Functional linkage between N acquisition strategies and aeration capacities of hydrophytes for efficient oxygen consumption in roots

We evaluated the specific strategies of hydrophytes for root O₂ consumption in relation to N acquisition and investigated whether the strategies varied depending on the aeration capacity. Aeration capacity of roots is an important factor for determining hypoxia tolerance in plants. However, some hydrophytes possessing quite different aeration capacities often co‐occur in wetlands, suggesting that root O₂ consumption also strongly affects hypoxia tolerance. We cultivated Phragmites australis with high aeration capacity and Zizania latifolia with low aeration capacity in hypoxic conditions with NH or NO treatment and compared the growth, N uptake, N assimilation and root respiration between the two species. In Z. latifolia grown with NH treatment, high N uptake activity and restrained root growth led to sufficient N acquisition and decrease in whole‐root respiration rate. These characteristics consequently compensated for the low aeration capacity. In contrast, in P. australis, low N uptake activity was compensated by active root growth, but the whole‐root respiration rate was high. This high root respiration rate was allowed by the high aeration capacity. The O₂ consumption‐related traits of hydrophyte roots were closely correlated with N acquisition strategies, which consequently led to a compensational relationship with the root aeration capacity. It is likely that this functional linkage plays an important role as a core mechanism in the adaptation of plants to hypoxic soils.     

Nitrogen metabolism of young barley plants as affected by NaCl-salinity and potassium

Summary In a solution culture experiment with 31 days old barley plants (var. Miura) the influence of NaCl-salinization (80 mM) and KCl addition (5 and 10 mM) on the uptake and turnover of labelled nitrogen (¹⁵NH₄ ¹⁵NO₃) was studied. Labelled N was applied for 24 h at the end of a 20 days' salinization period. Salinization impaired growth and incorporation of labelled N into the protein fraction paralleled by accumulation of labelle dinorganic N. All salt effects were much more pronounced in the shoots than in the roots.Potassium addition enhanced N uptake (total¹⁵N-content) and incorporation into protien, reduced the accumulation of inorganic N and improved the growth of salinized plants.The presented data support the point of view that impairment of protein (enzyme) metabolism is an important aspect of salt stress which is probably induced by the disturbance of the K/Na balance of the tissues under saline conditions.This work was supported by a grant from the Alexander von Humboldt foundation.     

Potassium and phosphate uptake of cucumber plants at different ammonia supply

Summary Experiments on cucumber plants grown in nutrient solution were conducted in order to study long and short time effects of ammonia on growth, nutrient element uptake and respiration of roots.Shoot yield and potassium concentration in tissue of plants treated 18 days with varied ammonia concentration were decreased. However, it was not assumed that K deficiency caused the yield reduction. The ammonia effect on K content was more pronounced in roots than in shoots.The decreased K concentration of plant tissue was linked to a diminished ability of plant roots to absorb potassium. The maximum rate of potassium uptake was lowered by ammonia during both, long- and short-time treatment. The results indicated that the NH₃ influence on potassium uptake was due to effects on metabolism and permeability of roots because changes of K uptake rate occurred immediately after starting the NH₃ treatment. Furthermore, it is shown that ammonia inhibited respiration of roots.During the short-time treatment net potassium efflux of roots was observed at higher NH₃ concentrations. The extent of K efflux depended on K concentration of both, root tissue and nutrient solution.Pretreating the plants for 12 hours with ammonia also resulted a decline in K uptake rate. However, plant roots subjected to ammonia concentrations up to 0.09 mM completely recovered during 24 hours after removing the NH₃ treatment whereas at higher NH₃ concentrations only a partial recovery occurred.Furthermore, it was shown that ammonia also influenced P uptake by plant roots.     

The effects of root surface charge and nitrogen forms on the adsorption of aluminum ions by the roots of rice with different aluminum tolerances

Aim

Our objectives were to compare effects of root charge properties on Al adsorption by the roots of rice that differed in Al-tolerance, and to examine effects of different nitrogen forms on charge properties of rice roots and Al adsorption.

Methods

Streaming potential and chemical methods were used to measure root zeta potential and investigate Al chemical forms adsorbed on the roots of rice obtained from solution culture experiments.

Results

Rice roots of the Al-sensitive variety Yangdao-6 carried greater negative charge than the Al-tolerant variety Wuyunjing-7, which meant the roots of Yangdao-6 adsorbed more exchangeable and complexed Al. When both rice varieties were grown in NH₄ ⁺-containing nutrient solutions, there were less functional groups and lower negative surface charge on their roots, which reduced Al adsorption compared to the rice grown in NO₃ ^? containing nutrient solutions. The decline in nutrient solution pH due to NH₄ ⁺ uptake by rice roots was responsible for the reduced numbers of functional groups and the lower negative surface charge on the roots compared to the rice grown in NO₃ ^? containing solutions.

Conclusions

Integrated root surface charge, as expressed by zeta potential, played an important role in Al adsorption by the roots of rice with different Al-tolerance.

     

Growth and development ofRumex roots as affected by hypoxic and anoxic conditions

The growth characteristics of threeRumex species were determined under different solution oxygen concentrations in hydroculture. These species all occur in a river foreland ecosystem and they were found to differ in their flood tolerance. The flood-tolerantR. maritimus undR. crispus developed a large number of new, aerenchymatous roots within a short period under low solution oxygen concentrations. Biomass production was not affected. In the flood-intolerantR. thyrsiflorus, however, only few slow-growing new roots were developed and biomass production was significantly reduced at solution oxygen concentrations below 2% (v:v). These different responses could be partly explained by a differential aerenchyma formation in new roots of the flood-tolerant species. Aerenchyma can relieve the oxygen stress of the root systems via internal aeration.The fast development of new roots of the flood-tolerantR. maritimus andR. crispus after the onset of anaerobiosis coincided with the reduction or cessation of growth of the primary roots. Notwithstanding the cessation of growth, however, primary roots of both species were able to recover following restoration of aerobic conditions after a 13-day anaerobic period. However, the roots ofR. thyrsiflorus ceased growing very soon after the onset of anaerobiosis. All had died within 10 days.The balance between the growth rates of the primary and the newly formed root system are discussed and related to the differential tolerance of theRumex species to transient flooding.     

Alteration of nitrogen metabolism in rice variety 'Nipponbare' induced by alkali stress

Aims

Alkali stress (AS) is an important agricultural contaminant and has complex effects on plant metabolism, specifically root physiology. The aim of this study was to test the role of nitrogen metabolism regulation in alkali tolerance of rice variety 'Nipponbare'.

Methods

In this study, the rice seedlings were subjected to salinity stress (SS) or AS. Growth, the contents of inorganic ions, NH ₄ ⁺ -nitrogen (free amino acids), and NO ₃ ^? -nitrogen in the stressed seedlings were then measured. The expression of some critical genes involved in nitrogen metabolism were also assayed to test their roles in the regulation of nitrogen metabolism during adaptation of rice variety 'Nipponbare' to AS.

Results

AS showed a stronger inhibiting effect on rice variety 'Nipponbare' growth than SS. AS may have more complex effects on nitrogen metabolism than SS.

Conclusions

Effects of AS on the nitrogen metabolism of rice variety 'Nipponbare' mainly comprised two mechanisms. Firstly, in roots, AS caused the reduction of NO ₃ ^? content, which caused two harmful consequences, the large downregulation of OsNR1 expression and the subsequent reduction of NH ₄ ⁺ production in roots. On the other hand, under AS (pH, 9.11), almost all the NH ₄ ⁺ was changed to NH₃, which caused a severe deficiency of NH ₄ ⁺ surrounding the roots. Both events might cause a severe deficiency of NH ₄ ⁺ in roots. Under AS, the increased expression of several OsAMT family members in roots might be an adaptative response to the reduction of NH ₄ ⁺ content in roots or the NH ₄ ⁺ deficiency in rhizosphere. Also, the down-regulation of OsNADH-GOGAT and OsGS1;2 in roots might be due to NH ₄ ⁺ deficiency in roots. Secondly, in shoots, AS caused a larger acuumulatiuon of Na⁺, which possibly affected photorespiration and led to a continuous decrease of NH ₄ ⁺ production in shoots, and inhibited the expression of OsFd-GOGAT and OsGS2 in chloroplasts.     

The Oxidation of d-1-Thioglucose by Cell-free Extract from Aspergillus niger

It was found that d-1-thioglucose was quantitatively converted to the corresponding disulfide with oxygen uptake by incubation with cell-free extract of Asp. niger. The molar ratio of the oxygen uptake to the d-1-thioglucose consumption was 0.22. The oxygen uptake was not affected by KCN, NH₂OH and NaN₃, but decreased by dialysis of the cell-free extract. The decrease of the oxygen uptake due to dialysis for 4 hr was recovered by the addition of FMN. From these results, a tentative mechanism of the oxidation of d-1-thioglucose was proposed.     

The Interaction of pH and Aeration in CI Uptake by Barley Roots

The uptake of Cl by excised roots of barley (Hordeum vulgare L.) from KC1 solution maintained at high pH was markedly reduced by high rates of aeration, whereas K uptake was scarcely affected. Aeration rate had relatively minor effects at low pH. The effect of high aeration rate at pH 9 could be overcome by the use of buffered solutions. In unbuffered solutions the H resulting from the excess cation uptake together with that produced from respiratory CO₂ was sufficient to materially reduce the pH of the solution. The reduction in pH favored the uptake of Cl which is adversely affected by high pH. The effect of aeration rate could be explained in terms of root induced pH changes and film diffusion involving the solution film adjacent to the root surface.     

Biofortification of rice grain with zinc through zinc fertilization in different countries

Aims and background

The ability to suppress soil nitrification through the release of nitrification inhibitors from plant roots is termed ‘biological nitrification inhibition’ (BNI). Earlier, we reported that sorghum roots release higher BNI-activity when grown with NH ₄ ⁺ , but not with NO ₃ ^- as N source. Also for BNI release, rhizosphere pH of <5.0 is needed; beyond this, a negative effect on BNI release was observed with nearly 80% loss of BNI activity at pH >7.0. This study is aimed at understanding the inter-functional relationships associated with NH ₄ ⁺ uptake, rhizosphere-pH and plasma membrane H⁺-ATPase (PM H⁺-ATPase) activity in regulating the release of BNIs (biological nitrification inhibitors) from sorghum roots.

Methods

Sorghum was grown hydroponically and root exudates were collected from intact plants using a pH-stat system to separate the secondary acidification effects by NH ₄ ⁺ uptake on BNIs release. A recombinant luminescent Nitrosomonas europaea bioassay was used to determine BNI-activity. Root plasma membrane was isolated using a two-phase partitioning system. Hydrolytic H⁺-ATPase activity was determined. Split-root system setup was deployed to understand the localized responses to NH ₄ ⁺ , H⁺-ATPase-stimulator (fusicoccin) or H⁺-ATPase-inhibitor (vanadates) on BNI release by sorghum.

Results

Presence of NH ₄ ⁺ in the rhizosphere stimulated the expression of H⁺-ATPase activity and enhanced the release of BNIs from sorghum roots. Fusicoccin, which stimulates H⁺-ATPase activity, also stimulated BNIs release in the absence of NH ₄ ⁺ ; vanadate, which suppresses H⁺-ATPase activity, also suppressed the release of BNIs. NH ₄ ⁺ levels (in rhizosphere) positively influenced BNIs release and root H⁺-ATPase activity in the concentration range of 0-1.0 mM, indicating a close relationship between BNI release and root H⁺-ATPase activity with a possible involvement of carrier-mediated transport for the release of BNIs in sorghum.

Conclusion

Our results suggest that NH ₄ ⁺ uptake, PM H⁺-ATPase activity, and rhizosphere acidification are functionally inter-connected with BNI release in sorghum. Such knowledge is critical to gain insights into why BNI function is more effective in light-textured, mildly acidic soils compared to other soil types.     

Nitrogen limitation in mycorrhizal Norway spruce (Picea abies) seedlings induced mycelial foraging for ammonium: implications for Ca and Mg uptake

Although many studies support the importance of the external mycelium for nutrient acquisition of ectomycorrhizal plants, direct evidence for a significant contribution to host nitrogen nutrition is still scarce. We grew nonmycorrhizal seedlings and seedlings mycorrhizal with Paxillus involutus (Batsch) Fr. in a sand culture system with two compartments separated by a 45-m Nylon mesh. Hyphae, but not roots, can penetrate this net. Nutrient solutions were designed to limit seedling growth by nitrogen. Hyphal density in the hyphal compartment, host N status and shoot growth of mycorrhizal seedlings significantly increased in response to NH₄ ⁺ addition to the hyphal compartment. Labeling the compartment only accessible to hyphae with ¹⁵NH₄ ⁺ showed that the increase in N uptake in the mycorrhizal seedlings was a result of hyphal N acquisition from the hyphal compartment. These results indicate that hyphae of P. involutus may actively forage into N-rich patches and improve host N status and growth. In the mycorrhizal seedlings, which received additional NH₄ ⁺ via their external mycelium, the increase in NH₄ ⁺ supply less negatively affected Ca and Mg uptake than in nonmycorrhizal seedlings, where the additional NH₄ ⁺ was directly supplied to the roots. This was most likely due to the close link of NH₄ ⁺ uptake and H⁺ extrusion, which, in the nonmycorrhizal seedlings, lead to a strong acidification in the root compartment, and subsequently reduced Ca and Mg uptake, whereas in the mycorrhizal seedlings the site of intensive NH₄ ⁺ uptake and acidification was in the hyphal and not in the root compartment. Our data support the idea that the ectomycorrhizal mycelium connected to an N-deficient host may actively forage for N. The mycelium may also be important as a biological buffer system ameliorating negative influence of high NH₄ ⁺ supply on cation uptake.     

Effect of salt stress on antioxidant system of plants as related to nitrogen nutrition

In plants of wheat (Triticum aestivum L.) grown in the media with nitrate (NO ₃ ^? plants), ammonium (NH ₄ ⁺ plants), and without nitrogen (N-deficient plants), the response to oxidative stress induced by the addition of 300 mM NaCl to the nutrient solution was investigated. Three-day-long salinization induced chlorophyll degradation and accumulation of malondialdehyde (MDA) in the leaves. These signs of oxidative stress were clearly expressed in NO ₃ ^? and N-deficient plants and weakly manifested in NH ₄ ⁺ plants. In none of the treatments, salinization induced the accumulation of MDA in the roots. Depending on the conditions of N nutrition, salt stress was accompanied by diverse changes in the activity of antioxidant enzymes in the leaves and roots. Resistance of leaves of NH ₄ ⁺ plants to oxidative stress correlated with a considerable increase in the activities of ascorbate peroxidase and glutathione reductase. Thus, wheat plants grown on the NH ₄ ⁺ -containing medium were more resistant to the development of oxidative stress in the leaves than those supplied with nitrate.     

Induction effect of PO 2 during continuous yeast production from ethanol in a multistage tower fermentor

Summary The effect of the periodic variation of the partial pressure of oxygen in the aeration gas on biomass concentrations, ethanol conversion, yield and productivity during continuous cultivations of the yeast Candida utilis in a multistage tower fermentor was studied. The results were compared with those obtained under aeration conditions with a constant P_{O 2} in the aeration gas. The results demonstrated that, with the optimum P_{O 2} in the aeration gas, the aeration procedure with the periodic variation of P_{O 2} in the gas phase permitted achievement of the same process parameters as those under constant P_{O 2}. Using this new aeration procedure, the consumption of pure oxygen can be lowered by 55% to 60%. In addition, the significance of the induction effect of P_{O 2} on growth characteristics in the individual stages of the fermentor was proved.Symbols Ac Concentration of acetic acid (g/l) - i Number of stage - P_{O 2} Partial pressure of oxygen in the aeration gas (torr) - P_R Productivity of the fermentor (g cell dwt/l/h) - S_R Ethanol concentration in the feed (g/l) - S Ethanol concentration in the cultivation broth (g/l) - t Time of continuous cultivation (h) - X Cell dry weight concentration (g/l) - (Y_X/S)_W Yield of cell dry weight from ethanol for the whole fermentor (g cell dwt/g ethanol) - Concentration interval in which parameters varied during the long-term cultivation at constant constant P_{O 2}=263.5 torr in the aeration gas - ₁ Concentration interval in which parameters varied during the long-term cultivation before the increase of P_{O 2} in the aeration gas - ₂ Concentration interval in which parameters varied during the long-term cultivation immediately after the decrtease of P_{O 2} in the aeration gas - ₃ Concentration interval in which parameters varied during the long-term cultivation about 24 h after the decrease of P_{O 2} in the aeration gas - ₄ Concentration interval in which parameters varied during the long-term cultivation about 48 h after the decrease of P_{O 2} in the aeration gas     

Soil potassium mobility and uptake by corn under differential soil moisture regimes

This study examined the effects of soil moisture on soil K mobility, dynamics of soil K, soil K fixation, plant growth and K uptake. A pot experiment, with and without corn (Zea maysL.), was conducted over a 16-d duration using a Yolo silt loam treated with two soil moisture regimes, i.e. constant moisture vs. wetting–drying (W–D) cycles. Soil K dynamics were determined using both ion exchange resin and direct extraction of soil solution. Soil K mobility increased significantly with soil moisture content (θ_v) and there was a positive curvilinear relationship between θ_v and effective diffusion coefficient (D_e), suggesting that more K⁺ can diffuse to the plant roots at sufficient soil moistures. Increase in D_e could be attributed to the decrease of impedance factor. During W–D cycles, soil solution K concentration increased as soil solution volume decreased, but soil solution K and NH₄ ⁺-extractable K pools decreased. In the constant moisture regime, available K pools decreased over the 16-d duration, but to a lesser extent than in W–D regime. The W–D cycles significantly enhanced K fixation and reduced available K pools in the soil in contrast to the constant moisture regime. Potassium fixation by the soil showed a biphasic pattern under the W–D regime, with a rapid fixation within the first 2 d after re-wetting, followed by a slower fixation. In the soil with constant moisture, K fixation was rapid during the first 8 h after wetting the soil, and then proceeded so slowly that no significant K fixation was observed after 4 d. The W–D cycles decreased root and shoot growth and K uptake by corn compared to constant moisture condition. Our results support the hypothesis that W–D cycles enhance soil K fixation, reduce soil K mobility and plant growth, and therefore reduce plant K⁺ uptake. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of different nitrogen forms and combination with foliar spraying with 6-benzylaminopurine on growth,transpiration, and water and potassium uptake and flow in tobacco

NH₄ ⁺-N can have inhibitory effects on plant growth. However, the mechanisms of these inhibitory effects are still poorly understood. In this study, effects of different N forms and a combination of ammonium + 6-benzylaminopurine (6-BA, a synthetic cytokinin) on growth, transpiration, uptake and flow of water and potassium in 88-days-old tobacco (Nicotiana tabacum L. K 326) plants were studied over a period of 12 days. Plants were supplied with equal amounts of N in different forms: NO₃ ^–, NH₄NO₃, NH₄ ⁺ or NH₄ ⁺+6-BA (foliar spraying every 2 days after onset of the treatments). For determining flows and partitioning upper, middle and lower strata of three leaves each were analysed. During the 12 days study period, 50% replacement of NO₃ ^–-N by NH₄ ⁺-N (NH₄NO₃) did not change growth, transpiration, uptake and flow of water and K⁺ compared with the NO₃ ^–-N treatment. However, NH₄ ⁺-N as the sole N-source caused: (i) a substantial decrease in dry weight gain to 42% and 46% of the NO₃ ^–-N and NH₄NO₃ treatments, respectively; (ii) a marked reduction in transpiration rate, due to reduced stomatal conductance, illustrated by more negative leaf carbon-isotope discrimination (¹³C) compared with the NO₃ ^– treatment, especially in upper leaves; (iii) a strong reduction both in total water uptake, and in the rate of water uptake by roots, likely due to a decrease in root hydraulic conductivity; (iv) a marked reduction of K⁺ uptake to 10%. Under NH₄ ⁺ nutrition the middle leaves accumulated 143%, and together with upper leaves 206% and the stem 227% of the K⁺ currently taken up, indicating massive mobilisation of K⁺ from lower leaves and even the roots. Phloem retranslocation of K⁺ from the shoot and cycling through the root contributed 67% to the xylem transport of K⁺, and this was 2.2 times more than concurrent uptake. Foliar 6-BA application could not suppress or reverse the inhibitory effects on growth, transpiration, uptake and flow of water and ions (K⁺) caused by NH₄ ⁺-N treatment, although positive effects by 6-BA application were observed, even when 6-BA (10^–8 M) was supplied in nutrient solution daily with watering. Possible roles of cytokinin to regulate growth and development of NH₄ ⁺-fed plants are discussed.     

Plant uptake of bicarbonate as measured with the11C isotope

Summary ¹¹C which is cyclotron produced by¹⁴N(P, )¹¹C(half-life 20.1M) was use as a tracer of bicarbonate to determine its movements from a nutrient solution through roots to stems and leaves of bush bean plants (Phaseolus vulgaris L. var. Improved Tendergreen). The short time involved and the high solution pH minimized the need for use of the Hederson Hasselbach equation for activity correction. Quantities of¹¹C did move into roots, stems and leaves with a sharp decreasing gradient (root/stem=14.5, stems/leaves=11.7) More¹¹C moved into plants with KHCO₃ than with NaHCO₃. The (NH₄)₂SO₄ enhanced¹¹C uptake and KNO₃ than with competition indicated possibility of some uptake of HCO ₃ ^– . In an experiment withGalenia pubescens (Eckl. and Zeyh.) Druce, the¹¹C was more readily moved to stems and leaves than in bush bean indicating substantial uptake of HCO ₃ ^– .     

Nitrogen uptake and allocation in <Emphasis Type="Italic">Populus simonii</Emphasis> in different seasons supplied with isotopically labeled ammonium or nitrate

Key message

The total uptake of ¹⁵ NO ₃ -N was twofold higher than that of ¹⁵ NH ₄ -N when supplied with ammonium and/or nitrate in different seasons; the seedlings fertilized with NO ₃ -N had good growth with high photosynthetic rate and total biomass.

Abstract

Appropriate fertilization is crucial for maximum plant growth and improving nitrogen use efficiency. Poplar is an important fast-growing tree species for biomass production, however, little is known about fertilizer management of poplar plantations growing on barren soil in different seasons. To understand nitrogen uptake and allocation of Populus simonii supplied with different forms of nitrogen in different seasons, we determined nitrogen uptake and allocation of P. simonii potted seedlings after a 4-day supply of ¹⁵NH₄-N, ¹⁵NO₃-N, ¹⁵NH₄NO₃, and NH ₄ ¹⁵ NO₃ in May, July, and September. The total ¹⁵N uptake was twofold higher when supplied with sole ¹⁵NO₃-N compared to sole ¹⁵NH₄-N in all the investigated seasons. In the presence of ammonium nitrate (¹⁵NH₄NO₃ and NH ₄ ¹⁵ NO₃), the total ¹⁵N uptake was two times higher when supplied with NH ₄ ¹⁵ NO₃ compared to ¹⁵NH₄NO₃. Per unit biomass, the ¹⁵N-uptake ability of fine roots was higher in May and July compared to that in September. ¹⁵N was present mainly in leaves in May and July, and was mainly stored in roots and stems in autumn. The effect of nitrogen on the growth of P. simonii seedlings was studied by fertilizing with NH₄-N, NO₃-N, and NH₄NO₃ for 8 weeks. The seedlings fertilized with NO₃-N had good growth with high photosynthetic rate and total biomass indicating that NO₃-N is crucial for P. simonii growth. These data contribute to understand the nitrogen uptake in different seasons in trees supplied with different forms of nitrogen. This provides important theoretical bases for fertilizer management of poplar plantations.

     

A comparative study of ammonium toxicity at different constatn pH of the nutrient solution

Seedlings of 14 species were grown for 14–28 days on nutrient solution with 6 mmol.l⁻¹ NH₄ as the sole nitrogen source. Solutions acidity was were kept constant at pH 4.0, 5.0, 6.0 and 7.0 by continuous titration with diluted KOH. The following species were used: barley, maize, oats sorghum, yellow and white lupin, pea, soybean, carrot, flax, castor-oil, spinach, sugarbeet and sunflower. Most plant species grew optimally at pH 6.0 with slight reductions at pH 5.0. Growth of many species was severely inhibited at pH 4.0, but this inhibition was not observed with the legume and cereal species. Yield depressions at pH 4.0 relative to pH 6.0 were well correlated with the respective relative decreases of the K concentration in their roots (P<0.002). In the roots of two species (sunflower and flax) total N concentrations were also strongly reduced at pH 4.0. apparently, the interactions between uptake of K, NH₄ and H ions become the prevalent problem at suboptimal pH. At pH 7.0, yields were also considerably decreased, with the exception of the lupines. At this pH, the roots of the growth inhibited plants were characterized by increased levels of total N and free NH₄. It is thought that the binding capacity of the roots for NH₄ is an important factor in preventing NH₄/NH₃ toxicity at supraoptimal pH.     

Nitrogen isotope discrimination in white spruce fed with low concentrations of ammonium and nitrate

Differences in ¹⁵N among ten white spruce [Picea glauca (Moench) Voss] families were examined in hydroponic experiments testing (1) three N sources [100 M N as (i) NH₄⁺, (ii) NO₃^– or (iii) NH₄NO₃] and (2) two supply regimes [200 M NH₄⁺ (i) maintained steadily or (ii) recurrently drawn-down]. In the N-source experiment, the NH₄⁺ treatment resulted in superior growth and lower C/N ratios. Whole plant ¹⁵N was higher on NH₄⁺ and NH₄NO₃, reflecting higher NH₄⁺ removal rates from the media. Families expressed differences in biomass, C/N, ¹⁵N and ¹³C. Family ¹⁵N and ¹³C were positively correlated in the NH₄NO₃ treatment and the steady-state regime. Supply regime did not affect total biomass, but higher root/shoot ratios implied N was more limiting under the draw-down regime. Family rank changed with supply regime, but not with N source. Analysis of media isotope enrichment during substrate depletion revealed relationships between net discrimination and external N concentration. Discrimination against ¹⁵NH₄⁺ was about twice that of ¹⁵NO₃^–. A simple model relating isotope discrimination to relative rates of ion efflux and influx predicted efflux/influx ratios consistent with published values for white spruce. We propose that genetic differences in discrimination are caused by different demands on assimilation and in uptake capacity which interact, influencing the balance between N influx and efflux.