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     

Influence of Abscisic Acid on Nitrate Accumulation and Nitrate Reductase Activity in Potato Tuber Slices

Abscisic acid (ABA) at concentrations of 1 to 10 µg.ml^–1suppressed development of nitrate reductase activity in freshtuber slices of Solanum tuberosum L. incubated in KNO₃. Suppressionof activity was evident after 3 hr and continued for 20 hr beforerecovery. This recovery may be due to inactivation of the hormone.Nitrate accumulation was enhanced by ABA. At exogenous NO₃ levelsof 0.1 to 5 mM, the hormone enhanced both NO₃ accumulation andnitrate reductase activity. When applied 24 hr after incubation in NO₃, ABA promoted a markeddecline in enzyme activity in the absence of exogenous NO₃,but was less effective in the presence of NO₃. Slices incubatedin NO₃ and ABA also exhibited increased loss of enzyme activityupon removal of NO₃. Preincubating slices in the hormone for24 hr in a NO₃- free medium resulted in stimulation of nitratereductase activity. Addition of NO₃ resulted in a marked stimulationof enzyme activity over a period of 8–10 hr. The ABA response is not related to tissue levels of free aminoacids and is not affected by different NO₃ sources. These resultssuggest the ABA effect on nitrate reductase activity is influencedby NO₃ status of the cells. Where external NO₃ levels are lowit stimulated NRA while it inhibited activity where NO₃ contentis high. (Received May 12, 1981; Accepted October 12, 1981)     

Effect of Nitrate, Ammonium and Some Amino Acids on Growth and Nitrate Reductase Activity in Suspension Cultures of Atropa belladonna

Growth and nitrate reductase activity (NRA) of Atropa belladonnacells were studied in medium supplemented with NaNO₃, NH₄NO₃,and amino acid precursors to tropane alkaloids. Growth and NRAwere stimulated by NH₄⁺ and by proline, by proline plus ornithine,but not by glutamate, in NO₃^–-containing medium. Testedamino acids inhibited neither utilization of inorganic nitrogennor growth. (Received September 30, 1988; Accepted August 28, 1989)     

Effects of Root Temperature and Form of Nitrogen Nutrition on Nitrate Reductase Activity in Oilseed Rape (Brassica napus L.)

The effect of root temperature and form of inorganic nitrogensupply on in vitro nitrate reductase activity (NRA) was studiedin oilseed rape (Brassica napus L. cv. bien venu). Plants weregrown initially in flowing nutrient solution containing 10 µMNH₄NO₃ and then supplied with either nitrate or ammonium for15 d at root temperatures of 3, 7, 11 or 17 °C. Shoot temperatureregime was similar for all plants; 20/15 °C, day/night.Root NRA was highest when roots were grown at 3 and 7 °C.In laminae and petioles NRA was highest when roots were 11 or17 °C. The plants supplied with ammonium had much lowerlevels of NRA in roots after 5 d than the plants supplied onlywith nitrate. NRA in the laminae of plants supplied with ammoniumwas low relative to that in plants supplied with nitrate onlywhen root temperature was 11 or 17 °C. Values of the apparent activation energy (E_a) of NR, calculatedfrom the Arrhenius equation, in laminae and petioles were differentfrom roots suggesting difference in enzyme conformation. Evidencethat the temperature at which roots were growing affected E_awas equivocal. Oilseed rape, Brassica napus L., activation energy, ammonium, Arrhenius equation, nitrate, root temperature, nitrate reductase     

Effect of Plant Growth Regulators on Nitrate Utilization by Roots of Nitrogen-Depleted Dwarf Bean

We examined the effect of pretreatments (18 h at 5 µmoldm^–3) with abscisic acid, the ethylene-releasing substance‘Ethephon’, gibberellic acid, indoleacetic acid,kinetin and zeatin on nitrate uptake and in vivo nitrate reductaseactivity (NRA) in roots of nitrogen-depleted Phaseolus vulgarisL. Nitrate uptake showed an apparent induction pattern witha steady state after about 6 h, in all treatments. The nitrateuptake rate after 6 h was unaffected or at most 30% lower aftertreatments with the plant growth regulators. Gibberellic acid, kinetin and zeatin induced substantial NRAin roots in the absence of nitrate, whereas Ethephon enhancedNRA only during nitrate nutrition. Kinetin-induced NRA (Ki-NRA)was maximal after a pretreatment at 1 µmol dm^–3,and showed a lag phase of 6–8 h. Ki-NRA was additive tonitrate-induced NRA (NO^–₃-NRA) for at least 24 h, independentof the induction sequence. After full induction, Ki-NRA approximated20% of NO-₃-NRA. Abscisic acid counteracted the developmentof Ki-NRA, but not of NO^–₃-NRA. Cycloheximide and tungstatewere equally effective to suppress the development of nitratereductase activity after supply of kinetin or NO^–₃. Our data are consistent with the operation of two independentenzyme fractions (Ki-NRA and NO^–₃-NRA) with apparentlyidentical properties but with separate control mechanisms. Theabsence of major effects of plant growth regulators on the time-courseand rate of nitrate uptake suggests that exogenous regulators,and possibly endogenous phytohormones are of minor importancefor initial nitrate uptake. The differential effect of someregulators on nitrate uptake and root NRA furthermore indicatesthat the processes of uptake and reduction of NO^–₃ arenot obligatory or exclusively coupled to each other.     

Time-course of Nitrate Uptake and Nitrate Reductase Activity in Nitrogen-depleted Dwarf Bean

The rate of nitrate uptake by N-depleted French dwarf bean (Phaseolus vulgaris L. cv. Witte Krombek) increased steadily during the first 6 h after addition of NO₃ -After this initial phase the rale remained constant for many hours. Detached root systems showed the same time-course of uptake as roots of intact plants. In vivo nitrate reductase activity (NRA) was assayed with or without exogenous NO₃^- in the incubation medium and the result ing activities were denoted potential and actual level, respectively. In roots the difference between actual and potential NRA disappeared within 15 min after addition of nitrate, and NRA increased for about 15 h. Both potential and actual NRA were initially very low. In leaves, however, potential NRA was initially very high and was not affected by ambient nitrate (0.1–5 mol m^-3) for about 10 h. Actual and potential leaf NRA became equal after the same period of time. In the course of nitrate nutrition, the two nitrate reductase activities in leaves were differentially inhibited by cycloheximide (3.6 mmol m^-3) and tungstate (1 mol m^-3). We suggest that initial potential NRA reflects the activity of pre-existing enzyme, whereas actual NRA depends on enzyme assembly during NO₃^- supply. Apparent induction of nitrate uptake and most (85%) of the actual in vivo NRA occurred in the root system during the first 6 h of nitrate utilization by dwarf bean.     

Developmental and Biochemical Regulation of 'Constitutive' Nitrate Reductase Activity in Leaves of Nodulating Soybean

The developmental profile of ‘constitutive’ nitratereductase activity (cNRA) in leaves of soybean (Glycine max(L.) cv. Bragg) plants at different ages is described. The youngestleaves had most cNRA and the activity dropped off as a newerleaf developed above it. Each leaf had its distinct active periodof in vivo cNRA. This pattern was different in urea-grown andsymbiotically-grown plants (inoculated with Bradyrhizobium japonicumstrain USDA 110), where the latter had no detectable in vivocNRA in older leaves. Urea-grown plants maintained considerablein vivo NRA in such older leaves. When symbiotically-grown plantshad their nodules removed, in vivo cNRA reappeared in olderleaves within 1 d of removal, nearly reaching levels of youngleaves at 3 d after nodule excision. Allantoic acid (ALL), oneof the known transport ureides of soybeans, was implicated asa possible signal molecule from nodules to leaves. Allantoicacid (100 µM) inhibited in vitro c₁ NRA significantly,with 400 µM ALL resulting in complete inhibition. In contrast,allantoin (ALN) had no inhibitive effect on NRA. Inhibitionof c₁NRA by ALL was by a competitive process, judging from Lineweaver-Burkeplots against nitrate. Kinetics showed a constant Vmax of around105 nmol NO₂ mg^–1 protein h^–1 and a K_m for nitrateof 15 mM, which increased to 60 mM in the presence of 200 µMallantoic acid. Non-specific (ionic and pH-related) influenceswere eliminated. Allantoic acid also had a slight stimulatingeffect of in vitro NRA (up about 25% at 400 µM). Thesefindings suggest that c1NRA may be involved in ureide metabolism,rather than in vivo nitrate metabolism. Key words: Root-shoot interaction, nitrogen metabolism, nodulation, symbiosis     

The Partitioning of Nitrate Assimilation Between Root and Shoot of a Range of Temperate Cereals and Pasture Grasses

Nitrate reductase activity (NRA, in vivo assay) and nitrate(NO^-₃) content of root and shoot and NO^-₃ and reduced nitrogencontent of xylem sap were measured in five temperate cerealssupplied with a range of NO^-₃ concentrations (0·1–20mol m^–3) and three temperate pasture grasses suppliedwith 0·5 or 5 0 mol m^–3 NO^-₃ For one cereal (Hordeumvulgare L ), in vitro NRA was also determined The effect ofexternal NO^-₃ concentration on the partitioning of NO^-₃ assimilationbetween root and shoot was assessed All measurements indicatedthat the root was the major site of NO3 assimilation in Avenasatwa L, Hordeum vulgare L, Secale cereale L, Tnticum aestivumL and x Triticosecale Wittm supplied with 0·1 to 1·0mol m^–3 NO^-₃ and that for all cereals, shoot assimilationincreased in importance as applied NO^-₃ concentration increasedfrom 1.0 to 20 mol m^–3 At 5.0–20 mol m^–3 NO3,the data indicated that the shoot played an important if notmajor role in NO^-₃ assimilation in all cereals studied Measurementson Lolium multiflorum Lam and L perenne L indicated that theroot was the main site of NO^-₃ assimilation at 0.5 mol m^–3NO^-₃ but shoot assimilation was predominant at 5.0 mol m^–3NO^-₃ Both NRA distribution data and xylem sap analysis indicatedthat shoot assimilation was predominant in Dactylis glomerataL supplied with 0.5 or 5.0 mol m^–3 NO^-₃ Avena sativa L., oats, Hordeum vulgare L., barley, Secale cereale L., rye, x Triticosecale Wittm., triticale, Triticum aestivum L., wheat, Dactylis glomerata L., cocksfoot, Lolium multiflorum Lam., Italian ryegrass, Lolium perenne L., perennial ryegrass, nitrate, nitrate assimilation, nitrate reductase activity, xylem sap     

Phosphate Regulation of Nitrate Assimilation in Soybean

It is known that phosphorus deficiency results in alterationsin the assimilation of nitrogen. An experiment was conductedto investigate mechanisms involved in altered ¹⁵NO^–₃ uptake,endogenous ¹⁵N translocation, and amino acid accumulation insoybean (Glycine max L. Merrill, cv. Ransom) plants deprivedof an external phosphorus supply for 20 d in solution culture.Phosphorus deprivation led to decreased rates of ¹⁵NO^–₃uptake and increased accumulation of absorbed ¹⁵N in the root.Both effects became more pronounced with time. Asparagine, theprimary transport amino acid in soybean, accumulated in largeexcess in roots and stems. In roots of phosphorus-deprived plants,concentrations of ATP and inorganic phosphate declined rapidly,but dry weight accumulation was similar to or above that ofthe control even after 20 d of treatment. Arginine accumulationin leaves was greatly enhanced, even though ¹⁵N partitioninginto the insoluble reduced-N fraction of leaves was unaffected.The results suggest that decreases in NO^–₃ uptake in lowphosphorus plants could be caused by feedback control factorsand by limited ATP availability. The decline in endogenous Ntransport from the root to the shoot may be associated withchanges in membrane properties, which also result in paralleleffects on hydraulic conductance and the upward flow of waterthrough the plant. Key words: Phosphorus stress, nitrate uptake, nitrate translocation, arginine     

The Effect of Short-Term H2S Fumigation on Nitrate Reductase Activity in Spinach Leaves

Short-term exposure of spinach plants to 250 ppb H₂S at a photonfluence rate of 35µmol m^–2s^–1 (within the400–700 nm range) in the ambient air did not affect invitro nitrate reductase activity (NRA) in the leaves. Likewise,H₂S exposure did not significantly affect in vivo NRA measuredunder anaerobic conditions. In vivo NRA of untreated plantswas apparently inhibited in the presence of oxygen. However,shortterm H₂S exposure increased in vivo "aerobic" NRA up tofive fold of that of untreated plants. H₂S induced increaseof in vivo "aerobic" NRA depended on the sulfide concentration.After 24 hours of exposure maximal increase (two to five fold)of in vivo NRA "aerobic" was observed at 220 ppb H₂S. It isproposed that H₂S inhibited NADH oxidizing enzymes, which resultedin an increase in NADH supply to nitrate reductase (NR) in thepresence of oxygen. It was unlikely that the increase in invivo "aerobic" NRA in sulfide exposed plants was due to an alteredcompetition between mitochondrial respiration and NR since leafrespiration was not affected by an exposure to 250 ppb H₂S (Received February 12, 1986; Accepted June 27, 1986)     

In vivo Nitrate Reductase Activity in Barley (Hordeum vulgare L.) during Ear Development

Experiments were conducted during the 1974–75 and 1975–76winter season with the barley (Hordeum vulgare L.) cultivarJyoti. From amongst the various plant parts, the flag leaf bladehad higher in vivo nitrate reductase (NR) activity than thelower two leaf blades, glumes, and grains. However, the potentialof a plant part to reduce NO₃^– is a function of its freshweight and the NR per unit fresh weight. On this basis, thesecond and third leaf blades could reduce more NO₃^– thanthe flag leaf blade. N fertilizer application resulted in enhancementof the activity of the leaf blades alone. N fertilizer appliedduring the reproductive phase was taken up and assimilated bythe various plant parts. The studies suggest that, even whenthe fertilizer is applied at optimum levels for obtaining maximumyields, the upper leaf blades have sub-optimal NR activity andthat there is a likelihood of either a preferential flow ofNO₃^– to the leaf blades or transnational barriers to NO₃^–movement to the ear.     

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

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

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     

The Influence of Nitrate and Ammonium Nutrition on the Growth of Wheat (Triticum aestivum) and Maize (Zea mays) Plants

The effects of NO^-₃ and NH⁺₄ nutrition on hydroponically grownwheat (Triticum aestivum L.) and maize (Zea mays L.) were assessedfrom measurements of growth, gas exchange and xylem sap nitrogencontents. Biomass accumulation and shoot moisture contents ofwheat and maize were lower with NH⁺₄ than with NO^-₃ nutrition.The shoot:root ratios of wheat plants were increased with NH⁺₄compared to NO^-₃ nutrition, while those of maize were unaffectedby the nitrogen source. Differences between NO^-₃ and NH⁺₄-fedplant biomasses were apparent soon after introduction of thenitrogen into the root medium of both wheat and maize, and thesedifferences were compounded during growth. Photosynthetic rates of 4 mM N-fed wheat were unaffected bythe form of nitrogen supplied whereas those of 12 mM NH⁺₄-fedwheat plants were reduced to 85% of those 12 mM NO^-₃-fed wheatplants. In maize supplied with 4 and 12 mM NH⁺₄ the photosyntheticrates were 87 and 82% respectively of those of NO^-₃-fed plants.Reduced photosynthetic rates of NH⁺₄ compared to NO^-₃-fed wheatand maize plants may thus partially explain reduced biomassaccumulation in plants supplied with NH⁺₄ compared to NO^-₃ nutrition.Differences in the partitioning of biomass between the shootsand roots of NO^-₃-and NH⁺₄-fed plants may also, however, arisefrom xylem translocation of carbon from the root to the shootin the form of amino compounds. The organic nitrogen contentof xylem sap was found to be considerably higher in NH⁺₄- thanin NO^-₃-fed plants. This may result in depletion of root carbohydrateresources through translocation of amino compounds to the shootin NH⁺₄-fed wheat plants. The concentration of carbon associatedwith organic nitrogen in the xylem sap of maize was considerablyhigher than that in wheat. This may indicate that the shootand root components of maize share a common carbon pool andthus differences induced by different forms of inorganic nitrogenare manifested as altered overall growth rather than changesin the shoot:root ratios.Copyright 1993, 1999 Academic Press Triticum aestivum, wheat, Zea mays, maize, nitrogen, growth, photosynthesis, amino acids, xylem     

Development of Nitrate Reductase Activity in Expanding Leaves of Nicotiana tabacum in Relation to the Concentration of Nitrate and Potassium

Up to 80% of the total nitrate reductase activity (NRA) determined in vivo in different parts of vegetative tobacco plant (Nicotiana tabacum) was located in the leaves. The NRA reached a peak when a leaf had expanded to 27% of its final weight and 33% of its final area. Thereafter, with advancing expansion and age of the leaf, the activity declined. This pattern of development of NRA during the ontogenesis of leaves was not influenced by raising the supply of NO₃⁻ from 3 to 6 milliequivalent per cubic decimeter in the substrate solution. The concentration of NO₃⁻ in leaves, stem and root was inversely related to NRA at both NO₃⁻ levels. Raising the supply of K⁺ from 1 to 6 milliequivalent per cubic decimeter at either concentration of NO₃⁻ slowed down the development of NRA in the initial stages of expansion, but promoted it subsequently. The peak of the activity which developed in a leaf of 62% of its final area was higher at the higher supply of K⁺. The higher activity was maintained thereafter in the expanding and in matured and older leaves. It was concluded that NRA and the pattern of its development in expanding leaves is related to the availability of metabolites and their incorporation into enzyme proteins. Both these processes are influenced by: (a) the vertical profile of concentration of K⁺ in the shoot and (b) the concentration of K⁺ in a leaf, which depend upon its supply.     

Internal Factors Regulating Nitrate and Chloride Influx in Plant Cells

The primary factor determining the observed decrease in activeC1^– influx during salt accumulation in carrot and barleyroot cells has been shown to be the concentration of C1^–+ NO₃^– in the vacuole. The relationship between C1^– influx and the vacuolar concentrationsof various substances was examined after the tissues had accumulatedions from various salt solutions. After accumulating K⁺ malate,C1^– influx was not reduced, but after accumulating C1^–or NO₃^– salts, C1^– influx was reduced by up to 90per cent. Considering all treatments, C1^– influx was notcorrelated with the vacuolar concentration of K⁺, Na⁺, (K⁺+Na⁺),reducing sugars, malate, C1^–, or NO₃^–, nor withthe cellular osmotic pressure. The correlation coefficient betweenCl^– influx and log (C1^– + NO₃^– concentrationin the vacuole) was highly significant, and accounted for allthe variation in C1^– influx in this experiment. Net NO₃^– influx is similarly reduced by a high C1^–concentration in the vacuole. External Cl^– and NO₃^–have quantitatively different, apparently competitive, effectson C1^– influx. These differ from the apparently negative-feedbackeffects of C1^– and NO₃^– in the vacuole, which arequantitatively similar. Decreasing the internal hydrostatic pressure by raising theexternal osmotic pressure increased active K⁺ influx in Valoniaventricosa, but had no effect on C1^– or K⁺ influx in carrotor maize root cells. Cl^– influx is not related to thereducing sugar concentration during ageing drifts in excisedcarrot root tissue. Acetazolamide did not inhibit C1^– influx to carrot tissue. The implications of this type of negative feedback regulation,and the relationship between C1^– and NO₃^– transportare discussed.     

Diurnal regulation of NO3-uptake in soybean plants II. Relationship with accumulation of NO and asparagine in the roots

Experiments were performed with soybean plants to test the hypothesisthat the inhibition of NO₃^– uptake in darkness is dueto feedback control by NO₃^– and/or Asn accumulating inthe roots. Xylem export of N compounds was shown to depend onwater flux in both excised root systems and ¹⁵N-labelled intactplants, suggesting that the shortage of transpiration in darknessmay be responsible for the retention of NO₃^– and Asn inthe roots. This was verified in experiments where the light/darkpattern of transpiration was modulated in intact plants by changingthe relative humidity of the atmosphere. Any decrease of transpirationat night was associated with a concurrent stimulation of NO₃^–and Asn accumulations in the roots. However, the light/darkrhythmicity of NO₃^– uptake was only marginally affectedby these treatments, and thusappeared quite independent fromtranspiration and root NO₃^– or Asn levels. Typically,the maintainance of a constant transpiration during the day/nightcycle did not suppress the inhibition of NO₃^– uptake indarkness, whereas it almost prevented the dark increase in rootNO₃^– and Asn contents. These data strongly support theconclusion that the effect of light on NO₃^– uptake isnot mediated by changes in translocation and accumulation ofN compounds. Key words: Glycine max, light/dark, cycles, nitrate uptake, transpiration, transport of N compounds, accumulation of N compounds     

Orthophosphate Relations of Root: NO-3 Effects on Orthophosphate Influx, Accumulation and Secretion into the Xylem

Lamaze, T., Sentenac, H. and Grignon, C. 1987. Orthophosphaterelations of root: NO^–₃effects on orthophosphate influx,accumulation and secretion into the xylem.—J. exp. Bot.38: 923–934. Orthophosphate (Pi) accumulation by barley (Hordeum vulgareL.) roots was specifically inhibited by NO^–₃ as comparedto Cl^– and SO₄^{2 –}, and Pi secretion into the xylemwas stimulated. The inhibition of Pi accumulation by NO^–₃was also observed in roots of intact photosynthesizing horsebean(Vicia faba L.), rice (Oryza sativa L.) and soybean (Glycinemax L.) plants. NO^–₃ effects on Pi transport by rootswere more thoroughly investigated with corn (Zea mays L.). Theywere due to intracellular NO^–₃. Pi secretion was stillstimulated by NO^–₃ after Pi withdrawal from the absorptionsolution. ³²Pi influx decreased during Pi accumulation, supportingthe hypothesis that this ion allosterically regulated its owntransport system by feedback control. This control was modulatedby other anions: the decrease was more pronounced in the presenceof nitrate. Chronologically, the depressive effect of NO^–₃on ³²Pi influx appeared after the inhibition of Pi accumulation.Furthermore, under conditions where Pi accumulation was notaffected by NO^–₃, ³²Pi influx and Pi secretion into thexylem became insensitive to the presence of nitrate. Our hypothesisis that the stimulative effect of NO^–₃ on Pi secretionand the depressive one on ³²Pi influx are the repercussionsof an increase in the Pi cytosolic concentration due to an NO^–₃-induced decrease in Pi uptake by the vacuoles. Key words: Root, orthophosphate fluxes, orthophosphate accumulation, nitrate, ionic interaction     

Nitrate Inhibition of Chloride Influx in Barley: Implications for a Proposed Chloride Homeostat

Net accumulation of Cl^– by intact barley plants was virtuallyeliminated in roots and reduced by 40% in shoots when externalmedia (0.5 mol m^–3 CaSO₄ plus 0–5 mol m^–3KCI) were supplemented with 0.25 mol m^– Ca(NO₃)₂. Plasmalemma³⁶Cl^– influx (_oc) was shown to be insensitive to externalNO₃^- in plants which had previously been grown in solutionslacking ^–3, but _oc became sensitive to NO₃^-after a lagperiod of 3–6 h. Kinetic analyses revealed that the inhibitionof 36C1^– influx by external NO₃^- was complex. At 0.25mol m^–3 NO₃^- the V_max for Cl^– influx was reducedby greater than 50%, with insignificant effects upon K_m. At0.5 mol m^–3 NO₃^- there was no further effect upon V_maxbut K_m for influx increased from 38±5 mmol m^–3to 116±26 mmol m^–3. By contrast, Cl^– effluxwas found to be insensitive to external NO₃^-. A model for theregulation of Cl^– influx is proposed which involves bothnegative feedback effects from vacuolar NO₃^- +Cl^–) concentrationand (external) NO₃^- inhibition of Cl^– influx at the plasmalemma.These combined effects serve to discriminate against Cl^–accumulation, favouring NO₃^- accumulation, when the latter ionis available. Such observations are inconsistent with recentproposals for the existence of bona fide homeostats for chlorideaccumulation in higher plants. Key words: Nitrate inhibition, Chloride influx, Barley