Effects of Nitrate Withdrawal and Resupply on the Assimilation of Nitrate in Leaves of Tomato

Nitrate reduction in leaves of tomato occurred at the same ratein plants grown in 8.0 mol m^–3 nitrate as in plants grownin 2.0 mol m^–3 nitrate, but at a much slower rate in plantsgrown in 0.1 mol m^–3 nitrate. However, the plants grownin 8.0 mol m^–3 nitrate had a larger leaf system than theplants grown in 2.0 mol m^–3 nitrate, and so the totalcapacity to assimilate nitrate was greater in the plants grownin the higher concentration. It was shown that plants grownin 8.0 mol m^–3 nitrate were better buffered against nitratewithdrawal than plants grown in 2.0 mol m^–3 nitrate asthe rate of nitrate reduction declined more slowly when plantswere transferred to 0.1 mol m^–3 nitrate from the higherconcentration than from the lower concentration. Furthermore,leaf expansion continued in the plants transferred from thehigher concentration, whereas it ceased abruptly in the plantstransferred from the lower concentration. It was concluded thatboth continuing expansion and continuing nitrate reduction wereaccompanied, and possibly caused by, a release of nitrate fromstorage pools in the lower part of the stem or the roots. Duringwithdrawal of nitrate the leaves were shown to maintain potentialactivity of the enzyme nitrate reductase although there wasno nitrate to be reduced. When nitrate was resupplied it couldbe reduced very quickly and reduction in the leaves was seento increase within 5 h of resupply. By 3 d after resupply furtherenzyme activity had been induced. Key words: Lycopersicon esculentum Mill, nitrate assimilation, nitrate reductase activity, nitrate withdrawal     

The uptake and flow of C, N and ions between roots and shoots in Ricinus communis L. III. Long-distance transport of abscisic acid depending on nitrogen nutrition and salt stress

Seedlings of Ricinus communis L. were cultivated in quartz sandand supplied with media which contained either different concentrationsof nitrate or ammonium nitrogen and were treated with a lowsalt stress. The concentration of ABA was determined in tissuesand in xylem and phloem saps. Between 41 and 51 day after sowing,abscisic acid (ABA) flows between roots and shoots were modelled.Long-distance transport of ABA was not stimulated under conditionsof nitrate deficiency (0.2 mol m^–3). However, when ammoniumwas given as the only N source (1.0 mol m^–3), ABA transportin both xylem and phloem was increased significantly. Mild saltstress (40 mol m^–3 NaCl) increased ABA transport in nitrate-fedplants, but not in ammonium-fed plants. The leaf conductancewas lowered by salt treatment with both nitrogen sources, butit was always lower in ammonium-fed compared to nitrate-fedplants. A negative correlation of leaf conductance to ABA levelsin leaves or flow in xylem was found only in comparison of ammonium-fedto nitrate-fed plants. Key words: Abscisic acid, ammonium, Ricinus communis, phloem, xylem, transport, nitrate, nitrogen nutrition     

Nitrate Effects on Pre-emergence Growth and Emergence Percentage of Wheat (Triticum aestivum L.) from Different Sowing Depths

The effects of a range of applied nitrate (NO₃^–) concentrations(0–20 mol m3) on germination and emergence percentageof Triticum aestivum L. cv. Otane were examined at 30, 60, 90and 120 mm sowing depths. Germination percentage was not affectedby either sowing depth or applied NO₃^– concentration whereasemergence percentage decreased with increased sowing depth regardlessof applied NO₃^– concentration. Nitrate did not affectemergence percentage at 30 mm sowing depth, but at 60 to 120mm depth, emergence percentage decreased sharply with an increasedapplied NO₃^– concentration of 0 to 1·0 mol m^–3then decreased only slightly with further increases in appliedNO₃^– of about 5·0 mol m^–3. Root and shoot growth, NO₃^– accumulation and nitrate reductaseactivity (NRA) of plants supplied with 0, 1·0 and 1·0mol m^–3 NO₃^– at a sowing depth of 60 mm were measuredprior to emergence. The coleoptile of all seedlings opened withinthe substrate. Prior to emergence from the substrate, shootextension growth was unaffected by additional NO₃^– butshoot fr. wt. and dry wt. were both greater at 1·0 and1·0 mol m^–3 NO₃^– than with zero NO₃^–.Root dry wt. was unaffected by NO₃^–. Nitrate concentrationand NRA in root and shoot were always low without NO₃^–.At 1·0 and 10 mol m3 NO₃^–, NO₃^– accumulatedin the root and shoot to concentrations substantially greaterthan that applied and caused the induction of NRA. Regardlessof the applied NO₃^– concentration, seedlings which failedto emerge still had substantial seed reserves one month afterplanting. Coleoptile length was substantially less for seedlingswhich did not emerge than for seedlings which emerged, but wasnot affected by NO₃^–. It is proposed that (a) decreasedemergence percentage with increased sowing depth was due tothe emergence of leaf I from the coleoptile within the substrateand (b) decreased emergence percentage with additional NO₃^–was due to the increased expansion of leaf 1 within the substrateresulting in greater folding and damage of the leaf. Key words: Triticum aestivwn L., nitrate, sowing depth, seedling growth, seedling emergence     

Seasonal nitrogen assimilation and carbon fixation in a fjordic sea loch

Carbon (C) fixation and nitrogen (N) assimilation rates havebeen estimated from ¹⁴C and ¹⁵N techniques for a 12 month periodin a Scottish sea loch. The maximum rate of nitrogen assimilated(29.92 mmol N m^–2 day^–1) was in April at the mostseaward station; similar high rates were experienced duringMay at the other stations. Carbon fixation rates were maximal(488–4047 mg C m^–2day^–1) at the time of highphytoplankton biomass (maximum 8.3 mg m^–3 chlorophylla) during May, whilst nitrate concentrations remained >0.7µ.mol l^–1. C:N assimilation ratios suggest nitrogenlimitation only during the peak of the spring bloom, althoughat times nitrogen (nitrate and ammonium) concentration fellto 0.2 µmol l^–1 in the following months. The verticalstability of the water column, influenced by tidal and riverineflushing, varied along the axis of the loch, resulting in markeddifferences between sampling stations. Although ammonium waspreferentially assimilated by phytoplankton, >50% of productionwas supported by nitrate uptake and only during the summer monthswas the assimilation of ammonium quantitatively important.     

Inhibition of Nodule Development in Soybean by Nitrate or Reduced Nitrogen

Imsande, J. 1986. Inhibition of nodule development in soybeanby nitrate or reduced nitrogen.—J. exp. Bot. 37: 348–355. Nodulation of hydroponically grown soybean plants [Glycine max(L.) Merr.] is inhibited by continuous growth in the presenceof 4· mol m^–3 KNO₃ The presence of 4·0 molm^–3 ‘starter nitrate’ for 3-6 d during noduledevelopment, however, subsequently stimulates nodule dry weightaccumulation and nitrogenase activity. These stimulations occureven though 4· mol m^–3 nitrate temporarily delaysnodule development, i.e. the late steps of nodule developmentare reversibly inhibited by a short-term exposure to 4·0mol m^–3 nitrate. On the other hand, treatment with 4·0mol m^–3 nitrate in excess of 14 d significantly reducesnodule dry weight Thus, extended growth in the presence of 4·0mol m^–3 KNO₃ seems to block both early and late stepsof nodule development. Nodulation of hydroponically grown soybeansis also inhibited by continuous growth in the presence of 2·0mol m^–3 (NH₄)₂SO₄ This inhibition is not caused by acidityof the growth medium. On the other hand, nodule development6 d after inoculation with Rhizoblum japonicum is not delayedby a 7-d exposure to 2·0 mol m^–3 (NH₄)₂SO₄ butis partially inhibited by a prolonged exposure to (NH₄)₂SO₄Because repression of nodulation by 4·0 mol m^–3KNO₃ is more severe than that by 2·0 mol m^–3 (NH₄)₂SO₄and because ammonium taken up by the soybean plant is not activelyoxidized to nitrate, it is suggested that there are at leasttwo mechanisms by which nitrate utilization represses noduleformation in soybean. Key words: Glycine max, nitrogen, nitrogen fixation, nodulation     

Carbon and Nitrogen Assimilation by Vicia faba L. at Low Temperature: the Importance of Concentration and Form of Applied-N

Low temperature (6 C) growth was examined in two cultivarsof Vicia faba L. supplied with 4 and 20 mol m^–3 N as nitrateor urea. Both cultivars showed similar growth responses to increasedapplied-N concentration regardless of N-form. Total leaf areaincreased, as did root, stem and leaf dry weight, total carboncontent and total nitrogen content. In contrast to findingsat higher growth temperatures, 20 mol m^–3 urea-N gavesubstantially greater growth (all parameters measured) than20 mol m^–3 nitrate-N. The increased carbon content per plant associated with increasedapplied nitrate or urea concentration, or with urea in comparisonto nitrate, was due to a greater leaf area per plant for CO₂uptake and not an increased CO₂, uptake per unit area, carbon,chlorophyll or dry weight, all of which either remained constantor decreased. Nitrate reductase activity was substantial inplants given nitrate but negligible in plants given urea. Neitherfree nitrate nor free urea contributed greatly to nitrogen levelsin plant tissues. It is concluded that there is no evidence for a restrictionin nitrate reduction at 6 C, and it is likely that urea givesgreater growth than nitrate because of greater rates of uptake. Vicia faba, broad bean, low temperature growth, carbon assimilation, nitrogen assimilation     

Bulb Development in Onion (Allium cepa L.) II. The Influence of Red: Far-red Spectral Ratio and of Photon Flux Density

Bulb development was followed in four onion cultivars growingin controlled environments under 17 h days in factorial combinationsof photon-flux densities (PFD) of 111 and 333 µmol m^–2s^–1 with red: far-red spectral ratios (R : FR) of 2.88and 1.41. A PFD of 111with R : FR of 0.86 was used in a secondexperiment. The lower the R : FR or the higher the PFD the higherthe mean bulbing ratio and the ratio of bulb plus sheath toleaf blade dry weight and the earlier the bulb swelling. LowerR : FR accelerated scale initiation as did the higher PFD underR : FR 2.88 but not under R : FR 1.41. In high PFD bulb swellingoccurred before scale initiation because many sheaths of bladedleaves thickened, but in low PFD and low R : FR bulb swellingand scale initiation were concurrent. In the low PFD a low R: FR increased soluble carbohydrate concentrations in sheathsand scales compared to high R : FR. In high PFD, soluble carbohydrateconcentrations did not vary with R : FR and were higher thanunder low PFD. Onion, Allium cepa L., bulb, photon flux density, red: far-red ratio, light spectral quality, soluble carbohydrate     

Rapid, Reversible Inhibition of Nitrate Influx in Barley by Ammonium

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

Salt Tolerance in the Halophyte Suaeda maritima L.Dum.: Abscisic Acid Concentrations in Response to Constant and Altered Salinity

Endogenous abscisic acid contents were measured by gas-liquidchromatography in shoots of Suaeda maritima growing both inthe steady state over a range of salinities and over a time-coursefollowing an increase in the culture solution salinity of betweenapproximately 100 and 400 mol m^–3 NaCl. In steady-stateplants, the ABA content was maximal in the absence of salt at41 ng g^–1 fr. wt., declining to a minimum at 200 mol m^–3NaCl of 24 ng g^–1 fr. wt. Increase of culture solutionsalinity resulted in a marked increase in shoot ABA which wasmaximal after 6 h or 24 h in plants previously growing at 200mol m^–3 NaCl and in the absence of salt, respectively.Additionally, culture solution water potentials were loweredby 1.0 MPa (equivalent to raising the salt concentration byaround 200 mol m^–3); this resulted in a similar increasein endogenous ABA content to that brought about by an iso-osmoticsalt increase. Results are discussed in relation to the possiblerole of ABA in halophyte salt tolerance mechanisms. Key words: Suaeda, halophyte, abscisic acid, salt tolerance     

The Long-Term Effects of Nitrate on the Growth and Nodule Structure of the Caesalpinioid Herbaceous Legume Chamaecrista fasciculata Michaux.

Various nitrate concentrations (0, 1, 2, 4, 8, 20, 50 mol m^–3)were applied at weekly intervals for 10 weeks to the caesalpinioidlegume Chamaecrista fasciculata. Microscopic techniques andgeneral growth studies showed that nitrate affected both theplant and its rhizobial symbiosis. As the nitrate concentrationwas increased, nodule structure became increasingly disruptedeven though nitrate remained limiting to plant growth until8 mol m^–3. Poly-ß-hydroxybutyrate (PHB) was observedusing transmission electron microscopy; as nitrate increasedfrom 0 to 2 mol m^–3, the PHB stores were utilized Key words: Chamaecrista fasciculata, poly-ß-hydroxybutyrate, nitrogen fixation     

Studies of the Uptake of Nitrate in Barley: III. COMPARTMENTATION OF NO3-

Compartmental analyses of intact roots of barley (Hordeum vulgareL. cv. Klondike) plants, grown with different levels of NO₃^–(up to 1·0 mol m^–3) in the external media, wereundertaken using ¹³NO₃^–. Two additional treatments, namelysodium dodecyl sulphate (SDS) or brief exposure to high temperature,designed to investigate the identity of the three NO₃^–compartments revealed by compartmental analyses, provided supportfor the identification of the latter as corresponding to superficialsolution, apoplasm, and cytoplasm. Half-lives for exchange ofthese compartments, 3 s, 30 s, and 7 mm, were unaffected bythe level of NO₃^– provided during growth. Independentestimates of ¹³NO₃^– fluxes obtained by direct methodsagreed well with values of fluxes calculated from the compartmentalanalyses. Cytoplasmic [NO₃^–], estimated from the compartmental analyses,were in the range from 1–37 mol m^–3, and increasedwith increasing [NO₃^–] of the medium. Such values forcytoplasmic [NO₃^–] are inconsistent with an earlier proposal(Siddiqi, Glass, Ruth, and Rufty, 1990; Glass, Siddiqi, Ruth,and Rufty, 1990) of passive NO₃^– uptake in the concentrationrange above 10 mol m^–3. A model, based upon localizeddistribution of nitrate reductase activity in epidermal cells,is proposed in which the proposed passive low affinity NO uptakeat high external [NO₃^–] is restricted to epidermal cells. During loading periods with ¹³NO₃^–, significant amountsof ¹³N were translocated to the shoot. Two pools of ¹³N, onebeing the root symplasm, appear to participate in the transferof labelled N to the shoot. Key words: Barley, compartmentation, nitrate, nitrate reductase, ¹³N     

Nitrogen Partitioning During Early Development of Supernodulating Soybean (Glycine max [L.] Merrill) Mutants and their Wild-Type Parent

The relative importance of fixed N², cotyledonary N, and nitratefor growth of seedlings of soybean cv. Bragg and two of itsnitrate tolerant supernodulating (nts) mutants (intermediatents 1116 and extreme nts1007) was investigated during symbioticdevelopment in the presence of nitrate (3.0 mol m^–3) using¹⁵N techniques. Newly-fixed N₂ and nitrate were both major sourcesof N for nodule development and nitrate principally supportedearly shoot and root growth in Bragg. In the nts mutants, however,all plant parts and nodules in particular, relied more on Nstored in the cotyledons. This resulted in later nodule maturityand a period of prolonged N-starvation for the seedlings ofthe extreme supernodulator, and could be responsible for theirsubsequently lowered biomass accumulation compared to the parentcultivar. Key words: Nodules, N partitioning, supernodulating soybeans     

Seedling Nitrate Reductase Activity and Nitrate Partitioning in Maize (Zea mays L.) Strains Divergently Selected For Post-anthesis Leaf-Lamina Nitrate Reductase Activity

Two experiments were conducted to evaluate the effects of phenotypicrecurrent selection for high and low post-anthesis leaf-laminain vivo NRA on nitrate uptake, nitrate partitioning and in vitroNRA of seedling roots and leaves. In Experiment 1, intact plantsof cycle 0, 4, and 6 of the high and low NRA strains were grownon NH₄-N for 11 d, then exposed to 1.0 mol m^–3 KNO₃, andcultures sampled at 6 h and 28 h (induction and post-inductionperiods). Nitrate uptake, tissue nitrate concentration and invitro NRA were determined. The pattern of response to selectionin seedling leaf NRA was similar to that observed for in vivoNRA of field grown plants. Leaf NRA increased between 6 h and28 h. Root NRA was not affected by selection or sampling time.Treatments differed in total fresh weight but not in reductionor uptake of nitrate per unit weight, indicating a lack of correspondencebetween NRA and reduction and supporting the idea that concomitantreduction by NR is not obligatorily linked to nitrate influxin the intact plant. In Experiment 2, dark-grown plants of cycle 0, and 6 of thehigh and low NRA strains were cultured without N, detopped onday 6, transferred the following day to 0-75 mol m^–3 KNO₃and sampled at 6 h and 28 h. In contrast to Experiment 1, selectionpopulations differed in nitrate reduction and root NRA, whichby 28 h reached higher average levels than root NRA of intactplants. Translocation and reduction were inversely related amongstrains within each sampling time. The high level of translocationin detopped plants of the low NRA strain was difficult to reconcilewith its low leaf NRA level of Experiment 1. It is suggestedthat nitrate transport in detopped roots is altered relativeto the intact system in a way which permits greater NRA inductionand nitrate reduction. The results indicate that nitrate partitioningby detopped root systems should be interpreted with caution. Key words: Zea, nitrate reductase activity, nitrate uptake, nitrate reduction, nitrate partitioning, selection     

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

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

Uptake of dissolved nitrogen by phytoplankton in a eutrophic subtropical lake

Uptake rates for ananonium, nitrate, urea and dinitrogen byphytoplankton in Lake Okeechobee ranged from 0.58 to 1.52 µmol1^–1 h^–1 among four representative stations duringa short-term study period. Ammonium accounted for 53% of theuptake rates, followed by nitrate (19%), urea (16%) and dinitrogen(12%). Half-saturation constants for nitrogen (N) uptake rangedfrom 8.70 µmol 1^–1 for ammonium, 2.07 iimol 1^–1for urea and 2.21 µmol 1^–1 for nitrate at Southstation. This study reveals spatially varying N uptake rates,particularly N fixation, within a large eutrophic lake.     

Whole Plant Studies using Radioactive 13-Nitrogen: II. A COMPARTMENTAL MODEL FOR THE UPTAKE AND TRANSPORT OF NITRATE IONS BY Zea mays

Nitrate ion uptake by the roots of hydroponically grown maizeseedlings was measured using the short-lived isotope ¹³N. Itis shown to be described by a four compartment model, recognizablynitrogen in the root bathing solution, nitrogen which is readilyexchangeable from the root, nitrogen bound in the root, andnitrogen transported from the root. Some of the absorbed activity leaks back into the root bathingsolution with the efflux from the root, as a fraction of theinflux, increasing with concentration to be greater than 0–8at external nitrate ion concentrations above about 1.0 mol m^–3.The capacity of the exchangeable root pool increases with externalnitrate ion concentration, approaching the expected cytoplasmicnitrate ion content at the highest external nitrate ion concentrationsstudied (70 mol m^–3). The investigation has highlighted the problems of interpretinguptake profiles in experiments for which the 10 min half-lifeof ¹³N dictates experimental times that are comparable withthe times for saturation of root pools. Key words: Zea mays, ¹³N, Compartmental model, Nitrate uptake     

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     

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

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

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

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

Proline and Polyamine Involvement in Chilling Tolerance of Maize Suspension Cultures

We have assessed the effect of various medium supplements inpromoting the ability of maize (Zea mays L.) inbred FR27rhmsuspension cultures to grow following a period of 4 °C chillingstress. Following a 4 week exposure to 4 °C in culture mediumwithout proline, no cell growth occurred upon subsequent incubationat 28°C for 2 weeks. This inhibition was reversed when 3to 48 mol m^–3 proline or 0.1 mol m^–3 putrescineor 0.01 mol m^–3 spermidine were present in the mediumduring the chilling stress. On the other hand, suspensions weremade more sensitive to 4°C by blocking polyamine biosynthesiswith 1.0 mol m^–3 methylglyoxal bis (guanylhydrazone) (MGBG)or a combination of 1.0 mol m^–3 difluoromethylornithine(DFMO) and 1.0 mol m^–3 difluoromethylarginine (DFMA).The addition of 10 mol m^–3 putrescine to the suspensioncontaining DFMO and DFMA prevented the increased chilling sensitivity.Electrolyte leakage studies conducted to assess membrane integrityafter 4 weeks at 4°C and a 2 week regrowth period showedthat cells treated with no polyamines (control), 0.01 mol m^–3spermidine, 1.0 mol m^–3 putrescine, or 1.0 mol m^–3MGBG lost 43, 32, 14, and 100% of the total electrolyte pool,respectively. These results suggest that proline and polyaminesare beneficial for inducing chilling tolerance in FR27rhm suspension. Key words: Proline, polyamine, chilling stress