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At least 19 recordsLinked to original sources
PubMed1976

In vivo nitrate reduction in relation to nitrate uptake, nitrate content, and in vitro nitrate reductase activity in intact barley seedlings.

A study was done to relate the in vivo reduction of nitrate to nitrate uptake, nitrate accumulation, and induction of nitrate reductase activity in intact barley seedlings (Hordeum vulgare L. var. ;Numar'). The characteristics of nitrate uptake in response to both time and ambient concentration of nitrate regulated reduction and accumulation. Uptake, accumulation, and in vivo reduction achieved steady state rates in 3 to 4 hours, whereas extractable (in vitro) nitrate reductase activity was still increasing at 12 hours. In vivo reduction of nitrate was better correlated exponentially than linearly over time with in vitro activity of nitrate reductase. A similar relationship occurred over increasing concentration of nitrate in the ambient solution. The results suggest that the rate of in vivo reduction of nitrate in barley seedlings may be regulated by the rate of uptake at the ambient concentrations of nitrate employed in the study.

Journal Article↗
PubMed1992

Studies of the uptake of nitrate in barley : v. Estimation of root cytoplasmic nitrate concentration using nitrate reductase activity-implications for nitrate influx.

The cytoplasmic NO(3) (-) concentration ([NO(3) (-)](c)) was estimated for roots of barley (Hordeum vulgare L. cv Klondike) using a technique based on measurement of in vivo nitrate reductase activity. At zero external NO(3) (-) concentration ([NO(3) (-)](o)), [NO(3) (-)](c) was estimated to be 0.66 mm for plants previously grown in 100 mum NO(3) (-). It increased linearly with [NO(3) (-)](o) between 2 and 20 mm, up to 3.9 mm at 20 mm [NO(3) (-)](o). The values obtained are much lower than previous estimates from compartmental analysis of barley roots. These observations support the suggestion (MY Siddiqi, ADM Glass, TJ Ruth [1991] J Exp Bot 42: 1455-1463) that the nitrate reductase-based technique and compartmental analysis determine [NO(3) (-)](c) for two separate pools; an active, nitrate reductase-containing pool (possibly located in the epidermal cells) and a larger, slowly metabolized storage pool (possibly in the cortical cells), respectively. Given the values obtained for [NO(3) (-)](c) and cell membrane potentials of -200 to -300 mV (ADM Glass, JE Schaff, LV Kochian [1992] Plant Physiol 99: 456-463), it is very unlikely that passive influx of NO(3) (-) is possible via the high-concentration, low-affinity transport system for NO(3) (-). This conclusion is consistent with the suggestion by Glass et al. that this system is thermodynamically active and capable of transporting NO(3) (-) against its electrochemical potential gradient.

Journal Article↗
PubMed2000

Discrepancy between nitrate reduction rates in intact leaves and nitrate reductase activity in leaf extracts: what limits nitrate reduction in situ?

Nitrate reductase (NR) activity in spinach leaf extracts prepared in the presence of a protein phosphatase inhibitor (50 microM cantharidine) was measured in the presence of Mg2+ (NRact) or EDTA (NRmax), under substrate saturation. These in-vitro activities were compared with nitrate reduction rates in leaves from nitrate-sufficient plants. Spinach leaves containing up to 60 micromol nitrate per g fresh weight were illuminated in air with their petiole in water. Their nitrate content decreased with time, permitting an estimation of nitrate reduction in situ. The initial rates (1-2 h) of nitrate consumption were usually lower than NRact, and with longer illumination time (4 h) the discrepancy grew even larger. When leaves were fed through their petiole with 30 mM nitrate, initial in-situ reduction rates calculated from nitrate uptake and consumption were still lower than NRact. However, nitrate feeding through the petiole maintained the in situ-nitrate reduction rate for a longer time. Initial rates of nitrate reduction in situ only matched NRact when leaves were illuminated in 5% CO2. In CO2-free air or in the dark, both NRact and in-situ nitrate reduction decreased, but NRact still exceeded in-situ reduction. More extremely, under anoxia or after feeding 5-amino-4-imidazole carboxyamide ribonucleoside in the dark, NR was activated to the high light level; yet in spite of that, nitrate reduction in the leaf remained very low. It was examined whether the standard assay for NRact would overestimate the in-situ rates due to a dissociation of the inactive phospho-NR-14-3-3 complex after extraction and dilution, but no evidence for that was found. In-situ NR obviously operates below substrate saturation, except in the light at high ambient CO2. It is suggested that in the short term (2 h), nitrate reduction in situ is mainly limited by cytosolic NADH, and cytosolic nitrate becomes limiting only after the vacuolar nitrate pool has been partially emptied.

14-3-3 Proteins↗
PubMed2004

Nitrite-responsive activation of the nitrate assimilation operon in Cyanobacteria plays an essential role in up-regulation of nitrate assimilation activities under nitrate-limited growth conditions.

NtcB of the cyanobacterium Synechococcus elongatus strain PCC 7942 is a LysR family protein that enhances expression of the nitrate assimilation operon (nirA operon) in response to the presence of nitrite, an intermediate of assimilatory nitrate reduction. Inactivation of ntcB in this cyanobacterium specifically abolishes the nitrite responsiveness of nirA operon expression, but under nitrate-replete conditions (wherein negative feedback by intracellularly generated ammonium prevails over the positive effect of nitrite) activity levels of the nitrate assimilation enzymes are marginally higher in the wild-type cells than in the mutant cells, raising the issue of whether the nitrite-promoted regulation has physiological importance. On the other hand, the strains carrying ntcB expressed much higher nitrate assimilation enzyme activities under nitrate-limited growth conditions than under nitrate-replete conditions whereas the ntcB-deficient strains showed levels of the enzyme activities lower than those seen under the nitrate-replete conditions. Although the ntcB mutant maintained a constant cell population in a nitrate-limited chemostat when grown as a single culture, it was diluted at a rate expected for nondividing cells when mixed with the wild-type cells and subjected to nitrate limitation in the chemostat culture system. These results demonstrated that the nitrite-promoted activation of the nitrate assimilation operon is essential for up-regulation of the nitrate assimilation activities under the conditions of nitrate limitation and for competitive utilization of nitrate.

Bacterial Proteins↗
PubMed2002

Nitrate signalling on the nitrate reductase gene promoter depends directly on the activity of the nitrate transport systems in Chlamydomonas.

Nitrate signalling on the nitrate reductase (Nia1) gene promoter from Chlamydomonas reinhardtii has been studied by using a construct of the Nia1 promoter transcriptionally fused to the Chlamydomonas arylsulphatase gene as a reporter in strains bearing different sets of nitrate/nitrite transport genes. The high-affinity nitrate transport (HANT) system I is required for efficient signalling by nitrate, even at submicromolar concentrations of the anion. In addition, the autogenous regulation of nitrate reductase has been found to depend on the presence of system I. The low-affinity nitrate transport system III promoted signalling optimally on the promoter at millimolar nitrate concentrations. The HANT system IV, which is insensitive to ammonium and active at low CO2, allowed nitrate signalling at micromolar concentrations even in the presence of ammonium, suggesting that the balance of these two effectors controls Nia1 transcription. Our data indicate that nitrate signalling on the Nia1 gene promoter occurs intracellularly and depends on the activity of nitrate transporters.

Animals↗
PubMed2003

[Induced activity of nitrate reductase by nitrate and cloning of nitrate reductase gene].

Excessive nitrate accumulated in plants affects vegetable quality severely and excessive nitrate ingestion would do harm to human health. Assimilatory NADH: nitrate reductase (NR, EC 1.6.6.1), a complex Mo-pterin-, cytochrome b(557)- and FAD-containing protein, catalyzes the regulated and rate-limiting step in the utilization of inorganic nitrogen by higher plants. Enhancing the activity of NR is conducive to reduce the concentration of nitrate in plants. The experiments were conducted to investigate the activity of nitrate reductase in different plant tissues and the relationship between external inducing solution concentration and NR activity (NRA) in plant leaves. Six plant seedlings growing in solution culture were deprived of an external nitrogen (N) supply for 2 weeks. On selected days, three of six plant seedlings were exposed to 50mmol/L NO3- for 0, 2, 5, 8, 11h, and four of the six plant seedlings were exposed to 0, 10, 30, 50mmol/L NO3- for 2h. The NRA was determined in vivo at 538nm using spectrophotometer. The results showed that NRA increased when those plant seedlings were induced by nitrate solution. The change trends of NRA in roots and in leaves of cole, pea and tomato were different during treating time. The NRA in cole leaves was higher than that in its root and in other two plants and increased along with inducing time, but the NRA in bea and tomato was highest when the treating time was 8h and 2h, respectively. The highest NRA in leaves of three kinds of Chinese cabbages and tomato was induced by different concentrations of KNO3 solution. In tomato leaves, the highest NRA was induced by 10 - 30mmol/L KNO3 solution. In three Chinese cabbages, Brassica chinensis L. cv. AJH, XBC and KR-605, the highest NRA was induced by 10, 30, 10mmol/L KNO3 solution, respectively. The results indicated that the response manners of NRA in plants to external nitrate solutions were different. According to these results, the level of NR mRNA in plants could be enhanced by nitrate inducement. The total RNA was isolated from tomato leaves and root which induced by 30mmol/L KNO3 solution for 2h, and NR cDNA was obtained by RT-PCR using the specific primers. The fragments of PCR products were cloned and sequenced. There are 2736 base pairs in the whole cDNA fragment. The deduced protein sequence contains 911 amino acids. The NR gene can be fused to the CaMV 35S promoter, then introduced to higher plants, such as vegetables. It is hoped to decrease drastically the nitrate content of the transgenic plants.

Brassica↗
PubMed1990

The effect of gastric achlorhydria on the urinary recovery of nitrate in man: relevance to urinary nitrate as a measure of dietary nitrate exposure.

It is important to have a reliable method of assessing the dietary nitrate exposure of populations for a proper understanding of the potential health effects of the endogenous metabolites of this ion to be gained from epidemiological studies. Recently we strongly advocated the use of the nitrate analysis of 24 h urine samples as being a superior method for such studies. Our previous observations and those of others relating to nitrate pharmacology in healthy human volunteers formed the basis of this judgement. The purpose of this study was to determine whether gastric achlorhydria or hypochlorhydria has any significant gross effects on the urinary recovery of dietary nitrate and to what extent the inclusion of such individuals would compromise the results of potential epidemiological studies. The results demonstrated a significantly greater loss of dietary nitrate as measured by urinary recovery in achlorhydrics than in normochlorhydrics, presumably as a consequence of bacterial metabolism in the colonized stomach. Thus the average urinary nitrate recovery of a 1.5 mmol challenge was 33% in individuals with reduced stomach acidity as compared to 56% in normal controls. This significant further loss of nitrate (average 23%) when intragastric conditions favour bacterial colonization clearly indicates that for valid assessments and comparisons of nitrate exposure between populations it would be wise to exclude individuals with low levels of stomach acid where this is likely to lead to significant gastric colonization by nitrate-reducing bacteria (i.e. pH less than 4-5).

Achlorhydria↗
PubMed1987

Nitrogen Utilization in Lemna: I. Relations between Net Nitrate Flux, Nitrate Reduction, and in Vitro Activity and Stability of Nitrate Reductase.

Cultures of Lemna gibba L. G3 were maintained at a constant, N-limited growth rate by adding nitrate daily in amounts calculated to sustain a rate of culture N increment of 0.20 day(-1). Nitrate added to the culture was consumed within 8 to 10 hours and the partitioning to reduction and accumulation during this phase corresponded to, on the average, 75 and 25% of net uptake, respectively. The calculated rate of nitrate reduction was stimulated by onset of net uptake without delay and decreased when net uptake ceased. NADH-nitrate reductase (NR) activity measured in vitro without inclusion of antiproteolytic agents more than doubled during the first hour after nitrate addition and then gradually fell to its original level over the rest of the 24 hour interval. In the presence of the proteinase inhibitor leupeptin during extraction, however, NR activity was in general much higher and without any apparent cycles. The relative stabilizing effect of leupeptin was greatest on NADH-NR and reduced flavin adenine mononucleotide-NR activities whereas the effect was less on NADH-cytochrome c reductase activity (diaphorase) and reduced methylviologen-NR activity. The constant nitrate reductase activity measured in the presence of proteinase inhibitors is assumed to reflect the physiological situation. It thus appeares that short-term changes in nitrate assimilation by N-limited Lemna is related to the flux of nitrate to the reducing site and not to changes in nitrate reductase activity.

Journal Article↗
PubMed1997

The YNT1 gene encoding the nitrate transporter in the yeast Hansenula polymorpha is clustered with genes YNI1 and YNR1 encoding nitrite reductase and nitrate reductase, and its disruption causes inability to grow in nitrate.

DNA sequencing in the phage lambda JA13 isolated from a lambda EMBL3 Hansenula polymorpha genomic DNA library containing the nitrate reductase-(YNR1) and nitrite reductase-(YNI1) encoding genes revealed an open reading frame (YNT1) of 1524 nucleotides encoding a putative protein of 508 amino acids with great similarity to the nitrate transporters from Aspergillus nidulans and Chlamydomonas reinhardtii. Disruption of the chromosomal YNT1 copy resulted in incapacity to grow in nitrate and a significant reduction in rate of nitrate uptake. The disrupted strain is still sensitive to chlorate, and, in the presence of 0.1 mM nitrate, the expression of YNR1 and YNI1 and the activity of nitrate reductase and nitrite reductase are significantly reduced compared with the wild-type. Northern-blot analysis showed that YNT1 is expressed when the yeast is grown in nitrate and nitrite but not in ammonium solution.

Amino Acid Sequence↗
PubMed1989

The effect of different sources of nitrate exposure on urinary nitrate recovery in humans and its relevance to the methods of estimating nitrate exposure in epidemiological studies.

It was demonstrated that nitrate concentration fluctuates dramatically in both urine and saliva throughout the study period and hence single time point ('spot') samples of either fluid give a poor measure of previous nitrate exposure. In contrast, the nitrate recovery in complete 24 h urine collections showed a strong positive correlation with previous nitrate exposure which was independent of the nature of the food matrix in which the nitrate was ingested. Regression analysis of the data showed that the apparent urinary recovery of 70% appears to consist of 55% arising from the original challenge and a background of approximately 0.22 mmol/day which is independent of the challenge and which may reflect endogenous mammalian synthesis of nitrate. It is therefore important that in future epidemiological studies, in which dietary nitrate exposures are to be determined, that analyses are conducted only on 24 h urine collections. A large inter-individual variation was found in the conversion of salivary nitrate to nitrite presumably mediated by the oral flora. The resulting differences in nitrite formation may be one of the factors determining the level of endogenous N-nitroso compound formation in an individual.

Diet↗
PubMed1997

Protection against oral and gastrointestinal diseases: importance of dietary nitrate intake, oral nitrate reduction and enterosalivary nitrate circulation.

Over the last 20 years, dietary nitrate has been implicated in the formation of methemoglobin and carcinogenic nitrosamines in humans. This has led to restrictions of nitrate and nitrite levels in food and drinking water. However, there is no epidemiological evidence for an increased risk of gastric and intestinal cancer in population groups with high dietary vegetable or nitrate intake. A reevaluation of our currently very negative perception of dietary nitrates comes from recent research into the metabolism and enterosalivary circulation of nitrate in mammals. These studies showed that nitrate is converted to nitrite in the oral cavity that then "fuels" an important mammalian resistance mechanism against infectious diseases. Moreover, there is now evidence that the conversion of nitrate into oxides of nitrogen prevents the formation carcinogenic nitrosamines.

Animals↗
PubMed2003

Electrophilic tetraalkylammonium nitrate nitration. II. Improved anhydrous aromatic and heteroaromatic mononitration with tetramethylammonium nitrate and triflic anhydride, including selected microwave examples.

A new one-pot nitration employing tetramethylammonium nitrate and trifluoromethanesulfonic anhydride in dichloromethane to provide a ready source of the nitronium triflate nitrating agent is presented. Rapid and selective nitration with a variety of aromatic and heteroaromatic substrates is achieved resulting in the synthesis of several novel organic compounds. A distinct advantage is the removal of undesired byproducts by aqueous workup. This very mild nitration permits large-scale syntheses and gives high isolated product yields that often require no further purification. This tetramethylammonium nitrate-based nitration also has been applied to microwave-assisted conditions, and the results with several compounds are outlined.

Journal Article↗
PubMed1988

Inhibition of nitrate transport by anti-nitrate reductase IgG fragments and the identification of plasma membrane associated nitrate reductase in roots of barley seedlings.

Membrane associated nitrate reductase (NR) was detected in plasma membrane (PM) fractions isolated by aqueous two-phase partitioning from barley (Hordeum vulgare L. var CM 72) roots. The PM associated NR was not removed by washing vesicles with 500 millimolar NaCl and 1 millimolar EDTA and represented up to 4% of the total root NR activity. PM associated NR was stimulated up to 20-fold by Triton X-100 whereas soluble NR was only increased 1.7-fold. The latency was a function of the solubilization of NR from the membrane. NR, solubilized from the PM fraction by Triton X-100 was inactivated by antiserum to Chlorella sorokiniana NR. Anti-NR immunoglobulin G fragments purified from the anti-NR serum inhibited NO3- uptake by more than 90% but had no effect on NO2- uptake. The inhibitory effect was only partially reversible; uptake recovered to 50% of the control after thorough rinsing of roots. Preimmune serum immunoglobulin G fragments inhibited NO3- uptake 36% but the effect was completely reversible by rinsing. Intact NR antiserum had no effect on NO3- uptake. The results present the possibility that NO3- uptake and NO3- reduction in the PM of barley roots may be related.

Biological Transport↗
PubMed1988

Development of accelerated net nitrate uptake : effects of nitrate concentration and exposure time.

Upon initial nitrate exposure, net nitrate uptake rates in roots of a wide variety of plants accelerate within 6 to 8 hours to substantially greater rates. Effects of solution nitrate concentrations and short pulses of nitrate (</=1 hour) upon ;nitrate-induced' acceleration of nitrate uptake in maize (Zea mays L.) were determined. Root cultures of dark-grown seedlings, grown without nitrate, were exposed to 250 micromolar nitrate for 0.25 to 1 hour or to various solution nitrate concentrations (10-250 micromolar) for 1 hour before returning them to a nitrate-free solution. Net nitrate uptake rates were assayed at various periods following nitrate exposure and compared to rates of roots grown either in the absence of nitrate (CaSO(4)-grown) or with continuous nitrate for at least 20 hours. Three hours after initial nitrate exposure, nitrate pulse treatments increased nitrate uptake rates three- to four-fold compared to the rates of CaSO(4)-grown roots. When cycloheximide (5 micrograms per milliliter) was included during a 1-hour pulse with 250 micromolar nitrate, development of the accelerated nitrate uptake state was delayed. Otherwise, nitrate uptake rates reached maximum values within 6 hours before declining. Maximum rates, however, were significantly less than those of roots exposed continuously for 20, 32, or 44 hours. Pulsing for only 0.25 hour with 250 micromolar nitrate and for 1 hour with 10 micromolar caused acceleration of nitrate uptake, but the rates attained were either less than or not sustained for a duration comparable to those of roots pulsed for 1 hour with 250 micromolar nitrate. These results indicate that substantial development of the nitrate-induced accelerated nitrate uptake state can be achieved by small endogenous accumulations of nitrate, which appear to moderate the activity or level of root nitrate uptake.

Journal Article↗
PubMed2001

Blood plasma response and urinary excretion of nitrite and nitrate in milk-fed calves after oral nitrite and nitrate administration.

There is marked endogenous production of nitrate in young calves. Here we have studied the contribution of exogenous nitrate and nitrite to plasma concentrations and urinary excretion of nitrite and nitrate in milk-fed calves. In experiment 1, calves were fed 0 or 200 &mgr;mol nitrate or nitrite/kg(0.75) or 100 &mgr;mol nitrite plus 100 &mgr;mol nitrate/kg(0.75) with milk for 3 d. In experiment 2, calves were fed 400 &mgr;mol nitrate or nitrite/kg(0.75) with milk for 1 d. Plasma nitrate rapidly and comparably increased after feeding nitrite, nitrate or nitrite plus nitrate. The rise of plasma nitrate was greater if 400 than 200 &mgr;mol nitrate or nitrite/kg(0.75) were fed. Plasma nitrate decreased slowly after the 3-d administration of 200 &mgr;mol nitrate or nitrite/kg(0.75) and reached pre-experimental concentrations 4 d later. Urinary nitrate excretions nearly identically increased if nitrate, nitrite or nitrite plus nitrate were administered and excreted amounts were greater if 400 than 200 &mgr;mol nitrate or nitrite/kg(0.75) were fed. After nitrite ingestion plasma nitrite only transiently increased after 2 and 4 h and urinary excretion rates remained unchanged. Plasma nitrate concentration remained unchanged if milk was not supplemented with nitrite or nitrate. Nitrate concentrations were stable for 24 h after addition of nitrite to full blood in vitro, whereas nitrite concentrations decreased within 2 h. In conclusion, plasma nitrate concentrations and urinary nitrate excretions are enhanced dose-dependently by feeding low amounts of nitrate and nitrite, whereas after ingested nitrite only a transient and small rise of plasma nitrite is observed because of rapid conversion to nitrate.

Journal Article↗
PubMed1976

Nitrate Reductase Activity in Maize (Zea mays L.) Leaves: I. Regulation by Nitrate Flux.

The roles that leaf nitrate content and nitrate flux play in regulating the levels of nitrate reductase activity (NRA) were investigated in 8- to 14-day old maize (Zea mays L.) plants containing high nitrate levels while other environmental and endogenous factors were constant. The nitrate flux of intact plants was measured from the product of the transpiration rate and the concentration of nitrate in the xylem. NRA decreased when the seedlings were deprived of nitrate. The nitrate flux and the leaf nitrate content also decreased. When nitrate was resupplied to the roots, all three parameters increased.Attempts to alter the nitrate flux by varying transpiration rates were unsuccessful due to a relatively constant rate of delivery of nitrate to the xylem as transpiration rates fell. However, cooling the roots resulted in a decrease in the nitrate flux. Plants with a lower nitrate flux rapidly lost NRA, although the leaf nitrate content was initially unaffected. If the roots remained cool for a long enough time, the leaf nitrate content eventually decreased. Rewarming the roots increased the nitrate flux, leaf nitrate content, and NRA to control levels. When the nitrate flux in excised shoots was varied in three separate ways, decreasing the nitrate flux to the leaves resulted in a rapid decrease in NRA, although leaf nitrate contents were unchanged.These experiments show that the nitrate flux to the leaves from the roots plays a much larger regulatory role than the leaf nitrate content in controlling the level of NRA in intact plants.

Journal Article↗
PubMed1981

Nitrate Accumulation, Assimilation, and Transport by Decapitated Corn Roots : EFFECTS OF PRIOR NITRATE NUTRITION.

The effects of accumulated [(14)N]nitrate and its utilization in decapitated, 5-day-old dark-grown corn roots on influx, accumulation, xylem deposition, and reduction of concurrently absorbed nitrate during an 18-hour exposure to 0.5 millimolar K(15)NO(3) nutrient solution were examined. A 20-hour pretreatment in 15.0 millimolar K(14)NO(3) high nitrate (HN) resulted in a 2-fold greater tissue nitrate level than pretreatment in 0.5 millimolar K(14)NO(3) low nitrate (LN). Upon transfer to the 0.5 millimolar K(15)NO(3) solution, the net nitrate uptake rate in HN roots after 2 hours was 52% of the LN rate, but increased to 93% at the end of the uptake period. Despite an enhanced [(14)N]nitrate efflux from HN roots to the uptake solution, the efflux differences between the two pretreatments did not compensate for the decrease in net nitrate uptake. The [(15)N]nitrate influx rate was initially restricted by 33% in the HN roots compared to LN roots, but it had decreased to 7% by the end of the 18-hour uptake period. At this time, the total tissue nitrate levels were similar for both pretreatments. The rate of accumulation of [(15)N]nitrate in the tissue was relatively constant for both pretreatments, but was 25% less in HN roots. Of the previously accumulated [(14)N]nitrate, 52 and 46% remained after 18 hours in the LN and HN roots, respectively. The [(14)N]nitrate decline for HN roots was initially more rapid than in the LN roots which was linear over time. Xylem transport and efflux more than accounted for the decline in [(14)N]nitrate of LN roots and all but 4% in the HN roots which was attributed to reduction. Compartmentation of the previously accumulated nitrate was evident from the higher atom per cent (15)N of xylem nitrate compared to that of the tissue nitrate of both LN and HN roots. During the first 2 hours, xylem transport of [(14)N]nitrate by the HN roots was 49% greater than for LN roots, while [(15)N]nitrate transport was 9% less in HN roots compared to LN roots. Even though the reduction of [(15)N]nitrate in HN roots was 31% less than LN roots during the first 2 hours, [(15)N]nitrate was reduced more rapidly than the previously accumulated [(14)N]nitrate. After the first 4 hours, the relative partitioning of absorbed [(15)N]nitrate between accumulation, reduction, and translocation was similar regardless of pretreatment.

Journal Article↗
PubMed1989

Nitrate transport is independent of NADH and NAD(P)H nitrate reductases in barley seedlings.

Barley (Hordeum vulgare L.) has NADH-specific and NAD(P)H-bispecific nitrate reductase isozymes. Four isogenic lines with different nitrate reductase isozyme combinations were used to determine the role of NADH and NAD(P)H nitrate reductases on nitrate transport and assimilation in barley seedlings. Both nitrate reductase isozymes were induced by nitrate and were required for maximum nitrate assimilation in barley seedlings. Genotypes lacking the NADH isozyme (Az12) or the NAD(P)H isozyme (Az70) assimilated 65 or 85%, respectively, as much nitrate as the wild type. Nitrate assimilation by genotype (Az12;Az70) which is deficient in both nitrate reductases, was only 13% of the wild type indicating that the NADH and NAD(P)H nitrate reductase isozymes are responsible for most of the nitrate reduction in barley seedlings. For all genotypes, nitrate assimilation rates in the dark were about 55% of the rates in light. Hypotheses that nitrate reductase has direct or indirect roles in nitrate uptake were not supported by this study. Induction of nitrate transporters and the kinetics of net nitrate uptake were the same for all four genotypes indicating that neither nitrate reductase isozyme has a direct role in nitrate uptake in barley seedlings.

Biological Transport↗