Simultaneous measurement of ammonium,nitrate and proton fluxes along the length of eucalypt roots

Knowledge of the preferred source of N for Eucalyptus nitens will lead to improved fertiliser management practices in plantations. Ion selective microelectrodes were used non-invasively to measure simultaneously net fluxes of NH₄ ⁺, NO₃ ^– and H⁺ along the tap root of solution-cultured E. nitens. Measurements were conducted in solutions containing 100 m NH₄NO₃. The pattern of fluxes was such that there was a large influx of NH₄ ⁺, a smaller influx of NO₃ ^– and large H⁺ efflux. The ratio of these fluxes was constant, according to the ratio 3:1:–6 (NH₄ ⁺:NO₃ ^–:H⁺). Within the region 20–60 mm from the root apex of E. nitens seedlings there was spatial and temporal variation in fluxes but flux patterns remained constant. Root hair density did not affect fluxes nor did proximity to lateral roots. Variation was less than that found in previous studies of localised root fluxes using similar high-resolution measurement techniques. It was concluded that small-scale spatial variation in fluxes may have confounded previous studies. There were associations between fluxes of all three ions, the strongest associations being between NH₄ ⁺ and H⁺, and NH₄ ⁺ and NO₃ ^–. Overall, these results are consistent with NH₄ ⁺ being the preferred source N for E. nitens.     

Effects of nitrate,ammonium and pH on the growth of conifer seedlings and their production of nitrate reductase

Summary Lodgepole pine (Pinus contorta Dougl.), Engelmann spruce (Picea engelmanni Parry), and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were grown in open-ended tube cultures of sand and perlite, irrigated with nitrate, ammonium, and a 1∶1 mixture of ammonium and nitrate, combined factorially with pH values of 4.6, 5.3 and 6.0 giving a total of nine treatments. Douglas-fir showed intolerance to ammonium which was especially marked in root weight. Lodgepole pine and Engelmann spruce made poor growth with nitrate, but showed little difference between ammonium and mixed sources. Only Douglas-fir showed a significant response to pH treatments with pH 5.3 plants being largest. Contamination of the sand with carbonate-bicarbonate, apparently caused seedlings grown in ammonium solutions to be larger in sand than in perlite. Douglas-fir grown in perlite cultures showed a growth response like the first experiment and nitrate reductase activity in the order nitrate > nitrateammonium mixture > ammonium. Plastic bead cultures had poor growth response due to low retention of water by the substrate, but the nitrate reductase assays produced results like the perlite cultures. Lodgepole pine grown in water culture demonstrated the well known pH shift associated with different nitrogen forms, and when assayed for nitrate reductase these seedlings had larger relative activities than Douglas-fir, but the order of activity remained nitrate > mixed source > ammonium.     

Net fluxes of ammonium and nitrate in association with H+ fluxes in fine roots of Populus popularis

Poplar plants are cultivated as woody crops, which are often fertilized by addition of ammonium (NH₄ ⁺) and/or nitrate (NO₃ ^?) to improve yields. However, little is known about net NH₄ ⁺/NO₃ ^? fluxes and their relation with H⁺ fluxes in poplar roots. In this study, net NH₄ ⁺/NO₃ ^? fluxes in association with H⁺ fluxes were measured non-invasively using scanning ion-selective electrode technique in fine roots of Populus popularis. Spatial variability of NH₄ ⁺ and NO₃ ^? fluxes was found along root tips of P. popularis. The maximal net uptake of NH₄ ⁺ and NO₃ ^? occurred, respectively, at 10 and 15 mm from poplar root tips. Net NH₄ ⁺ uptake was induced by ca. 48 % with provision of NO₃ ^? together, but net NO₃ ^? uptake was inhibited by ca. 39 % with the presence of NH₄ ⁺ in poplar roots. Furthermore, inactivation of plasma membrane (PM) H⁺-ATPases by orthovanadate markedly inhibited net NH₄ ⁺/NO₃ ^? uptake and even led to net NH₄ ⁺ release with NO₃ ^? co-provision. Linear correlations were observed between net NH₄ ⁺/NO₃ ^? and H⁺ fluxes in poplar roots except that no correlation was found between net NH₄ ⁺ and H⁺ fluxes in roots exposed to NH₄Cl and 0 mM vanadate. These results indicate that root tips play a key role in NH₄ ⁺/NO₃ ^? uptake and that net NH₄ ⁺/NO₃ ^? fluxes and the interaction of net fluxes of both ions are tightly associated with H⁺ fluxes in poplar roots.     

Ammonium, nitrate, and proton fluxes along the maize root

Ion-selective microelectrodes were used to measure NH₄⁺, NO₃^– and H⁺ fluxes along the primary root of maize seedlings. Plants were exposed to nutrient solutions containing NH₄⁺, NO₃^– or both ions. Nitrogen fluxes along the root varied substantially among the different treatments. Net NH₄⁺ and NO₃^– uptake and H⁺ extrusion were low at the very apex of the root and generally increased in the more basal regions. In the absence of nitrogen or in the presence of NO₃^– alone, net H⁺ uptake (and root surface alkalinization) occurred at the root tip (0–1 mm), whereas net H⁺ extrusion occurred in all other regions. In the presence of NH₄⁺ alone, a dramatic increase in net H⁺ extrusion was detected in all regions except for the region 6–11 mm from the apex. In contrast, when NO₃^– alone was supplied, net H⁺ extrusion was depressed at all locations except for the tip (0–1 mm). When both NH₄⁺ and NO₃^– were supplied, NO₃^– uptake was suppressed at all locations while net H⁺ extrusion was increased relative to NO₃^– alone. The capacities to absorb NH₄⁺ and NO₃^– at the tip were similar, as indicated by flux rates when NH₄⁺ or NO₃^– were supplied as sole sources, but when supplied together, net NO₃^– uptake was half that of net NH₄⁺ uptake, indicating that NH₄⁺ may satisfy the nitrogen requirements of the poorly vascularized apical tissue in the most energy-efficient way. The high spatial resolution of the measurements enabled us to establish that acidification in the root expansion zone is maintained regardless of nitrogen source.     

A comparison of ammonium, nitrate and proton net fluxes along seedling roots of Douglas-fir and lodgepole pine grown and measured with different inorganic nitrogen sources

Significant spatial variability in NH4+, NO3- and H+ net fluxes was measured in roots of young seedlings of Douglas-fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta) with ion-selective microelectrodes. Seedlings were grown with NH4+, NO3-, NH4NO3 or no nitrogen (N), and were measured in solutions containing one or both N ions, or no N in a full factorial design. Net NO3- and NH4+ uptake and H+ efflux were greater in Douglas-fir than lodgepole pine and in roots not exposed to N in pretreatment. In general, the rates of net NH4+ uptake were the same in the presence or absence of NO3-, and vice versa. The highest NO3- influx occurred 0-30 mm from the root apex in Douglas-fir and 0-10 mm from the apex in lodgepole pine. Net NH4+ flux was zero or negative (efflux) at Douglas-fir root tips, and the highest NH4+ influx occurred 5-20 mm from the root tip. Lodgepole pine had some NH4+ influx at the root tips, and the maximum net uptake 5 mm from the root tip. Net H+ efflux was greatest in the first 10 mm of roots of both species. This study demonstrates that nutrient uptake by conifer roots can vary significantly across different regions of the root, and indicates that ion flux profiles along the roots may be influenced by rates of root growth and maturation.     

Measurement of net fluxes of ammonium and nitrate at the surface of barley roots using ion-selective microelectrodes

Neutral carrier-based liquid membrane ion-selective microelectrodes for NH₄⁺ and NO₃⁻ were developed and used to investigate inorganic nitrogen acquisition in two varieties of barley, Hordeum vulgare L. cv Olli and H. vulgare L. cv Prato, originating in cold and warm climates, respectively. In the present paper, the methods used in the fabrication of ammonium- and nitrate-selective microelectrodes are described, and their application in the study of inorganic nitrogen uptake is demonstrated. Net ionic fluxes of NH₄⁺ and NO₃⁻ were measured in the unstirred layer of solution immediately external to the root surface. The preference for the uptake of a particular ionic form was examined by measuring the net flux of the predominant form of inorganic nitrogen, with and without the alternative ion in solution. Net flux of NH₄⁺ into the cold-adapted variety remained unchanged when equimolar concentrations (200 micromolar) of NH₄⁺ and NO₃⁻ were present. Similarly, net flux of NO₃⁻ into the warm-adapted variety was not affected when NH₄⁺ was also present in solution. The high temporal and spatial resolution afforded by ammonium- and nitrate-selective microelectrodes permits a detailed examination of inorganic nitrogen acquisition and its component ionic interactions.     

Response of conifer seedlings to nitrate and ammonium sources of nitrogen

Summary Differences in growth responses of Douglas fir, western hemlock, Sitka spruce, and white spruce to nitrate and ammonium N sources were examined in sand culture and artificial soil culture. Effects of the two forms of N on growth, needle area, and N uptake of three Douglas fir halb-sib progenies were examined in a second sand culture. Response of Douglas fir to the two forms of N was followed over two years in nursery soil of different pH levels. In sand culture 1 mean seedling dry weight of all species, except hemlock, was greatest when ammonium N and nitrate N were provided in equal amounts. In all species, except Sitka spruce, ammonium alone resulted in greater growth than nitrate alone. Use of ammonium N resulted in greater growth of all species, than was obtained with nitrate N, at pH values in the region 5.4 and 7.5 in artificial soil culture. Only Douglas fir showed substantial differences due to N source below pH 5. Growth of all species was greater at pH 5.4 than at 7.5 in each N source treatment. Growth of Douglas fir seedlings was greatest with ammonium N and least with nitrate N in sand culture 2. Supply of nitrate and ammonium in equal proportions resulted in intermediate growth. Leaf area/plant weight ratio was unaffected by N source. Analysis of nutrient solutions showed appreciable nitrification of ammonium N during the 7 days between solution changes. In the three greenhouse experiments, with little exception, increase in proportion of ammonium in N supply resulted in increase of seedling tissue N concentration. This effect was more pronounced in roots than shoots. Total N uptake by ammonium fed seedlings was about double the N uptake of nitrate fed seedlings in sand culture 2. Nursery grown Douglas fir seedlings showed greater growth response to ammonium sulphate than to calcium nitrate, and this appeared due entirely to form of N supply in the first year. A similar response in the second year was partly due to greater soil acidification by ammonium sulphate. Compared with calcium nitrate, ammonium sulphate increased N concentration of one-year old shoots, but this difference was not detected by foliar analysis of two-year old seedlings.     

Nitrate supply affects ammonium transport in canola roots

Plants may suffer from ammonium (NH4+) toxicity when NH4+ is the sole nitrogen source. Nitrate (NO3-) is known to alleviate NH4+ toxicity, but the mechanisms are unknown. This study has evaluated possible mechanisms of NO3- alleviation of NH4+ toxicity in canola (Brassica napus L.). Dynamics of net fluxes of NH4+, H+, K+ and Ca2+ were assessed, using a non-invasive microelectrode (MIFE) technique, in plants having different NO3- supplies, after single or several subsequent increases in external NH4Cl concentration. After an increase in external NH4Cl without NO3-, NH4+ net fluxes demonstrated three distinct stages: release (tau1), return to uptake (tau2), and a decrease in uptake rate (tau3). The presence of NO3- in the bathing medium prevented the tau1 release and also resulted in slower activation of the tau3 stage. Net fluxes of Ca2+ were in the opposite direction to NH4+ net fluxes, regardless of NO3- supply. In contrast, H+ and K+ net fluxes and change in external pH were not correlated with NH4+ net fluxes. It is concluded that (i) NO3- primarily affects the NH4+ low-affinity influx system; and (ii) NH4+ transport is inversely linked to Ca2+ net flux.     

Shoot excision has no effect on net flux of protons, ammonium or nitrate in seedling roots of a conifer and three crop species

A high-flux region, 5mm from the root tips of seedlings of coastal Douglas-fir (Pseudotsuga menziesii), soybean (Glycine max), zucchini (Cucurbita pepo) and pea (Pisum sativum), was monitored using a microelectrode ion flux measurement system, for changes in the net fluxes of H(+), NH(4)(+) and NO(3)(-) in response to shoot removal. In all species, careful excision of the seedling shoot had no significant effect on the net fluxes of H(+), NH(4)(+) or NO(3)(-) measured 5mm from the root tip. Experiments were carried out for up to 80min after shoot removal, and no temporal interactions were noted.     

Release of carboxylic anions and protons by tomato roots in response to ammonium nitrate ratio and pH in nutrient solution

The exudation of certain organic anions and protons by roots which may affect solubility of metals and P and uptake by plants, is affected by nitrogen form and pH. The objective of this work was to study exudation of carboxylates and H⁺/OH^– by tomato plants in response to NH₄/NO₃ ratio and pH in nutrient solution. Four NH₄/(NH₄+NO₃) ratios (R= 0, 0.33, 0.67 and 1) and constant vs. variable solution pH treatments were investigated. The sum of the exudation rates of all carboxylates tended to decline with increasing R, particularly tri- and dicarboxylates. The molar fraction of the exuded tri- and dicarboxylates, averaged over all treatments and plant ages, increased in the order tartarate 2%), malate (6%), succinate (15%), citrate (26%) and fumarate (46%). At R=1 the solution pH dropped from 5.2 to 3 and at R=0 increased to 8. The R corresponding to the pH stat of tomato plant was 0.3. For the constant solution pH treatment, the effect of solution pH on carboxylate exudation rate was small as compared to the effect of R. The exudation of citrate and H⁺ efflux which were initiated when NO₃ and NH₄ uptake rates per plant exceeded certain threshold values, increased with plant age.     

Iron deficiency differently affects metabolic responses in soybean roots

Iron deficiency responses were investigated in roots of soybean, a Strategy I plant species. Soybean responds to iron deficiency by decreasing growth, both at the root and shoot level. Chlorotic symptoms in younger leaves were evident after a few days of iron deficiency, with chlorophyll content being dramatically decreased. Moreover, several important differences were found as compared with other species belonging to the same Strategy I. The main differences are (i) a lower capacity to acidify the hydroponic culture medium, that was also reflected by a lower H(+)-ATPase activity as determined in a plasma membrane-enriched fraction isolated from the roots; (ii) a drastically reduced activity of the phosphoenolpyruvate carboxylase enzyme; (iii) a decrease in both cytosolic and vacuolar pHs; (iv) an increase in the vacuolar phosphate concentration, and (v) an increased exudation of organic carbon, particularly citrate, phenolics, and amino acids. Apparently, in soybean roots, some of the responses to iron deficiency, such as the acidification of the rhizosphere and other related processes, do not occur or occur only at a lower degree. These results suggest that the biochemical mechanisms induced by this nutritional disorder are differently regulated in this plant. A possible role of inorganic phosphate in the balance of intracellular pHs is also discussed.     

pH control in nitrate uptake studies with excised roots

Summary In the investigation of NO ₃ ^– uptake by excised roots, pH of salt solutions can be effectively controlled by periodic additions of a cation exchange resin, Amberlite IR-120, in the H⁺ form. This method of pH control provides for the addition of H⁺ to systems without an accompanying anion which might influence the absorption of NO ₃ ^– . The resin appeared to provide a favorable buffering capacity during an extended period and the method was especially useful for NO ₃ ^– uptake studies conducted at high root-to-solution ratios.Contribution from the Plant Nutrition Laboratory, Department of Botany, The University of Michigan, paper number 39. This work was supported by research grant GM-07339-02 from the National Institutes of Health, U. S. Public Health Service.     

Characterization of ammonium and nitrate uptake and assimilation in roots of tea plants

It has been pointed out that tea (Camellia sinensis (L.) O. Kuntze) prefers ammonium (NH ₄ ⁺ ) over nitrate (NO ₃ ^? ) as an inorganic nitrogen (N) source. ¹⁵N studies were conducted using hydroponically grown tea plants to clarify the characteristics of uptake and assimilation of NH ₄ ⁺ and NO ₃ ^? by tea roots. The total ¹⁵N was detected, and kinetic parameters were calculated after feeding ¹⁵NH ₄ ⁺ or ¹⁵NO ₃ ^? to tea plants. The process of N assimilation was studied by monitoring the dynamic ¹⁵N abundance in the free amino acids of tea plant roots by GC-MS. Tea plants supplied with ¹⁵NH ₄ ⁺ absorbed significantly more ¹⁵N than those supplied with ¹⁵NO ₃ ^? . The kinetics of ¹⁵NH ₄ ⁺ and ¹⁵NO ₃ ^? influx into tea plants followed a classic biphasic pattern, demonstrating the action of a high affinity transport system (HATS) and a low affinity transport system (LATS). The V _max value for NH ₄ ⁺ uptake was 54.5 nmol/(g dry wt min), which was higher than that observed for NO ₃ ^? (39.3 nmol/(g dry wt min)). K_M estimates were approximately 0.06 mM for NH ₄ ⁺ and 0.16 mM for NO ₃ ^? , indicating a higher rate of NH ₄ ⁺ absorption by tea plant roots. Tea plants fed with ¹⁵NH ₄ ⁺ accumulated larger amounts of assimilated N, especially glutamine (Gln), compared with those fed with ¹⁵NO ₃ ^? . Gln, Glu, theanine (Thea), Ser, and Asp were the main free amino acids that were labeled with ¹⁵N under both conditions. The rate of N assimilation into Thea in the roots of NO ₃ ^? -supplied tea plants was quicker than in NH ₄ ⁺ -supplied tea plants. NO ₃ ^? uptake by roots, rather than reduction or transport within the plant, seems to be the main factor limiting the growth of tea plants supplied with NO ₃ ^? as the sole N source. The NH ₄ ⁺ absorbed by tea plants directly, as well as that produced by NO ₃ ^? reduction, was assimilated through the glutamine synthetase-glutamine oxoglutarate aminotransferase pathway in tea plant roots. The ¹⁵N labeling experiments showed that there was no direct relationship between the Thea synthesis and the preference of tea plants for NH ₄ ⁺ .     

The effect of some ammonium salts on nitrate reductase level,onin vivo nitrate reduction and on nitrate content in excisedpisum sativum roots

The effect of some ammonium salts on nitrate reductase (NR) level, onin vivo nitrate reduction and on nitrate content was followed in the presence of nitrate in the medium, under changing experimental conditions, in excisedPisum sativum roots, and their effect was compared with that of KNO₃, Ca(NO₃)₂ and NaNO₃ at 15 mM NO₃ ^- concentration, i.e. at a concentration which considerably exceeded the level of saturation with nitrate with respect to nitrate reductase. The effect of ammonium salts on NR level is indirect and changes from a positive one to a strongly negative one which is dependent on the time of action of the salt, on the presence of other cations, on pH of the solution of the ammonium salt and on the nature of the anion of the ammonium salt. A positive effect on the enzyme level can be observed in the presence of other cations than NH₄ ⁺ at suitable concentrations of those ammonium salts, the solutions of which have their pH values in the acid region (i.e. NH₄H₂PO₄, (NH₄)₂SO₄ and NH₄NO₃). However their positive effect is independent of the presence of NH₄ ⁺ ions, and it is obviously the result of an increased concentration of H⁺ ions. A clear-cut negative effect on NR level can be observed after 24 h in one-salt NH₄NO₃ solution where NH₄ ⁺ is not balanced with other cations and thus certainly can adversely influence many metabolic processes, and in the solutions containing neutral (pH 6.2) and dibasic ammonium phosphates in which dissolved undissociated ammonia [(NH₃). (H₂O) which can also affect many metabolic processes incl. proteosynthesis] probably has a toxic influence. Thein vivo nitrate reduction is always depressed in excised pea roots in the presence of ammonium salts in the medium, regardless of the level of nitrate reductase. Under the described conditions, no relationship could be established between the enzyme level and the so-called metabolic NO₃ ^- pool (i.e. NO₂ ^- production under anaerobic conditions), nor between NR level and the total nitrate content in the roots. One-salt solutions of NaNO₃, Ca(NO₃)₂ and KNO₃ exert different effects on the level of nitrate reductase and on the content of NO₃ ^- in the roots, but the in vivo NO₃ ^- reduction shows the same trend as NR level in the roots influenced by these salts. Cl^- ions, supplied in NH₄C1, depress both NR level and NO₃ ^- content in the roots at higher concentrations, but they do not significantly affect the in vivo nitrate reduction in comparison with other ammonium salts. These results indicate that NR level,in vivo nitrate reduction, and nitrate uptake can be regulated in pea roots independently of each other.     

Influence of ammonium and nitrate nutrition on enzymatic activity in soybean and sunflower

Under conditions of controlled pH, nitrate and ammonium are equally effective in supporting the growth of young soybean (Glycine max var. Bansei) and sunflower (Helianthus annuus L. var., Mammoth Russian) plans. Soybean contains an active nitrate reductase in roots and leaves, but the low specific activity of this enzyme in sunflower leaves indicates a dependency upon the roots for nitrate reduction. Suppression of nitrate reductase activity in sunflower leaves may be due to high concentrations of ammonia received from the roots. Nitrate reductase activity in leaves of nitrate-supplied soybean and sunflower follows closely the distribution of nitrate reductase. For the roots of both species, glutamic acid dehydrogenase activity was greater with ammonium than with nitrate. The glutamic acid dehydrogenase of ammonium roots is wholly NADH-dependent, whereas that of nitrate roots is active with NADH and NADPH. In leaves, an NADPH-dependent glutamic acid dehydrogenase appears to be responsible for the assimilation of translocated ammonia and ammonia formed by nitrate reduction.     

Salt stress enhances proline utilization in the apical region of barley roots

Accumulation of an osmoprotectant, proline, is enhanced in response to salinity in plants. Here, by immunohistochemical analysis, we demonstrated that proline transporter (HvProT) was highly expressed in the apical region of barley roots under salt stress. Free proline was accumulated more in the basal region than in the apical region of barley roots under salt stress, although expression level of HvProT was higher in the apical region. On the other hand, salt stress increased proline and hydroxyproline contents in the cell wall fraction of the root apical region, suggesting increment of proline utilization. Expression of the genes encoding cell wall proteins (proline rich protein and extensin) and cellulose synthase was induced in barley roots by salt stress. These findings indicated that free proline transported by HvProT presumably behaved as a component of cell wall synthesis in the apical region of barley roots under salt stress.     

A comparative study of fluxes and compartmentation of nitrate and ammonium in early-successional tree species

¹³NO₃^– and ¹³NH₄⁺ compartmental analyses were carried out in seedling roots of trembling aspen (Populus tremuloides Michx.), lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) and interior Douglas-fir (Pseudotsuga menziesii var. glauca [Beissn.] Franco) at 0·1 and 1·5 mol m^–3 external NO₃^– or NH₄⁺ concentrations ([NO₃^–]_o or [NH₄⁺]_o, respectively). At the lower [NO₃^–]_o, the capacities and efficiencies of acquisition and accumulation of NO₃^–, based upon NO₃^– fluxes and cytoplasmic NO₃^– concentrations ([NO₃^–]_c), were in the order aspen >> Douglas-fir > pine. At 1·5 mol m^–3[NO₃^–]_o, the NO₃^– influx increased 18-fold in pine, four-fold in Douglas-fir and approximately 1·4-fold in aspen; in fact, at 1·5 mol m^–3[NO₃^–]_o, the NO₃^– influx in pine was higher than in aspen. However, at high [NO₃^–]_o, efflux also increased in the two conifers to a much greater extent than in aspen. In aspen, at both [NO₃^–]_o, approximately 30% of the ¹³N absorbed was translocated to the shoot during 57 min of ¹³N loading and elution, compared with less than 10% in the conifers. At 0·1 mol m^–3[NH₄⁺]_o, influx and net flux were in the order: aspen > pine > Douglas-fir but the differences were much less than in NO₃^– fluxes. At 1·5 mol m^–3[NH₄⁺]_o, NH₄⁺ influx, efflux and [NH₄⁺]_c greatly increased in aspen and Douglas-fir and, to a much lesser extent, in pine. In aspen, 29 and 12% of the ¹³N absorbed was translocated to the shoot at 0·1 and 1·5 mol m^–3[NH₄⁺]_o, respectively, compared with 5 to 7% in the conifers at either [NH₄⁺]_o. These patterns of nitrogen (N) uptake, particularly in the case of NO₃^–, and the observed concentration responses of NO₃^– uptake, reflect the availability of N in the ecological niches, to which these species are adapted.     

Effect of ammonium on nitrate utilization by roots of dwarf bean

The effect of exogenous NH₄⁺ on NO₃⁻ uptake and in vivo NO₃⁻ reductase activity (NRA) in roots of Phaseolus vulgaris L. cv Witte Krombek was studied before, during, and after the apparent induction of root NRA and NO₃⁻ uptake. Pretreatment with NH₄Cl (0.15-50 millimolar) affected neither the time pattern nor the steady state rate of NO₃⁻ uptake.

When NH₄⁺ was given at the start of NO₃⁻ nutrition, the time pattern of NO₃⁻ uptake was the same as in plants receiving no NH₄⁺. After 6 hours, however, the NO₃⁻ uptake rate (NUR) and root NRA were inhibited by NH₄⁺ to a maximum of 45% and 60%, respectively.

The response of the NUR of NO₃⁻-induced plants depended on the NH₄Cl concentration. Below 1 millimolar NH₄⁺, the NUR declined immediately and some restoration occurred in the second hour. In the third hour, the NUR became constant. In contrast, NH₄⁺ at 2 millimolar and above caused a rapid and transient stimulation of NO₃⁻ uptake, followed again by a decrease in the first, a recovery in the second, and a steady state in the third hour. Maximal inhibition of steady state NUR was 50%. With NO₃⁻-induced plants, root NRA responded less and more slowly to NH₄⁺ than did NUR.

Methionine sulfoximine and azaserine, inhibitors of glutamine synthetase and glutamate synthase, respectively, relieved the NH₄⁺ inhibition of the NUR of NO₃⁻-induced plants. We conclude that repression of the NUR by NH₄⁺ depends on NH₄⁺ assimilation. The repression by NH₄⁺ was least at the lowest and highest NH₄⁺ levels tested (0.04 and 25 millimolar).

     

Deposition of ammonium and nitrate in the roots of maize seedlings supplied with different nitrogen salts

This study measured total osmolarity and concentrations of NH(4)(+), NO(3)(-), K(+), soluble carbohydrates, and organic acids in maize seminal roots as a function of distance from the apex, and NH(4)(+) and NO(3)(-) in xylem sap for plants receiving NH(4)(+) or NO(3)(-) as a sole N-source, NH(4)(+) plus NO(3)(-), or no nitrogen at all. The disparity between net deposition rates and net exogenous influx of NH(4)(+) indicated that growing cells imported NH(4)(+) from more mature tissue, whereas more mature root tissues assimilated or translocated a portion of the NH(4)(+) absorbed. Net root NO(3)(-) influx under Ca(NO(3))(2) nutrition was adequate to account for pools found in the growth zone and provided twice as much as was deposited locally throughout the non-growing tissue. In contrast, net root NO(3)(-) influx under NH(4)NO(3) was less than the local deposition rate in the growth zone, indicating that additional NO(3)(-) was imported or metabolically produced. The profile of NO(3)(-) deposition rate in the growth zone, however, was similar for the plants receiving Ca(NO(3))(2) or NH(4)NO(3). These results suggest that NO(3)(-) may serve a major role as an osmoticant for supporting root elongation in the basal part of the growth zone and maintaining root function in the young mature tissues.