Nitrate Transport Characteristics in Hordeum vulgare L. Seedlings using Three Different Tracer Techniques

Deane-Drummond, C. E. and Thayer, J.R. 1986. Nitrate transportcharacteristics in Hordeum vulgare L. seedlings using threedifferent tracer techniques.—J. exp. Bot. 37: 429–439. and have been used to investigate various properties of nitrate uptake and translocation intoHordeum vulgare L seedlings. Short term / influx into seed lings grown in CaSO⁴ was stimulated by after a lag of 2 h. The apparent kinetics of shortterm / influx over the concentration range 0?0–0?7mol m fitted Michaelis-Menten equations The apparent V_max didnot change when seedlings were used that had been pretreatedin 10 or 100 mmol m^–3 and V^max=3.77 and 3?56µmol g^–1 fr. wt. h^–1respectively. The apparent Michaelis constants were also similarand K_m=0?47 and 0?45 mol m^–3 respectively. Longer term pulse chase experiments with the heavy isotope ¹⁵Nhave shown that feeding roots with resulted in the preferential appearance of ¹⁵N labelled aminoacids in the xylem sap. Pulse chase experiments with the radioisotope¹³N have shown that feeding shoots with resulted in a radial pattern of distnbution of labelin the leaf veins, which can be detected using autoradiography. The limitations and advantages of all three techniques are comparedby reference to other known experimental data. Key words: ³⁶Chlorate, ¹³nitrate, ¹⁵nitrate, Hordeum vulgare     

Relative Uptake Rates of Inorganic Nutrients by NO3- and NH4+ Grown Ricinus communis and by two Plantago Species

Allen, S., Thomas, G. E. and Raven, J. A. 1986. Relative uptakerates of inorganic nutrients by and grown Ricinus communis and by two Plantago species.—J. exp. Bot. 37: 419–428. The relative rates of uptake and assimilation of C, N, P, S,Cl^–, K⁺ , Na⁺ Ca²⁺ and Mg²⁺ by and grown Ricinus conimunisand by NH₄NO₃- grown Plantago lanceolata and P. major were calculatedfrom data presented elsewhere. Results showed that for grown Ricinus the short term relativeuptake rates, for each nutrient X did not change significantly over the steady-state periodof exponential growth. The average gave , the mean relative uptake rate during exponential growth, for each nutrient. The amountof each nutrient taken up from a nutrient solution over a periodof time could, therefore, be calculated. For and -grown R. communis,the relative uptake rate of each nutrient was a constant fractionof the relative rate of carbon assimilation. It is suggestedthat this is typical of plants of cauline habit. For both Plantago spp., the relative rates of nitrogen uptakeand assimilation fell significantly during the exponential growthphase It is suggested that this could be a characteristic ofthe growth habit of the rosette plant. Key words: Relative uptake rates, Ricinus, Plantago, ammonium, nitrate, cauline, rosette     

Differential rates of inhibition of N2 fixation by sustained low concentrations of NH4+ and NO3- in northern ecotypes of white clover (Trifolium repens L.)

An experimental study in flowing solution culture compared threeNorwegian ecotypes (from Saerheim, Pasvik and Bod) and a commercialcultivar (Ac51) of white clover (Trifolium repens L.) and demonstrateddifferences with respect to the rate and intensity with whichN₂ fixation was affected when or were supplied at 20 mmol m^–3 over 14 d. Plants were nodulated and N₂ fixation wasestablished over 17 d prior to supplying mineral N, with shoottemperature 25/15 C day/night and root temperature adjustedprogressively to 12C. Control plants received no mineral Nthroughout, and did not grow as rapidly as those supplied with or . Mineral N generally depressed the total N₂ fixation per plantrelative to control plants, with two exceptions. The effectof on N₂ fixation was moresevere than that of and, over the 14 d, -fed plants fixed more N₂ than the comparable -fed plants. Interpolated daily rates of N₂ fixation per plant andnodule dry wts were used to calculate specific rates of N₂ fixation.These showed that (1) addition of either or initially stimulated fixation relative to control plants and to mineral N uptake;(2) this stimulation was greatest and was delayed by 1-2 d in-fed plants; and (3) nutrition sustained higher residualrates of N₂ fixation after 8 d compared with nutrition, under which fixation all but ceased after10 d in three of the genotypes. Ecotype Bode showed by far themost severe induced depression of N₂ fixation. Key words: N₂ fixation, nitrate, ammonium, white clover, northern ecotypes     

The Titrimetric Assay of OH- and HCO3- Excreted by Ricinus Cultivated on NO3--containing Nutrient Media: The Influence of Ionic Strength, End Point pH and CO2 Supersaturation

Allen, S. and Allen, C. R. 1987. The titrimetric assay of OH^–and excreted by Ricinus cultivatedon -containing nutrient media: the influence of ionic strength, end point pH and CO² supersaturation.–J. exp. Bot. 38: 607–617. When spent -containing nutrient media were titrated to the starting pH of 6.5 the titre wasequivalent to 50%orless of the base (i.e. ) excreted. Calculation of the total amount of baseexcreted could only be made from data obtained by titrationto pH 4–5. An accurate calculation of the amount of baserequired the inclusion of activity coefficients, estimated fromionic strength, in the calculations. Spent nutrient media contained from four to ten times the concentrationof CO₂ predicted from equilibrium values. It is probable that is very slow. Key words: Ricinus, nitrate-N nutrient medium, base excretion, ionic activity coefficients, carbon dioxide supersaturation     

Nitrate Uptake and Assimilation following Nitrate Deprivation

Upon first exposure to , the uptake and reduction capacities of dark-grown corn (Zea maysL.) roots are initially low, but increase markedly within 6h. The development of the accelerated uptake rate appears to be substrate ‘induced’ as is reductase (NR), the first enzyme in the assimilatory pathway. However, the ‘induction’of uptake is independent of NR induction. The effect of deprivation was studied to determine the role of endogenous on subsequent uptake and reduction. Corn roots were ‘induced’ for 24 h in 0–5 mol m^–3 nutrient solution and then exposed for 0 to 32 h to -free nutrient solution. Uptake and reduction of were determined periodically by exposing sets of roots to a1 h pulse of 0.5 mol m^–3 . Neither uptake (4.57 µmol root^–1 h^–1)nor the percentage of absorbed reduced (27%) was changed significantly (P 0.05) by exogenous deprivation. However, the estimated ‘induced’ componentof uptake decreased significantly (50% after 32 h). Concurrently, the ‘non-induced’ basal componentof uptake increased. Previously accumulated decreased from 23 to 4.5 µmol root^–1 after 32 h of exogenous deprivation. Nearly equivalent quantities of endogenous were used for translocation and reduction during deprivation. During each 1 h pulse, the amounts of translocation and net efflux of to the uptake solution were similar. Net efflux of was strongly correlated (r = 0.991) to the amount of endogenous . The remaining endogenous and its rate of utilization were apparently sufficient to minimize a rapid declineor complete loss in both the ‘induced’ uptake state and the rate of in vivo assimilation. Key words: reduction, translocation, efflux, root, Zea mays L     

An analysis of the physiological basis of commonality between diurnal patterns of NH4+, NO3- and K+ uptake by Phleum pratense and Festuca pratensis

Commonality in diurnal variation in net uptake of by the grasses Phleum pratense L. cv. Bodin (timothy),and Festuca pratensis Huds. cv. Salten (fescue) was evaluatedin flowing solution culture under a semi-natural light regime.Hourly uptake rates from constant 20 mmol m^–3 concentrationsof each ion were measured concurrently over 7 d, without physicaldisturbance. The light period was 11 h, natural light supplementedby constant artificial illumination, with a step-transitionto a 13 h dark period. Uptake of all three ions showed a broadlysimilar pattern of diurnal variation, rates increasing duringthe light period and decreasing during the darkness to a minimumwithin 2 h of the end of the dark period. The amplitude wasgreatest during high-irradiance days, and both proportionatelyand absolutely greater for (mean min:max uptake?0.24) than (min:max=0.41) or K⁺ (min:max?0.34). There were significantdifferences between and the other two ions whose behaviour coincided in almost all respects, in timing of maximum and minimumrates, and acceleration in uptake during the light period. Preferentialuptake of over increased sharply during the first half of the lightperiod; in relative terms uptake accelerated twice as fast as uptake. Fescue always absorbed more than but timothy showed a preference for during part of the dark period. The results are interpreted in terms ofthe hypothesis that diurnally fluctuating ‘sink-strength’for nutrients is the primary determinant of nutrient transportrates, although uptake may become temporarily uncoupled fromnutrient demand during periods of physiological stress or perturbation,such as towards the end of the dark period, during which uptakerates are determined by factors (e.g. carbohydrate supply) otherthan current nutrient demand. Key words: Ammonium, diurnal variation, Gramineae, ion uptake, nitrate, potassium, regulation     

Nitrate Metabolism in Alfalfa Root Nodules under Water Stress

Three-month-old plants (vegetative stage) of alfalfa (Medicagosaliva L cv. Aragon) were supplied for one week with 1.0dm³(uniformly distributed) nutrient solutions containing 0 or 20mol m^–3 . One week after initiation of treatment, the plants were subjected to drought by withholding water. Bacteroidsand cytosol of nodules were obtained at different stages ofstress, and used for enzyme assays and for determination of, and . Proteins of bacteroids were more stable than cytosolic proteinswith respect to the detrimental effects of water stress and. Protein contents of bacteroids and cytosol were inversely related to proteolytic activitiesagainst azocasein in both nodule fractions. Specific nitrate reductase activity (NRA) and nitrite reductaseactivity (NiRA) of bacteroids from -treated plants were inhibited by c. 70% and 45%, respectively, as leafwater potential (_w) declined from –0.5 MPa (control) to–1.8 MPa. At still lower _w both activities began to increase:NRA was doubled, whereas NiRA only returned to its control level.Cytosolic NRA was strongly inhibited by drought, but the correspondingNiRA remained constant. Ammonia concentration in bacteroids and nodule cytosol keptbasically constant, whereas accumulated in the cytosol at severe stress, due to the activationof bacteroid nitrate reductase. Results indicate that nitrate and nitrite reductases of thebacteroids and the nodule cytosol act in different form: assimilatory,the cytosolic enzymes; and dissimilatory, the enzymes of bacteroidsat low _w The possibility that assimilation of also occurs in bacteroids at control or mild waterstress conditions is suggested. Key words: Assimilatory and dissimilatory reduction, bacteroids, Medicago saliva, nodule cytosol, water stress     

Stage of development is an important determinant in the effect of nitrate on photoassimilate (13C) partitioning in chicory (Cichorium intybus)

The effect of nitrogen supply to chicory plants on carbon partitioningbetween shoot, root and tuberized root was studied at differentstages of vegetative growth, using long-term ¹³CO₂ labelling-chaseexperiments. This approach was complemented by measurement ofstorage carbohydrates and activities of enzymes involved inroot sucrose metabolism (sucrose-sucrose fructosyl transferase(SST), sucrose synthase, invertase). In both young and matureplants, low resulted in a 30–35% decrease in ¹³C assimilation. However, the partitioningof ¹³C between shoot and root was affected differently at differentstages of development. In young plants, in which carbohydrateswere being used for structural root and shoot growth, neither¹³C shoot/¹³C root ratio nor root activities of the above enzymeswere modified by supply. In contrast, in mature plants storing large amounts of carbohydratesas fructan in the tuberized root, low caused the ratio to decrease from 0.6 to 0.2, despiteunchanged net flux of ¹³C from shoot to root. The extractableactivity of SST was elevated in mature plants, compared to youngplants, at both low and high , consistent with its role in fructan synthesis. However, matureplants grown at low exhibited SST activity double that of plants grown at high . From these results, it is concluded that the observeddecrease in shoot/root dry weight ratio at low supply is caused by increased utilization of carbohydratesfor storage due to elevated root SST activity. Key words: Chicory, nitrate, ¹³C, shoot/root ratio, fructans, SST     

Evidence for N feedback regulation of N2 fixation in Alnus glutinosa L.

Treatments were applied to vary C and N availability in Alnusglutinosa L. and plant growth, nodule activity (including acetylenereduction) and amino acid composition of the xylem sap weremeasured. Removing the buds, a sink for N, caused a decreasein nodule activity. Flushing root systems daily with 100% O₂destroyed nitrogenase activity and substantially decreased theamount of citrulline in the xylem sap. The amino acid compositionof xylem saps also altered according to the mode of N nutrition.In plants fed , xylem sap composition was similar to N₂-fixing plants, however, when plants were fed, citrulline content increased. The assimilation and subsequent distribution of nitrate wasfollowed in an experiment in which labelled ¹⁵ was added to the base of plant pots. After 12 h7% of root N was from applied ¹⁵ and this increased to 75% at 7 d; substantial enrichment ofN from ¹⁵ also occurred in stems, buds and leaves. After 7 d, 3.5% of nodule N was from¹⁵, consistent with some N being supplied by recycling of shoot N. Xylem saps were alsocollected and after 12 h, glutamate and aspartate were enrichedwith ¹⁵N to 53% and 37% increasing after 7 d to 80% and 49%,respectively. Citrulline content of the xylem sap increasedfrom 3 to 9 µmol cm^–3 following addition of ¹⁵ and at 7 d 80% of the N in the citrullinehad been derived from ¹⁵N. It is hypothesized that the growthand activity of A. glutinosa root nodules is sensitive to theN status of the plant and that the level of citrulline (or otheramino acids) returning to the nodules may feed back to regulatenodule growth and activity. Key words: Alnus glutinosa, citrulline, nitrate, feedback mechanism, N₂-fixation.     

Salt-Stimulated Bicarbonate-Dependent Photosynthesis in the Marine Angiosperm Zostera muelleri

Millhouse, J. and Strother, S. 1986. Salt-stimulated bicarbonate-dependentphotosynthesis in the marine angiosperm Zostera muelleri.—J.exp. Bot. 37: 965–976. Photosynthetic oxygen evolution in the seagrass Zostera muelleriIrmisch ex Aschers. was inhibited in iso-osmotic sucrose. Theapparent affinity of the leaves for CO₂ in seawater increasesfrom pH 8?2 to 8?9 indicating that as well as CO₂ may act as a substrate for photosynthesis. Theaffinity for CO₂ was lower in iso-osmotic sucrose and was notaffected by pH. Under these conditions was not a substrate for photosynthesis. The differencebetween the photosynthetic rate in seawater and iso-osmoticsucrose at the same concentration of CO₂ was used to estimate assimilation. The Briggs-Maskell equation, which allows for an unstirred layer around the tissuewas more appropriate than the Michaelis-Menten theory for calculatingthe apparent affinity of the leaf slices for CO₂. The apparentK_m CO₂ was calculated as 116 mmol m^–3 at pH 8?2 by Michaelis-Mentenkinetics but only 8?10 mmol m^–3 by the Briggs-Maskellequation. The stimulation by various ions in Seawater of use was investigated. The cations,in decreasing order of effectiveness were Ca²⁺, Mg²⁺, K⁺ andNa⁺ Anions were ineffective. No single cation at its concentrationin seawater was capable of supporting use at the rate observed in seawater. Acetazolamide,an inhibitor of carbonic anhydrase, inhibited the use of for photosynthesis but had littleeffect on CO₂ photosynthesis. Thus, carbonic anhydrase activityis required for -dependent photosynthesis. Key words: Zostera muelleri, photosynthesis, salinity     

Expression of Characteristics of Ammonium Nutrition as Affected by pH of the Root Medium

To study the effect of root-zone pH on characteristic responsesof -fed plants, soybeans (Glycine max {L.}Merr. cv. Ransom) were grown in flowing solution culture for21 d on four sources of N (1.0 mol m^–3 , 0.67 mol m^–3 plus 0.33 mol m^–3, 0.33 mol m^–3 plus 0.67 mol m^–3 , and 1.0 mol m^–3) with nutrient solutions maintained at pH 6.0, 5.5, 5.0, and 4.5. Amino acid concentration increased inplants grown with as the sole source of N at all pH levels. Total amino acid concentration in the rootsof -fed plants was 8 to 10 times higher than in -fed plants, with asparagine accounting for more than 70% of the total in the roots of these plants.The concentration of soluble carbohydrates in the leaves of-fed plants was greater than that of -fed plants, but was lower in roots of -fed plants, regardless of pH. Starch concentration was only slightlyaffected by N source or root-zone pH. At all levels of pH tested,organic acid concentration in leaves was much lower when was the sole N source than when all or part of theN was supplied as . Plants grown with mixed plus N sources were generally intermediate between - and -fed plants. Thus, changes in tissue compositioncharacteristic of nutrition when root-zone pH was maintained at 4.5 and growth was reduced, still occurredwhen pH was maintained at 5.0 or above, where growth was notaffected. The changes were slightly greater at pH 4.5 than athigher pH levels. Key words: Ammonium, nitrogen nutrition, root-zone pH, soybean, tissue composition     

Interactions Between the Uptake and Assimilation of Inorganic Nitrogen and Carbon in Amphidinium spp. (Dinophyceae)

The rate of exponential growth of Amphidinium carterae Hulburtwas the same (0.025 h^–1) with either or as sole N-source. Nevertheless, in short-term experiments, cells growing exponentially with as N-source took up added 5–6 times faster than . accumulated in the cells. Addition of inhibited, reversibly, disappearance of from the medium; prior N-deprivation of the cells did not affect this inhibition. N-repIetecells, grown with as N-source, took up for several hours both in light and darkness, butthe uptake by such cells soon ceased in darkness although it continued in light. When uptake ceased, the cells could still take up rapidly in darkness. Ammonium taken up was assimilated rapidly into organic-N includingglutamine, other amino acids and protein. Ammonium uptake in darkness was accompanied by the utilizationof cellular polysaccharide, mainly glucose polysaccharide. Mostof this carbon was unavailable for the dark assimilation of. Addition of the analogue, methylammonium, did not initiate polysacchande catabolism. Itis suggested that a control mechanism is in operation, througha product of assimilation, which operates on one or more of the enzymes concerned with polysaccharidebreakdown, for example, -amylase or phosphorylase. Uptake of was accompanied by a high rate of dark ¹⁴CO₂ fixation and with both Amphidintum klebsii Kof.et Swezy and A. carterae N-deprivation led to a marked increasein this rate following addition of ; addition of had much less effect. The possible implications of these findings for the eco-physiology of marine dinoflagellatesare discussed. Key words: Ammonium, nitrate, CO₂ fixation     

Comparison of the Effects of Root Temperature on Nitrate and Ammonium Nutrition of Oilseed Rape (Brassica napus L.) in Flowing Solution Culture: I. GROWTH UPTAKE OF NITROGEN

Oilseed rape plants (Brassica napus L. cv. Bien venu) were grownin flowing nutrient solution and acclimatized over 49 d to lowroot temperature (5 ?C) prior to treatment at root temperaturesof 3, 7, 11 or 17?C for 14 d, with common air temperature of20/15?C day/night and solution pH 6.0. The effects of root temperatureon growth, uptake and assimilation of N were compared with Nsupplied either as or at 10 mmol m^–3. Plant fresh weight, dry weight and leafarea increased with root temperature. Q₁₀ values for specificgrowth rates between 7?C and 17?C were 1.10 and 1.33, respectively,under and nutrition. Fresh weight and leaf area were favoured by nutrition, particularly at 11 ?C and 17 ?C. At each temperaturethe net uptake of over 14 d was similar to that of , increasing 4-fold between 3?C and 17 ?C. Unit absorption rates of increased over the first 3 or 4 d, to an extent that increased with roottemperature. Thereafter, near constant values were maintainedat 3, 7 and 11 ?C. Unit absorption rates of at 3 ?C and 7 ?C did not increase over the first 3 or 4 d. Plantsat 17?C responded differently from those at other temperatures,with steep increases in unit absorption rates of and after day 8. The tendency for plants to adjust unit absorption rates of N over 14 d to become independentof root temperature was limited to the range 7–11 ?C.Q¹⁰ values between 7 ?C and 17 ?C for unit absorption rateswere 1.7—1.8 and 1.4—1.5, respectively, for and . Plants at 7, 11 and 17?C absorbed more than was subsequently assimilated, but at 3 ?C the absorption of was insufficient to meet the assimilatory demand. The results are discussed comparativelyfor and and in terms of the factors that determine unit absorption rates of N inresponse to a change in root temperature. Key words: Brassica napus, oilseed rape, root temperature, nitrate, ammonium, ion uptake rate     

Reduced15N-nitrogen Transport Through Arbuscular Mycorrhizal Hyphae to Triticum aestivum L. Supplied with Ammonium vs. Nitrate Nutrition

This study compared the influence of or nutrition on the contribution of extraradical hyphae of the arbuscular mycorrhizal fungusGlomus mosseae(Nicol. and Gerd.) Gerd. and Trappe (BEG 107)to or uptake byTriticum aestivum L. ‘Hano’(summer wheat) with sufficient or insufficient N supply in semi-hydroponicculture. Roots and root-distant hyphae were spatially separatedin compartmentalized pots. Although -fed plants supplied with sufficient N had higher N concentrationsthan their -fed counterparts, this did not favourably affect colonization rates, hyphal lengthdensities or¹⁵N amounts transported via hyphae to the plants.Ammonium supply did not result in higher P or reduced carbohydrateconcentrations in the plants, so these factors could not explainthe reduced hyphal lengths. It was concluded that the effectof supply on hyphal length may be related to the reduced root growth and/or a direct effectof on hyphal growth. Plant N deficiency reduced the percentage root length colonized, hyphallength, total¹⁵N uptake by hyphae and dry weight of both - and -fed mycorrhizal plants. This was more obvious for -fed plants because plant biomass and hyphal lengthsof -fed plants were relatively low in mycorrhizal plants irrespective of the N concentrationsupplied. Copyright 2001 Annals of Botany Company Ammonium, arbuscular mycorrhiza, Glomus mosseae, nitrate, nitrogen uptake,¹⁵N studies, Triticum aestivum, wheat     

Translocation of Some Anions, Cations, and Acids in Turnip (Brassica napus L.) Mitochondria

The translocation of various anions, cations, and acids acrossthe membrane system of turnip mitchondria has been investigatedby light scattering techniques. The results imply that the membranesof turnip mitochondria are not permeable to choline, K⁺, Na⁺,, , , and . It has been established that Cl^–,, and SCN– passively permeate the mitochondrial membrane. It is probable that permeates as NH₃ and that and cross the membrane either as the corresponding acid or on ananion/ hydroxide antiporter. The alkali metals Na⁺ and K⁺ equilibrateacross the membrane via specific H⁺/Na⁺ and H⁺/K⁺ antiporters,having similar activity at neutral pH.     

Cyclic Variations in Nitrogen Uptake Rate of Soybean Plants: Effects of External Nitrate Concentration

Net uptake of by non-nodulated soybean plants [Glycine max (L.) Merr.cv. Ransom] growing in flowing hydroponiccultures containing 0–5, 1.0 and 10-0 mol m^–3 was measured daily during a 24-d period of vegetativedevelopment to determine if amplitude of maximum and minimumrates of net uptake are responsive to external concentrations of . Removal of from the replenished solutions during each 24-h period was determinedby ion chromatography. Neither dry matter accumulation nor theperiodicity of oscillations in net uptakerate was altered bythe external concentrations. The maxima of the oscillations in net uptake rate, however, increased nearly3-fold in response to external concentrations. The maxima and minima, respectively, changed from 4.0 and 0–6mmol per gram root dry weight per day at an external solution level of 05 mol m^–3 to 15–2and -2–7 mmol per gram root dry weight per day at an external solution level of 10–0mol m^–3 . The negative values for minimum net uptake rate from10–0 mol m^–3 solutions show that net efflux was occurring andindicate thatthe magnitude of the efflux component of net uptake was responsiveto external concentration of . Key words: Nitrate uptake rate, carbon-nitrogen partitioning, flowing solution culture     

The Titrimetric Assay of H+ Excreted by Ricinus communis Cultivated on NH4+-N Nutrient Media: The Effect of Ionic Strength and Nutrient Phosphate Concentration

Allen, C. R. and Allen, S. 1987. The titrimetric assay of H⁺excreted by Ricinus communis cultivated on -N nutrient media; the effect of ionic strengthand nutrient phosphate concentration.— J. exp. Bot. 38:597–606. The simple titrimetric assay of H⁺ excreted by -N plants into spent nutrient solution was criticallyassessed on the basis of equations for charge balance, conservationof matter and chemical equilibrium (including activity coefficients).For Ricinus communis, a simple volumetric calculation from back-titrationof the spent medium to 6–5 underestimated proton excretionby 3%, compared with the corresponding error of up to 50% fortitrimetric assay of base excreted into spent -N media. Key words: Ricinus, ammonium-N nutrient medium, proton excretion, ionic activity coefficients     

Plant response to nitrate starvation is determined by N storage capacity matched by nitrate uptake capacity in two Arabidopsis genotypes

In a low-input agricultural context, plants facing temporalnutrient deficiencies need to be efficient. By comparing theeffects of NO-starvation in two lines of Arabidopsis thaliana (RIL282 and 432 from the Bay-0xShahdarapopulation), this study aimed to screen the physiological mechanismsallowing one genotype to withstand NO-deprivation better than another and to rate the relative importance of processessuch as nitrate uptake, storage, and recycling. These two lines,chosen because of their contrasted shoot N contents for identicalshoot biomass under N-replete conditions, underwent a 10 d nitratestarvation after 28 d of culture at 5 mM NO. It was demonstrated that line 432 coped betterwith NO-starvation, producing higher shoot and root biomass and sustaining maximal growthfor a longer time. However, both lines exhibited similar featuresunder NO-starvation conditions. In particular, the nitrate pool underwent the same drastic andearly depletion, whereas the protein pool was increased to asimilar extent. Nitrate remobilization rate was identical too.It was proportional to nitrate content in both shoots and roots,but it was higher in roots. One difference emerged: line 432had a higher nitrate content at the beginning of the starvationphase. This suggests that to overcome NO-starvation, line 432 did not directly rely on theN pool composition, nor on nitrate remobilization efficiency,but on higher nitrate storage capacities prior to NO-starvation. Moreover, the higher resistanceof 432 corresponded to a higher nitrate uptake capacity anda 2–9-fold higher expression of AtNRT1.1, AtNRT2.1, andAtNRT2.4 genes, suggesting that the corresponding nitrate transportersmay be preferentially involved under fluctuating N supply conditions. Key words: Arabidopsis thaliana, genetic variability, N partitioning, N recycling, N use efficiency, nitrate deficiency, nitrate remobilization rate, nitrate transporter gene expression, nitrogen reserves, plant development Received 12 July 2007; Revised 21 November 2007 Accepted 17 December 2007     

Diurnal variation in the simultaneous uptake and 'sink' allocation of NH4+ and NO3- by Lolium perenne in flowing solution culture

Perennial ryegrass (Lolium perenne L.) was grown in flowingsolution culture under artificial illumination with a 10 h lightperiod. The diurnal fluctuations in the net uptake of and from a 20 mmol m^–3 NH₄NO₃ supply were measured (i) fromthe amounts of these ions supplied automatically to maintainconcentrations in solution, and (ii) by ¹⁵N pulse-labellingthroughout the diurnal cycle. Diurnal variation in leaf extension,translocation and allocation of assimilated C (¹⁴CO₂ pulse-labelling)and N (¹⁵N pulse and steadystate labelling) were also followed.The apparent ‘sinkstrength’ of different organsfor recently assimilated C and N was calculated from the concentrationof label recovered in the tissues. Ammonium and showed qualitatively similar diurnal patterns ofnet uptake, with minimum and maximum rates, respectively, atthe start and end of the photoperiod; but uptake showed a proportionately steeper declineduring the dark period. This trend was mirrored by the decreasein translocation of N from roots to shoots during the dark,concurrent with an increase in the relative allocation of -N to expanding leaves and young tillers,and in -N to older expanded leaves. Overall the apparent ‘sink-strength’ ofexpanding leaves for N declined during the dark period. Pulse-chasesof ¹⁴CO₂ fed to the youngest fully expanded leaf at the startand end of the photoperiod showed that translocation of ¹⁴Cto the roots continued throughout the dark period, but thatthe apparent rate was halved after 9 h of darkness. The resultswere interpreted as contrary to the ‘carbohydrate supplylimitation’ hypothesis for the dark-related decline inN uptake, but compatible with regulation by ‘sink-strength’for N. Key words: Nitrate, ammonium, diurnal variation, perennial ryegrass, translocation     

Nitrate transport in Zea mays cell suspension cultures: the effect of solution composition and cell age

Nitrate transport characteristics of an amino acid-grown Zeamays P3377 cell culture line were studied. Age (days after subculturing)of the cells was shown to have a significant effect on transport; older stationary phasecultures did not absorb nitrate from the medium as rapidly asyounger growing cultures. Solution composition had a pronouncedimpact on induction of accelerated nitrate transport and transportrates. Maximum uptake rates required fresh culture media ratherthan simple solutions. Differences in ionic strength among uptakesolutions of equal concentration were shown to affect the apparent uptake rates by changing theactivity coefficient of . The uptake kinetics were established by following uptake for 24h in a wide range of nitrate concentrations. Uptake patternsof cells in solutions ranging from 0.02 to 2 mM were as typicallyreported for plants. The kinetic constants for the Zea mayscell suspension cultures concurred with reports of other solution-culturedcells. When cells were placed in solutions containing greater than 2 mM, uptake patternssuggested a significant passive uptake component. Passive diffusionof was estimated by Nernst analysis and indicated to be an important component of nitrateuptake in maize cell suspension cultures grown in the absenceof nitrate then transferred into nitrate-containing media. Key words: Cell suspension culture, nitrate, passive uptake, Zea mays