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PubMed1991

[Postoperative nitrogen loss].

Total nitrogen excretion was studied prospectively in 54 surgical patients during the first and second postoperative day by the Chemiluminescence method. The results were compared to three different methods for approximation of the total nitrogen excretion. Daily total nitrogen excretion was 13.13 +/- 5.7 g after peripheral vascular procedures, 15.97 +/- 8.1 g after major surgical procedures and 20.42 +/- 5.2 g in septic patients. The assessment of total nitrogen excretion by the three other methods underestimated real nitrogen losses by 20-52%. The nitrogen concentration in various secretions was relatively constant. We conclude that measuring the urine nitrogen losses only and adjusting them for the calculation of the total nitrogen excretion by several factors in incorrect. Direct measurement of total nitrogen excretion is too costly for the use in daily clinical practice.

Adult↗
PubMed1990

Basal and stimulated formation and release of L-arginine-derived nitrogen oxides from cultured endothelial cells.

Endothelium-derived nitrogen oxides, i.e., nitrogen oxide and/or nitrogen oxide-containing compounds, formed from L-arginine account, at least in part, for the biological activity of endothelium-derived relaxing factor(s). We have developed a rapid and sensitive assay to determine the basal and stimulated release of defined breakdown products of endothelium-derived nitrogen oxides, i.e., nitrite and nitrate. Formation and/or release of these nitrogen oxides was time- and concentration-dependently stimulated by various endothelium-dependent vasodilators and was decreased by NG-methyl-L-arginine and L-canavanine, compounds that most likely inhibit the endothelial enzymatic conversion of L-arginine into nitrogen oxides. In addition to nitrogen oxide, hydroxylamine was shown to have pharmacological and physico-chemical properties similar to endothelium-derived relaxing factor(s). Moreover, hydroxylamine release was detected in stimulated bovine aortic endothelial cells. We suggest that hydroxylamine represents one endothelium-derived nitrogen oxide within the group of biologically active nitrogen oxides.

Acetylcholine↗
PubMed1990

Endogenous nitrogen excretion in alcoholic patients on an enteral diet.

1. Obligatory nitrogen losses were evaluated in a group of 9 chronic alcoholic adult males of low socioeconomic status, with an intake of approximately 1 liter/day of a Brazilian beverage (45% ethanol by weight) for at least 5 years. 2. The patients were fed a nitrogen-free diet orally or by tube for 7 days. Since endogenous nitrogen excretion was statistically similar for all subjects, the data were pooled for overall analysis. 3. A stable rate of urinary nitrogen output of 28.42 +/- 11.93 mg N kg-1 day-1 was reached in about 3 days both after oral and tube feeding. The obligatory fecal losses for days 4 to 7 were 17.20 +/- 8.86 mg N kg-1 day-1. 4. Based on the factorial approach, considering the skin and other minor nitrogen losses to be 10 mg N kg-1 day-1, the total endogenous obligatory nitrogen loss for this group of alcoholic patients was 55.62 +/- 18.57 mg N kg-1 day-1. 5. The present results are discussed on the basis of the controversy about the nitrogen requirement of alcoholic patients. The endogenous nitrogen excretion demonstrated does not suggest the need for increased nitrogen intake, but the protein malnutrition of the patients points to the need for more protein.

Adult↗
PubMed1989

[Effects of various caloric intakes on nitrogen metabolism of the body, the muscles and the viscera during total parenteral nutrition with aminoacidic solution enriched with branched-chain amino acids].

Nitrogen metabolism was studied in ten injured/septic patients by means of a two compartment model, differentiating muscle from non muscle (central) tissue. During fasting muscle tissue loses a consistent amount of aminoacids which is retained in part by central tissue, giving rise to a positive nitrogen balance, whilst three methyl histidine and body nitrogen output are elevated. Total parenteral nutrition (glucose 15 kcal kg-1 day-1, nitrogen 0.30 g kg-1 die-1) improved body nitrogen balance and three methyl histidine excretion, but did not affect significantly either muscle or central nitrogen balance. Increasing caloric support to 30 kcal kg-1 day-1 did not showed any further effect on body nitrogen balance and three methyl histidine, while it improved significantly vs basal muscle nitrogen balance, but did not affect central nitrogen balance.

Amino Acids, Branched-Chain↗
PubMed1987

Nitrogen in fecal bacterial, fiber, and soluble fractions of patients with cirrhosis: effects of lactulose and lactulose plus neomycin.

To determine how lactulose and lactulose plus neomycin might alter nitrogen metabolism in the colon we investigated the effect of these agents on the distribution of nitrogen in the bacterial, soluble, and fiber fractions of stool. The alterations in fecal nitrogen excretion were additionally correlated with changes in total body urea synthesis and degradation rates. Six patients with stable cirrhosis received a control diet alone followed by the administration of lactulose (56 +/- 6 gm/day), and eight similar patients received lactulose alone (63 +/- 5 gm/day) followed by the addition of neomycin (4 gm/day). Their feces were partitioned into individual fractions by physical separation. Lactulose administration increased nitrogen excreted in the bacterial fraction by 165% (from 0.52 +/- 0.14 gm/day to 1.38 +/- 0.21 gm/day) and by 135% in the soluble fraction (from 0.58 +/- 0.08 gm/day to 1.36 +/- 0.23 gm/day). When lactulose was supplemented with neomycin, the nitrogen content of the bacterial fraction decreased by 28%. Lactulose caused a 23% reduction in the urea production rate that was mainly accounted for by increase in fecal nitrogen excretion. The addition of neomycin caused a further reduction in urea production that was explained by an inhibition of urea degradation. These results demonstrate that a major effect of lactulose was to augment the incorporation of nitrogen into fecal bacteria although nitrogen in the soluble fraction also increased. The additional nitrogen excreted in the fecal bacterial and soluble fractions caused a reduction in urea synthesis.

Aged↗
PubMedDate not supplied

[Nitrogen fractions of human milk during the first week of lactation. Comparison of milk from mothers of term and premature newborn infants].

Nitrogen fractions in human milk (total nitrogen, non protein nitrogen, casein nitrogen and soluble protein nitrogen) were determined by direct dosage or by calculation during the first 7 days of lactation in 61 mothers feeding artificially. Twenty-five delivered at term and 36 prematurely. In all cases a decrease in all nitrogen fractions was observed. The amounts of total nitrogen and soluble proteins were similar in both groups. A difference in the distribution of the nitrogen fractions occurs only on the 8th day following delivery. Milk from mothers who deliver prematurely contains more casein and less non protein nitrogen amounts.

Caseins↗
PubMed1984

Intravenous nitrogen intake requirements in full-term newborns undergoing surgery.

To determine the intravenous intake of nitrogen needed to duplicate the weight gain and nitrogen retention observed in healthy infants fed human milk, full-term infants had weight change and nitrogen balance measured postoperatively while they were receiving two different parenteral regimens which provided adequate energy (87 kcal/kg/d) and varying intakes of nitrogen as crystalline amino acids (290 to 579 mg/kg/d) over a six-day period. Weight change was similar to that observed in breast-fed infants (35 g/d) and was independent of nitrogen intake. A significant correlation was observed between nitrogen intake and retention (r = .71, P less than .01). Substituting the rate of nitrogen retention observed in thriving full-term infants fed human milk into the regression equation describing the relationship between nitrogen intake and retention, the nitrogen intake required by parenterally fed infants in order to duplicate the accretion rate of their milk-fed counterparts was 280 mg/kg/d. After correcting for individual variation, recommended intravenous amino acid intakes range from 2.3 to 2.7 g/kg/d depending on the commercial amino acid formulation chosen and providing that adequate amounts of energy are concurrently infused.

Amino Acids, Essential↗
PubMed1981

Comparative effects of lactulose and magnesium sulfate on urea metabolism and nitrogen excretion in cirrhotic subjects.

In previous studies with cirrhotic subjects administration of oral lactulose caused a reduction in the urea production rate associated with an increase in fecal nitrogen excretion. The change in urea production rate in response to lactulose therapy was an indirect measure of a reduction in total gut ammonia production. In this study, the effect of magnesium sulfate administration was compared with lactulose therapy in 5 cirrhotic subjects to determine whether the effects of lactulose on nitrogen metabolism might be attributed to a nonspecific, cathartic effect. Both magnesium sulfate (5-15 g/day) and lactulose (40-80 g/day) caused significant and comparable increases in stool weight, solids, and total nitrogen. Only lactulose caused a reduction in fecal pH. Magnesium sulfate had no significant effect on the urea production rate or urinary nitrogen excretion, whereas lactulose caused a 25% reduction in the urea production rate and an 18% reduction in urinary nitrogen excretion. The latter was accounted for by a fall in urinary urea. Nitrogen balance was more negative during magnesium sulfate than during control or lactulose periods since magnesium sulfate increased fecal nitrogen without altering urinary nitrogen excretion. These data demonstrated that the effects of lactulose on nitrogen excretion and urea metabolism were not duplicated by equivalent cathartic doses of magnesium sulfate.

Disaccharides↗
PubMed1993

Unresponsiveness of hepatic nitrogen metabolism to glucagon infusion in patients with cirrhosis: dependence on liver cell failure.

Glucagon exerts an up-regulatory effect on hepatic nitrogen metabolism in healthy subjects, but its potential role in the presence of liver failure is uncertain. The effects of glucagon on urea synthesis and hepatic nitrogen clearance during alanine infusion were studied in five control subjects and six cirrhotic patients in paired experiments at spontaneous glucagon concentrations and at high physiological glucagon levels (approximately 300 to 500 pmol.L-1) induced by a 7.5-hr continuous glucagon infusion. In all experiments the urea nitrogen synthesis rate increased linearly with increasing alpha-amino-nitrogen concentrations. At spontaneous glucagon concentrations the dynamics of alpha-amino nitrogen to urea nitrogen conversion (functional hepatic nitrogen clearance) were significantly reduced in cirrhosis (23.2 +/- 6.7 L.hr-1 vs. 35.3 +/- 8.0 L.hr-1, p < 0.05) in relation to decreased liver function. Glucagon superinfusion caused a 63% increase in the dynamics of the process in controls (57.7 +/- 11.0 L.hr-1; p vs. spontaneous glucagon, p < 0.01), whereas in cirrhosis it increased on average by only 15% (26.7 +/- 10.7; p = NS). The glucagon-induced change in functional hepatic nitrogen clearance significantly correlated with galactose elimination capacity and antipyrine clearance (r = 0.905 and 0.964, respectively). Glucagon, in high physiological concentrations achieved with glucagon infusion, does not produce significant effects on hepatic nitrogen metabolism in cirrhosis. The reduced sensitivity of the cirrhotic liver to glucagon seems to be dependent on decreased hepatocellular function. These data do not support the role of glucagon as a "catabolic" hormone in cirrhosis.

Adult↗
PubMed1996

The nature of nitrogen: an overview.

Most of the nitrogen available to the biosphere exists as N2 in the atmosphere, and is not useful to most organisms until it is "fixed" either biologically or abiotically (by lightning or aurorae, or industrially). Once it is fixed into NH3, usually it is either assimilated and transformed into organic N or nitrified into NO3-. Organic N can be transformed back into NH3 by ammonification. Nitrate can be converted into N2O by nitrification and denitrification, and to N2 by denitrification. Such N2O and N2 production results in nitrogen loss from ecosystems and a nitrogen gain to the atmospheric nitrogen reservoir. The different steps of the nitrogen cycle require different environmental conditions. These differences result in a spatial distribution pattern of the different nitrogen transformation reactions. Biological nitrogen fixation occurs universally in soils, sediments, fresh water, and marine systems that are both aerobic and anaerobic. Nitrification occurs primarily in aerobic habitats, whereas denitrification predominates in anaerobic habitats such as sediments and water-logged soils. In closed systems, such as plant growth chambers for CELSS, denitrification and nitrification result in a loss of fixed nitrogen available to plants.

Ammonia↗
PubMed2002

Quantitative trait loci analysis of growth response to varying nitrogen sources in Arabidopsis thaliana.

Nitrogen absorption and assimilation is variable among plants as a result of two factors: the source of nitrogen available and the genetic variation among species within the resulting nitrogen pathways. Several genes involved in nitrogen cycling have been identified, yet little is known about the genes that control quantitative responses to different nitrogen sources. With quantitative trait loci (QTL) mapping in Arabidopsis thaliana recombinant inbred lines (Columbia x Landsberg erecta) we have identified chromosomal regions controlling aerial mass, root mass, and root length when plants are grown in nitrate, ammonium, ammonium nitrate, or low nitrogen treatments. A total of 16 QTL ( P < 0.01) were identified among the nitrogen treatments. Most of the QTL were specific to a single treatment. The percentage additive genetic effects of significant QTL were as high as 17%. Five significant QTL corresponded to the locations of candidate genes associated with nitrogen assimilation, while a few QTL corresponded with candidate genes in the developmental pathways. Most QTL were not shared across treatments, suggesting that there is no optimal genotype for all nitrogen sources.

Journal Article↗
PubMed1988

Nitrogen saturation of terrestrial ecosystems.

Nitrogen saturation, in the sense that nitrogen additions to an ecosystem lead to losses of the same order of magnitude, is analyzed as an interplay between a plant subsystem and a soil subsystem. The plant system is defined by its nitrogen productivity, which allows calculations of the maximum amount of nitrogen that can be held in, and the maximum nitrogen flux density that can be utilized by, the plant subsystem. The most important response of the soil subsystem is a change in the microbial nitrogen concentration, from which the nitrogen absorption capacity can be derived. It is shown that of the two subsystems the soil must always saturate first. The time to reach saturation depends strongly on site history in terms of the sources of litter forming the soil organic matter and on the ratio between the external nitrogen inflows and the litter nitrogen flow.

Journal Article↗
PubMed1998

Human motile sperm recovery after cryopreservation: freezing in nitrogen vapor vs the direct plunge technique.

Objectives: The following will be tested: 1) Freezing in nitrogen vapor prior to submerging will have a higher motile sperm recovery rate compared to directly submerging in liquid nitrogen, 2) The length of time a sample spends in nitrogen vapor will not affect the motile sperm recovery, and 3) The volume of sample to be frozen will not affect the recovery of motile sperm.Design: This is a prospective study performed through a university infertility clinic. The semen samples were obtained from 20 male partners of couples being screened for in vitro fertilization.Materials and Methods: Semen was collected and washed with 5 volumes of modified Ham's F10 medium and centrifuged at 120xg for 10 minutes. The pellet was resuspended in 5 mL of medium and centrifuged in a similar manner. The swim-up sperm was prepared with the second pellet overlaid with 1 mL of medium, containing 1% human serum albumin, and incubated for 1 hour at 37 C. Equal volumes of sperm suspension and cryoprotectant were mixed by gentle stirring. Sperm density and motility were recorded before freezing. Each vial contained 1 mL of the sperm/cryoprotectant mixture. All vials were eventually submerged in liquid nitrogen for 40 minutes and thawed at room temperature for 1 hour. The percent of motile sperm recovered was assessed. Hypothesis 1: Comparing freezing in nitrogen vapor versus direct plunge, one vial was frozen for 6 minutes in the nitrogen vapor 2 cm above the liquid surface prior to submerging. The paired vial was kept at room temperature for 6 minutes before submerging. Hypothesis 2: Two vials were frozen in vapor for 6 and 12 minutes prior to submerging them. Hypothesis 3: To assess the effect of sample volume on motile sperm recovery, three separate vials were prepared, each containing either 1.0, 0.5, or 0.25 mL of the sperm/cryoprotectant mixture. Each vial was suspended in vapor for 6 minutes prior to submerging into the liquid. Paired t test analysis was used for objectives 1 and 2, with repeated measures analysis of variance used to determine the significance of objective 3.Results: Pure motile sperm density and motility were assessed for 20 samples after the addition of cryoprotectant. The sperm density and motility ranged from 0.9 to 120 million sperm/mL (33.2, SD +/- 35.0) and 78-100% (86.5, SD +/- 12.4), respectively. The mean motile sperm recovered for vapor freezing prior to plunge was 54.9% (SD +/- 15.4), compared to submerging directly, which was 21.5% (SD +/- 10.0); this was significant with a P <.05. The initial sperm density, <20 or >/=20 million/mL, did not influence the percent of motile sperm recovered (P >.05). Motile sperm recovered from samples in vapor for 6 and 12 minutes was 49.7% (SD +/- 8.71) and 51.8% (SD +/- 10.4), respectively; this was not significant. The volume of purified motile sperm frozen did not significantly alter the percent of motile sperm recovered.Conclusion: Directly submerging a sample from room temperature into liquid nitrogen has shown a reduction of motile sperm recovery by 65.0%, compared to exposing the sample to liquid nitrogen vapor prior to submerging, which only showed a reduction of 31.7%. Motility appears to be influenced by the rate at which the sample is frozen. There was no significant impact on motile sperm recovery compared to the amount of sperm preserved, length of time the sample was in liquid nitrogen vapor, or the initial sperm density. This is the first study to isolate the motile faction from semen and examine the recovery rate between the two cryopreservation techniques. The clinical application implies that increasing the recovery of motile sperm could improve pregnancy rates and this simplified technique is possible without obtaining special equipment that could impose a significant financial burden with similar results.

Journal Article↗
PubMed2000

Sensitivity to nitrogen mustard relates to the ability of processing DNA damage in Chinese hamster ovary cells.

The hallmark of the excision repair pathways is the removal of DNA adducts by excision of the damaged nucleotides. In the course of repair, transient DNA strand breaks occur, which can be measured by the Comet assay. We have investigated the processing of DNA damage, mediated by nitrogen mustard, in wild-type AA8 Chinese hamster ovary cells, and in UV5, UV20 and UV41 DNA repair deficient cell lines. Whereas DNA repair could not be detected by unscheduled DNA synthesis at nitrogen mustard doses below 10 microM, processing of nitrogen mustard-mediated DNA damage was observed by the Comet assay at a 100-times lower concentration. Wild-type Chinese hamster ovary AA8 cells were able to process nitrogen mustard-mediated DNA damage within 4-24 hr depending on the dose of nitrogen mustard (0.1-10 microM). None of the repair-deficient cell lines was able to completely process the DNA damage induced by 10 microM nitrogen mustard. At nitrogen mustard doses that conferred 10% colony forming ability, the repair-deficient cells had an altered processing of nitrogen mustard-mediated DNA damage: In the AA8, UV20, and UV41 cells, the amplitude of strand breaks peaked early (within 4 hr), the level of strand breaks in the nitrogen mustard exposed UV20 and UV41 cells did not return to the baseline of the unexposed reference culture, and the peak in strand breaks in the UV5 cell line occurred after 4 hr. Our results indicate that the single cell gel electrophoresis (Comet) assay is suitable for assessing repair capability of DNA alkylations.

Alkylating Agents↗
PubMed2006

A low fraction of nitrogen in molecular form in a dark cloud.

Nitrogen is the fifth most abundant element in the Universe. In the interstellar medium, it has been thought to be mostly molecular (N2). However, N2 has no observable rotational or vibrational transitions, so its abundance in the interstellar medium remains poorly known. In comets, the N2 abundance is very low, while the elemental nitrogen abundance is deficient with respect to the solar value. Moreover, large nitrogen isotopic anomalies are observed in meteorites and interstellar dust particles. Here we report the N2H+ (and by inference the N2) abundance inside a cold dark molecular cloud. We find that only a small fraction of nitrogen in the gas phase is molecular, with most of it being atomic. Because the compositions of comets probably reflect those of dark clouds, this result explains the low N2 abundance in comets. We argue that the elemental nitrogen abundance deficiency in comets can be understood if the atomic oxygen abundance is lower than predicted by present chemical models. Furthermore, the lack of molecular nitrogen in molecular clouds explains the nitrogen anomalies in meteorites and interstellar dust particles, as nitrogen fractionation is enhanced if gaseous nitrogen is atomic.

Journal Article↗
PubMed1998

Interactive effects of nitrogen and water availabilities on gas exchange and whole-plant carbon allocation in poplar.

Cuttings of balsam spire hybrid poplar (Populus trichocarpa var. Hastata Henry x Populus balsamifera var. Michauxii (Dode) Farwell) were grown in sand culture and irrigated every 2 (W) or 10 (w) days with a solution containing either 3.0 (N) or 0.5 (n) mol nitrogen m(-3) for 90 days. Trees in the WN (control) and wn treatments had stable leaf nitrogen concentrations averaging 19.4 and 8.4 mg g(-1), respectively, over the course of the experiment. Trees in the Wn and wN treatments had a similar leaf nitrogen concentration, which increased from 12.0 to 15.8 mg g(-1) during the experiment. By the final harvest, mean stomatal conductances of trees in the wN and wn treatments were less than those of trees in the Wn and WN treatments (1.8 versus 4.6 mm s(-1)). Compared to the WN treatment, biomass at the final harvest was reduced by 61, 72 and 75% in the Wn, wN and wn treatments, respectively. At the final harvest, WN trees had a mean total leaf area of 4750 +/- 380 cm(2) tree(-1) and carried 164 +/- 8 leaves tree(-1) with a specific leaf area of 181 +/- 16 cm(2) g(-1), whereas Wn trees had a smaller mean total leaf area (1310 +/- 30 cm(2) tree(-1)), because of the production of fewer leaves (41 +/- 6) with a smaller specific leaf area (154 +/- 2 cm(2) g(-1)). A greater proportion of biomass was allocated to roots in Wn trees than in WN trees, but component nitrogen concentrations adjusted such that there was no Wn treatment effect on nitrogen allocation. Compared with WN trees, rates of photosynthesis and respiration per unit weight of tissue of Wn trees decreased by 28 and 31%, respectively, but the rate of photosynthesis per unit leaf nitrogen remained unaltered. The wN and Wn trees had similar leaf nitrogen concentrations; however, compared with the Wn treatment, the wN treatment decreased mean total leaf area (750 +/- 50 cm(2) tree(-1)), number of leaves per tree (29 +/- 2) and specific leaf area (140 +/- 6 cm(2) g(-1)), but increased the allocation of biomass and nitrogen to roots. Net photosynthetic rate per unit leaf nitrogen was 45% lower in the wN treatment than in the other treatments. Rates of net photosynthesis and respiration per unit weight of tissue were 48 and 33% less, respectively, in wN trees than in Wn trees.

Journal Article↗
PubMed1992

Distinctive Responses of Ribulose-1,5-Bisphosphate Carboxylase and Carbonic Anhydrase in Wheat Leaves to Nitrogen Nutrition and their Possible Relationships to CO(2)-Transfer Resistance.

The amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), total chlorophyll (Chl), and total leaf nitrogen were measured in fully expanded, young leaves of wheat (Triticum aestivum L.), rice (Oryza sativa L.), spinach (Spinacia oleracea L.), bean (Phaseolus vulgaris L.), and pea (Pisum sativum L.). In addition, the activities of whole-chain electron transport and carbonic anhydrase were measured. All plants were grown hydroponically at different nitrogen concentrations. Although a greater than proportional increase in Rubisco content relative to leaf nitrogen content and Chl was found with increasing nitrogen supply for rice, spinach, bean, and pea, the ratio of Rubisco to total leaf nitrogen or Chl in wheat was essentially independent of nitrogen treatment. In addition, the ratio of Rubisco to electron transport activities remained constant only in wheat. Nevertheless, gas-exchange analysis showed that the in vivo balance between the capacities of Rubisco and electron transport in wheat, rice, and spinach remained almost constant, irrespective of nitrogen treatment. The in vitro carbonic anhydrase activity in wheat was very low and strongly responsive to increasing nitrogen content. Such a response was not found for the other C(3) plants examined, which had 10- to 30-fold higher carbonic anhydrase activity than wheat at any leaf-nitrogen content. These distinctive responses of carbonic anhydrase activity in wheat were discussed in relation to CO(2)-transfer resistance and the in vivo balance between the capacities of Rubisco and electron transport.

Journal Article↗
PubMed2006

A central role for the nitrate transporter NRT2.1 in the integrated morphological and physiological responses of the root system to nitrogen limitation in Arabidopsis.

Up-regulation of the high-affinity transport system (HATS) for NO(3)(-) and stimulation of lateral root (LR) growth are two important adaptive responses of the root system to nitrogen limitation. Up-regulation of the NO(3)(-) HATS by nitrogen starvation is suppressed in the atnrt2.1-1 mutant of Arabidopsis (Arabidopsis thaliana), deleted for both NRT2.1 and NRT2.2 nitrate transporter genes. We then used this mutant to determine whether lack of HATS stimulation affected the response of the root system architecture (RSA) to low NO(3)(-) availability. In Wassilewskija (Ws) wild-type plants, transfer from high to low NO(3)(-) medium resulted in contrasting responses of RSA, depending on the level of nitrogen limitation. Moderate nitrogen limitation (transfer from 10 mm to 1 or 0.5 mm NO(3)(-)) mostly led to an increase in the number of visible laterals, while severe nitrogen stress (transfer from 10 mm to 0.1 or 0.05 mm NO(3)(-)) promoted mean LR length. The RSA response of the atnrt2.1-1 mutant to low NO(3)(-) was markedly different. After transfer from 10 to 0.5 mm NO(3)(-), the stimulated appearance of LRs was abolished in atnrt2.1-1 plants, whereas the increase in mean LR length was much more pronounced than in Ws. These modifications of RSA mimicked those of Ws plants subjected to severe nitrogen stress and could be fully explained by the lowered NO(3)(-) uptake measured in the mutant. This suggests that the uptake rate of NO(3)(-), rather than its external concentration, is the key factor triggering the observed changes in RSA. However, the mutation of NRT2.1 was also found to inhibit initiation of LR primordia in plants subjected to nitrogen limitation independently of the rate of NO(3)(-) uptake by the whole root system and even of the presence of added NO(3)(-) in the external medium. This indicates a direct stimulatory role for NRT2.1 in this particular step of LR development. Thus, it is concluded that NRT2.1 has a key dual function in coordinating root development with external NO(3)(-) availability, both indirectly through its role as a major NO(3)(-) uptake system that determines the nitrogen uptake-dependent RSA responses, and directly through a specific action on LR initiation under nitrogen-limited conditions.

Anion Transport Proteins↗