Effect of ammonium nutrition on the activity of nitrate reductase in the roots of apple seedlings

Active extracts of nitrate reductase were prepared from theroots of apple seedlings c.v. Granny Smith which were grownin nutrient solution under controlled enviromental conditions.The nutrient solutions contained various ratios of nitrate andammonium ions but all the treatments contained a total of 112ppm nitrogen. Maximum nitrate reductase activity in the roots was obtainedwhen plants were supplied with nitrate as the sole source ofnitrogen. Roots grown in solution containing only ammonium nitrogenhad little or no activity. When plants were supplied with bothforms of nitrogen in the nutrient solution, the presence ofammonium ions markedly lowered the activity of nitrate reductasein the roots. Plants supplied with 98 ppm nitrate nitrogen plus14 ppm ammonium nitrogen had activities only half those of plantsgrown in nitrate alone. Plants supplied with equal amounts ofammonium and nitrate nitrogen had activities less than one sixththose of plants grown in nitrate alone. (Received June 3, 1972; )     

Growth, water use efficiency, and proline content of hydroponically grown tomato plants as affected by nitrogen source and nutrient concentration

Tomato plants (Lycopersicon esculentum Mill. cv. Counter) were grown in 12-L polyethylene containers in aerated and CaCO₃-buffered nutrient solutions containing different concentrations of complete stock solutions with either nitrate (stock solution N) or ammonium (stock solution A) as the only nitrogen source (X1 = standard concentration with 5 mM NO₃ ^–-N or NH₄ ⁺-N, and X3, X5.5, X8 and X11 = 3, 5.5, 8, 11 times the standard concentration), or a mixture of both stock solutions (N:A ratio = 100:0, 75:25, 50:50, 25:75, 0:100) at moderate nutrient concentration (X3). Total dry matter production and fruit dry weight were only slightly affected by increasing nutrient concentration if nitrate was supplied as the sole nitrogen source. Compared to nitrate, ammonium nitrogen caused a decrease in total dry weight (32–86% between X1 and X11), but led to an increase in both total dry weight and fruit dry weight (11% and 30%) at low concentration if supplied in addition to nitrate nitrogen (N:A ratio = 75:25). Dry matter partitioning in plants was affected by the strength of the nutrient solution, but even more by ammonium nitrogen. Fruits accumulated relatively less dry matter than did the vegetative parts of tomato plants when supplied with nutrient solutions containing ammonium as the only nitrogen source (fruit dry weight to total dry weight ratio 0.37 and 0.15 at low and high nutrient concentration), while nitrate nitrogen rather supported an increase in dry matter accumulation in the reproductive organs (fruit dry weight to total dry weight ratio 0.39–0.46). The water use efficiency (WUE) was only slightly affected by the strength of the nutrient solutions containing nitrate nitrogen (2.9–3.4 g DW (kg H₂O)^–1), while ammonium nitrogen led to a decrease in WUE from 2.4 to 1.3 g DW (kg H₂O)^–1at low (X1) and high (X11) nutrient concentration, respectively. The proline content of leaves fluctuated (0.1–5.0 mol (g fresh weight)^–1) according to nutrient concentration and global radiation, and reflected enhanced sensitivity of plants to these potential stress factors if ammonium was the predominant N source supplied. It was concluded, that proline is a reliable indicator of the environmental stress imposed on hydroponically grown tomato plants.     

Importance of seed Zn content for wheat growth on Zn-deficient soil

Eggplants (Solanum melongena L. cv. Bonica) were grown in a glasshouse during summer under natural light with one unbranched shoot or one shoot with 3 to 4 branches and with or without fruit in quartz sand buffered and not buffered with 0.5% CaCO₃ (w : v), respectively. Nutrient solutions supplied contained nitrate or ammonium as the sole nitrogen source. Compared with nutrient solutions containing nitrate (10 mM), solutions containing ammonium (10 mM) caused a decrease in net photosynthesis of eggplants during early stages of vegetative growth when grown in quartz sand not buffered with CaCO₃. The decrease was not observed before leaves showed interveinal chlorosis. In contrast, net photosynthesis after bloom at first increased more rapidly in eggplants supplied with ammonium than with nitrate nitrogen. However, even in this case, net photosynthesis decreased four weeks later when ammonium nutrition was continued. The decrease was accompanied by epinasty and interveinal chlorosis on the lower leaves and later by severe wilting, leaf drop, stem lesions, and hampered growth of stems, roots, and fruits. These symptoms appeared later on plants not bearing fruits than on plants bearing fruits. If nutrient solutions containing increasing concentrations of ammonium (0.5–30 mM) were supplied after the time of first fruit ripening, shoot growth and set of later flowers and fruits were promoted. In contrast, vegetative growth and reproduction was only slightly affected by increasing the concentration of nitrate in the nutrient solutions. In quartz sand buffered with CaCO₃ ammonium nutrition caused deleterious effects only under low light conditions (shade) and on young plants during rapid fruit growth. If eggplants were supplied with ammonium nitrogen before bloom, vegetative growth was promoted, and set of flowers and fruit occurred earlier than on plants supplied with nitrate. Furthermore, the number of flowers and fruit yield increased. These effects of ammonium nutrition were more pronounced when plants were grown with branched shoots than with unbranched shoots. The results indicate that vegetative and reproductive growth of eggplants may be manipulated without causing injury to the plants by supplying ammonium nitrogen as long as the age of the plants, carbohydrate reserves of the roots, quantity of ammonium nitrogen supplied, and pH of the growth medium are favourable. T W Rufty Section editor     

The Influence of Urea and Other Nitrogen Sources on Growth Rate of Scots Pine Seedlings

The growth rate and water content of urea-fed seedlings of Pinus silvestris L. were compared with those of nitrate-and ammonium-fed seedlings grown in continuously renewed nutrient solutions, in which the hydrolysis of urea to ammonia and carbon dioxide was minimized. The growth rate of seedlings grown in an ammonium nutrient solution, in an urea nutrient solution and in a nitrate nutrient solution was about 90 per cent, 75 per cent and 60 per cent, respectively, of that of seedlings grown in a mixture of ammonium and nitrate. Seedlings with urea as the sole nitrogen source developed very severe chlorosis of the needles, the old roots were dark-coloured, the whole root system was very fragile, and the lateral roots of the third order were missing. Urea-grown seedlings had the highest nitrogen contents, closely followed by the ammonium and the ammonium + nitrate seedlings. The lowest nitrogen level was in nitrate seedlings. The low growth rate and the chlorosis of urea-fed seedlings were suggested to be the result of a hydrolysis of urea inside the root, causing an increase in pH and an accumulation of ammonia in the root.     

Ammonium Nutrition Enhances Chlorophyll and Glaucousness in Kohlrabi

Kohlrabi (Brassica oleracea var.gongylodes) plants were grownin the greenhouse under autumn conditions and fertilized eitherwith pellets containing nitrogen as 40% ammonium sulphate and60% urea or with nutrient solution containing nitrogen predominantlyas nitrate. Plants given nitrogen as ammonium ions developedglaucous leaves compared to those supplied with nitrate whichformed glossy leaves. Ammonium-induced glaucousness was theresult of a two-fold increase in the amount of epicuticularwax and a markedly altered fine structure. Leaves from ammoniumfertilized kohlrabi plants also showed a 21% increase in chlorophyllcontent together with a reduction in the chlorophyll a:b ratioand decreased ground state fluorescence compared to plants suppliedwith nitrate. Photosynthesis and stomatal transpiration wereunaffected by the form of supplied nitrogen. Brassica oleracea ; chlorophyll; chlorophyll fluorescence; epicuticular wax; glaucousness; photosynthesis; transpiration     

Ammonium and nitrate uptake by Eucalyptus nitens: effects of pH and temperature

Ammonium and nitrate uptake by roots of Eucalyptus nitens was characterised with respect to pH and temperature. Uptake of ammonium and nitrate was measured as depletion from solutions by roots of intact 11 week old solution-cultured seedlings. Uptake rates of ammonium were consistently higher than those of nitrate in all experiments. Uptake rates for ammonium were 200% higher at pH 4 than at pH 6, but for nitrate were unchanged. Uptake rates of ammonium and nitrate were both reduced to a similar extent (70%) with a decrease in temperature from 20 °C to 10 °C. For ammonium uptake, there was rapid (<24 hr) adaptation to a reduction in root temperature. The apparent preference shown here for ammonium over nitrate could be indicative of E. nitens growing in cold, acidic forest soils where ammonium is commonly more available than nitrate. These results suggest that N uptake rates of E. nitens may be maximised under a wide variety of conditions if N is supplied predominantly in the ammonium form. This revised version was published online in June 2006 with corrections to the Cover Date.     

Contamination of Ammonium-Based Nutrient Solutions by Nitrifying Organisms and the Conversion of Ammonium to Nitrate

Conversion of ammonium to nitrate and contamination by nitrifying organisms are often assumed not to be significant in ammonium-based nutrient solutions. To assess this assumption, maize (Zea mays) and pea (Pisum sativum) were grown under greenhouse conditions in aeroponic, hydroponic, and sand-culture systems containing 2 mM ammonium chloride as the sole nitrogen source and evaluated for the activity of contaminating nitrifying organisms. In all three culture systems, root colonization by nitrifying organisms was detected within 5 d, and nitrate was detected in the nutrient solution within 10 d after seedling transfer. In sand culture, solution nitrate concentration reached 0.35 mM by the end of the 17-d experiment. Consistent with the microbial ammonium oxidation sequence, nitrite was detected earlier than nitrate and remained at lower levels throughout the experiment. Nitrate was found in significant quantities in root and shoot tissues from seedlings grown in ammonium-based nutrient solutions in all of the solution culture systems. Maize seedlings grown in an ammonium-based hydroponic system contained nitrate concentrations at 40% of that found in plants grown in nitrate-based solution. Determination of nitrate (or nitrite) levels in the nutrient solution was the weakest indicator of the activity of nitrifying organisms. A bioassay for the presence of nitrifying organisms in combination with tissue analysis for nitrate was a better indicator of microbial conversion of ammonium to nitrate in nutrient solution culture. The results have implications for the use of ammonium-based nutrient solutions to obtain plants suitable for research on induction of nitrate uptake and reduction or for research using solution culture to compare ammonium versus nitrate fertilization.     

Mineral Nutrient Requirements of Vaccinium vitis idaea and V. myrtillus

The mineral nutrient requirements of Vaccinium vitis idaea L. and V. myrtillus L. have been studied. It is found that the optimum nutrient proportions are similar to those of birch seedlings except for a relatively low potassium requirement. The nitrogen sources ammonium and nitrate are both usable at low concentrations. A system with both sources present seems preferable. The ammonium uptake is then much more rapid than the nitrate uptake. The optimum total concentration falls within the low salt range and is about the same as found for birch seedlings, but the sensitivity to high salt concentrations is more pronounced in the Vaccinium species. A very marked characteristic of the nutrient absorption is the rapid calcium uptake. The results are discussed in relation to similar experiments with birch and cucumber seedlings, representing species with a wide or calcareous ecological adaptation.     

Root Development and Absorption of Ammonium and Nitrate from the Rhizosphere

Plant roots operate in an environment that is extremely heterogeneous, both spatially and temporally. Nonetheless, under conditions of limited diffusion and against intense competition from soil microorganisms, plant roots locate and acquire vital nitrogen resources. Several factors influence the mechanisms by which roots respond to ammonium and nitrate. Nitrogen that is required for cell division and expansion derives primarily from the apex itself absorbing rhizosphere ammonium and nitrate. Root density and extension are greater in nutrient solutions containing ammonium than in those containing nitrate as the sole nitrogen source. Root nitrogen acquisition alters rhizosphere pH and redox potential, which in turn regulate root cell proliferation and mechanical properties. The net result is that roots proliferate in soil zones rich in nitrogen. Moreover, plants develop thinner and longer roots when ammonium is the primary nitrogen source, an appropriate strategy for a relatively immobile nitrogen form. Present address of Alison R. Taylor: The Marine Biological Association, The Laboratory, Citadel Hill, Plymouth PL1 2PB, UK.     

Mineral Nutrient Requirements of Pinus silvestris and Picea abies Seedlings

The mineral nutrient requirements of Pinus silvestris L. and Picea abies Karst. were studied according to previously published methods applied to a series of various plant species. The optimum nutrient proportions are similar to those of Vaccinium, with a lower relative potassium requirement than birch and other broad-leaf species. Various ratios between ammonium and nitrate nitrogen were about equally efficient except for a minor growth reduction with pure nitrate, which gave a comparatively low nitrogen content and a high cation/nitrogen ratio. The rate of ammonium uptake was much higher than that of nitrate when both sources were supplied. The required total concentration in the nutrient solution for maximum growth is lower in pine than in spruce, but both fall within the low salt range. Both species, especially pine, are sensitive to high salt concentrations. Although pine and spruce grow on the same type of soils as Vaccinium— leached soils with low base saturation – accumulation of calcium or other cations is not as pronounced as in Vaccinium, especially not in pine. The results are compared with results from similar experiments with a series of other conifers. All the conifers have more flexible cation uptake mechanisms than Vaccinium but the results indicate tendencies to accumulation of anions, nitrate and phosphate. Recommended fertilizer compositions for forest fertilization and nurseries are discussed.     

Does the Response of Perennial Ryegrass to Elevated CO2 Concentration Depend on the Form of the Supplied Nitrogen?

To test whether different nitrogen form (nitrate or ammonium) in substrate can alter the response to elevated partial pressure of CO₂ (pCO₂) plants of perennial ryegrass (Lolium perenne cv. Bastion) were grown from seeds in growth chambers under pCO₂ of either 35 Pa (ambient, CA) or 70 Pa (elevated, CE) in a hydroponic system (with nutrient and pH control) for 24 d. Nitrogen was supplied as ammonium, nitrate or an equimolar mixture of both N forms. Under CE plants grew faster than their counterparts under CA during the first 14 d but after 23 d of cultivation stimulation disappeared. Despite the strong positive effect of mixed forms of N on plant growth, the beneficial effect of CE was similar to that in the other two N treatments. However, the almost alike final growth response to CE had different underlying mechanisms in different N treatments. Plants supplied with nitrate as a sole source of nitrogen had lower leaf mass ratio but much higher specific leaf area compared to plants supplied with ammonium. The decrease in the content of leaf organic N (per unit of structural dry mass) under CE was found only in leaves of plants supplied with ammonium on day 14. Nevertheless, the available form of N evidently contributes to changes of leaf N content under CE. The high levels of N and non-structural saccharides in plants supplied with ammonium at CE suggest that the CO₂ response of these plants was controlled by factors other than amount of available carbon and nitrogen.     

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.     

Comparison of acetylene-reduction and nitrogen-15 techniques for the determination of nitrogen fixation by field bean (Vicia faba) nodules

Field bean (Vicia faba L.) cv. Maris Bead seeds were inoculated with Rhizobium Catalogue No. 1001, supplied by Rothamsted Experimental Station, and grown in sand culture supplied with a complete nutrient solution which included nitrate at either 1.5 or 6.0 mM. Nodules were detached from the roots at intervals during plant development and their rates of nitrogen fixation estimated by both acetylene reduction and ¹⁵N gas technique. There was a constant relationship, independent of nitrate supply, between the results obtained by these two methods at all samplings. The amounts of acetylene reduced divided by a factor of 5.75 gave the amount of true nitrogen fixation; this factor is about twice the theoretical value. It is suggested that this discrepancy arose because, with acetylene, all the electrons available to the nitrogenase were used to form ethylene, whereas during normal fixation only about half the electron supply was used to fix nitrogen, the remainder having been consumed in the production of hydrogen gas.     

Ammonium and urea as nitrogen sources for bromeliads

Epiphytic bromeliads have no contact with the pedosphere, so they need to draw their nutrients from the atmosphere as well as from the host tree and animal debris. Terrestrial bromeliads, like Ananas comosus, generally depend on the soil as their main nutrient source. The aim of this study was to investigate and compare some aspects of the nitrogen metabolism of two bromeliads with different growth habits: Ananas comosus, a terrestrial bromeliad, and Vriesea gigantea, an epiphytic tank bromeliad. Nitrogen-starved plants were grown in vitro for 3, 7, 15, 30, and 60 days, either with 5 mmol L⁻¹ ammonium [(NH₄)₂SO₄] or urea as the sole nitrogen source. When NH₄⁺ was supplied to the plants, it stimulated a faster increase of chlorophyll content in A. comosus than in V. gigantea. In the presence of urea, after 15 days of the plants in culture, there was a significant increase in tissue free-NH₄⁺ and total amino acids for V. gigantea only. V. gigantea presented a higher level of total free amino acids than A. comosus when nitrogen was supplied to the plants. Asparagine was the main amino acid accumulated in both bromeliads when plants were transferred to the medium with nitrogen. When the ratio of the main individual free amino acids between the bromeliads grown in NH₄⁺ and urea was compared, values such as 7.2 for asparagine, 5.3 for glutamate, and 1.8 for aspartate in A. comosus, and values such as 2.3 for asparagine, 1.1 for glutamate and 0.7 for aspartate in V. gigantea were observed, demonstrating that the last is more efficient in assimilating urea. The results prompted us to support the idea that V. gigantea, an epiphytic tank bromeliad, is better adapted to absorb and assimilate organic nitrogen, such as urea, while A. comosus, a terrestrial plant, is better adapted to inorganic nitrogen forms, such as ammonium. The natural exposure of tank bromeliads to urea is discussed in the paper.     

Nitrogen and phosphorus uptake in seedlings of the seagrass Amphibolis antarctica in Western Australia

The uptake of nitrate, ammonium and phosphate was examined in vitro in seedlings of the seagrass Amphibolis antarctica ((Labill.) Sonder ex Aschers.). Uptake of all three nutrients was significantly correlated with external concentration up to 800 µ g l^–1. The uptake of nitrate (0–200 µ g NO₃-N g dry wt^–1 h^–1) was significantly lower than the uptake of ammonium (0–500 µ g NH₄-N g dry wt^–1 h^–1), suggesting that the seedlings have a higher affinity for this form of nitrogen in the water column.Data were in general agreement with uptake rates recorded for other seagrasses, notably Zostera marina. In comparison to the dominant macroalgae for the same region, seedlings had either similar or higher uptake rates in relation to external concentration, lending support to the hypothesis that seedlings, which do not possess roots, behave like macroalgae in terms of nutrient acquisition from the water column.A comparison with literature data on adult seagrass suggests, however, that seagrasses show lower uptake rates than macroalgae suggesting that the macroalgae, which are totally reliant on the water column for nutrients, are more efficient at uptake than seagrasses which may potentially use the sediment for a nutrient source.     

Differential Effects of Ammonium and Nitrate on Growth Performance of <i>Glechoma longituba</i> under Heterogeneous Cd Stress

Water, minerals, nutrients, etc., can be shared by physiological integration among inter-connected ramets of clonal plants. Nitrogen plays an important role in alleviating cadmium (Cd) stress for clonal plants. But how different forms of nitrogen affect growth performance of clonal plants subjected to heterogeneous Cd stress still remains poorly understood. A pot experiment was conducted to investigate the differential effects of ammonium and nitrate on growth performance of Glechoma longituba under heterogeneous Cd stress. In the experiment, parent ramets of Glechoma longituba clonal fragments were respectively supplied with modified Hoagland solution containing 7.5 mM ammonium, 7.5 mM nitrate or the same volume of nutrient solution without nitrogen. Cd solution with different concentrations (0, 0.1 or 2.0 mM) was applied to offspring ramets of the clonal fragments. Compared with control (N-free), nitrogen addition to parent ramets, especially ammonium, significantly improved antioxidant capacity [glutathione (GSH), proline (Pro), peroxidase (POD,) superoxide dismutase (SOD) and catalase (CAT)], PSII activity [maximum quantum yield of PSII (F_v/F_m) and effective quantum yield of PSII (ΦPSII)], chlorophyll content and biomass accumulation of the offspring ramets suffering from Cd stress. In addition, negative effects of nitrate on growth performance of whole clonal fragments were observed under Cd stress with high concentration (2.0 mM). Transportation or sharing of nitrogen, especially ammonium, can improve growth performance of clonal plants under heterogeneous Cd stress. The experiment provides insight into transmission mechanism of nitrogen among ramets of clonal plants suffering from heterogeneous nutrient supply. Physiological integration might be an important ecological strategy for clonal plants adapting to heterogeneous environment stress conditions.     

Effects of atrazine on the assimilation of inorganic nitrogen in cereals

The inclusion of sub-lethal amounts ofthe herbicide atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] in the nutrient solution supplied to maize and barley increased the growth of the root and shoot and the uptake of nitrate. The activities of nitrate and nitrite reductases, glutamine synthetase and glutamate synthase were enhanced and the amino acid and nitrate contents of the xylem sap increased. All these effects of atrazine were found only in plants grown with nitrate as the nitrogen source. The uptake of ¹⁵NO₃^? and its incorporation into protein in the root and shoot of maize and barley seedlings was significantly greater in the atrazine treated plants. However, a stimulation in the incorporation of leucine-[¹⁴C] into TCA-precipitable protein of detached leaves from 7-day-old barley seedlings was obtained only in the absence of a supply of combined nitrogen either in the culture medium or in the in vitro incubation mixture containing the labelled amino acid.     

Formation of aerenchyma in roots of Zea mays in aerated solutions, and its relation to nutrient supply

The formation of lysigenous cavities (aerenchyma) in the root cortex of maize, Zea mays L. cv. Capella, under well-aerated conditions has been studied in relation to the composition of the nutrient solutions. Nitrogen, either supplied as nitrate or as ammonium, reduced the cavity formation by the roots. This reduction was most apparent at nitrate concentrations above 2 mM. Cavities were increasingly formed when the nitrate concentration was decreased and they reached their largest dimensions in roots growing in water. Thus, inadequate availability of nitrogen leads, under acrated conditions, to deterioration of cortex cells and cavity formation in the maize roots. It is suggested that cavity formation in these roots is connected with reduced nitrogen assimilation.     

Content of Oxalate in Actinidia Deliciosa Plants Grown in Nutrient Solutions with Different Nitrogen Forms

Kiwifruit plants (Actinidia deliciosa cv. Hayward) were grown in Hoagland nutrient solution with calcium nitrate, potassium nitrate, ammonium nitrate or ammonium chloride as the nitrogen source. Plants grown in the solution with nitrate nitrogen displayed a higher oxalate content, greater shoot length and leaf area, and higher content of ascorbic acid and NO₃ ^– ions in the leaves. Plants grown in the solution with ammonium nitrate, and particularly with ammonium chloride, showed low oxalate content, low content of ascorbic acid and NO₃ ^–, high content of Cl^– and Na⁺, low shoot length and leaf area. Oxalate formation appeared to be connected with the assimulation of nitrate, more precisely with nitrate reduction, while ammonium nitrogen assimilation did not induce the synthesis of oxalic acid.     

Regulation of the High-Affinity Nitrate Transport System in Wheat Roots by Exogenous Abscisic Acid and Glutamine

Nitrate is a major nitrogen （N） source for most crops. Nitrate uptake by root cells is a key step of nitrogen metabolism and has been widely studied at the physiological and molecular levels. Understanding how nitrate uptake is regulated will help us engineer crops with improved nitrate uptake efficiency. The present study investigated the regulation of the high-affinity nitrate transport system （HATS） by exogenous abscisic acid （ABA） and glutamine （Gin） in wheat （Triticum aestivum L.） roots. Wheat seedlings grown in nutrient solution containing 2 mmol/L nitrate as the only nitrogen source for 2weeks were deprived of N for 4d and were then transferred to nutrient solution containing 50 μmol/L ABA, and 1 mmol/L Gin in the presence or absence of 2 mmol/L nitrate for 0, 0.5, 1, 2, 4, and 8 h. Treated wheat plants were then divided into two groups. One group of plants was used to investigate the mRNA levels of the HATS components NRT2 and NAR2 genes in roots through semi-quantitative RT-PCR approach, and the other set of plants were used to measure high-affinity nitrate influx rates in a nutrient solution containing 0.2 mmol/L ^15N-labeled nitrate. The results showed that exogenous ABA induced the expression of the TaNRT2.1, TaNRT2.2, TaNRT2.3, TaNAR2.1, and TaNAR2.2 genes in roots when nitrate was not present in the nutrient solution, but did not further enhance the induction of these genes by nitrate. Glutamine, which has been shown to inhibit the expression of NRT2 genes when nitrate is present in the growth media, did not inhibit this induction. When Gin was supplied to a nitrate-free nutrient solution, the expression of these five genes in roots was induced. These results imply that the inhibition by Gin of NRT2 expression occurs only when nitrate is present in the growth media. Although exogenous ABA and Gin induced HATS genes in the roots of wheat, they did not induce nitrate influx.