Potassium influx into maize root systems : influence of root potassium concentration and ambient ammonium

Potassium influx into roots of dark-grown decapitated maize seedling (Zea mays L., cv Pioneer 3369A) was examined in presence and absence of ambient ammonium and at various root potassium concentrations. Six-day old seedlings which were dependent on the endosperm reserves for their energy source were exposed to KCl (labeled with ⁸⁶Rb) ranging from 5 to 200 micromolar. At both low (13 micromoles per gram fresh weight) and high (100 micromoles per gram fresh weight) root potassium concentration, isotherms indicated two potassium influx systems, one approaching saturation at 50 to 100 micromolar potassium and an additional one tentatively considered to be linear. A mixed-type inhibition by ammonium for the low-concentration saturable system was indicated by a concomitant decrease in V_max and increase in K_m. High root potassium concentration decreased V_max but had little effect on K_m of this system. The rate constant for the second quasilinear system was decreased by ambient ammonium and by high root potassium status. Transfer of high potassium roots to potassium-free solutions resulted in an increase in influx within 2 hours; by 24 hours influx significantly exceeded that of roots not previously exposed to potassium. In roots of both low and high root potassium concentrations, potassium influx was restricted progressively as ambient ammonium increased to about 100 micromolar, but there was little further inhibition as ammonium concentrations increased beyond that to 500 micromolar. The data imply that potassium influx has two components, one subject to inhibition by ambient ammonium and one relatively resistant.     

Endogenous Ammonium Generation in Maize Roots and its Relationship to other Ammonium Fluxes

An investigation to determine the magnitude of the back reactionswhich occur during net ammonium uptake by roots and during netammonium assimilation within roots was undertaken with maize(Zea mays L.). Ten-day-old seedlings, which had been grown on250 mmol m^–3 ammonium at pH 4 or 6, were pretreated for3 h in the absence or presence of 500 mmol m ^–3 MSX (methionine-DL-sulphoximine),an inhibitor of the glutamine synthetase-catalysed pathway ofammonium assimilation. They were then exposed for 2 h to 99A% ¹⁵N-ammonium ± MSX. Substantial ammonium cycling occurredduring net ammonium uptake. Efflux was enhanced by MSX treatment,reflecting a 2- to 3-fold accumulation of ammonium in the roottissue. Influx of ammonium was also increased by treatment withMSX, indicating that influx was enhanced when products of ammoniumassimilation were dissipated. The decline in root ¹⁴N-ammoniumaccounted for only a small fraction of the ¹⁴N-ammonium recoveredin the ambient ¹⁵N-ammonium solution, revealing a substantialgeneration of endogenous ¹⁴N-ammonium during the 2 h exposure.The net quantity of ammonium generated was increased appreciablywhen assimilation of ammonium was restricted by MSX and it wasestimated to occur at least 50% faster than net ammonium uptake.Presence of MSX severely decreased translocation of ¹⁵N to shootsbut had a smaller influence on incorporation of ¹⁵N into macromoleculesof the root tissue. The various ammonium flux rates were notgreatly affected by growth at pH 4.0, implying a considerableresistance of ammonium assimilation processes in these maizeroots to the high ambient acidity commonly induced by exposureto ammonium Key words: Ammonium generation, uptake, assimilation     

The effect of ammonium ions on membrane potential and anion flux in roots of barley and tomato

The effects of ammonium (0–5 mol m^?3) on root hair membrane potential and on the influx of nitrate and phosphate were investigated in roots of intact barley and tomato plants. In both species, addition of ammonium to the medium bathing the roots caused an almost immediate depolarization of the membrane potential; the depolarization was greater at higher concentrations of ammonium. Influx of ¹³NC₃^? and ³²Pi was inhibited over the same time scale and concentration range. In tomato roots, there was little further depolarization of the membrane potential or inhibition of anion influx at ammonium concentrations above 0.4 mol m^?3. In barley roots, the inhibition of nitrate influx and the depolarization of the membrane potential did not saturate below 5 mol m^?3 ammonium.     

Simultaneous influx of ammonium and potassium into maize roots: kinetics and interactions

The interaction between ammonium and potassium during influx was examined in roots of dark-grown decapitated corn seedlings (Zea mays L., cv. Pioneer 3369A). Influx was measured during a 10-min exposure to either (¹⁵NH₄)₂SO₄ ranging from 10 to 200 M NH ₄ ⁺ with and without 200 M K(⁸⁶Rb)Cl or to K(⁸⁶Rb)Cl ranging from 10 to 200 M K⁺ with and without 200 M NH ₄ ⁺ as (¹⁵NH₄)₂SO₄. The simple Michaelis-Menten model described the data well only for potassium influx in the presence of ambient ammonium. For the other three instances, the data were improved by assuming that a second influx mechanism became operative as the low-concentration phase approached saturation. Two distinct mechanisms are thus indicated for both ammonium and potassium influx within the range of 10 to 200 M.The influx mechanism operating at low concentrations showed greater affinity for potassium than for ammonium, even though the capacity for ammonium transport was twice as large as that for potassium. It is suggested that this phase involved a common transport system for the two ions and that localized low acidity next to the internal surface, following H⁺ extrusion, favored ammonium deprotonation and dissociation from the transport system-ammonium complex. Parallel decreases in V _max and increases in K_m of the low-concentration saturable phase occurred for ammonium influx when ambient potassium was present and for potassium influx when ambient ammonium was present. The data support a mixed-type inhibition in each case. Simultaneous measurement of potassium and ammonium influx showed that they were highly negatively correlated at the lower concentrations, indicating that the extent to which influx of the inhibited ion was restricted was associated with influx of the inhibitor ion. Presence of ambient ammonium eliminated the second phase of potassium influx. In contrast, the presence of ambient potassium decreased the concentration at which the second phase of ammonium influx was initiated but did not restrict the rate.Paper no. 11131 of the Journal Series of the North Carolina Agricultural Research ServiceThe use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products named, nor criticism of similar ones not mentioned     

A comparison of regulatory effects of chloride on nitrate uptake, and of nitrate on chloride uptake into Pisum sativum seedlings

Net nitrate uptake, ³⁶ClO^?₃/NO^?₃ influx and ³⁶Cl^? influx into Pisum sativum L. cv. Feltham First seedlings have been examined following growth in culture medium containing different combinations of chloride and nitrate. When young (6 days old) seedlings, that had been grown in the absence of N were used, nitrate accumulation stimulated net nitrate uptake and ³⁶ClO^?₃/NO^?₃ influx (r²= 0.99) while chloride accumulation inhibited nitrate uptake and ³⁶ClO^?₃/NO^?₃ influx (r²= 0.65). When nitrate was provided during growth there was no effect of chloride pretreatment on net nitrate uptake and there was little effect of total [NO^?₃+ Cl^?]_i on ³⁶ClO^?₃/NO^?₃ influx (r²= 0.26). A direct effect of Cl^? on ³⁶ClO^?₃/NO^?₃ influx was only found when seedlings had been starved of N for more prolonged periods (14 days). When moderate chloride was supplied during growth, ³⁶Cl^? influx was insensitive to nitrate or chloride accumulated, but significantly correlated with log_e [NO^?₃+ Cl^?]_i (r²= 0.75). When trace amounts of Cl^? were supplied during growth ³⁶Cl^? influx was inhibited by (a) NO^?₃ in the external medium and (b) Cl^? pretreatment, but was insensitive to NO^?₃ pretreatment. The sensitivity of ³⁶Cl^? influx to external nitrate was not found following Cl^? pretreatment in the absence of nitrate. The possibility that there are two populations of chloride carriers which differ in their sensitivity to external nitrate is discussed. Tentative schematic models to account for the regulation of nitrate and chloride uptake are proposed in the context of current hypotheses for regulation of ion transport and control systems theory.     

Effects of ouabain and isoproterenol on potassium influx in the turkey erythrocyte: Quantitative relation to ligand binding and cyclic AMP generation

Studies have been carried out in the turkey erythrocyte to examine: (1) the influence of external K⁺ concentration on both [³H]ouabain binding and the sensitivity of potassium influx to inhibition by ouabain and (2) the quantitative relation between β-adrenergic receptor site occupancy, agonist-directed cyclic AMP generation and potassium influx rate. Both [³H]ouabain binding and the ability of ouabain to inhibit potassium influx are markedly reduced at increasing external K⁺ concentrations, and at each K⁺ concentration the concentrations of ouabain required for half-maximal binding to the erythrocyte membrane and for half-maximal inhibition of potassium influx are identical. Both basal and isoproterenol-stimulated potassium influx rise with increasing external K⁺ concentrations. In contrast to basal potassium influx, which is 50–70% inhibitable by ouabain, the isoproterenol-stimulated component of potassium influx is entirely insensitive to ouabain. At all concentrations of K⁺, inhibition of basal potassium influx by ouabain is linear with ouabain binding, indicating that the rate of transport per unoccupied ouabain binding site is unaffected by simultaneous occupancy of other sites by ouabain. Similarly, the rate of isoproterenol-stimulated cyclic AMP synthesis is directly proportional to β-adrenergic receptor occupancy over the entire concentration-response relationship for isoproterenol, showing that at all levels of occupancy β-adrenergic receptor sites function independently of each other.Analysis of the relation of catecholamine-dependent potassium transport to the number of β-adrenergic receptor sites occupied indicates an extremely sensitive physiological system, in which 50%-maximal stimulation of potassium transport is achieved at less than 3% receptor occupancy, corresponding to fewer than ten occupied receptors per cell.     

Inward and outward transport of ammonium in roots of maize and sorghum: contrasting effects of methionine sulphoximine

To examine the adaptive responses of ammonium efflux of nitrate-grownplants during first exposure to ammonium nutrition, isotopicflows of ammonium in and out of root tissue of maize (Zea maysL.) and sorghum (Sorghum bicolor L.) were delineated. Appreciableefflux of ¹⁴N-ammonium occurred during the first 2 min of exposure,suggesting that ammonium efflux is characteristic of plantsgrown with nitrate as the sole source of nitrogen. Treatmentwith methionine sulphoximine (MSX) enhanced the initial ¹⁴N-ammoniumefflux 5-fold in maize but less than 2-fold in sorghum. Increasingambient acidity from pH 6 to pH 4 had relatively little effecton 14N-ammonium efflux in either species. Initial influx ofexogenous ¹⁵N-ammonium was stimulated by MSX in maize (slightlyless at pH 4 than at pH 6), but either was not affected (atpH 6) or was inhibited (at pH 4) in sorghum. Assuming that therate of ammonium influx remained constant during the 2 h adaptationperiod, and that efflux of ¹⁵N-ammonium was negligible duringthe 0-2 min period, permitted estimates to be made of the unidirectionalinward and outward flow of the two isotopic ammonium species.The decline in efflux of endogenous ¹⁴N-ammonium after the 2min exposure was more than compensated for by an increase inefflux of the exo-genously-supplied 15N-ammonium. As a consequence,total ammonium efflux increased significantly during the adaptationperiod. By this analysis about 60% of the total efflux in maizewas as 1BN-ammonium within the 2-5 min period whereas in sorghumit was 20% to 40%. The data indicate that appreciable ammoniumefflux occurs in nitrate-grown seedlings in spite of low rootammonium concentrations, and that substantial futile cyclingof ammonium occurs in both maize and sorghum during net transportof ammonium into their root tissue. Key words: Ammonium, cycling, transport, maize, sorghum     

Effect of methionine-sulfoximine on nitrate uptake and photosynthesis in the cyanobacteriumAnabaena doliolum Bharadwaja

l-Methionine-dl-sulfoximine (MSX) stimulated nitrate uptake but inhibited¹⁴CO₂ fixation and O₂ evolution inAnabaena doliolum. Nitrate uptake was inhibited by ammonium (NH ₄ ⁺ ) in the absence of MSX, but not in the presence of MSX. Glutamine or a derivative of it appears to be the actual negative effector of nitrate utilization. In presence of nitrate, MSX-treated cells ofA. doliolum evolve more O₂ than do untreated cells. Our results suggest a close relation between photoassimilation of carbon and utilization of nitrogen.     

Inhibition of nitrate influx by glutamine in Lolium perenne depends upon the contribution of the HATS to the total influx

Plants of Lolium perenne L. were grown in sterile solution culture supplied with 2 mol m(-3) nitrogen as either nitrate or ammonium. Glutamine at 5 mol m(-3) was added to the nutrient solution of half the plants for 24 h. Root nitrate influx (at external nitrate concentrations 0-2000 mmol m(-3)) and amino acid concentrations were determined. In a second experiment the concentration of the added glutamine was varied from 0-5 mol m(-3) and nitrate influx determined at 250 and 2000 mmol m(-3). The maximum rate of influx attributed to the high affinity transport system (HATS) was reduced by 66% by the presence of glutamine achieved through an 84% reduction in its constitutive component and a 59% reduction in its inducible component. Influx attributed to LATS was unaffected by the addition of glutamine. The inhibition of total nitrate influx by glutamine was positively related to the contribution of HATS to the total influx. In both nitrate- and ammonium-grown plants, the concentration of glutamine required to inhibit nitrate influx significantly was lower when influx was determined at 250 mmol m(-3) compared with 2000 mmol m(-3) nitrate. The addition of glutamine increased its concentrations in root tissue. However, the results cannot be attributed to changes in glutamine alone as its addition also resulted in increased concentrations of other amino acids. Implications for plants growing under field conditions are discussed.     

Short-term ammonium inhibition of nitrate utilization by Anacystis nidulans and other cyanobacteria

Ammonium at low concentrations caused a rapid and effective inhibition of nitrate utilization in the light by the cyanobacterium Anacystis nidulans without affecting the cellular level of nitrate reductase activity. The inhibition was reversible, and the ability of the cells to utilize nitrate was restored immediately after ammonium had been exhausted. The inhibitory effect was dependent on consumption by the cells of the added ammonium which was rapidly incorporated into amino acids. In the presence of L-methionine-d,l-sulfoximine (MSX) or azaserine, inhibitors of the glutamine synthetase-glutamate synthase pathway, ammonium did not exhibit any inhibitory effect on nitrate utilization. Ammonium assimilation, rather than ammonium itself, seems to regulate nitrate utilization in A. nidulans. Short-term inhibition by ammonium of nitrate utilization and its prevention by MSX were also demonstrated in the filamentous cyanobacteria Anabaena and Nostoc.Abbreviations MSX L-Methionine-d-l-sulfoximine     

Effect of ouabain upon diuretic-sensitive K+ transport in cultured cells evidence for separate modes of operation of the transporter

(1) Unidirectional K⁺ (⁸⁶Rb) influx and efflux were measured in subconfluent layers of MDCK renal epithelial cells and HeLa carcinoma cells. (2) In both MDCK and HeLa cells, the furosemide-inhibitable and chloride-dependent component of K⁺ influx/efflux was stimulated 2-fold by a 30 min incubation in 1 · 10^?3 M ouabain. (3) Measurements of net K⁺ loss and Na⁺ gain in ouabain-treated cells at 1 h failed to show any diuretic sensitive component, confirming the exchange character of the diuretic-sensitive fluxes. (4) Prolonged incubations for 2.5 h in ouabain revealed a furosemide- and anion-dependent K⁺ (Cl^?) outward net flux uncoupled from net Na⁺ movement. Net K⁺ (Cl^?) outward flux was half-maximally inhibited by 2 μM furosemide. (5) After 2.5 h ouabain treatment, the anion and cation dependence of the diuretic-sensitive K⁺ influx/efflux were essentially unchanged when compared to untreated controls.     

Restricted nitrate influx and reduction in corn seedlings exposed to ammonium

The effect of ambient ammonium (0.5 millimolar [¹⁴NH₄]₂SO₄) added to a nutrient solution containing 1.0 millimolar K¹⁵NO₃, 99 atom per cent ¹⁵N, upon [¹⁵N]nitrate assimilation and utilization of previously accumulated [¹⁴N]nitrate was investigated. Corn seedlings, 5-day-old dark-grown decapitated (experiment I) and 10-day-old light-grown intact (experiment II), which had previously been grown on K¹⁴NO₃ nutrient solution, were used. In both experiments, the presence of ambient ammonium decreased [¹⁵N]nitrate influx (20% after 6 hours) without significantly affecting the efflux of previously accumulated [¹⁴N]nitrate. In experiment I, relative reduction of [¹⁵N]nitrate (reduction as a percentage of influx) was inhibited more than was [¹⁵N]nitrate influx. Nevertheless, in experiment I, where all reduction could be assigned to the root system, the absolute inhibition of reduction during the 12 hours (13 micromoles/root) was less than the absolute inhibition in influx (24 micromoles/root). The data suggest that the influence of ammonium on [¹⁵N]nitrate influx could not be totally accounted for by the decrease in the potential driving force which resulted from restricted reduction; an additional impact on the influx process is indicated. Reduction of [¹⁵N]nitrate in experiment II after 6 hours accounted for 30 and 18% of the tissue excess ¹⁵N in the control and ammonium treatments, respectively. Relative distribution of ¹⁵N between roots and exudate (experiment I), or between roots and shoots (experiment II) was not affected by ammonium. On the other hand, the accumulation of [¹⁵N]nitrate in roots, shoots, and xylem exudate was enhanced by ammonium treatment compared to the control, whereas the accumulation of reduced ¹⁵N was inhibited.     

APPLIED ISSUES The effect of ammonium nitrate on the feeding and development of larvae of the smooth newt, Triturus vulgaris (L.), and on the behaviour of its food source, Daphnia

1. Larvae of the smooth newt, Triturus vulgaris, were exposed to four concentrations of ammonium nitrate in artificial pond water (50, 100, 200 and 500 mg 1-^?1) under controlled laboratory conditions. 2. Larvae exposed to a 50 mg 1-^?1 solution of ammonium nitrate showed no significant difference to control larvae in feeding rate, mass at metamorphosis or time to metamorphosis. However, larvae exposed to 200 or 500 mg 1-^?1 ammonium nitrate, for a single period of 24, 48 or 72 h, were significantly smaller than controls at metamorphosis. Larvae exposed to 100 mg 1-^?1 had a significantly higher feeding rate than those reared under control conditions or those exposed to 200 or 500 mg 1-^?1 ammonium nitrate, but this was not reflected in their size at metamorphosis, which was not significantly different from the controls. Larval survival remained high in all trials. 3. In separate trials, Daphnia, used as a food source for the newt larvae, were exposed to the same series of ammonium nitrate concentrations. Daphnia behaviour was affected by exposure to 200 and 500 mg 1-^?1 ammonium nitrate, and in both cases animals were significantly more likely to be found at the top of the water column than those exposed to 50 or 100 mg 1-^?1 ammonium nitrate or the control medium.     

Utilization of nitrate,nitrite and ammonium by Chlamydomonas reinhardii

In phototrophically grown Chlamydomonas cells, ammonium strongly inhibited the utilization of nitrate or nitrite. Under darkness, or in the presence of an uncoupler or inhibitor of the non-cyclic photosynthetic electron flow, the utilization of nitrate, nitrite or ammonium was suppressed. l-Methionine-d,l-sulfoximine (MSX) or azaserine, which blocks the assimilation of ammonium, inhibited the consumption of nitrate, but not nitrite, by the cells. Ammonium produced an immediate inhibition of the permease for nitrate in Chlamydomonas growing with nitrate, while ammonium-grown cells lacked this permease. The synthesis of nitrate-reductase activity was dependent on an active permease. In N-starved Chlamydomonas cells, previously treated with MSX, the permease for nitrate was insensitive to inhibition by ammonium, and a significant amount of nitrate reductase was synthetized. These cells photoproduce ammonium by reducing nitrate. Nitrogen-repleted cells, treated with MSX, actively photoproduced ammonium by reducing nitrite, but not nitrate.Abbreviations DCMU N-(3,4-dichlorophenyl)N,N-di-methyl-urea - PCCP Carbonylcyanid-p-trifluoromethoxy-phenylhydrazone - Mops 2-(N-morpholino)propanesulfonic acid - MSX l-Methionine-d,l-sulfoximine     

Biodegradation of Diesel Fuel in Soil Under Various Nitrogen Addition Regimes

Bioremediation is a growing technology for treating fuel-contaminated soils. Many biological, physical, and chemical parameters control the rate and efficiency of this process, including type and concentration of contaminants, temperature, oxygen content, and nutrient status. This study investigated the effect that nitrogen sources and concentrations had on the degradation rate of diesel fuel in nutrient limited soil at two carbon-to-nitrogen ratios. The different sources of nitrogen studied were ammonium nitrate, ammonium sulfate, potassium nitrate, urea, and urea oligomers (control release fertilizer). Laboratory experiments were conducted on field-contaminated soil using sealed bioreactors at a controlled temperature of 25°C. For both carbon-to-nitrogen ratios tested, hydrocarbon degradation rates were the highest for the ammonium sulfate (20:1 at 0.032?d^?1; 40:1 at 0.019d^?1) and urea treatments (20:1 at 0.025?d^?1; 40:1 at0.011?d^?1). A degradation rate correlation as a function of nitrate and ammonia concentrations was developed. The correlation suggests the occurrence of nitrate inhibition at elevated nitrate concentrations.     

The Effect of Acetylcholine Release on Choline Fluxes in Isolated Synaptic Terminals

Abstract: As in intact tissues, choline influx into synaptosomes is enhanced after a period of depolarization induced release of acetylcholine. The activation of uptake is dependent on the presence of Ca²⁺ and inhibited by high Mg²⁺ concentrations in the medium during depolarization. Choline transport in erythrocytes was not activated by prior treatment with potassium. The permeability constant of the synaptosome membrane to choline was found to be 2.7 × 10^?8 cm·s^?1 and to acetylcholine 1.8 ′ 10^?8 cm·s^?1. Choline influx has been studied after pre-loading synaptosomes with choline. Different radiolabels were used to measure efflux of preloaded choline and influx simultaneously. Isotopic dilution in flux studies was estimated and corrected for. Influx was stimulated by high internal concentrations of choline, and efflux similarly stimulated by high outside concentrations of choline. The maximal influx and efflux at saturating opposite concentrations of choline were equal with a value of about 500 pmol·min^?1 per mg synaptosomal protein. A reciprocating carrier would explain the equality of the maximal influx and efflux. Acetylcholine competes with choline for binding to the carrier but is itself hardly transported. Increased acetylcholine concentrations were shown to inhibit both choline influx and efflux from the trans position. Raising intrasynaptosomal acetylcholine concentrations by pre-loading abolished the stimulation of influx by prior depolarization. It is proposed that high concentrations of acetylcholine immobilize the carrier on the inside of the synaptic membrane. The stimulation of choline influx consequent upon depolarization is caused by release of ACh which results in relief of this immobilisation. The enhanced supply of choline achieved by this mechanism is likely to be important in maintaining stores of the acetylcholine in vivo.     

Nitrite Uptake Patterns in Wheat Seedlings as Influenced by Nitrate and Ammonium

Nitrite and nitrate uptake by wheat (Triticum vulgare) from 0.5 mM potassium solutions both showed an apparent induction pattern characterized by a slow initial rate followed by an accelerated rate. The accelerated phase was more rapid for nitrate uptake, was initiated earlier, and was seriously restricted by the presence of equimolar nitrite. The accelerated phase of nitrite uptake was restricted by nitrate to a lesser extent. The two anions seem not to be absorbed by identical mechanisms. Ammonium pretreatments or prior growth with ammonium had relatively little influence on the pattern of nitrite uptake. However, prior growth with nitrate eliminated the slow initial phase and induced development of the accelerated phase of nitrite uptake. A beneficial effect was noted after 3 h nitrate pretreatment and full development had occurred by 12 h nitrate pretreatment. The evidence suggests that a small amount of tissue nitrite, which could be supplied either by absorption or by nitrate reduction, was specifically required for induction of the accelerated phase of nitrite uptake. Cycloheximide (2 μg ml^?1) seriously restricted development of the accelerated phase of nitrite uptake, but its effect was not as severe when it was added after the accelerated phase had been induced by prior exposure to nitrite or nitrate. However, translocation of ¹⁵N from the absorbed nitrite was sharply decreased under the latter conditions, indicating a difference in sensitivity of the uptake and translocation processes to cycloheximide. Potassium uptake was greater from KNO₃ than from KNO₂ and in both instances it was enhanced during the early stages of the accelerated phase of anion uptake. Moreover, addition of NaNO₃ to KNO₂ substantially increased potassium uptake. A coupling between anion and potassium uptake was therefore evident, but the coupling was not obligatory because the accelerated phase of nitrite uptake could occur in absence of rapid potassium uptake.     

Kinetics of Myo-inositol transport in rat ocular lens

Myo-inositol (MI) influx as a function of concentration in rat lens consisted of a saturable component, fit by a rectangular hyperbola, and a linear component which was more distinct at high myo-inositol concentrations suggesting passive diffusion. The hyperbolic component was half-maximally saturated (K_t) at 61.3 μM and had a maximal transport rate (J_max) of 44.6 μMol/kg wet wt/h. The linear component had an apparent permeability coefficient of 1.44 × 10^?6 s^?1. Sorbitol, which distributed rapidly in the extracellular space (6.83 ml/100 g wet wt), also appeared to enter the intracellular space with a permeability coefficient of 1.37 × 10^?6 s^?1, similar to that of myo-inositol. The influx of myo-inositol was critically dependent on the concentration of extracellular sodium consistent with a sodium-myo-inositol contransport. The kinetics of influx activation by sodium suggested an apparent 2:1 coupling ratio for sodium and myo-inositol. When potassium was used as sodium substitute, a significantly stronger influx inhibition was observed than with nondepolarizing sodium substitutes, indicating that myoinositol was driven by the electrochemicl gradient of sodium rather than the chemical gradient only. Reducing the extracellular Na concentration increased the MI concentration at which transport was half-maximally activated, suggesting an ordered binding sequence of Na followed by MI. Myo-inositol influx was competitively inhibited by phlorizin with an inhibitory coefficient (K_i) of 35 μM. Phloretin also was capable of inhibition but with a much lesser efficacy. Myoinositol desaturates from the lens at a rate of 0.00862 h^?1. Approximately 19% of the efflux can be inhibited with phlorizin, suggesting that it represents carrier-mediated flux. The phlorizin insensitive flux has a rate of 0.00695 h^?1 or 1.93 × 10^?6 s^?1, similar to the Na-independent passive influx. MI influx is due to a Na-dependent, phlorizin-sensitive active transport while the efflux consists largely of a phlorizin-independent passive leakage. © 1995 Wiley-Liss, Inc.     

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.