K+-Na+ Exchange and Selectivity in Barley Root Cells: Effect of Na+ on the Na+ Fluxes

Using the compartmental analysis the unidirectional Na⁺ fluxesin cortical cells of barley roots, the cytoplasmic and vacuolarNa⁺ contents Q_c and Q_v, and the trans-root Na⁺ transport R'have been studied as a function of the external Na⁺ concentration.Using the re-elution technique the effect of low K⁺ concentrationson the plasmalemma efflux _co of Na⁺ (K+-Na+ exchange) and onR' was investigated at different Na+ concentrations and correspondinglydifferent values of the cytoplasmic sodium content Qc. The relationof the K+-dependent Na+ efflux coK+-dep to Qc or to the cytoplasmicNa+ concentration obeyed Michaelis-Menten kinetics. This isconsistent with a linkage of co, K+-dep to K+ influx by a K⁺-Na⁺exchange system. The apparent Km corresponded to a cytoplasmicNa⁺ concentration of 28 mM at 0·2 mM K⁺ and about 0·2mM Na⁺ in the external solution. 0·2 mM K⁺ stimulatedthe plasma-lemma efflux of Na⁺ and inhibited Na⁺ transport selectivelyeven in the presence of 10 mM Na⁺ in the external medium showingthe high efficiency of the K⁺-Na⁺ exchange system. However,_co, K⁺-dep was inhibited at 10 mM Na¹ compared to lower Na¹concentrations suggesting some competition of Na¹ with K¹ atthe external site of the exchange system. The effect of theNa+ concentration on Na¹ influx _oc is discussed with respectto kinetic models of uuptake.     

Some Effects of Root Anatomy on K, Na and Cl Loading of Citrus Roots and Leaves

Loading of K, Na and Cl into fibrous roots of salt-treated citrusgenotypes, Rangpur lime (Citrus reticulata var. austera hybrid?)and Etrog citron (C. medica L.) was investigated in relationto root anatomy, in particular, the differentiation of the epidermal-hypodermallayers with distance from the root tip. The influence of durationof salinity treatment on the characteristics of K, Na and Claccumulation in leaves of the two genotypes was explored intwo experiments respectively, covering the short term (14 d)and long term (12 weeks). This study focused on two regions of the fibrous root, a segment2–12 mm from the root tip, immediately basipetal to thezone of elongation and a differentiated region of the maturesuberized, fibrous root, 40–50 mm from the root tip. Inthe distal root segment (2–12 mm) the epidermis and hypodermisof both genotypes was observed as two closely packed, uniseriatelayers of living cells. In the proximal root segment (40–50mm) the differentiated hypodermis was evident as a uniseriatelayer of thick-walled lumina interspersed with ‘passagecells’ which were frequently associated with clustersof viable epidermal cells. The characteristics of Na and Cl loading in the two root zonesdiffered profoundly during the short term loading (acclimation)phase. Attainment of quasi-steady-states for Na and Cl in thedistal region (with the exception of Na in Etrog citron) wasrapid as was Na equilibration of the proximal root segmentsin both genotypes. In contrast, Cl loading in the proximal regiontook c. 14 d to reach a quasi-steady-state by which time Cllevels were 2 to 3 times higher in the proximal than in thedistal root segments. The superior tolerance of Rangpur lime to long term salinitywas highly correlated to Cl exclusion from the leaves. However,during the first 14 d of acclimation to 50 mol m^–3 NaClthere was no segregation of the two genotypes based upon leafCl levels. Expression of differential accumulation of Cl inleaves appeared to be a time dependent process and was manifestonly after Cl saturation of the proximal root which representsthe bulk of the fibrous root system. The salt tolerance of Rangpurwas also associated with high selectivity of fibrous roots forK. over Na. A pronounced loss of K from cortical cells in theproximal root segment of salt-stressed Etrog citron was alsoevident by X-ray microanalysis. Key words: Citrus, anatomy, salinity, roots, X-ray microanalysis     

Observations on the Exchange of Salt Between the Xylem and Neighbouring Cells in Zea mays Primary Roots

Comparison of two preparations of Zea mays primary roots hasshown that the salt concentration in the xylem fluid is alteredas it passes through a region of the root where no absorptionof salts or water from the external bathing medium can occur.Salts are secreted to the xylem by such a region, presumablyfrom the vacuoles of the cortical cells. It has also been demonstratedthat longitudinal water movement through the root cortex isnegligibly small during root-pressure exudation.     

Evidence for a Na/H Antiporter in Membrane Vesicles Isolated from Roots of the Halophyte Atriplex nummularia

The ATP-dependent establishment of a positive membrane potential (measured as S¹⁴CN⁻-accumulation) in membrane vesicles isolated from the roots of Atriplex nummularia Lindl. was not inhibited by NaMes and KMes at concentrations up to 140 millimolar. On the other hand, the formation of ΔpH (measured as ¹⁴C-methylamine accumulation or quenching of quinacrine fluorescence), was depressed by NaMes concentrations as low as 30 millimolar. Supply of NaMes after the ΔpH had been established brought about partial dissipation within 30 seconds. Extent of dissipation of ΔpH increased with NaMes concentration over the range tested (up to 180 millimolar). The H⁺/Na⁺ exchange indicated by these results was not due to the creation of a Na⁺ diffusion potential. Formation of ΔpH in these vesicles was stable to NO₃⁻ up to 100 millimolar; further, the dissipating effect of Na⁺ supply was apparent on a ΔpH formed in the presence of 30 millimolar NO₃⁻. Additional evidence that the origin of the membrane vesicles observed in this investigation was not the tonoplast and was probably the plasmalemma included the vanadate sensitivity of the establishment of the membrane potential.     

Substitution of Na+ by K+ in Tissues and Root Vacuoles of Barley (Hordeum vulgare L. cv. Aramir)

The aim of this paper was to investigate whether Na⁺ accumulated in the vacuoles of barley roots can be replaced by K⁺, or else remain irreversibly trapped.Barley seedlings (Hordeum vulgare L. cv. Aramir) were grown for 2 days on nutrient solutions and then salinized over 24 h by a linear increase in NaCl concentration (50 mol · m⁻³). NaCl was replaced by an equimolar concentration of KCl and 1 day later the KCl concentration was gradually decreased over 23 hours. After this the plants were grown for 2 further days on basic nutrient solution. Plant material was harvested at each experimental step in order to measure plant growth and K⁺ and Na⁺ concentrations in vacuoles and tissues.The results from quantitated, energy dispersive X-ray analyses on frozen, hydrated vacuoles of the rhizodermls and 3 subsequent cortical cell rows, point to a proportional replacement of K⁺ by Na⁺ after NaCl stress. A replacement of Na⁺ by K⁺ was clearly found in the rhizodermal vacuoles after KCl treatment. Flame emission spectrometric data from corresponding root segments support these results.     

NaCl Uptake in Roots of Zea mays Seedlings: Comparison of Root Pressure Probe and EDX Data

The extent by which salinity affects plant growth depends partlyon the ability of the plant to exclude NaCl. To study the uptakeof NaCl into excised roots of Zea mays L. cv. ‘Tanker’,two different techniques were applied. A root pressure probewas used to record steady state as well as transient valuesof root (xylem) pressure upon exposure of the root to mediacontaining NaCl and KCl as osmotic solutes. In treatments withNaCl, pressure/time responses of the root indicated a significantuptake of NaCl into the xylem. NaCl induced kinetics were completelyreversible when the NaCl solution was replaced by an isosmoticKCl solution. This indicated a passive movement of Na⁺-saltsacross the root cylinder. Root samples were taken at differenttimes of exposure to NaCl and prepared for X-ray microanalysis(EDX analysis). Radial profiles of ion concentrations (Na⁺,K⁺, Cl^–) were measured in cell vacuoles and xylem vesselsalong the root axis. Na⁺ appeared rapidly in mature xylem (earlymetaxylem) and living xylem (late metaxylem) before it was detectablein vacuoles of the root cortex. EDX results confirmed that thekinetics observed by the pressure probe technique correspondedmainly to an influx of Na⁺-salts into early metaxylem. In latemetaxylem, the uptake of Na⁺ was associated with a decline ofK⁺. The Na⁺/K⁺ exchange indicated a mechanism to reduce sodiumfrom the transpiration stream. Ion localization, ion transport, maize, root pressure, salinity, water relations, X-ray microanalysis, Zea mays     

The Combined Effects of Salinity and Root Anoxia on Growth and Net Na+ and K+-accumulation in Zea mays Grown in Solution Culture

The effects of excess salinity and oxygen deficiency on growthand solute relations in Zea mays L. cv. Pioneer 3906 were examinedin greenhouse experiments. The roots of plants 14 d old growingin nutrient solution containing additions of NaCl in the range1.0–200 mol m^–3 were either exposed to a severedeficiency of O₂ by bubbling with nitrogen gas (N₂ treatment),or maintained with a supply of air (controls), for a periodof 1–7 d. The threshold NaCl concentration resulting inappreciable inhibition of leaf extension, and shoot f. wt gainin controls was between 10 and 25 mol m^–3. At 25 mol m^–3NaCl the ratio of Na+/K+ transported to shoots was about 20times greater than in plants in 1.0 mol m^–3 NaCl. Theeffect of addition of NaCl to the nutrient solution was to enhanceNa+ movement but simultaneously depress the rate of K+ transportto shoots (per g f. wt roots). Interactions between NaCl levels and aeration treatment wereshown by analyses of variance to be statistically significantfor leaf extension, shoot and root f. wt gains, Na+ and K+ concentrationsin shoots and roots. When roots were N₂-treated, shoot and rootgrowth were depressed, the effect of aeration treatment beinggreatest at NaCl concentrations of 50 mol m^–3 or less.Additionally, N₂-treatment greatly accelerated Na- transportto shoots while depressing K+ transport still further, so thatat 10 mol m^–3 NaCl the ratio Na+/K+ acquired by the shootswas 230 times greater than in controls. Over the concentrationrange 1.0 to 50 mol m^–3 NaCl, the ratio Na+/K+ transportedto shoots by anoxic roots increased by a factor of 860. Mechanisms controlling changes in solute flux to the shoot,and the significance in relation to plant tolerance of excesssalts or oxygen deficiency are discussed. Anaerobic, corn, flooding, maize, oxygen-deficiency, salinity     

Effects of internal Na and external K concentrations on Na/K coupling of Na,K-pump in frog skeletal muscle

Summary To clarify the dependency of the Na/K coupling of the Na,K-pump on internal Na and external K concentrations in skeletal muscle, the ouabain-induced change in membrane potential, the ouabain-induced change in Na efflux and the membrane resistance were measured at various internal Na and external K concentrations in bullfrog sartorius muscle.Upon raising the internal Na concentration from 6 mmol/kg muscle water to 20 mmol/kg muscle water, the magnitude of the ouabain-induced change in membrane potential increased about eightfold and the magnitude of the ouabain-induced change in Na efflux increased about fivefold while the membrane resistance was not significantly changed. As the external K concentration increased from 1 to 10mm, the magnitude of the ouabain-induced change in membrane potential decreased (1/5.5 fold), while the magnitude of the ouabain-induced change in Na efflux increased (about 1.5-fold). The membrane resistance decreased upon raising the external K concentration from 1 to 10mm (1/2-fold). These observations imply that the values of the Na/K coupling of the Na,K-pump increases upon raising the internal Na concentration and decreases upon raising the external K concentration.     

The Kinetics of Bicarbonate and Malate Exchange in Carrot and Barley Root Cells

The time course of loss of¹⁴C from H¹⁴CO₃-labelled carrot tissuehas been measured. The graph of log (¹⁴C remaining in the cells)versus time can be fitted by two exponential components. Thegraph of log (rate of ¹⁴C loss over successive periods) versustime can also be fitted by two exponential components usingthe same fitting procedure. However, the half times of the componentsfitting the two types of graph are not the same, and thereforethe apparently good fit is not valid. Three exponential componentscan be fitted to the two types of plot such that their rateconstants are equal and their intercepts in the correct theoreticalrelationship. Their rate constants are about 7 h^–1, 1.5h^–1, and less than 0.1 h^–1, and they probably correspondto the total -CO₂ in the cell, the malatein the cytoplasm, and the malate in the vacuole, respectively.From these data it is shown that one can calculate the influxof -CO₂ across the plasmalemma, and the influxof malate across the tonoplast during accumulation of endogenouslyproduced malate. The time course of uptake of ¹⁴C-malate is estimated as thesum of ¹⁴C accumulated in the tissue and ¹⁴C evolved as CO₂.At 1 mM external malate total uptake is linear with time, suggestingthat uptake is limited by and equal to the influx across theplasmalemma. At higher external malate concentrations the evolutionof ¹⁴CO₂ saturates but accumulation in the tissue continuesto rise. Under these conditions it is concluded that the tonoplastinflux of malate can be calculated and that this influx doesnot saturate at high external malate concentrations. The net efflux of malate is very small but measurable. The effluxof ¹⁴C-labelled malate is not stimulated by external malate^–,Cl^–, or . There are therefore no plasmalemma systems exchanging internal malatefor these anions.     

影响玉米根中K+、Na+离子向地上部输送的因素（简报）

培养液中含有 1mmol·L-1K 、Na 离子的玉米根运往木质部渗出液中的K /Na 比最大 ;低温不影响渗出液中的K /Na 比 ,但极大地减少了根部向地上部输送的K 、Na 总量 ;钙的加入可以大大提高渗出液中的K /Na 比 ,减少由根部向地上部输送的Na 量。     

Root Growth, Secondary Root Formation and Root Gravitropism in Carotenoid-deficient Seedlings of Zea mays L.

The effect of ABA on root growth, secondary-root formation androot gravitropism in seedlings of Zea mays was investigatedby using Fluridone-treated seedlings and a viviparous mutant,both of which lack carotenoids and ABA. Primary roots of seedlingsgrown in the presence of Fluridone grew significantly slowerthan those of control (i.e. untreated) roots. Elongation ofFluridone-treated roots was inhibited significantly by the exogenousapplication of 1 mM ABA. Exogenous application of 1 µMand 1 nM ABA had either no effect or only a slight stimulatoryeffect on root elongation, depending on the method of application.The absence of ABA in Fluridone-treated plants was not an importantfactor in secondary-root formation in seedlings less than 9–10d old. However, ABA may suppress secondary-root formation inolder seedlings, since 11-d-old control seedlings had significantlyfewer secondary roots than Fluridone-treated seedlings. Rootsof Fluridone-treated and control seedlings were graviresponsive.Similar data were obtained for vp-9 mutants of Z. mays, whichare phenotypically identical to Fluridone-treated seedlings.These results indicate that ABA is necessary for neither secondary-rootformation nor for positive gravitropism by primary roots. Zea mays, gravitropism, carotenoid-deficient, Fluridone, root growth, vp-9 mutant     

Water Transport Properties of Roots and Root Cortical Cells in Proton- and Al-Stressed Maize Varieties

Root and root cell pressure-probe techniques were used to investigate the possible relationship between Al- or H+-induced alterations of the hydraulic conductivity of root cells (LPc) and whole-root water conductivity (LPr) in maize (Zea mays L.) plants. To distinguish between H+ and Al effects two varieties that differ in H+ and Al tolerance were assayed. Based on root elongation rates after 24 h in nutrient solution of pH 6.0, pH 4.5, or pH 4.5 plus 50 [mu]M Al, the variety Adour 250 was found to be H+-sensitive and Al-tolerant, whereas the variety BR 201 F was found to be H+-tolerant but Al-sensitive. No Al-induced decrease of root pressure and root cell turgor was observed in Al-sensitive BR 201 F, indicating that Al toxicity did not cause a general breakdown of membrane integrity and that ion pumping to the stele was maintained. Al reduced LPc more than LPr in Al-sensitive BR 201 F. Proton toxicity in Adour 250 affected LPr more than LPc. In this Al-tolerant variety LPc was increased by Al. Nevertheless, this positive effect on LPc did not render higher LPr values. In conclusion, there were no direct relationships between Al- or H+-induced decreases of LPr and the effects on LPc. To our knowledge, this is the first time that the influence of H+ and Al on root and root cell water relations has been directly measured by pressure-probe techniques.     

The Occurrence of Vascular Cavities and Specialized Parenchyma Cells in the Roots of Cool-season Legumes

The seeds of 20 legume species were grown in the greenhouse or in growth chambers at different temperatures. Under warm temperature conditions (above 15 °C), six species, pea (Pisum sativum), broadbean (Vicia faba), chickpea (Cicer arietinum), lentil (Lens culinaris), wild lupine (Lupinus latifolius), and soybean (Glycine max), formed cavities in the vascular cylinder of their primary roots, which in turn became filled by the ingrowth of specialized parenchyma cells (SP cells). When these species were grown at low temperature (below 15 °C), however, a “normal” vascular cylinder formed in the primary roots with late-maturing metaxylem vessel members differentiating in the center. These species were all cool-season legumes except soybean, a warm-season legume, which sometimes also formed cavities and SP cell ingrowths. The occurrence of cavities and SP cells therefore was restricted to the cool-season legumes (except soybean) when they were grown under warm temperature conditions. The position and size of cavities varied among these species. Pea and broadbean usually formed large, axially elongated cavities in the central vascular cylinder, or in the xylem poles. Others formed smaller cavities of various lengths.     

Occurrence and Ecological Significance of GTP in the Ocean and in Microbial Cells

A comparison between the ATP concentrations based on peak height light emission values (0 to 3 s) and integrated light flux determinations (15 to 75 s) for a variety of seawater samples revealed that the integrated method of light detection consistently yielded higher ATP concentrations, ranging from 1.38 to 2.35 times larger than the corresponding peak ATP values. A significant correlation (r = 0.923) was observed for a plot of ΔADP (i.e., integrated ATP - peak ATP) versus GTP + UTP, suggesting that the analytical interference on the ATP assay was the result of the presence of non-adenine nucleotide triphosphates. Size-fractionation studies revealed an enrichment of the non-adenine nucleotide triphosphates, relative to ATP, in the smallest size fraction analyzed (<10 μm). Investigations were conducted with 20 species of unicellular marine algae to determine their intracellular nucleotide concentrations, and these determinations were compared to the levels measured in lab cultures of the marine bacterium Serratia marinorubra. These results indicated that the intracellular GTP/ATP ratios in S. marinorubra increase in direct proportion to the rate of cell growth, and that the GTP/ATP ratios in bacteria are much greater than in growing algae, presumably due to the differences in rates of cellular biosynthesis. It is concluded that quantitative determinations of GTP/ATP ratios in environmental sample extracts may be useful for measuring microbial growth.     

Membrane Potential Hyperpolarization in Mammalian Cardiac Cells by Synchronization Modulation of Na/K Pumps

In previously reported work, we developed a new technique, synchronization modulation, to electrically activate Na/K pump molecules. The fundamental mechanism involved in this technique is a dynamic entrainment procedure of the pump molecules, carried out in a stepwise pattern. The entrainment procedure consists of two steps: synchronization and modulation. We theoretically predicted that the pump functions can be activated exponentially as a function of the membrane potential. We have experimentally demonstrated synchronization of the Na/K pump molecules and acceleration of their pumping rates by many fold through use of voltage-clamp techniques, directly monitoring the pump currents. We further applied this technique to intact skeletal muscle fibers from amphibians and found significant effects on the membrane resting potential. Here, we extend our study to intact mammalian cardiomyocytes. We employed a noninvasive confocal microscopic fluorescent imaging technique to monitor electric field–induced changes in ionic concentration gradient and membrane resting potential. Our results further confirm that the well-designed synchronization modulation electric field can effectively accelerate the Na/K pumping rate, increasing the ionic concentration gradient across the cell membrane and hyperpolarizing the membrane resting potential.     

Effects of Bafilomycin A1 and Metabolic Inhibitors on the Maintenance of Vacuolar Acidity in Maize Root Hair Cells

Proton pumps of tonoplast membranes have been studied extensively in vitro, but data concerning their regulation in vivo are lacking. Effects of either anoxia, or the addition of KCN, 2-deoxy-d-glucose (deoxy-glucose), or bafilomycin-A1 (BAF) on vacuolar pH of maize (Zea mays L.) root hair cells were followed by fluorescence microscopy after loading of 2[prime]7[prime]-bis-(2-carboxyethyl)-5-(and-6) carboxyfluorescein. Root hair cells were able to maintain vacuolar acidity for at least 2 h in the presence of either 10 mM KCN or 50 mM deoxy-glucose or during anoxia. Treatments with either deoxy-glucose or KCN reduced total tissue ATP more than anoxia. ADP accumulated during anoxia and treatment with KCN as detected by in vivo 31P-NMR spectroscopy, but not during deoxy-glucose treatment. With control roots and roots treated with deoxy-glucose, the presence of BAF, a specific inhibitor of the V-type ATPase, caused alkalization of the vacuolar pH. However, either in the presence of KCN or under anoxic conditions, BAF was relatively ineffective in dissipating vacuolar acidity. Therefore, under anoxia or in the presence of KCN, unlike the situation with air or deoxy-glucose, the V-type ATPase apparently is not required for maintenance of vacuolar acidity.     

Regulation of apical K and Na channels and Na/K pumps in rat cortical collecting tubule by dietary K

The patch-clamp technique was used to study the properties and the density of conducting K and Na channels in the apical membrane of rat cortical collecting tubule. The predominant K channel observed in cell- attached patches (SK channels) had an outward single-channel conductance (with LiCl in the pipette) of 10 pS. The inward conductance (with KCl in the pipette) was 42 pS. The channel had a high open probability that increased with depolarization. Kinetic analysis indicated the presence of a single open state and two closed states. Increasing K intake by maintaining animals on a high K diet for 12-16 d increased the number of SK channels per patch by threefold (0.7- 2.0/patch) over control levels. In addition, conducting Na-selective channels, which were not observed in control animals, were seen at low density (0.5/patch). These channels had properties similar to those observed when the animals were on a low Na diet, except that the mean open probability (0.84) was higher. In other experiments, the whole- cell patch clamp technique was used to measure Na channel activity (as amiloride-sensitive current, INa) and Na pump activity (as ouabain- sensitive current, Ipump). In animals on a high K diet, INa was greater than in controls but much less than in rats on a low Na diet. Ipump was greater after K loading than in controls or Na-depleted animals. These K diet-dependent effects were not accompanied by a significant increase in plasma aldosterone concentrations. To further investigate the relationship between K channel activity and mineralocorticoids, rats were maintained on a low Na diet to increase endogenous aldosterone secretion. Under these conditions, no increase in SK channel density was observed, although there was a large increase in the number of Na channels (to 2.7/patch). Aldosterone was also administered exogenously through osmotic minipumps. As with the low Na diet, there was no change in the density of conducting SK channels, although Na channel activity was induced. These results suggest that SK channels, Na channels and Na/K pumps are regulated during changes in K intake by factors other than aldosterone.     

Role of the Plasmalemma H-ATPase in Pseudomonas syringae-Induced K/H Exchange in Suspension-Cultured Tobacco Cells

Activation of a host plasma membrane K⁺ efflux/net H⁺ uptake exchange by pathogenic pseudomonads plays an important role in the development of hypersensitivity in tobacco (Nicotiana tabacum). Involvement of the plasmalemma H⁺-pumping ATPase in this response was investigated. The exchange response of suspension-cultured tobacco cells to Pseudomonas syringae pv syringae was reduced 90% or more by ATPase inhibitors including vanadate, N-ethylmaleimide, and N,N′-dicyclohexylcarbodiimide. The exchange was also strongly inhibited by the protonophore carbonyl cyanide m-chlorophenylhydrazone and by slightly alkaline external pH. Respiratory inhibitors such as oligomycin and sodium azide reduced the exchange by 50% to 75%, while glycolysis inhibitors such as sodium arsenite and sodium iodoacetate decreased exchange by approximately 90%. These results suggest that plasmalemma H⁺-ATPase activity is required for the exchange response and that this may reflect a requirement for a plasmalemma pH and/or electrical potential gradient.     

K/Na selectivity at the plasmalemma of cortical root cells and preferential release of sodium to the xylem vessels in roots of Atriplex hortensis

Using excised roots of Atriplex hortensis L., cv. Gelbe Gartenmelde, the uptake, accumulation and xylem transport of K⁺ and Na⁺ have been measured. Influx as well as xylem transport proved to discriminate little between K⁺ and Na⁺, when considered in relation to the external solution. Both K⁺ and Na⁺ inhibited the uptake and xylem transport of each other to about the same degree. Measurements of intracel-lular Na⁺ fluxes by means of compartment analysis indicated that the low degree of K/Na discrimination during uptake was due to low influx selectivity. Moreover, K⁺/Na⁺ exchange at the plasmalemma was not very efficient in Atriplex roots. In order to establish the basis of the low K/Na discrimination in xylem transport, the rates of K⁺ and Na⁺ transport were related to the cytoplasmic K⁺ and Na⁺ concentrations to yield the selectivity ratio of transport, S(transport) = (φ_cx(K) × [Na⁺]_c)/(φ_cx(Na) × [K⁺]_c). Under all conditions this ratio was far below one indicating that Na⁺ was favoured during xylem release in excised roots of Atriplex at low external concentrations. The implications of this discrimination in favour of Na+ are discussed with respect to salt tolerance of A. hortensis .