The Inducible Formation and Stability of Nitrate Reductase in Higher Plants: II. EFFECTS OF ENVIRONMENTAL FACTORS, ANTIMETABOLITES, AND AMINO-ACIDS ON INDUCTION

The induction of nitrate reductase by molybdenum or nitratein excised tissues of cauliflower leaf was dependent on temperature;for the range 2^? to 12^? C, Q₁₀ was about 2; for the range 12^?to 22^? C, Q₁₀ was greater than 3. Enzyme formation was initiallymost rapid at 32^? C but did not continue for as long as it didat 22^? or 24^? C. Decreased oxygen supply lessened the rate ofenzyme formation. The effects on enzyme formation of a widerange of natural and synthetic antimetabolites were tested withrespect to induction by either nitrate or molybdenum, when introducedat the same time by infiltration. Actidione (cycloheximide),patulin, cycloserine, polymyxin B, L-2-thiolhistidine D-methionine,L-dihydroxyphenylalanine, D,L--methylglutamic acid, sarcosineand 1 ,2-dichloro-4-(p-nitrobenzenesulphonylamido)-5-nitrobenzene(DCDNS) were the most inhibitory compounds tested. Serine stimulatedproduction of enzyme activity; kinetin, benzimidazole, and p-fluorophenylalanine,3--methyltryptophane and the 4- isomer, chloramphenicol, gramicidin,and several thio- and^aza- derivatives of purines or pyrimidineswere practically without effect. Differential effects of inhibitorson enzyme formation in response to nitrate or molybdenum wererarely observed, and no deductions regarding the possible sequencein which the substrate and prosthetic metal induce activitycould be inferred from the results.     

Accumulation of Oxalate in Tissues of Sugar-beet, and the Effect of Nitrogen Supply

In field-grown sugar-beet concentration of insoluble oxalatewas low in roots and high (about 12 per cent of ethanol insolublematerial) in leaves, and for a particular leaf the concentrationincreased continuously during its life. Of the insoluble oxalate,15–30 per cent was present as the magnesium salt and theremainder as the calcium salt. Oxalate contents of plants grownin culture solutions with nitrate as nitrogen source were similarto those of plants grown in soil, but when nitrogen was suppliedas ammonium sulphate or ammonium nitrate both soluble and insolubleoxalate were low. Plants grown in soil with regular additionsof ammonium sulphate or ammonium nitrate also had very low concentrationsof soluble oxalate although insoluble oxalate was only slightlylower than with nitrate nitrogen. Disks of root or leaf tissuewashed for several days in distilled water lost insoluble oxalatebut when washed in tap water insoluble oxalate increased morethan twofold. Addition of calcium and nitrate to the distilledwater caused an increase of insoluble oxalate, while additionof potassium caused a decrease. Use of ¹⁴C labelled oxalateand washing experiments showed that oxalate can be metabolizedby tissue disks and so is not necessarily a final product ofmetabolism. The accumulation of oxalate appears to be connectedwith the assimilation of nitrate and the preservation of thecation-anion balance of the plant.     

Incorporation of radioactive molybdenum into protein during nitrate reductase formation and effect of molybdenum on nitrate reductase and diaphorase activities of spinach (Spinacea oleracea L.)

Spinach plants grown without molybdenum lack nitrate reductaseand when plants are deprived of nitrate existing activity islost. Transfer of molybdenum-deficient plants to a solutioncontaining (NH₄)₂⁹⁹MoO₄) or nitrate-starved plants to NaNO₃solution induced enzyme activity in 24 hr. After purificationby selective adsorption, precipitation and disc electrophoresis,the protein from molybdenum-deficient plants given ⁹⁹Mo showedradioactivity only where nitrate reductase was revealed on theacrylamide gel. Molybdenum was similarly selectively concentratedinto the enzyme as a result of induction by nitrate in plantsgrown with sub-optimal molybdenum supply in order to minimizeeffects of isotope dilution on measurement of ⁹⁹Mo incorporation. There was no exchange in vitro between ⁹⁹Mo and purified activeenzyme in the resting state over 18 hr at 4°C, or with functioningenzyme held at room temperature for 24 hr. There was evidenceeither for possible in vivo exchange of ⁹⁹Mo andenzyme boundMo or for slight synthesis of fresh enzyme under conditionsof net loss of enzyme in nitrate starved plants. Five NADH₂ and two NADPH₂ reactive diaphorases which could beseparated by electrophoresis were present in extracts. Onlyone of these having strong NADH₂ and weak NADPH₂ activity wasdirectly associated with nitrate reductase. The same complexalso showed the only benzyl viologen (BV.) reactive nitratereductase. Nitrate reductase in spinach is therefore considered to be amolybdenum-dependant and molybdenum-containing protein in whichNADH₂ (with weak NADPH₂) and BVelectron donor functions anddiaphorase/reductase activities remain closely associated duringpurification and electrophoresis. The techniques provide a simple means for the production andpurification of enzyme containing radioactively labelled Moapplicable to investigations on the structure of the enzyme. (Received January 16, 1971; )     

Depression of Nitrate and Ammonium Transport in Barley Plants with Diminished Sulphate Status. Evidence of Co-regulation of Nitrogen and Sulphate Intake

When barley plants were grown in a solution with nitrate asthe sole N-source but deprived of sulphate (–Splants)for 1 to 5 d, the capacity for sulphate transport by the rootsincreased very markedly; subsequent measurement of influx using³⁵S-labelled showed increases of > 10-fold compared to plants continuously supplied with sulphate (+S plants).There were only small effects on plant growth over a 5 d periodand yet the influx of , labelled with the short-lived tracer ¹³N, was diminished by approximately 30%.By contrast, the influx of phosphate was little affected bysulphate-deprivation. When a sulphate supply was restored to– S plants, the sulphate influx was quickly repressedover the subsequent 24 h and the nitrate influx was restoredto >90% of the value in +S plants. When plants were grown in a solution with a mixed nitrate andammonium supply and deprived of sulphate for 1 d or 5 d thedepression of nitrate influx was more strongly marked (up to55% depression). The influx of ammonium was also depressed after5 d of sulphate-deprivation, but not at 1 d, nor where the concentrationof ammonium in the uptake solution was lowered to 20 mmol m^–3or less. Additional measurements with ¹⁵N-labelled nitrate and ammoniumover longer periods were used to determine net uptake. Net uptakeof nitrate was depressed to a similar extent to efflux, butnet ammonium uptake was depressed only in unbuffered uptakesolution where the pH decreased to pH 4.9 during the uptakeperiod. The ¹⁵N-tracer experiments showed that the translocationof label to the shoot, from both nitrate and ammonium, was depressedto a greater extent than net uptake in –S plants. Thedepression of nitrate influx, caused by 5 d of sulphate deprivation,could be relieved almost completely by providing plants with1.0 mol m^–3 L-methionine during the day prior to influxmeasurement. This treatment substantially decreased sulphateand potassium (⁸⁶Rb-labelled) influx in both +S and –Splants, but greatly increased total S-status of the plants.This methionine treatment had no effect on ammonium influx ornet uptake in – S plants but increased influx significantlyin +S ones. When plants were grown with sulphate but deprived of nitratefor 4 d there was a marked depression of the sulphate influx(by 48–65%) but a smaller effect on phosphate influx (21–37%of +N). The results are discussed in relation to the effects of sulphate-deprivationon growth rate and the root: shoot weight ratio. It is concludedthat the effects on influx and net uptake of nitrogen are moresevere than could be accounted for by these factors. The decreasedtranslocation of either nitrate, or the products of nitrateand ammonium assimilation from the roots, is suggested as areason for the depression of influx. The restoration of nitrateinflux and net uptake by methionine suggests that, for thision at least, a shortage of S-amino acids within the plant maylead to the accumulation of inhibitory concentrations of non-Samino acids in the transport pool. Key words: ¹³N, sulphate, nitrate, ammonium, ion-uptake, barley     

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     

Nitrate Reduction and Partitioning of Nitrogen in Komatsuna (Brassica campestris L. var. rapa) Plants: Compartmental Analysis in Combination with 15N Tracer Experiments

Komatsuna (Brassica campestris L. var. rapa cv. Misugi) is aleafy vegetable that readily accumulates nitrate in its tissues.Plants grown hydroponically with 2 mM nitrate in a greenhousewere fed ¹⁵N-labeled nitrate for 2 h, followed with nonlabelednitrate for 8 h. At intervals of 2 h, the plants were sampledand analyzed for the distribution of 15N in the nitrate, aminoacids, and proteins in the tissues of roots, petioles plus midribs,and leaves. Nitrate reduction and nitrogen fluxes were examinedusing a compartmental analysis with 19 compartments and 28 transferrate constants. Nitrate existed in the three types of tissues as a large storagepool and a small metabolic pool. Nitrogen reduction was observedin these tissues, but mainly in the leaf tissue. Nitrate uptakeand reduction rates were smaller in the dark than in light,and particularly nitrate reduction in the shoot was less inthe dark. The rate of protein synthesis was much greater inthe light. The simulation, using compartment models and ¹⁵Ndistribution data, may be useful for estimating actual ratesof nitrogen transport and metabolism in the whole plant system. (Received October 15, 1986; Accepted March 26, 1987)     

Triadimenol Increases Nitrate Levels and Nitrate Reductase Activity in Canola Leaves

Nitrate reductase activity in the first true leaves of canola(Brassica napus L.) seedlings grown in one-quarter strengthHoagland's solution from seeds pretreated with triadimenol (0.3or 30 g (a.i.) kg^–1 of seed) was higher than controlsduring the growth period of 15 to 25 d after planting. Triadimenolalso increased chlorophyll levels, the increase being more pronouncedat its lower concentration. The treatment also increased theweight and nitrate content of the leaves. When seedlings weregrown in nutrient solution containing 1 to 20 mM nitrate, theincrease in nitrate reductase activity by triadimenol was higherat lower rather than at higher nitrate concentrations. The nitratelevels and Kjeldahl nitrogen in the triadimenol-treated leaveswas higher than the controls at concentrations of added nitrateabove 2 mM. Addition of nitrate to plants grown in ammonium,increased nitrate reductase activity more in plants grown fromtriadimenol-treated seeds than controls. However, addition of10µM triadimenol for 24 h to ammonium-grown plants hadlittle effect on enzyme activity, both in the absence as wellas the presence of nitrate. This study demonstrates that triadimenolincreases nitrate reductase activity and nitrate accumulationin the leaves and at least part of the increased enzyme activityis independent of nitrate accumulation. Key words: Triazoles, nitrate content, nitrate reductase activity     

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     

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

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

Nitrate Reductase in the Field Pea (Pisutn arvense L.)

The extraction and assay of a soluble, NADH-requiring nitratereductase is described. Induction of the enzyme by nitrate isdemonstrated in the roots and shoot of young seedlings and inthe leaves of older, nodulated plants grown without inorganicnitrogen. Induction occurs most readily in actively growingtissues. Studies with sterile cultures of excised roots confirmthe presence of an endogenous enzyme system within the root. Assays of the enzyme in vitro are combined with analyses offree nitrate and organic compounds of nitrogen in the bleedingsap. Either the root or shoot may act as the main centre forreduction of incoming nitrate. The extent to which these organsfunction is apparently influenced by environmental factors,particularly the level of nitrate in the rooting medium. Thediurnal rhythm of export of nitrogenous substances from theroot is correlated with observations on daily fluctuations inthe level of extractable enzyme in root and leaves. The activity of the enzyme is studied in different ages of leafof plants grown on a constant supply of nitrate. Assays suggestthat the enzyme is most active just as a leaf is fully expanded.Thereafter enzyme activity falls sharply, although small amountsof active enzyme may be recovered until a leaf becomes senescent.     

Nitrate Nutrition and Temperature Effects on Wheat: Enzyme Composition, Nitrate and Total Amino Acid Content of Leaves

Lawlor, D. W., Boyle, F. A., Kendall, A. C. and Keys, A. J.1987. Nitrate nutrition and temperature effects on wheat: Enzymecomposition, nitrate and total amino acid content of leaves.—J.exp. Bot. 38: 378–392. Wheat plants were grown in controlled environments in two temperatureregimes with two rates of nitrate fertilization. In some experimentstwo light intensities were combined with the nitrogen and temperaturetreatments. The composition of the third leaf was studied fromsoon after emergence until early senescence. The amounts ofchlorophyll, soluble protein, ribulose bisphosphate carboxylase-oxygenase(RuBPc-o) protein, nitrate, and total amino acids were measuredtogether with the activities of RuBPc-o, fructose- 1,6-bisphosphatase,glycolate oxidase, carbonic anhydrase, nitrate reductase, glutaminesynthetase and serine- and glutamate-glyoxylate aminotransferases.Additional nitrate supply increased the amounts, per unit leafarea, of chlorophyll, total soluble protein and RuBPc-o proteinand the activities of all the enzymes. The ratio of RuBP carboxylaseto RuBP oxygenase activity, when measured at constant CO²/O²ratio and temperature, was unaffected by growth conditions orleaf age. Leaves grown at the lower temperature, especiallywith more nitrate, contained much more soluble protein, nitratereductase, fructose bisphosphatase and free amino acids perunit area than the plants grown in the warmer conditions. However,young leaves grown in the warm contained more nitrate than thosegrown in the cool. Amounts of protein, amino acids and chlorophylland most enzyme activities reached maxima near full leaf expansionand decreased with age; additional nitrate slowed the decreaseand senescence was delayed. Nitrate content and nitrate reductaseactivities were highest in leaves before full expansion andthen fell rapidly after full expansion. Increased light intensityincreased the content of RuBPc-o protein at the higher rateof nitrate supply. Chloroplast components and, to a lesser extent,peroxisomal enzymes associated with photosynthetic nitrogenassimilation changed in proportion with different treatmentsbut nitrate reductase activity was not closely related to chloroplastenzymes. Control of tissue composition in relation to environmentalconditions is discussed. Key words: Nitrate nutrition, temperature, wheat, enzyme, amino acid, leaves, ribulose bisphosphate carboxylase oxygenase, nitrate reductase     

Intracellular Localization and Properties of NADH-Glutamate Dehydrogenase from Vitis vinifera L.: Purification and Characterization of the Major Leaf Isoenzyme

Glutamate dehydrogenase was partially purified from grapevine(Vitis vinifera L. cv. Soultanina) tissues and its activityand isoenzymic pattern were studied. Seven anodal migratingisoenzymes were revealed after PAGE. Leaf protoplasts were isolatedfrom in vitro-grown axenic shoot cultures and used to studythe intracellular localization of GDH. Results revealed thatthe enzyme was associated with the mitochondrial fraction. Theisoenzyme with the lowest electrophoretic mobility, which accountedfor 35 to 40% of total activity, was purified 2050-fold to homogeneityfrom leaves. The purification method included ammonium sulphatefractionation, DEAE-cellulose chromatography, Sephadex G-200gel filtration and NAD-sepharose affinity chromatography. Themolecular weight of the native enzyme was estimated to be 252kDa and it consisted of identical 42.5 kDa subunits. pH optimumfor the aminating reaction was 8.0 and for the deaminating reaction9.3. At optimum pH conditions the apparent K_m values for ammonium,as ammonium chloride and ammonium sulphate, -ketoglutarate,NADH, glutamate, and NAD⁺ were 45.0, 13.0, 2.1, 0.069, 18.0,and 0.195 mM, respectively. The amination reaction of GDH wasfully activated with about 100 µM Ca²⁺ while the deaminationreaction was not affected by the addition of Ca²⁺. The isoenzymesof GDH showed different magnitude of their activating responseto calcium ions. Key words: Vitis vinifera L., glutamate dehydrogenase     

Salt Tolerance in the Halophyte Suaeda maritima L. Dum.: Intracellular Compartmentation of Ions

The time-course of exchange of sodium and potassium ions fromroot and leaf material of the halophyte Suaeda maritima hasbeen followed and the data analysed according to the phenomenologyof efflux, or compartmental, analysis. Sodium ions were exchangedmuch more slowly (c. 4 times) from the vacuoles of leaf cellsof plants grown in sodium chloride than were potassium ionsfrom the vacuoles of leaf cells of plants grown either in similarconcentrations of potassium chloride or in low concentrationsof potassium. In plants grown in sodium chloride, sodium ionswere exchanged 9 times more slowly from the vacuoles of leafcells than from the vacuoles of root cells. The concentration of sodium ions in the cytoplasm of leaf cellsof plants growing in 340 mol m^–3 sodium chloride was estimatedto be 165 mol m^–3 when the average concentration in theleaf tissue was about 600 mol m^–3. As measured by movement from mature to developing leaves inintact plants; there was less in vivo retranslocation of ²²Naand ³⁶CI in plants growing in sodium chloride than there wasof ⁸⁶Rb in plants growing either in potassium chloride or innon-saline conditions. The results are discussed in terms of the concept and energeticsof compartmentation of ions in the cells of halophytes.     

Changes in the Morphology of Roots and Leaves of Carob Seedlings Induced by Nitrogen Source and Atmospheric Carbon Dioxide

Carob seedlings were grown hydroponically for 9 weeks under360 and 800 µl l^-1CO₂. One of two nitrogen sources, nitrateor ammonium, was added to the nutrient medium at concentrationsof 3 mol m^-3. Root systems of the developing plants suppliedwith nitrate compared to those supplied with ammonium were characterizedby:(a)more biomass on the lower part of the root;(b)fewer lateralroots of first and second order;(c)longer roots;(d)higher specificroot length;(e)a smaller root diameter. The morphology of theroot systems of nitrate-fed plants changed in the presence ofelevated carbon dioxide concentrations, resembling, more closely,that of ammonium-fed plants. Total leaf area was higher in ammonium-than in nitrate-fed plants. Nitrate-fed plants had greater totalleaf area in the presence of high carbon dioxide than in normalCO₂, due to an increase in epidermal cell size that led to developmentof larger leaflets with lower stomatal frequency. The observedchanges in the morphology of roots and shoots agreed with theresults observed for total biomass production. Nitrate-fed plantsincreased their biomass production by 100% in the presence ofelevated CO₂compared to 15% in ammonium-fed plants, indicatingthat the response of carob to high CO₂concentrations is verydependent on the nitrogen source. Under elevated CO₂, nitrate-grownplants had a larger content of sucrose in both roots and shoots,while no significant difference was observed in the contentof sucrose in ammonium-grown plants, whether in ambient or enrichedcarbon dioxide. Hence, the differences in soluble carbohydratecontents can, at least partly, account for differences in rootand shoot morphology.Copyright 1997 Annals of Botany Company Ceratonia siliquaL.; carob; ammonium; carbohydrate; carbon dioxide; nitrate; morphology; sucrose     

Diagnosis of nitrogen deficiency and toxicity of Eucalyptus globulus seedlings by foliar analysis

The relationship between shoot growth and foliar nitrogen (N) in E. globulus seedlings was studied in the glasshouse to determine standard values for N deficiency and toxicity diagnosis. Seedlings were grown for 9 weeks in yellow sand, at 10 rates of N, applied as ammonium sulphate, calcium nitrate or ammonium nitrate. Shoot dry weight (DW) increased linearly with N rate for all forms of N in the deficiency range. Seedlings continued to respond to higher rates of ammonium and ammonium nitrate than to nitrate. Maximum shoot DW for nitrate fed plants and ammonium nitrate fed plants were 51% and 84% respectively of ammonium fed plants. Total N concentration in the youngest fully expanded leaf (YFEL) ranged from 1.0% to 3.3% in deficient and adequate plants. The critical N concentration for deficiency diagnosis (corresponding to 90% maximum yield) in the YFEL, determined from these growth response curves averaged over all N forms, was 2.6% N. For ammonium nitrate fed plants, total N concentration in the YFEL for the severely deficient, deficient, adequate, and toxic ranges were <1.4%, 1.4–2.5%, 2.6–3.5%, > 4.3%. High total N concentrations were associated with growth depression and toxicity symptoms, which differed with N form. For nitrate fed plants, a total N concentration above 3.3% in the YFEL was associated with severe growth depression, and leaf tip necrosis. The adequate concentration range for ammonium nitrate was similar to values found on a field trial with 7 month old E. globulus trees grown on an exforest site.     

Regulation of aldehyde oxidase and nitrate reductase in roots of barley (Hordeum vulgare L.) by nitrogen source and salinity

The molybdenum cofactor (MoCo) is a component of aldehyde oxidase (AO EC 1.2.3.1), xanthine dehydrogenase (XDH EC 1.2.1.37) and nitrate reductase (NR, EC 1.6.6.1). The activity of AO, which catalyses the last step of the synthesis of abscisic acid (ABA), was studied in leaves and roots of barley (Hordeum vulgare L.) plants grown on nitrate or ammonia with or without salinity. The activity of AO in roots was enhanced in plants grown with ammonium while nitrate-grown plants exhibited only traces. Root AO in barley was enhanced by salinity in the presence of nitrate or ammonia in the nutrient medium while leaf AO was not significantly affected by the nitrogen source or salinity of the medium.Salinity and ammonium decreased NR activity in roots while increasing the overall MoCo content of the tissue. The highest level of AO in barley roots was observed in plants grown with ammonium and NaCl, treatments that had only a marginal effect on leaf AO. ABA concentration in leaves of plants increased with salinity and ammonium.Keywords: ABA, aldehyde oxidase, ammonium, nitrate, salinity.      

Sulphate supply and its regulation of transport in roots of a tropical legume Macroptilium atropurpureum cv. Sirato

During a period of sulphate deprivation, roots of Macroptiliumatropurpureum responded by increasing their uptake capacityat the plasma membrane. This effect was apparent both in intactplants and in tissues excised prior to uptake. In experiments using excised root systems previousy labelledwith ³⁵SO₄^2- the rate of tracer transport to the xylem was muchgreater in roots subsequently deprived of external sulphatethan in those supplied with unlabelled sulphate. Removing theexternal sulphate to the external solution. Additionally, compartmentalanalysis of tracer exchange kinetics showed that the flux ofsulphate from the cytoplasm to the xylem(     

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     

Control of Net Uptake of Nutrients by Regulation of Influx in Barley Plants Recovering from Nutrient Deficiency

Rates of influx and net uptake of nitrate, phosphate and sulphatewere measured in intact barley plants, and concurrent effluxwas obtained by difference. Net uptake of these anions variedwidely depending on the nutrient status of the plants, and thedifferences in net uptake could be accounted for almost entirelyby changes in influx. Efflux played only a minor role in regulatingnet uptake of nitrate, phosphate or sulphate during recoveryfrom N-, P-, or S-deficiency. Nitrate influx and short-term ammonium absorption by N-deficientbarley plants were closely correlated, and varied in parallelwith rates of net uptake of nitrate or ammonium by similar plants.Again, it would seem that net uptake of ammonium is controlledpredominantly by changes in the rate of influx.Copyright 1993,1999 Academic Press Hordeum vulgare, barley, nutrient absorption, influx, nitrate, phosphate, sulphate, ammonium     

In vitro Propagation of Red Cabbage (Brassica oleracea L. var. capitata)

By manipulation of various growth regulators and physical conditions,plants have been regenerated from excised roots, stem segments,cotyledons, leaves, and callus cultures of red cabbage (Brassicaoleracea var. capitata) grown under in vitro conditions. Shootbuds were induced on isolated root segments (1 cm long) culturedon Murashige and Skoog's medium and the frequency of bud formationwas greatly enhanced by the addition of kinetin (0.5 part 10^–6).Callus obtained from the seeds, cotyledons, and hypocotyl segmentscultured on a medium fortified with 2,4-D (1 part 10^–6),kinetin (0.1 part 10^–6), and coconut milk (10%, v/v) hasbeen repeatedly subcultured. The callus is slow growing, andon transference to a kinetin (2 parts 10^–6) and IAA (2parts 10^–6) medium underwent morphogenesis to give riseto plants. The significance of the propagation of red cabbageby in vitro culture is pointed out.