Relationship between biomass and enzymatic activity of a bloom-forming dinoflagellate (Dinophyceae) in southern Chile (41{degrees} S): a field approach

In a coastal area of southern Chile (41° S), the major ammoniumassimilating enzyme glutamine synthetase (GS) was detected ina green dinoflagellate bloom during April 2003. High chlorophylla concentrations (1000 µg L^–1) attributable to Gymnodiniumcf. chlorophorum in surface waters were associated with highand very low nitrate reductase activities. Coincident with thebloom, dissolved inorganic nitrogen concentrations were nearthe detection limit (NO₃^– + NH₄⁺ <0.5 µM). SinceGS correlates with the use of ammonium as an external nitrogensource, we suggest that GS activity seems to be a good indicatorof ammonium utilization in a period dominated by a single dinoflagellatespecies.     

Growth of Actinorhizal Plants as Influenced by the Form of N with Special Reference to Myrica gale and Alnus incana

Growth of two actinorhizal species was studied in relation tothe form of N supply in water culture. Non-nodulated bog myrtle(Myrica gale) and grey alder (Alnus incana) were grown withNH₄⁺, NH₄NO₃^– or NO₃^– (4 mol m^–3 N). A nodulatedseries of bog myrtle was also cultivated in N-free medium. Relative growth rate (RGR), utilization rate of N, and shoot/rootratio were highest for the two species with the N completelysupplied as NH₄⁺. In both species, nitrate was largely reducedin the roots and the presence of NO₃^– in combined-N supplyalways affected the RGR and N utilization rate negatively. BothN₂ fixation and complete NO₃^– nutrition represented conditionsof relative N-deficiency resulting in relatively low tissue-Nconcentrations and a greater allocation of dry mass to the roots.The physiological N status of nodulated M. gale plants was comparativelygood, as indicated by a normal nodule weight ratio and a relativelyhigh N₂-fixing rate per unit nodule mass. However, whole-plantN₂-fixing capacity remained relatively low in comparison withacquisition rates of N in combined-N plants. The anion charge from the nitrate reduction was largely directlyexcreted as an OH^– efflux. H ⁺ /N ratios generally agreedwith the theory. In comparison with NH₄⁺ nutrition, carboxylateconcentrations were higher in N₂-fixing M. gale plants and theH ⁺ /N ratio in nodulated plants was less than unity below thevalue for ammonium plants as previously found for other actinorhizalspecies. Therefore, NH₄⁺ should be an energetically more attractiveN source for actinorhizal plants than N₂. The results agree with commonly accepted views on energeticsof N uptake and assimilation in higher plants and support theconcept of a basically similar physiological behaviour betweennon-legumes and legumes. Key words: Actinorhizal symbioses, ammonium, H⁺/OH^– efflux, nitrate, N₂ fixation, NRA     

The Influence of the Concentration of Gaseous Ammonia on its Uptake by the Leaves of Italian Ryegrass, with and without an Adequate Supply of Nitrogen to the Roots

Whitehead, D. C. and Lockyer, D. R. 1986. The influence of theconcentration of gaseous ammonia on its uptake by the leavesof Italian ryegrass, with and without an adequate supply ofnitrogen to the roots.—J. exp. Bot. 38: 818–827. Plants of Italian ryegrass (Lolium multiflorum Lam.) were grownin pots of soil with two rates of ¹⁵N-labclled nitrate, oneproviding adequate, and the other less than adequate, N formaximum growth. After 25 d in a controlled environment cabinet,the plants were transferred to chambers and exposed for 33 dto NH₃in the air at one of nine concentrations ranging from14 to 709 µg NH₃ m^–3. Increasing the concentrationof NH₃ in the air increased the dry weight of the shoots ofplants grown at the lower but not the higher rate of nitrate.The content of total N in the plant shoots (% dry weight) waslinearly related to NH₃ concentration; at 709 µg NH₃ andin both sets of plants it was more than double the content at14 µg NH₃ m^–3. Calculations, based on ¹⁵N enrichment,indicated that the amount of N taken up from the NH₃ per unitleaf area increased linearly with increasing concentration ofNH₃ in the air uptake (µg dm^–2 h^–1) = 0.1009xat the lower rate of nitrate and 0-0829x at the higher rateof nitrate, where x is the concentration of NH₃ in the air expressedas µg NH₃m^–3. The proportion of the total plant N that was derived from theNH₃ ranged from 4?0% at a concentration of 14 µg NH₃ m^–3with the higher rate of nitrate addition to 77?5% at a concentrationof 709 µg m^–3 with the lower rate of nitrate addition.The proportions of the total N in the water-insoluble proteinof the leaf tissue that were derived from nitrate and gaseousNH₃ were similar to the proportions in the whole leaf material. Key words: Ammonia, nitrogen, leaf sorption, Lolium multiflorum     

Response of young barley plants to CO2 enrichment

Barley (Hordeum vulgare L. cv. Digger) was grown for 22 d inenclosed chambers with a CO₂ enrichment of 35, 155, 400 or 675µmol CO₂ mol1. CO₂ enrichment increased photosyntheticcapacity in the plants grown at either of the two highest levelsof pCO₂. A CO₂ enrichment of 675µmol CO₂ caused a significantincrement of shoot dry weight, whereas no changes were observedin fresh weight, chlorophyll or protein levels. At a light intensityof 860µmol m^–2s^–1 CO₂ enrichment caused photosyntheticcapacity to increase by 250%, whereas no effect was observedat 80 µmol m^–2 s^–1. Over time, photosynthesisdecreased by 70% independent of CO₂. A time-dependent increasein the level of extractable fructose was observed whereas totalextractable carbohydrate only changed slightly. Key words: Carbohydrates, CO₂ enrichment, Hordeum vulgare, photosynthesis, respiration     

Ammonium Nutrition in Ricinus communis: Its Effect on Plant Growth and the Chemical Composition of the Whole Plant, Xylem and Phloem Saps

Allen, S. and Smith, J A. C. 1986. Ammonium nutrition in Ricinuscommunis: its effect on plantgrowth and the chemical compositionof the whole plant, xylem and phloem saps.—J. exp. Bot.37: 1599–1610. The growth and chemical composition of Ricinus communis cultivatedhydroponically on 12 mol m ^{– 3} NO₃^–-N were comparedwith plants raised on a range of NH₄⁺-N concentrations. At NH₄⁺-Nconcentrations between 0·5 and 4·0 mol m^–3,fresh- and dry-weight yields of 62-d-old plants were not significantlydifferent from those of the NO₃^–-N controls. Growth wasreduced at 0·2 mol m^–3 NH₄⁺-N and was associatedwith increased root. shoot and C: organic N ratios, suggestingthat the plants were N-limited. At 8·0 mol m^–3NH₄⁺-N, growth was greatly restricted and the plants exhibitedsymptoms of severe ‘NH₄⁺ toxicity’. Plants growingon NH₄⁺-N showed marked acidification of the rooting medium,this effect being greatest on media supporting the highest growthrates. Shoot carboxylate content per unit dry weight was lower at mostNH₄⁺-N concentrations than in the NO₃^–-N controls, althoughit increased at the lowest NH₄⁺-N levels. Root carboxylate contentwas comparable on the two N sources, but also increased substantiallyat the lowest NH₄⁺-N levels. N source had little effect on inorganic-cationcontent at the whole-plant level, while NO₃^– and carboxylatewere replaced by Cl as the dominant anion in the NH₄⁺-N plants.This was reflected in the ionic composition of the xylem andleaf-cell saps, the latter containing about 100 mol m^–3Cl^– in plants on 8·0 mol m^–3 NH₄⁺. Xylem-saporganic-N concentration increased more than threefold with NH₄⁺-N(with glutamine being the dominant compound irrespective ofN source) while in leaf-cell sap it increased more than 12-foldon NH₄⁺-N media (with arginine becoming the dominant species).In the phloem, N source had little or no effect on inorganic-cation,sucrose or organic-N concentrations or sap pH, but sap fromNH₄⁺-N plants contained high levels of Cl^– and serine. Collectively, the results suggested that the toxic effects ofhigh NH₄⁺ concentrations were not the result of medium acidification,reduced inorganic-cation or carboxylate levels, or restrictedcarbohydrate availability, as is commonly supposed. Rather,NH₄⁺ toxicity in R. communis is probably the result of changesin protein N turnover and impairment of the photorespiratoryN cycle. Key words: Ricinus, ammonium nutrition, nitrate, whole-plant composition, xylem, Phloem, amino acids, carboxylate     

A System for Measuring Effects of Sulphur Dioxide on Gas Exchange of Plants

Apparatus is described for exposing plants to low concentrationsof SO₂ (50–500µg m^–3 in air) and for measuringeffects on photosynthesis, dark respiration, and transpiration.Temperature, humidity, and irradiance in the chambers were controlledindependently, and fans ensured that leaf boundary layer resistanceswere low. Experiments with plants of Vicia faba in clean andpolluted air showed that: (i) a depression of net photosynthesisby 50 µg m^–3 SO₂ depended on boundary layer resistanceand on irradiance; (ii) stomatal resistance was increased ordecreased by 50 µg m^–3 SO₂ when relative humidityin the chambers was low (35% r.h., 22 °C) or high (50% r.h.,22 °C) respectively.     

Nitrogen Utilization in N-limited Barley During Vegetative and Generative Growth: III. POST-ANTHESIS KINETICS OF NET NITRATE UPTAKE AND THE ROLE OF THE RELATIVE ROOT SIZE IN DETERMINING THE CAPACITY FOR NITRATE ACQUISITION

Barley (Hordeum vulgare L., cvs Golf, Mette, and Laevigatum)was grown under nitrogen limitation in solution culture untilnear maturity. Three different nitrogen addition regimes wereused: in the ‘HN’ culture the relative rate of nitrate-Naddition (RA) was 0·08 d^–1 until day 48 and thendecreased stepwise to, finally, 0·005 d^–1 duringgrain-filling; the ‘LN’ culture received 45% ofthe nitrogen added in HN; the ‘CN’ culture was maintainedat RA 0·0375 d^–1 throughout. Kinetics of net nitrateuptake were measured during ontogeny at 30 to 150 mmol m^–3external nitrate. V_max (which is argued to reflect the maximuminflux rate in these plants) declined with age in both HN andLN cultures. A pronounced transient drop was observed just beforeanthesis, which correlated in time with a peak in root nitrateconcentration. Similar, but less pronounced, trends were observedin CN. The relative V_max (unit nitrogen taken up per unit nitrogenin plants and day) in all three cultures declined from 1·3–2·3d^–1 during vegetative growth to 0·1–0·7d^–1 during generative growth. These values are in HN andLN cultures 15- to more than 100-fold in excess of the demandset by growth rates throughout ontogeny. Predicted balancingnitrate concentrations (defined as the nitrate concentrationrequired to support the observed rate of growth) were below6·0 mmol m^–3 in HN and LN cultures before anthesisand then decreased during ontogeny. In CN cultures the balancingnitrate concentration increased during grain-filling. Apartfrom the transient decline during anthesis, most of the effectof ageing on relative V_max can be explained in terms of reducedcontribution of roots to total biomass (R:T). The loss in uptakeper unit root weight is largely compensated for by the declinewith time in average tissue nitrogen concentrations. The quantitativerelationships between relative V_max and R:T in ageing plantsare similar to those observed for vegetative plants culturedat different RAs. The data support the contention that the capacity for nitrateacquisition in N-limited plants is under general growth control,rather than controlled by specific regulation of the biochemicalpathway of nitrate assimilation. Key words: Barley, nitrogen concentration, root: total plant biomass ratio, V_max     

Response of Glutamine Synthetase and Glutamate Synthase Isoforms to Nitrogen Sources in Rice Cell Cultures

As a model system with no photorespiration and no long distancetransport, rice cell cultures (Oryza saliva L. cv Sasanishiki)were used to investigate the effect of nitrogen sources on thelevels of isoforms of glutamine synthetase (GS) and glutamatesynthase (GOGAT). Isoforms of GS and GOGAT were analyzed byimmunoblotting methods and their activities in early growthphase of the cells. Cytosolic type GS (41 kDa subunit) and NADH-GOGATwere the major isoforms in the rice cells grown in normal R-2medium. However, contents of plastid type GS (44 kDa subunit)and Fd-GOGAT increased in response to NO_–³ supply. NADH-GOGATactivity also increased following the supply of NO_–³.In vitro translated products from poly(A)⁺RNA prepared fromthe cells showed that the precursor of plastid type GS (49 kDa)was detected at 48 h after the inoculation. Supply of NH₊⁴ resultedin an increase in NADH-GOGAT activity but had no effect on thelevels of Fd-GOGAT, of polypeptides of the plastid type GS orof the corresponding mRNAs. (Received May 30, 1990; Accepted August 23, 1990)     

Carbon Dioxide and Ammonia Exchange in the Trachypogon Savannas of the Orinoco Llanos

Atmospheric carbon dioxide (CO₂) and ammonia (NH₂) exchangeswere determined in the Trachypogon savannas of the Orinoco Llanosusing the energy balance approach. Total dry mass and separatedry mass values of plant parts were used for a growth analysisof the community and for measurements of nitrogen content. Duringthe growth period, the net assimilation (P₄₀) ranged from 0.102to 0.127 MJ m^–2 d^–1 (6.6–7.9 g dry mass m^–2d^–1). These figures were similar to mean crop growth ratemeasured using the mass balance approach (2.8–6.9 g drymass m^–2 d^–1). Analysis of the daily trend of theCO₂ assimilated by the community showed a low total energy conversionof net photosynthesis (_x = 0.7) compared with the values reportedfor tropical grasses. During the dry season, the community conserved71% of the maximum N accumulated during the previous wet season.Sixty-eight per cent of the community nitrogen content was lostas volatile NH₃ from the community during the reproductive period.Results suggested that the predominant net NH₃ efflux from thevegetation was determined by the low concentration of NH₃ inthe atmosphere ( 1.8 µg m^–2) and the compensationconcentration point. However, N losses were balanced by annualnitrogen input to the community from precipitation and biologicalfixation. Thus, a redistribution rather than a loss of nitrogenseems to be occurring in the ecosystem. Carbon dioxide fluxes, ammonia fluxes, Trachypogon savannas, energy balance, growth analysis     

CAM Photosynthesis under Drought Conditions in Kalanchoe blossfeldiana Grown with Nitrate or Ammonium as the Sole Nitrogen Source

K. blossfeldiana Poelln. cv. Hikan was grown in vermiculite,supplied daily with nutrient solution containing 1 mM (or 10mM) nitrate or ammonium as the sole nitrogen source. The nitrate-grownplants had more activity of CAM (Crassulacean acid metabolism)photosynthesis (nocturnal CO₂ uptake in the shoot and nocturnalincreases of titratable acidity and malate content in the leaves)than the ammonium-grown plants. Interruption of the solutionsupply for 5 or more days (drought conditions) increased theactivity of CAM photosynthesis in nitrate- or ammonium-grownplants, and the diurnal CO₂ uptake pattern in the nitrate-grownplants shifted from ‘weak-CAM’ to ‘full-CAM’.The difference in the activity of CAM photosynthesis betweennitrate- and ammonium-grown plants increased under the droughtconditions. When the solution was resupplied, the activity ofCAM photosynthesis rapidly decreased to the levels before theinterruption. The physiological mechanism and ecological significanceof the effect of the nitrogen source on CAM photosynthesis arediscussed (Received January 5, 1988; Accepted April 13, 1988)     

Interactions of NH4+ and L-glutamate with NO3- transport processes of non-mycorrhizal Fagus sylvatica roots

The processes of NO₃^– uptake and transport and the effectsof NH₄⁺ or L-glutamate on these processes were investigatedwith excised non-mycorrhizal beech (Fagus sylvatica L.) roots.NO₃^– net uptake followed uniphasic Michaelis-Menten kineticsin a concentration range of 10µM to 1 mM with an apparentK_m of 9.2 µM and a V_max of 366 nmol g^–1 FW h^–1.NH₄⁺, when present in excess to NO₃^–, or 10 mM L-glutamateinhibited the net uptake of NO₃^– Apparently, part of NO₃^–taken up was loaded into the xylem. Relative xylem loading ofNO₃^– ranged from 3.21.6 to 6.45.1% of NO₃^– netuptake. It was not affected by treatment with NH₄⁺ or L-glutamate.¹⁶N/¹³N double labelling experiments showed that NO₃^–efflux from roots increased with increasing influx of NO₃^–and, therefore, declined if influx was reduced by NH₄⁺ or L-glutamateexposure. From these results it is concluded that NO₃^–net uptake by non-mycorrhizal beech roots is reduced by NH₄⁺or L-glutamate at the level of influx and not at the level ofefflux. Key words: Nitrate transport, net uptake, influx, efflux, ammonium, Fagus, Fagaceae     

Metabolism of Glycollate by Lemna minor L. Grown on Nitrate or Ammonium as Nitrogen Source

Marques, I. A., Oberholzer, M. J. and Erismann, K. H. 1985.Metabolism of glycollate by Lemna minor L. grown on nitrateor ammonium as nitrogen source.—J. exp. Bot. 36: 1685–1697. Duckweed, Lemna minor L., grown on inorganic nutrient solutionscontaining either NH₄⁺ or NO₃^– as nitrogen source wasallowed to assimilate [1-¹⁴C]- or [2-¹⁴C]glycollate during a20 min period in darkness or in light. The incorporation ofradioactivity into water-soluble metabolites, the insolublefraction, and into the CO₂ released was measured. In additionthe extractable activity of phosphoenolpyruvate carboxylasewas determined. During the metabolism of [2-¹⁴C]glycollate in darkness, as wellas in the light, NH₄⁺ grown plants evolved more ¹⁴CO₂ than NO₃^–grown plants. Formate was labelled only from [2-¹⁴C]glycollateand in NH₄⁺ grown plants it was significantly less labelledin light than in darkness. In NO₃^– grown plants formateshowed similar radioactivity after dark and light labelling.The radioactivity in glycine was little influenced by the nitrogensource. Amounts of radioactivity in serine implied that thefurther metabolism of serine was reduced in darkness comparedwith its metabolism in the light under both nitrogen regimes.In illuminated NH₄⁺ plants, serine was labelled through a pathwaystarting from phosphoglycerate. After [1-¹⁴C]glycollate feedingNH₄⁺ grown plants contained markedly more radioactive aspartateand malate than NO₃^– plants indicating a stimulated phosphoenolpyruvatecarboxylation in plants grown on NH₄⁺. Key words: Photorespiration, glycollate, nitrogen, Lemna     

Influence of Elevated CO2 and Temperature on the Photosynthesis and Respiration of White Clover Dependent on N2 Fixation

Single clonal plants of white clover (Trifolium repens L) grownfrom explants in a Perlite rooting medium, and dependent fornitrogen on N₂ fixation in root nodules, were grown for severalweeks in controlled environments which provided two regimesof CO₂, and temperature 23/18 °C day/night temperaturesat 680 µmol mol^–1 CO₂, (C680), and 20/15 °Cday/night temperatures at 340 µmol mol^–1 CO₂ (C340)After 3–4 weeks of growth, when the plants were acclimatedto the environmental regimes, leaf and whole-plant photosynthesisand respiration were measured using conventional infra-red gasanalysis techniques Elevated CO₂ and temperature increased ratesof photosynthesis of young, fully expanded leaves at the growthirradiance by 17–29%, despite decreased stomatal conductancesand transpiration rates Water use efficiency (mol CO₂ mol H₂O^–1)was also significantly increased Plants acclimated to elevatedCO₂, and temperature exhibited rates of leaf photosynthesisvery similar to those of C340 leaves ‘instantaneously’exposed to the C680 regime However, leaves developed in theC680 regime photosynthesised less rapidly than C340 leaves whenboth were exposed to a normal CO₂, and temperature environmentIn measurements where irradiance was varied, the enhancementof photosynthesis in elevated CO₂ at 23 °C increased graduallyfrom approx 10 % at 100 µmol m^–1 s^–1 to >27 % at 1170 µmol m^–2 s^–1 In parallel, wateruse efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 In parallel,water use efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 to approx100 % at the highest irradiance Elevated CO₂, and temperatureincreased whole-plant photosynthesis by > 40 %, when expressedin terms of shoot surface area or shoot weight No effects ofelevated CO₂ and temperature on rate of tissue respiration,either during growth or measurement, were established for singleleaves or for whole plants Dependence on N₂, fixation in rootnodules appeared to have no detrimental effect on photosyntheticperformance in elevated CO₂, and temperature Trifolium repens, white clover, photosynthesis, respiration, elevated CO₂, elevated temperature, water use efficiency, N₂ fixation     

Growth and Preferences for Ammonium or Nitrate Uptake by Barley in Relation to Root Termperature

Barley plants (Hordewn vulgare L. cv. Atem) were grown fromseed for 28 d in flowing solution culture, during which timeroot temperature was lowered decrementally to 5?C. Plants werethen subjected to root temperatures of 3, 5, 7, 9, 11, 13, 17or 25 ?C, with common air temperature of 25/15 ?C (day/night).Changes in growth, plant total N, and NO₃^– levels, andnet uptake of NH₄⁺ and NO₃^– from a maintained concentrationof 10 mmol m^–3 NH₄NO₃ were measured over 14 d. Dry matterproduction increased 6-fold with increasing root temperaturebetween 3–25 ?C. The growth response was biphasic followingan increase in root temperature. Phase I, lasting about 5 d,was characterized by high root specific growth rates relativeto those of the shoot, particularly on a fresh weight basis.During Phase I the shoot dry weight specific growth rates wereinversely related to root temperature between 3–13 ?C.Phase 2, from 5–14 d, was characterized by the approachtowards, and/or attainment of, balanced exponential growth betweenshoots and roots. Concentrations of total N in plant dry matterincreased with root temperature between 3–25 ?C, moreso in the shoots than roots and most acutely in the youngestfully expanded leaf (2?l–6?9% N). When N contents wereexpressed on a tissue fresh weight basis the variation withtemperature lessened and the highest concentration in the shootwas at 11 ?C. Uptake of N increased with root temperature, andat all temperatures uptake of NH₄⁺, exceeded that of NO₃^–,irrespective of time. The proportions of total N uptake over14 d absorbed in the form of NH₄⁺ were (%): 86, 91, 75, 77,76, 73, 77, and 80, respectively, at 3, 5, 7, 9, Il, 13, 17,and 25 ?C. At all temperatures the preference for NH₄⁺ overNO₃^– uptake increased with time. An inverse relationshipbetween root temperature (3–11 ?C) and the uptake of NH₄⁺as a proportion of total N uptake was apparent during PhaseI. The possible mechanisms by which root temperature limitsgrowth and influences N uptake are discussed. Key words: Hordeum vulgare, root temperature, ammonium, nitrate, ion uptake, growth rate     

Photosynthetic Nitrogen Metabolism in High and Low CO2-adapted Scenedesmus: II. EFFECT OF AMMONIUM AND METHIONINE SULPHOXIMINE ON NITRATE UTILIZATION

Larsson, C.-M., Larsson, M. and Guerrero, M. G. 1985. Photosyntheticnitrogen metabolism in high and low CO²-adapted Scenedesmus.II. Effect of ammonium and methionine sulphoximine on nitrateutilization.—J. exp. Bot. 36: 1387–1395 In 3% CO²-grown Scenedesmus obtusiusculus Chod. utilizing NO₃^–J as the N source, NH₄⁺ addition caused a prompt inhibitionof NO₃^– utilization. Nitrate reductase (NR) activity declinedrapidly in response to the presence of NO₄⁺, but the cessationof NO₃^– utilization was too rapid to be accounted forby the loss in NR activity. The first site of NO₄⁺ inhibitionin these cells seems to be the entrance of NO₃^– into thecells. Upon exhaustion of NO₄⁺ from the medium, NO₃^– utilizationwas rapidly restored and NR activity increased. Air-grown cellswere much less sensitive to the effect of NO₄⁺, more than 30min being required for added NO₄⁺ to cause complete inhibitionof NO₃^– utilization. In these cells, NO₃^– uptakeand NR activity decreased in parallel in response to NO₄⁺ addition.In 3% CO²-grown cells simultaneously subjected to NO₄⁺ and air-levelof CO², NO₄⁺ initially inhibited NO₃^– utilization completely,but a slight recovery took place after approximately 20 min The glutamine synthetase (GS) inhibitor L-methionine D, L-sulphoximine(MSO) behaved as a potent inhibitor of NO₃^– uptake in3% CO²-grown cells, but had considerably less effect in air-growncells, although the time-course of the MSO-induced inhibitionof GS was the same in both cases Key words: Ammonium, nitrate utilization, Scenedesmus     

The Effects of Temperature and Form of Nitrogen Supply on the Relative Contribution of Root Uptake and Remobilization in Supplying Nitrogen for Laminae Regrowth of Lolium perenne L.

Plants of Lolium perenne L. cv. S23 were grown in sand culturesupplied with either ammonium (NH₄⁺) or nitrate (NO₃^–)in an otherwise complete nutrient solution at 12°C or 20°C.Three weeks after germination, plants were clipped weekly tosimulate grazing. After 10 weeks growth all nitrogen (N) wassupplied enriched with ¹⁵N to quantify the effects of form ofN supply and temperature on the relative ability of currentroot uptake and remobilization to supply N for laminae regrowth. The form of N supply had no effect on the dry matter partitioning,while at 20°C more dry weight was allocated to laminae regrowthand less to the remaining plant material. The current root uptakeof N, which subsequently appeared in the laminae regrowth, wassimilar for plants supplied with NH₄⁺ or NO₃^–, and bothwere equally reduced at the lower temperature of growth. Remobilizationof N to laminae regrowth was greater for plants receiving NH₄⁺than NO₃^–; remobilization with either form of N supplywas reduced at the lower temperature of growth. Remobilizationwas reduced to a lesser extent at 12°C than current rootuptake. It was concluded that remobilization became relativelymore important in supplying N for regrowth of laminae at lowertemperatures. Key words: Lolium perenne, ammonium, nitrate, temperature, remobilization     

Nitrogen Utilization in N-limited Barley during Vegetative and Generative Growth: I. GROWTH AND NITRATE UPTAKE KINETICS IN VEGETATIVE CULTURES GROWN AT DIFFERENT RELATIVE ADDITION RATES OF NITRATE-N

Growth and nitrate uptake kinetics in vegetatively growing barley(Hordeum vulgare L., cvs Laevigatum, Golf, and Mette) were investigatedin solution culture under long-term limitations of externalnitrogen availability. Nitrate was fed to the cultures at relativeaddition rates (RA) ranging from 0.02 to 0.2 d^–1. Therelative growth rate (RG, calculated for total plant dry weight)correlated well with RA in the range 0.02 to 0.07 d^–1.In the RA range from 0.07 to 0.2 d^–1 RG continued to increase,but an increasing fraction of nitrogen, added and absorbed,was apparently stored rather than used for structural growth.The RG of the roots was less affected by RA. V_max, for net nitrateuptake increased with RA up to 0.11 d^–1, but decreasedat higher RA. The decline in V_max coincided with a build-upof nitrate stores in both roots and shoots. V_max, expressedper unit nitrogen in the plants (the relative V_max, was higherthan required for maintenance of growth (up to 30-fold) at lowRA, whereas at higher RA the relative V_max decreased. Kineticpredictions of steady-state external nitrate concentrationsduring N-limited growth ranged from 0.2 to 5.0 mmol m^–3over the RG range 0.02 to 0.11 d_–1. It is suggested thatthe nitrate uptake system is not under specific regulation atlow RA, but co-ordinated with root protein synthesis and growthin general. At RA higher than 0.11 d_–1, however, specificregulation of nitrate uptake, possibly via root nitrate pools,become important. The three cultivars showed very similar growthand nitrate uptake characteristics. Key words: Barley, growth, nitrogen limitation, nitrate uptake, kinetics     

The Uptake and Flow of C, N and lons between Roots and Shoots in Ricinus communis L.

Seedlings of Ricinus communis L. cultivated in quartz sand weresupplied with a nutrient solution containing either 1 mol m^–3NO^–₃ or 1 mol m^–3 NH⁺₄ as the nitrogen source. Duringthe period between 41 and 51 d after sowing, the flows of N,C and inorganic ions between root and shoot were modelled andexpressed on a fresh weight basis. Plant growth was clearlyinhibited in the presence of NH⁺₄. In the xylem sap the majornitrogenous solutes were nitrate (74%) or glutamine (78%) innitrate or ammonium-fed plants, respectively. The pattern ofamino acids was not markedly influenced by nitrogen nutrition;glutamine was the dominant compound in both cases. NH⁺₄ wasnot transported in significant amounts in both treatments. Inthe phloem, nitrogen was transported almost exclusively in organicform, glutamine being the dominant nitrogenous solute, but theN-source affected the amino acids transported. Uptake of nitrogenand carbon per unit fresh weight was only slightly decreasedby ammonium. The partitioning of nitrogen was independent ofthe form of N-nutrition, although the flow of nitrogen and carbonin the phloem was enhanced in ammonium-fed plants. Cation uptakerates were halved in the presence of ammonium and lower quantitiesof K⁺, Na⁺ and Ca²⁺ but not of Mg²⁺ were transported to theshoot. As NH⁺₄ was balanced by a 30-fold increase in chloride in thesolution, chloride uptake was increased 6-fold under ammoniumnutrition. We concluded that ammonium was predominantly assimilated inthe root. Nitrate reduction and assimilation occurred in bothshoot and root. The assimilation of ammonium in roots of ammonium-fedplants was associated with a higher respiration rate. Key words: Ricinus communis, nitrogen nutrition (nitrate/ammonium), phloem, xylem, transport, partitioning, nitrogen, carbon, potassium, sodium, magnesium, calcium, chloride     

Xylem fluxes of fixed N through nodules of the legume Acacia littorea and haustoria of an associated N-dependent root hemiparasite Olax phyllanthi

Nodulated 1-1.5-year-old plants of Acacia littorea grown inminus nitrogen culture were each partnered with a single seedlingof the root hemiparasite Olax phyllanthi. Partitioning of fixedN between plant organs of the host and parasite was studiedfor the period 4–8 months after introducing the parasite.N fluxes through nodules of Acacia and xylem-tapping haustoriaof Olax were compared using measured xylem flows of fixed Nand anatomical information for the two organs. N₂ fixation duringthe study interval (635 µg N g FW nodules^–1 d^–1)corresponded to a xylem loading flux of 0.20 µg N mm^–2d^–1 across the secretory membranes of the pencycle parenchymaof the nodule vascular strands. A much higher flux of N (4891µg mm^–2 d^–1) exited through xylem at the junctionof nodule and root. The corresponding flux of N from host xylemacross absorptive membranes of the endophyte parenchyma of Olaxhaustorium was 1.15 µg N mm^–1 d^–1, six timesthe loading flux in nodules. The exit flux from haustorium toparasite rootlet was 20.0 pg N mm^–1 d^–1, 200-foldless than that passing through xylem elements of the nodule.Fluxes of individual amino compounds in xylem of nodule andhaustorium were assessed on a molar and N basis. N flux valuesare related to data for transpiration and partitioning of Cand N of the association recorded in a companion paper. Key words: Olax phyllanthi, host-parasite relationships, N flux, Acacia, N₂ fixation     

Concurrent Rates of N2 Fixation, Nitrate and Ammonium Uptake by White Clover in Response to Growth at Different Root Temperatures

Nodulated white clover plants (Trifolium repens L. cv. Huia)were grown for 71 d in flowing nutrient solutions containingN as 10 mmol m_–3 NH₄NO₃, under artificial illumination,with shoots at 20/15°C day/night temperatures and root temperaturereduced decrementally from 20 to 5°C. Root temperatureswere then changed to 3, 7, 9, 11, 13, 17 or 25°C, and theacquisition of N by N₂ fixation, NH₄+ and NO₃^– uptakewas measured over 14 d. Shoot specific growth rates (d. wt)doubled with increasing temperature between 7 and 17°C,whilst root specific growth rates showed little response; shoot:root ratios increased with root temperature, and over time at11°C. Net uptake of total N per plant (N₂ fixation + NH₄++ NO₃^–) over 14 d increased three-fold between 3 and 17°C.The proportion contributed by N₂ fixation decreased with increasingtemperature from 51% at 5°C to 18% at 25°C. Uptake ofNH₄+ as a proportion of NH₄+ + NO₃^– uptake over 14 d variedlittle (55–62%) with root temperature between 3 and 25°C,although it increased with time at most temperatures. Mean ratesof total N uptake per unit shoot f. wt over 14 d changed littlebetween 9 and 25°C, but decreased progressively with temperaturebelow 9°C, due to the decline in the rates of NH₄+ and NO₃^–uptake, even though N₂ fixation increased. The results suggestthat N₂ fixation in the presence of sustained low concentrationsof NH₄+ and NO₄^– is less sensitive to low root temperaturethan are either NH₄+ or NO₃^– uptake systems. White clover, Trifolium repens L. cv. Huia, root temperature, nitrogen fixation, ammonium, nitrate