The Route of Nitrate-Nitrogen Assimilation in the Root of Datura stramonium L.

Datura roots were pressure-infiltrated with 400 µg ml^–115N-nitrate feeding solutions with and without the additionof 7 mM L-methionine-DL-sulphoximine (MSO), a glutamine synthetaseinhibitor. Over a 30 min time course the main diversion of newlyreduced ¹⁵N in MSO untreated roots was to glutamine. In MSO-treatedroots ammonia assimilation into amino compounds was completelysuppressed, with resultant accumulation of a large ¹⁵N ammoniapool. This treatment also caused marked concentrational changesin the free amino compound pools, suggesting that conditionsof nitrogen stress had been induced. Glutamate dehydrogenaseactivity was unaffected by the MSO treatment. The results are consistent with the concept that the glutaminesynthetase/glutamate synthase pathway is the major route ofnewly reduced nitrogen assimilation in Datura roots.     

Endogenous Ammonium Generation in Maize Roots and its Relationship to other Ammonium Fluxes

An investigation to determine the magnitude of the back reactionswhich occur during net ammonium uptake by roots and during netammonium assimilation within roots was undertaken with maize(Zea mays L.). Ten-day-old seedlings, which had been grown on250 mmol m^–3 ammonium at pH 4 or 6, were pretreated for3 h in the absence or presence of 500 mmol m ^–3 MSX (methionine-DL-sulphoximine),an inhibitor of the glutamine synthetase-catalysed pathway ofammonium assimilation. They were then exposed for 2 h to 99A% ¹⁵N-ammonium ± MSX. Substantial ammonium cycling occurredduring net ammonium uptake. Efflux was enhanced by MSX treatment,reflecting a 2- to 3-fold accumulation of ammonium in the roottissue. Influx of ammonium was also increased by treatment withMSX, indicating that influx was enhanced when products of ammoniumassimilation were dissipated. The decline in root ¹⁴N-ammoniumaccounted for only a small fraction of the ¹⁴N-ammonium recoveredin the ambient ¹⁵N-ammonium solution, revealing a substantialgeneration of endogenous ¹⁴N-ammonium during the 2 h exposure.The net quantity of ammonium generated was increased appreciablywhen assimilation of ammonium was restricted by MSX and it wasestimated to occur at least 50% faster than net ammonium uptake.Presence of MSX severely decreased translocation of ¹⁵N to shootsbut had a smaller influence on incorporation of ¹⁵N into macromoleculesof the root tissue. The various ammonium flux rates were notgreatly affected by growth at pH 4.0, implying a considerableresistance of ammonium assimilation processes in these maizeroots to the high ambient acidity commonly induced by exposureto ammonium Key words: Ammonium generation, uptake, assimilation     

Effect of u.v.-B irradiance on the response of 15N-nitrate uptake of Lauderia annulate and Synedra planctonica

The marine diatoms Lauderia annulata and Synedra planctonicaharvested during exponential growth were exposed to differentdoses of u.v.-B (286, 439 and 710 J m^–2 d^–1) for2 days. Uptake of ¹⁵N-nitrate was estimated before, during andafter u.v.-B radiation over 2 days. Exposure to high levelsof u.v.-B (710 J m^–2 d^–1) caused irreversible damageat the second daily irradiance. Lauderia cells were less affectedby u.v.-B stress than Synedra cells. ¹⁵N-nitrate uptake wasreduced under u.v.-B irradiance but could be reactivated within1 day following exposure to a low dose (286 J m^–2 d^–1).Higher levels of u.v.-B radiation (710 J m^–2 d^–1)led to irreversible damage. The pattern of ¹⁵N-incorporationinto several amino acids of Lauderia varied after 2 days ofu.v.-B radiation. ¹⁵N enrichment of glutamine increased markedlyafter u.v.-B stress (717 J m^–2 d^–1) whereas ^I5Nexcess of aspartic acid was significantly reduced. Results arediscussed with reference to the u.v.-B damage of the nitratetransport system.     

Inward and Outward Movement of Ammonium in Root Systems: Transient Responses during Recovery from Nitrogen Deprivation in Presence of Ammonium

Morgan, M. A. and Jackson, W. A. 1988. Inward and outward movementof ammonium in root systems: transient responses during recoveryfrom nitrogen deprivation in presence of ammonium.— J.exp. Bot. 39: 179-191. Net ammonium uptake by 20-d-old wheat (Triticum aestivum cv.Kleiber) and oat (Avena sativa cv. Tarok) seedlings was increased5- to 10-fold when the seedlings were deprived of nitrate duringthe 14-20 d period. The effect of nitrogen deprivation was toincrease net ¹⁵N-ammonium influx and decrease net ¹⁴N-ammoniumefflux during a 1 h assay period. The sizeable rate of net ¹⁵N-ammoniuminflux resulting from nitrogen deprivation was stimulated furtherby prior exposure of the seedlings to ¹⁴N-ammonium for 5 h.Additional exposure to ¹⁴N-ammonium caused the stimulated rateof ¹⁵N-ammonium influx to decline. During the 1 h assays in¹⁵N-ammonium, net ¹⁴N-ammonium efflux increased after 2 h exposureto ¹⁴N-ammonium, peaked at 5–10 h, and then declined.The consequence of the differential response of the influx andefflux processes in wheat was a marked decrease in net ammoniumuptake in the initial 2–5 h, followed by a recovery which,in turn, was followed by a slow decline. In oat, there was norecovery in net ammonium uptake after 2–5 h. Interference in ammonium assimilation by presence of methioninesulphoximine after 5 h did not inhibit expression of the ammonium-stimulatednet ¹⁵N-ammonium influx at 10 h but did substantially increasenet ¹⁴N-ammonium efflux. In nitrogen depleted seedlings, andin those exposed to ¹⁴N-ammonium for 2 h, subsequent net ¹⁴N-ammoniumefflux during 1 h in ¹⁵N-ammonium exceeded the quantity of ¹⁴N-ammoniuminitially in the roots. The increase in ¹⁵N-ammonium influx upon nitrogen deprivation,its further stimulation with 5-10 h exposure to ammonium andits subsequent decline, are discussed as possibly resultingfrom (a) the operation of two ammonium influx systems (b) theinterplay of tissue ammonium and a product of its assimilationrespectively acting as positive and negative effectors of asingle influx system and (c) variations in energy supply fromthe shoots. Key words: Net ammonium uptake, stimulated ammonium influx, ammonium efflux, tissue ammonium     

Reciprocal Ammonium Transport Into and Out of Plant Roots: Modifications by Plant Nitrogen Status and Elevated Root Ammonium Concentration

Wheat and oat were grown for 20 d on a nitrate-containing solution(nitrogen-replete plants) or for the last 6 d of this periodon a nitrate-free solution (nitrogen-depleted plants). Exposureof the nitrogen-depleted plants on day 20 to nitrate-free solutionscontaining 500 mmol m^–3 ammonium (96 A% ¹⁵N) resultedin a cumulative net influx of ¹⁵N-ammonium over an 8 h periodthat was appreciably greater than that of the nitrogen-repleteplants. Both the initial rate and the more restricted rate afterthe first hour were enhanced by nitrogen deprivation. In thenitrogen-replete plants, cumulative net efflux of endogenous¹⁴N-ammonium was approximately equivalent to net ammonium uptakeduring the first hour, and was essentially complete after 1–2h. Pretreating nitrogen-depleted plants for 5 h in 500 mmolm^{–3 15}N-ammonium (99 A% ¹⁵N) resulted in root ammoniumconcentrations of 12.7?1.1 and 16.0?0.4 µmol for wheat and oat, respectively. Subsequent net efflux of ¹⁵N-ammoniumto 500 mmol m^–3 exogenous ¹⁴N-ammonium exceeded theseinitial amounts within 2 h. Increasing ambient ¹⁴N-ammoniumto 5000 mmol m^–3 increased net ¹⁵N-ammonium efflux suchthat net loss of the maximal original amount in the root tissuewas exceeded within 0.75 h. The data for both species indicatesubstantial reciprocal transfers of ammonium into and out ofroots of ammonium-treated plants and a significant degradationof recently synthesized products of ammonium assimilation concurrentwith ammonium assimilation. Key words: Accumulation, ammonium, efflux, oat, root, uptake, wheat     

Evidence of ammonium assimilation via the glutamine synthetase-glutamate synthase system in rice seedling roots

When rice seedling roots were fed ¹⁵N-ammonium for 1 hr, theamide nitrogen of glutamine showed the highest ¹⁵N abundance.Moreover, glutamine amino, glutamic acid, aspartic acid andalanine showed higher ¹⁵N abundance than ammonium did. In roots whose GS activity was inhibited with MS, both the amountof ammonium and its ¹⁵N abundance were increased. In contrast,both the amount of all examined amino acids containing glutamicacid and their ¹⁵N abundance decreased in roots whose GS activitywas inhibited. From these results, it could be concluded thatthe first step of ammonium assimilation in rice seedling rootswas mainly glutamine synthesis by GS and the second was glutamicacid formation by the GOGAT system. The results of an experiment using ¹⁵N glutamine also supportedthis conclusion. (Received February 23, 1977; )     

15N-ammonium and 15N-nitrate uptake of a 15-year-old Picea abies plantation

Throughfall nitrogen of a 15-year-old Picea abies (L.) Karst. (Norway spruce) stand in the Fichtelgebirge, Germany, was labeled with either ¹⁵N-ammonium or ¹⁵N-nitrate and uptake of these two tracers was followed during two successive growing seasons (1991 and 1992). ¹⁵N-labeling (62 mg ¹⁵N m^-2 under conditions of 1.5 g N m^-2 atmospheric nitrogen deposition) did not increase N concentrations in plant tissues. The ¹⁵N recovery within the entire stand (including soils) was 94%±6% of the applied ¹⁵N-ammonium tracer and 100%±6% of the applied ¹⁵N-nitrate tracer during the 1st year of investigation. This decreased to 80%±24% and 83%±20%, respectively, during the 2nd year. After 11 days, the ¹⁵N tracer was detectable in 1-year-old spruce needles and leaves of understory species. After 1 month, tracer was detectable in needle litter fall. At the end of the first growing season, more than 50% of the ¹⁵N taken up by spruce was assimilated in needles, and more than 20% in twigs. The relative distribution of recovered tracer of both ¹⁵N-ammonium and ¹⁵N-nitrate was similar within the different foliage age classes (recent to 11-year-old) and other compartments of the trees. ¹⁵N enrichment generally decreased with increasing tissue age. Roots accounted for up to 20% of the recovered ¹⁵N in spruce; no enrichment could be detected in stem wood. Although ¹⁵N-ammonium and ¹⁵N-nitrate were applied in the same molar quantities (¹⁵NH ₄ ⁺ : ¹⁵NO ₃ ^- =1:1), the tracers were diluted differently in the inorganic soil N pools (¹⁵NH ₄ ⁺ /NH ₄ ⁺ : ¹⁵NO ₃ ^- /NO ₃ ^- =1:9). Therefore the measured ¹⁵N amounts retained by the vegetation do not represent the actual fluxes of ammonium and nitrate in the soil solution. Use of the molar ammonium-to-nitrate ratio of 9:1 in the soil water extract to estimate ¹⁵N uptake from inorganic N pools resulted in a 2–4 times higher ammonium than nitrate uptake by P. abies.     

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     

Translocation of Nitrogenous Compounds in Symbiotic and Nitrate-Fed Amide-Exporting Legumes

Peoples, M. B., Sudin, M. N. and Herridge, D. F. 1987. Translocationof nitrogenous compounds insymbiotic and nitrate-fed amide-exportinglegumes.–J. exp. Bot. 38: 567–579. The transport of nitrogen from the roots and nodules of chickpea(Cicer anetinum L.), lentil (Lens culinaris Medic), faba bean(Vicia faba L.) and pea (Pisum sativum L.) was examined in glasshouse-grownplants supplied either with nitrate-free nutrients or with nutrientssupplemented with 1,2,4 or 8 mol m^-3153N-nitrate. A sixth treatmentcomprised uninoculated plants supplied with 8–0 mol m^-31513N-nitrate. For each species, more than 75% of the nitrogenwas exported from the nodules as the amides, asparagine andglutamine. In fully symbiotic plants, the amides also dominatednitrogen transport to the shoot When N2 fixation activity wasdecreased by the addition of nitrate to the rooting medium,the N-composition of xylem exudate and stem solutes changedconsiderably. The relative concentrations of asparagine tendedto increase in the xylem whilst those of glutamine were reduced;the levels of nitrate increased in both xylem exudate and thesoluble nitrogen pool of the stem with a rise in nitrate supply.The changes in relative nitrate contents reflected generallythe contributions of root and shoot to overall nitrate reductaseactivity at the different levels of nitrate used. The relationshipsbetween the relative contents of xylary or stem nitrate andamino nitrogen and the plants' reliance on N2 fixation (determinedby the ¹⁵N isotope dilution procedure) were examined. Data suggestthat compositional relationships based on nitrate may be reasonableindicators of symbiotic dependence for all species under studyexcept faba bean when greater than 25% of plant nitrogen wasderived from N2 fixation. Key words: Nitrogen, translocation, legumes     

Nitrogen assimilation in citrus trees

Assimilation of ¹⁵N-ammonium and ¹⁵N-nitrate was examined in 3-year-old satsuma mandarin (Citrus unshiu Marcovitch) trees. Experiments were designed to establish the time course of incorporation of nitrogen just taken up into amino compounds. In fine roots, absorbed ¹⁵N-ammonium was actively incorporated into glutamine and then into glutamic acid and asparagine. When feeding ¹⁵N-nitrate, glutamic acid and asparagine were actively synthesized, but glutamine synthesis was comparatively low as compared with that in ammonium feeding. In current leaves and fruits, a clear difference in the labelling patterns of amino acids was found between the ammonium and nitrate feedings. The amino acid most markedly labelled was asparagine in the ammonium feeding and glutamine in the nitrate feeding. Considering the most heavily labelled component in leaves and fruits, the main form of the nitrogen components transported upward in the xylem was discussed.     

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     

Nitrogen-13 Studies of Nitrate Fluxes in Barley Roots: II. EFFECT OF PLANT N-STATUS ON THE KINETIC PARAMETERS OF NITRATE INFLUX

Influx of nitrate into the roots of intact barley plants wasfollowed over periods of 1–15 min using nitrogen-13 asa tracer. Based on measurements taken over 15 min from a rangeof external nitrate concentrations (0·2–250 mmolm^–3), the kinetic parameters of influx, I_max and K_m, werecalculated. Compared with plants grown in the presence of nitrate throughout,plants that had been starved of N for 3 d showed a significantlygreater value ofI_max for ¹³N-nitrate influx (by a factor of1·4–1·8), but a similar value of K_m (12–14mmol m^–3). Pre-treating N-starved plants with nitratefor about 5 h further increased the subsequent rate of ¹³N-nitrateinflux, but had little effect in the unstarved controls. Allowingfor this induction of additional nitrate transport, the differencein rates of nitrate influx in control and N-starved plants wassufficient to account for the previously-observed differencein net uptake by the two groups of plants. In barley plants grown without any exposure to nitrate, butwith ammonium as N-source, both I_max and K_m for subsequent ¹³N-nitrateinflux were significantly decreased (by about one-half) comparedwith the corresponding nitrate-grown controls. The importance of changes in the rate of influx in the regulationof net uptake of nitrate is discussed. Key words: Ion transport, nitrate, influx, kinetic parameters, N-deficiency     

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     

Nitrogen-13 Studies of Nitrate Fluxes in Barley Roots: I. COMPARTMENTAL ANALYSIS FROM MEASUREMENTS OF 13N EFFLUX

The short-lived radio-isotope nitrogen-13 (half-life 10 min)was used as a tracer in studying fluxes of N in the roots ofintact barley plants. After supplying the plants with ¹³N-nitratefor 30 min, efflux of ¹³N into an unlabelled (wash) solutionwas followed under steady-state conditions for a further 10min. Tests with ion exchange resins suggested that all of the¹³N released during this period was in the form of nitrate. In addition to nitrate from a surface film of solution and fromthe free space of the roots, efflux from another compartmentwas detected, tentatively identified as the cytoplasmic nitratepool. In plants grown with nitrate as the only external N-source,efflux from this compartment decreased with a rate constantabout 0·17 min^–1 (half-time 4 min). Adding ammoniumsulphate to the wash solution alone did not significantly affecteither the initial rate, or the rate constant, of efflux of¹³N from these roots. However, ¹³N efflux decreased more rapidly(rate constant about 0·32 min^–1, half-time 2·2min) in roots grown in, and subsequently washed with, solutioncontaining ammonium nitrate. In barley plants grown with 1·5 mol m^–3 nitrate,the cytoplasmic nitrate pool was estimated to contain about2% of the total nitrate in the roots, corresponding to a cytoplasmicnitrate concentration 26 mol m^–3. Nitrate efflux was equivalentto almost 40% of nitrate influx in the roots of these plants. Key words: Ion transport, nitrate, ammonium, efflux analysis, compartmentation     

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     

A field study on the fate of 15N-ammonium to demonstrate nitrification of atmospheric ammonium in an acid forest soil

To demonstrate the contribution of atmospheric ammonium to soil acidification in acid forest soils, a field study with¹³N-ammonium as tracer was performed in an oak-birch forest soil. Monitoring and analysis of soil solutions from various depths on the¹³N-ammonium and¹⁵N-nitrate contents, showed that about 54% of the applied¹⁵N-ammonium was oxidized to nitrate in the forest floor. Over a period of one year about 20% of the¹⁵N remained as organic nitrogen in this layer. The percentage¹⁵N enrichment in ammonium and nitrate were in the same range in all the forest floor percolates, indicating that even in extremely acid forest soils (pH < 4) nitrate formation from ammonium can occur. Clearly, atmospheric ammonium can contribute to soil acidification even at low soil pH.     

Rapid, Reversible Inhibition of Nitrate Influx in Barley by Ammonium

The rate of influx of nitrate into the roots of intact barleyplants was measured over a period of 3–5 min from externalnitrate concentrations of 1–150 mmol m^–3, using¹³N-labelled nitrate as tracer. Ammonium at external concentrationsof 0.005–50 mol m^–3 inhibited nitrate influx ina manner which did not conform to a simple kinetic model butincreased approximately as the logarithm of the ammonium concentration.At any particular ammonium concentration, inhibition of nitrateinflux reached its full extent within 3 min of the ammoniumbeing supplied and was not made more severe by up to 17 minpre-treatment with ammonium. On removing the external ammonium,nitrate influx returned to its original rate within about 3min. Potassium at 0.005–50 mol m^–3 did not reproducethe rapid effect of ammonium on nitrate influx. Net uptake of nitrate also decreased when ammonium was supplied,over a similar timescale and to a similar extent as nitrateinflux. The decrease in nitrate influx caused by ammonium wassufficient to account for the observed reduction in net uptake,without necessitating any acceleration of nitrate efflux. Key words: Hordeum vulgare, roots, ion transport, short-lived isotopes, ¹³N     

N2 Fixation and the Respiratory Costs of Nodules, Nitrogenase Activity, and Nodule Growth and Maintenance in Fiskeby Soyabean

The respiratory effluxes of nodules and of roots of FiskebyV soyabean (Glycine max (L.) Merr.), grown in a controlled environment,were measured at intervals in air and 3% O₂ from shortly afterthe onset of N₂ fixation until plant senescence. The respiratoryburdens linked with nitrogenase plus ammonia metabolism, andnodule growth and maintenance, were calculated from gas exchangedata and related to the concurrent rates of N₂ fixation. The specific respiration rates of nodules increased to a maximumof 21 mg CO₂ g^–1 h^–1 at the time pods began development:the equivalent maximum for roots was c. 4.5 mg CO₂ g^–1h^–1. Maximum nodule and root respiration rates per plantwere attained about 25 d later at the time N₂ fixation peakedat 15 mg N d^–1 plant^–1. The relationship between nodule respiration and N₂ fixationindicated an average respiratory cost of 13.2 mg CO₂ mg^–1N until the last few days of plant development Separation ofnodule respiration into the two components: nitrogenase (+ NH₃metabolism) respiration and nodule growth and maintenance respiration,indicated that the latter efflux accounted for c. 20% of nodulerespiration while N₂ fixation was increasing and new noduletissue was being formed. When nodule growth ceased and N₂ fixationdeclined, this component of respiration also declined. The respiratorycost of nitrogenase activity plus the associated metabolismof NH₃ varied between 11 mg CO₂ mg^–1 N during vegetativeand early reproductive growth, to 12.5 mg CO₂ mg^–1 N duringthe later stages of pod development. Key words: N2 fixation, Respiration, Nodules, Nitrogenase     

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     

UV Effects on Pigments and Assimilation of 15N-Ammonium and 15N-Nitrate by Natural Marine Phytoplankton of the North Sea

Effects of ambient solar UV radiation in the field and of artifical UV irradiation under controlled laboratory conditions were studied with natural phytoplankton populations from Helgoland, German Bight, North Sea. The pattern of pigments varied after UV-A or UV-B plus a low dose of UV-A radiation: UV-A usually induced a stimulation of pigment biosynthesis; whereas UV-B plus UV-A led to a reduction of the contents of chlorophyll a, diadinoxanthin, fucoxanthin, peridinin and an unknown carotenoid; content of diatoxanthin was significantly enhanced. The damaging effect on nitrogen assimilation by UV was more pronounced after artificial UV-B plus UV-A irradiance compared to the influence of ambient solar UV under field conditions. The uptake of inorganic nitrogen was dependent on the dose and exposure time of UV radiation as well as on the species composition. The uptake of ¹⁵N-nitrate by natural phytoplankton collected in spring was more sensitive to UV irradiation than the assimilation of ¹⁵N-ammonium. UV-A radiation with a small part of shorter wavelengths at 315 nm (Philips-lamps in conjunction with the cut-off filter WG 320) caused a reduction of up to 12% whereas a stimulation of the ¹⁵NH₄⁺ uptake was observed after exposure to UV-A without any UV-B (Philips lamps TL 60W/09N). Pattern of ¹⁵N-incorporation into free amino acids and pool sizes varied in dependence on the applied nitrogen compound and on the irradiation conditions. The impact of UV radiation on the pattern of ¹⁵N-Iabelled free amino acids and the pool sizes was different. ¹⁵N enrichment into all the tested amino acids was reduced after 5 h UV-B plus UV-A exposure and after application of ¹⁵NH₄⁺. A depression of the glutamate and glutamine pools was observed after addition of ¹⁵N-nitrate alone. Pools of all main amino acids from phytoplankton in summer 1993/94 were inhibited by UV irradiance. Results are discussed with reference to the UV target (e.g. enzymes, pigments) and the adaptation to the environmental conditions.