Nutrient-Limited Growth Rates: Roles of Nutrient-Use Efficiency and of Adaptations to Increase Uptake Rate

When stressed by low nutrient availability, young sunflowerplants (Helianthus annuus) showed responses seen in many otherspecies: increases in root uptake capacity (V^max, l/K_m), root:shoot ratio, and putative nutrient-use efficiency, nUE=l/(tissuenutrient content). A straightforward mechanistic model is derivedfor relative growth rate (RGR) in solution culture in termsof these factors. A linear regression based on the model indicatesa negative role for nUE, which violates a premise of the model.A revised model proposes that primary adaptations are only inuptake rate and growth or nutrient allocations, and these actthrough the photosynthetic utility of nutrient. The tissue nutrientcontent and associated nUE become dependent quantities. Thepredictions for RGR, as tested by linear regression, are improved.The model predicts that nUE can increase as external solutionconcentration decreases, but decreases with increased uptakeadaptations in one given environment. The decrease in nUE compromisespotential gains in RGR from uptake adaptations, and makes increasesin root: shoot ratio a nearly insignificant contributor to earlyRGR. The model and associated regression analyses are generalizedfor additional adaptations such as increased root fineness andfor different quantitative ways that a nutrient may limit photosynthesis.The model and analyses are also generalized to plant growthin soil and growth without functional balance between root andshoot. Key words: Relative growth rate, Helianthus annuus, nutrient stress, nutrient use efficiency, functional balance     

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     

Suppression of the High Affinity Phosphate Uptake System: A Mechanism of Arsenate Tolerance in Holcus lanatus L.

In Holcus lanatus L. phosphate and arsenate are taken up bythe same transport system. Short-term uptake kinetics of thehigh affinity arsenate transport system were determined in excisedroots of arsenate-tolerant and non-tolerant genotypes. In tolerantplants the V_max of ion uptake in plants grown in phosphate-freemedia was decreased compared to non-tolerant plants, and theaffinity of the uptake system was lower than in the non-tolerantplants. Both the reduction in V_max and the increase in K_m ledto reduced arsenate influx into tolerant roots. When the twogenotypes were grown in nutrient solution containing high levelsof phosphate, there was little change in the uptake kineticsin tolerant plants. In non-tolerant plants, however, there wasa marked decrease in the V_max to the level of the tolerant plantsbut with little change in the K_m. This suggests that the lowrate of arsenate uptake over a wide range of differing rootphosphate status is due to loss of induction of the synthesisof the arsenate (phosphate) carrier. Key words: Arsenate, Holcus lanatus L., phosphate uptake, tolerance mechanisms, uptake mechanisms     

Sagebrush (Artemisia tridentata Nutt.) roots maintain nutrient uptake capacity under water stress

Phosphorus and nitrogen uptake capacities were assessed during36–58 d drying cycles to determine whether the abilityof sagebrush (Artemisia tridentata Nutt.) to absorb these nutrientschanged as the roots were subjected to increasing levels ofwater stress. Water was withheld from mature plants in large(6 I) containers and the uptake capacity of excised roots insolution was determined as soil water potentials decreased from–0.03 MPa to –5.0 MPa. Phosphorus uptake rates of excised roots at given substrateconcentrations increased as preharvest soil water potentialsdecreased to –5.0 MPa. V_max and K_m also increased as soilwater potentials declined. Declining soil water potentials depressednitrogen uptake at set substrate concentrations, but uptakecapacity, calculated as the sum V_max for both NH⁺₄+NO^–₃,did not change significantly with drying. The sum V_max correlatedwith root nitrogen concentration. Root uptake capacity for nitrogen and phosphorus was extremelystable under severe water stress in this aridland shrub. Maintenanceof uptake capacity, coupled with a previously demonstrated abilityto conduct hydraulic lift, may enable A. tridentata better tomaintain nitrogen and phosphorus uptake as soil water availabilitydeclines. These mechanisms may be important in the ability ofA. tridentata to maintain growth, complete reproduction, andgain an advantage against competitors late in the season whenthe soil layers with higher nutrient availability are dry. Key words: Kinetics, nitrogen, phosphorus, roots, water stress     

Effects of nitrogen nutrition on salt-stressed Nicotiana tabacum var. Samsun in vitro plantlets

Tobacco shoots were grown in vitro for 35 d, in MS culture mediummodified to include various sources (nitrate-N, ammonium-N ora mixture) and levels (0–120 mM) of N, and in the presenceof 0–180 mM NaCI or iso-osmotic concentrations of mannitol.Growth of control plantlets was significantly inhibited whenNH₄⁺-N was the sole N source, and at high (120 mM) NO^–₃-N supply. Under conditions of salt stress (90 and 180 mM NaCI)growth was repressed, with roots being more severely affectedthan shoots. Salinity also inhibited root emergence in vitro.The only alleviation of the salt stress by nitrate nutritionobserved in this study was on shoot growth parameters of plantletsgrown on 60 mM NO^–₃-N and 90 mM NaCI. Although both weresignificantly inhibited by NaCI, nitrate reduc-tase activitywas more severely affected than nitrate uptake. When mannitolreplaced NaCI in the culture medium, similar Inhibition of growth,nutrient uptake and enzyme activity were recorded. These observations,together with the relatively low recorded values for Na⁺ andCI^– uptake, indicate that under in vitro salt stress conditionsthe negative effects of NaCI are primarily osmotic. Key words: Growth, nitrogen metabolism, osmotic stress, salinity     

Nutrient Relations of Groundsel (Senecio vulgaris) Infected by Rust (Puccinia lagenophorae) at a Range of Nutrient Concentrations II. Uptake of N, P and K and Shoot-Root Interactions

Groundsel (Senecio vulgaris), healthy or infected with rust,Puccinia lagenophorae, was grown at a range of nutrient concentrationsin sand culture. Specific absorption rates calculated on thebasis of root dry weight (SAR_W) were greater in rusted thancontrol groundsel for nitrogen, potassium and phosphorus. Whilethe magnitudes of these stimulations varied, they occurred acrossthe whole range of nutrient concentrations. By contrast, specificabsorption rate on the basis of root length (SAR_L) were littlechanged by rust at any external nutrient concentration; SAR_Lfor phosphate and potassium were slightly reduced when nutrientswere freely available. Water flux per unit dry root weight and length was stimulatedby rust because transpiration per unit leaf area was more rapidin infected plants after fungal sporulation. However, water-fluxand the rate of uptake of nutrients were correlated only whenexpressed on the basis of root weight and increased transpirationdid not appear to be the mechanism underlying increased rootactivity. Rather, increased SAR_W for N, P and K could very largelybe attributed to increased shoot demand per unit root, whichresulted from the higher shoot: root (S: R) ratios of infectedindividuals. Changes in S: R accounted for 92, 81 and 57% oftotal variation in SAR_W for K, P and N respectively. Greatervalues for SAR_W were possible because specific root length (SRL)increased, producing more functional root per unit root weight.The lack of stimulation in SAR_L in response to rust could beexplained since the higher SRL of infected plants resulted instable values of shoot weight per unit root length, i.e. shootdemand was not increased by infection on this basis. Senecio vulgaris, Puccinia lagenophorae, rust infection, nutrient uptake, water uptake, shoot: root interactions     

The Response of Oxygen and Carbon Dioxide Exchanges and Root Activity to Short Term Water Stress in Soybean

The gross and net O₂ evolution together with O₂ uptake, CO₂assimilation, transpiration, shoot dark respiration, root respirationand ion uptake of a soybean plant were studied during 19 d whichincluded two periods of water stress. O₂ uptake was measuredusing ¹⁸O₂ as a tracer. Short term water stress induced immediateand lasting effects: (1) reduction of light interception bywilting, (2) limitation of the total reducing equivalent producedby the electron transport chain, (3) decrease of stomatal conductancereducing both losses of water and the entry of CO₂ for assimilation,(4) relative stimulation of O₂ uptake. The ratio of O₂ uptaketo CO₂ assimilation changed from 1.0 before stress to 1.4 forseveral days after. Root respiration was less affected by thestress than ion uptake and shoot gas exchanges. Key words: Photosynthesis, Photorespiration, Transpiration, Shoot and root respiration, Ion uptake, Water stress, Glycine max. L.     

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     

Plant Growth in Relation to the Supply and Uptake of NO3-: A Comparison Between Relative Addition Rate and External Concentration as Driving Variables

Two approaches to quantifying relationships between nutrientsupply and plant growth were compared with respect to growth,partitioning, uptake and assimilation of NO₃^– by non-nodulatedpea (Pisum sativum L. cv. Marma). Plants grown in flowing solutionculture were supplied with NO₃^– at relative addition rates(RAR) of 0·03, 0·06, 0·12, and 0·18d^–1, or constant external concentrations ([NO₃^–)of 3, 10, 20, and 100 mmol m^–3 over 19 d. Following acclimation,relative growth rates (RGR)approached the corresponding RARbetween 0·03–0.12 d^-1, although growth was notlimited by N supply at RAR =0.18 d^-1. Growth rates showed littlechange with [NO₃–] between 10–100 mmol m^–3(RGR=0·15 –0·16 d^-1). The absence of growthlimitation over this range was suggested by high unit absorptionrates of NO₃^–, accumulation of NO₃^– in tissues andprogressive increases in shoot: root ratio. Rates of net uptakeof NO₃^– from 1 mol m^–3 solutions were assessed relativeto the growth-related requirement for NO₃^–, showing thatthe relative uptake capacity increased with RGR between 0·03–0·06d^–1 , but decreased thereafter to a theoretical minimumvalue at RGR     

Compensatory Roles of Nitrogen Uptake and Photosynthetic N-use Efficiency in Determining Plant Growth Response to Elevated CO2: Evaluation Using a Functional Balance Model

We used a modified functional balance (FB) model to predictgrowth response of Helianthus annuus L. to elevated CO₂. Modelpredictions were evaluated against measurements obtained twiceduring the experiment. There was a good agreement between modelpredictions of relative growth rate (RGR) responses to elevatedCO₂and observations, particularly at the second harvest. Themodel was then used to compare the relative effects of biomassallocation to roots, nitrogen (N) uptake and photosyntheticN-use efficiency (PNUE) in determining plant growth responseto elevated CO₂. The model predicted that a rather substantialincrease in biomass allocation to root growth had little effecton whole plant growth response to elevated CO₂, suggesting thatplasticity in root allocation is relatively unimportant in determininggrowth response. Average N uptake rate at elevated comparedto ambient CO₂was decreased by 21–29%. In contrast, elevatedCO₂increased PNUE by approx. 50% due to a corresponding risein the CO₂-saturation factor for carboxylation at elevated CO₂.The model predicted that the decreased N uptake rate at elevatedCO₂lowered RGR modestly, but this effect was counterbalancedby an increase in PNUE resulting in a positive CO₂effect ongrowth. Increased PNUE may also explain why in many experimentselevated CO₂enhances biomass accumulation despite a significantdrop in tissue nitrogen concentration. The formulation of theFB model as presented here successfully predicted plant growthresponses to elevated CO₂. It also proved effective in resolvingwhich plant properties had the greatest leverage on such responses.Copyright 2000 Annals of Botany Company Elevated CO₂, functional balance model, Helianthus annuus L., N uptake, photosynthetic nitrogen use efficiency, root:shoot ratio     

Effects of P deficiency on the uptake, flows and utilization of C, N and H2O within intact plants of Ricinus communis L.

The influence of P deficiency on the uptake, flow and utilizationof C, N and H₂0 by intact NO₃-fed castor bean plants {Ricinuscommunis L.) was studied over a 9 d period in the middle oftheir vegetative growth. The modelling techniques incorporateddata on net increments or losses of C, N and H₂O in plant parts,photosynthetic gains in and respiratory losses of C, molar C:Nratios of solutes in phloem and xylem sap and transpirationallosses of H₂0. Plant growth was inhibited within 3 d of withholdingP supply and dry matter production was less than one-third ofthe controls. Leaf growth was particularly depressed, whileroot growth was much less affected than that of the shoot. Shoot:rootratio of low-P plants was 1.5 compared with 2.6 under P supply.Over the 9 d study period total plant C and N increased by 560and 47 mmol, respectively, in the controls, but by only 113and 6.9 mmol in the low-P treatment. The particularly low incrementof N in P-deficient plants was due principally to decreasedN0₃^- uptake. Flows of C and N during the study period were markedlydifferent between control and P-deficient plants. The partitioningprofile for C in P-deficient plants showed a dramatic inhibitionof net photosynthesis and attendant photoassimilate flow. Proportionaldownward to upward allocation of carbon increased with increasein sink size of the root relative to shoot. This was reflectedin greater relative allocation of C to root dry matter and rootrespiration than in P-sufficient plants, and suppressed cyclingof C from root to shoot via xylem. Nitrogen intake and xylemtransport to the shoot of P-deficient plants were only 15% ofthe control and, as in the case of C, downward allocation ofN predominated over upward phloem translocation. Apart fromthese severe changes, however, the basic patterns of N flowsincluding xylem-to-phloem and xylem-to-xylem transfer of N werenot changed, a feature highlighting the vital nature of thesetransfer processes even under deficiency conditions. The alterationsin flows and partitioning of C, N and H₂O in response to low-Pconditions are discussed in relation to the corresponding effectsof moderate salt stress in Ricinus and the conclusion is reachedthat changes in nutrient flows under P deficiency were morehighly co-ordinated than when plants experience salt stress.Flow profiles under P deficiency which favour root growth andactivity are viewed as a means for increasing the potentialcapability of the plant to acquire P from the nutrient medium. Key words: Ricinus communis L., P deficiency, carbon, nitrogen, water, partitioning, xylem transport, phloem transport     

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     

Uptake of 13C and 15N (ammonium, nitrate and urea) by Microcystis in Lake Kasumigaura

The uptake rate of carbon and nitrogen (ammonium, nitrate andurea) by the Microcystis predominating among phytoplankton wasinvestigated in the summer of 1984 in Takahamaira Bay of LakeKasumigaura. The V_max values of Microcystis for nitrate (0.025–0.046h^–1) and ammonium (0.15–0.17 h^–1) were considerablyhigher than other natural phytoplankton. The ammonium, nitrateand urea uptake by Microcystis was light dependent and was notinhibited with nigh light intensity. The K₁ values were farlower than the I_k values. The carbon uptake was not influencedby nitrogen enrichment. Microcystis accelerated the uptake rateby changing V_max/K _s value when nitrogen versus carbon contentin cells declined. Nitrate was scarcely existent in TakahamairiBay during the summer, when Microcystis usually used ammoniumas the nitrogen source. However, the standing stock of ammoniumin the water was far lower than the daily ammonium uptake rates.Therefore, the ammonium in this water had to be supplied becauseof its rapid turn-over time (–0.7–2.6 h).     

The Effect of Root Temperature on Growth and Uptake of Ammonium and Nitrate by Brassica napus L. in Flowing Solution Culture: I. GROWTH

Macduff, J. H., Hopper, M. J. and Wild, A. 1987. The effectof root temperature on growth and uptake of ammonium and nitrateby Brassica napus L. in flowing solution culture. I. Growth.—J.exp. Bot. 38: 42–52 Oilseed rape (Brassica napus L. cv. Bien venu) was grown for49 d in flowing nutrient solution at pH 6?0 with root temperaturedecrementally reduced from 20?C to 5?C; and then exposed todifferent root temperatures (3, 5, 7, 9, 11, 13,17 or 25?C)held constant for 14 d. The air temperature was 20/15?C day/nightand nitrogen was supplied automatically to maintain 10 mmolm^–3 NH₄NO₃ in solution. Total dry matter production wasexponential with time and similar at all root temperatures givinga specific growth rate of 0?0784 g g^–1 d^–1. Partitioningof dry matter was influenced by root temperature; shoot: rootratios increased during treatment at 17?C and 25?C but decreasedafter 5 d at 3?C and 5?C. The ratio of shoot specific growthrate: root specific growth rate increased with the ratio ofwater soluble carbohydrates (shoot: root). Concentrations ofwater soluble carbohydrates in shoot and root were inverselyrelated to root temperature; at 3, 5 and 7?C they increasedin stem + petioles throughout treatment, coinciding with a decreasein the weight of tissue water per unit dry matter. These resultssuggest that the accumulation of soluble carbohydrates at lowtemperature is the result of metabolic imbalance and of osmoticadjustment to water stress. Key words: Brassica napus, oilseed rape, root temperature, specific growth rate     

Regulation of K+ Influx in Barley: Evidence for a Direct Control of Influx by K+ Concentration of Root Cells

Siddiqi, M. Y. and Glass, A. D. M. 1987. Regulation of K⁺ influxin barley: Evidence for a direct control of influx by K⁺ concentrationof root cells.—J. exp. Bot. 38: 935–947. The kinetics of K⁺ (⁸⁶Rb⁺) influx into intact roots of barley(Hordeum vulgare L. cv. Fergus) seedlings having different combinationsof root and shoot [K⁺], different growth rates and differentroot:shoot weight ratios were studied. K⁺ influx was stronglycorrelated with root [K⁺]; shoot [K⁺], growth rates, and root:shoot ratios appeared to have little effect on K⁺ influx. Adetailed study showed that both V_max and K_m for K⁺ influx wereaffected by root [K⁺] but not by shoot [K⁺]. We have suggestedthat factors such as growth rates and root: shoot ratio mayaffect K⁺ influx indirectly primarily via their influence onroot factors such as root [K⁺]. We have reiterated that othertypes of kinetic control, e.g. increased or decreased synthesisof ‘carrier systems’, may operate in addition todirect (allosteric?) control of K⁺ influx by root [K⁺]. Thenegative feedback signal from root [K⁺] appeared to be the primeeffector in the regulation of K⁺ influx. Key words: Barley, K⁺ influx     

The Distribution of Nitrate Assimilation Between Root and Shoot in Vicia faba L.

Nitrate assimilation was examined in two cultivars (Banner Winterand Herz Freya) of Vicia faba L. supplied with a range of nitrateconcentrations. The distribution between root and shoot wasassessed. The cultivars showed responses to increased applied nitrateconcentration. Total plant dry weight and carbon content remainedconstant while shoot: root dry weight ratio, total plant nitrogen,total plant leaf area and specific leaf area (SLA) all increased.The proportion of total plant nitrate and nitrate reductase(NR) activity found in the shoot of both cultivars increasedwith applied nitrate concentrations as did NO₃^–: Kjeldahl-Nratios of xylem sap. The cultivars differed in that a greaterproportion of total plant NR activity occurred in the shootof cv. Herz Freya at all applied nitrate concentrations, andits xylem sap NO₃^–: Kjeldahl-N ratio and SLA were consistentlygreater. It is concluded that the distribution of nitrate assimilationbetween root and shoot of V. faba varies both with cultivarand with external nitrate concentration. Vicia faba L., field bean, nitrate assimilation, nitrate reductase, xylem sap analysis     

Assimilate Partitioning in Red and White Clover Either Dependent on N2 Fixation in Root Nodules or Utilizing Nitrate Nitrogen

During vegetative growth in controlled environments, the patternof distribution of ¹⁴C-labelled assimilates to shoot and root,and to the meristems of the shoot, was measured in red and whiteclover plants either wholly dependent on N₂ fixation in rootnodules or receiving abundant nitrate nitrogen but lacking nodules. In experiments where single leaves on the primary shoot wereexposed to ¹⁴CO₂, nodulated plants of both clovers generallyexported more of their labelled assimilates to root (+nodules),than equivalent plants utilizing nitrate nitrogen, and thiswas offset by reduced export to branches (red clover) or stolons(white clover). The intensity of these effects varied with experiment.The export of labelled assimilate to growing leaves at the terminalmeristem of the donor shoot was not influenced by source ofnitrogen. Internode elongation in the donor shoot utilized nolabelled assimilate. Whole plants of white clover exposed to ¹⁴CO₂ on seven occasionsover 32 days exhibited the same effect on export to root (+nodules),which increased slightly in intensity with increasing plantage. Nodulated plants had larger root: shoot ratios than theirequivalents utilizing nitrate nitrogen. Trifolium repens, Trifolium pratense, red clover, white clover, nitrogen fixation, nitrate utilization, assimilate partitioning     

Root system adjustments: regulation of plant nutrient uptake and growth responses to elevated CO2

Nutrients such as nitrogen (N) and phosphorus (P) often limit plant growth rate and production in natural and agricultural ecosystems. Limited availability of these nutrients is also a major factor influencing long-term plant and ecosystem responses to rising atmospheric CO₂ levels, i.e., the commonly observed short-term increase in plant biomass may not be sustained over the long-term. Therefore, it is critical to obtain a mechanistic understanding of whether elevated CO₂ can elicit compensatory adjustments such that acquisition capacity for minerals increases in concert with carbon (C) uptake. Compensatory adjustments such as increases in (a) root mycorrhizal infection, (b) root-to-shoot ratio and changes in root morphology and architecture, (c) root nutrient absorption capacity, and (d) nutrient-use efficiency can enable plants to meet an increased nutrient demand under high CO₂. Here we examine the literature to assess the extent to which these mechanisms have been shown to respond to high CO₂. The literature survey reveals no consistent pattern either in direction or magnitude of responses of these mechanisms to high CO₂. This apparent lack of a pattern may represent variations in experimental protocol and/or interspecific differences. We found that in addressing nutrient uptake responses to high CO₂ most investigators have examined these mechanisms in isolation. Because such mechanisms can potentially counterbalance one another, a more reliable prediction of elevated CO₂ responses requires experimental designs that integrate all mechanisms simultaneously. Finally, we present a functional balance (FB) model as an example of how root system adjustments and nitrogen-use efficiency can be integrated to assess growth responses to high CO₂. The FB model suggests that the mechanisms of increased N uptake highlighted here have different weights in determining overall plant responses to high CO₂. For example, while changes in root-to-shoot biomass allocation, r, have a small effect on growth, adjustments in uptake rate per unit root mass, [`(n)]\bar \nu , and photosynthetic N use efficiency, p*, have a significantly greater leverage on growth responses to elevated CO₂ except when relative growth rate (RGR) reaches its developmental limit, maximum RGR (RGR_max).     

Nitrate Uptake and Reduction of Aseptically Cultivated Spruce Seedlings, Picea abies (L.) Karst

Spruce (Picea abies (L.) Karst.) seedlings were asepticallycultivated and the effects of different N-nutrition on net uptakeand reduction of nitrate were investigated. The characteristicsof nitrate uptake were calculated, K_s as 0?2 mol m^–3 andV_max as 18 µmol g^–1 d^–1. Low pH, and Al³⁺ in the medium caused adecrease in nitrate uptake rate. An in vivo assay was set upwhich allowed the measurement of NRA in both roots and needlesof spruce seedlings. The in vivo nitrate reductase activitywas repressed by ammonium and stimulated by nitrate. Nitratereduction was similar to nitrate uptake, negatively affectedby low pH and ammonium. Therefore, a limited N-supply to spruceseemed to occur when pH was low in the rhizosphere combinedwith the presence of Al³⁺ and . Key words: Spruce, nitrate uptake, nitrate reduction     

Kinetics of nitrate uptake by different species from nutrient-rich and nutrient-poor habitats as affected by the nutrient supply

The species Urtica dioica L., Plantago major ssp. major L., Plantago lanceolata L., Hypochaeris radicata L. ssp. radicata and Hypochaeris radicata ssp. ericetorum Van Soest were grown under high and low nutrient conditions (1/4 Hoagland and 2% of 1/4 Hoagland further called the 100% and 2% treatment, containing 3.75 mM NO^-₃ and 0.075 mM NO^-₃, respectively). After a certain period half of the plants were transferred from low to high or high to low nutrients, yielding the 100%/2% and the 2%/100% treatments. The kinetics of nitrate uptake in the range of system I of the five species grown under the different nutrient conditions were measured during a three week experimental period. The nitrate uptake of all the species showed the characteristic features of Michaelis-Menten kinetics. Under low nutrient conditions the apparent V_max of U. dioica expressed per g dry root was lower than under high nutrient conditions. For H. radicata ssp. radicata and for H. radicata ssp. ericetorum the reverse was found. The V_max values of P. major ssp. major were almost the same for the two treatments. The apparent V_max in young plants of P. lanceolata was higher in the 100% treatment than in 2%; whereas the reverse was found in mature plants. The results are explained in relation to the relative growth rate, the shoot to root ratio and the natural environment of the species. The apparent K_m values were not influenced by the different treatments. Differences in K_m between the species, if any, were very small. It is suggested that the V_max is a more important parameter for the distribution of plant species in the field than the K_m. The rate of nitrogen accumulation was calculated from growth data and the contents of nitrate and reduced nitrogen. It is concluded that the V_max of system I for nitrate uptake in most cases was sufficient to explain the observed growth rates.