Fate of nitrate during initial nitrate utilization by nitrogen-depleted dwarf bean

The fate of nitrate and nitrogen-15 was followed during the apparent induction phase (6h) for nitrate uptake by N-depleted dwarf bean (Phaseolus vulgaris L. ev. Witte Krombek). Experiments were done with intact plants and with detached root systems. Qualitatively and quantitatively, xylem exudation from detached roots was a bad estimate of the export of NO^?₃ or NO^?₃-¹⁵N from roots of intact plants. In vivo nitrate reductase activity (NRA) agreed well with in situ reduction, calculated as the difference between uptake and accumulation in whole plants, provided NRA was assayed with merely endogenous nitrate as substrate (‘actual’ NRA). The majority (75%) of the entering nitrate remained unmetabolized. Both nitrate reduction and nitrate accumulation occurred predominantly in the root system. Some (< 25%) of the root-reduced nitrate-N was translocated to the shoot. Nitrate uptake occurred against the concentration gradient between medium and root cells, and probably against the gradient of the electro-chemical potential of nitrate. Part of the energy expended for NO^?₃ absorption came from the tops, since decapitation and ringing at the stem base restricted nitrate uptake.     

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.     

Critical evaluation of thein situ nitrate reductase assay

Anin situ method, derived from anin vivo method, was used to determine nitrate reductase activity (NRA) in:i) excised barley and corn shoots and excised soybean leaves during a N-depletion experiment and; ii) roots and shoots of N-depleted barley and corn seedlings during induction of nitrate, reductase (NR). Nitrate reduction, calculated from thesein situ RNA measurements, was compared with estimates of each organ's nitrate reduction in light aerobic conditions from NO ₃ ⁻ consumption and a¹⁵N model (Gojonet al., 1986b). Thein situ RNA of roots strongly underestimated their¹⁵NO ₃ ⁻ reduction. In contrast, in barley and corn shoots and in the first trifoliolate leaves from 26-day-old, soybean, thein situ NRA assay gave a fair approximation of the true NO ₃ ⁻ reduction rate (relative differences ranging from −14 to +32%). In young soybean leaves (from 20-day-old plants), however, thein situ NRA strongly underestimated the actual NO ₃ ⁻ reduction. The physiological significance of thein situ NRA assay in shoots and roots, and its value for field studies are discussed from these results.     

The nitrogen balance of Raphanus sativus X raphanistrum plants. I. Daily nitrogen use under high nitrate supply

Abstract Growth-chamber cultivated Raphanus plants accumulate nitrate during their vegetative growth. After 25 days of growth at a constant supply to the roots of 1 mol m^?3 (NO^?₃) in a balanced nutrient solution, the oldest leaves (eight-leaf stage) accumulated 2.5% NO^?₃-nitrogen (NO₃-N) in their lamina, and almost 5% NO₃-N in their petioles on a dry weight basis. This is equivalent to approximately 190 and 400 mol^?3 m^?3 concentration of NO^?₃ in the lamina and the petiole, respectively, as calculated on a total tissue water content basis. Measurements were made of root NO^?₃ uptake, NO^?₃ fluxes in the xylem, nitrate uptake by the mesophyll cells, and nitrate reduction as measured by an in vivo test. NO^?₃ uptake by roots and mesophyll cells was greater in the light than in the dark. The NO^?₃ concentration in the xylem fluid was constant with leaf age, but showed a distinct daily variation as a result of the independent fluxes of root uptake, transpiration and mesophyll uptake. NO^?₃ was reduced in the leaf at a higher rate in the light than in the dark. The reduction was inhibited at the high concentrations calculated to exist in the mesophyll vacuoles, but reduction continued at a low rate, even when there was no supply from the incubation medium. Sixty-four per cent of the NO^?₃ influx was turned into organic nitrogen, with the remaining NO^?₃ accumulating in both the light and the dark.     

Induction of nitrate uptake and nitrate reductase activity in trembling aspen and lodgepole pine

¹³NO₃^– influx into the roots and in vivo nitrate reductase activity (NRA) in the roots and leaves have been measured in trembling aspen (Populus tremuloides Michx.) and lodgepole pine (Pinus contorta Dougl.) seedlings after exposure to either 0·1 or 1·5 mol m^–3 NO₃^– for varying periods up to 20 d. Both NO₃^– influx and NRA were inducible in these species and, in trembling aspen, peak induction of nitrate influx and NRA were achieved within 12 h, compared to 2–4 d for influx and 4–12 d for NRA in lodgepole pine. In trembling aspen, ≈ 30% of the total ¹³N absorbed during a 10 min influx period followed by 2 min of desorption was translocated to the shoot. In lodgepole pine, by contrast, translocation of ¹³N to the shoot was undetectable during the same time period. Root NRA as well as NO₃^– influx from 0·1 mol m^–3 NO₃^– were substantially higher in trembling aspen than in lodgepole pine at all stages of NO₃^– exposure, i.e. during the uninduced, the peak induction, and steady-state stages. In order to examine whether the lower rates of NO₃^– influx and NRA were related to proportionately fewer young (unsuberized) roots in lodgepole pine, we determined these parameters in young and old (suberized) roots of this species separately. Induction of influx and NRA were initially greater in young roots but at steady-state there were only minor differences between the young and the old roots. However, even the elevated initial rates in the young roots of lodgepole pine were substantially lower than those of aspen. In pine, influx at 1·5 mol m^–3 NO₃^– was ~ 6-fold higher than at 0·1 mol m^–3 NO₃^– and appeared to be mostly via a constitutive system. By contrast, in aspen, steady-state influxes at 0·1 and 1·5 mol m^–3 were not significantly different, being similar to the rate attained by pine at only the higher [NO₃^–]. In aspen, leaf NRA was ~ 2-fold higher than that of roots. In lodgepole pine NRA of the needles was below the detection limit. These results show that trembling aspen seedlings are better adapted for NO₃^– acquisition and utilization than lodgepole pine seedlings.     

The effect of calcium deficiency on nitrate absorption and assimilation in pumpkin plants

The absorption of nitrate and the activity of nitrate reductase were much lower in Ca-deficient plants ofCururbita pepo L., cv. ‘Kveta’ than in normal plants grown in complete nutrient solution for a period of 8 days. After the addition of nitrate to the nutrient medium, nitrate reductase activity in the roots of NO₃-deficient plants sharply rose during the first 6 h and then remained constant during the following 6 h; the content of endogenous NO₃ ^? rose slowly and continuously. These processes were depressed in (Ca, NO₃)-deficient plants independently of the addition of Ca²⁺ to the medium in the variant with NO₃ ^?. Thus it seems that the whole nitrogen metabolism,i.e. both NO₃ ^? absorption and the synthesis of nitrate reductase, is impaired in Ca-deficient plants.     

Humic acid differentially improves nitrate kinetics under low‐ and high‐affinity systems and alters the expression of plasma membrane H+‐ATPases and nitrate transporters in rice

Humic acids (HAs) have a major effect on nutrient uptake, metabolism, growth and development in plants. Here, we evaluated the effect of HA pretreatment applied with a nutrient solution on the uptake kinetics of nitrate nitrogen (N‐NO₃^?) and the metabolism of nitrogen (N) in rice under conditions of high and low NO₃^? supply. In addition, the kinetic parameters of NO₃^? uptake, N metabolites, and nitrate transporters (NRTs) and the plasma membrane (PM) H⁺‐ATPase gene expression were examined. The plants were grown in a growth chamber with modified Hoagland and Arnon solution until 21 days after germination (DAG), and they were then transferred to a solution without N for 48 h and then to another solution without N and with and without the addition of HAs for another 48 h. After this period of N deprivation, the plants received new nutrient solutions containing 0.2 and 2.0 mM N‐NO₃^?. Treatment of rice plants with HA promoted the induction of the genes OsNRT2.1‐2.2/OsNAR2.1 and some isoforms PM H⁺‐ATPase in roots. The application of HAs differentially modified the parameters of the uptake kinetics of NO₃^? under both concentrations. When grown with 0.2 mM NO₃^?, the plants pretreated with HA had lower K_m and C_min values as well as a higher V_max/K_m ratio. When grown with 2 mM NO₃^?, the plants pretreated with HA had a higher V_max value, a greater root and shoot mass, and a lower root/shoot ratio. The N fractions were also altered by pretreatment with HA, and a greater accumulation of NO₃^? and N‐amino was observed in the roots and shoots, respectively, of plants pretreated with HA. The results suggest that pretreatment with HA modifies root morphology and gene expression of PM H⁺‐ATPases and NO₃^? transporters, resulting in a greater efficiency of NO₃^? acquisition by high‐ and low‐affinity systems.     

Effects of soil applied herbicides on leaf nitrate reductase and crude protein in the leaf and seed of two cowpea cultivars

A glass-house study was conducted to determine the effects of four commonly used herbicides (pendimethalin, metobromuron, metolachlor and prometryne) applied pre-emergence at rates of 0, 0.125, 0.625 and 1.25 kg ha^–1, on leaf nitrate concentration (NO₃–C), nitrate reductase activity (NRA), leaf crude protein and seed protein in two cowpea cultivars, 60 day (60D) and Ife brown (IB).Control and treated plants of both cultivars showed separate peaks for NO₃–C and NRA, 49 days after planting (DAP) and 35 DAP for 60D and IB respectively. Herbicide treatment generally enhanced NO₃–C but tended to decrease NRA in both cultivars. Howver, metobromuron at 0.625 kg ha^–1 increased NRA throughout the growth period with an optimum increase of 52.5%, over the control, at 35 DAP. Pendimethalin increased NO₃–C NRA and leaf protein but did not influence seed protein appreciably. In contrast metobromuron increased NO₃–C, decreased NRA, but increased seed protein by 29.6% over the control at 0.125 kg ha^–1 in 60D. Metolachlor and prometryne were most inhibitory to seed protein development. In addition, metolachlor reversed the interdependence of NO₃–C and NRA.     

Nitrate reductase and nitrate accumulation in relation to nitrate toxicity in Boronia megastigma

Moderate levels of N were toxic to the native Australian plant boronia (Boronia megastigma Nees). As NO^-₃ is the major N form available for plants under cultivated conditions, NO^-₃ reduction and accumulation patterns in boronia were examined following the supply of various levels of NO^-₃ to understand the physiological basis of this toxicity. At a low level of supplied NO^-₃ [15 mmol (plant)^-1], NO^-₃ was reduced without any detectable accumulation and without nitrate reductase activity (NRA) reaching its maximum capacity. When higher NO^-₃ levels [≥25 mmol (plant)^-1] were supplied, both NRA and NO^-₃ accumulation increased further. However, NRA increased to a maximum of ca 500 nmol NO^-₃ (g fresh weight)^-1 h^-1, both in the roots and leaves, irrespective of a 4-fold difference in the levels of supplied NO^-₃, whereas NO^-₃ continued to accumulate in proportion to the level of supplied NO^-₃. Chlorotic toxicity symptoms appeared on the leaves at an accumulation of ca 32 μmol NO^-₃ (g fresh weight)^-1. High endogenous NO^-₃ concentrations inhibited NRA. The low level of NRA in boronia was not limited by NO^-₃ or electron donor availability. It is concluded that the low NR enzyme activity is a genetic adaptation to the low NO^-₃ availability in the native soils of boronia. Thus, when NO^-₃ supply is high, the plat cannot reduce it at high rates, leading to large and toxic accumulations of the ion in the leaf tissues.     

The influence of enriched rhizosphere CO2 on N uptake and metabolism in wild-type and NR-deficient barley plants

Positive influences of high concentrations of dissolved inorganic carbon (DIC) in the growth medium of salinity-stressed plants are associated with carbon assimilation through phosphoenolpyruvate carboxylase (PEPc) activity in roots; and also in salinity-stressed tomato plants, enriched CO₂ in the rhizosphere increases NO^?₃uptake. In the present study, wild-type and nitrate reductase-deficient plants of barley (Hordeum vulgare L. cv. Steptoe) were used to determine whether the influence of enriched CO₂ on NO^?₃ uptake and metabolism is dependent on the activity of nitrate reductase (NR) in the plant. Plants grown in NH₄⁺and aerated with ambient air, were transferred to either NO₃^? or NH₄⁺ solutions and aerated with air containing between 0 and 6 500 μmol mol^?1 CO₂. Nitrogen uptake and tissue concentrations of NO₃^? and NH₄⁺ were measured as well as activities of NR and PEPc. The uptake of NO^?₃ by the wild-type was increased by increasing CO₂. This was associated with increased in vitro NR activity, but increased uptake of NO₃^? was found also in the NR-deficient genotype when exposed to high CO₂ concentrations; so that the influence of CO₂ on NO₃^? uptake was independent of the reduction of NO₃^? and assimilation into amino acids. The increase in uptake of NO₃^? in wild-type plants with enriched CO₂ was the same at pH 7 as at pH 5, indicating that the relative abundance of HCO₃^? or CO₂ in the medium did not influence NO₃^? uptake. Uptake of NH₄⁺ was decreased by enriched CO₂ in a pH (5 or 7) independent fashion. Thus NO₃^? and NH⁺₄ uptakes are influenced by the CO₂ component of DIC independently of anaplerotic carbon provision for amino acid synthesis, and CO₂ may directly affect the uptake of NO₃^? and NH₄⁺ in ways unrelated to the NR activity in the tissue.     

Translocation of nitrogen in osmotically stressed wheat seedlings

Wheat (Triticum aestivum L., cv. Drabant) seedlings were grown in a ‘split root’ system where either the whole root system or one root half was subjected to osmotic stress for 24 h, using 200 g polyethylene glycol (PEG, molecular weight 4000) dm^?3 nutrient solution. ¹⁵N-Labelled nitrate was fed to one of the root compartments and total N and ¹⁵N-labelling were measured in plant material and xylem sap. Untreated plants translocated 87% of the N taken up to the shoot, and 10% of this was then retranslocated back to the root. Recalculated on a root nitrogen basis, 36% of the label recovered in the root after 24 h had passed through the shoot. Significant labelling of xylem sap collected from non-labelled roots indicated cycling of organic N through the roots. PEG-treatment of the whole root system caused significant water loss in both roots and shoots. Uptake of nitrate and retranslocation of N to roots were inhibited, whereas cycling of organic nitrogen through the root was still measurable. Treatment of half the root system with PEG had minor effects on shoot water content, but reduced the water content of the treated root part. The total uptake of nitrate by the root system was unaffected, and the effect on the treated root half was comparatively small. Nitrate reductase activity (NRA) declined in PEG-treated roots even if high nitrate uptake rates were maintained. Shoot NRA was unaffected by osmotic stress. The data indicate that the reduction in water content of the root per se has only small effects on nitrate uptake. Major inhibition of nitrate uptake was observed only after treatment of a sufficiently large portion of the root system to given an effect on shoot water content.     

Differences in nitrate and ammonium uptake between Scots pine and European larch

In forest soils, ammonium is usually the predominant form of inorganic nitrogen. However, the capacity of trees to utilize both NO₃ ^- and NH₃ ⁺ may provide greater flexibility in responding to changes of nitrogen supply from the environment. Such capacity has been studied in seedlings of Scots pine (Pinus sylvestris L.) and European larch (Larix decidua Mill.) grown in the presence or absence of either nitrate or ammonium. Nitrate-induced plants showed a higher nitrate uptake rate than non-induced plants; this difference was almost negligible after 24 h of exposure to NO₃ ^-. Ammonium uptake in both species was consistently higher than that of nitrate, regardless of prior nitrogen provision. In both nutrient conditions, larch showed a more efficient transport system in comparison with Scots pine, with higher ammonium and nitrate uptake rates in both induced and non-induced plants. This was consistent also with the activity of nitrate reductase, measured in vivo in roots and leaves. This revised version was published online in June 2006 with corrections to the Cover Date.     

Nitrogen Fixation and Nitrogen Assimilation in a Temperate Saline Ecosystem

As nitrogen is known to be a limiting factor for plant growth, we were interested in the relationship between soil microbial activity and the nitrogen assimilation of 5 different halophytes from 4 saline sites near the lake “Neusiedlersee”, Austria. The following were studied between May and October 1985: nitrogen fixation (¹⁵N₂ and acetylene reduction): N-mineralization; several soil characteristics and in vivo nitrate reductase activity of roots and shoots of these plants. NO^?₃, org. N- and carboxylate contents of both roots and shoots, as well as the effect of NO^?₃-fertilization on the amounts of these substances, were determined on plants growing in the field during a 3-day period in September 1985. Fertilization led to a decrease in acetylene reduction activity at most sites, and an increase in the nitrate reductase activity of the shoots of all plants. Overall, carboxylate and organic nitrogen contents of these halophytes did not change in response to fertilization. Only in the roots of Aster tripolium and Atriplex hastata was there a marked increase in the nitrate reductase activity in response to fertilization. Species growing at the same site, such as Plantago maritima and Lepidium crassifolium showed contrasting levels of assimilatory activity. Apparent low rates of ammonification and nitrification were detected in soils from the 4 sites. The results are discussed in relation to the nitrogen and carbon economies of the microorganisms and plants.     

ISOLATION AND PARTIAL CHARACTERIZATION OF NITRATE ASSIMILATION MUTANTS OF DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE)1

Fifteen nitrate assimilation-deficient mutants of the euryhaline green alga, Dunaliella tertiolecta Butcher were selected by their chlorate resistance. Ten mutants, unable to grow on NO₃^? but able to grow on NO₂^?, had no detectable nitrate reductase activity. Five mutants, unable to grow on either NO₃^? or NO₂^?, had depressed levels of both nitrate and nitrite reductase. A method for assaying methyl viologen-nitrate reductase in the presence of nitrite reductase is described.     

Effects of sodium on mineral nutrition in rose plants

The effects of sodium (Na⁺) ion concentration on shoot elongation, uptake of ammonium (NH₄⁺) and nitrate (NO₃^?) and the activities of nitrate reductase (NR) and glutamine synthetase (GS) were studied in rose plants (Rosa hybrida cv. “Lambada”). The results showed that shoot elongation was negatively correlated with sodium concentration, although no external symptoms of toxicity were observed. Nitrate uptake decreased at high sodium levels, specifically at 30 meq litre4 of sodium. As flower development was normal under high saline conditions, this could suggest that nitrogen was being mobilised from shoot and leaf reserves. Ammonium uptake was not affected by any of the salt treatments applied probably because it diffuses through the cell membrane at low concentrations. Nitrate reductase activity was reduced by 50% at 30 meq litre 1 compared with control treatment, probably due to a decrease in the free nitrate related to nitrate uptake pattern. None of the salt treatments used affected total leaf GS activity (both chloroplastic and cytosolic isoforms) or leaf NPK mineral contents. Nitrate reductase activity in leaves increased at 10 meq litre^?1 of sodium and GS activity in roots (cytosolic isoform only) followed the same pattern as NR. It is suggested that the activation of both enzymes at low salt level could be attributed to the beneficial effect of increased sulphur in the nutrient solutions.     

Nitrate reductase activity and nitrogen compounds in xylem exudate of Juglans nigra seedlings: relation to nitrogen source and supply

Nitrogen (N) limits plant productivity and its uptake and assimilation may be regulated by N source, N availability, and nitrate reductase activity (NRA). Knowledge of how these factors interact to affect N uptake and assimilation processes in woody angiosperms is limited. We fertilized 1-year-old, half-sib black walnut (Juglans nigra L.) seedlings with ammonium (NH₄ ⁺) [as (NH₄)₂SO₄], nitrate (NO₃ ⁻) (as NaNO₃), or a mixed N source (NH₄NO₃) at 0, 800, or 1,600 mg N plant⁻¹ season⁻¹. Two months following final fertilization, growth, in vivo NRA, plant N status, and xylem exudate N composition were assessed. Specific leaf NRA was higher in NO₃ ⁻-fed and NH₄NO₃-fed plants compared to observed responses in NH₄ ⁺-fed seedlings. Regardless of N source, N addition increased the proportion of amino acids (AA) in xylem exudate, inferring greater NRA in roots, which suggests higher energy cost to plants. Root total NRA was 37% higher in NO₃ ⁻-fed than in NH₄ ⁺-fed plants. Exogenous NO₃ ⁻ was assimilated in roots or stored, so no difference was observed in NO₃ ⁻ levels transported in xylem. Black walnut seedling growth and physiology were generally favored by the mixed N source over NO₃ ⁻ or NH₄ ⁺ alone, suggesting NH₄NO₃ is required to maximize productivity in black walnut. Our findings indicate that black walnut seedling responses to N source and level contrast markedly with results noted for woody gymnosperms or herbaceous angiosperms.     

Sink-stimulated photosynthesis and sink-dependent increase in nitrate uptake: nitrogen and carbon relations of the parasitic association Cuscuta reflexa–Ricinus communis

Ricinus communis L. was grown under limiting N supply in quartz sand culture, fed with 0.2, 1 or 5 mol m^?3 NO₃^?, or in liquid culture with 0.022, 0.05 or 0.5 mol m^?3 NO₃^?. Some of the plants were infected with Cuscuta reflexa Roxb. As occurred for the host, dry matter production and growth of C. reflexa were severely depressed with decreasing N supply to the host. When parasitized by C. reflexa, the shoot and root dry weight of Ricinus was diminished at all levels of N nutrition, but the total dry weight of host plus parasite was almost the same as that of uninfected Ricinus. In contrast to the situation in Lupinus albus (Jeschke et al. 1994b), infection by Cuscuta resulted in increased tissue N levels in the host and the N content of the system Ricinus plus C. reflexa was the same or even somewhat larger than that of uninfected plants. This indicated a sink-dependent stimulation of nitrate uptake. As a result of decreased root weights, nitrate uptake g^?1 FW was stimulated by 80, 60 or only 40% at 0.2, 1 or 5 mol m^?3 nitrate supply. Increased nitrate uptake was reflected, particularly at low N supply, in xylem transport; xylem sap nitrate concentrations were substantially elevated, while those of amino acids were decreased in parasitized plants. This indicated an inhibition of nitrate assimilation in roots of parasitized plants under limiting N supply. Besides these effects on N relations, C. reflexa induced a substantial sink-dependent stimulation of net photosynthesis in host leaves and a concomitant increase in stomatal opening and transpiration. This stimulation depended on the relative sink size induced by Cuscuta, on nitrogen nutrition and on leaf age, indicating that delayed senescence of leaves contributes to the overall effects of Cuscuta on its host. The Cuscuta-induced inhibition of nitrate assimilation in the roots and the increase in nitrate uptake suggest that nitrate reduction was shifted towards the leaves in the presence of C. reflexa. The stimulating effects of C. reflexa in the Ricinus-Cuscuta association are compared with the strongly inhibitory effects occurring in the tripartite association L. albus–Rhizobium–Cuscuta reflexa.     

Effect of Previous Nitrate Deprivation on 15N-nitrate Absorption and Assimilation by Wheat Seedlings

Nitrate uptake and assimilation were examined in intact 18 days old wheat (Triticum aestivum, cv Capitole) seedlings either permanently grown on nitrate (high-N seedlings) or N-stressed by transfer to an 0 N-solution for the final 7 days (low-N seedlings). The N-stressed seedlings were characterized by a lower organic N content (2.5 mg instead of 4.9 mg per seedling) and an increased root dry weight.The seedlings received ¹⁵NO₃K for 7 h in the light. Nitrate uptake was 2.8 times higher in low-N than in high-N seedlings. The assimilation rate was 35 and 16 μmol NO₃^?·^h?1· g^?1 dry weight respectively. Partitioning of NO₃^? to reduction and assimilation was the very same in both kinds of seedlings. The results support the view that 50 % of the nitrate reduction in Triticum aestivum, cv Capitole could be achieved in the roots.The present observations are interpreted as evidence that factors closely associated with the seedling N-status may have a major role in regulating NO₃^? uptake and assimilation. In low-N seedlings, the high amount of carbohydrates in roots may add its stimulus to the specific inducing effect of nitrate whereas in high-N seedlings, excess of nitrate or amino-acids may set the pace by negative feedback control.     

Nitrate regulation of nitrate uptake and nitrate reductase expression in barley grown at different nitrate:ammonium ratios at constant relative nitrogen addition rate

Barley (Hordeum vulgare L. cv. Golf) was cultured using the relative addition rate technique, where nitrogen is added in a fixed relation to the nitrogen already bound in biomass. The relative rate of total nitrogen addition was 0.09 day^?1 (growth limiting by 35%), while the nitrate addition was varied by means of different nitrate: ammonium ratios. In 3- to 4-week-old plants, these ratios of nitrate to ammonium supported nitrate fluxes ranging from 0 to 22 μmol g^?1 root dry weight h^?1, whereas the total N flux was 21.8 ± 0.25 μmol g^?1 root dry weight h^?1 for all treatments. The external nitrate concentrations varied between 0.18 and 1.5 μM. The relative growth rate, root to total biomass dry weight ratios, as well as Kjeldahl nitrogen in roots and shoots were unaffected by the nitrate:ammonium ratio. Tissue nitrate concentration in roots were comparable in all treatments. Shoot nitrate concentration increased with increasing nitrate supply, indicating increased translocation of nitrate to the shoot. The apparent V_max for net nitrate uptake increased with increased nitrate fluxes. Uptake activity was recorded also after growth at zero nitrate addition. This activity may have been induced by the small, but detectable, nitrate concentration in the medium under these conditions. In contrast, nitrate reductase (NR) activity in roots was unaffected by different nitrate fluxes, whereas NR activity in the shoot increased with increased nitrate supply. NR-mRNA was detected in roots from all cultures and showed no significant response to the nitrate flux, corroborating the data for NR activity. The data show that an extremely low amount of nitrate is required to elicit expression of NR and uptake activity. However, the uptake system and root NR respond differentially to increased nitrate flux at constant total N nutrition. It appears that root NR expression under these conditions is additionally controlled by factors related to the total N flux or the internal N status of the root and/or plant. The method used in this study may facilitate separation of nitrate-specific responses from the nutritional effect of nitrate.