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

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.

Estimating NOx removal capacity of urban trees using stable isotope method: A case study of Beijing, China

It is widely recognized that green infrastructures in urban ecosystems provides important ecosystem services, including air purification. The potential absorption of nitrogen oxides (NO_x) by urban trees has not been fully quantified, although it is important for air pollution mitigation and the well-being of urban residents. In this study, four common tree species (Sophora japonica L., Fraxinus chinensis Roxb., Populus tomentosa Carrière, Sabina chinensis (L.)) in Beijing, China, were studied. The dual stable isotopes (¹⁵N and ¹⁸O) and a Bayesian isotope mixing model were applied to estimate the sources contributions of potential nitrogen sources to the roadside trees based on leaf and soil sampling in urban regions. The following order of sources contributions was determined: soil > dry deposition > traffic-related NO_x. The capacity of urban trees for NO_x removal in the city was estimated using a remote sensing and GIS approach, and the removal capacity was found to range from 0.79 to 1.11 g m^-2 a^-1 across administrative regions, indicating that 1304 tons of NO_x could be potentially removed by urban trees in 2019. Our finding qualified the potential NO_x removal by urban trees in terms of atmospheric pollution mitigation, highlighting the role of green infrastructure in air purification, which should be taken into account by stakeholders to manage green infrastructure as the basis of a nature-based approach.

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.

Wheat exposure to cerium oxide nanoparticles over three generations reveals transmissible changes in nutrition, biochemical pools, and response to soil N

This study investigated the effects of third generation exposure to cerium oxide nanoparticles (CeO₂-NPs) on biomass, elemental and ¹⁵N uptake, and fatty acid contents of wheat (Triticum aestivum). At low or high nitrogen treatment (48 or 112 mg N), seeds exposed for two generations to 0 or 500 mg CeO₂-NPs per kg soil treatment were cultivated for third year in soil amended with 0 or 500 mg CeO₂-NPs per kg soil. The results showed that parental and current exposures to CeO₂-NPs increased the root biomass in daughter plants with greater magnitude of increase at low N than high N. When wheat received CeO₂-NPs in year 3, root elemental contents increased primarily at low N, suggesting an important role of soil N availability in altering root nutrient acquisition. The δ¹⁵N ratios, previously shown to be altered by CeO₂-NPs, were only affected by current and not parental exposure, indicating effects on N uptake and/or metabolism are not transferred from one generation to the next. Seed fatty acid composition was also influenced both by prior and current exposure to CeO₂-NPs. The results suggest that risk assessments of NP exposure may need to include longer-term, transgenerational effects on growth and grain quality of agronomic crops.

Fertilizer Rate-Associated Increase in Foliar Jasmonate Burst Observed in Wounded Arabidopsis thaliana Leaves is Attenuated at eCO2

The predicted future increase in tropospheric carbon dioxide (CO2) levels will have major effects on C3 plants and their interactions with other organisms in the biosphere. In response to attack by chewing arthropod herbivores or nectrotrophic pathogens, many plants mount a rapid and intense increase in jasmonate-related phytohormones that results in a robust defense response; however, previous studies have shown that C3 plants grown at elevated CO2 may have lower induced jasmonate levels, particularly in well nitrate-fertilized plants. Given the relationship between atmospheric CO2, photorespiration, cellular reductant and redox status, nitrogen assimilation and phytohormones, we compared wound-induced responses of the C3 plant Arabidopsis thaliana. These plants were fertilized at two different rates (1 or 10 mM) with nitrate or ammonium and grown at ambient or elevated CO2. In response to artificial wounding, an increase in cellular oxidative status leads to a strong increase in jasmonate phytohormones. At ambient CO2, increased oxidative state of nitrate-fertilized plants leads to a robust 7-iso-jasmonyl-L-isoleucine increase; however, the strong fertilizer rate-associated increase is alleviated in plants grown at elevated CO2. As well, the changes in ascorbate in response to wounding and wound-induced salicylic acid levels may also contribute to the suppression of the jasmonate burst. Understanding the mechanism underlying the attenuation of the jasmonate burst at elevated CO2 has important implications for fertilization strategies under future predicted climatic conditions.

Quantifying N-loss by root abscission: consequences for wheat N budgets and δ15N values

Lower plant δ¹⁵N values relative to source δ¹⁵N are commonly attributed to ¹⁵N efflux. We determined the extent to which root abscission contributes to plant N-loss and consequences for plant δ¹⁵N. Wheat (Triticum aestivum L. cv. SST015) was grown in hydroponics with direct aeration, aeration constrained within a pipe and circulation of nutrient solution through sand, representing three levels of stability for root growth. The δ¹⁵N of nutrient solutions and root fragments were periodically determined, as well as root and shoot δ¹⁵N. Plants in solution had significantly more negative δ¹⁵N (-8.9 and -9.2‰) than plants in sand (-6.9‰), suggesting greater ¹⁵N-loss; root fragments were major biomass- (six-fold greater than root dry weight) and N-loss (two-fold greater than plant net N uptake) pathways in solution. These plants had more ephemeral roots and two-fold more root tips than the sand treatment. We estimated that root fragment loss decreased plant δ¹⁵N by at least -3.7, -2.6 and -1.0‰ in the direct, pipe and sand treatments, respectively. Positive nutrient solution δ¹⁵N in all treatments relative to the source δ¹⁵N suggests that plant N, probably derived from efflux, was present in solution. Despite this, root abscission and root turnover are also important N-loss pathways in plants, while plant δ¹⁵N values are probably influenced by a combination of root abscission and N efflux.

Shifts in N and δ15N in wheat and barley exposed to cerium oxide nanoparticles

The effects of cerium oxide nanoparticles (CeO2-NPs) on 15N/14N ratio (δ15N) in wheat and barley were investigated. Seedlings were exposed to 0 and 500 mg CeO2-NPs/L (Ce-0 and Ce-500, respectively) in hydroponic suspension supplied with NH4NO3, NH4 +, or NO3 -. N uptake and δ15N discrimination (i.e. differences in δ15N of plant and δ15N of N source) were measured. Results showed that N content and 15N abundance decreased in wheat but increased in barley. Ce-500 only induced whole-plant δ15N discrimination (-1.48‰, P ≤ 0.10) with a simultaneous decrease (P ≤ 0.05) in whole-plant δ15N (-3.24‰) compared to Ce-0 (-2.74‰) in wheat in NH4 +. Ce-500 decreased (P ≤ 0.01) root δ15N of wheat in NH4NO3 and NH4 + (3.23 and -2.25‰, respectively) compared to Ce-0 (4.96 and -1.27‰, respectively), but increased (P ≤ 0.05) root δ15N of wheat in NO3 - (3.27‰) compared to Ce-0 (2.60‰). Synchrotron micro-XRF revealed the presence of CeO2-NPs in shoots of wheat and barley regardless of N source. Although the longer-term consequences of CeO2-NP exposure on N uptake and metabolism are unknown, the results clearly show the potential for ENMs to interfere with plant metabolism of critical plant nutrients such as N even when toxicity is not observed.

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.

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.

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

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

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.