Inorganic nitrogen uptake and regeneration in perennially icecovered Lakes Fryxell and Vanda, Antarctica

The isotope ¹⁵N was used to examine nitrogen dynamics in LakesFryxell and Vanda, two permanently ice-covered Antarctic lakes.Half-saturation constants for NH₄⁺. uptake in the shallow watersof both lakes were <10 µg N l^–1; uptake kineticexperiments on populations forming the deep-chlorophyll layersof these lakes showed zero-order kinetics and could not be fittedwith the Michaelis-Menten equation. Elevated uptake within thefirst few minutes following pulses of NH₄⁺. and NO₃^– occurredin both lakes. NH₄⁺ regeneration, determined from isotope dilutionexperiments, exceeded uptake at 4.6 m in Lake Fryxell, was lessthan uptake at 9 m in Lake Fryxell and was equal to uptake at10 m in Lake Vanda under the experimental conditions. NO₃^–uptake was suppressed by NH₄⁺ levels as low as 2 µg NH₄⁺-N l^–1 in Lake Fryxell; the suppression was strongestin the near-surface populations. Substrate-saturated C:N uptakeratios (g:g) in Lake Fryxell decreased from 8.4 near the surfaceto 1.8 at the bottom of the trophogenic zone. Overall, the nitrogendynamics in these lakes are similar to other lakes and the openocean in that biological productivity during the austral summeris supported by regenerated nutrients.     

Can the 15N Dilution Technique be used to Study N2 Fixation in Tropical Tree Symbioses as Affected by Water Deficit?

Three methods were used to study N₂ fixation and effects ofwater deficit on N₂ fixation: C₂H₂ reduction assay (ARA), ¹⁵Ndilution technique and accumulated N content. In addition, ¹⁵Ndilution was calculated both in a traditional way and in a modifiedway, which takes into consideration N and ¹⁵N content for theplants before the experiment started. The three methods wereapplied on the following Rhizobium-symbioses: Acacia albidaDel (Faidherbia albida (Del) A. Chev.) and Leucaena leucocephala(Lam) de Wit., and the Frankia-symbiosis Casuarina equisetifoliaL. The plants wereabout 4-months-old when they were harvested. Nitrogen derived from N₂ fixation in control plants of Acaciaalbida was 54·2 mg as measured with ARA, while it was28·5 mg as measured with the ¹⁵N dilution technique,compared to 30·7 mg calculated as accumulated N. In comparison,L. leucocephala fixed 41·6 mg N (ARA), 53·5 mgN(15N dilution technique) and 56·3 mg N (accumulatedN). The Frankia-symbiosis had fixed 27·4 mg N as measuredby ARA, 8·1 mg N as measured by ¹⁵N dilution techniqueand 12·3 mg N as accumulated N. There were no differencesbetween the estimates based ontraditional and modified waysof calculating ¹⁵N dilution. The immediate effect of water deficit treatment on N₂ fixationwas continuously measured inall species with ARA, which startedto decrease approximately 10 d after the initiation of the treatment,and declined to less than 5% of the initial level after 21–28d. The decrease in the amount of N derived from N₂ fixation wasstudied in L. leucocephala during the period of treatment. Therewas a 26% decrease in amount of N derived from N₂ fixation asresult of water deficit (as measured with ARA), while the decreasewas 23% when measured withboth the ¹⁵N dilution method and asaccumulated N. The three different methods for measuring N₂ fixation and effectsof water deficit on N₂ fixation are discussed. Key words: Acacia albida, ARA, Casuarina equisetifolia, Leucaena leucocephala, ¹⁵N dilution, N₂N fixation, water deficit     

Symbiotic N2 fixation of Alnus incana ssp. rugosa in shrub wetlands of the Adirondack Mountains, New York, USA

Surface waters in forested watersheds in the Adirondack Mountains and northern New York State are susceptible to nitrogen (N) saturation. Atmospheric deposition of N to watersheds in this region has been measured but the extent of internal N inputs from symbiotic N₂ fixation in alder-dominated wetlands is not known. We estimated N₂ fixation by speckled alder in these wetlands by the ¹⁵N natural abundance method and by acetylene reduction using a flow-through system. Foliar N derived from fixation (%N_dfa) was estimated for five wetlands. The '¹⁵N of speckled alder foliage from four of the five sites did not differ significantly (PА.05) from that of nodulated speckled alders grown in N-free water culture (-1.2ǂ.1‰). Estimates from the ¹⁵N natural abundance method indicated that alders at these sites derive 85-100% of their foliar N from N₂ fixation. At one of the sites, we also measured biomass and N content and estimated that the alder foliage contained 43 kg N ha^-1 of fixed N in 1997. This estimate was based on a foliar N content of 55.4lj kg N ha^-1 (mean-SE), 86dž%N_dfa, and an assumption that 10% of foliar N was derived from reserves in woody tissues. At this site, we further estimated via acetylene reduction that 37ᆞ kg N ha^-1 was fixed by speckled alders in 1998. This estimate used the theoretical 4:1 C₂H₂ reduction to N₂ fixation ratio and assumed no night-time fixation late in the season. Nitrogen inputs in wet and dry deposition at this site are approximately 8 kg N ha^-1 year^-1. We conclude that speckled alder in wetlands of northern New York State relies heavily on N₂ fixation to meet N demands, and symbiotic N₂ fixation in speckled alders adds substantial amounts of N to alder-dominated wetlands in the Adirondack Mountains. These additions may be important for watershed N budgets, where alder-dominated wetlands occupy a large proportion of watershed area.     

Salinity Reduces Water Use and Nitrate-N-use Efficiency of Citrus

Five-month-old Cleopatra mandarin (Citrus reticulata Blanco)(CM) and Volkamer lemon (Citrus volkameriana Ten. and Pasq.)(VL) seedlings were grown in a glasshouse in 2·3-1 potsof Candler fine sand. Plants were irrigated with either non-saline(EC_e = 0·23 dS m^-1) or saline (6·13 dS m^-1) waterusing 3:1 NaCl:CaCl₂ solution over a 4-week period. A singleapplication of K¹⁵NO₃ (19·64 atom % excess ¹⁵N) at 212mg N1^-1, was substituted for a normal weekly fertilization after3 weeks and plants were harvested 7 d later. The transpirationrate, uptake of nitrogen, growth and nitrogen-use efficiency(NUE) on a dry weight basis (mg d. wt mg^-1 N) of both specieswas reduced by salinity. Based on growth, water-use and chloride(Cl) accumulation in leaves, VL was more salt-sensitive thanCM, but ¹⁵N uptake was equally reduced by salinity in both species.Salinity reduced ¹⁵N uptake relatively more than shoot growthover the 7-d period, such that the ¹⁵NUE (mg d. wt µg^-115N) of new shoot growth of both species increased. There wasno evidence of Cl antagonism of nitrate (NO₃) uptake but totalplant ¹⁵NO₃ uptake was positively correlated with whole planttranspiration in both species. Thus, it appears that reductionsin NO₃ uptake are more strongly related to reduced water usethan to Cl antagonism from salt stress.Copyright 1993, 1999Academic Press Sodium, chloride, salinity, calcium, nitrate, ¹⁵NO₃ uptake, nitrogen allocation, nitrogen-use efficiency, water use, Citrus reticulata, Citrus volkameriana     

Contribution from Stolons and Roots to Estimates of the Total Amount of N2Fixed by White Clover (Trifolium repensL.)

Growth and N-accumulation rates in leaves, stolons and rootsof individual white clover plants were studied in three experimentsusing two methods. In a growth chamber experiment, the relativedifferences between tissues were found to be almost constantfor a wide range of clover plant sizes. The stolon dry matter(DM) production was 56% and the root DM production 40% of theDM production in leaves. The N yield of stolons was 30% whileN yield in roots was 34% of N yield in leaves. The effect ofN application on these relations was investigated in a glasshouseexperiment. Application of N reduced the root:shoot N ratiofrom 0.50 to 0.28, whereas the stolon+root:leaf N ratio (i.e.for abovevs.below cutting-height tissues) was only reduced from0.97 to 0.80. In a field trial with two contrasting N regimes,growth and N accumulation were measured on individual cloverplants. Dinitrogen fixation was estimated by¹⁵N isotope dilutionbased on analysis of leaves-only or by including stolons. Usingleaves-only did not affect the calculation of percentage ofclover N derived from N₂fixation (% Ndfa) since the¹⁵N enrichmentwas found to be uniform in all parts of the clover. A correctionfactor of 1.7 to account for N in below cutting-height tissueis suggested when N₂fixation in white clover is estimated byharvesting the leaves only.Copyright 1997 Annals of Botany Company Leaves; N accumulation; N₂fixation; ¹⁵N isotope dilution; pastures; roots; root/shoot ratio; stolons; Trifolium repensL.; white clover     

Variation in nitrogen-15 natural abundance and nitrogen uptake traits among co-occurring alpine species: do species partition by nitrogen form?

In the N-limited alpine tundra, plants may utilize a diversity of N sources (organic and inorganic N) in order to meet their nutritional requirements. To characterize species-level differences in traits related to N acquisition, we analyzed foliar '¹⁵N, nitrate reductase activity (NRA) and mycorrhizal infection in co-occurring alpine species during the first half of the growing season and compared these traits to patterns of N uptake using a ¹⁵N (¹⁵N-NH₄⁺, ¹⁵N-NO₃^-) or ¹³C,¹⁵N ([1]-¹³C-¹⁵N-glycine) tracer addition in the greenhouse. ¹³C enrichment in belowground tissue indicated that all species were capable of taking up labeled glycine, although only one species showed uptake of glycine potentially exceeding that of inorganic N. Species showing the most depleted foliar '¹⁵N and elevated NRA in the field also tended to show relatively high rates of NO₃^- uptake in the greenhouse. Likewise, species showing the most enriched foliar '¹⁵N also showed high rates of NH₄⁺ uptake. The ratio of NO₃^-:NH₄⁺ uptake rates and growth rate explained 64% and 72% of the variance in foliar '¹⁵N, respectively, suggesting that species differ in the ability to take up NO₃^- and NH₄⁺ in the field and that such differences may enable species to partition soil N on the basis of N form.     

Development of the Xylem Ureide Assay for the Measurement of Nitrogen Fixation by Pigeonpea (Cajanus cajan (L.) Millsp.)

Quantification of N₂ fixation by pigeonpea (Cajanus cajan (L.)Millsp.) in the field has proved difficult using techniquessuch as ¹⁵N isotope dilution, acetylene reduction and N difference.We report experiments to develop the ureide assay of N₂ fixationbased on extraction and analysis of xylem exudate. Plants ofpigeonpea cv. Quantum, inoculated with effective Rhizobium spp.CB756, were grown in a temperature-controlled glasshouse inlarge pots filled with a sand: vermiculite mixture, in waterculture and in a slightly acidic, red-brown earth in replicatedfield plots. Xylem exudate was collected as bleeding sap fromboth nodulated and unnodulated roots, and from detached nodules.Exudate was extracted also from detached shoots and stems ofpigeonpea using a mild vacuum (60–70 kN m^–2). Largedifferences in the composition of N solutes exported from rootsof N₂-dependent and nitrate-dependent plants suggested thatshifts in plant dependence on N₂ fixation may be reflected byconcomitant changes in N solutes. Thus, nodulated plants weresupplied throughout growth with either N-free nutrients or nutrientssupplemented with 1, 2, 5, 5, 10, or 20 mol m^{–3 15}. Plants were harvested at regular intervals fordry matter and vacuum-extracted exudate. The relative abundanceof ureides ([ureide-N/ureide-N + nitrate-N + -amino-N] ? 100)in the exudate was highly correlated with the proportion ofplant N (calculated using a ¹⁵N isotope dilution technique)derived from N₂ fixation. Two distinct phases of plant growthwere recognized and standard curves were prepared for each.The relationship between proportional dependence of plants onN₂ and xylem relative ureides was unaffected by mineral-N source,i.e. nitrate or ammonium. This result is discussed in relationto interpretation of material from field-grown plants. The effectsof plant genotype, strain of rhizobia, section of stem extracted,removal of leaves, time delay between shoot detachment and extraction,and diurnal characteristics were examined in order to identifypotential sources of error and to optimize sampling procedures. Key words: Ureides, allantoin, allantoic acid, N₂ fixation, pigeonpea, Cajanus cajan     

Diurnal regulation of NO3- uptake in soybean plants I. Changes in NO3- influx, efflux, and N utilization in the plant during the day/night cycle

The effect of light on NO₃^– utilization was investigatedin non-nodulated soybean (Clycine max L. Merr., cv. Kingsoy)plants during a 14/10 h light/dark period at a constant temperatureof 26C. A 30–50% decrease of net NO₃^– uptake ratewas observed 2–6 h after the lights were turned off. Thiswas specifically due to an inhibition of NO₃^– influx asmeasured by ¹⁵N incorporation during 5 min. The absolute valuesof NO₃^– efflux depended on whether the labelling protocolinvolved manipulation of the plants or not, but were not affectedby illumination of the shoots. Darkness had an even more markedeffect in lowering the reduction of ¹⁵NO₃^– in both rootsand shoots, as well as xylem transport of ¹⁵NO₃^– and reduced¹⁵N. Concurrently with this slowing down of transport and metabolicprocesses, accumulations of NO₃^– and Asn were significantlystimulated in roots during the dark period. These data are discussedin view of the hypothesis that darkness adversely affects NO₃^–uptake through specific feedback control, in response to alterationsin the later steps of N utilization which are more directlydependent on light. Key words: Glycine max, light/dark cycles, nitrate uptake, nitrate reduction     

Phosphate Regulation of Nitrate Assimilation in Soybean

It is known that phosphorus deficiency results in alterationsin the assimilation of nitrogen. An experiment was conductedto investigate mechanisms involved in altered ¹⁵NO^–₃ uptake,endogenous ¹⁵N translocation, and amino acid accumulation insoybean (Glycine max L. Merrill, cv. Ransom) plants deprivedof an external phosphorus supply for 20 d in solution culture.Phosphorus deprivation led to decreased rates of ¹⁵NO^–₃uptake and increased accumulation of absorbed ¹⁵N in the root.Both effects became more pronounced with time. Asparagine, theprimary transport amino acid in soybean, accumulated in largeexcess in roots and stems. In roots of phosphorus-deprived plants,concentrations of ATP and inorganic phosphate declined rapidly,but dry weight accumulation was similar to or above that ofthe control even after 20 d of treatment. Arginine accumulationin leaves was greatly enhanced, even though ¹⁵N partitioninginto the insoluble reduced-N fraction of leaves was unaffected.The results suggest that decreases in NO^–₃ uptake in lowphosphorus plants could be caused by feedback control factorsand by limited ATP availability. The decline in endogenous Ntransport from the root to the shoot may be associated withchanges in membrane properties, which also result in paralleleffects on hydraulic conductance and the upward flow of waterthrough the plant. Key words: Phosphorus stress, nitrate uptake, nitrate translocation, arginine     

N2 Fixation and Nitrate Uptake by White Clover Swards in Response to Root Temperature in Flowing Solution Culture

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown as simulated swards for 71 d in flowing nutrientsolutions with roots at 11 C and shoots at 20/15 C, day/night,under natural illumination. Root temperatures were then changedto 3, 5, 7, 11, 13, 17 or 25 C and the total N₂, fixation over21 d was measured in the absence of a supply mineral N. Alltreatments were subsequently supplied with 10 mmol m^–2NO₂ ^– in the flowing solutions for 14 d, and the relativeuptake of N by N₂, fixation and NO₃ ^– uptake was compared.Net uptake of K⁺ was measured on a daily basis. Root temperature had little effect on root d. wt over the 35-dexperimental period, but shoot d. wt increased by a factor of3.5 between 3 and 25 C, with the sharpest increase occurringat 7–11 C. Shoot: root d. wt ratios increased from 25to 68 with increasing temperature at 7–25 C. N₂-fixationper plant (in the absence of NO₂ ^–) increased with roottemperature at 3–13C, but showed little change above13 C. The ratios of N₂ fixation: NO₂ ^– uptake over 14d (mol N: mol N) were 0.47–0.77 at 3–7 C, 092–154at 11–17 C, and 046 at 25 C, reflecting the dominanceof NO₃ ^– uptake over N₂ fixation at extremes of high andlow root temperature. The total uptake of N varied only slightlyat 11–25 –C (095–110 mmol N plant^–1),the decline in N₂ fixation as root temperature increased above11 C was compensated for by the increase in NO^– ₃ uptake.The % N in shoot dry matter declined with decreasing root temperature,from 32% at 13 C to 15% at 3 C. In contrast, concentrationsof N expressed on a shoot water content basis showed a modestdecrease with increasing temperature, from 345 mol m^–3at 3 C to 290 mol m^–3 at 25 C. Trifolium repens L, white clover, root temperature, N₂ fixation, potassium uptake, nitrate uptake, flowing solution culture     

Cortical Cell Fluxes of Ammonium and Nitrate in Excised Root Segments ofAllium cepa L; Studies using15N

From compartmental analysis of ¹⁵N elution measurements, concentrationsand fluxes of NH⁺₄ and NO₃^– were estimated for corticalcells in excised root segments, when bathed in a complete nutrientsolution, in which 20 mol m^–3 NH₄⁺ or NO₃^– werethe single N sources. The results were compared with those fornutrient solution containing 20 mol m^–3 NH₄NO₃. No nitratereductase activity was detected in the roots but rapid assimilationof NH₄⁺ occurred, due to glutamine synthetase activity. Theefflux curves for NH₄⁺, on a 'µg ¹⁵N remaining againsttime' basis, deviated from the criteria determining conformityto first order kinetics, since the slowest rate constant wasan order of magnitude lower than that exhibited by the curvefor efflux versus time. The data were transformed to conformto the appropriate criteria, revealing a large slowly exchangingpool equated with assimilated NH₄⁺. The presence of NO₃^–had little effect on NH₄⁺ uptake and exchange, but NH₄⁺ suppressedNOj uptake and reduced exchange across plasmalemma and tonoplast.It was established that NH₄⁺ absorption was an active process.However, NH₄⁺ entering and leaving the vacuole was overestimated,since the flux equation used did not differentiate between total¹⁵NH₄ influx at the plasmalemma and that at the tonoplast, afterassimilation. The only active NO₃^– transfer was influxat the plasmalemma. The results were compared with those ofothers using¹³N and ³⁶C1O₃ analogues to measure NH₄⁺ and NO₃^–fluxes in cereal roots. Key words: Ammonium, nitrate, compartmental analysis, ¹⁵N, active transport     

The Apparent Induction of Nitrate Uptake by Chara corallina Cells following Pretreatment with or without Nitrate and Chlorate

Nitrate provision has been found to regulate the capacity forChara corallina cells to take up nitrate. When nitrate was suppliedto N sufficient cells maximum nitrate uptake was reached after8 h. Prolonged treatment of the cells in the absence of N alsoresulted in the apparent ability of these cells to take up nitrate.Chlorate was found to substitute partially for nitrate in the‘induction’ step. The effects on nitrate reductionwere separated from those on nitrate uptake by experiments usingtungstate. Tungstate pretreatment had no effect on NO^–₃uptake ‘induced’ by N starvation, but inhibitedNO^–₃ uptake associated with NO^–₃ pretreatment. Chloridepretreatment similarly had no effect on NO^–₃ uptake ‘induced’by N deprivation, but inhibited NO^–₃ uptake followingNO^–₃ pretreatment. The data suggest that there are atleast two mechanisms responsible for the ‘induction’of nitrate uptake by Chara cells, one associated with NO^–₃reduction and ‘induced’ by CIO^–₃ or NO^–₃and one associated with N deprivation. Key words: Nitrate, Chlorate, Chara corallina, Induction     

Nitrate Stimulation of Mobilization of Seed Reserves in Temperate Cereals: Importance of Water Uptake

Relationships between nitrate (NO^-₃) supply, uptake and assimilation,water uptake and the rate of mobilization of seed reserves wereexamined for the five main temperate cereals prior to emergencefrom the substrate. For all species, 21 d after sowing (DAS),residual seed dry weight (d.wt) decreased while shoot plus rootd.wt increased (15–30%) with increased applied NO^-₃concentrationfrom 0 to 5–20 mM . Nitrogen (N) uptake and assimilationwere as great with addition of 5 mM ammonium (NH⁺₄) or 5 mMNO^-₃but NH⁺₄did not affect the rate of mobilization of seedreserves. Chloride (Cl^-) was similar to NO^-₃in its effect onmobilization of seed reserves of barley (Hordeum vulgare L.).Increased rate of mobilization of seed reserves with additionalNO^-₃or Cl^-was associated with increases in shoot, root and residualseed anion content, total seedling water and residual seed watercontent (% water) 21 DAS. Addition of NH⁺₄did not affect totalseedling water or residual seed water content. For barley suppliedwith different concentrations of NO^-₃or mannitol, the rate ofmobilization of seed reserves was positively correlated (r >0.95)with total seedling water and residual seed water content. Therate of mobilization of seed reserves of barley was greaterfor high N content seed than for low N content seed. Seed watercontent was greater for high N seed than for low N seed, 2 DAS.Additional NO^-₃did not affect total seedling water or residualseed water content until 10–14 DAS. The effects of seedN and NO^-₃on mobilization of seed reserves were detected 10and 14 DAS, respectively. It is proposed that the increasedrate of mobilization of seed reserves of temperate cereals withadditional NO^-₃is due to increased water uptake by the seedlingwhile the seed N effect is due to increased water uptake bythe seed directly. Avena sativa L.; oat; Hordeum vulgare L.; barley; Secale cereale L.; rye; xTriticosecale Wittm.; triticale; Triticum aestivum L.; wheat; nitrate; seed; germination; seed reserve mobilization     

Influence of Chloride and Transpiration on Net15NO3-Uptake Rate byCitrus Roots

Three-month-old Carrizo citrange (hybrid of Citrus sinensisL. OsbeckxPoncirus trifoliata Blanco) seedlings were grown incontrolled environment chambers in pots of fine sand. Plantswere irrigated with either non-saline or saline solutions overa 3-week period. After these treatments, plants were transferredto vessels containing a 5 m M¹⁵NO₃K (96% atom excess¹⁵N) solution,and transpiration as well as concentration of¹⁵N and Cl^-in roots,stem and leaves were measured after 24 h. Transpiration and¹⁵NO₃^-uptakerates were inhibited after exposure to NaCl and the concentrationof salt pre-treatment determined the intensity of this inhibitoryeffect. To determine the effect of transpiration on NO₃^-absorption,net¹⁵NO₃^-uptake rate was measured in salt stressed and non-stressedplants exposed to different light intensities or relative humiditiesand also in detached roots. Reduction in NO₃^-uptake was moreclosely related to Cl^-antagonism from salt stress than to reducedtranspiration rate. Copyright 1999 Annals of Botany Company Nitrate, absorption, inhibition transport system, salt, light and humidity.     

The Influence of Plant Nitrogen Status on NO2 Uptake, NO2 Assimilation and on the Gas Exchange Characteristics of Barley Plants Exposed to Atmospheric NO2

Barley plants were grown in nutrient solution at two contrastingnitrate concentrations to produce plants of low or high nitrogen(N) status. Leaves were then exposed continuously to either0.3 mm³ dm^–3 NO₂ or clean air, with the roots and rootingmedium isolated from the polluted air. Uptake of NO₂ was measuredin two ways; as depletion from an air stream containing thegas and using ¹⁵N-labelled NO₂. Results from the two methodsagreed well and demonstrated that the flux of NO₂ into the leavesof N-deficient barley was lower than that of N-sufficient plants.Nevertheless, the relative contribution of¹⁵N derived from ¹⁵NO₂to the N status of the plant was greater in the plants suppliedwith low nitrate. A major factor in regulating NO₂ uptake bybarley leaves appeared to be stomatal conductance, althoughinternal conductance may also be involved. The effects of NO₂exposure of barley on carbon dioxide exchange rates, transpirationand water vapour conductance were also influenced by the N statusof the plant. Key words: Hordeum vulgare, ¹⁵N-labelled NO₂, carbon dioxide exchange, transpiration     

Diurnal regulation of NO3-uptake in soybean plants II. Relationship with accumulation of NO and asparagine in the roots

Experiments were performed with soybean plants to test the hypothesisthat the inhibition of NO₃^– uptake in darkness is dueto feedback control by NO₃^– and/or Asn accumulating inthe roots. Xylem export of N compounds was shown to depend onwater flux in both excised root systems and ¹⁵N-labelled intactplants, suggesting that the shortage of transpiration in darknessmay be responsible for the retention of NO₃^– and Asn inthe roots. This was verified in experiments where the light/darkpattern of transpiration was modulated in intact plants by changingthe relative humidity of the atmosphere. Any decrease of transpirationat night was associated with a concurrent stimulation of NO₃^–and Asn accumulations in the roots. However, the light/darkrhythmicity of NO₃^– uptake was only marginally affectedby these treatments, and thusappeared quite independent fromtranspiration and root NO₃^– or Asn levels. Typically,the maintainance of a constant transpiration during the day/nightcycle did not suppress the inhibition of NO₃^– uptake indarkness, whereas it almost prevented the dark increase in rootNO₃^– and Asn contents. These data strongly support theconclusion that the effect of light on NO₃^– uptake isnot mediated by changes in translocation and accumulation ofN compounds. Key words: Glycine max, light/dark, cycles, nitrate uptake, transpiration, transport of N compounds, accumulation of N compounds     

Influx and Efflux of Nitrate and Ammonium in Italian Ryegrass and White Clover Roots: Comparisons Between Effects of Darkness and Defoliation

Seedlings of Italian ryegrass (Lolium multiflorum Lam. cv. RVP)and clonal stolon cuttings of white clover (Trifolium repensL. cv. Blanca) were grown for 19 d in flowing solution culture,with N supplied as either 250 mmol m^–3 NO₃^– or NH₃⁺.Rates of net uptake, influx and translocation of NO₃^–and NH₄⁺ were then determined using ¹⁵N and ¹³N labelling techniques:between 3–5 h into the photoperiod following 8 h darknessfor white clover (CL), and for ryegrass plants that were eitherentire (IL) or with shoots excised 90 min prior to ¹³N influx(IC); and 75 min into the photoperiod following 37–39h darkness for ryegrass (ID). Rates of net uptake, influx andefflux of NH₄⁺ exceeded those of NO₃^– in IL and IC ryegrassplants: the opposite occurred in white clover (CL). The decreasein net uptake following defoliation of ryegrass was greaterfor NH₄⁺ (62%) than NO₃^– (40%). For NH₄⁺ this was associatedwith a large decrease in influx from 110 to 6.0µmol h^–1g^–1 root fr. wt; but for NO₃^–, influx only decreasedfrom 42 to 37 µmol h^–1 g^–1. Prolonged exposureto darkness (ID plants) also lowered net uptake of NO₃^–and NH₄⁺ by, respectively, 86% and 95% of IL levels. For NH₄⁺this was characterized by a large decrease in influx and a smalldecrease in efflux; whilst for NO₃^– the effect of a largedecrease in influx was reinforced by a smaller increase in efflux. The data were used to estimate the translocatory fluxes of NO₃^–(03–20µmol h^–1 g^–1) and NH₄⁺ (003–0.4µmolh^–1 g^–1), assimilation in the roots of NO₃^–(02–26µmol h^–1 g^–1) and NH⁺⁴ (05–89 µmolh^–1 g^–1), and the concentrations of NO₃^– (9–15mol m^–3) in the cytoplasmic compartment of the roots.The relevance of variable influx and efflux to models for theregulation of N uptake is discussed. Key words: Lolium multiflorum, Trifolium repens, influx, efflux, nitrate, ammonium, ¹³N     

Assimilation of 15N2 and 15NO3- by Partially Nitrate-Tolerant Nodulation Mutants of Soybean

Growth-chamber studies were conducted to evaluate nitrogen assimilationby three hypernodulated soybean [Glycine max (L.) Merr.] mutants(NOD1–3, NOD2–4, NOD3–7) and the Williamsparent. Seeds were inoculated at planting and transplanted atday 7 to nutrient solution with 1 mol m^–3 urea (optimizesnodule formation) or 5 mol m^–3 NO₃^– (inhibits noduleformation). At 25 d after planting, separate plants were exposedto ¹⁵NO₂ or ¹⁵NO₃^– for 3 to 48 h to evaluate N₂ fixationand NO₃^– assimilation. Plant growth was less for hypernodulatedmutants than for Williams with both NO₃^– and urea nutrition.The major portion of symbiotically fixed ¹⁵N was rapidly assimilated(30 min) into an ethanol-soluble fraction, but by 24 h aftertreatment the ethanolinsoluble fraction in each plant part wasmost strongly labelled. Distribution patterns of ¹⁵N among organswere very similar among lines for both N growth treatments aftera 24 h ¹⁵N₂ fixation period; approximate distributions were40% in nodules, 12% in roots, 14% in stems, and 34% in leaves.With urea-grown plants the totalmg ¹⁵N fixed plant^–1 24h^–1 was 1·18 (Williams), 1·40 (N0D1-3),107 (NOD2-4), and 0·80 (NOD3-7). The 5 mol m^-3 NO₃^- treatmentresulted in a 95 to 97% decrease in nodule mass and ¹⁵N₂ fixationby Williams, while the three mutants retained 30 to 40% of thenodule mass and 17 to 19% of the ¹⁵N₂ fixation of respectiveurea-grown controls. The hypernodulated mutants, which had restrictedroot growth, absorbed less ¹⁵NO₃^- than Williams, irrespectiveof prior N growthcondition. The ¹⁵N from ¹⁵NO₃^- was primarilyretained in the soluble fraction of all plant parts through24 h. The ¹⁵N incorporation studies confirmed that nodule developmentis less sensitive to external NO₃^- in mutant lines than in theWilliams parent, and provide evidence that subsequent metabolismand distribution within the plant was not different among lines.These results further confirm that the hypernodulated mutantsof Williams are similar in many respects to the hyper- or supernodulatedmutants in the Bragg background, and suggest that a common mutationalevent affectingautoregulatory control of nodulation has beentargeted. Key words: Glycine max (L.) Merr., soybean, N₂fixation, nitrate assimilation, nodulation mutants, ¹⁵N isotope     

Microbial ammonium cycling in the Mississippi River plume during the drought spring of 2000

Microbial potential uptake and regeneration rates of ammonium(NH₄⁺) were studied along a salinity gradient (salinities 0.2–34.4)in the Mississippi River plume during an extreme drought inspring 2000. Chlorophyll concentrations up to 30 µg L^–1were highest in the low- and mid-salinity regions (salinities8.5–28.2) and comparable to records of other years butextended over smaller areas than during periods of normal riverflow. Bacterial biomass (5.1–28.3 µg C L^–1)was at the low end of the range observed in normal flow years,decreased with distance from the river mouth and did not peakwith chlorophyll. Heterotrophic nanoflagellate abundance (1.4–4.0µg C L^–1) did not reflect phytoplankton and bacterialspatial distribution but peaked at 9.2 µg C L^–1at salinity 8.5. Microbial NH₄⁺ regeneration rates were estimatedby ¹⁵NH₄⁺ isotope dilution experiments for the whole microbialcommunity, under light and dark conditions, and for the <2µm bacterium-dominated size fraction. Microbial NH₄⁺ regenerationrates (0.018–0.124 µmol N L^–1 h^–1) werelow relative to previous reports and peaked at salinity 28.Total NH₄⁺ regeneration rates were higher than those in the<2 µm size fraction at only four stations, suggestingthat bacterial mineralization was a significant component ofNH₄⁺ recycling in some parts of the river plume. Higher NH₄⁺regeneration in whole-water samples versus <2 µm fractionsprovided evidence for microbial grazing in regions where chlorophylland regeneration rates peaked and at two full-salinity stations.     

Soil nitrogen form and plant nitrogen uptake along a boreal forest productivity gradient

We present results from a study of soil solution concentrations of ammonium (NH₄⁺), nitrate (NO₃^-), and amino acid N over one growing season along a local 90-m-long plant productivity gradient in a boreal forest. Three forest types are found along the gradient: an ericaceous dwarf-shrub type between 0 and 40 m, a low-herb type between 40 and 80 m, and a tall-herb type at 90 m. Soil sampling of the mor layer was performed in June, July, August and October in the three forest types. In addition, plant uptake of NH₄⁺, NO₃^- and the amino acid glycine was investigated. A mixture of the three N forms was injected into the soil; one N form at a time was labeled with ¹⁵N, and in the case of glycine also with ¹³C. In the dwarf-shrub forest, where plant productivity was low, the soil N pool was strongly dominated by amino acid N. There, plants took up more NH₄⁺ than NO₃^-. Glycine uptake did not differ significantly from either NH₄⁺ or NO₃^- uptake. Along the gradient, soil concentrations of NH₄⁺ and NO₃^- increased, as did plant productivity. In the low-herb forest NH₄⁺ comprised a major portion of the soil N pool, and plants took up more NH₄⁺ than NO₃^- or glycine. In the tall-herb forest, NO₃^- was as abundant as NH₄⁺, and together these two N forms dominated the soil N pool. Here, plants took up nearly equal amounts of NO₃^- and NH₄⁺, and this uptake exceeded that of glycine severalfold. Apart from the overall preference for NH₄⁺ that plants exhibited throughout the gradient, the results show a correlation between soil concentrations of amino acids and NO₃^- and plant preferences for these N forms.