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

Nitrogen isotope discrimination in open-pollinated and hybrid canola suggests indirect selection for enhanced ammonium utilization

Nitrogen isotope discrimination (Δ¹⁵N) may have utility as an indicator of nitrogen use in plants. A simple Δ¹⁵N-based isotope mass balance (IMB) model has been proposed to provide estimates of efflux/influx (E/I) ratios across root plasma membranes, the proportion of inorganic nitrogen assimilation in roots (P _root) and translocation of inorganic nitrogen to shoots (Ti/Tt) under steady-state conditions. We used the IMB model to investigate whether direct selection for yield in canola (Brassica napus L.) has resulted in indirect selection in traits related to nitrogen use. We selected 23 canola lines developed from 1942 to 2017, including open-pollinated (OP) lines developed prior to 2005 as well as more recent commercial hybrids (CH), and in three separate experiments grew them under hydroponic conditions in a greenhouse with either 0.5 mM ammonium, 0.5 mM nitrate, or 5 mM nitrate. Across all lines, E/I, P_root and Ti/Tt averaged 0.09±0.03, 0.82±0.05 and 0.23±0.06 in the low nitrate experiment, and 0.31±0.06, 0.71±0.07 and 0.42±0.12 in the high nitrate experiment, respectively. In contrast, in the ammonium experiment average E/I was 0.40±0.05 while Ti/Tt averaged 0.07±0.04 and P_root averaged 0.97±0.02. Although there were few consistent differences between OP and CH under nitrate nutrition, commercial hybrids were collectively better able to utilize ammonium as their sole nitrogen source, demonstrating significantly greater overall biomass and a lower P_root and a higher Ti/Tt, suggesting a somewhat greater flux of ammonium to the shoot. Average root and whole-plant Δ¹⁵N were also slightly higher in CH lines, suggesting a small increase in E/I. An increased ability to tolerate and/or utilize ammonium in modern canola hybrids may have arisen under intensive mono-cropping.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

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.

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.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Genotypic variation in nitrogen isotope discrimination in Populus balsamifera L. clones grown with either nitrate or ammonium

Intraspecific variability in nitrogen use has not been comprehensively assessed in a natural poplar species. Here, a nitrogen isotope mass balance approach was used to assess variability in nitrogen uptake, assimilation and allocation traits in 25 genotypes from five climatically dispersed provenances of Populus balsamifera L. grown hydroponically with either nitrate or ammonium. Balsam poplar was able to grow well with either ammonium or nitrate as the sole nitrogen source. Variation within provenances exceeded significant provenance level variation. Interestingly, genotypes with rapid growth on nitrate achieved similar growth with ammonium. In most cases, the root:shoot ratio was greater in plants grown with ammonium. However, there were genotypes where root:shoot ratio was lower for some genotypes grown with ammonium compared to nitrate. Tissue nitrogen concentration was greater in the leaves and stems but not the roots for plants grown with ammonium compared to nitrate. There was extensive genotypic variation in organ-level nitrogen isotope composition. Root nitrogen isotope discrimination was greater under nitrate than ammonium, but leaf nitrogen isotope discrimination was not significantly different between plants on different sources. This can indicate variation in partitioning of nitrogen assimilation, efflux/influx (E/I) and root or leaf assimilation rates. The proportion of nitrogen assimilated in roots was lower under nitrate than ammonium. E/I was lower for nitrate than ammonium. With the exception of E/I, genotype-level variations in nitrogen-use traits for nitrate were correlated with the same traits when grown with ammonium. Using the nitrogen isotope mass balance model, a high degree of genotypic variation in nitrogen use traits was identified at both the provenance and, more extensively, the genotypic level.

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

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

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.