Effects of water availability, nitrogen supply and atmospheric CO2 concentrations on plant nitrogen natural abundance values

Shifts in carbon and nitrogen stable isotope composition and epicuticular lipids in leaves reflect early water-stress in vineyards

Changes in leaf carbon and nitrogen isotope composition (δ¹³C and δ¹⁵N values) and the accumulation of epicuticular lipids have been associated with plant responses to water stress. We investigated their potential use as indicators of early plant water deficit in two grapevine (Vitis vinifera L.) cultivars, Chasselas and Pinot noir, that were field-grown under well-watered and water-deficient conditions. We tested the hypothesis that the bulk δ¹³C and δ¹⁵N values and the concentrations of epicuticular fatty acids may change in leaves of similar age with the soil water availability. For this purpose, leaves were sampled at the same position in the canopy at different times (phenological stages) during the 2014 growing season. Bulk dry matter of young leaves from flowering to veraison had higher δ¹³C values, higher total nitrogen content, and lower δ¹⁵N values than old leaves. In both cultivars, δ¹⁵N values were strongly correlated with plant water deficiency, demonstrating their integration of the plant water stress response. δ¹³C values recorded the water deficiency only in those plants that had not received foliar organic fertilization. The soil water deficiency triggered the accumulation of C_>26 fatty acids in the cuticular waxes. The compound-specific isotope analysis (CSIA) of fatty acids from old leaves showed an increase in δ¹³C among the C₁₆-C₂₂ chains, including stress signaling linoleic and linolenic acids. Our results provide evidence for leaf ¹³C-enrichment, ¹⁵N-depletion, and enhanced FA-chain elongation and epicuticular accumulation in the grapevine response to water stress. The leaf δ¹³C and δ¹⁵N values, and the concentration of epicuticular fatty acids can be used as reliable and sensitive indicators of plant water deficit even when the level of water stress is low to moderate. They could also be used, particularly the more cost-efficient δ¹³C and δ¹⁵N measurements, for periodic biogeochemical mapping of the plant water availability at the vineyard and regional scale.

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.

Effects of two centuries of global environmental variation on phenology and physiology of Arabidopsis thaliana

Intraspecific trait variation is caused by genetic and plastic responses to environment. This intraspecific diversity is captured in immense natural history collections, giving us a window into trait variation across continents and through centuries of environmental shifts. Here we tested if hypotheses based on life history and the leaf economics spectrum explain intraspecific trait changes across global spatiotemporal environmental gradients. We measured phenotypes on a 216-year time series of Arabidopsis thaliana accessions from across its native range and applied spatially varying coefficient models to quantify region-specific trends in trait coordination and trait responses to climate gradients. All traits exhibited significant change across space or through time. For example, δ¹⁵ N decreased over time across much of the range and leaf C:N increased, consistent with predictions based on anthropogenic changes in land use and atmosphere. Plants were collected later in the growing season in more recent years in many regions, possibly because populations shifted toward more spring germination and summer flowering as opposed to fall germination and spring flowering. When climate variables were considered, collection dates were earlier in warmer years, while summer rainfall had opposing associations with collection date depending on regions. There was only a modest correlation among traits, indicating a lack of a single life history/physiology axis. Nevertheless, leaf C:N was low for summer- versus spring-collected plants, consistent with a life history-physiology axis from slow-growing winter annuals to fast-growing spring/summer annuals. Regional heterogeneity in phenotype trends indicates complex responses to spatiotemporal environmental gradients potentially due to geographic genetic variation and climate interactions with other aspects of environment. Our study demonstrates how natural history collections can be used to broadly characterize trait responses to environment, revealing heterogeneity in response to anthropogenic change.

Patterns of univariate and multivariate plasticity to elevated carbon dioxide in six European populations of Arabidopsis thaliana

The impact of elevated carbon dioxide on plants is a growing concern in evolutionary ecology and global change biology. Characterizing patterns of phenotypic integration and multivariate plasticity to elevated carbon dioxide can provide insights into ecological and evolutionary dynamics in future human-altered environments. Here, we examined univariate and multivariate responses to carbon enrichment in six functional traits among six European accessions of Arabidopsis thaliana. We detected phenotypic plasticity in both univariate and multivariate phenotypes, but did not find significant variation in plasticity (genotype by environment interactions) within or among accessions. Eigenvector, eigenvalue variance, and common principal components analyses showed that elevated carbon dioxide altered patterns of trait covariance, reduced the strength of phenotypic integration, and decreased population-level differentiation in the multivariate phenotype. Our data suggest that future carbon dioxide conditions may influence evolutionary dynamics in natural populations of A. thaliana.

Plant community change mediates the response of foliar δ(15)N to CO 2 enrichment in mesic grasslands

Rising atmospheric CO2 concentration may change the isotopic signature of plant N by altering plant and microbial processes involved in the N cycle. CO2 may increase leaf δ(15)N by increasing plant community productivity, C input to soil, and, ultimately, microbial mineralization of old, (15)N-enriched organic matter. We predicted that CO2 would increase aboveground productivity (ANPP; g biomass m(-2)) and foliar δ(15)N values of two grassland communities in Texas, USA: (1) a pasture dominated by a C4 exotic grass, and (2) assemblages of tallgrass prairie species, the latter grown on clay, sandy loam, and silty clay soils. Grasslands were exposed in separate experiments to a pre-industrial to elevated CO2 gradient for 4 years. CO2 stimulated ANPP of pasture and of prairie assemblages on each of the three soils, but increased leaf δ(15)N only for prairie plants on a silty clay. δ(15)N increased linearly as mineral-associated soil C declined on the silty clay. Mineral-associated C declined as ANPP increased. Structural equation modeling indicted that CO2 increased ANPP partly by favoring a tallgrass (Sorghastrum nutans) over a mid-grass species (Bouteloua curtipendula). CO2 may have increased foliar δ(15)N on the silty clay by reducing fractionation during N uptake and assimilation. However, we interpret the soil-specific, δ(15)N-CO2 response as resulting from increased ANPP that stimulated mineralization from recalcitrant organic matter. By contrast, CO2 favored a forb species (Solanum dimidiatum) with higher δ(15)N than the dominant grass (Bothriochloa ischaemum) in pasture. CO2 enrichment changed grassland δ(15)N by shifting species relative abundances.

Influence of N doses and form on ¹⁵N natural abundance of pepper plants: considerations for using δ¹⁵N values as indicator of N source

BACKGROUND

The aim of this study was to ascertain the effect of the N form (NO(3) (-) , NH(4) (+) and organic N) and N concentration on plant isotopic fractionation and on the contribution of the different N sources to the plant N budget, in order to evaluate the feasibility of using plant δ(15) N values for discriminating between conventional and organic crops. To this end, different N concentrations (applied as NO(3) (-) ), N forms (NO(3) (-) versus NH(4) (+) ), and increasing NO(3) (-) applications to an organic N-based fertilization regime were studied.

RESULTS

When using NO(3) (-) as N source, intra-plant fractionation was significant and tended to increase when NO(3) (-) concentration increased in the root medium. However, negligible net isotopic fractionation was observed when comparing theoretical and measured plant δ(15) N values. On the other hand, when plants are fertilized with a mixture of NO(3) (-) and NH(4) (+) , differences in uptake patterns for both salts could result in variation in plant δ(15) N regarding to the expected value. Finally, the application of NO(3) (-) to plants was detected when it was combined with high levels of organic N sources, from 99:1 organic:inorganic N ratio.

CONCLUSION

Under certain conditions and following some considerations concerning sampling, δ(15) N values can be considered to be a potential tool to guarantee the authenticity of organic products.

Thirteen decades of foliar isotopes indicate declining nitrogen availability in central North American grasslands

*Humans are increasing both the deposition of reactive nitrogen (N) and concentrations of atmospheric CO(2) on Earth, but the combined effects on terrestrial ecosystems are not clear. In the absence of historical records, it is difficult to know if N availability is currently increasing or decreasing on regional scales. *To determine the nature and timing of past changes in grassland ecosystem dynamics, we measured the composition of stable carbon (C) and N isotopes in leaf tissue from 545 herbarium specimens of 24 vascular plant species collected in Kansas, USA from 1876 to 2008. We also parameterized a simple model of the terrestrial N cycle coupled with a stable isotope simulator to constrain processes consistent with observed patterns. *A prolonged decline in foliar N concentrations began in 1926, while a prolonged decline in foliar delta(15)N values began in 1940. Changes in the difference between foliar and atmospheric C isotopes reveal slightly increased photosynthetic water use efficiency since 1876. *The declines in foliar N concentrations and foliar delta(15)N suggest declining N availability in these grasslands during the 20th century despite decades of anthropogenic N deposition. Our results are consistent with progressive-nitrogen-limitation-type hypotheses where declines in N availability are driven by increased ecosystem N storage as a result of increased atmospheric CO(2).

Competition intensity at local versus regional spatial scales

Studies of competition intensity over natural (i.e. topographic) gradients often contradict the results from studies where artificial (i.e. fertilizer) gradients have been used. Why should the type of gradient matter? To explore the possibilities, we performed experiments to measure competition intensity experienced by tree seedlings from grass competitors across a natural resource gradient, and simultaneously across artificial soil nutrient (fertiliser) gradients. We measured various functional traits (i.e. specific leaf area, leaf area, leaf nitrogen content, delta(15)N, delta(13)C, RGR) to gain mechanistic insight into the nature of competition across these gradients. Competition intensity increased with increasing resource availability, unequivocally at the local scale (i.e. with fertilizer application) but not at the regional scale (i.e. across the natural productivity gradient). Our measurements of plant traits were generally consistent with measurements of competition intensity, and demonstrate that competition occurs even when resource levels are low. Competition mainly acted to reduce the growth of Eucalyptus seedlings. Functional (physiological) traits in the Eucalyptus seedlings were not strongly affected by competitors, with the possible exception of delta(15)N, which may effectively integrate information on soil nutrient, moisture and leaf processes.

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.

Carbon partitioning in N2 fixing Medicago sativa plants exposed to different CO2 and temperature conditions

Many of the studies analysing the CO₂ effect on plant development have been conducted in optimal growth conditions. Furthermore, although some of those studies suggest that legumes might show a steady productivity increase with rising CO₂, the role of nodule activity on the plant responsiveness to predicted atmospheric CO₂ enhancement is not well understood. In this study, C (metabolism and allocation) and N (nodule activity) interaction between the plant and the bacterial symbiont during the photosynthetic acclimation of N₂-fixing alfalfa (Medicago sativa L. cv. Aragón) plants exposed to elevated CO₂ and temperature conditions was analysed. The plants were grown in temperature gradient greenhouses (TGG) where, in the case of elevated CO₂ treatments, the isotopic ¹³C/¹²C composition (δ¹³C) inside the TGG was modified. Compared with the corresponding temperature treatment, exposure to 700 μmol mol^-1 CO₂ enhanced dry mass (DM) of plants in elevated temperature treatments (26%), whereas no significant effect was detected in ambient temperature treatments. The δ¹³C data revealed that although all the carbon corresponding to leaf total organic matter (TOM) came from newly assimilated C, plants exposed to elevated CO₂ did not develop strong sink activity (especially in ambient temperature conditions). Leaf carbohydrate build-up induced reduction in the Rubisco (E.C. 4.1.1.39) carboxylation capacity of plants. Despite this reduction in Rubisco content, plants exposed to elevated CO₂ conditions maintained (at ambient temperature) or increased (at elevated temperature) photosynthetic rates (measured at growth conditions) by increasing N use efficiency. The larger C sink strength of nodules in plants grown at elevated CO₂ and temperature conditions did not contribute towards overcoming photosynthetic acclimation. Further, the inhibitory effect of CO₂ on nodule total activity was caused by a large depletion in total soluble protein (TSP) of nodules. Depletion of leaf N demand, together with the reduction in nodule carbohydrate availability (as reflected by the nodule starch concentration), negatively affected the nodule TSP content and enzymatic activity.

Plastic Trait Integration across a CO2Gradient inArabidopsis thaliana

Shifts across environments in patterns of trait integration may govern or alter adaptive responses. Changes in resource supply rates may be an especially important cause of plasticity of trait integration because they can lead to shifts in colimitation and coregulation of traits. Traditional evolutionary genetic characterization of trait integration relies on covariance analyses. Structural equation modeling (SEM) can complement such analyses. The SEM provides insights into causal structure not possible with a covariance analysis, thereby providing mechanistic understanding of shifts in integration and suggesting likely foci of selection in changing environments. We tested for changes in trait integration by growing 35 genotypes of Arabidopsis thaliana (Brassicaceae; mouse-eared cress) from throughout the species' range in four atmospheric CO2 concentrations: 250 (past), 355 (approximately recent CO2), 530, and 710 (future) microM M(-1). SEM revealed significant shifts in the integration of N, C, and H2O use and their effects on reproductive dry mass across the CO2 gradient. The low CO2 stress of 250 microM M(-1) had the most divergent integration structures. Standardized total effects of C assimilation, water loss, and early N mass changed in sign across the C supply gradient, and the total effect of quantum yield decreased from significant to nonsignificant values across the gradient. Transpiration exhibited significant genetic variation and is thus a candidate target for selection and adaptation under novel growth CO2 concentrations. The strength of the correlation between C assimilation and transpiration declined by 19% from 250 to 710 microM M(-1), indicating a partial decoupling of their current mutual evolutionary constraint in the atmosphere of the future. Structural equation analysis of functional integration provides unique insights into the mechanisms through which changes in limiting resources can alter the nature of trait integration.

Carbon and nitrogen isotope variations in tree-rings as records of perturbations in regional carbon and nitrogen cycles

Increasing anthropogenic pollution from urban centers and fossil fuel combustion can impact the carbon and nitrogen cycles in forests. To assess the impact of twentieth century anthropogenic pollution on forested system carbon and nitrogen cycles, variations in the carbon and nitrogen isotopic compositions of tree-rings were measured. Individual annual growth rings in trees from six sites across Ontario and one in New Brunswick, Canada were used to develop site chronologies of tree-ring delta 15N and delta 13C values. Tree-ring 615N values were approximately 0.5% per hundred higher and correlated with contemporaneous foliar samples from the same tree, but not with delta 15N values of soil samples. Temporal trends in carbon and nitrogen isotopic compositions of these tree-rings are consistent with increasing anthropogenic influence on both the carbon and nitrogen cycles since 1945. Tree-ring delta 13C values and delta 15N values are correlated at both remote and urban-proximal sites, with delta 15N values decreasing since 1945 and converging on 1% per hundred at urban-proximal sites and decreasing but not converging on a single delta 15N value in remote sites. These results indicate that temporal trends in tree-ring nitrogen and carbon isotopic compositions record the regional extent of pollution.