Demonstration of a K+/H+ exchange activity in a photodenitrifier, Rhodopseudomonas sphaeroides forma sp. denitrificans

Abstract Washed cells of Rhodopseudomonas sphaeroides forma sp. denitrificans , grown under photodenitrifying conditions, exhibited K⁺ uptake dependent on the transmembrane proton gradient (Δ pH). These cells also acidified the suspension medium in response to K⁺ pulses both aerobically and anaerobically in light and in the dark. The results indicate that the photodenitrifier has a reversible K⁺/H⁺ exchange activity which reflects its role in regulating the intracellular K⁺ concentration, as well as intracellular pH. The acidification of the external medium resulting from K⁺ pulses was inhibited by carbonyl cyanide- m -chlorophenylhydrazone (CCCP) indicating that the antiporter is energy-dependent. Addition of KCl to washed cells depolarized the membrane potential (Δψ) with a concomitant increase in ΔpH, indicating that the K⁺/H⁺ antiporter was electrogenic.     

pH homeostasis and bioenergetic work in alkalophiles

Abstract A Na⁺/H⁺ antiporter catalyses coupled Na⁺ extrusion and H⁺ uptake across the membranes of extremely alkalophilic bacilli. This exchange is electrogenic, with H⁺ translocated inward > Na⁺ extruded. It is energized by the Δψ ² component of the ΔμH⁺ that is established during primary proton pumping by the alkalophile respiratory chain complexes. These complexes abound in the membranes of extreme alkalophiles. Combined activity of the respiratory chain, the antiporter, and solute transport systems that are coupled to Na⁺ re-entry, allow the alkalophiles to maintain a cytoplasmic pH that is several pH units more acidic than optimal external pH values for growth. There is no compelling evidence for a specific and necessary role for any ion other than sodium in pH homeostasis, and although there is very high cytoplasmic buffering capacity in the alkaline range, active mechanisms for pH homeostasis are crucial. Energization of the antiporter as well as the proton translocating F ₁ F ₀-ATPase that catalyses ATP synthesis in the extreme alkalophiles must accommodate the problem of the low net ΔμH⁺ and the very low concentrations of protons, per se, in the external medium. This problem is by-passed by other bioenergetic work functions, such as solute uptake or motility, that utilize sodium ions for energy-coupling in the place of protons.     

The active uptake of carbon dioxide by the unicellular green algae Chlorella saccharophila and C. ellipsoidea

Abstract. Mass spectrometry has been used to measure the rates of CO₂ uptake of acid- and alkali-grown cells of the green algae Chlorella ellipsoidea (UTEX 20) and C. saccharophila (UTEX 27). The time course of CO₂ formation on addition of 100mmol m⁻³ K₂CO₃ to cells in the dark was used as an assay for external carbonic anhydrase (CA). No external CA was detected in acid-grown cells of either species or in alkali-grown cells of C. ellipsoidea but was present in alkali-grown C. saccharophila . In the absence of external CA, or when it was inhibited by 5mmol m⁻³ acetazolamide, cells of both species, on illumination, rapidly depleted the free CO₂ in the medium at pH 7.5 to near zero concentrations before maximum photosynthetic O₂ evolution rates were established. Addition of bovine CA rapidly restored the equilibrium CO₂ concentration in the medium, indicating that the cells were selectively taking up CO₂. Transfer of cells to the dark caused a rapid increase in the CO₂ concentration in the medium largely due to the efflux of inorganic carbon from the cells as CO₂. This rapid light-dependent CO₂ uptake takes place against pH and concentration gradients and, thus, has the characteristics of active transport.     

Neuropeptide Y inhibits [Ca2+]i changes in rat retinal neurons through NPY Y1, Y4, and Y5 receptors

Neuropeptide Y (NPY) and NPY receptors are widely distributed in the CNS, including the retina, but the role of NPY in the retina is largely unknown. The aim of this study was to investigate whether NPY modulates intracellular calcium concentration ([Ca²⁺]_i) changes in retinal neurons and identify the NPY receptors involved. As NPY decreased the [Ca²⁺]_i amplitudes evoked by 30 mM KCl in only 50% of neurons analyzed, we divided them in two populations: NPY-non-responsive neurons (Δ2/Δ1 ≥ 0.80) and NPY-responsive neurons (Δ2/Δ1 < 0.80), being the Δ2/Δ1 the ratio between the amplitude of [Ca²⁺]_i increase evoked by the second (Δ2) and the first (Δ1) stimuli of KCl. The NPY Y₁/Y₅, Y₄, and Y₅ receptor agonists (100 nM), but not the Y₂ receptor agonist (300 nM), inhibited the [Ca²⁺]_i increase induced by KCl. In addition, the inhibitory effect of NPY on evoked-[Ca²⁺]_i changes was reduced in the presence of the Y₁ or the Y₅ receptor antagonists. In conclusion, NPY inhibits KCl-evoked [Ca²⁺]_i increase in retinal neurons through the activation of NPY Y₁, Y₄, and Y₅ receptors. This effect may be viewed as a potential neuroprotective mechanism of NPY against retinal neurodegeneration.     

Role of external calcium in homeostasis of intracellular pH in the cyanobacterium Anabaena sp. strain PCC7120 exposed to low pH

The role of external Ca²⁺ in the homeostasis of intracellular pH (pHi) of Anabaena sp. strain PCC7120 in response to a decrease in the external pH (pHex) has been studied in cell suspensions. Increase in cytoplasmic pH after acid shock is dependent on the presence of Ca²⁺ in the medium. The observed Ca²⁺-mediated alkalization of the cytoplasm depends on the extent of the shift in external pH. Acid pH shifts resulted in an increased permeability of the cytoplasmic membrane to protons, which could be reversed by increasing the concentration of Ca²⁺ in the medium. Thus, the ability of Ca²⁺ to increase cytoplasmic pH might be correlated with an inhibition of net proton uptake by increasing concentrations of external Ca²⁺ under these conditions. This combined response resulted in the generation and maintenance of a larger pH gradient (ΔpH) at acid external pH values. All Ca²⁺ channel blockers tested, such as verapamil and LaCl₃, inhibited the observed Ca²⁺-mediated response. On the other hand, the Ca ionophore calcimycin (compound A23187) was agonistic, and stimulated both cytoplasmic alkalization and inhibition of net proton uptake. The protonophorous uncoupler carbonylcyanide m -chlorophenyl hydrazone, inhibited this Ca²⁺-mediated response, whereas monensin, an inhibitor of the Na⁺/H⁺ antiporter, had no significant effect. The results of the present study suggest that an influx of Ca²⁺ from the extracellular space is required for the regulation of cytoplasmic pH in Anabaena sp. strain PCC7120 exposed to low external pH values.     

The regulation of Mn2+ and Cu2+ uptake in cells of the yeast Candida utilis grown in continuous culture

Abstract Transport of Mn²⁺ was repressed in Candida utilis cells grown in continuous culture in high-Mn²⁺ (100 μM Mn²⁺) medium as compared to cells grown in basic (0.45 μM Mn²⁺) and low-Mn²⁺ (< 0.05 μM Mn²⁺) media. In contrast, no repression of Cu²⁺ uptake occurred in high-Cu²⁺-grown (25 μM Cu²⁺) cells as compared to cells grown in basic medium (0.54 μM Cu²⁺). Cu²⁺-limited cells did not hyperaccumulate Cu²⁺ and there was not significant difference in initial uptake rates for all 3 Cu²⁺ conditions. Mn²⁺ uptake appears to be regulated by a mechanism sensitive to the external Mn²⁺ concentration, whereas Cu²⁺ transport is not governed in this way by the external Cu²⁺.     

Chilling induces a decrease in pyrophosphate-dependent H+-accumulation associated with a DeltapH(vac)-stat in mung bean, a chill-sensitive plant

Chilling leads to cytoplasmic acidification in chill-sensitive plants. A possible explanation for this observation is that a ΔpH-stat between the cytosol and vacuole (ΔpH_vac-stat) is perturbed by chilling. To understand the nature of this ΔpH_vac-stat, the effect of temperature, between 20 and 0 °C, on pyrophosphate (PPi)- or ATP-dependent acidification of vacuolar vesicles, isolated from mung bean hypocotyls, was determined. Over the temperature range investigated, the H⁺-influx mediated by PPase was balanced with the H⁺-efflux, which was PPi-dependently suppressed, and consequently a constant pH in vesicles (pH_in) of ca. 5 was maintained against temperature changes. However, the ΔpH_in driven by ATP decreased as the temperature dropped. Thus, the PPi-dependent H⁺-accumulation may function as an essential factor to form a ΔpH_vac-stat against temperature changes. Next, to study the chilling sensitivity of PPi-dependent H⁺-accumulation, vacuolar vesicles were isolated from control seedlings or from seedlings chilled at 0 °C for 1 d. Chilling treatment resulted in a decrease in the H⁺-accumulation rate and in the steady-state ΔpH_in formed by PPi, the causes of which were enhanced by PPi-dependent H⁺-efflux and reduced by H⁺-influx driven by PPase. Together, the results suggest that the decrease of PPi-dependent H⁺-accumulation associated with the ΔpH_vac-stat could result in cytoplasmic acidification.     

High-frequency field measurements of diurnal carbon isotope discrimination and internal conductance in a semi-arid species, Juniperus monosperma

We present field observations of carbon isotope discrimination (Δ) and internal conductance of CO₂ ( g _i) collected using tunable diode laser spectroscopy (TDL). Δ ranged from 12.0 to 27.4‰ over diurnal periods with daily means from 16.3 ± 0.2‰ during drought to 19.0 ± 0.5‰ during monsoon conditions. We observed a large range in g _i, with most estimates between 0.04 and 4.0  µ mol m⁻² s⁻¹ Pa⁻¹. We tested the comprehensive Farquhar, O'Leary and Berry model of Δ (Δ_comp), a simplified form of Δ_comp (Δ_simple) and a recently suggested amendment (Δ_revised). Sensitivity analyses demonstrated that varying g _i had a substantial effect on Δ_comp, resulting in mean differences between observed Δ (Δ_obs) and Δ_comp ranging from 0.04 to 9.6‰. First-order regressions adequately described the relationship between Δ and the ratio of substomatal to atmospheric CO₂ partial pressure ( p _i/ p _a) on all 3 d, but second-order models better described the relationship in July and August. The three tested models each best predicted Δ_obs on different days. In June, Δ_simple outperformed Δ_comp and Δ_revised, but incorporating g _i and all non-photosynthetic fractionations improved model predictions in July and August.     

Na+-driven ATP synthesis of a psychrophilic marine bacterium, Vibrio sp. strain ABE-1

Abstract: In vivo ATP synthesis of a psychrophilic marine bacterium, Vibrio sp. strain ABE-1, derived from endogenous respiration, was examined. ATP was synthesized at both pH 6.5 and 8.5 after the start of the endogenous respiration by supplying O₂ to the anaerobic cell suspension. The ATP synthesis at pH 6.5, but not at pH 8.5, was completely inhibited by a H⁺ conductor, carbonylcyanide m -chlorophenylhydrazone (CCCP). The CCCP-resistant ATP synthesis at pH 8.5 was strongly inhibited by an inhibitor of the respiration-dependent primary Na⁺ pump, 2- n -heptyl-4-hydroxyquinoline N -oxide, and essentially required Na⁺. These results show that this bacterium synthesizes ATP at pH 6.5 by electrochemical potentials across the membrane Δ ∼ μ _H+, whereas at pH 8.5 by Δ ∼ μ _Na+ but not Δ ∼ μ _H+.     

Effects of amiodarone on K+, internal pH and Ca2+ homeostasis in Saccharomyces cerevisiae

In this study, amiodarone, at very low concentrations, produced a clear efflux of K⁺. Increasing concentrations also produced an influx of protons, resulting in an increase of the external pH and a decrease of the internal pH. The K⁺ efflux resulted in an increased plasma membrane potential difference, responsible for the entrance of Ca²⁺ and H⁺, the efflux of anions and the subsequent changes resulting from the increased cytoplasmic Ca²⁺ concentration, as well as the decreased internal pH. The Δ tok1 and Δ nha1 mutations resulted in a smaller effect of amiodarone, and Δ trk1 and Δ trk2 showed a higher increase of the plasma membrane potential. Higher concentrations of amiodarone also produced full inhibition of respiration, insensitive to uncouplers and a partial inhibition of fermentation. This phenomenon appears to be common to a large series of cationic molecules that can produce the efflux of K⁺, through the reduction of the negative surface charge of the cell membrane, and the concentration of this cation directly available to the monovalent cation carriers, and/or producing a disorganization of the membrane and altering the functioning of the carriers, probably not only in yeast.     

Food limitation of planktonic rotifers: field experiments in two mountain ponds

1. Resource competition is thought to be important in controlling zooplankton population dynamics and structuring zooplankton communities. Resource competition requires that resources are limiting. Ten field experiments were conducted to determine the presence and intensity of resource limitation of herbivorous planktonic rotifers in two mountain ponds. The intensity of food limitation was measured as Δ r , the difference between population growth rates in food supplemented versus control enclosures.
2. Rotifers were significantly food limited (Δ r  > 0) in most experiments. The intensity of food limitation varied between species. In Grady Lake, mean Δ r was 0.26 day⁻¹ for Polyarthra vulgaris and 0.43 day⁻¹ for an unidentified Synchaeta sp. In L1 Pond, mean Δ r was 0.07 day⁻¹ for Keratella cochlearis and 0.28 day⁻¹ for S. oblonga . The frequent and intense food limitation suggests that edible phytoplankton in both ponds were often present at low densities, were of low nutritional quality, or both. The intensity of food limitation often changed rapidly over time, indicating rapid temporal variation in resource availability.
3. For P. vulgaris , Δ r was not correlated with its population growth rate in Grady Lake. Thus, although food limitation was common, population dynamics may have been more strongly affected by other factors. For K. cochlearis , Δ r increased as population density increased, suggesting the existence of intraspecific resource competition that may regulate population dynamics.     

H+/PPi stoichiometry of a membrane‐bound pyrophosphatase of plant mitochondria

The H⁺/PP_i stoichiometry of the mitochondrial H⁺‐PP_iase from pea ( Pisum sativum L.) stem was determined by two kinetic approaches, and compared with the H⁺/substrate stoichiometries of the mitochondrial H⁺‐ATPase, and the vacuolar H⁺‐PP_iase and H⁺‐ATPase. Using sub‐mitochondrial particles or preparations enriched in vacuolar membranes, the rates of substrate‐dependent H⁺‐transport were evaluated: by a mathematical model, describing the time‐course of H⁺‐gradient (ΔpH) formation; or by determining the rate of H⁺‐leakage following H⁺‐pumping inhibition by EDTA at the steady‐state ΔpH. When the H⁺‐transport rates were divided by those of PP_i or ATP hydrolysis, measured under identical conditions, apparent stoichiometries of ca 2 were determined for the mitochondrial H⁺‐PP_iase and H⁺‐ATPase, and for the vacuolar H⁺‐ATPase. The stoichiometry of the vacuolar H⁺‐PP_iase was found to be ca 1. From these results, it is suggested that the mitochondrial H⁺‐PP_iase may, in theory, function as a primary H⁺‐pump poised towards synthesis of PP_i and, therefore, acting in parallel with the main H⁺‐ATPase.     

Spatial variations in nutrient supply to the red algae Eucheuma serra (J. Agardh) J. Agardh

The nutrient supply rates within the canopy of the economically important red algal species, Eucheuma serra J. Agardh were determined experimentally in a recirculating flow-chamber. A single individual was placed in the working section of the 2000 × 200 × 250 mm³ acrylic flow-chamber and subjected to unidirectional water velocities from 1.0 to 9.3 cm s⁻¹. Rates of nutrient supply were determined using 9.7 mm diameter CaSO₄ (gypsum) spheres that were attached to the thallus inside and outside of the canopy. The supply rates within the canopy were 56% less than outside of the canopy. Increases in internal and external water velocity asymptotically increased the nutrient supply rates regardless of location. A model was developed to examine how changes in ammonium and nitrate supply compared with the physiologically maximum uptake rates of these nutrients. The results suggest that when the ammonium concentration in the water was 20 µmol L⁻¹ uptake rates were limited by the supply rate especially at velocities below 5 cm s⁻¹, whereas in the case of 20 µmol L⁻¹ of nitrate, the supply of nitrate was more than adequate to maximize nutrient uptake.     

Inhibition of Listeria monocytogenes by acetate, benzoate and sorbate: weak acid tolerance is not influenced by the glutamate decarboxylase system

Aims:  Weak acids are widely used by the food industry to prevent spoilage and to inhibit the growth of pathogenic micro-organisms. In this study the inhibitory effects of three commonly used weak acids, acetic acid, benzoic acid and sorbic acid, on the growth of Listeria monocytogenes were investigated.
Methods and Results:  In a chemically defined medium at pH 6·4 benzoic acid had the greatest inhibitory effect (50% inhibition of growth at 4 mmol l⁻¹), while acetate was the least inhibitory (50% inhibition of growth at 50 mmol l⁻¹). Mutants lacking either sigmaB (Δ sigB ) or two of the glutamate decarboxylase systems (Δ gadAB ) were used to investigate the contribution these systems make to weak acid tolerance in L. monocytogenes .
Conclusions:  The stress-inducible sigma factor sigmaB (σ^B) was not required for protection against acetate and played only a minor role in tolerating benzoate and sorbate. The glutamate decarboxylase system, which plays an important role in tolerating inorganic acids, played no significant role in the ability of L. monocytogenes to tolerate these weak acids, and neither did the presence of glutamate in the growth medium.
Significance and Impact of the Study:  These results suggest that the effectiveness of weak acid preservatives in food will not be compromised by the presence of glutamate, at least under mildly acidic conditions.     

Evidence for a Na+ / H+ electrogenic antiporter in an alkaliphilic cyanobacterium Synechocystis

Abstract An alkaliphilic cyanobacterium characterized as a Synechocystis species was purified from a soil sample taken from a village in Java, Indonesia, by its preferential growth at elevated pH; it grew optimally at pH 9.5. Phosphorus nuclear magnetic resonance studies showed that the organism can maintain a ΔpH of over 2 pH units at an external pH of 10. It was observed that the viability of the organism in the dark was dependent on sodium ions. Evidence from experiments in which the extrusion of Na⁺ was measured from cells subjected to an alkali shock suggests that the organism possesses a Na⁺ / H⁺ electrogenic antiporter which is used for the maintenance of pH homeostasis.     

High external phosphate (Pi) increases sodium ion uptake and reduces salt tolerance of 'Pi-tolerant' soybean

Previous studies on the interaction between environmental inorganic phosphate (Pi) and salinity stress using soybean cultivars sensitive to high external Pi had two limitations: (1) the phenotype was dominated by overaccumulation of phosphorus (P); and (2) no detailed analysis was performed for sodium ion uptake. In this study, we focused on the effects of high external Pi on the sodium ion uptake in 'Pi-tolerant' soybean cultivars. The P accumulation in Pi-tolerant soybean Union was much lower [9.0 mg g⁻¹ dry weight (DW); contrasting to 38–76 mg g⁻¹ DW in the 'Pi-sensitive' soybean cultivars]. At in planta level, high level of external Pi significantly ( P  < 0.001) increased net sodium ion uptake and aggravated salinity stress symptoms. The effects of high external Pi diminished when de-rooted plants were used, suggesting that root is the primary organ interacting with Pi in the growth medium. Two-cell models, including soybean suspension cells and the tobacco Bright Yellow-2 cell line, were also employed to study the effects of high external Pi at the cellular level. Consistent to in planta results, high external Pi uplifted cellular sodium ion uptake and reduced cell viability under salinity stress. Gene expression analyses further showed that HPi (2 m M Pi supplements; excessive level of Pi) could reduce the fold of induction of GmSOS1 and GmCNGC under salinity stress, suggesting that they may be possible molecular targets involved in the interaction between high external Pi and Na⁺ uptake.     

THE CHEMICAL FORM OF DISSOLVED SI TAKEN UP BY MARINE DIATOMS

Results of past studies of the pH-dependent Si uptake kinetics of Phaeodactylum tricornutum Bohlin suggested that the anion SiO(OH)     is the chemical form of dissolved Si taken up by marine diatoms. We determined the chemical form of Si taken up by three other marine diatom species and P. tricornutum by examining the kinetics of Si use under two dramatically different SiO(OH)     :Si(OH)₄ ratios in seawater by varying pH from ≈8 to ≈9.6. Uptake rates were determined using a precise and sensitive ³²Si tracer methodology. The pH-dependent uptake kinetics obtained for all species except P. tricornutum suggest that marine diatoms transport Si(OH)₄. The half-saturation constant (K _m ) varies strongly as a function of pH for all species when the substrate of transport is assumed to be SiO(OH)     . Kinetic curves for Thalassiosira pseudonana (Hustedt) Hasle et Heimdal, Thalassiosira weissflogii (Grunow) G. Fryxell et Hasle, and Cylindrotheca fusiformis Reimann et Lewin have statistically identical values of K _m at each pH when the substrate for transport is assumed to be Si(OH)₄ ( T. pseudonana and T. weissflogii ) or total dissolved silicon ( C. fusiformis ). In contrast, P. tricornutum exhibits unusual biphasic uptake kinetics: uptake conforms to Michaelis–Menten kinetics up to 15 to 25 μM, above which uptake increases linearly. This enigmatic response may have biased conclusions drawn from past experiments using this species. However, based on the consistency of the results for the three other species, a new model of Si transport in marine diatoms is proposed on the basis of the direct formation of a complex between the Si-transport protein and Si(OH)₄.     

Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature

Ananas comosus L. (Merr.) (pineapple) was grown at three day/night temperatures and 350 (ambient) and 700 (elevated) μ mol mol^–1 CO₂ to examine the interactive effects of these factors on leaf gas exchange and stable carbon isotope discrimination ( Δ ,‰). All data were collected on the youngest mature leaf for 24 h every 6 weeks. CO₂ uptake (mmol m^–2 d^–1) at ambient and elevated CO₂, respectively, were 306 and 352 at 30/20 °C, 175 and 346 at 30/25 °C and 187 and 343 at 35/25 °C. CO₂ enrichment enhanced CO₂ uptake substantially in the day in all environments. Uptake at night at elevated CO₂, relative to that at ambient CO₂, was unchanged at 30/20 °C, but was 80% higher at 30/25 °C and 44% higher at 35/25 °C suggesting that phosphoenolpyruvate carboxylase was not CO₂-saturated at ambient CO₂ levels and a 25 °C night temperature. Photosynthetic water use efficiency (WUE) was higher at elevated than at ambient CO₂. Leaf Δ -values were higher at elevated than at ambient CO₂ due to relatively higher assimilation in the light. Leaf Δ was significantly and linearly related to the fraction of total CO₂ assimilated at night. The data suggest that a simultaneous increase in CO₂ level and temperature associated with global warming would enhance carbon assimilation, increase WUE, and reduce the temperature dependence of CO₂ uptake by A. comosus .     

The PhoE porin and transmission of the chemical stimulus for induction of acid resistance (acid habituation) in Escherichia coli

R. J. ROWBURY, M. GOODSON AND A.D. WALLACE. 1992. Escherichia coli K12 becomes resistant to killing by acid (habituates to acid) in a few minutes at pH 5.0. Habituation involves protein synthesis-dependent and -independent stages; both must occur at an habituating pH. The habituation sensor does not detect increased ΔpH (or decreased Δψ) nor an increased difference between pH_o and periplasmic pH but probably detects a fall in either external or periplasmic pH. Phosphate ions inhibit habituation, at any stage, probably by interfering with outer membrane passage of hydrogen ions. Most outer membrane components tested are not required for habituation but phoE deletion mutants habituated poorly and are acid-resistant. Strains derepressed for phoE , in contrast, showed increased acid sensitivity. These and other results suggest that habituation involves hydrogen ions or protonated carriers crossing the outer membrane preferentially via the PhoE pore, a process inhibited by phosphate and other anions. Stimulation by phosphate of the poor growth of E. coli at pH 5.0 is in accord with the above. Acetate did not enhance acid killing of pH 5.0 cells, suggesting that their resistance does not depend on maintaining pH_i near to neutrality at an acidic pH_o level.     

Na+/H+ antiport activity in tonoplast vesicles from roots of the salt-tolerant Plantago maritima and the salt-sensitive Plantago media

Plantago species differ in their strategy towards salt stress, a major difference being the uptake and distribution of Na⁺ ions. A salt-sensitive ( Plantago media L.) and a salt-tolerant ( P. maritima L.) species were compared with respect to Na⁺/H⁺ antiport activities at the tonoplast. After exposure of the plants to 50 m M NaCl for 6 days isolated tonoplast vesicles of P. maritima showed Na⁺/H⁺ antiport activity with saturation kinetics and a K_m of 2.4 m M Na⁺, NaCl-grown P. media and the control plants of both species showed no antiport activity. Selectivity of the antiport system for Na⁺ was high and was determined by adding different chloride salts after formation of a Δ pH in the vesicles. Specific tonoplast ATPase activities were similar in the two species and did not alter after exposure to NaCl stress.