Biophysical and Biochemical Regulation of Cytoplasmic pH in Chara corallina during Acid Loads

The contribution of membrane transport to regulation of cytoplasmicpH in Chara corallina has been measured during proton-loadingby uptake of butyric acid. In the short-term (i.e. up to 20min) uptake of butyric acid is not affected by removal of externalK⁺, Na⁺ or Cl^– but over longer periods uptake is decreased(by 20–50% in different experiments) in the absence ofexternal Na⁺ or, sometimes, K⁺. Influxes of both Na⁺ and K⁺increase temporarily after addition of butyrate, Na⁺ immediatelyand K⁺ after a lag. Effects on Cl^– influx are small butCl^– efflux increases enormously after a short lag. Anapproximate comparison of internal butyrate with changes inthe concentration of K⁺, Na⁺, and Cl^– suggests that initially(i.e. for a few min) cytoplasmic pH is determined by bufferingand possibly by some decarboxylation of organic acids (biochemicalpH regulation), and that biophysical pH regulation involvingefflux of H⁺ balanced by influxes of K⁺, Na⁺ and especiallyefflux of Cl^– progressively becomes dominant. When butyric acid is washed out of the cells, cytoplasmic pHis restored completely or partially (depending on the butyrateconcentration used) and this is independent of the presenceor absence of external Cl^–. Where Cl^– is present,its influx is relatively small. It is suggested that cytoplasmicpH is then controlled biochemically, involving the synthesisof an (unidentified) organic acid and the accumulation of acidicanions in place of butyurate lost from the cell. During thesecond application of butyrate, net Cl^– efflux is small:it is suggested that control of cytoplasmic pH then involvesdecarboxylation of the organic acid anions. The questions of the source of Cl^– lost from the cell(cytoplasm or vacuole) and of possible cytoplasmic swellingassociated with the accumulation of butyrate are discussed. Key words: Chara corallina, butyric acid, cytoplasmic pH, membrane transport     

Effect of Divalent Cations on Na+ Permeability of Chara corallina and Freshwater Grown Chara buckellii

The effect of elevated Na⁺ concentration on Na⁺ permeability(P_Na) and Na⁺ influx in the presence of two levels of externaldivalent cations was determined in Chara corallina and freshwater-culturedChara buckellii. When Na⁺ in the medium was increased from 1.0to 70 mol m^–3, Na⁺ influx increased in both species ifCa²⁺ was low (0.1 mol m–3). If Ca²⁺ was increased to 7.0mol m^–3 when Na⁺ was increased, Na⁺ influx remained atthe low control level in C. corallina, and showed only a temporaryincrease in C. buckellii. Mg²⁺ was a better substitute for Ca²⁺in C. buckellii than in C. corallina. Na⁺ permeability data suggest that when the external Ca²⁺ concentrationis low, P_Na does not increase in the presence of elevated NaCl;the increase in Na⁺ influx appears to be due to the increasein external Na⁺ concentration alone. Ca^{2 +} supplementation appearsto decrease P_Na whereas supplemental Mg²⁺ has no effect. Na⁺ effluxes were computed from previously determined net fluxesand the influxes. It was found that for both species, fluxesin both directions were stimulated in response to all experimentaltreatments, but Na⁺ influx always exceeded efflux. This resultedin net Na⁺ accumulation in the vacuoles of both species. The results are discussed with reference to net flux and electrophysiologicaldata obtained previously under identical conditions, as wellas the comparative salinity tolerance of both species and theNa⁺/divalent cation ratio. Key words: Na⁺ influx, Na⁺ tolerance, membrane potential, permeability, Chara     

Effect of K+, its Counter Anion, and pH on Sodium Efflux from Barley Root Tips

Sodium efflux from ²²Na⁺-loaded root tips root tips of Hordeumvulgare L. was markedly increased by replacing 10mM Na₂SO₄ inthe washing solution by K₂SO₄ with the same electrical conductivity.This increase was inhibited by both an uncoupler and an inhibitorof oxidative phosphorylation but not by ouabain. Potassium ionsdid not enhance Na⁺ efflux in the presence of a rapidly absorbedcounter anion, such as Cl^–, instead of . Efflux of ²²Na⁺ could also be enhanced by a low pH in theabsence of K⁺; this was prevented by uncouplers, but not byan inhibitor of the mitochondrial ATPase. It seems that K⁺ indirectly enhances Na⁺ efflux. It is suggestedthat metabolic K⁺ uptake in excess of the counter anion resultsin a proton gradient across the plasmalemma (acid outside) inducingH⁺/Na⁺ antiport.     

Effect of Potassium on Photosynthate Unloading from Seed Coats of Phaseolus vulgaris L. Specificity and Membrane Location

Rates of ¹⁴C-photosynthate unloading from excised seed-coathalves of Phaseolus vulgaris L. plants were stimulated by externalKCI concentrations in excess of 10 mM with an optimal responseat 100–150 mM KCI. The cellular pattern of ¹⁴C-photosynthatemetabolism was not altered by KCI but the treatment preferentiallystimulated the release of sucrose from the seed-coats. Photosynthateunloading was insensitive to Cl^– and was stimulated bya range of membrane-permeable cations (Na⁺, Mg²⁺ and tetraphenylphosphoniumion) in addition to K⁺. The K⁺ ionophore, valinomycin, abolishedthe K⁺ stimulation of ¹⁴C-photosynthate unloading. A switchto a wash solution containing K⁺ elicited a rapid burst of ¹⁴C-photosynthateunloading; the rate constant for the final phase of ¹⁴C-efflux(probably across the tonoplast) was unaffected by K⁺. The KCItreatment did not change the passive permeability of eitherthe plasmalemma or tonoplast. While sucrose influx across theplasmalemma was insensitive to K⁺, sucrose transfer to the vacuolewas slowed. The results obtained support the postulate thatK⁺ (and other membrane permeable cations) preferentially stimulatesucrose efflux across the plasmalemma of the unloading cellsby serving to carry positive charge in the opposite direction. Phaseolus vulgaris, bean, photosynthate unloading, potassium stimulation, seed-coat     

Potassium Transport in Enteromorpha intestinalis (L.) Link: II. EFFECTS OF MEDIUM COMPOSITION AND METABOLIC INHIBITORS

In springwater (25.5 mol m^–3 Cl^–, 20.4 mol m^–3Na⁺, 0.14 mol m^–3 K⁺) Enteromorpha intestinalis couldnot survive for more than a few weeks unless provided with 0.5mol m^–3 K⁺ in the medium or alternatively exposed to seawaterfor 1 day per week. Maintenance of a cytoplasmic K⁺ level ofabout 200 mol m^–3 is critical for the maintenance of normalmetabolic activity. Net gains of intracellular K⁺ occurred whenthe plants were transferred from low-salinity to seawater; converselylarge net losses occurred when plants were transferred fromseawater to springwater. These two processes were not simplythe reverse of one another; net gain of K⁺ involved a largeincrease in the tracer flux both into and out of the cell butnet loss of K⁺ virtually halted the tracer flux into the cell.Any injury incurred by rapid salinity changes was short-lived;plants were rapidly able to adjust intracellular [K₁.K⁺). K⁺(orto some extent Rb⁺) was found to be necessary in the effluxmedium for ⁴²K⁺ exchange to occur. The osmotic concentrationof the medium was also important but extracellular Na⁺ and Cl^–concentrationswere not critical. K⁺ influx and efflux in both springwaterand seawater were largely independent of light and were sensitivein varying degrees to a range of common metabolic inhibitorsand uncouplers. The results are best explained by the presenceof an active K⁺ influx, generated by an ATP-dependent K⁺ pumpat the plasmalemma. Key words: Enteromorpha, Potassium transport, Salinity changes, Uncouplers, Inhibitors     

Uptake of Imidazole and its Effects on the Intracellular pH and Ionic Relations of Chara corallina

Ammonia (pK_a 9.25) and methylamine (pK_a, 10.65) increase cytoplasmicpH and stimulate Cl^– influx in Chara corallina, theseeffects being associated with influx of the amine cations ona specific porter. The weak base imidazole (pK_a 6.96) has similareffects but diffuses passively into the cell both as an unionizedbase and as a cation. When the external pH is greater than 6.0influx of the unionized species predominates. Imidazole accumulates to high concentrations in the vacuole,where it is protonated. Cytoplasmic pH and vacuolar pH riseby only 0.2–0.3 units, suggesting a large balancing protoninflux across the plasma membrane. Balance of electric chargeis partially maintained by net efflux of K⁺ and net influx ofCl^–. Calculation of vacuolar concentrations of imidazole(from (¹⁴C] imidazole uptake, assuming that there is no metabolism)plus K⁺ and Na⁺ indicates an excess of cations over inorganicanions (Cl^–). However, although the osmotic potentialof the cells increases, also indicating increased solute concentrations,the increase is less than that predicted by the calculated ionicconcentrations. This discrepancy remains to be resolved. Becausethe osmotic potential also increases when imidazole is absorbedfrom Cl^–-free solutions it is likely that maintenanceof charge-balance can also involve synthesis and vacuolar storageof organic or amino acids. Key words: Imidazole, potassium, intracellular pH, membrane transport, Chara     

Effects of Fusicoccin and Abscisic Acid on Sugar and lon Transport from Plant Glands

Fusicoccin (FC) inhibited net excretion of Cl^– by theglands of the pitchers of the carnivorous plant Nepenthes hookeriana;of Na⁺ and Cl^– by the salt glands of the halophytes Limoniumvulgare and L. pectinatum and of K⁺ in the nectar of Acer platanoidesflowers. It had no effect on K⁺ elimination with nectar of Impatienswalleriana (extrafloral nectaries) and Abutilon striatum. Abscisicacid (ABA) stimulated net excretion of K⁺ and Cl^– in Nepenthesand of Na⁺ and Cl^– in Limonium but had no effects on K⁺in nectar. Thus, FC and ABA had opposing effects on ion excretionby the salt eliminating glands of Limonium and Nepenthes. Bothcompounds, however, had similar effects on sugar secretion ofnectary glands which was either inhibited or unaffected by FCand ABA. It is suggested that the effects of FC and ABA on ion excretionby gland cells could be reconciled with literature showing FC-stimulationand possible ABA-inhibition of proton pumps at the plasmalemmaof plant cells. Nepenthes hookeriana, Limonium vulgare, Limonium pectinatum, Acer platanoides, salt-glands, nectaries, excretion, fusicoccin, abscisic acid, proton pump     

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.     

Internal Factors Regulating Nitrate and Chloride Influx in Plant Cells

The primary factor determining the observed decrease in activeC1^– influx during salt accumulation in carrot and barleyroot cells has been shown to be the concentration of C1^–+ NO₃^– in the vacuole. The relationship between C1^– influx and the vacuolar concentrationsof various substances was examined after the tissues had accumulatedions from various salt solutions. After accumulating K⁺ malate,C1^– influx was not reduced, but after accumulating C1^–or NO₃^– salts, C1^– influx was reduced by up to 90per cent. Considering all treatments, C1^– influx was notcorrelated with the vacuolar concentration of K⁺, Na⁺, (K⁺+Na⁺),reducing sugars, malate, C1^–, or NO₃^–, nor withthe cellular osmotic pressure. The correlation coefficient betweenCl^– influx and log (C1^– + NO₃^– concentrationin the vacuole) was highly significant, and accounted for allthe variation in C1^– influx in this experiment. Net NO₃^– influx is similarly reduced by a high C1^–concentration in the vacuole. External Cl^– and NO₃^–have quantitatively different, apparently competitive, effectson C1^– influx. These differ from the apparently negative-feedbackeffects of C1^– and NO₃^– in the vacuole, which arequantitatively similar. Decreasing the internal hydrostatic pressure by raising theexternal osmotic pressure increased active K⁺ influx in Valoniaventricosa, but had no effect on C1^– or K⁺ influx in carrotor maize root cells. Cl^– influx is not related to thereducing sugar concentration during ageing drifts in excisedcarrot root tissue. Acetazolamide did not inhibit C1^– influx to carrot tissue. The implications of this type of negative feedback regulation,and the relationship between C1^– and NO₃^– transportare discussed.     

Inhibition of Cl- Channel Activation in Chara corallina Membrane by Lanthanum Ion

We examined a role of Ca²⁺ in the activation of the two majorion channels, i.e., Cl^– and K⁺ channels at the excitationof the characean plasmalemma. The current-voltage relation (I-Vcurve) of the Chara membrane was compared under the ramp voltageclamp condition before and after external application of 20µMof La³⁺ (a Ca²⁺ channel blocker). The transient inward currentcomponent, which is carried mainly by the efflux of Cl^–,disappeared almost completely in about 30 min with La³⁺ treatment.On the other hand, no effect was observed on the late largeoutward current, which is mainly carried by the efflux of K⁺in a large depolarization region (less negative than –50mV). These results suggest that the Cl^– channel in theChara plasmalemma is activated by Ca²⁺ influx, while the K⁺channel is simply activated by depolarization. (Received April 7, 1986; Accepted June 6, 1986)     

Coupling of Proton Fluxes in the Polar Leaves of Potamogeton lucens L

An attempt has been made to quantify the light-induced H⁺ effluxand influx observed in polar leaves of Potamogeton lucens.Theseproton fluxes are spatially separated. The H⁺ efflux, mediatedby a plasmalemma bound H⁺ –ATPase, occurs across theplasmamembrane at the morphological lower epidermis and is accompaniedby an H⁺ influx (or OH^– efflux) at the upper side oftheleaf. As a result, these leaves exhibit a remarkable pH–polarityin the light. The pH near the lower epidermis may drop to avalueas low as 3.5, while a pH of about 10.5 can be observed at theupper epidermis. Obviously this phenomenon requires theco–ordinationof transport processes in the different cell layers of the leaftissue. These observations led to quantitative studies oftherelation between the H⁺ fluxes at either plasmalemma. Thesefluxes were calculated from the pH values recorded at twodistancesfrom the leaf surface. Although the H⁺ influx always exceededthe efflux, a coupling between the transport processesacrosseither plasma membrane became evident from the time–coursesof the two fluxes. Key words: Potamogeton lucens, proton flux, flux coupling, pH–;polarity     

The Ionic Relations of Acetabularia mediterranea

The concentrations of K⁺, Na⁺, and Cl^– in the cytoplasmand the vacuole of Acetabularia mediterranea have been measured,as have the vacuolar concentrations of SO₄^–– andoxalate. The electrical potential difference between externalsolution, and vacuole and cytoplasm has been measured. The resultsindicate that Cl^– and SO₄^–– are probably transportedactively into the cell, and that active transport of Na⁺ isoutwards. The results for K⁺ are equivocal. The fluxes of K⁺,Na⁺, Cl^–, and S0₄^–– into the cell and theeffluxes of Na⁺ and Cl^– have been determined. The Cl^–fluxes are extremely large. In all cases the plasmalemma isthe rate-limiting membrane for ion movement. A technique isdescribed for the preparation of large, completely viable cellfragments containing only cytoplasm, with no vacuole.     

Measurements of Ion Concentrations and Fluxes in Dunaliella parva

Measurements of K⁺, Na⁺, and Cl^– were made on a halotolerantstrain of Dunaliella growing at 500 mM NaCl, 25 ?C, and a relativelylow light intensity (6000 Lx). Much effort was spent in searchingfor a means of measuring the extracellular volume of fluid trappedbetween the cells of centrifuged pellets. All of the sugarstried as markers were rejected because they were found to bedigested in the cell suspension. The most suitable marker wasfound to be [¹⁴C]polyethylene glycol² (mol. wt. 4000); althoughthis substance was apparently adsorbed to the cell exterior,it was found possible to correct for adsorption and then obtaina reasonable figure for the trapped fluid. The final concentrationsof cell K⁺ and Na⁺ were 128 ? 53 mM and 131 ? 117 mM respectively.Cl^– balanced the sum of K⁺ Na⁺. Influxes of ²²Na⁺, ⁴²K⁺,and ³⁶C1^– were measured in cells in which the ions werein the steady state. Averages of 610 and 6.6 nmol m^–2s^–1 were obtained for Na⁺ and K⁺ respectively. Cl^–influx was divided into 2 phases with values of 1540 and 178mmol m^–2 s^–1. The faster influx was considered tobe across the outer cell membrane. The membrane responsiblefor the slower influx has not been identified. By comparingvalues of the potential difference calculated from the Nernstand Goldman equations, it was concluded that Na⁺ and K⁺ areprobably controlled by active mechanisms, whereas cell Cl^–is likely to be at thermodynamic equilibrium with the medium.     

Entry of Methylammonium and Ammonium Ions into Chara Internodal Cells

Low concentrations of ammonia and methylamine greatly affectmembrane transport by, and the electrical properties of, cellsof Chara corallina (=C. australis). In the presence of theseamines, influx of Cl^– and efflux of K⁺ increase and alarge depolarizing current flows through the cell membrane. Measurements with [¹⁴C]methylamine show that methylamine isabsorbed rapidly over a wide pH range, and that the absorptionisotherm is complex. Methylamine influx is not affected by presenceor absence of Cl^–, K⁺, or Na⁺, but is decreased by additionof . The depolarizing current is associated with a small increase in membrane conductance, except at highpH, and both these effects are reversible. The current showssaturation with increasing amine concentration; when methylamineis 10–12 times more concentrated than ammonia, it producesa current of the same magnitude. The results show that the amines enter the cells as cations( or CH₃) except where external pH is high, and that a specific, selective electrogenicporter is involved. There is no need to invoke active transport,as there is no evidence that internal amine concentrations exceedthe equilibrium (Nernst) concentrations.     

Na+, K+ and Cl- in Xylem Sap Flowing to Shoots of NaCl-Treated Barley

Munns, R. 1985. Na⁺, K⁺ and Cl^– in xylem sap flowing toshoots of NaCl-treated barley.—J. exp. Bot. 36: 1032–1042. Na⁺, Cl^– and K⁺ concentrations were measured in xylemsap obtained by applying pressure to the roots of decapitatedbarley plants grown at external [NaCl] of 0, 25, 50, 100, 150and 200 mol m^–3. For any given NaCl treatment, ion concentrationsin the xylem sap were hyperbolically related to the flux ofwater. Ion concentrations in sap collected at very low volumefluxes (without applied pressure) were 5–10 times higherthan in sap collected at moderate fluxes (under pressure). Fora given moderate volume flux, Na+ concentration in the xylemsap, [Na⁺]_x, was only 4.0 mol m^–3 at external [NaCl] of25–150 mol m^–3, and increased to 7.0 mol m^–3at 200 mol m^–3. [Cl-]x showed a similar pattern. Thisshows there would be little difference in the rate of uptaketo the shoot of plants at 25–150 mol m^–3 externalNaCl and indicates little change even at 200 mol m^-3 NaCl becausetranspiration rates would be much lower. Thus the reduced growthof the shoot of plants at high NaCl concentrations is not dueto higher uptake rates of Na⁺ or Cl^–. The fluxes of Na⁺, Cl^– and K^– increased non-linearlywith increasing volume flux indicating little movement of saltin the apoplast. The flux of K⁺ increased even when [K⁺]_x wasgreater than external [K⁺], indicating that membrane transportprocesses modify the K⁺ concentration in the transpiration streamas it flows through the root system. Key words: -Xylem sap, Na⁺, K⁺, Cl^– fluxes, salinity, barley     

Determination of Volume of Dunaliella Cells by Lithium Dilution Measurements and Derivation of Internal Solute Concentrations

A method has been developed to measure the cell volume of theunicellular green alga Dunaliella parva 19/9 using Li⁺ measurementsonly. Concentrations of internal solutes can also be calculatedif they are assayed in the same samples as Li⁺. We found thatD. parva cells grown in 0.4 kmol m^–3 NaCl have an averageaqueous cell volume of 65.1 ?2.9 µm³, a K⁺ concentrationof 126?6 mol m^–3, a Na⁺ concentration of 11?11 mol m^–3and a glycerol concentration of 615?27 mol m–3 (n= 12).Algae grown in 1.5 kmol m^–3 NaCl have an average aqueouscell volume of 131 ?7.5 µm³, a K⁺ concentration of 109?4mol m^–3, a Na⁺ concentration of 10?39 mol m^–3 anda glycerol concentration of 1 425?59 mol m^–3 (n = 12).These results indicate that D. parva cells adapted to high salinitieshave larger cell volumes than those adapted to lower salinities.However, there is no evidence for a significant difference ininternal Na⁺ concentration, despite the almost 4-fold differencein the concentration of external NaCl. The intracellular glycerolconcentration alone accounts for 65% and 54%, respectively,of the osmotic balance in low and high salt grown cells. Key words: Dunaliella, cell volume, intracellular solutes     

Interactions of NH4+ and L-glutamate with NO3- transport processes of non-mycorrhizal Fagus sylvatica roots

The processes of NO₃^– uptake and transport and the effectsof NH₄⁺ or L-glutamate on these processes were investigatedwith excised non-mycorrhizal beech (Fagus sylvatica L.) roots.NO₃^– net uptake followed uniphasic Michaelis-Menten kineticsin a concentration range of 10µM to 1 mM with an apparentK_m of 9.2 µM and a V_max of 366 nmol g^–1 FW h^–1.NH₄⁺, when present in excess to NO₃^–, or 10 mM L-glutamateinhibited the net uptake of NO₃^– Apparently, part of NO₃^–taken up was loaded into the xylem. Relative xylem loading ofNO₃^– ranged from 3.21.6 to 6.45.1% of NO₃^– netuptake. It was not affected by treatment with NH₄⁺ or L-glutamate.¹⁶N/¹³N double labelling experiments showed that NO₃^–efflux from roots increased with increasing influx of NO₃^–and, therefore, declined if influx was reduced by NH₄⁺ or L-glutamateexposure. From these results it is concluded that NO₃^–net uptake by non-mycorrhizal beech roots is reduced by NH₄⁺or L-glutamate at the level of influx and not at the level ofefflux. Key words: Nitrate transport, net uptake, influx, efflux, ammonium, Fagus, Fagaceae     

An Examination, by Compartmental Flux Analysis, of the Development of Sodium and Chloride Absorption Capacities in Beetroot Disks

Using beetroot, Beta vulgaris L. var. Avon Early, grown in radioactivelylabelled nutrient solutions, concentrations and fluxes of Na⁺and Cl^– were estimated for cells of freshly cut storageroot disks, by compartmental analysis of radioisotope elutionmeasurements. These values were compared with results obtainedin a similar manner from beetroot grown in non-labelled nutrientsolution and loaded instead during an aging period in ²²Na-or ³⁶Cl- labelled 1 mM NaCl solution. In accord with the generallyaccepted, but never properly tested, view, it was found thatnet Na⁺ influx followed from a reduction in efflux with agingand net Cl^– uptake depended on a marked increase in influx.However, both these important changes took place at the tonoplast,and, although aging led to a reduction of plasmalemma fluxes,at no time was entry of Cl^– into the cytoplasm, or lossof Na⁺ from the cytoplasm, significantly restricted.     

Electrophysiological Measurements on the Root of Atriplex hastata

The ion contents and membrane potentials of the cells of young,hydroponically cultured seedlings of Atriplex hastata L. var.salina, Wallr. have been measured at several different NaClconcentrations. The total tissue concentrations of Na⁺ and Cl^–increase as external NaCl increases, but there is always a markedexcess of internal Na⁺ over Cl^–; this is balanced by endogenousorganic anion formation with a concomitant extrusion of H⁺ tothe bathing solution. Membrane potentials of the root cells remain essentially invariantwith changes in external NaCl at approx. –130 mV; thereis no evidence of a radial gradient of potential across theroot. The potential seems to contain a cyanide-sensitive electrogeniccomponent, also invariant with NaCl concentration, of about–70 mV, and a diffusion component. The electrogenic componentseems likely to be a H⁺ efflux, probably through a H⁺ uniportATPase.     

Salinity-induced Malate Accumulation in Chara

Ion absorption by Chara corallina from solutions containingpredominantly KC1 or RbCl at up to 100 mol m^–3 resultedin accumulation of salts and turgor regulation. Turgor regulationdid not occur in solutions containing Na⁺ or Li⁺salts. Duringion absorption from various salts of K⁺ and Rb⁺ vacuolar cationconcentration exceeded Cl^– concentration. This differencewas shown to be balanced by the synthesis and accumulation ofmalate. Vacuolar malate concentration reached 48 mol m₃^–,with accumulation occurring at rates of up to 0.45 mol m^–3h^–1. Malate accumulation was inhibited by low externalpH and was dependent upon external HCO₃^– concentration.The synthesis of malic acid and its subsequent dissociationimposed a severe acid load on the cell. Biophysical regulationof cellular pH was achieved by a H⁺efflux at a rate of about40 nmol m^–2 s^–1from the cell. The results presentedargue against cytoplasmic Cl^–, HCO₃^– or pH regulatingmalate accumulation in Chara and it is suggested that malatetransport across the tonoplast may regulate malate accumulation. Key words: Malate, Chara corallina, pH regulation, salinity