The dynamics of neutral sugars in the rhizosphere of wheat. An approach by13C pulse-labelling and GC/C/IRMS

Rhizodeposition, i.e. the release of carbon into the soil by growing roots, is an important part of the terrestrial carbon cycle. However thein situ nature and dynamics of root-derived carbon in the soil are still poorly understood. Here we made an investigation of the latter in laboratory experiments using¹³CO₂ pulse chase labelling of wheat (Triticum aestivum L.). We analyzed the kinetics of¹³C-labelled carbon and more specially¹³C carbohydrates in the rhizosphere. Wheat seedlings-soil mesocosms were exposed to¹³CO₂ for 5 hours in controlled chambers and sampled repeatedly during two weeks for¹³C/C analysis of organic carbon. After a two-step separation of the soil from the roots, the amount of total organic¹³C was determined by isotope ratio mass spectrometry as well as the amounts of¹³C in arabinose, fructose, fucose, glucose, galactose, mannose, rhamnose and xylose. The amount and isotopic ratio of monosaccharides were obtained by capillary gas chromatography coupled with isotope ratio mass spectrometry (GC/C/IRMS) after trimethyl-silyl derivatization. Two fractions were analyzed : total (hydrolysable) and soluble monomeric (water extractable) soil sugars. The amount of organic¹³C found in the soil, expressed as a percentage of the total photosynthetically fixed¹³C at the end of the labelling period, reached 16% in the day following labelling and stabilised at 9% after one week. We concluded that glucose under the form of polymers was the dominant moietie of rhizodeposits. Soluble glucose and fructose were also present. But after 2 days, these soluble sugars had disappeared. Forty percent of the root-derived carbon was in the form of neutral sugars, and exhibited a time-increasing signature of microbial sugars. The composition of rhizospheric sugars rapidly tended towards that of bulk soil organic matter.     

Estimating seasonal and annual carbon inputs,and root decomposition rates in a temperate pasture following field 14C pulse-labelling

Using a ¹⁴C pulse-labelling technique, we studied the seasonal changes in assimilation and partitioning of photoassimilated C in the plant–root–soil components of a temperate pasture. Pasture and soil samples were taken after 4-h, and 35-day chase periods, to examine these seasonal ¹⁴C fluxes. Total C and ¹⁴C were determined in the shoot, root and soil system. The amounts of C translocated annually to roots and soil were also estimated from the seasonal ¹⁴C distribution and pasture growth. The in situ field decomposition of newly formed roots during different seasons, also using ¹⁴C-labelling, was studied for one year in undisturbed rhizosphere soil. The ¹⁴C-labelled roots were sampled five times and decomposition rates were calculated assuming first-order decomposition.Annual pasture production at the site was 16 020 kg DM ha^–1, and pasture growth varied with season being highest (75–79 kg ha^–1 d^–1) in spring and lowest (18–20 kg ha^–1 d^–1) in winter. The above- and below-ground partitioning of ¹⁴C also varied with the season. The respiratory ¹⁴C–CO₂ losses, calculated as the difference between the total amounts of ¹⁴C recovered in the soil-plant system at 4 h and 35 days, were high (66–70%) during the summer, autumn and winter season, and low (37–39%) during the spring and late-spring season. Pasture plants partitioned more C below-ground during spring compared with summer, autumn and winter seasons. Overall, at this high fertility dairy pasture site, 18 220 kg C/ha was respired, 6490 kg remained above-ground in the shoot, and 6820 kg was translocated to roots and 1320 kg to soil. Root decomposition rate constant (k) differed widely with the season and were the highest for the autumn roots. The half-life was highest (111 days) for autumn roots and lowest (64 days) for spring roots. About one-third of the root label measured in the spring season disappeared in the first 5 weeks after the initial 35 Day of allocation period. The late spring, summer, late summer and winter roots had intermediate half-lives (88–94 days). These results indicate that seasonal changes in root growth and decomposition should be accounted for to give a better quantification of root turnover.     

Carbon allocation and carbon transfer between t Betula papyrifera and t Pseudotsuga menziesii seedlings using a 13C pulse-labeling method

Here we describe a simple method for pulse-labeling tree seedlings with ¹³CO_2(gas), and then apply the method in two related experiments: t (i) comparison of carbon allocation patterns between t Betula papyrifera Marsh. and t Pseudotsuga menziesii (Mirb.) Franco, and t (ii) measurement of one-way belowground carbon transfer from t B. papyrifera to t P. menziesii. Intraspecific carbon allocation patterns and interspecific carbon transfer both influence resource allocation, and consequently development, in mixed communities of t B. papyrifera and t P. menziesii.In preparation for the two experiments, we first identified the appropriate ¹³CO_2(gas) pulse-chase regime for labeling seedlings: a range of pulse (100-mL and 200-mL 99 atom%¹³ CO_2(gas)) and chase (0, 3 and 6 d) treatments were applied to one year-old t B. papyrifera and t P. menziesii seedlings. The amount of ¹³CO₂ fixed immediately after 1.5 h exposure was greatest for both t B. papyrifera (40.8 mg excess ¹³C) and t P. menziesii (22.9 mg excess ¹³C) with the 200-mL pulse, but higher ¹³C loss and high sample variability resulted in little difference in excess¹³ C content between pulse treatments after 3 d for either species. The average excess ¹³C root/shoot ratio of t B. papyrifera and t P. menziesii changed from 0.00 immediately following the pulse to 0.61 and 0.87 three and six days later, which reflected translocation of 75% of fixed isotope out of foliage within 3 d following the pulse and continued enrichment in fine roots over 6 d. Based on these results, the 100-mL CO_2(gas) and 6-d chase were considered appropriate for the carbon allocation and belowground transfer experiments.In the carbon allocation experiment, we found after 6 d that t B. papyrifera allocated 49% (average 9.5 mg) and t P. menziesii 41% (average 5.8 mg) of fixed isotope to roots, of which over 55% occurred in fine roots in both species. Species differences in isotope allocation patterns paralleled differences in tissue biomass distribution. The greater pulse labeling efficiency of t B. papyrifera compared to t P. menziesii was associated with its two-fold and 13- fold greater leaf and whole seedling net photosynthetic rates, respectively, 53% greater biomass, and 35% greater root/shoot ratio.For the carbon transfer experiment, t B. papyrifera and t P. menziesii were grown together in laboratory rootboxes, with their roots intimately mingled. A pulse of 100 mL¹³ CO_2(gas) was applied to paper birch and one-way transfer to neighboring t P. menziesii was measured after 6 d. Of the excess ¹³C fixed by t B. papyrifera, 4.7% was transferred to neighboring t P. menziesii, which distributed the isotope evenly between roots and shoots. Of the isotope received by t P. menziesii, we estimated that 93% was taken up through belowground pathways, and the remaining 7% taken up by foliage as¹³ CO_2(gas) respired by t B. papyrifera shoots. These two experiments indicate that t B. papyrifera fixes more total carbon and allocates a greater proportion to its root system than does t P. menziesii, giving it a competitive edge in resource gathering; however, below-ground carbon sharing is of sufficient magnitude that it may help ensure co-existence of the two species in mixed communities.     

Contribution of flexible allocation priorities to herbivory tolerance in C4 perennial grasses: an evaluation with 13C labeling

The ability of plants to rapidly replace photosynthetic tissues following defoliation represents a resistance strategy referred to as herbivory tolerance. Rapid reprioritization of carbon allocation to regrowing shoots at the expense of roots following defoliation is a widely documented tolerance mechanism. An experiment was conducted in a controlled environment to test the hypothesis that herbivory-sensitive perennial grasses display less flexibility in reprioritizing carbon allocation in response to defoliation than do grasses possessing greater herbivory tolerance. An equivalent proportion of shoot biomass (60% dry weight) was removed from two C₄ perennial grasses recognized as herbivory-sensitive, Andropogon gerardii and Schizachyrium scoparium, and two C₄ perennial grasses recognized as herbivory-tolerant, Aristida purpurea and Bouteloua rigidiseta. Both defoliated and undefoliated plants were exposed to ¹³CO₂ for 30 min, five plants per species were harvested at 6, 72 and 168 h following labeling, and biomass was analyzed by isotope ratio mass spectrometry. The tallgrass, A. geraiddii, exhibited inflexible allocation priorities while the shortgrass, B. rigidiseta, exhibited flexible allocation priorities in response to defoliation which corresponded with their initial designations as herbivory-sensitive and herbivory-tolerant species, respectively. A. gerardii had the greatest percentage and concentration of ¹³C within roots and lowest percentage of ¹³C within regrowth of the four species evaluated. In contrast, B. rigidiseta had a greater percentage of ¹³C within regrowth than did A. gerardii, the greatest percentage of ¹³C within new leaves of defoliated plants, and the lowest concentration of ¹³C within roots follwing defoliation. Although both midgrasses, S. scoparium and A. purpurea, demonstrated flexible allocation priorities in response to defoliation, they were counter to those stated in the initial hypothesis. The concentration of ¹³C within new leaves of S. scoparium increased in response to a single defoliation while the percentage and concentration of ¹³C within roots was reduced. A. purpurea was the only species in which the percentate of ¹³C within new leaves decreased while the percentage of ¹³C within roots increased following defoliation. The most plausible alternative hypothesis to explain the inconsistency between the demonstrated responsiveness of allocation priorities to defoliation and the recognized herbivory resistance of S. scoparium and A. purpurea is that the relative ability of these species to avoid herbivory may make an equal or greater contribution to their overall herbivory resistance than does herbivory tolerance. Selective herbivory may contribute to S. scoparium's designation as a herbivorysensitive species even though it possesses flexible allocation priorities in response to defoliation. Alternatively, the recognized herbivory resistance of A. purpurea may be a consequence of infrequent and/or lenient herbivory associated with the expression of avoidance mechanisms, rather than the expression of tolerance mechanisms. A greater understanding of the relative contribution of tolerance and avoidance strategies of herbivory resistance are required to accurately interpret how herbivory influences plant function, competitive interactions, and species abundance in grazed communities.     

An Enzymatic Preparation of UDP (U-13C) Glucose

Uniformly labeled uridine diphosphoglucose (UDP(U-¹³C)G) was prepared by a two-step enzymatic synthesis. (U-¹³C) G-6-P was prepared quantitatively by incubating (U-¹³C) glucose, ATP, MgS0₄, and hexokinase. UDP(U-¹³C) Glucose was prepared by incubation of (U-¹³C)G-6-P with UDPG pyrophosphorylase, phosphoglucomutase, inorganic pyrophosphatase, UTP, and glucose-1, 6-diphosphate in pH 7.5, 100 mM Tris-HCl buffer. After purification over Biogel P-2 and subsequent preparative HPLC, UDP (U-¹³C)G was obtained in 50% yield. UDP(U-¹³C)G was characterized by ¹³C NMR and FAB-MS.     

Effect of grazing on carbon stocks and assimilate partitioning in a Tibetan montane pasture revealed by 13CO2 pulse labeling

Since the late 1950s, governmental rangeland policies have changed the grazing management on the Tibetan Plateau (TP). Increasing grazing pressure and, since the 1980s, the privatization and fencing of pastures near villages has led to land degradation, whereas remote pastures have recovered from stronger overgrazing. To clarify the effect of moderate grazing on the carbon (C) cycle of the TP, we investigated differences in below‐ground C stocks and C allocation using in situ ¹³CO₂ pulse labeling of (i) a montane Kobresia winter pasture of yaks, with moderate grazing regime and (ii) a 7‐year‐old grazing exclosure plot, both in 3440 m asl. Twenty‐seven days after the labeling, ¹³C incorporated into shoots did not differ between the grazed (43% of recovered ¹³C) and ungrazed (38%) plots. In the grazed plots, however, less C was lost by shoot respiration (17% vs. 42%), and more was translocated below‐ground (40% vs. 20%). Within the below‐ground pools, <2% of ¹³C was incorporated into living root tissue of both land use types. In the grazed plots about twice the amount of ¹³C remained in soil (18%) and was mineralized to CO₂ (20%) as compared to the ungrazed plots (soil 10%; CO₂ 9%). Despite the higher contribution of root‐derived C to CO₂ efflux, total CO₂ efflux did not differ between the two land use types. C stocks in the soil layers 0–5 and 5–15 cm under grazed grassland were significantly larger than in the ungrazed grassland. However, C stocks below 15 cm were not affected after 7 years without grazing. We conclude that the larger below‐ground C allocation of plants, the larger amount of recently assimilated C remaining in the soil, and less soil organic matter‐derived CO₂ efflux create a positive effect of moderate grazing on soil C input and C sequestration.     

Natural 13C distribution in oil palm (Elaeis guineensis Jacq.) and consequences for allocation pattern

Oil palm has now become one of the most important crops, palm oil representing nearly 25% of global plant oil consumption. Many studies have thus addressed oil palm ecophysiology and photosynthesis‐based models of carbon allocation have been used. However, there is a lack of experimental data on carbon fixation and redistribution within palm trees, and important C‐sinks have not been fully characterized yet. Here, we carried out extensive measurement of natural ¹³C‐abundance (δ¹³C) in oil palm tissues, including fruits at different maturation stages. We find a ¹³C‐enrichment in heterotrophic organs compared to mature leaves, with roots being the most ¹³C‐enriched. The δ¹³C in fruits decreased during maturation, reflecting the accumulation in ¹³C‐depleted lipids. We further used observed δ¹³C values to compute plausible carbon fluxes using a steady‐state model of ¹³C‐distribution including metabolic isotope effects (¹²v/¹³v). The results suggest that fruits represent a major respiratory loss (≈39% of total tree respiration) and that sink organs such as fruits are fed by sucrose from leaves. That is, glucose appears to be a quantitatively important compound in palm tissues, but computations indicate that it is involved in dynamic starch metabolism rather that C‐exchange between organs.     

Fate of carbon in upland grassland subjected to liming using in situ 13CO2 pulse-labelling

Knowledge of the fate of plant assimilate is fundamental to our understanding of the terrestrial carbon cycle, particularly if we are to predict the effects of changes in climate and land management practices on agroecosystems. Pulse-labelling experiments have revealed that some of the carbon fixed by plants is rapidly allocated below-ground and released back into the atmosphere in respiration. However, little is known about the fate of plant assimilate, not accounted for in soil respiration, in the longer term and how current management practices such as liming may affect this. In southern Scotland, UK, limed and unlimed acid grassland plots were pulse-labelled with ¹³CO₂ and the turnover of ¹³C was studied one and two years after labelling. In this study the amount of labelled carbon remaining in shoot, root, and bulk soil pools, and how this differed between limed and unlimed plots was investigated. The results indicated that plant-root turnover was faster, and plants invested less nitrogen in the roots in the limed plots than in the unlimed plots. More ¹³C remained in the soil in the unlimed treatment compared to the limed treatment, but the main difference was found in the particulate organic matter, which turned over relatively quickly. The label was still above natural abundance one and two years after labelling in many cases. In addition, the results demonstrate that a ¹³CO₂ pulse-label administered for only a few hours can be a useful approach for investigating turnover of carbon several years later.     

Cultivation effects on the nature of organic matter in soils and water extracts using CP/MAS 13C NMR spectroscopy

The objective of this study was to examine the chemical structure of the organic matter (SOM) of Oxisols soils in slash and burn agriculture, in relation to its biological properties and soil fertility. The CP/MAS ¹³C technique was used to identify the main structural groups in litter and fine roots as SOM precursors; to identify the changes on the nature of the SOM upon cultivation and the proportion of labile and stable components; and to identify the nature of the organics present in water extracts (DOC). Carbohydrates were the main structural components in litter whereas components such as carbonyl C, carboxyl C,O-alkyl C and alkyl C were more common in SOM. Phenolic C and the degree of aromaticity were similar in litter and SOM. Cultivation resulted in a small decrease in the relative proportion of carbohydrates in SOM, little change in the levels of O-alkyl C and carbonyl C, but an increase in carboxyl C, phenolic C and aromaticity of the SOM. The level of alkyl C in soil was higher than the level of O-alkyl C, indicating the importance of long-chain aliphatics along with lignins in the stabilization of the SOM in Oxisols. The SOM of Mollisols from the Canadian Prairies differed from the Oxisol, with a generally stronger expression of aromatic structures, particularly in a cultivated soil in relation to a native equivalent. Carbohydrate components were the predominant structures in the DOC, indicating their importance in nutrient cycling and vertical translocations in the Oxisol.     

Pulse-labelling a cover crop with 13C to follow its decomposition in soil under field conditions

A preliminary study was conducted using the stable isotope ¹³C to pulse label the cover crop phacelia (Phacelia tanacetifolia) to examine its decomposition in soil, under field conditions. Plants were grown, in pots, in the greenhouse and after four weeks of growth were labelled with ¹³CO₂ six times, at 1–2 week intervals. A single chamber was placed over the pots, and ¹³CO₂ was generated, inside the chamber, by injecting lactic acid into sodium carbonate (99 atom % ¹³C). For calculating the quantity of Na₂CO₃ required, a target enrichment of 5 atom% ¹³C within the shoots of plants, assuming no respiration losses, was used. When harvested, at flowering, the mean enrichment of the shoot material was 3.0466 atom% ¹³C, or 1.9654 atom% excess ¹³C. To assess uniformity of labelling within plants, the shoot of a single plant was divided into leaves and stem from three sections of equal length. Ninety-three percent of this plant's dry matter had a ¹³C enrichment within 20 % of the weighted mean. At a field site with sandy soil, ¹³C labelled shoot and root material were combined and mixed with soil (0–15 cm). The soil was sampled 16 and 179 days later to determine the recovery of the added excess ¹³C in soil total C. The recoveries in soil (0–30 cm) were, respectively, 78 and 40 % at 16 and 179 days; there was appreciable variation associated with the recovery data from day 16, much less so at day 179. Methodological procedures for (i) enhancing the uniformity of labelling with ¹³C within plants, and (ii) minimising variability in the recovery of ¹³C from soil are suggested. ei]R Merckx     

(13)C CP MAS NMR and crystal structure of methyl glycopyranosides

The X-ray diffraction analysis, (13)C CP MAS NMR spectra and powder X-ray diffraction patterns were obtained for selected methyl glycosides: alpha- and beta-d-lyxopyranosides (1, 2), alpha- and beta-l-arabinopyranosides (3, 4), alpha- and beta-d-xylopyranosides (5, 6) and beta-d-ribopyranoside (7) and the results were confirmed by GIAO DFT calculations of shielding constants. In X-ray diffraction analysis of 1 and 2, a characteristic shortening and lengthening of selected bonds was observed in molecules of 1 due to anomeric effect and, in crystal lattice of 1 and 2, hydrogen bonds of different patterns were present. Also, an additional intramolecular hydrogen bond with the participation of ring oxygen atom was observed in 1. The observed differences in chemical shifts between solid state and solution come from conformational effects and formation of various intermolecular hydrogen bonds. The changes in chemical shifts originating from intermolecular hydrogen bonds were smaller in magnitude than conformational effects. Furthermore, the powder X-ray diffraction (PXRD) performed for 4, 5 and 7 revealed that 7 existed as a mixture of two polymorphs, and one of them probably consisted of two non-equivalent molecules.     

A (13)C NMR study on the interactions of calcium chloride/potassium chloride with pyranosides in D(2)O

The (13)C NMR spectra of methyl beta-d-glucopyranoside, methyl beta-d-galactopyranoside, methyl beta-d-xylopyranoside, and methyl beta-l-arabinopyranoside were recorded in CaCl(2)/KCl+D(2)O mixtures and in D(2)O. The chemical shifts of C-1, C-3, and C-5 in the methyl beta-d-glucopyranoside and methyl beta-d-galactopyranoside decrease rapidly as molalities of CaCl(2)/KCl increase, while those of C-1, C-2, and C-3 in the methyl beta-d-xylopyranoside and methyl beta-l-arabinopyranoside decrease rapidly as molalities of CaCl(2)/KCl increase. Cations (Ca(2+)/K(+)) can weakly complex with O in OMe of the pyranosides studied. Results are discussed in terms of the stereochemistry of the pyranoside molecules and the structural properties of the ions.     

Rat brain slices oxidize glucose at high rates: a (13)C NMR study

Since glucose is the main cerebral substrate, we have characterized the metabolism of various ¹³C glucose isotopomers in rat brain slices. For this, we have used our cellular metabolomic approach that combines enzymatic and carbon 13 NMR techniques with mathematical models of metabolic pathways. We identified the fate and the pathways of the conversion of glucose carbons into various products (pyruvate, lactate, alanine, aspartate, glutamate, GABA, glutamine and CO₂) and determined absolute fluxes through pathways of glucose metabolism. After 60 min of incubation, lactate and CO₂ were the main end-products of the metabolism of glucose which was avidly metabolized by the slices. Lactate was also used at high rates by the slices and mainly converted into CO₂. High values of flux through pyruvate carboxylase, which were similar with glucose and lactate as substrate, were observed. The addition of glutamine, but not of acetate, stimulated pyruvate carboxylation, the conversion of glutamate into succinate and fluxes through succinate dehydrogenase, malic enzyme, glutamine synthetase and aspartate aminotransferase. It is concluded that, unlike brain cells in culture, and consistent with high fluxes through PDH and enzymes of the tricarboxylic acid cycle, rat brain slices oxidized both glucose and lactate at high rates.     

Assimilate partitioning affects 13C fractionation of recently assimilated carbon in maize

Coupling ¹³C natural abundance and ¹⁴C pulse labelling enabled us to investigate the dependence of ¹³C fractionation on assimilate partitioning between shoots, roots, exudates, and CO₂ respired by maize roots. The amount of recently assimilated C in these four pools was controlled by three levels of nutrient supply: full nutrient supply (NS), 10 times diluted nutrient supply (DNS), and deionised water (DW). After pulse labelling of maize shoots in a ¹⁴CO₂ atmosphere, ¹⁴C was traced to determine the amounts of recently assimilated C in the four pools and the δ¹³C values of the four pools were measured. Increasing amounts of recently assimilated C in the roots (from 8% to 10% of recovered ¹⁴C in NS and DNS treatments) led to a 0.3‰ ¹³C enrichment from NS to DNS treatments. A further increase of C allocation in the roots (from 10% to 13% of recovered ¹⁴C in DNS and DW treatments) resulted in an additional enrichment of the roots from DNS to DW treatments by 0.3‰. These findings support the hypothesis that ¹³C enrichment in a pool increases with an increasing amount of C transferred into that pool. δ¹³C of CO₂ evolved by root respiration was similar to that of the roots in DNS and DW treatments. However, if the amount of recently assimilated C in root respiration was reduced (NS treatment), the respired CO₂ became 0.7‰ ¹³C depleted compared to roots. Increasing amounts of recently assimilated C in the CO₂ from NS via DNS to DW treatments resulted in a 1.6‰ δ¹³C increase of root respired CO₂ from NS to DW treatments. Thus, for both pools, i.e. roots and root respiration, increasing amounts of recently assimilated C in the pool led to a δ¹³C increase. In DW and DNS plants there was no ¹³C fractionation between roots and exudates. However, high nutrient supply decreased the amount of recently assimilated C in exudates compared to the other two treatments and led to a 5.3‰ ¹³C enrichment in exudates compared to roots. We conclude that ¹³C discrimination between plant pools and within processes such as exudation and root respiration is not constant but strongly depends on the amount of C in the respective pool and on partitioning of recently assimilated C between plant pools. Section Editor: H. Lambers     

Time-saving methods for heteronuclear multidimensional NMR of (13C, 15N) doubly labeled proteins

Summary Heteronuclear 2D (¹³C, ¹H) and (¹⁵N, ¹H) correlation spectra of (¹³C, ¹⁵N) fully enriched proteins can be acquired simultaneously with virtually no sensitivity loss or increase in artefact levels. Three pulse sequences are described, for 2D time-shared or TS-HSQC, 2D TS-HMQC and 2D TS-HSMQC spectra, respectively. Independent spectral widths can be sampled for both heteronuclei. The sequences can be greatly improved by combining them with field-gradient methods. By applying the sequences to 3D and 4D NMR spectroscopy, considerable time savings can be obtained. The method is demonstrated for the 18 kDa HU protein.Abbreviations HMQC heteronuclear multiple-quantum coherence spectroscopy - HSQC heteronuclear single-quantum coherence spectroscopy - HSMQC heteronuclear single- and multiple-quantum coherence spectroscopy - NOESY nuclear Overhauser enhancement spectroscopy     

Metabolism of [1-13C]glucose in a synaptosomally enriched fraction of rat cerebrum studied by1H/13C magnetic resonance spectroscopy

This study explored the utility of¹H and¹³C magnetic resonance spectroscopy to study a standard synaptosomally enriched fraction (P2 pellet) made from rat cerebrum. The preparations contained high concentrations of N-acetylaspartate and -aminobutyric acid and low concentrations of glutamine, indicating that they were in fact rich in neuronal cytosol. The metabolic competence of the preparation was assessed by quantitative measurements of its ability to convert [1-¹³C]glucose into lactate, glutamate, aspartate, and other metabolites under well oxygenated conditions in 30 minutes. The minimum mean glycolytic rate was 0.8 mM glucose/min and the flow through the tricarboxylic acid cycle was equivalent to 0.2 mM glucose/min.Abbreviations ppm parts per million (chemical shift scale) - NMR nuclear magnetic resonance - GABA -aminobutyric acid - PBS phosphate-buffered normal saline solution - TSP 3-trimethylsilylpropionate During the performance of these studies Dr. A.P. Burlina was on leave from Instituto di Clinica delle Malattie Nervose e Mentali, University of Padua, Padua, Italy.     

Two-dimensional rotating-frame Overhauser spectroscopy (ROESY) and (13)C NMR study of the interactions between maltodextrin and an anionic surfactant

Rotational frame nuclear Overhauser effect spectroscopy (ROESY) and (13)C NMR measurements were carried out to study the molecular interaction between maltodextrin, a digestive byproduct of starch, and an anionic surfactant. Significant differences in chemical shifts were observed when sodium dodecyl sulfate (SDS) was introduced into the maltodextrin (DE 10) solutions. (13)C NMR measurement indicated that there were downfield shifts and broadening of peaks, especially in the region of 75-81 and 100-103 ppm, which were assigned to carbons 1 and 4 of the d-glucopyranose residues of maltodextrin, respectively. ROESY spectra indicated cross-peaks between the SDS and maltodextrin protons. These peaks can arise only in the case of the designated SDS protons and maltodextrin protons being less than 0.5 nm apart for a substantial period of time. The most intense cross-peaks are those between the central CH(2) protons of SDS near 1.2 ppm and the maltodextrin protons ranging from 3.5 to 3.9 ppm. The SDS-H3 CH(2) protons were resolved from the bulk of the SDS protons, with peaks and shoulders at 1.25 ppm, which indicated an especially strong interaction of the SDS hydrophobic tail with MD6 and some less intense interactions with MD2, 4, and 5.     

Compound-specific deltaD-delta13C analyses of n-alkanes extracted from terrestrial and aquatic plants

Stable hydrogen and carbon isotopic compositions of individual n-alkanes were determined for various terrestrial plants (33 samples including 27 species) and aquatic plants (six species) in natural environments from Japan and Thailand. In C3 plants, n-alkanes extracted from angiosperms have a deltaD value of -152+/-26 per thousand (relative to Standard Mean Ocean Water [SMOW]) and delta13C value of -36.1+/-2.7 per thousand (relative to Peedde Belemnite [PDB]), and those from gymnosperms have a deltaD value of -149+/-16 per thousand and delta13C value of -31.6+/-1.7 per thousand. Angiosperms have n-alkanes depleted in 13C relative to gymnosperms. n-Alkanes from C4 plants have a deltaD value of -171+/-12 per thousand and delta13C value of -20.5+/-2.1 per thousand, being a little depleted in D and much enriched in 13C compared to C3 plants. n-Alkanes of CAM plants are a little depleted in D and vary widely in delta13C relative to those of C3 and C4 plants. In aquatic plants, n-alkanes from freshwater plants have a deltaD value of -187+/-16 per thousand and delta13C value of -25.3+/-1.9 per thousand, and those from seaweeds have a deltaD value of -155+/-34 per thousand and delta13C value of -22.8+/-1.0 per thousand. All n-alkanes from various plant classes are more depleted in D and 13C relative to environmental water and bulk tissue, respectively. In addition, the hydrogen and carbon isotopic fractionations during n-alkane synthesis are distinctive for these various plant classes. While C3 plants have smaller isotopic fractionations in both D and 13C, seaweed has larger isotopic fractionations.     

Present dynamics of the savanna-forest boundary in the Congolese Mayombe: a pedological,botanical and isotopic (13C and 14C) study

Isolated savannas enclosed by forest are especially abundant in the eastern part of the Congolese Mayombe. They are about 3000 years old, and were more extensive some centuries ago. The boundary between forest and savanna is very abrupt, as a consequence of the numerous savanna fires lit by hunters. Floristic composition and vegetation structure data, organic carbon ratios, ¹⁴C and ¹³C measurements presented here show that forest is spreading over savanna at the present time and suggest that the rate of forest encroachment is is currently between 14 and 75 m per century, and more probably about 20–50 m per century. As most savannas are less than 1 km across, such rates mean, assuming there are no changes in environmental conditions, that enclosed savannas could completely disappear in the Mayombe in about 1000–2000 years.