Mapping of the K+/Na+ discrimination locus Kna1 in wheat

In saline environments, bread wheat, Triticum aestivum L. (genomes AABBDD), accumulates less Na⁺ and more K⁺ in expanding and young leaves than durum wheat, T. turgidum L. (genomes AABB). Higher K⁺/Na⁺ ratios in leaves of bread wheat correlate with its higher salt tolerance. Chromosome 4D from bread wheat was shown in previous work to play an important role in the control of this trait and was recombined with chromosome 4B in the absence of the Ph1 locus. A population of plants disomic for 4D/4B recombined chromosomes in the genetic background of T. turgidum was developed to investigate the genetic control of K⁺/Na⁺ discrimination by chromosome 4D. Evidence was obtained that the trait is controlled by a single locus, designated Kna1, in the long arm of chromosome 4D. In the present work, K⁺/Na⁺ discrimination was determined for additional families with 4D/4B chromosomes. The concentrations of Na⁺ and K⁺/Na⁺ ratios in the youngest leaf blades clustered in two nonoverlapping classes, and all recombinant families could be unequivocally assigned to Kna1 and kna1 classes. The Kna1 locus scored this way was mapped on a short region in the 4DL arm and was completely linked to Xwg199, Xabc305, Xbcd.402, Xpsr567, and Xpsr375; it was also mapped as a quantitative trait. The results of the QTL analysis, based on the K⁺/Na⁺ ratios in the young leaves of greenhousegrown plants and flag leaves of field-grown plants, agreed with the position of Knal determined as a qualitative trait. Several aspects of gene introgression by manipulation of the Ph1 locus are discussed.     

Selective Effect of Salt Stress on the Activity of Two ATPases in the Cell Membrane of Nostoc muscorum

ATPase activity in the cell membrane of a salt-stressed cyanobacterium,Nostoc muscorum M-14, was examined cytochemically by three differentstaining protocols. Application of Hulstaert's method resultedin distinct precipitation of the reaction products of ATPaseinside the cell membrane exclusively. No reaction products wereformed when ATP was replaced by GTP or when dicyclohexylcarbodiimideor N-ethylmaleimide was present in the reaction mixture. Bycontrast, low levels were detectable after the reaction in thepresence of ouabain. Bafilomycin did not affect the formationof products. Mayahara's method, which is considered to demonstratethe reaction of Na⁺,K⁺-ATPase activity, revealed the presenceof a ouabain-sensitive Na⁺,K⁺-ATPase in the cell membrane, whileWachstein-Meisel's method revealed the presence of an ATPaseactivity that was resistant to ouabain. It appears, therefore,that cell membranes of Nostoc muscorum contain both ouabain-sensitiveATPase and ouabain-insensitive ATPase. Comparison of the stainingprofiles of salt-stressed cells with those of control cellssuggested that a high-salt environment activates the ouabain-sensitiveNa⁺,K⁺-ATPase, which seems likely to be involved in the effluxof Na⁺ ions. (Received February 7, 1995; Accepted August 9, 1995)     

Enhancement of the salt tolerance of Triticum turgidum L. by the Kna1 locus transferred from the Triticum aestivum L. chromosome 4D by homoeologous recombination

Durum wheat, Triticum turgidum L. (2n= 4x=28, genome formula AABB) is inferior to bread wheat, T. aestivum L. (2n=6x=42, genome formula AABBDD), in the ability to exclude Na⁺ under salt strees, in the ratio of the accumulated K⁺ to Na⁺ in the leaves under salt stress, and in tolerance of salt stress. Previous work showed that chromosome 4D has a major effect on Na⁺ and K⁺ accumulation in the leaves of bread wheat. The 4D chromosome was recombined with chromosome 4B in the genetic background of durum wheat. The recombinants showed that Na⁺ exclusion and enhanced K⁺/Na⁺ ratio in the shoots were controlled by a single locus, Kna1, in the long arm of chromosome 4D. The recombinant families were grown in the field under non-saline conditions and two levels of salinity to determine whether Kna1 confers salt tolerance. Under salt stress, the Kna1 families had higher K⁺/Na⁺ ratios in the flag leaves and higher yields of grain and biomass than the Kna1 ^- families and the parental cultivars. Kna1 is, therefore, one of the factors responsible for the higher salt tolerance of bread wheat relative to durum wheat. The present work provides conceptual evidence that tolerance of salt stress can be transferred between species in the tribe Triticeae.     

Salt, nutrient uptake and transport, and ABA of Populus euphratica; a hybrid in response to increasing soil NaCl

The uptake and transport of salt ions (Na⁺, Cl^-), macronutrients (K⁺, Ca²⁺, Mg²⁺) and abscisic acid (ABA) response to increasing soil salinity were examined in 2-year-old seedlings of Populus euphratica and a hybrid, P. talassica Kom 2 (P. euphratica + Salix alba L.). Leaf burn symptoms appeared in the hybrid after 8 days of exposure to salinity when soil NaCl concentration increased to 206 mM, whereas P. euphratica exhibited leaf damage after day 21 when soil NaCl exceeded 354 mM. Leaf necrosis was the result of excess salt accumulation since the injury followed an abrupt increase of endogenous salt levels. Compared with the hybrid, P. euphratica exhibited a greater capacity to exclude salt ions from leaves under increasing salinity, especially Cl^-. Salt treatment altered nutrient balance of the hybrid, leaf K⁺, Ca²⁺ and Mg²⁺ concentrations significantly declined and the same trends were observed in roots with the exception of K⁺. Although K⁺ levels decreased in salinised P. euphratica, increasing salinity did not affect the levels of Ca²⁺ and Mg²⁺ in leaves, but did increase the uptake of these nutrients when salt stress was initiated. NaCl-induced increase of ABA concentration in xylem sap [ABA] was observed in the two tested genotypes, however xylem [ABA] increased more rapidly in P. euphratica and a fivefold increase of xylem [ABA] was recorded after the first day of exposure to salt stress. Therefore, we conclude that the increase of Ca²⁺ uptake may be associated with the rise of ABA, and thus contributes to membrane integrity maintenance, which enables P. euphratica to regulate uptake and transport of salt ions under high levels of external salinity in the longer term.     

Ion transport systems in the kidney and urinary bladder of two Antarctic teleosts, Chionodraco hamatus and Trematomus bernacchii

Na⁺-K⁺-Cl^- cotransport and Na⁺/K⁺ATPase were studied by immunohistochemistry in the kidney and urinary bladder of Trematomus bernacchii and Chionodraco hamatus. The activity was correlated to the density of mitochondria. The first segment of the renal proximal tubule was more active than the second one. In T. bernacchii and the temperate marine teleost Pagellus bogaraveo, the immunoreactivity for the antibody to cotransporters and to the !-subunit of the sodium pump was stronger than in the icefish. This difference indicates in the kidney of the icefish, a weaker secretory activity, a consequent lower osmolarity in the lumen and lower water loss, which correlates well with the need for a greater blood volume in the icefish. The epithelium of the urinary bladder in T. bernacchii, where intense immunostaining was observed, was composed of columnar cells. In C. hamatus the columnar cells, where the immunostaining was weaker, lined only a portion of the urinary bladder, the other region being composed of cuboidal cells.     

Hypoxia affects root sodium and chloride concentrations and alters water conductance in salt-treated jack pine (<Emphasis Type="Italic">Pinus banksiana</Emphasis>) seedlings

The effects of NaCl were studied in 6-month-old jack pine (Pinus banksiana Lamb.) seedlings growing in solution culture under hypoxic (approximately 2 mg l^у O₂) and well-aerated (approximately 8 mg l^у O₂) conditions. The results showed that hypoxia led to further reduction of stomatal conductance (g_s) in plants treated with 45 mM NaCl. This effect was likely due to a reduction in root hydraulic conductance by both stresses. When applied individually or together, neither 45 mM NaCl nor hypoxia affected cell membrane integrity of needles as measured by tissue electrolyte leakage. Hypoxia did not alter shoot Na⁺ and Cl^m concentrations in NaCl-treated plants. However, root Na⁺ concentrations were lower in NaCl-treated hypoxic plants, suggesting that hypoxia affected the ability of roots to store Na⁺. Hypoxia also induced root electrolyte leakage from NaCl-treated and control plants. The higher root Cl^m concentrations compared with Na⁺ and the positive correlation between root Cl^m concentrations and electrolyte leakage suggest that Cl^m played a major role in salt injury observed in jack pine seedlings. Roots of well-aerated plants treated for 1 week with NaCl contained almost two-fold higher concentration of total non-structural carbohydrates compared with plants from other experimental treatments and these concentrations decreased in subsequent weeks. We suggest that under prolonged hypoxic conditions, roots lose the ability to prevent Cl^m uptake resulting in the increase in root Cl^m concentration, which has damaging effects on root cell membranes.     

Seawater tolerance in first-time migrants of anadromous Arctic charr (Salvelinus alpinus)

Anadromous juvenile Arctic charr (Salvelinus alpinus) were caught in the Sila River (66°21'N, 13°10'E, Nordland County, Norway) by use of a fish trap during their migration towards the sea in May/June. The absence of tags (not captured before) and examination of otoliths from sacrificed fish revealed that none of the experimental charr had previously encountered seawater. During 4 days of exposure to seawater, only minor changes in blood plasma osmolality, and blood plasma concentrations of Na⁺, Cl^- and Mg²⁺ were detected. An increase in Na⁺-K⁺-ATPase activity was measured during the exposure period. These results verified that first-time migrants exhibit hypoosmoregulatory capacity similar to that of smoltified Atlantic salmon (Salmo salar). Accordingly, the Arctic charr first-time migratory behaviour, including late descent and a possible stay within an estuary, cannot be taken as evidence of a poorly developed seawater tolerance.     

Quantitative analyses of shade-shoot architecture of conifers native to New Zealand

Shoot architecture was quantified by measuring the "maximum silhouette area ratio" (R_max). R_max was calculated from the maximum silhouette area (or projected area) of the intact shoot, divided by the silhouette area of the leaves or phylloclades (leaf-like flattened stems) when they are removed from the shoot and laid out flat. Like conifers of the Northern Hemisphere (NH) with non-appressed foliage, the R_max of shade-adapted shoots ranged from 0.5 to 1.0 in New Zealand (NZ) conifers with non-appressed foliage. Defining a "leaf" to mean either a true leaf or a phylloclade, the following was found: leaf area/leaf dry weight, leaf area/shoot dry weight, and leaf dry weight/shoot dry weight, were all similar in the shade-shoots of NZ and NH conifers. None of these variables were significantly correlated with R_max in the NZ conifers, unless species with leaves averaging less than 4 mm² in size were excluded from the analyses. Foliage dry weight/shoot projected area was strongly correlated with R_max. NZ conifers had both smaller and larger mean leaf sizes in comparison to NH conifers. The mean projected area per shade-adapted leaf of NZ conifers varied from 2.7 to 436 mm². In NH conifers, the mean projected area per shade leaf varied from 12 to 83 mm². Except for the strikingly larger range in leaf size in NZ conifers, the data support a hypothesis of strong convergent evolution of shade-shoot architecture in NZ and NH conifers. The results are discussed in relation to photosynthesis, stand production, and the ecological distribution of conifers.     

Plants feed ants: food bodies of myrmecophytic Piper and their significance for the interaction with Pheidole bicornis ants

Several species of Piper (Piperaceae) live in symbiosis with Pheidole bicornis (Formicidae-Myrmicinae) on the southern Pacific slope of Costa Rica. These plants produce small single-celled food bodies (FBs) in leaf domatia, formed by the petiole bases and roofing leaf sheaths. In the present study the dependency of ants on FBs of Piper fimbriulatum as a food source was analysed by comparing the natural abundance of ¹³C and ¹⁵N in ants and FBs. Both '¹³C and '¹⁵N values were very similar between FBs and Pheidole bicornis ants but differed substantially between the plant and other ant species. Therefore we suggest that FBs are a main food source for Pheidole bicornis ants. To strengthen this suggestion, the chemical composition of FBs of four myrmecophytic Piper species was analysed, with special emphasis on the nutritional requirements of inhabiting Pheidole bicornis ants. Standard chemical methods were modified and combined to a novel analysis scheme by which all major FB constituents could be quantified from minute [3-10 mg dry mass (DM)] quantities. Piper FBs mainly consisted of lipids (41-48% of DM) and proteins (17-24% of DM). Soluble carbohydrates and amino acids proved to be quantitatively unimportant. N was predominantly stored as soluble protein and, thus, was easily available to the ants. FBs proved to be a high-energy food source (up to 23 kJ g^-1 DM), with a chemical composition that meets well the nutritional needs of the inhabiting ants.     

Na(+)/H(+) exchanger NHE1 as plasma membrane scaffold in the assembly of signaling complexes

The plasma membrane Na⁺/H⁺ exchanger NHE1 has an established function in intracellular pH and cell volume homeostasis by catalyzing electroneutral influx of extracellular Na⁺ and efflux of intracellular H⁺. A second function of NHE1 as a structural anchor for actin filaments through its direct binding of the ezrin, radixin, and moesin (ERM) family of actin-binding proteins was recently identified. ERM protein binding and actin anchoring by NHE1 are necessary to retain the localization of NHE1 in specialized plasma membrane domains and to promote cytoskeleton-dependent processes, including actin filament bundling and cell-substrate adhesions. This review explores a third function of NHE1, as a plasma membrane scaffold in the assembly of signaling complexes. Through its coordinate functions in H⁺ efflux, actin anchoring, and scaffolding, we propose that NHE1 promotes protein interactions and activities, assembles signaling complexes in specialized plasma membrane domains, and coordinates divergent signaling pathways. hydrogen ion efflux; intracellular pH; molecular scaffold     

Loss of calcineurin homologous protein-1 in chicken B lymphoma DT40 cells destabilizes Na+/H+ exchanger isoform-1 protein

NHE1/SLC9A1 is a ubiquitous isoform of vertebrate Na⁺/H⁺ exchangers (NHEs) functioning in maintaining intracellular concentrations of Na⁺ and H⁺ ions. Calcineurin homologous protein-1 (CHP1) binds to the hydrophilic region of NHE1 and regulates NHE1 activity but reportedly does not play a role in translocating NHE1 from the endoplasmic reticulum to the plasma membrane. However, an antiport function of NHE1 requiring CHP1 remains to be clarified. Here we established CHP1-deficient chicken B lymphoma DT40 cells by gene targeting to address CHP1 function. CHP1-deficient cells showed extensive decreases in Na⁺/H⁺ activities in intact cells. Although NHE1 mRNA levels were not affected, NHE1 protein levels were significantly reduced not only in the plasma membrane but in whole cells. The expression of a CHP1 transgene in CHP1-deficient cells rescued NHE1 protein expression. Expression of mutant forms of CHP1 defective in Ca²⁺ binding or myristoylation also partially decreased NHE1 protein levels. Knockdown of CHP1 also caused a moderate decrease in NHE1 protein in HeLa cells. These data indicate that CHP1 primarily plays an essential role in stabilization of NHE1 for reaching of NHE1 to the plasma membrane and its exchange activity. membrane protein; transporter; antiporter; quality control; degradation     

Varietal differences in sodium uptake in barley cultivars exposed to soil salinity or salt spray

Barley varieties are known to differ in the extent of Na⁺ andCl^– accumulation in leaves when grown in saline soil orhydroponic culture. In particular, the cv. Chevron accumulatesmore Na⁺ than the more salt-tolerant cv. CM67, and has lowerleaf K⁺ concentrations. When salt was applied as a spray tothe leaves, CM67 accumulated more Na+ than Chevron, and theselection Sinis 27 (from a landrace collected on the Sinis Peninsulaof Sardinia) accumulated more Na⁺ than Sinis 28. In some casesleaf K⁺ concentrations decreased in response to high concentrationsof salt sprayed on to the leaves. Accumulation of Na⁺ was greaterin the 4th leaf than in the flag leaf. Added CaCI₂ had oppositeeffects when added to the salt applied to the soil or to thesaline spray. In the soil, CaCI₂ reduced Na⁺ uptake; appliedto the leaf it increased Na⁺ uptake. Pre-wetting the leavesbefore the salt spray, or washing the leaves with non-salinewater 1 h after the salt spray, reduced the uptake of Na⁺ andCl^–. It is clear that tolerance to salt applied as saltspray or in the soil are different characteristics. Key words: Barley, salt, foliar uptake, calcium     

Physiological functions of the regulatory domains of the cardiac Na(+)/Ca(2+) exchanger NCX1

Physiologicalfunctions of the intracellular regulatory domains of theNa⁺/Ca²⁺ exchanger NCX1 were studied byexamining Ca²⁺ handling in CCL39 cells expressing alow-affinity Ca²⁺ regulatory site mutant (D447V/D498I), anexchanger inhibitory peptide (XIP) region mutant displaying noNa⁺ inactivation (XIP-4YW), or a mutant lacking most of thecentral cytoplasmic loop (246-672). We found that D447V/D498Iwas unable to efficiently extrude Ca²⁺ from the cytoplasm,particularly during a small rise in intracellular Ca²⁺concentration induced by the physiological agonist -thrombin orthapsigargin. The same mutant took up Ca²⁺ much lessefficiently than the wild-type NCX1 in Na⁺-free medium whentransfectants were not loaded with Na⁺, although itappeared to take up Ca²⁺ normally in transfectantspreloaded with Na⁺. XIP-4YW and, to a lesser extent,246-672, but not NCX1 and D447V/D498I, markedly accelerated theloss of viability of Na⁺-loaded transfectants. Furthermore,XIP-4YW was not activated by phorbol ester, whereas XIP-4YW andD447V/D498I were resistant to inhibition by ATP depletion. The resultssuggest that these regulatory domains play important roles in thephysiological and pathological Ca²⁺ handling by NCX1, aswell as in the regulation of NCX1 by protein kinase C or ATP depletion.

     

The COOH termini of NBC3 and the 56-kDa H+-ATPase subunit are PDZ motifs involved in their interaction

The electroneutralsodium bicarbonate cotransporter 3 (NBC3) coimmunoprecipitates fromrenal lysates with the vacuolar H⁺-ATPase. In renal type Aand B intercalated cells, NBC3 colocalizes with the vacuolarH⁺-ATPase. The involvement of the COOH termini of NBC3 andthe 56-kDa subunit of the proton pump in the interaction of theseproteins was investigated. The intact and modified COOH termini of NBC3 and the 56-kDa subunit of the proton pump were synthesized, coupled toSepharose beads, and used to pull down kidney membrane proteins. Boththe 56- and the 70-kDa subunits of the proton pump, as well as a PDZdomain containing protein Na⁺/H⁺ exchangerregulatory factor 1 (NHERF-1), were bound to the intact 18 amino acidNBC3 COOH terminus. A peptide truncated by five COOH-terminal aminoacids did not bind these proteins. Replacement of the COOH-terminalleucine with glycine blocked binding of both the proton pump subunitsbut did not affect binding of NHERF-1. The 18 amino acid COOH terminusof the 56-kDa subunit of the proton pump bound NHERF-1 and NBC3, butthe truncated and modified peptide did not. A complex of NBC3, the56-kDa subunit of the proton pump, and NHERF-1 was identified in ratkidney. The data indicate that the COOH termini of NBC3 and the 56-kDasubunit of the vacuolar proton pump are PDZ-interacting motifs that arenecessary for the interaction of these proteins. NHERF-1 is involved inthe interaction of NBC3 and the vacuolar proton pump.

     

SGK1 activates Na+-K+-ATPase in amphibian renal epithelial cells

Serum- and glucocorticoid-induced kinase 1 (SGK1) is thought to be an important regulator of Na⁺ reabsorption in the kidney. It has been proposed that SGK1 mediates the effects of aldosterone on transepithelial Na⁺ transport. Previous studies have shown that SGK1 increases Na⁺ transport and epithelial Na⁺ channel (ENaC) activity in the apical membrane of renal epithelial cells. SGK1 has also been implicated in the modulation of Na⁺-K⁺-ATPase activity, the transporter responsible for basolateral Na⁺ efflux, although this observation has not been confirmed in renal epithelial cells. We examined Na⁺-K⁺-ATPase function in an A6 renal epithelial cell line that expresses SGK1 under the control of a tetracycline-inducible promoter. The results showed that expression of a constitutively active mutant of SGK1 (SGK1^T_S425D) increased the transport activity of Na⁺-K⁺-ATPase 2.5-fold. The increase in activity was a direct consequence of activation of the pump itself. The onset of Na⁺-K⁺-ATPase activation was observed between 6 and 24 h after induction of SGK1 expression, a delay that is significantly longer than that required for activation of ENaC in the same cell line (1 h). SGK1 and aldosterone stimulated the Na⁺ pump synergistically, indicating that the pathways mediated by these molecules operate independently. This observation was confirmed by demonstrating that aldosterone, but not SGK1^T_S425D, induced an 2.5-fold increase in total protein and plasma membrane Na⁺-K⁺-ATPase ₁-subunit abundance. We conclude that aldosterone increases the abundance of Na⁺-K⁺-ATPase, whereas SGK1 may activate existing pumps in the membrane in response to chronic or slowly acting stimuli. sodium transport; serum- and glucocorticoid-induced kinase; A6 cells; sodium pump     

Ionic Relations of Garden Orache, A triplex hortensis L.: Growth and Ion Distribution at Moderate Salinity and the Function of Bladder Hairs

The growth of garden orache, A triplex hortensis was studiedunder conditions of mild NaCl or Na₂SO₄ salinity. Growth, drymatter production and leaf size were substantially stimulatedat 10 mM and 50 mM Na⁺ salts. Increased growth, however, appearedto be due to a K⁺-sparing effect of Na⁺ rather than to salinityper se. The distribution of K⁺ and Na⁺ in the plant revealeda remarkable preference for K+ in the roots and the hypocotyl.In the shoot the K/Na ratio decreased strongly with leaf age.However, the inverse changes in K⁺ and Na⁺ content with leafage were dependent on the presence of bladder hairs, which removedalmost all of the Na⁺ from the young leaf lamina. Measurementsof net fluxes of K⁺ and Na⁺ into roots and shoots of growingAtriplex plants showed a higher K/Na selectivity of the netion flux to the root compared to the shoot. With increasingsalinity the selectivity ratio S_{K, Na}^* of net ion fluxes tothe roots and to the shoots was increased. The data suggestthat recirculation of K⁺ from leaves to roots is an importantlink in establishing the K/Na selectivity in A. hortensis plants.The importance of K⁺ recirculation and phloem transport forsalt tolerance is discussed. Key words: Atriplex hortensis, Salinity, Potassium, Sodium, K⁺ retranslocation, Bladder hairs, Growth stimulation     

The Role of the Stem in the Partitioning of Na+ and K+ in Salt-Treated Barley

Hordeum vulgare cv. California Mariout was grown for 50 d insand culture at 100 mol m^–3 NaCl. Xylem sap was collectedthrough incisions at the base of individual leaves along thestem axis by applying pressure to the root system. K⁺ concentrationsin the xylem sap reaching individual leaves increased towardsthe apex, while concentrations of Na⁺, NO₃^–, and Cl^–declined. Phloem exudate was obtained by collecting into Li₂EDTAfrom the base of excised leaves. K/Na ratios of phloem exudatesincreased from older to younger leaves. K/Na ratios in xylem sap and phloem exudate were combined withchanges in ion content between two harvests (38 and 45 d aftergermination) and the direction of phloem export from individualleaves, to construct an empirical model of K⁺ and Na⁺ net flowswithin the xylem and phloem of the whole plant. This model indicatesthat in old leaves, phloem export of K⁺ greatly exceeded xylemimport. In contrast, Na⁺ export was small compared to importand Na⁺ once imported was retained within the leaf. The direction of export strongly depended on leaf age. Old,basal leaves preferentially supplied the root, and most of theK⁺ retranslocated to the roots was transferred to the xylemand subsequently became available to the shoot. Upper leavesexported to the apex. Young organs were supplied by xylem andphloem, with the xylem preferentially delivering Na⁺ , and thephloem most of the K⁺ . For the young ear, which was still coveredby the sheath of the flag leaf, our calculation predicts phloemimport of ions to such an extent that the surplus must havebeen removed by an outward flow in the xylem. Within the culm,indications for specific transfers of K⁺ and Na⁺ between xylemand phloem and release or absorption of these ions by the tissuewere obtained. The sum of these processes in stem internodes and leaves ledto a non-uniform distribution of Na⁺ and K⁺ within the shoot,Na⁺ being retained in old leaves and basal stem internodes,and K⁺ being available for growth and expansion of young tissues. Key words: Hordeum vulgare L., K⁺, Na⁺, stem, salt stress     

Na+ fluxes in Chara under salt stress

The influx and efflux of Na⁺ across the plasma membrane of Characorallina and Chara longifolia were examined under mild saltstress conditions. Na⁺ influx was found to be rapid in bothspecies with the freely exchangeable cytoplasmic Na⁺ cominginto isotopic equilibrium with external ²²Na⁺ within 1 h ofexposure to isotope. Cytoplasmlc Na⁺ concentration and Na⁺ influxwere greater in C. corallina than in C. longifolla under thesame conditions. Na⁺ influx across the tonoplast was much lowerthan the flux across the plasma membrane. Na⁺ efflux was stimulatedat pH 5 relative to pH 7 by 218% in C. coralllna and 320% inC. longifolia. In both species externally applied Li⁺ inhibitedNa⁺ efflux at pH 5 but not at pH 7. Na⁺ etflux was not significantlyinhibited by amiloride. Key words: Na⁺ influx, Na⁺ efflux, Na⁺/H⁺ antiport, Chara     

A Sodium Pump in the Plasma Membrane of the Marine Alga Heterosigma akashiwo

ATP-dependent transport of ²²Na⁺ into liposomes reconstitutedfrom plasma membrane proteins of Heterosigma akashiwo was examined.The apparent K^m values for transport of Na⁺ were 400 µMfor ATP and 7 mM for Na⁺. ATP-dependent transport of ²²Na⁺ wasnot inhibited by a protonophore or a membrane-permeable cationbut was inhibited by an inhibitor of P-type ATPases. (Received October 2, 1995; Accepted February 1, 1996)