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

Bulk and Compound-Specific Stable Isotope Analysis for the Authentication of Walnuts (Juglans regia) Origins

Walnuts are grown in various countries, and as product origin information is becoming more important to consumers, new techniques to differentiate walnut geographical authenticity are needed. We conducted bulk stable isotope analysis (BSIA) and compound-specific stable isotope analysis (CSIA) on walnuts grown in seven countries. The BSIA consisted of δ13Cbulk, δ15Nbulk, and δ34Sbulk, and CSIA covered δ2Hfatty acid, δ13Cfatty acid, δ13Camino acid, δ15Namino acid, and δ2Hamino acid. Analysis of variance (ANOVA) and linear discriminant analysis (LDA) were used for statistical analysis to compare samples from the USA and China. Parameters that yielded significant variations are δ2HC18:1n-9, δ13CC18:2n-6, δ13CC18:3n-3, δ13CGly, δ13CLeu, δ13CVal, δ2HGlu, δ2HIle, δ2HLeu, and δ2HThr. Our findings suggested that CSIA of fatty acids and amino acids can be useful to differentiate the geographical provenance of walnuts.

Partial substitution of chemical fertilizer by Trichoderma biofertilizer improved nitrogen use efficiency in wolfberry (Lycium chinense) in coastal saline land

A two-year field trial was conducted to investigate the effects of partial substitution of chemical fertilizer (CF) by Trichoderma biofertilizer (TF) on nitrogen (N) use efficiency and associated mechanisms in wolfberry (Lycium chinense) in coastal saline land. As with plant biomass and fruit yield, apparent N use efficiency and plant N accumulation were also higher with TF plus 75% CF than 100% CF, indicating that TF substitution promoted plant growth and N uptake. As a reason, TF substitution stabilized soil N supply by mitigating steep deceases in soil NH4 +-N and NO3 -N concentrations in the second half of growing seasons. TF substitution also increased carbon (C) fixation according to higher photosynthetic rate (Pn) and stable 13C abundance with TF plus 75% CF than 100% CF. Importantly, leaf N accumulation significantly and positively related with Pn, biomass, and fruit yield, and structural equation modeling also confirmed the importance of the causal relation of N accumulation coupled with C fixation for biomass and yield formation. Consequently, physiological and agronomical N use efficiencies were significantly higher with TF plus 75% CF than 100% CF. Overall, partial substitution of CF by TF improved N use efficiency in wolfberry in coastal saline land by stabilizing soil N supply and coupling N accumulation with C fixation.

Compound-Specific 14N/15N Analysis of Amino Acid Trimethylsilylated Derivatives from Plant Seed Proteins

Isotopic analyses of plant samples are now of considerable importance for food certification and plant physiology. In fact, the natural nitrogen isotope composition (δ¹⁵N) is extremely useful to examine metabolic pathways of N nutrition involving isotope fractionations. However, δ¹⁵N analysis of amino acids is not straightforward and involves specific derivatization procedures to yield volatile derivatives that can be analysed by gas chromatography coupled to isotope ratio mass spectrometry (GC-C-IRMS). Derivatizations other than trimethylsilylation are commonly used since they are believed to be more reliable and accurate. Their major drawback is that they are not associated with metabolite databases allowing identification of derivatives and by-products. Here, we revisit the potential of trimethylsilylated derivatives via concurrent analysis of δ¹⁵N and exact mass GC-MS of plant seed protein samples, allowing facile identification of derivatives using a database used for metabolomics. When multiple silylated derivatives of several amino acids are accounted for, there is a good agreement between theoretical and observed N mole fractions, and δ¹⁵N values are satisfactory, with little fractionation during derivatization. Overall, this technique may be suitable for compound-specific δ¹⁵N analysis, with pros and cons.

Stable Isotope Fractionation of Metals and Metalloids in Plants: A Review

This work critically reviews stable isotope fractionation of essential (B, Mg, K, Ca, Fe, Ni, Cu, Zn, Mo), beneficial (Si), and non-essential (Cd, Tl) metals and metalloids in plants. The review (i) provides basic principles and methodologies for non-traditional isotope analyses, (ii) compiles isotope fractionation for uptake and translocation for each element and connects them to physiological processes, and (iii) interlinks knowledge from different elements to identify common and contrasting drivers of isotope fractionation. Different biological and physico-chemical processes drive isotope fractionation in plants. During uptake, Ca and Mg fractionate through root apoplast adsorption, Si through diffusion during membrane passage, Fe and Cu through reduction prior to membrane transport in strategy I plants, and Zn, Cu, and Cd through membrane transport. During translocation and utilization, isotopes fractionate through precipitation into insoluble forms, such as phytoliths (Si) or oxalate (Ca), structural binding to cell walls (Ca), and membrane transport and binding to soluble organic ligands (Zn, Cd). These processes can lead to similar (Cu, Fe) and opposing (Ca vs. Mg, Zn vs. Cd) isotope fractionation patterns of chemically similar elements in plants. Isotope fractionation in plants is influenced by biotic factors, such as phenological stages and plant genetics, as well as abiotic factors. Different nutrient supply induced shifts in isotope fractionation patterns for Mg, Cu, and Zn, suggesting that isotope process tracing can be used as a tool to detect and quantify different uptake pathways in response to abiotic stresses. However, the interpretation of isotope fractionation in plants is challenging because many isotope fractionation factors associated with specific processes are unknown and experiments are often exploratory. To overcome these limitations, fundamental geochemical research should expand the database of isotope fractionation factors and disentangle kinetic and equilibrium fractionation. In addition, plant growth studies should further shift toward hypothesis-driven experiments, for example, by integrating contrasting nutrient supplies, using established model plants, genetic approaches, and by combining isotope analyses with complementary speciation techniques. To fully exploit the potential of isotope process tracing in plants, the interdisciplinary expertise of plant and isotope geochemical scientists is required.

Genotypic variation in C and N isotope discrimination suggests local adaptation of heart-leaved willow

Plants acquire multiple resources from the environment and may need to adjust and/or balance their respective resource-use efficiencies to maximize grow and survival, in a locally adaptive manner. In this study, tissue and whole-plant carbon (C) isotopic composition (δ13C) and carbon/nitrogen (C/N) ratios provided long-term measures of use efficiencies for water (WUE) and nitrogen (NUE), and a nitrogen (N) isotopic composition (δ15N)-based mass balance model was used to estimate traits related to N uptake and assimilation in heart-leaved willow (Salix eriocephala Michx.). In an initial common garden experiment consisting of 34 populations, we found population-level variation in δ13C, C/N ratio and δ15N, indicating different patterns in WUE, NUE and N uptake and assimilation. Although there was no relationship between foliar δ13C and C/N ratios among populations, there was a significant negative correlation between these measures across all individuals, implying a genetic and/or plastic trade-off between WUE and NUE not associated with local adaptation. To eliminate any environmental effect, we grew a subset of 21 genotypes hydroponically with nitrate as the sole N source and detected significant variation in δ13C, δ15N and C/N ratios. Variation in δ15N was mainly due to genotypic differences in the nitrate efflux/influx ratio (E/I) at the root. Both experiments suggested clinal variation in δ15N (and thus N uptake efficiency) with latitude of origin, which may relate to water availability and could contribute to global patterns in ecosystem δ15N. There was a tendency for genotypes with higher WUE to come from more water-replete sites with shorter and cooler growing seasons. We found that δ13C, C/N ratio and E/I were not inter-correlated, suggesting that the selection of growth, WUE, NUE and N uptake efficiency can occur without trade-off.

Carbon and nitrogen stable isotope variations in leaves of two grapevine cultivars (Chasselas and Pinot noir): Implications for ecophysiological studies

We investigated the within- and between-leaf variability in the carbon and nitrogen isotope composition (δ¹³C and δ¹⁵N) and total nitrogen (TN) content in two grapevine cultivars (Vitis vinifera cv. Chasselas and Pinot noir) field-grown under rain-fed conditions. The within-leaf variability was studied in discs sampled from base-to-tip and left and right regions from the margin to midrib. The intra- and interplant variability was studied by comparing leaves at different positions along the shoot (basal, median, apical). In leaves from both cultivars, a decrease in δ¹³C from base to tip was observed, which is in line with an upward gradient of stomatal density and chlorophyll concentration. Less important, but still significant differences were observed between the right and left discs. The leaf TN and δ¹⁵N values differed between cultivars, showed smaller variations than the δ¹³C values, and no systematic spatial trends. The intraleaf variations in δ¹³C, δ¹⁵N, and TN suggest that stomatal behavior, CO₂ fixation, chlorophyll concentrations, and the chemical composition of leaf components were heterogeneous in the leaves. At the canopy scale, the apical leaves had less ¹³C and more ¹⁵N and TN than the basal leaves, indicating differences in their photosynthetic capacity and remobilizations from old, senescing leaves to younger leaves. Overall, this study demonstrates patchiness in the δ¹³C and δ¹⁵N values of grapevine leaves and species-specificity of the nitrogen assimilation and ¹⁵N fractionation. These findings suggest that care must be taken not to overinterpret foliar δ¹³C and δ¹⁵N values in studies based on fragmented material as markers of physiological and biochemical responses to environmental factors.

Effect of irrigation salinity and ecotype on the growth, physiological indicators and seed yield and quality of Salicornia europaea

The euhalophyte species Salicornia europaea is cultivated for oilseed and as a fodder crop in various parts of the world. In saline coastal environments it possesses great potential for the subsistence of the most disadvantaged farmers. We investigated the effect of salinity levels in irrigation water on the germination capacity, shoot biomass and seed productivity as well as diverse quality traits (nitrogen content in shoots and seeds and fatty acids, in seeds) and physiological traits (stable carbon and nitrogen isotopes and ion content) of two accessions collected in the United Arab Emirates (UAE). The three salinity levels tested were irrigation with fresh water (0.3 dS m^-1), brackish water (25 dS m^-1) and sea water (40 dS m^-1). In addition, a hypersaline condition (80 dS m^-1) was also tested for germination. The best germination rates were achieved with seeds exposed to fresh and brackish water, while imbibition with sea water decreased germination by half and hypersaline water inhibited it almost totally. However, the best irrigation regime in terms of biomass and seed yield involved brackish water. Moreover, rising salinity in the irrigation increased the stable isotope composition of carbon (δ¹³C) and nitrogen (δ¹⁵N), together with the Na⁺ and K⁺ of shoots and seeds, and the lipid levels of seeds, while the total nitrogen content and the profile of major fatty acids of seeds did not change. Differences between the two ecotypes existed for growth and seed yield with the best ecotype exhibiting lower δ¹³C and higher K⁺ in both shoots and seeds, lower Na⁺ and higher δ¹⁵N in shoots, and lower N in seeds, together with differences in major fatty acids. Physiological mechanisms behind the response to irrigation salinity and the ecotypic differences are discussed in terms of photosynthetic carbon and nitrogen metabolism.

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.

Isotopic and Water Relation Responses to Ozone and Water Stress in Seedlings of Three Oak Species with Different Adaptation Strategies

The impact of global changes on forest ecosystem processes is based on the species-specific responses of trees to the combined effect of multiple stressors and the capacity of each species to acclimate and cope with the environment modification. Combined environmental constraints can severely affect plant and ecological processes involved in plant functionality. This study provides novel insights into the impact of a simultaneous pairing of abiotic stresses (i.e., water and ozone (O3) stress) on the responses of oak species. Water stress (using 40 and 100% of soil water content at field capacity—WS and WW treatments, respectively) and O3 exposure (1.0, 1.2, and 1.4 times the ambient concentration—AA, 1.2AA, and 1.4AA, respectively) were carried out on Quercus robur L., Quercus ilex L., and Quercus pubescens Willd. seedlings, to study physiological traits (1. isotope signature [δ13C, δ18O and δ15N], 2. water relation [leaf water potential, leaf water content], 3. leaf gas exchange [light-saturated net photosynthesis, Asat, and stomatal conductance, gs]) for adaptation strategies in a Free-Air Controlled Exposure (FACE) experiment. Ozone decreased Asat in Q. robur and Q. pubescens while water stress decreased it in all three oak species. Ozone did not affect δ13C, whereas δ18O was influenced by O3 especially in Q. robur. This may reflect a reduction of gs with the concomitant reduction in photosynthetic capacity. However, the effect of elevated O3 on leaf gas exchange as indicated by the combined analysis of stable isotopes was much lower than that of water stress. Water stress was detectable by δ13C and by δ18O in all three oak species, while δ15N did not define plant response to stress conditions in any species. The δ13C signal was correlated to leaf water content (LWC) in Q. robur and Q. ilex, showing isohydric and anisohydric strategy, respectively, at increasing stress intensity (low value of LWC). No interactive effect of water stress and O3 exposure on the isotopic responses was found, suggesting no cross-protection on seasonal carbon assimilation independently on the species adaptation strategy.

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.

An investigation on possible effect of leaching fractions physiological responses of hot pepper plants to irrigation water salinity

BACKGROUND

The modification effect of leaching fraction (LF) on the physiological responses of plants to irrigation water salinity (ECiw) remains unknown. Here, leaf gas exchange, photosynthetic light-response and CO2-response curves, and total carbon (C) and nitrogen (N) accumulation in hot pepper leaves were investigated under three ECiw levels (0.9, 4.7 and 7.0 dS m- 1) and two LFs treatments (0.17 and 0.29).

RESULTS

Leaf stomatal conductance was more sensitive to ECiw than the net photosynthesis rate, leading to higher intrinsic water use efficiency (WUE) in higher ECiw, whereas the LF did not affect the intrinsic WUE. Carbon isotope discrimination was inhibited by ECiw, but was not affected by LF. ECiw reduced the carboxylation efficiency, photosynthetic capacity, photorespiration rate, apparent quantum yield of CO2 and irradiance-saturated rate of gross photosynthesis; however, LF did not influence any of these responses. Total C and N accumulation in plants leaves was markedly increased with either decreasing ECiw or increasing LF.

CONCLUSIONS

The present study shows that higher ECiw depressed leaf gas exchange, photosynthesis capacity and total C and N accumulation in leaves, but enhanced intrinsic WUE. Somewhat surprisingly, higher LF did not affect the intrinsic WUE but enhanced the total C and N accumulation in leaves.

Leaf Age Compared to Tree Age Plays a Dominant Role in Leaf δ13C and δ15N of Qinghai Spruce (Picea crassifolia Kom.)

Leaf stable isotope compositions (δ13C and δ15N) are influenced by various abiotic and biotic factors. Qinghai spruce (Picea crassifolia Kom.) as one of the dominant tree species in Qilian Mountains plays a key role in the ecological stability of arid region in the northwest of China. However, our knowledge of the relative importance of multiple factors on leaf δ13C and δ15N remains incomplete. In this work, we investigated the relationships of δ13C and δ15N to leaf age, tree age and leaf nutrients to examine the patterns and controls of leaf δ13C and δ15N variation of Picea crassifolia. Results showed that 13C and 15N of current-year leaves were more enriched than older ones at each tree age level. There was no significant difference in leaf δ13C values among trees of different ages, while juvenile trees (<50 years old) were 15N depleted compared to middle-aged trees (50–100 years old) at each leaf age level except for 1-year-old leaves. Meanwhile, relative importance analysis has demonstrated that leaf age was one of the most important indicators for leaf δ13C and δ15N. Moreover, leaf N concentrations played a dominant role in the variations of δ13C and δ15N. Above all, these results provide valuable information on the eco-physiological responses of P. crassifolia in arid and semi-arid regions.

A study of transcriptome in leaf rust infected bread wheat involving seedling resistance gene Lr28

Leaf rust disease causes severe yield losses in wheat throughout the world. During the present study, high-throughput RNA-Seq analysis was used to gain insights into the role of Lr28 gene in imparting seedling leaf rust resistance in wheat. Differential expression analysis was conducted using a pair of near-isogenic lines (NILs) (HD 2329 and HD 2329+Lr28) at early (0h before inoculation (hbi), 24 and 48h after inoculation (hai)) and late stages (72, 96 and 168 hai) after inoculation with a virulent pathotype of pathogen Puccinia triticina. Expression of a large number of genes was found to be affected due to the presence/absence of Lr28. Gene ontology analysis of the differentially expressed transcripts suggested enrichment of transcripts involved in carbohydrate and amino acid metabolism, oxidative stress and hormone metabolism, in resistant and/or susceptible NILs. Genes encoding receptor like kinases (RLKs) (including ATP binding; serine threonine kinases) and other kinases were the most abundant class of genes, whose expression was affected. Genes involved in reactive oxygen species (ROS) homeostasis and several genes encoding transcription factors (TFs) (most abundant being WRKY TFs) were also identified along with some ncRNAs and histone variants. Quantitative real-time PCR was also used for validation of 39 representative selected genes. In the long term, the present study should prove useful in developing leaf rust resistant wheat cultivars through molecular breeding.

Compound-specific δ15N composition of free amino acids in moss as indicators of atmospheric nitrogen sources

Haplocladium microphyllum moss samples were collected in Nanchang, China. Free amino acid (FAA) concentrations and N isotope compositions (δ15NFAA) in the samples were determined and compared with the bulk N concentrations and δ15Nbulk values. The aim was to determine whether δ15NFAA values in moss (which are very variable) indicate the sources of atmospheric N. The δ15NFAA values among individual FAA varied widely (from -19.3‰ to +16.1‰), possibly because of the different sources of N and isotope fractionation in amino acids metabolic pathways. Total 15N-enrichment for the individual FAAs was equal to total 15N-depletion relative to δ15Nbulk. The concentration-weighted mean δ15N value for total FAAs (TFAA) (δ15NTFAA) was -3.1‰ ± 3.2‰, which was similar to δ15Nbulk (-4.0‰ ± 2.9‰). We concluded that a N isotope balance occurred during amino acid metabolism and that little isotope disparity occurred between the concentration-weighted TFAA and bulk N. We concluded that δ15NTFAA ≈ δ15Nbulk ≈ δ15Nsource. The mean δ15Nalanine (-4.1‰), δ15Nglutamate (-4.2‰), and δ15Nlysine (-4.0‰) were similar to the mean δ15Nbulk, which we attributed to little isotope fractionation occurring during their in situ the metabolic pathways. This suggests that δ15Nalanine, δ15Nglutamate, and δ15Nlysine in moss can be used to indicate the sources of atmospheric N deposition.

Distinct Carbon and Nitrogen Metabolism of Two Contrasting Poplar Species in Response to Different N Supply Levels

Poplars have evolved various strategies to optimize acclimation responses to environmental conditions. However, how poplars balance growth and nitrogen deficiency remains to be elucidated. In the present study, changes in root development, carbon and nitrogen physiology, and the transcript abundance of associated genes were investigated in slow-growing Populus simonii (Ps) and fast-growing Populus euramericana (Pe) saplings treated with low, medium, and high nitrogen supply. The slow-growing Ps showed a flourishing system, higher δ15N, accelerated C export, lower N uptake and assimilation, and less sensitive transcriptional regulation in response to low N supply. The slow-growing Ps also had greater resistance to N deficiency due to the transport of photosynthate to the roots and the stimulation of root development, which allows survival. To support its rapid metabolism and growth, compared with the slow-growing Ps, the fast-growing Pe showed greater root development, C/N uptake and assimilation capacity, and more responsive transcriptional regulation with greater N supply. These data suggest that poplars can differentially manage C/N metabolism and photosynthate allocation under different N supply conditions.

Interactive Effects of CO2 Concentration and Water Regime on Stable Isotope Signatures, Nitrogen Assimilation and Growth in Sweet Pepper

Sweet pepper is among the most widely cultivated horticultural crops in the Mediterranean basin, being frequently grown hydroponically under cover in combination with CO2 fertilization and water conditions ranging from optimal to suboptimal. The aim of this study is to develop a simple model, based on the analysis of plant stable isotopes in their natural abundance, gas exchange traits and N concentration, to assess sweet pepper growth. Plants were grown in a growth chamber for near 6 weeks. Two [CO2] (400 and 800 μmol mol-1), three water regimes (control and mild and moderate water stress) and four genotypes were assayed. For each combination of genotype, [CO2] and water regime five plants were evaluated. Water stress applied caused significant decreases in water potential, net assimilation, stomatal conductance, intercellular to atmospheric [CO2], and significant increases in water use efficiency, leaf chlorophyll content and carbon isotope composition, while the relative water content, the osmotic potential and the content of anthocyanins did change not under stress compared to control conditions support this statement. Nevertheless, water regime affects plant growth via nitrogen assimilation, which is associated with the transpiration stream, particularly at high [CO2], while the lower N concentration caused by rising [CO2] is not associated with stomatal closure. The stable isotope composition of carbon, oxygen, and nitrogen (δ13C, δ18O, and δ15N) in plant matter are affected not only by water regime but also by rising [CO2]. Thus, δ18O increased probably as response to decreases in transpiration, while the increase in δ15N may reflect not only a lower stomatal conductance but a higher nitrogen demand in leaves or shifts in nitrogen metabolism associated with decreases in photorespiration. The way that δ13C explains differences in plant growth across water regimes within a given [CO2], seems to be mediated through its direct relationship with N accumulation in leaves. The changes in the profile and amount of amino acids caused by water stress and high [CO2] support this conclusion. However, the results do not support the use of δ18O as an indicator of the effect of water regime on plant growth.

Functional and DNA-protein binding studies of WRKY transcription factors and their expression analysis in response to biotic and abiotic stress in wheat (Triticum aestivum L.)

WRKY, a plant-specific transcription factor family, plays vital roles in pathogen defense, abiotic stress, and phytohormone signalling. Little is known about the roles and function of WRKY transcription factors in response to rust diseases in wheat. In the present study, three TaWRKY genes encoding complete protein sequences were cloned. They belonged to class II and III WRKY based on the number of WRKY domains and the pattern of zinc finger structures. Twenty-two DNA-protein binding docking complexes predicted stable interactions of WRKY domain with W-box. Quantitative real-time-PCR using wheat near-isogenic lines with or without Lr28 gene revealed differential up- or down-regulation in response to biotic and abiotic stress treatments which could be responsible for their functional divergence in wheat. TaWRKY62 was found to be induced upon treatment with JA, MJ, and SA and reduced after ABA treatments. Maximum induction of six out of seven genes occurred at 48 h post inoculation due to pathogen inoculation. Hence, TaWRKY (49, 50, 52, 55, 57, and 62) can be considered as potential candidate genes for further functional validation as well as for crop improvement programs for stress resistance. The results of the present study will enhance knowledge towards understanding the molecular basis of mode of action of WRKY transcription factor genes in wheat and their role during leaf rust pathogenesis in particular.

Carbon and nitrogen allocation strategy in Posidonia oceanica is altered by seawater acidification

Rising atmospheric CO₂ causes ocean acidification that represents one of the major ecological threats for marine biota. We tested the hypothesis that long-term exposure to increased CO₂ level and acidification in a natural CO₂ vent system alters carbon (C) and nitrogen (N) metabolism in Posidonia oceanica L. (Delile), affecting its resilience, or capability to restore the physiological homeostasis, and the nutritional quality of organic matter available for grazers. Seawater acidification decreased the C to N ratio in P. oceanica tissues and increased grazing rate, shoot density, leaf proteins and asparagine accumulation in rhizomes, while the maximum photochemical efficiency of photosystem II was unaffected. The ¹³C-dilution in both structural and non-structural C metabolites in the acidified site indicated quali-quantitative changes of C source and/or increased isotopic fractionation during C uptake and carboxylation associated with the higher CO₂ level. The decreased C:N ratio in the acidified site suggests an increased N availability, leading to a greater storage of ¹⁵N-enriched compounds in rhizomes. The amount of the more dynamic C storage form, sucrose, decreased in rhizomes of the acidified site in response to the enhanced energy demand due to higher shoot recruitment and N compound synthesis, without affecting starch reserves. The ability to modulate the balance between stable and dynamic C reserves could represent a key ecophysiological mechanism for P. oceanica resilience under environmental perturbation. Finally, alteration in C and N dynamics promoted a positive contribution of this seagrass to the local food web.

Manipulation of Auxin and Cytokinin Balance During the Plasmodiophora brassicae-Arabidopsis thaliana Interaction

The symptoms of the clubroot disease on Brassica species caused by the obligate biotrophic protist Plasmodiophora brassicae relies, among other factors, on the modulation of plant hormones. Signaling, transport as well as biosynthesis and metabolism are key features how the levels of auxins and cytokinins are controlled. We here describe (a) how to inoculate the model plant Arabidopsis thaliana with P. brassicae, (b) qualitative and quantitative methods to evaluate disease severity in auxin and cytokinin mutants,

Interactive Effects of Elevated [CO2] and Water Stress on Physiological Traits and Gene Expression during Vegetative Growth in Four Durum Wheat Genotypes

The interaction of elevated [CO2] and water stress will have an effect on the adaptation of durum wheat to future climate scenarios. For the Mediterranean basin these scenarios include the rising occurrence of water stress during the first part of the crop cycle. In this study, we evaluated the interactive effects of elevated [CO2] and moderate to severe water stress during the first part of the growth cycle on physiological traits and gene expression in four modern durum wheat genotypes. Physiological data showed that elevated [CO2] promoted plant growth but reduced N content. This was related to a down-regulation of Rubisco and N assimilation genes and up-regulation of genes that take part in C-N remobilization, which might suggest a higher N efficiency. Water restriction limited the stimulation of plant biomass under elevated [CO2], especially at severe water stress, while stomatal conductance and carbon isotope signature revealed a water saving strategy. Transcript profiles under water stress suggested an inhibition of primary C fixation and N assimilation. Nevertheless, the interactive effects of elevated [CO2] and water stress depended on the genotype and the severity of the water stress, especially for the expression of drought stress-responsive genes such as dehydrins, catalase, and superoxide dismutase. The network analysis of physiological traits and transcript levels showed coordinated shifts between both categories of parameters and between C and N metabolism at the transcript level, indicating potential genes and traits that could be used as markers for early vigor in durum wheat under future climate change scenarios. Overall the results showed that greater plant growth was linked to an increase in N content and expression of N metabolism-related genes and down-regulation of genes related to the antioxidant system. The combination of elevated [CO2] and severe water stress was highly dependent on the genotypic variability, suggesting specific genotypic adaptation strategies to environmental conditions.

Comparative effect of salinity on growth, grain yield, water use efficiency, δ(13)C and δ(15)N of landraces and improved durum wheat varieties

Supplemental irrigation with low-quality water will be paramount in Mediterranean agriculture in the future, where durum wheat is a major crop. Breeding for salinity tolerance may contribute towards improving resilience to irrigation with brackish water. However, identification of appropriate phenotyping traits remains a bottleneck in breeding. A set of 25 genotypes, including 19 landraces and 6 improved varieties most cultivated in Tunisia, were grown in the field and irrigated with brackish water (6, 13 and 18dSm(-1)). Improved genotypes exhibited higher grain yield (GY) and water use efficiency at the crop level (WUEyield or 'water productivity'), shorter days to flowering (DTF), lower N concentration (N) and carbon isotope composition (δ(13)C) in mature kernels and lower nitrogen isotope composition (δ(15)N) in the flag leaf compared with landraces. GY was negatively correlated with DTF and the δ(13)C and N of mature kernels and was positively correlated with the δ(15)N of the flag leaf. Moreover, δ(13)C of mature kernels was negatively correlated with WUEyield. The results highlight the importance of shorter phenology together with photosynthetic resilience to salt-induced water stress (lower δ(13)C) and nitrogen metabolism (higher N and δ(15)N) for assessing genotypic performance to salinity.

Can stable isotope mass spectrometry replace ‎radiolabelled approaches in metabolic studies?

Metabolic pathways and the key regulatory points thereof can be deduced using isotopically labelled substrates. One prerequisite is the accurate measurement of the labeling pattern of targeted metabolites. The subsequent estimation of metabolic fluxes following incubation in radiolabelled substrates has been extensively used. Radiolabelling is a sensitive approach and allows determination of total label uptake since the total radiolabel content is easy to detect. However, the incubation of cells, tissues or the whole plant in a stable isotope enriched environment and the use of either mass spectrometry or nuclear magnetic resonance techniques to determine label incorporation within specific metabolites offers the possibility to readily obtain metabolic information with higher resolution. It additionally also offers an important complement to other post-genomic strategies such as metabolite profiling providing insights into the regulation of the metabolic network and thus allowing a more thorough description of plant cellular function. Thus, although safety concerns mean that stable isotope feeding is generally preferred, the techniques are in truth highly complementary and application of both approaches in tandem currently probably provides the best route towards a comprehensive understanding of plant cellular metabolism.

Relationship Between Photochemical Quenching and Non-Photochemical Quenching in Six Species of Cyanobacteria Reveals Species Difference in Redox State and Species Commonality in Energy Dissipation

Although the photosynthetic reaction center is well conserved among different cyanobacterial species, the modes of metabolism, e.g. respiratory, nitrogen and carbon metabolism and their mutual interaction, are quite diverse. To explore such uniformity and diversity among cyanobacteria, here we compare the influence of the light environment on the condition of photosynthetic electron transport through Chl fluorescence measurement of six cyanobacterial species grown under the same photon flux densities and at the same temperature. In the dark or under weak light, up to growth light, a large difference in the plastoquinone (PQ) redox condition was observed among different cyanobacterial species. The observed difference indicates that the degree of interaction between respiratory electron transfer and photosynthetic electron transfer differs among different cyanobacterial species. The variation could not be ascribed to the phylogenetic differences but possibly to the light environment of the original habitat. On the other hand, changes in the redox condition of PQ were essentially identical among different species at photon flux densities higher than the growth light. We further analyzed the response to high light by using a typical energy allocation model and found that 'non-regulated' thermal dissipation was increased under high-light conditions in all cyanobacterial species tested. We assume that such 'non-regulated' thermal dissipation may be an important 'regulatory' mechanism in the acclimation of cyanobacterial cells to high-light conditions.

Isotopic evidence for nitrogen exchange between autotrophic and heterotrophic tissues in variegated leaves

Many plant species or cultivars form variegated leaves in which blades are made of green and white sectors. On the one hand, there is little photosynthetic CO2 assimilation in white tissue simply because of the lack of functional chloroplasts and thus, leaf white tissue is heterotrophic and fed by photosynthates exported by leaf green tissue. On the other hand, it has been previously shown that the white tissue is enriched in nitrogenous compounds such as amino acids and polyamines, which can, in turn, be remobilised upon nitrogen deficiency. However, the origin of organic nitrogen in leaf white tissue, including the possible requirement for N-reduction in leaf green tissue before export to white tissue, has not been examined. Here, we took advantage of isotopic methods to investigate the source of nitrogen in the white tissue. A survey of natural isotope abundance (δ15N) and elemental composition (%N) in various variegated species shows no visible difference between white and green tissues, suggesting a common N source. However, there is a tendency for N-rich white tissue to be naturally 15N-enriched whereas in the model species Pelargonium×hortorum, white sectors are naturally 15N-depleted, indicating that changes in metabolic composition and/or N-partitioning may occur. Isotopic labelling with 15N-nitrate on illuminated leaf discs clearly shows that the white tissue assimilates little nitrogen and thus relies on nitrate reduction and metabolism in the green tissue. The N-sink represented by the white tissue is considerable, accounting for nearly 50% of total assimilated nitrate.

Leaf δ(15)N as a physiological indicator of the responsiveness of N2-fixing alfalfa plants to elevated [CO2], temperature and low water availability

The natural (15)N/(14)N isotope composition (δ(15)N) of a tissue is a consequence of its N source and N physiological mechanisms in response to the environment. It could potentially be used as a tracer of N metabolism in plants under changing environmental conditions, where primary N metabolism may be complex, and losses and gains of N fluctuate over time. In order to test the utility of δ(15)N as an indicator of plant N status in N2-fixing plants grown under various environmental conditions, alfalfa (Medicago sativa L.) plants were subjected to distinct conditions of [CO2] (400 vs. 700 μmol mol(-1)), temperature (ambient vs. ambient +4°C) and water availability (fully watered vs. water deficiency-WD). As expected, increased [CO2] and temperature stimulated photosynthetic rates and plant growth, whereas these parameters were negatively affected by WD. The determination of δ(15)N in leaves, stems, roots, and nodules showed that leaves were the most representative organs of the plant response to increased [CO2] and WD. Depletion of heavier N isotopes in plants grown under higher [CO2] and WD conditions reflected decreased transpiration rates, but could also be related to a higher N demand in leaves, as suggested by the decreased leaf N and total soluble protein (TSP) contents detected at 700 μmol mol(-1) [CO2] and WD conditions. In summary, leaf δ(15)N provides relevant information integrating parameters which condition plant responsiveness (e.g., photosynthesis, TSP, N demand, and water transpiration) to environmental conditions.

Carbon and nitrogen allocation and partitioning in traditional and modern wheat genotypes under pre-industrial and future CO₂ conditions

The results of a simultaneous (13)C and (15)N labelling experiment with two different durum wheat cultivars, Blanqueta (a traditional wheat) and Sula (modern), are presented. Plants were grown from the seedling stage in three fully controllable plant growth chambers for one growing season and at three different CO₂ levels (i.e. 260, 400 and 700 ppm). Short-term isotopic labelling (ca. 3 days) was performed at the anthesis stage using (13)CO₂ supplied with the chamber air and (15)NH₄₋(15)NO₃ applied with the nutrient solution, thereby making it possible to track the allocation and partitioning of (13)C and (15) N in the different plant organs. We found that photosynthesis was up-regulated at pre-industrial CO₂ levels, whereas down-regulation occurred under future CO₂ conditions. (13)C labelling revealed that at pre-industrial CO₂ carbon investment by plants was higher in shoots, whereas at future CO₂ levels more C was invested in roots. Furthermore, the modern genotype invested more C in spikes than did the traditional genotype, which in turn invested more in non-reproductive shoot tissue. (15)N labelling revealed that the modern genotype was better adapted to assimilating N at higher CO₂ levels, whereas the traditional genotype was able to assimilate N more efficiently at lower CO₂ levels.

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.

Investigating patterns of symbiotic nitrogen fixation during vegetation change from grassland to woodland using fine scale δ(15) N measurements

Biological nitrogen fixation (BNF) in woody plants is often investigated using foliar measurements of δ(15) N and is of particular interest in ecosystems experiencing increases in BNF due to woody plant encroachment. We sampled δ(15) N along the entire N uptake pathway including soil solution, xylem sap and foliage to (1) test assumptions inherent to the use of foliar δ(15) N as a proxy for BNF; (2) determine whether seasonal divergences occur between δ(15) Nxylem sap and δ(15) Nsoil inorganic N that could be used to infer variation in BNF; and (3) assess patterns of δ(15) N with tree age as indicators of shifting BNF or N cycling. Measurements of woody N-fixing Prosopis glandulosa and paired reference non-fixing Zanthoxylum fagara at three seasonal time points showed that δ(15) Nsoil inorganic N varied temporally and spatially between species. Fractionation between xylem and foliar δ(15) N was consistently opposite in direction between species and varied on average by 2.4‰. Accounting for these sources of variation caused percent nitrogen derived from fixation values for Prosopis to vary by up to ∼70%. Soil-xylem δ(15) N separation varied temporally and increased with Prosopis age, suggesting seasonal variation in N cycling and BNF and potential long-term increases in BNF not apparent through foliar sampling alone.

Natural abundance (δ¹⁵N) indicates shifts in nitrogen relations of woody taxa along a savanna-woodland continental rainfall gradient

Water and nitrogen (N) interact to influence soil N cycling and plant N acquisition. We studied indices of soil N availability and acquisition by woody plant taxa with distinct nutritional specialisations along a north Australian rainfall gradient from monsoonal savanna (1,600-1,300 mm annual rainfall) to semi-arid woodland (600-250 mm). Aridity resulted in increased 'openness' of N cycling, indicated by increasing δ(15)N(soil) and nitrate:ammonium ratios, as plant communities transitioned from N to water limitation. In this context, we tested the hypothesis that δ(15)N(root) xylem sap provides a more direct measure of plant N acquisition than δ(15)N(foliage). We found highly variable offsets between δ(15)N(foliage) and δ(15)N(root) xylem sap, both between taxa at a single site (1.3-3.4 ‰) and within taxa across sites (0.8-3.4 ‰). As a result, δ(15)N(foliage) overlapped between N-fixing Acacia and non-fixing Eucalyptus/Corymbia and could not be used to reliably identify biological N fixation (BNF). However, Acacia δ(15)N(root) xylem sap indicated a decline in BNF with aridity corroborated by absence of root nodules and increasing xylem sap nitrate concentrations and consistent with shifting resource limitation. Acacia dominance at arid sites may be attributed to flexibility in N acquisition rather than BNF capacity. δ(15)N(root) xylem sap showed no evidence of shifting N acquisition in non-mycorrhizal Hakea/Grevillea and indicated only minor shifts in Eucalyptus/Corymbia consistent with enrichment of δ(15)N(soil) and/or decreasing mycorrhizal colonisation with aridity. We propose that δ(15)N(root) xylem sap is a more direct indicator of N source than δ(15)N(foliage), with calibration required before it could be applied to quantify BNF.

Transamination governs nitrogen isotope heterogeneity of amino acids in rats

The nitrogen isotope composition (δ¹⁵N) of different amino acids carries different dietary information. We hypothesized that transamination and de novo synthesis create three groups that largely explain their dietary information. Rats were fed with ¹⁵N-labeled amino acids. The redistribution of the dietary ¹⁵N labels among the muscular amino acids was analyzed. Subsequently, the labeling was changed and the nitrogen isotope turnover was analyzed. The amino acids had a common nitrogen half-life of ∼20 d, but differed in δ¹⁵N. Nontransaminating and essential amino acids largely conserved the δ¹⁵N of the source and, hence, trace the origin in heterogeneous diets. Nonessential and nontransaminating amino acids showed a nitrogen isotope composition between their dietary composition and that of their de novo synthesis pool, likely indicating their fraction of de novo synthesis. The bulk of amino acids, which are transaminating, derived their N from a common N pool and hence their δ¹⁵N was similar.

Nitrogen isotope discrimination as an integrated measure of nitrogen fluxes, assimilation and allocation in plants

Fractionation of nitrogen isotopes between a plant and its environment occurs during uptake and assimilation of inorganic nitrogen. Fractionation can also occur between roots and the shoot. Under controlled nitrogen conditions, whole-plant and organ-level nitrogen isotope discrimination (Δ(15) N) is suggested to primarily be a function of three factors: nitrogen efflux back to the substrate relative to gross influx at the root (efflux/influx), the proportion of net influx assimilated in the roots and the export of remaining inorganic nitrogen for assimilation in the leaves. Here, an isotope discrimination model combining measurements of δ(15) N and nitrogen content is proposed to explain whole-plant and organ-level variation in δ(15) N under steady-state conditions and prior to any significant retranslocation. We show evidence that nitrogen isotope discrimination varies in accordance with changes to nitrogen supply or demand. Increased whole-plant discrimination (greater Δ(15) N or more negative δ(15) N relative to the source nitrogen δ(15) N) indicates increased turnover of the cytosolic inorganic nitrogen pool and a greater efflux/influx ratio. A greater difference between shoot and root δ(15) N indicates a greater proportion of inorganic nitrogen being assimilated in the leaves. In addition to calculations of integrated nitrogen-use traits, knowledge of biomass partitioning and nitrogen concentrations in different plant organs provides a spatially and temporally integrated, whole-plant phenotyping approach for measuring nitrogen-use in plants. This approach can be used to complement instantaneous cell- and tissue-specific measures of nitrogen use currently used in nitrogen uptake and assimilation studies.

Complexities of nitrogen isotope biogeochemistry in plant-soil systems: implications for the study of ancient agricultural and animal management practices

Nitrogen isotopic studies have the potential to shed light on the structure of ancient ecosystems, agropastoral regimes, and human-environment interactions. Until relatively recently, however, little attention was paid to the complexities of nitrogen transformations in ancient plant-soil systems and their potential impact on plant and animal tissue nitrogen isotopic compositions. This paper discusses the importance of understanding nitrogen dynamics in ancient contexts, and highlights several key areas of archaeology where a more detailed understanding of these processes may enable us to answer some fundamental questions. This paper explores two larger themes that are prominent in archaeological studies using stable nitrogen isotope analysis: (1) agricultural practices (use of animal fertilizers, burning of vegetation or shifting cultivation, and tillage) and (2) animal domestication and husbandry (grazing intensity/stocking rate and the foddering of domestic animals with cultigens). The paucity of plant material in ancient deposits necessitates that these issues are addressed primarily through the isotopic analysis of skeletal material rather than the plants themselves, but the interpretation of these data hinges on a thorough understanding of the underlying biogeochemical processes in plant-soil systems. Building on studies conducted in modern ecosystems and under controlled conditions, these processes are reviewed, and their relevance discussed for ancient contexts.

Cereal grain, rachis and pulse seed amino acid δ15N values as indicators of plant nitrogen metabolism

Natural abundance δ(15)N values of plant tissue amino acids (AAs) reflect the cycling of N into and within plants, providing an opportunity to better understand environmental and anthropogenic effects on plant metabolism. In this study, the AA δ(15)N values of barley (Hordeum vulgare) and bread wheat (Triticum aestivum) grains and rachis and broad bean (Vicia faba) and pea (Pisum sativum) seeds, grown at the experimental farm stations of Rothamsted, UK and Bad Lauchstädt, Germany, were determined by GC-C-IRMS. It was found that the δ(15)N values of cereal grain and rachis AAs could be largely attributed to metabolic pathways involved in their biosynthesis and catabolism. The relative (15)N-enrichment of phenylalanine can be attributed to its involvement in the phenylpropanoid pathway and glutamate has a δ(15)N value which is an average of the other AAs due to its central role in AA-N cycling. The relative AA δ(15)N values of broad bean and pea seeds were very different from one another, providing evidence for differences in the metabolic routing of AAs to the developing seeds in these leguminous plants. This study has shown that AA δ(15)N values relate to known AA biosynthetic pathways in plants and thus have the potential to aid understanding of how various external factors, such as source of assimilated N, influence metabolic cycling of N within plants.

Experimental evidence for diel δ15N-patterns in different tissues, xylem and phloem saps of castor bean (Ricinus communis L.)

Nitrogen isotope signatures in plants might give insights in the metabolism and allocation of nitrogen. To obtain a deeper understanding of the modifications of the nitrogen isotope signatures, we determined δ(15)N in transport saps and in different fractions of leaves, axes and roots during a diel course along the plant axis. The most significant diel variations were observed in xylem and phloem saps where δ(15)N was significantly higher during the day compared with during the night. However in xylem saps, this was observed only in the canopy, but not at the hypocotyl positions. In the canopy, δ(15)N was correlated fairly well between phloem and xylem saps. These variations in δ(15)N in transport saps can be attributed to nitrate reduction in leaves during the photoperiod as well as to (15)N-enriched glutamine acting as transport form of N. δ(15)N of the water soluble fraction of roots and leaves partially affected δ(15)N of phloem and xylems saps. δ(15)N patterns are likely the result of a complex set of interactions and N-fluxes between plant organs. Furthermore, the natural nitrogen isotope abundance in plant tissue is not constant during the diel course - a fact that needs to be taken into account when sampling for isotopic studies.

Nitrogen metabolism of two contrasting poplar species during acclimation to limiting nitrogen availability

To investigate N metabolism of two contrasting Populus species in acclimation to low N availability, saplings of slow-growing species (Populus popularis, Pp) and a fast-growing species (Populus alba × Populus glandulosa, Pg) were exposed to 10, 100, or 1000 μM NH4NO3. Despite greater root biomass and fine root surface area in Pp, lower net influxes of NH4(+) and NO3(-) at the root surface were detected in Pp compared to those in Pg, corresponding well to lower NH4(+) and NO3(-) content and total N concentration in Pp roots. Meanwhile, higher stable N isotope composition (δ(15)N) in roots and stronger responsiveness of transcriptional regulation of 18 genes involved in N metabolism were found in roots and leaves of Pp compared to those of Pg. These results indicate that the N metabolism of Pp is more sensitive to decreasing N availability than that of Pg. In both species, low N treatments decreased net influxes of NH4(+) and NO3(-), root NH4(+) and foliar NO3(-) content, root NR activities, total N concentration in roots and leaves, and transcript levels of most ammonium (AMTs) and nitrate (NRTs) transporter genes in leaves and genes involved in N assimilation in roots and leaves. Low N availability increased fine root surface area, foliar starch concentration, δ(15)N in roots and leaves, and transcript abundance of several AMTs (e.g. AMT1;2) and NRTs (e.g. NRT1;2 and NRT2;4B) in roots of both species. These data indicate that poplar species slow down processes of N acquisition and assimilation in acclimation to limiting N supply.

The influence of leaf-atmosphere NH3(g ) exchange on the isotopic composition of nitrogen in plants and the atmosphere

The distribution of nitrogen isotopes in the biosphere has the potential to offer insights into the past, present and future of the nitrogen cycle, but it is challenging to unravel the processes controlling patterns of mixing and fractionation. We present a mathematical model describing a previously overlooked process: nitrogen isotope fractionation during leaf-atmosphere NH3(g ) exchange. The model predicts that when leaf-atmosphere exchange of NH3(g ) occurs in a closed system, the atmospheric reservoir of NH3(g ) equilibrates at a concentration equal to the ammonia compensation point and an isotopic composition 8.1‰ lighter than nitrogen in protein. In an open system, when atmospheric concentrations of NH3(g ) fall below or rise above the compensation point, protein can be isotopically enriched by net efflux of NH3(g ) or depleted by net uptake. Comparison of model output with existing measurements in the literature suggests that this process contributes to variation in the isotopic composition of nitrogen in plants as well as NH3(g ) in the atmosphere, and should be considered in future analyses of nitrogen isotope circulation. The matrix-based modelling approach that is introduced may be useful for quantifying isotope dynamics in other complex systems that can be described by first-order kinetics.

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.

Quantifying remobilization of pre-existing nitrogen from cuttings to new growth of woody plants using 15N at natural abundance

BACKGROUND

For measurements of nitrogen isotope composition at natural abundance, carry-over of pre-existing nitrogen remobilized to new plant growth can cause deviation of measured isotope composition (δ15N) from the δ15Nof newly acquired nitrogen. To account for this problem, a two-step approach was proposed to quantify and correct for remobilized nitrogen from vegetative cuttings of Populus balsamifera L. grown with either nitrate (δ15N = 58.5‰) or ammonium (δ15N = -0.96‰). First, the fraction of carry-over nitrogen remaining in the cutting was estimated by isotope mass balance. Then measured δ15N values were adjusted for the fraction of pre-existing nitrogen remobilized to the plant.

RESULTS

Mean plant δ15N prior to correction was 49‰ and -5.8‰ under nitrate and ammonium, respectively. Plant δ15N was non-linearly correlated to biomass (r2 = 0.331 and 0.249 for nitrate and ammonium, respectively; P < 0.05) where the δ15N of plants with low biomass approached the δ15N of the pre-existing nitrogen. Approximately 50% of cutting nitrogen was not remobilized, irrespective of size. The proportion of carry-over nitrogen in new growth was not different between sources but ranged from less than 1% to 21% and was dependent on plant biomass and, to a lesser degree, the size of the cutting. The δ15N of newly acquired nitrogen averaged 52.7‰ and -6.4‰ for nitrate and ammonium-grown plants, respectively; both lower than their source values, as expected. Since there was a greater difference in δ15N between the carried-over pre-existing and newly assimilated nitrogen where nitrate was the source, the difference between measured δ15N and adjusted δ15N was also greater. There was no significant relationship between biomass and plant δ15N with either ammonium or nitrate after adjusting for carry-over nitrogen.

CONCLUSION

Here, we provide evidence of remobilized pre-existing nitrogen influencing δ15N of new growth of P. balsamifera L. A simple, though approximate, correction is proposed that can account for the remobilized fraction in the plant. With careful sampling to quantify pre-existing nitrogen, this method can more accurately determine changes in nitrogen isotope discrimination in plants.

Comparative performance of δ13C, δ18O and δ15N for phenotyping durum wheat adaptation to a dryland environment

Grain yield and the natural abundance of the stable isotope compositions of carbon (δ13C), oxygen (δ18O) and nitrogen (δ15N) of mature kernels were measured during 3 consecutive years in 10 durum wheat genotypes (five landraces and five modern cultivars) subjected to different water and N availabilities in a Mediterranean location and encompassing a total of 12 trials. Water limitation was the main environmental factor affecting yield, δ13C and δ18O, whereas N fertilisation had a major effect on δ15N. The genotypic effect was significant for yield, yield components, δ13C, δ18O and δ15N. Landraces exhibited a higher δ13C and δ15N than cultivars. Phenotypic correlations of δ13C and δ18O with grain yield were negative, suggesting that genotypes able to sustain a higher water use and stomatal conductance were the most productive and best adapted; δ15N was also negatively correlated with grain yield regardless of the growing conditions. δ13C was the best isotopic trait in terms of genetic correlation with yield and heritability, whereas δ18O was the worst of the three isotopic abundances. The physiological basis for the different performance of the three isotopes explaining the genotypic variability in yield is discussed.

Comparative response of δ13C, δ18O and δ15N in durum wheat exposed to salinity at the vegetative and reproductive stages

This study compared the performance of the stable isotope composition of carbon (δ(13) C), oxygen (δ(18) O) and nitrogen (δ(15) N) by tracking plant response and genotypic variability of durum wheat to different salinity conditions. To that end, δ(13) C, δ(18) O and δ(15) N were analysed in dry matter (dm) and the water-soluble fraction (wsf) of leaves from plants exposed to salinity, either soon after plant emergence or at anthesis. The δ(13) C and δ(18) O of the wsf recorded the recent growing conditions, including changes in evaporative conditions. Regardless of the plant part (dm or wsf), δ(13) C and δ(18) O increased and δ(15) N decreased in response to stress. When the stress conditions were established just after emergence, δ(15) N and δ(13) C correlated positively with genotypic differences in biomass, whereas δ(18) O correlated negatively in the most severe treatment. When the stress conditions were imposed at anthesis, relationships between the three isotope signatures and biomass were only significant and positive within the most severe treatments. The results show that nitrogen metabolism, together with stomatal limitation, is involved in the genotypic response to salinity, with the relative importance of each factor depending on the severity and duration of the stress as well as the phenological stage that the stress occurs.

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.

Root metabolic response of rice (Oryza sativa L.) genotypes with contrasting tolerance to zinc deficiency and bicarbonate excess

Plants are routinely subjected to multiple environmental stresses that constrain growth. Zinc (Zn) deficiency and high bicarbonate are two examples that co-occur in many soils used for rice production. Here, the utility of metabolomics in diagnosing the effect of each stress alone and in combination on rice root function is demonstrated, with potential stress tolerance indicators identified through the use of contrasting genotypes. Responses to the dual stress of combined Zn deficiency and bicarbonate excess included greater root solute leakage, reduced dry matter production, lower monosaccharide accumulation and increased concentrations of hydrogen peroxide, phenolics, peroxidase and N-rich metabolites in roots. Both hydrogen peroxide concentration and root solute leakage were correlated with higher levels of citrate, allantoin and stigmasterol. Zn stress resulted in lower levels of the tricarboxylic acid (TCA) cycle intermediate succinate and the aromatic amino acid tyrosine. Bicarbonate stress reduced shoot iron (Fe) concentrations, which was reflected by lower Fe-dependent ascorbate peroxidase activity. Bicarbonate stress also favoured the accumulation of the TCA cycle intermediates malate, fumarate and succinate, along with the non-polar amino acid tyrosine. Genotypic differentiation revealed constitutively higher levels of D-gluconate, 2-oxoglutarate and two unidentified compounds in the Zn-efficient line RIL46 than the Zn-inefficient cultivar IR74, suggesting a possible role for these metabolites in overcoming oxidative stress or improving metal re-distribution.

Phenotyping for abiotic stress tolerance in maize

The ability to quickly develop germplasm having tolerance to several complex polygenic inherited abiotic and biotic stresses combined is critical to the resilience of cropping systems in the face of climate change. Molecular breeding offers the tools to accelerate cereal breeding; however, suitable phenotyping protocols are essential to ensure that the much-anticipated benefits of molecular breeding can be realized. To facilitate the full potential of molecular tools, greater emphasis needs to be given to reducing the within-experimental site variability, application of stress and characterization of the environment and appropriate phenotyping tools. Yield is a function of many processes throughout the plant cycle, and thus integrative traits that encompass crop performance over time or organization level (i.e. canopy level) will provide a better alternative to instantaneous measurements which provide only a snapshot of a given plant process. Many new phenotyping tools based on remote sensing are now available including non-destructive measurements of growth-related parameters based on spectral reflectance and infrared thermometry to estimate plant water status. Here we describe key field phenotyping protocols for maize with emphasis on tolerance to drought and low nitrogen.

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.

(12)C/(13)C fractionations in plant primary metabolism

Natural (13)C abundance is now an unavoidable tool to study ecosystem and plant carbon economies. A growing number of studies take advantage of isotopic fractionation between carbon pools or (13)C abundance in respiratory CO(2) to examine the carbon source of respiration, plant biomass production or organic matter sequestration in soils. (12)C/(13)C isotope effects associated with plant metabolism are thus essential to understand natural isotopic signals. However, isotope effects of enzymes do not influence metabolites separately, but combine to yield a (12)C/(13)C isotopologue redistribution orchestrated by metabolic flux patterns. In this review, we summarise key metabolic isotope effects and integrate them into the corpus of plant primary carbon metabolism.