioning of acid and ammonia excretion in the larval tiger salamander,Ambystoma tigrinum,following ingestion of ammonium salts

Summary The skin/gills and the kidneys of aquatic amphibians are potential sites of acid-base regulation. The roles of these organs in acid-base balance were examined in larval Ambystoma tigrinum following gastric infusion of ammonium salts. A single dose of 1.75 mEq NH₄Cl·100 g^-1 produced a mixed acidosis by 1 h after gavage. By 8 h after ingestion, pH and HCO ₃ ^– had increased and PCO₂ had decreased as the animals recovered. A prolonged acidosis was developed in a second group by gavage of an initial dose (1.5 mEq·100 g^-1), followed by periodic maintenance doses (0.25 mEq·100 g^-1) to prolong the disturbance for 8 h. The magnitude of the acidosis during this period was similar to that seen at 1 h after ingestion in the time-course study. A third group of larvae were given NaCl as a control for salt loading, which induced a small but significant respiratory acidosis. Unidirectional fluxes of Na⁺ and Cl^- were examined during these serial ingestions. Salt loading inhibited the influx of the ingested ion. Na⁺ influx increased during the NH₄Cl-induced acidosis. A fourth group of larvae were used to partition acid and ammonia excretion between the skin and the kidneys. These animals were given (NH₄)₂SO₄ to allow re-examination of Cl^- flux rates under non-Cl^--loaded conditions. The ensuing acidosis had a reduced respiratory component and, therefore, pH did not decrease as much. Cl^- influx rates did decrease significantly under these conditions. In both control and acidotic conditions, the majority of the acid efflux was as ammonia and the skin was the primary site of acid excretion. However, both the skin and the kidneys increased total acid excretion, although the efflux across the skin showed a much greater increase. This suggests a primary role for the skin in acid-base regulation in aquatic amphibians.Abbreviations GFR glomerular filtration rate - PO₂ partial pressure of oxygen - PCO₂ partial pressure of carbon dioxide - SITS 4-acetamido-4-isothiocynanatostilbene-2,2-disulfonic acid - TA titratible acidity Present address: Department of Organismal Biology and Anatomy, University of Chicago, 1025 E. 57th St., Chicago, IL 60637, USA     

Single point mutations of the sodium channel drastically reduce the pore permeability without preventing its gating

1. Two mutants of the sodium channel II have been expressed inXenopus oocytes and have been investigated using the patch-clamp technique. In mutant E387Q the glutamic acid at position 387 has been replaced by glutamine, and in mutant D384N the aspartic acid at position 384 has been replaced by asparagine.2. Mutant E387Q, previously shown to be resistant to block by tetrodotoxin (Noda et al. 1989), has a single-channel conductance of 4 pS, that can be easily measured only using noise analysis. At variance with the wild-type, the openchannel current-voltage relationship of mutant E387Q is linear over a wide voltage range even under asymmetrical ionic conditions.3. Mutant D384N has a very low permeability for any of the following ions: Cl^–, Na⁺, K⁺, Li⁺, Rb⁺, Ca²⁺, Mg²⁺, NH₄ ⁺ , TMA⁺, TEA⁺. However, asymmetric charge movements similar to the gating currents of the Na⁺-selective wild-type are still observed.4. These results suggest that residues E387 and D384 interact directly with the pathway of the ions permeating the open channel.Abbreviations TTX tetrodotoxin; Na⁺, sodium; K⁺, potassium; - NFR normal frog Ringer - HEPES N-2-hydroxylethyl piperazine-N-2-ethanesulfonic acid - EGTA ethyleneglycol-bis(-amino-ethyl ether) N,N,N',N'-tetra acetic acid - TEA tetraethylammonium - TMA tetramethylammonium;I _g , gating current; , single-channel conductance     

Seasonal variability of apoplastic NH4 + and pH in an intensively managed grassland

The stomatal compensation point of ammonia (_s) is a major factor controlling the exchange of atmospheric ammonia (NH₃) with vegetation. It is known to depend on the supply of nitrogen and to vary among plant species, but its seasonal variation has not yet been reported for grassland. In this study, we present the temporal variation of apoplastic NH₄ ⁺ concentration ([NH₄ ⁺]_apo) and pH (pH_apo) measured in leaves of Lolium perenne L. in a grassland, through two periods of cutting / fertilisation, followed by a livestock grazing period. The total free NH₄ ⁺ concentration measured in foliage ([NH₄ ⁺]_fol), and soil mineral NH₄ ⁺ and NO₃ ^– concentration are also presented. The value of [NH₄ ⁺]_apo varied from less than 0.01 mM to a maximum of 0.5 mM occurring just after fertilisation, whereas the apoplastic pH ranged from pH 6 to 6.5 for most of the time and increased up to pH 7.8, 9 days after the second fertilisation, when grazing started. [NH₄ ⁺]_fol varied between 20 and 50 g N-NH₄ ⁺ g^–1 f.w. The compensation point at 20°C, ranged from 0.02 g NH₃ m^–3 between the fertilisations to 10 g NH₃ m^–3 just after the second fertilisation. The reasons for these seasonal changes are discussed, with respect to plant metabolism and the concentration of ammonium and nitrate in the soil.     

Regulation of the Na+-K+(NH)-2Cl− cotransporter of rat submandibular glands

A cellular suspension from rat submandibular glands was exposed to different concentrations of NH₄Cl, and the variations of the intracellular concentration of calcium ([Ca²⁺]_i) and the intracellular pH (pH_i) were measured using fura-2 and 2′,7′-bis-(2-carboxy-ethyl)-5(6)-carboxyfluorescein. More than 5 mmol/l NH₄Cl significantly increased the [Ca²⁺]_i without affecting the response to 100 µmol/l carbachol. When exposed to 1 and 5 mmol/l NH₄Cl, the cells acidified immediately. At 30 mmol/l, NH₄Cl first alkalinized the cells and the pH_i subsequently dropped. This drop reflects the uptake of NH ions that dissociate to NH₃ and H⁺ in the cytosol. These protons are exchanged for extracellular sodium by the Na⁺/H⁺ exchanger because the presence of an inhibitor of the exchanger in the medium increased the acidification induced by 1 mmol/l NH₄Cl. Ouabain partly blocked the uptake of NH. In the combined presence of ouabain and bumetanide (an inhibitor of the Na⁺-K⁺-2Cl⁻ cotransporter), 1 mmol/l NH₄Cl alkalinized the cells. The contribution of the Na/K ATPase and the Na⁺-K⁺-2Cl⁻ cotransporter in the uptake of NH was independent of the presence of calcium in the medium. Isoproterenol increased the uptake of NH by the cotransporter. Conversely, 1 mmol/l extracellular ATP blocked the basal uptake of NH by the cotransporter. This inhibition was reversed by extracellular magnesium or Coomassie Blue. It was mimicked by benzoyl-ATP but not by CTP, GTP, UTP, ADP, or ADPβS. ATP only slightly inhibited the increase of cyclic AMP (−22%) by isoproterenol but fully blocked the stimulation of the cotransporter by the β-adrenergic agonist. ATP increased the release of ³H-arachidonic acid from prelabeled cells but SK&F 96365, an imidazole-based cytochrome P450 inhibitor, did not affect the inhibition by ATP. It is concluded that the activation of a purinoceptor inhibits the basal and the cyclic AMP-stimulated activity of the Na⁺-K⁺-2Cl⁻ cotransporter. J. Cell. Physiol. 180:422–430, 1999. © 1999 Wiley-Liss, Inc.     

Interactions of elevated CO2, NH3 and O3 on mycorrhizal infection,gas exchange and N metabolism in saplings of Scots pine

Four-year-old saplings of Scots pine (Pinus sylvestris) (L.) were exposed for 11 weeks in controlled-environment chambers to charcoad-filtered air, or to charcoal-filtered air supplemented with NH₃ (40 g m^–3), O₃ (110 g m^–3 during day/ 40 g m^–3 during night) or NH₃+O₃. All treatments were carried out at ambient (259 L L^–1) and at elevated CO₂ concentration (700 L L^–1). Total tree biomass, mycorrhizal infection, net CO₂ assimilation (P_n), stomatal conductance (g_s), transpiration of the shoots and NH₃ metabolization of the needles were measured. In ambient CO₂ (1) gaseous NH₃ decreased mycorrhizal infection, without significantly affecting tree biomass or N concentration and it enhanced the activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in one-year-old needles; (2) ozone decreased mycorrhizal infection and the acitivity of GS in the needles, while it increased the activity og GDH; (3) exposure to NH₃+O₃ lessened the effects of single exposures to NH₃ and O₃ on reduction of mycorrhizal infection and on increase in GDH activity. Similar lessing effects on mycorrhizal infection as observed in trees exposed to NH₃+O₃ at ambient CO₂, were measured in trees exposed to NH₃+O₃ at elevated CO₂. Exposure to elevated CO₂ without pollutants did not significantly affect any of the parameters studied, except for a decrease in the concentration of soluble proteins in the needles. Elevated CO₂ _NH₃ strongly decreased root branching and mycorrhizal infection and temporarily stimulated P_n and g_s. The exposure to elevated CO₂+NH₃+O₃ also transiently stimulated P_n. The possible mechanisms underlying and integrating these effects are discussed. Elevated CO₂ clearly did not alleviate the negative effects of NH₃ and O₃ mycoorhiral infection. The significant reduction of mycorrhizal infection after exposure to NH₃ or O₃, observed before significant changes in gas exchange or growth occurred, suggest the use of mycorrhizal infection as an early indicator for NH₃ and O₃ induced stress.Abbreviations DW dry weight - FA filtered air - FA_a filtered air at ambient CO₂ - FA_e filtered air at elevated CO₂ - FW fresh weight - GDH glutamate dehydrogenase - GS glutamine synthetase - g_s stomatal conductance - P_n net CO₂ assimilation - RWR root weight ratio - SRL specific root length     

The influence of NO3 - and NH4 + nutrition on the carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) plants

The carbon and nitrogen partitioning characteristics of wheat (Triticum aestivum L.) and maize (Zea mays L.) grown hydroponically at a constant pH on either 4 mM or 12 mM NO₃ ^- or NH₄ ⁺ nutrition were investigated using either ¹⁴C or ¹⁵N techniques. Greater allocation of ¹⁴C to amino-N fractions occurred at the expense of allocation of ¹⁴C to carbohydrate fractions in NH₄ ⁺-compared to NO₃ ^--fed plants. The [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were 1.5-fold and 2.0-fold greater in shoots and roots respectively of 12 mM NO₃ ^--compared to 12 mM NH₄ ⁺-fed wheat. In both 4 mM and 12 mM N-fed maize the [¹⁴C]carbohydrate:[¹⁴C]amino-N ratios were approximately 1.7-fold and 2.0-fold greater in shoots and roots respectively of NO₃ ^--compared to NH₄ ⁺-fed plants. Similar results were observed in roots of wheat and maize grown in split-root culture with one root-half in NO₃ ^--and the other in NH₄ ⁺-containing nutrient media. Thus the allocation of carbon to the amino-N fractions occurred at the expense of carbohydrate fractions, particularly within the root. Allocation of ¹⁴N and ¹⁵N within separate sets of plants confirmed that NH₄ ^--fed plants accumulated more amino-N compounds than NO₃ ^--fed plants. Wheat roots supplied with ¹⁵NH₄ ⁺ for 8 h were found to accumulate ¹⁵NH₄ ⁺ (8.5 g ¹⁵N g^-1 h^-1) whereas in maize roots very little ¹⁵NH₄ ⁺ accumulated (1.5 g ¹⁵N g^-1 h^-1)It is proposed that the observed accumulation of ¹⁵NH₄ ⁺ in wheat roots in these experiments is the result of limited availability of carbon within the roots of the wheat plants for the detoxification of NH₄ ⁺, in contrast to the situation in maize. Higher photosynthetic capacity and lower shoot: root ratios of the C₄ maize plants ensure greater carbon availability to the root than in the C₃ wheat plants. These differences in carbon and nitrogen partitioning between NO₃ ^--and NH₄ ⁺-fed wheat and maize could be responsible for different responses of wheat and maize root growth to NO₃ ^- and NH₄ ⁺ nutrition.     

Use of chemical ionization for GC–MS metabolite profiling

Metabolite profiling is commonly performed by GC–MS of methoximated trimethylsilyl derivatives. The popularity of this technique owes much to the robust, library searchable spectra produced by electron ionization (EI). However, due to extensive fragmentation, EI spectra of trimethylsilyl derivatives are commonly dominated by trimethylsilyl fragments (e.g. m/z 73 and 147) and higher m/z fragment ions with structural information are at low abundance. Consequently different metabolites can have similar EI spectra, and this presents problems for identification of “unknowns” and the detection and deconvolution of overlapping peaks. The aim of this work is to explore use of positive chemical ionization (CI) as an adjunct to EI for GC–MS metabolite profiling. Two reagent gases differing in proton affinity (CH₄ and NH₃) were used to analyse 111 metabolite standards and extracts from plant samples. NH₃-CI mass spectra were simple and generally dominated by [MH]⁺ and/or the adduct [M+NH₄]⁺. For the 111 metabolite standards, m/z 73 and 147 were less than 3% of basepeak in NH₃-CI and less than 30% of basepeak in CH₄-CI. With CH₄-CI, [MH]⁺ was generally present but at lower relative abundance than for NH₃-CI. CH₄-CI spectra were commonly dominated by losses of CH₄ [M+1-16]⁺, 1–3 TMSOH [M+1-nx90]⁺, and combinations of CH₄ and TMSOH losses [M+1-nx90-16]⁺. CH₄-CI and NH₃-CI mass spectra are presented for 111 common metabolites, and CI is used with real samples to help identify overlapping peaks and aid identification via determination of the pseudomolecular ion with NH₃-CI and structural information with CH₄-CI.     

Ion-molecule condensation reactions: A mechanism for organic synthesis in ionized reducing atmospheres

The CH₃ ⁺ ion, formed in ionized methane, undergoes consecutive eliminative condensation reactions with methane to form the carbonium ions C₂H₅ ⁺, i-C₃H₇ ⁺ and t-C₄H₉ ⁺. AtT<500°K, \(N_{CH_4 } \) ?10¹⁶ cm^?3 these ions react with NH₃ in competitive condensation-H⁺ transfer reactions, e.g. $$\begin{gathered} C_2 H_5 ^ + + NH_3 \xrightarrow{M} C_2 H_5 NH_3 ^ + \hfill \\ - - - \to NH_4 ^ + + C_2 H_4 \hfill \\ \end{gathered} $$ At particle densities of \(N_{CH_4 } \) <10¹⁶ cm^?3 proton transfer is the only significant reaction channel. At \(N_{CH_4 } \) >10¹⁷ cm^?3 condensation constitutes 5–20% of the overall reactions. The product of the condensation reaction further associates with CO₂ to form C₂H₅NH₃ ⁺·CO₂; the atomic composition of this cluster ion is identical with the protonated amino acid alanine. The carbonium ions i-C₃H₇ ⁺ and t-C₄H₉ ⁺ condense also with HCN to yield protonated isocyanides. HCNH⁺ also appears to condense with HCN atT>570°K, and form cluster ions with HCN at lower temperatures. The rate constants of the condensation reactions vary with temperature and pressure in a complex manner. Under conditions similar to those on Titan at an altitude of 100 km (T=100–150°K, \(N_{CH_4 } \) ≈10¹⁸ cm^?3), with a methane atmosphere containing 1% H₂ and traces of NH₃ and H₂O, ion-molecule condensation reactions followed by H⁺ transfer are expected to lead to the atmospheric synthesis of C₂H₆, C₃H₈, CH₃OH, C₂H₅OH and the terminal ions NH₄ ⁺, CH₃NH₃ ⁺ and C₂H₅NH₃ ⁺. At higher temperatures (250°K<T<400°K), the synthesis of i-C₄H₁₀, i-C₃H₇OH and t-C₄H₉OH and of the ions i-C₃H₇NH₃ ⁺ and t-C₄H₉NH₃ ⁺ is also expected. Electron recombination of the terminal ions may yield amines, imines and nitriles. Cycles of protonation and dissociative recombination of the alkanes and alcohols produced in condensation reactions will also produce unsaturated hydrocarbons, ketones and aldehydes in the ionized atmosphere.     

Ganglioside changes in the chicken optic lobes and cerebrum during embryonic development

Summary The developmental profiles of 15 different gangliosides of the optic lobes and cerebrum of the chicken were followed from the 6 th day of incubation to hatching and correlated to morphological development. Five of these gangliosides appearing in both structures between the sixth and tenth day, have not been reported previously in higher vertebrates. Three chromatographed on TLC-plates similarly to G_T3, G_T2, and G_T1c gangliosides, which have been demonstrated in fish brain. One fraction moved just below G_Q1b and is suggested, to contain G_Q1c. These novel gangliosides, which are possibly related to a recently proposed separate and probably phylogenetically older biosynthetic pathway, contained up to 20% of total ganglioside sialic acid. The fifth novel fraction, containing up to 16% of total ganglioside-sialic acid, moved below the penta-sialoganglioside G_P1 and is suggested to contain hexa-sialogangliosides.There were two main changes in ganglioside synthesis, which were identical in both structures.The first occurred from the sixth to the eleventh day, parallel to decreased proliferation, maximal cell migration and neuroblast differentiation, G_D3 and G_D2 decreased rapidly in favour of G_Q1b, G_P1, and to the novel fractions, described above.The second occurred from the eleventh to the eighteenth day, parallel to increased growth and arborization of dendrites and axons as well as functional establishment of synaptic contacts, there was a sharp rise in the amount of G_D1b, G_T1b, and G_D1a. Concomitantly the novel gangliosides decreased. At hatching G_D1a was the predominant ganglioside. G_M3, G_M2, and G_M1 were always minor fractions, each accounting for less than 4% of total ganglioside-sialic acid. G_M4 was never detected, indicating neglegible myelinisation until hatching.Abbreviations NeuAc N-Acetylneuraminic acid - G_M4 I³NeuAc-GalCer - G_M3 II³NeuAc-LacCer - G_M2 II³NeuAc-G_gOse₃Cer - G_M1 II³NeuAc-G_gOse₄Cer - G_D3 II³(NeuAc)₂LacCer - G_D1a IV³NeuAc, II³NeuAc G_gOse₄Cer - G_T3 II³ (NeuAc)₃LacCer - G_D2 II³(NeuAc)₂G_gOse₃Cer - G_D1b II³(NeuAc)₂G_gOse₃Cer - G_D1b II³(NeuAc)₂G_gOse₄Cer - G_T2 II³(NeuAc)₃G_gOse₃Cer - G_T1b IV³NeuAc, II³(NeuAc)₂G_gOse₄Cer - G_T1c II³(NeuAc)₃G_gOse₄Cer - G_Q1b IV³(NeuAc)₂ II³(NeuAc)₂G_gOse₄Cer - G_Q1c IV³NeuAc, II³(NeuAc)₃G_gOse₄Cer - G_P1 IV³(NeuAc)₂, II³(NeuAc)₃G_gOse₄Cer - G_H(?) IV₃(NeuAc)₃, II³(NeuAc)₃G_gOse₄Cer     

Human SLC4A11 Is a Novel NH3/H+ Co-transporter

SLC4A11 has been proposed to be an electrogenic membrane transporter, permeable to Na⁺, H⁺ (OH⁻), bicarbonate, borate, and NH₄⁺. Recent studies indicate, however, that neither bicarbonate or borate is a substrate. Here, we examined potential NH₄⁺, Na⁺, and H⁺ contributions to electrogenic ion transport through SLC4A11 stably expressed in Na⁺/H⁺ exchanger-deficient PS120 fibroblasts. Inward currents observed during exposure to NH₄Cl were determined by the [NH₃]_o, not [NH₄⁺]_o, and current amplitudes varied with the [H⁺] gradient. These currents were relatively unaffected by removal of Na⁺, K⁺, or Cl⁻ from the bath but could be reduced by inclusion of NH₄Cl in the pipette solution. Bath pH changes alone did not generate significant currents through SLC4A11, except immediately following exposure to NH₄Cl. Reversal potential shifts in response to changing [NH₃]_o and pH_o suggested an NH₃/H⁺-coupled transport mode for SLC4A11. Proton flux through SLC4A11 in the absence of ammonia was relatively small, suggesting that ammonia transport is of more physiological relevance. Methylammonia produced currents similar to NH₃ but with reduced amplitude. Estimated stoichiometry of SLC4A11 transport was 1:2 (NH₃/H⁺). NH₃-dependent currents were insensitive to 10 μm ethyl-isopropyl amiloride or 100 μm 4,4′- diisothiocyanatostilbene-2,2′-disulfonic acid. We propose that SLC4A11 is an NH₃/2H⁺ co-transporter exhibiting unique characteristics.     

Rapid Ammonia Gas Transport Accounts for Futile Transmembrane Cycling under NH3/NH4+ Toxicity in Plant Roots

Futile transmembrane NH₃/NH₄⁺ cycling in plant root cells, characterized by extremely rapid fluxes and high efflux to influx ratios, has been successfully linked to NH₃/NH₄⁺ toxicity. Surprisingly, the fundamental question of which species of the conjugate pair (NH₃ or NH₄⁺) participates in such fluxes is unresolved. Using flux analyses with the short-lived radioisotope ¹³N and electrophysiological, respiratory, and histochemical measurements, we show that futile cycling in roots of barley (Hordeum vulgare) seedlings is predominately of the gaseous NH₃ species, rather than the NH₄⁺ ion. Influx of ¹³NH₃/¹³NH₄⁺, which exceeded 200 µmol g^–1 h^–1, was not commensurate with membrane depolarization or increases in root respiration, suggesting electroneutral NH₃ transport. Influx followed Michaelis-Menten kinetics for NH₃ (but not NH₄⁺), as a function of external concentration (K_m = 152 µm, V_max = 205 µmol g^–1 h^–1). Efflux of ¹³NH₃/¹³NH₄⁺ responded with a nearly identical K_m. Pharmacological characterization of influx and efflux suggests mediation by aquaporins. Our study fundamentally revises the futile-cycling model by demonstrating that NH₃ is the major permeating species across both plasmalemma and tonoplast of root cells under toxicity conditions.Ammonia/ammonium (NH₃/NH₄⁺) toxicity in higher plants has resulted in crop reduction and forest decline (Pearson and Stewart, 1993; Vitousek et al., 1997; Britto and Kronzucker, 2002), biodiversity loss (Stevens et al., 2004; Bobbink et al., 2010), and species extirpation (de Graaf et al., 1998; McClean et al., 2011). These major ecological and economic problems have been aggravated by an accelerated global nitrogen (N) cycle caused primarily by the industrialized production and use of N fertilizers (Galloway et al., 2008; Gruber and Galloway, 2008). With increasing global population and demands on agricultural production, there is no sign of this trend easing: anthropogenic N fixation has reached 210 teragrams year^–1, an approximately 12% increase from 2005 and an approximately 1,300% rise from 150 years ago (Galloway et al., 2008; Fowler et al., 2013).Although considerable knowledge of the causes and mechanisms of NH₃/NH₄⁺ toxicity has accrued in recent years, our understanding of the key processes remains rudimentary (Gerendas et al., 1997; Britto and Kronzucker, 2002). A major hypothesis is that of futile transmembrane NH₄⁺ cycling, which proposes a pathological inability of root cells to restrict the primary entry of NH₄⁺ at high external concentrations ([NH₄+]_ext); many downstream toxicological events are contingent upon this entry (Britto et al., 2001b). In this model, a rapid, thermodynamically passive influx of NH₄⁺ is coupled to an active efflux of NH₄⁺ that is nearly as rapid, constraining normal cellular function and energetics and resulting in plant growth decline and mortality. This phenomenon is thought to occur in NH₄⁺-sensitive species such as barley (Hordeum vulgare) and, to a lesser extent, in tolerant species such as rice (Oryza sativa), which can be susceptible at higher thresholds (Balkos et al., 2010; Chen et al., 2013).Most soils are typically acidic, especially when [NH₄⁺] is high (i.e. in the millimolar range; Van Breemen et al., 1982; Bobbink et al., 1998; Britto and Kronzucker, 2002), and given the pKa of 9.25 for the conjugate pair NH₃/NH₄⁺, [NH₃] is generally low (Izaurralde et al., 1990; Weise et al., 2013). Consequently, the fluxes of NH₃ have largely been considered negligible (Britto et al., 2001a; Britto and Kronzucker, 2002; Loqué and von Wirén, 2004), in contrast to NH₄⁺ fluxes, which are well characterized physiologically (Lee and Ayling, 1993; Wang et al., 1993a, 1993b; Kronzucker et al., 1996) and at the molecular level (Rawat et al., 1999; von Wirén et al., 2000; Ludewig et al., 2007), at least at lower concentrations. However, the transport of NH₃ across membranes has received new attention in the light of evidence that some members of the aquaporin (AQP) family of transporters, a diverse and ubiquitous class of major intrinsic proteins (Maurel et al., 2008; Hove and Bhave, 2011), can mediate NH₃ fluxes in single-cell systems (Jahn et al., 2004; Holm et al., 2005; Loqué et al., 2005; Saparov et al., 2007). However, a convincing demonstration that AQPs transport NH₃ in planta is currently lacking. Given the unusually high capacity of AQP-mediated fluxes relative to those of ion channels and other transporters (Kozono et al., 2002), it is possible that sizable NH₃ fluxes can be conducted through AQPs, even at very low external NH₃ concentration ([NH₃]_ext).Here, we have critically reexamined the hypothesis that futile cycling is composed of cationic NH₄⁺ fluxes across the plasmalemma, of which an active efflux mechanism accounts for energetic demands directly contributing to toxicity (Britto et al., 2001b). We present evidence for the following alternative scenario: 1) futile cycling consists mainly of the passive electroneutral flux of the conjugate base NH₃; 2) such fluxes rapidly span both major membrane systems in root cells (i.e. plasmalemma and tonoplast); 3) AQPs mediate such fluxes; and 4) a thermodynamic equilibrium of NH₃ is established throughout the cell, resulting in hyperaccumulation of NH₄⁺ in the acidic vacuole. This evidence comes primarily from positron emission tracing with the short-lived radioisotope ¹³N, used to characterize the component fluxes of futile cycling at the cellular level in the model species barley. We have coupled this with ⁴²K⁺ radiotracing, to provide comparison with a well-understood cationic flux, as well as electrophysiological, respiratory, pharmacological, and histochemical analyses.     

Longitudinal assessment of the effect of concentration on stream N uptake rates in an urbanizing watershed

We examined the effect of concentration on nitrogen uptake patterns for a suburban stream in Maryland and addressed the question: How does NO₃ ^? uptake change as a function of concentration and how do uptake patterns compare with those found for NH₄ ⁺? We applied a longitudinal (stream channel corridor) approach in a forested stream section and conducted short-term nutrient addition experiments in late summer 2004. In the downstream direction, NO₃ ^? concentrations decreased because of residential development in headwaters and downstream dilution; NH₄ ⁺ concentrations slightly increased. The uptake patterns for NO₃ ^? were very different from NH₄ ⁺. While NH₄ ⁺ had a typical negative relationship between first-order uptake rate constant (K _c ) and stream size, NO₃ ^? had a reverse pattern. We found differences for other metrics, including uptake velocity (V _f ) and areal uptake rate (U). We attributed these differences to a stream size effect, a concentration effect and a biological uptake capacity effect. For NO₃ ^? these combined effects produced a downstream increase in K _c , V _f and U; for NH₄ ⁺ they produced a downstream decrease in K _c and V _f , and a not well defined pattern for U. We attributed a downstream increase in NO₃ ^? uptake capacity to an increase in hyporheic exchange and a likely increase in carbon availability. We also found that K _c and V _f were indirectly related with concentration. Similar evidence of ‘nutrient saturation’ has been reported in other recent studies. Our results suggest that higher-order uptake models might be warranted when scaling NO₃ ^? uptake across watersheds that are subject to increased nitrogen loading.     

The effect of copper toxicity on the contents of nitrogen compounds in Silene vulgaris (Moench) Garcke

The effect of copper on the uptake of nitrogen and the tissue contents of inorganic nitrogen, amino acids and proteins were studied in cooper-sensitive Silene vulgaris (Moench) Garcke, grown at different nitrogen sources (NH₄ ⁺ or NO₃ ^-). All the toxic copper levels tested, i.e. 4, 8, 16 M Cu²⁺, strongly inhibited the uptake of nitrogen, especially of NO₃ ^-, and decreased the content of NO₃ ^-, amino acids and proteins. Especially at 4 and 8 M Cu²⁺, NH₄ ⁺ accumulated in the plants, suggesting that the conversion of NH₄ ^- into amino acids was inhibited.     

Analysis of applicability of triplet-state emission of molecular hydrogen for spectral diagnostics of a DC discharge

The applicability of emission of the N ³Λ_σ triplet states of molecular hydrogen for spectral diagnostics of the positive column of a dc glow discharge in hydrogen at translational gas temperatures of 360–600 K, specific absorbed powers of 0.8–4.25 W/cm, gas pressures of p = 0.3–15.0 Torr, reduced fields of E/N = 30–130 Td, and electron densities of n _e = 4.0 × 10⁹–6.5 × 10¹⁰ cm^–3 is analyzed by using an advanced level-based semi-empirical collisional?radiative model. It is found that secondary processes make the main contribution to the population and decay of the N ³Λ_σ = a ³Σ⁺ _g , c ³Π _u , g ³Σ⁺ _g , h ³Σ⁺ _g , and i ³Π _g triplet states. The dipole-allowed transitions e ³Σ⁺ _g → a ³Σ⁺ _g , f ³Σ⁺ _g → a ³Σ⁺ _g , g ³Σ⁺ _g and k ³Π _u → a ³Σ⁺ _g can be used for spectral diagnostics of a dc discharge within a simplified coronal model.