Viewpoint: Isotopic fractionation by plant nitrate reductase, twenty years later

Quantification of Glucose Metabolism and Nitrogen Utilization in Two Brassicaceae Species under Bicarbonate and Variable Ammonium Soil Conditions

In karst habitats under drought conditions, high bicarbonate (high pH), and an abundant nitrate soil environment, bicarbonate regulates the glycolysis (EMP) and pentose phosphate pathways (PPP), which distribute ATP and NADPH, affecting nitrate (NO3-) and ammonium (NH4+) utilization in plants. However, the relationship between EMP PPP and NO3-, and NH4+ utilization and their responses to bicarbonate and variable ammonium still remains elusive. In this study, we used Brassica napus (Bn, a non-karst-adaptable plant) and Orychophragmus violaceus (Ov, a karst-adaptable plant) as plant materials, employed a bidirectional nitrogen-isotope-tracing method, and performed the quantification of the contribution of EMP and PPP. We found that bicarbonate and ammonium inhibited glucose metabolism and nitrogen utilization in Bn under simulated karst habitats. On the other hand, it resulted in a shift from EMP to PPP to promote ammonium utilization in Ov under high ammonium stress in karst habitats. Compared with Bn, bicarbonate promoted glucose metabolism and nitrogen utilization in Ov at low ammonium levels, leading to an increase in photosynthesis, the PPP, carbon and nitrogen metabolizing enzyme activities, nitrate/ammonium utilization, and total inorganic nitrogen assimilation capacity. Moreover, bicarbonate significantly reduced the growth inhibition of Ov by high ammonium, resulting in an improved PPP, RCRUBP, and ammonium utilization to maintain growth. Quantifying the relationships between EMP, PPP, NO3-, and NH4+ utilization can aid the accurate analysis of carbon and nitrogen use efficiency changes in plant species. Therefore, it provides a new prospect to optimize the nitrate/ammonium utilization in plants and further reveals the differential responses of inorganic carbon and nitrogen (C-N) metabolism to bicarbonate and variable ammonium in karst habitats.

Implication of quantifying nitrate utilization and CO2 assimilation of Brassica napus plantlets in vitro under variable ammonium/nitrate ratios

BACKGROUND

Plantlets grown in vitro with a mixed nitrogen source utilize sucrose and CO₂ as carbon sources for growth. However, it is very difficult to obtain the correct utilization proportions of nitrate, ammonium, sucrose and CO₂ for plantlets. Consequently, the biological effect of ammonium/nitrate utilization, the biological effect of sucrose/CO₂ utilization, and the ammonium/nitrate use efficiency for new C input derived from CO₂ assimilation/sucrose utilization are still unclear for plantlets.

RESULTS

The bidirectional stable nitrogen isotope tracer technique quantified the proportions of assimilated nitrate and ammonium in Brassica napus plantlets grown at different ammonium/nitrate ratios. The utilization proportions of sucrose and CO₂ could be quantified by a two end-member isotope mixing model for Bn plantlets grown at different ammonium/nitrate ratios. Under the condition that each treatment contained 20 mM ammonium, the proportion of assimilated nitrate did not show a linear increase with increasing nitrate concentration for Bn plantlets. Moreover, the proportion of assimilated CO₂ did not show a linear relationship with the nitrate concentration for Bn plantlets. Increasing the nitrate concentration contributed to promoting the assimilation of ammonium and markedly enhanced the ammonium utilization coefficient for Bn plantlets. With increasing nitrate concentration, the amount of nitrogen in leaves derived from nitrate assimilation increased gradually, while the nitrate utilization coefficient underwent no distinct change for Bn plantlets.

CONCLUSIONS

Quantifying the utilization proportions of nitrate and ammonium can reveal the energy efficiency for N assimilation in plantlets grown in mixed N sources. Quantifying the utilization proportion of CO₂ contributes to evaluating the photosynthetic capacity of plantlets grown with variable ammonium/nitrate ratios. Quantifying the utilization proportions of nitrate, ammonium, sucrose and CO₂ can reveal the difference in the ammonium/nitrate use efficiency for new C input derived from CO₂ assimilation/sucrose utilization for plantlets grown at variable ammonium/nitrate ratios.

N uptake, assimilation and isotopic fractioning control δ 15N dynamics in plant DNA: A heavy labelling experiment on Brassica napus L

In last decades, a large body of evidence clarified nitrogen isotope composition (δ¹⁵N) patterns in plant leaves, roots and metabolites, showing isotopic fractionation along N uptake and assimilation pathways, in relation to N source and use efficiency, also suggesting ¹⁵N depletion in plant DNA. Here we present a manipulative experiment on Brassica napus var. oleracea, where we monitored δ ¹⁵N of purified, lyophilized DNA and source leaf and root materials, over a 60-days growth period starting at d 60 after germination, in plants initially supplied with a heavy labelled (δ ¹⁵N_Air-N2 = 2100 mUr) ammonium nitrate solution covering nutrient requirements for the whole observation period (470 mg N per plant) and controlling for the labelled N species (ṄH₄, ṄO₃ and both). Dynamics of Isotopic Ratio Mass Spectrometry (IRMS) data for the three treatments showed that: (1) leaf and root δ ¹⁵N dynamics strictly depend on the labelled chemical species, with ṄH₄, ṄO₃ and ṄH₄ṄO₃ plants initially showing higher, lower and intermediate values, respectively, then converging due to the progressive NH₄⁺ depletion from the nutrient solution; (2) in ṄH₄ṄO₃, where δ¹⁵N was not affected by the labelled chemical species, we did not observe isotopic fractionation associated to inorganic N uptake; (3) δ¹⁵N values in roots compared to leaves did not fully support patterns predicted by differences in assimilation rates of NH₄⁺ and NO₃^-; (4) DNA is depleted in ¹⁵N compared to the total N pools of roots and leaves, likely due to enzymatic discrimination during purine biosynthesis. In conclusion, while our experimental setup did not allow to assess the fractionation coefficient (ε) associated to DNA bases biosynthesis, this is the first study specifically reporting on dynamics of specific plant molecular pools such as nucleic acids over a long observation period with a heavy labelling technique.

Isotopic composition and concentration of total nitrogen and nitrate in xylem sap under near steady-state hydroponics

After root uptake, nitrate is effluxed back to the medium, assimilated locally, or translocated to shoots. Rooted black cottonwood (Populus trichocarpa) scions were supplied with a NO₃^- -based (0.5 mM) nutrient medium of known isotopic composition (δ¹⁵ N), and xylem sap was collected by pressure bombing. To establish a sampling protocol, sap was collected from lower and upper stem sections at 0.1-0.2 MPa above the balancing pressure, and after increasing the pressure by a further 0.5 MPa. Xylem sap from upper stem sections was partially diluted at higher pressure. Further analysis was restricted to sap obtained from intact shoots at low pressure. Total-, NO₃^- -N and, by difference, organic-N concentrations ranged from 6.1-11.0, 1.2-2.4, and 4.6-9.4 mM, while discrimination relative to the nutrient medium was -6.3 to 0.5‰, -23.3 to -11.5‰ and - 1.3 to 4.9‰, respectively. There was diurnal variation in δ¹⁵ N of total- and organic-N, but not NO₃^- . The difference in δ¹⁵ N between xylem NO₃^- and organic-N suggests that discrimination by nitrate reductase is near 25.1 ± 1.6‰. When this value was used in an isotope mass balance model, the predicted xylem sap NO₃^- -N to total-N ratio closely matched direct measurement.

δ15 N values in plants are determined by both nitrate assimilation and circulation

Nitrogen (N) assimilation is associated with ¹⁴ N/¹⁵ N fractionation such that plant tissues are generally ¹⁵ N-depleted compared to source nitrate. In addition to nitrate concentration, the δ¹⁵ N value in plants is also influenced by isotopic heterogeneity amongst organs and metabolites. However, our current understanding of δ¹⁵ N values in nitrate is limited by the relatively small number of compound-specific data. We extensively measured δ¹⁵ N in nitrate at different time points, in sunflower and oil palm grown at fixed nitrate concentration, with nitrate circulation being varied using potassium (K) conditions and waterlogging. There were strong interorgan δ¹⁵ N differences for contrasting situations between the two species, and a high ¹⁵ N-enrichment in root nitrate. Modelling shows that this ¹⁵ N-enrichment can be explained by nitrate circulation and compartmentalisation whereby despite a numerically small flux value, the backflow of nitrate to roots via the phloem can lead to a c. 30‰ difference between leaves and roots. Accordingly, waterlogging and low K conditions, which down-regulate sap circulation, cause a decrease in the leaf-to-root isotopic difference. Our study thus suggests that plant δ¹⁵ N can be used as a natural tracer of N fluxes between organs and highlights the potential importance of δ¹⁵ N of circulating phloem nitrate.

Nitrogen fertilisation influences low CO2 effects on plant performance

Low atmospheric CO2 conditions prevailed for most of the recent evolutionary history of plants. Such concentrations reduce plant growth compared with modern levels, but low-CO2 effects on plant performance may also be affected by nitrogen availability, since low leaf nitrogen decreases photosynthesis, and CO2 concentrations influence nitrogen assimilation. To investigate the influence of N availability on plant performance at low CO2, we grew Elymus canadensis at ambient (~400 μmol mol-1) and subambient (~180 μmol mol-1) CO2 levels, under four N-treatments: nitrate only; ammonium only; a full and a half mix of nitrate and ammonium. Growth at low CO2 decreased biomass in the full and nitrate treatments, but not in ammonium and half plants. Low CO2 effects on photosynthetic and maximum electron transport rates were influenced by fertilisation, with photosynthesis being most strongly impacted by low CO2 in full plants. Low CO2 reduced stomatal index in half plants, suggesting that the use of this indicator in paleo-inferences can be influenced by N availability. Under low CO2 concentrations, nitrate plants discriminated more against 15N whereas half plants discriminated less against 15N compared with the full treatment, suggesting that N availability should be considered when using N isotopes as paleo-indicators.

Foliar uptake of atmospheric nitrate by two dominant subalpine plants: insights from in situ triple-isotope analysis

The significance of foliar uptake of nitrogen (N) compounds in natural conditions is not well understood, despite growing evidence of its importance to plant nutrition. In subalpine meadows, N-limitation fosters the dominance of specific subalpine plant species, which in turn ensures the provision of essential ecosystems services. Understanding how these plants absorb N and from which sources is important in predicting ecological consequences of increasing N deposition. Here, we investigate the sources of N to plants from subalpine meadows with distinct land-use history in the French Alps, using the triple isotopes (Δ¹⁷ O, δ¹⁸ O, and δ¹⁵ N) of plant tissue nitrate (NO₃^- ). We use this approach to evaluate the significance of foliar uptake of atmospheric NO₃^- (NO₃^-_atm ). The foliar uptake of NO₃^-_atm accounted for 4-16% of the leaf NO₃^- content, and contributed more to the leaf NO₃^- pool after peak biomass. Additionally, the gradual ¹⁵ N enrichment of NO₃^- from the soil to the leaves reflected the contribution of NO₃^-_atm assimilation to plants' metabolism. The present study confirms that foliar uptake is a potentially important pathway for NO₃^-_atm into subalpine plants. This is of major significance as N emissions (and deposition) are predicted to increase globally in the future.

Differential contributions of NO3 -/NH4 + to nitrogen use in response to a variable inorganic nitrogen supply in plantlets of two Brassicaceae species in vitro

BACKGROUND

The primary sources of nitrogen for plants have been suggested to be nitrate (NO3 -) and ammonium (NH4 +). However, when both nitrate and ammonium are simultaneously available to plants, it is very difficult to differentially quantify NO3 -/NH4 + utilization in culture media or soil. Consequently, the contribution of NO3 -/NH4 + to total inorganic nitrogen assimilation cannot be determined.

RESULTS

We developed a method called the bidirectional stable nitrogen isotope tracer to differentially quantify the nitrate and ammonium utilization by Orychophragmus violaceus (Ov) and Brassica napus (Bn) plantlets in vitro. The utilization efficiency of nitrate was markedly lower than the utilization efficiency of ammonium for plantlets of both Ov and Bn. In both Ov and Bn, the proportion of NO3 -/NH4 + utilization did not show a linear relationship with inorganic nitrogen supply. The Ov plantlets assimilated more nitrate than the Bn plantlets at the lowest inorganic nitrogen concentration.

CONCLUSIONS

Quantifying the utilization of nitrate and ammonium can reveal the differences in nitrate and ammonium assimilation among plants at different inorganic nitrogen supply levels and provide an alternate way to conveniently optimize the supply of inorganic nitrogen in culture media.

Nitrate is an important nitrogen source for Arctic tundra plants

Plant nitrogen (N) use is a key component of the N cycle in terrestrial ecosystems. The supply of N to plants affects community species composition and ecosystem processes such as photosynthesis and carbon (C) accumulation. However, the availabilities and relative importance of different N forms to plants are not well understood. While nitrate (NO3-) is a major N form used by plants worldwide, it is discounted as a N source for Arctic tundra plants because of extremely low NO3- concentrations in Arctic tundra soils, undetectable soil nitrification, and plant-tissue NO3- that is typically below detection limits. Here we reexamine NO3- use by tundra plants using a sensitive denitrifier method to analyze plant-tissue NO3- Soil-derived NO3- was detected in tundra plant tissues, and tundra plants took up soil NO3- at comparable rates to plants from relatively NO3--rich ecosystems in other biomes. Nitrate assimilation determined by 15N enrichments of leaf NO3- relative to soil NO3- accounted for 4 to 52% (as estimated by a Bayesian isotope-mixing model) of species-specific total leaf N of Alaskan tundra plants. Our finding that in situ soil NO3- availability for tundra plants is high has important implications for Arctic ecosystems, not only in determining species compositions, but also in determining the loss of N from soils via leaching and denitrification. Plant N uptake and soil N losses can strongly influence C uptake and accumulation in tundra soils. Accordingly, this evidence of NO3- availability in tundra soils is crucial for predicting C storage in tundra.

Variation in fluxes estimated from nitrogen isotope discrimination corresponds with independent measures of nitrogen flux in Populus balsamifera L

Acquisition of mineral nitrogen by roots from the surrounding environment is often not completely efficient, in which a variable amount of leakage (efflux) relative to gross uptake (influx) occurs. The efflux/influx ratio (E/I) is, therefore, inversely related to the efficiency of nutrient uptake at the root level. Time-integrated estimates of E/I and other nitrogen-use traits may be obtainable from variation in stable isotope ratios or through compartmental analysis of tracer efflux (CATE) using radioactive or stable isotopes. To compare these two methods, Populus balsamifera L. genotypes were selected, a priori, for high or low nitrogen isotope discrimination. Vegetative cuttings were grown hydroponically, and E/I was calculated using an isotope mass balance model (IMB) and compared to E/I calculated using (15) N CATE. Both methods indicated that plants grown with ammonium had greater E/I than nitrate-grown plants. Genotypes with high or low E/I using CATE also had similarly high or low estimates of E/I using IMB, respectively. Genotype-specific means were linearly correlated (r = 0.77; P = 0.0065). Discrepancies in E/I between methods may reflect uncertainties in discrimination factors for the assimilatory enzymes, or temporal differences in uptake patterns. By utilizing genotypes with known variation in nitrogen isotope discrimination, a relationship between nitrogen isotope discrimination and bidirectional nitrogen fluxes at the root level was observed.

Multi-factorial in vivo stable isotope fractionation: causes, correlations, consequences and applications

Many physical and chemical processes in living systems are accompanied by isotope fractionation on H, C, N, O and S. Although kinetic or thermodynamic isotope effects are always the basis, their in vivo manifestation is often modulated by secondary influences. These include metabolic branching events or metabolite channeling, metabolite pool sizes, reaction mechanisms, anatomical properties and compartmentation of plants and animals, and climatological or environmental conditions. In the present contribution, the fundamentals of isotope effects and their manifestation under in vivo conditions are outlined. The knowledge about and the understanding of these interferences provide a potent tool for the reconstruction of physiological events in plants and animals, their geographical origin, the history of bulk biomass and the biosynthesis of defined representatives. It allows the use of isotope characteristics of biomass for the elucidation of biochemical pathways and reaction mechanisms and for the reconstruction of climatic, physiological, ecological and environmental conditions during biosynthesis. Thus, it can be used for the origin and authenticity control of food, the study of ecosystems and animal physiology, the reconstruction of present and prehistoric nutrition chains and paleaoclimatological conditions. This is demonstrated by the outline of fundamental and application-orientated examples for all bio-elements. The aim of the review is to inform (advanced) students from various disciplines about the whole potential and the scope of stable isotope characteristics and fractionations and to provide them with a comprehensive introduction to the literature on fundamental aspects and applications.

Nitrate dynamics in natural plants: insights based on the concentration and natural isotope abundances of tissue nitrate

The dynamics of nitrate (NO(-) 3), a major nitrogen (N) source for natural plants, has been studied mostly through experimental N addition, enzymatic assay, isotope labeling, and genetic expression. However, artificial N supply may not reasonably reflect the N strategies in natural plants because NO(-) 3 uptake and reduction may vary with external N availability. Due to abrupt application and short operation time, field N addition, and isotopic labeling hinder the elucidation of in situ NO(-) 3-use mechanisms. The concentration and natural isotopes of tissue NO(-) 3 can offer insights into the plant NO(-) 3 sources and dynamics in a natural context. Furthermore, they facilitate the exploration of plant NO(-) 3 utilization and its interaction with N pollution and ecosystem N cycles without disturbing the N pools. The present study was conducted to review the application of the denitrifier method for concentration and isotope analyses of NO(-) 3 in plants. Moreover, this study highlights the utility and advantages of these parameters in interpreting NO(-) 3 sources and dynamics in natural plants. We summarize the major sources and reduction processes of NO(-) 3 in plants, and discuss the implications of NO(-) 3 concentration in plant tissues based on existing data. Particular emphasis was laid on the regulation of soil NO(-) 3 and plant ecophysiological functions in interspecific and intra-plant NO(-) 3 variations. We introduce N and O isotope systematics of NO(-) 3 in plants and discuss the principles and feasibilities of using isotopic enrichment and fractionation factors; the correlation between concentration and isotopes (N and O isotopes: δ(18)O and Δ(17)O); and isotope mass-balance calculations to constrain sources and reduction of NO(-) 3 in possible scenarios for natural plants are deliberated. Finally, we offer a preliminary framework of intraplant δ(18)O-NO(-) 3 variation, and summarize the uncertainties in using tissue NO(-) 3 parameters to interpret plant NO(-) 3 utilization.

Nitrate reductase (15)N discrimination in Arabidopsis thaliana, Zea mays, Aspergillus niger, Pichea angusta, and Escherichia coli

Stable (15)N isotopes have been used to examine movement of nitrogen (N) through various pools of the global N cycle. A central reaction in the cycle involves the reduction of nitrate (NO(-) 3) to nitrite (NO(-) 2) catalyzed by nitrate reductase (NR). Discrimination against (15)N by NR is a major determinant of isotopic differences among N pools. Here, we measured in vitro (15)N discrimination by several NRs purified from plants, fungi, and a bacterium to determine the intrinsic (15)N discrimination by the enzyme and to evaluate the validity of measurements made using (15)N-enriched NO(-) 3. Observed NR isotope discrimination ranged from 22 to 32‰ (kinetic isotope effects of 1.022-1.032) among the different isozymes at natural abundance (15)N (0.37%). As the fractional (15)N content of substrate NO(-) 3 increased from natural abundance, the product (15)N fraction deviated significantly from that expected based on substrate enrichment and (15)N discrimination measured at natural abundance. Additionally, isotopic discrimination by denitrifying bacteria used to reduce NO(-) 3 and NO(-) 2 in some protocols became a greater source of error as (15)N enrichment increased. We briefly discuss potential causes of the experimental artifacts with enriched (15)N and recommend against the use of highly enriched (15)N tracers to study N discrimination in plants or soils.

Nitrogen isotope effects induced by anammox bacteria

Nitrogen (N) isotope ratios ((15)N/(14)N) provide integrative constraints on the N inventory of the modern ocean. Anaerobic ammonium oxidation (anammox), which converts ammonium and nitrite to dinitrogen gas (N2) and nitrate, is an important fixed N sink in marine ecosystems. We studied the so far unknown N isotope effects of anammox in batch culture experiments. Anammox preferentially removes (14)N from the ammonium pool with an isotope effect of +23.5‰ to +29.1‰, depending on factors controlling reversibility. The N isotope effects during the conversion of nitrite to N2 and nitrate are (i) inverse kinetic N isotope fractionation associated with the oxidation of nitrite to nitrate (-31.1 ± 3.9‰), (ii) normal kinetic N isotope fractionation during the reduction of nitrite to N2 (+16.0 ± 4.5‰), and (iii) an equilibrium N isotope effect between nitrate and nitrite (-60.5 ± 1.0‰), induced when anammox is exposed to environmental stress, leading to the superposition of N isotope exchange effects upon kinetic N isotope fractionation. Our findings indicate that anammox may be responsible for the unresolved large N isotope offsets between nitrate and nitrite in oceanic oxygen minimum zones. Irrespective of the extent of N isotope exchange between nitrate and nitrite, N removed from the combined nitrite and nitrate (NOx) pool is depleted in (15)N relative to NOx. This net N isotope effect by anammox is superimposed on the N isotope fractionation by the co-occurring reduction of nitrate to nitrite in suboxic waters, possibly enhancing the overall N isotope effect for N loss from oxygen minimum zones.

Whole-plant and organ-level nitrogen isotope discrimination indicates modification of partitioning of assimilation, fluxes and allocation of nitrogen in knockout lines of Arabidopsis thaliana

The nitrogen isotope composition (δ¹⁵N) of plants has potential to provide time-integrated information on nitrogen uptake, assimilation and allocation. Here, we take advantage of existing T-DNA and γ-ray mutant lines of Arabidopsis thaliana to modify whole-plant and organ-level nitrogen isotope composition. Nitrate reductase 2 (nia2), nitrate reductase 1 (nia1) and nitrate transporter (nrt2) mutant lines and the Col-0 wild type were grown hydroponically under steady-state NO₃⁻ conditions at either 100 or 1000 μM NO₃⁻ for 35 days. There were no significant effects on whole-plant discrimination and growth in the assimilatory mutants (nia2 and nia1). Pronounced root vs leaf differences in δ¹⁵N, however, indicated that nia2 had an increased proportion of nitrogen assimilation of NO₃⁻ in leaves while nia1 had an increased proportion of assimilation in roots. These observations are consistent with reported ratios of nia1 and nia2 gene expression levels in leaves and roots. Greater whole-plant discrimination in nrt2 indicated an increase in efflux of unassimilated NO₃⁻ back to the rooting medium. This phenotype was associated with an overall reduction in NO₃⁻ uptake, assimilation and decreased partitioning of NO₃⁻ assimilation to the leaves, presumably because of decreased symplastic intercellular movement of NO₃⁻ in the root. Although the results were more varied than expected, they are interpretable within the context of expected mechanisms of whole-plant and organ-level nitrogen isotope discrimination that indicate variation in nitrogen fluxes, assimilation and allocation between lines.

Metabolic origin of δ15 N values in nitrogenous compounds from Brassica napus L. leaves

Nitrogen isotope composition (δ(15) N) in plant organic matter is currently used as a natural tracer of nitrogen acquisition efficiency. However, the δ(15) N value of whole leaf material does not properly reflect the way in which N is assimilated because isotope fractionations along metabolic reactions may cause substantial differences among leaf compounds. In other words, any change in metabolic composition or allocation pattern may cause undesirable variability in leaf δ(15) N. Here, we investigated the δ(15) N in different leaf fractions and individual metabolites from rapeseed (Brassica napus) leaves. We show that there were substantial differences in δ(15) N between nitrogenous compounds (up to 30‰) and the content in ((15) N enriched) nitrate had a clear influence on leaf δ(15) N. Using a simple steady-state model of day metabolism, we suggest that the δ(15) N value in major amino acids was mostly explained by isotope fractionation associated with isotope effects on enzyme-catalysed reactions in primary nitrogen metabolism. δ(15) N values were further influenced by light versus dark conditions and the probable occurrence of alternative biosynthetic pathways. We conclude that both biochemical pathways (that fractionate between isotopes) and nitrogen sources (used for amino acid production) should be considered when interpreting the δ(15) N value of leaf nitrogenous compounds.

Pitfalls and new mechanisms in moss isotope biomonitoring of atmospheric nitrogen deposition

Moss N isotope (δ(15)N(bulk)) has been used to monitor N deposition, but it remains questionable whether inhibition of nitrate reductase activity (NRA) by reduced dissolved N (RDN) engenders overestimation of RDN in deposition when using moss δ(15)N(bulk). We tested this question by investigation of δ(15)N(bulk) and δ(15)NO(3)(-) in mosses under the dominance of RDN in N depositions of Guiyang, SW China. The δ(15)N(bulk) of mosses on bare rock (-7.9‰) was unable to integrate total dissolved N (TDN) (δ(15)N = -6.3‰), but it reflected δ(15)N-RDN (-7.5‰) exactly. Moreover, δ(15)N-NO(3)(-) in mosses (-1.7‰) resembled that of wet deposition (-1.9‰). These isotopic approximations, together with low isotopic enrichment with moss [NO(3)(-)] variations, suggest the inhibition of moss NRA by RDN. Moreover, isotopic mixing modeling indicated a negligible contribution from NO(3)(-) to moss δ(15)N(bulk) when the RDN/NO(3)(-) reaches 3.8, at which maximum overestimation (21%) of RDN in N deposition can be generated using moss δ(15)N(bulk) as δ(15)N-TDN. Moss δ(15)N-NO(3)(-) can indicate atmospheric NO(3)(-) under distinctly high RDN/NO(3)(-) in deposition, although moss δ(15)N(bulk) can reflect only the RDN therein. These results reveal pitfalls and new mechanisms associated with moss isotope monitoring of N deposition and underscore the importance of biotic N dynamics in biomonitoring studies.

Eukaryotic assimilatory nitrate reductase fractionates N and O isotopes with a ratio near unity

In order to (i) establish the biological systematics necessary to interpret nitrogen (N) and oxygen (O) isotope ratios of nitrate ((15)N/(14)N and (18)O/(16)O) in the environment and (ii) investigate the potential for isotopes to elucidate the mechanism of a key N cycle enzyme, we measured the nitrate N and O isotope effects ((15)ε and (18)ε) for nitrate reduction by two assimilatory eukaryotic nitrate reductase (eukNR) enzymes. The (15)ε for purified extracts of NADPH eukNR from the fungus Aspergillus niger and the (15)ε for NADH eukNR from cell homogenates of the marine diatom Thalassiosira weissflogii were indistinguishable, yielding a mean (15)ε for the enzyme of 26.6 ± 0.2‰. Both forms of eukNR imparted near equivalent fractionation on N and O isotopes. The increase in (18)O/(16)O versus the increase in (15)N/(14)N (relative to their natural abundances) was 0.96 ± 0.01 for NADPH eukNR and 1.09 ± 0.03 for NADH eukNR. These results are the first reliable measurements of the coupled N and O isotope effects for any form of eukNR. They support the prevailing view that intracellular reduction by eukNR is the dominant step in isotope fractionation during nitrate assimilation and that it drives the (18)ε:(15)ε ≈ 1 observed in phytoplankton cultures, suggesting that this O-to-N isotope signature will apply broadly in the environment. Our measured (15)ε and (18)ε may represent the intrinsic isotope effects for eukNR-mediated N-O bond rupture, a potential constraint on the nature of the enzyme's transition state.

Combined use of δ¹³C, δ18O and δ15N tracks nitrogen metabolism and genotypic adaptation of durum wheat to salinity and water deficit

• Accurate phenotyping remains a bottleneck in breeding for salinity and drought resistance. Here the combined use of stable isotope compositions of carbon (δ¹³C), oxygen (δ¹⁸O) and nitrogen (δ¹⁵N) in dry matter is aimed at assessing genotypic responses of durum wheat under different combinations of these stresses. • Two tolerant and two susceptible genotypes to salinity were grown under five combinations of salinity and irrigation regimes. Plant biomass, δ¹³C, δ¹⁸O and δ¹⁵N, gas-exchange parameters, ion and N concentrations, and nitrate reductase (NR) and glutamine synthetase (GS) activities were measured. • Stresses significantly affected all traits studied. However, only δ¹³C, δ¹⁸O, δ¹⁵N, GS and NR activities, and N concentration allowed for clear differentiation between tolerant and susceptible genotypes. Further, a conceptual model explaining differences in biomass based on such traits was developed for each growing condition. • Differences in acclimation responses among durum wheat genotypes under different stress treatments were associated with δ¹³C. However, except for the most severe stress, δ¹³C did not have a direct (negative) relationship to biomass, being mediated through factors affecting δ¹⁸O or N metabolism. Based upon these results, the key role of N metabolism in durum wheat adaptation to salinity and water stress is highlighted.

Stable isotope biogeochemistry of seabird guano fertilization: results from growth chamber studies with maize (Zea mays)

BACKGROUND

Stable isotope analysis is being utilized with increasing regularity to examine a wide range of issues (diet, habitat use, migration) in ecology, geology, archaeology, and related disciplines. A crucial component to these studies is a thorough understanding of the range and causes of baseline isotopic variation, which is relatively poorly understood for nitrogen (δ(15)N). Animal excrement is known to impact plant δ(15)N values, but the effects of seabird guano have not been systematically studied from an agricultural or horticultural standpoint.

METHODOLOGY/PRINCIPAL FINDINGS

This paper presents isotopic (δ(13)C and δ(15)N) and vital data for maize (Zea mays) fertilized with Peruvian seabird guano under controlled conditions. The level of (15)N enrichment in fertilized plants is very large, with δ(15)N values ranging between 25.5 and 44.7‰ depending on the tissue and amount of fertilizer applied; comparatively, control plant δ(15)N values ranged between -0.3 and 5.7‰. Intraplant and temporal variability in δ(15)N values were large, particularly for the guano-fertilized plants, which can be attributed to changes in the availability of guano-derived N over time, and the reliance of stored vs. absorbed N. Plant δ(13)C values were not significantly impacted by guano fertilization. High concentrations of seabird guano inhibited maize germination and maize growth. Moreover, high levels of seabird guano greatly impacted the N metabolism of the plants, resulting in significantly higher tissue N content, particularly in the stalk.

CONCLUSIONS/SIGNIFICANCE

The results presented in this study demonstrate the very large impact of seabird guano on maize δ(15)N values. The use of seabird guano as a fertilizer can thus be traced using stable isotope analysis in food chemistry applications (certification of organic inputs). Furthermore, the fertilization of maize with seabird guano creates an isotopic signature very similar to a high-trophic level marine resource, which must be considered when interpreting isotopic data from archaeological material.

Preliminary insights into δ15N and δ18O of nitrate in natural mosses: a new application of the denitrifier method

Natural mosses have been employed as reactive and accumulative indicators of atmospheric pollutants. Using the denitrifier method, the concentration, δ(15)N and δ(18)O of moss nitrate (NO(3)(-)) were measured to elucidate the sources of NO(3)(-) trapped in natural mosses. Oven drying at 55-70 °C, not lyophilization, was recommended to dry mosses for NO(3)(-) analyses. An investigation from urban to mountain sites in western Tokyo suggested that moss [NO(3)(-)] can respond to NO(3)(-) availability in different habitats. NO(3)(-) in terricolous mosses showed isotopic ratios as close to those of soil NO(3)(-), reflecting the utilization of soil NO(3)(-). Isotopic signatures of NO(3)(-) in corticolous and epilithic mosses elucidated atmospheric NO(3)(-) sources and strength from the urban (vehicle NO(x) emission) to mountain area (wet-deposition NO(3)(-)). However, mechanisms and isotopic effects of moss NO(3)(-) utilization must be further verified to enable the application of moss NO(3)(-) isotopes for source identification.

Aphids alter host-plant nitrogen isotope fractionation

Plant sap-feeding insects and blood-feeding parasites are frequently depleted in (15)N relative to their diet. Unfortunately, most fluid-feeder/host nitrogen stable-isotope studies simply report stable-isotope signatures, but few attempt to elucidate the mechanism of isotopic trophic depletion. Here we address this deficit by investigating the nitrogen stable-isotope dynamics of a fluid-feeding herbivore-host plant system: the green peach aphid, Myzus persicae, feeding on multiple brassicaceous host plants. M. persicae was consistently more than 6‰ depleted in (15)N relative to their hosts, although aphid colonized plants were 1.5‰ to 2.0‰ enriched in (15)N relative to uncolonized control plants. Isotopic depletion of aphids relative to hosts was strongly related to host nitrogen content. We tested whether the concomitant aphid (15)N depletion and host (15)N enrichment was coupled by isotopic mass balance and determined that aphid (15)N depletion and host (15)N enrichment are uncoupled processes. We hypothesized that colonized plants would have higher nitrate reductase activity than uncolonized plants because previous studies had demonstrated that high nitrate reductase activity under substrate-limiting conditions can result in increased plant δ(15)N values. Consistent with our hypothesis, nitrate reductase activity in colonized plants was twice that of uncolonized plants. This study offers two important insights that are likely applicable to understanding nitrogen dynamics in fluid-feeder/host systems. First, isotopic separation of aphid and host depends on nitrogen availability. Second, aphid colonization alters host nitrogen metabolism and subsequently host nitrogen stable-isotope signature. Notably, this work establishes a metabolic framework for future hypothesis-driven studies focused on aphid manipulation of host nitrogen metabolism.

Carbon dioxide enrichment inhibits nitrate assimilation in wheat and Arabidopsis

The concentration of carbon dioxide in Earth's atmosphere may double by the end of the 21st century. The response of higher plants to a carbon dioxide doubling often includes a decline in their nitrogen status, but the reasons for this decline have been uncertain. We used five independent methods with wheat and Arabidopsis to show that atmospheric carbon dioxide enrichment inhibited the assimilation of nitrate into organic nitrogen compounds. This inhibition may be largely responsible for carbon dioxide acclimation, the decrease in photosynthesis and growth of plants conducting C(3) carbon fixation after long exposures (days to years) to carbon dioxide enrichment. These results suggest that the relative availability of soil ammonium and nitrate to most plants will become increasingly important in determining their productivity as well as their quality as food.

Natural 15N/14N isotope composition in C3 leaves: are enzymatic isotope effects informative for predicting the 15N-abundance in key metabolites?

Although nitrogen isotopes are viewed as important tools for understanding plant N acquisition and allocation, the current interpretation of natural 15N-abundances (δ15N values) is often impaired by substantial variability among individuals or between species. Such variability is likely to stem from the fact that 15N-abundance of assimilated N is not preserved during N metabolism and redistribution within the plant; that is, 14N/15N isotope effects associated with N metabolic reactions are certainly responsible for isotopic shifts between organic-N (amino acids) and absorbed inorganic N (nitrate). Therefore, to gain insights into the metabolic origin of 15N-abundance in plants, the present paper reviews enzymatic isotope effects and integrates them into a metabolic model at the leaf level. Using simple steady-state equations which satisfactorily predict the δ15N value of amino acids, it is shown that the sensitivity of δ15N values to both photorespiratory and N-input (reduction by nitrate reductase) rates is quite high. In other words, the variability in δ15N values observed in nature might originate from subtle changes in metabolic fluxes or environment-driven effects, such as stomatal closure that in turn changes v0, the Rubisco-catalysed oxygenation rate.

How stable isotopes may help to elucidate primary nitrogen metabolism and its interaction with (photo)respiration in C3 leaves

Intense efforts are currently devoted to elucidate the metabolic networks of plants, in which nitrogen assimilation is of particular importance because it is strongly related to plant growth. In addition, at the leaf level, primary nitrogen metabolism interacts with photosynthesis, day respiration, and photorespiration, simply because nitrogen assimilation needs energy, reductant, and carbon skeletons which are provided by these processes. While some recent studies have focused on metabolomics and genomics of plant leaves, the actual metabolic fluxes associated with nitrogen metabolism operating in leaves are not very well known. In the present paper, it is emphasized that (12)C/(13)C and (14)N/(15)N stable isotopes have proved to be useful tools to investigate such metabolic fluxes and isotopic data are reviewed in the light of some recent advances in this area. Although the potential of stable isotopes remains high, it is somewhat limited by our knowledge of some isotope effects associated with enzymatic reactions. Therefore, this paper should be viewed as a call for more fundamental studies on isotope effects by plant enzymes.

On the 16O/18O isotope effect associated with photosynthetic O2 production

While photosynthetically evolved O₂ has been repeatedly shown to have nearly the same oxygen isotope composition as source water so that there is no corresponding ¹⁶O/¹⁸O isotope effect, some recent ¹⁸O-enrichment studies suggest that a large isotope effect may occur, thus feeding a debate in the literature. Here, the classical theory of isotope effects was applied to show that a very small isotope effect is indeed expected during O₂ production. Explanations of the conflicting results are briefly discussed.

Methods for improving the visualization and deconvolution of isotopic signals

Stable isotopes and their associated mechanistic frameworks have provided the means to model biological transformations from inorganic sources to organic sinks, and their impact on long-term terrestrial carbon reservoirs. However, stable isotopes have also the potential to diagnose the mechanisms by which biological systems operate, when using 'multidimensional' analyses of the isotopic outputs. For this purpose, we suggest that isotopic signals may be treated as mathematical vectors to reveal their interrelationships. These visualizations may be achieved, thanks to multidimensional representations (the 'isotopology' method), or by appreciating the colinearity of isotopic vectors to develop a clustering analysis (the 'isotopomic' method) similar to that used in molecular biology. Both methods converge to the same mathematical form, i.e. both lead to a covariation approach. Using simple practical examples, we argue that such procedures allow the deconvolution of plant biological systems by revealing the hierarchy of contributory physiological processes.

Viewpoint: How large is the carbon isotope fractionation of the photorespiratory enzyme glycine decarboxylase?

Despite the intense effort developed over the past 10 years to determine the ¹²C / ¹³C isotope fractionation associated with photorespiration, much uncertainty remains about the amplitude, and even the sign, of the ¹²C / ¹³C isotope fractionation of glycine decarboxylase, the enzyme that produces CO₂ during the photorespiratory cycle. In fact, leaf gas-exchange data have repeatedly indicated that CO₂ evolved by photorespiration is depleted in ¹³C compared with the source material, while glycine decarboxylase has mostly favoured ¹³C in vitro. Here I give theoretical insights on the glycine decarboxylase reaction and show that (i), both photorespiration and glycine decarboxylation must favour the same carbon isotope - the in vitro measurements being probably adulterated by the high sensitivity of the enzyme to assay conditions and the possible reversibility of the reaction in these conditions, and (ii), simplified quantum chemistry considerations as well as comparisons with other pyridoxal 5'-phosphate-dependent decarboxylases indicate that the carbon isotope fractionation favour the ¹²C isotope by ~20‰, a value that is consistent with the value of the photorespiratory fractionation (f) obtained by gas-exchange experiments.