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     

Diurnal Rb+ Transport from Roots to the Laminar Pulvinus in Phaseolus vulgaris L.

The primary leaves of kidney bean (Phaseolus vulgaris L.) openunder light and close in the dark by the deformation of thepulvinus resulting from diurnal distribution changes of K⁺,Cl^–, organic acid (or H⁺) and NO₃^–. When Rb⁺ was added as a tracer of K⁺ to the seedlings throughtheir roots, it was transported to the pulvinus cells duringthe light period but not during the dark period. Transpirationoccurred vigorously in the light but almost stopped in the dark.We concluded that Rb⁺ absorbed by the roots was carried to thepulvinus by the transpiration stream. Phaseolus vulgaris L., pulvinus, Rb⁺, diurnal transport transpiration stream     

Ecotypic Variation in the Osmotic Responses of Enteromorpha intestinalis (L.) Link.

Young, A. J., Collins, J. C. and Russell, G. 1987. Ecotypicvariation in the osmotic responses of Enteromorpha intestinalis(L.) Link.—J. exp. Bot. 38: 1309–1324. The physiological basis for salt tolerance has been studiedin the euryhaline marine alga Enteromorpha intestinalis. Adaptationto dilute and concentrated seawaters has been investigated inthree separate populations of this alga: marine, rock pool andestuarine. Internal K⁺, Na⁺ and Cl^– levels have been determined usingtracer efflux analyses. K⁺ has been shown to be the major osmoticsolute within this alga. Cellular levels of Cl^– and, inparticular, Na⁺ are low although levels in the cell wall arehigh. Levels of these ions varied considerably between the separateplants; K⁺ levels within marine plants of E. intestinalis aretwo to four times those found in the other populations. Thetertiary sulphonium compound ß-dimethylsulphonio-propionateis maintained at relatively high levels, although it remainsfairly insensitive to change in the external salinity. Changes in the tissue water content and cell volume are large,particularly within the estuarine plants. The thin cell wallsof these plants allow large changes in volume in the diluteconditions experienced in an estuary, while low turgor preventscell rupture. Thicker cell walls and small cells of the marineand rock pool plants assist in tolerating high and low externalosmotic potential—the estuarine plants respond poorlyto concentrated seawater. Key words: Enteromorpha, osmoregulation, ecotypes     

Chloride Transport in Chara: I. KINETICS AND CURRENT-VOLTAGE CURVES FOR A PROBABLE PROTON SYMPORT

Chara cells show an inward positive electric current acrossthe plasmalemma when exposed to Cl^– under voltage-clampconditions. The rapid rise of this current suggests that itis directly associated with the inward transport of Cl^–.The dependence of the current on Cl^– concentration showssaturation, the data fitting the Michaelis-Menten equation withV_m up to 100 nmol m^–2 s^–1 (for Cl^–starvedcells) with K_M 10–20 µM, and with some allowancefor an unstirred layer of water adjacent to the membrane. Theeffects on the current of clamp potential, illumination, withdrawalof alkali metal cations, and addition of amine were also investigated.These results suggest that the mechanism is the symport of 2H⁺ with each Cl^–, and that the actions of light, externalK⁺, and amine in stimulating Cl^–, influx are indirect.     

Influence of Cytoplasmic Streaming and Turgor Pressure Gradient on the Transnodal Transport of Rubidium and Electrical Conductance in Chara corallina

In order to study the transnodal transport of Rb⁺ in internodalcells of Chara corallina, a low-temperature loading system wasestablished to separate the loading process from the transportprocess. Tandem cells, consisting of internode-node-internode,were isolated from algal plants. Treatment of a single internodewith 100 mM RbCl at 5°C for 30 min caused an accumulationof 43 mM Rb⁺ in the cytoplasm of this cell (= source cell),but no Rb⁺ was found in the other internode (= sink cell) ofthe tandem cells. In 40 min after a return to 25°C, about12% of the Rb⁺ loaded in the source cell was transported intothe sink cell. The apparent transnodal permeability of Rb⁺ wascalculated to be 4.6 x 10^–7 m.s^–1. Under the assumptionthat the total cross-sectional area of plasmodesmata occupies10% of the nodal area, the diffusion coefficient of RbCl throughplasmodesmata was calculated to be 2.3 x 10^–11 m².s^–1which is about 1% of the free diffusion coefficient in water(2 x 10^–9m².s^–1). The transnodal transport of Rb⁺ was intimately correlated withthe rate of cytoplasmic streaming. The rate of streaming inboth the source and sink cells was varied either by treatingthe cells with cytochalasin B (CB) or by lowering the temperature.The transport rate correlated with the streaming rate irrespectiveof the method used. Since the level of ATP was not influencedby CB or low temperature, the transnodal transport is assumedto be the result of passive diffusion process through plasmodesmata. A turgor pressure gradient across the node decreased both thenodal electrical conductance and the transnodal transport ofRb⁺. By contrast, the exposure of both internodal cells to asolution of sorbitol had no effect on either of them. A turgorpressure gradient of 240 mOsm decreased the transport of Rb⁺in the first hour to 3% of the control, while it decreased thenodal conductance to about 50%. The increase in the electricalresistance occurred on the junction side between the node andthe internode that was treated with sorbitol. Cytochalasin Ehad no effect on the nodal electrical resistance. It is assumedthat plasmodesmata are equipped with a valve-like mechanismwhich is sensitive to the gradient of turgor pressure acrossthe node and is not regulated by an actomyosin system. (Received February 15, 1989; Accepted April 20, 1989)     

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     

Properties of Phosphoenolpyruvate Carboxylase and Carbohydrate Accumulation in the C3-CAM Intermediate Sedum telephium L. Grown Under Different Light and Watering Regimes

A comparison of the activity and properties of the enzyme phosphoenolpyruvatecarboxylase (PEPC) was made for plants of Sedum telephium L.grown under low (70 µmol m^–2 s^–1) or high(500µmol m^–2 s^–1) PPFD and subjected to varyingdegrees of water stress. Under well-watered conditions onlyplants grown under high PPFD accumulated titratable acidityovernight and the extractable activity of PEPC was almost 2-foldhigher in these plants than in plants grown under low PPFD.Increasing drought stress resulted in a substantial increasein the activity of PEPC extracted both during the light anddark periods and a decrease in the sensitivity to inhibitionby malic acid. The magnitude of these changes was determinedby the severity and duration of drought and by light intensity.A comparison of the kinetic properties of PEPC from severelydroughted plants revealed that plants droughted under high PPFDhad a lower K_m for PEP than plants under low PPFD. Additionof 2·0 mol m^–3 malate resulted in an increase inthe K_m for PEP, with plants draughted under low PPFD havinga significantly higher K_m in the presence of malic acid comparedto those under high _PPFD. Response to the activator glc-6-P,which lowered the K_m for PEP, also varied between plants grownunder the two light regimes. Under well-watered conditions PEPCextracted from plants under high PPFD was more sensitive toactivation by glc-6-P than those under low PPFD. After the severedrought treatment, however, the K_m for PEP in the presence ofglc-6-P was similar for enzyme extracted from plants grown underboth light regimes. Soluble sugars and starch were depletedovernight and were both possible sources of substrate for PEPC.With increasing drought, however, the depletion of starch relativeto soluble sugars increased under both light regimes. The propertiesof PEPC and the characteristics of carbohydrate accumulation/depletionare discussed in relation to the regulation of CAM in S. telephiumgrown under different light and watering regimes. Key words: PEP carboxylase, CAM, carbohydrates, Sedum telephium     

Proton Fluxes and the Activity of a Stelar Proton Pump in Onion Roots

The xylem vessels of excised adventitious roots of onion, Alliumcepa, were perfused with unbuffered nutrient solution adjustedinitially to either pH 9·3 or 3·9; the pH of thesolution after passage through the xylem, at rates not lessthan 2 xylem volume changes min^–1, was close to pH 6·5in both instances. The flux of H⁺ across the xylem/symplastboundary into mildly alkaline, phosphate-buffered solutionsperfusing the vessels could be increased greatly with increasingbuffer strength, up to a maximum value between 0·5–1·0pmol H⁺ mm^–2 s^–1. The apparent neutralization ofacidic malic acid buffers had a slightly lower maximum capacity,equivalent to –0·3 to –0·5 pmol H⁺mm^–2 s^–1. The addition of 5·0 pmol m^–3fusicoccin (FC) to the xylem perfusion solution stimulated theentry of H⁺ into the xylem; in unbuffered perfusion solutionsthe pH fell to pH 3·6 after a lag of 25–35 min.FC additions to phosphate-buffered solutions also stimulatedthe H⁺ flux to an extent similar to that in unbuffered solution,viz. 0·2–0·4 pmol mm^–2 s^–1. The release of K⁺ (³⁶Rb-labelled) into xylem sap transientlyincreased as the [K⁺] in weakly buffered perfusion solutionswas raised stepwise; a very marked increase being seen whenthe concentration was raised to 100 mol m^–3 from 40 molm^–3. The addition of 5·0 mmol m^–3 FC to theperfusing solution containing 100 mol m^–3 K⁺ rapidly decreasedthe K⁺ flux to the xylem as the H⁺ flux increased. Fusicoccinalso inhibited the flux of K⁺ into unbuffered perfusion solutionsbut the effect appeared reversible. Addition of 10 mmol m^–3abscisic acid (ABA) to the perfusion solution quickly producedtransient increases in both K⁺ and H⁺ fluxes into the xylem.In this and other experiments using weakly phosphate-bufferedperfusing solutions, H⁺ fluxes were comparable in size to thoseof K⁺ The results are consistent with the idea that the stele of onionroots contains a proton trarislocating ATPase whose activityresponds to the pH of the xylem sap. It is evident that theactivity of the proton secreting and proton neutralizing mechanismsin the xylem parenchyma control the movement of other ions acrossthe xylem/symplast boundary. Key words: Xylem perfusion, fusicoccin, abscisic acid, pH gradient     

Further Characteristics of Salt-Dependent Bicarbonate Use by the Seagrass Zostera muelleri

Millhouse, J. and Strother, S. 1987. Further characteristicsof salt-dependent bicarbonate use by the seagrass Zostera muelleri.—J.exp. Bot. 38: 1055–1068. The contribution of HCO^–₃to photosynthetic O₂ evolutionin the seagrass Zostera muelleri Irmisch ex Aschers. increasedwith increasing salinity of the bathing seawater when the inorganiccarbon concentration was kept constant. K_1/2 (seawater salts)for HCO^–₃ -dependent photosynthesis was 66% of seawatersalinity. Both short- and long-term pretreatment at low salinitiesstimulated photosynthesis in full strength seawater. Twentyfour hours pre-incubation of seagrass plants in 3·0 molm^–3 NaHCO₃ resulted in increased photosynthesis at allsalinities, apparently due to stimulation of HCO^–₃ use(K_1/2 (seawater salts) = 26%). V_max (HCO^–₃) was not affectedby low salinity pretreatment. The kinetics of HCO^–₃ stimulationby the major seawater cations was investigated. Ca²⁺ was themost effective cation with the highest V_max (HCO^–₃) andwith K_1/2(Ca²⁺) = 14 mol m^–3. Mg²⁺ was also very effectiveat less than 50 mol m^–3 but higher concentrations wereinhibitory. This inhibition cannot be accounted for solely byprecipitation of MgCO₃. Na⁺ and K⁺ were both capable of stimulatingHCO^–₃ use. Stimulation was in two distinct parts. Up to500 mol m^–3, both citrate and chloride salts gave similarresults (K_1/2(Na⁺) 81 mol m^–3, V_max(HCO^–₃) 0·26µmol O₂ mg^–1 chl min^–1), but use of citratesalts above 500 mol m^–2 caused a second stimulation ofHCO^–₃ use (K_1/2(Na⁺) 830 mol m^–3, V_max(HCO^–₃)0·68 µmol O₂ mg^–1 chl min^–1). V_max(HCO^–₃)for the second-phase Na⁺ or K⁺ stimulation was of the same orderas for Ca²⁺-stimulated HCO^–₃ use. To further characterizesalt-dependent HCO^–₃ use, the sensitivity of photosynthesisto Tris and TES buffers was investigated. The effects of Trisappear to be due to the action of Tris⁺ causing stimulationof HCO^–₃ -dependent photosynthesis in the absence of salt,but inhibition of HCO^–₃ use in saline media. TES has noeffect on photosynthesis. External carbonic anhydrase, althoughimplicated in salt-dependent HCO^–₃ use in Z. muelleri,could not be detected in whole leaves. Key words: Zostera muelleri, HCO^–₃ use, salinity     

Ion Transport and the Effects of Acetic Acid in White Clover: II. POTASSIUM ABSORPTION

Dunlop, J., Knighton, M. V. and White, D. W. R. 1988. Ion transportand the effects of acetic acid in white clover. II. Potassiumabsorption.—J. exp. Bot. 39: 89–96. Acetic acid stimulated K⁺ influx into cells of white cloverin suspension culture and net K⁺ influx by roots of intact whiteclover plants. At concentrations of acetic acid up to 2·0mol m^–3 the stimulation continued unabated for at least2 h. However, at higher concentrations the rate of absorptiondeclined to near zero values after 2 h. When acetic acid wasremoved from the solution there was a net efflux of K⁺. Thestimulation was pH dependent with a maximum at pH 5·0.There was maximum stimulation at an acetic acid concentrationof 2·0 mol m^–3 Net H⁺ efflux was reduced by 1·0 mol m^–3 aceticacid. When Rb⁺ uptake and H⁺ efflux were measured at a rangeof RbCl concentrations Rb⁺ uptake increased with concentrationwhereas H⁺ efflux was maximum in the absence of Rb⁺ (and K⁺)and decreased as RbCl concentration was increased Acetic acid caused a hyperpolarization of the membrane electricalpotential difference (E) of about 25 mV. In 1·0 mol m^–3acetic acid the hyperpolarization persisted for at least 2 hwhereas at 10 mol m^–3 d E subsequently depolarized tovalues around –80 mV. With a slight lag, the time courseof the stimulation of the rate of K⁺ absorption followed thepolarization and depolarization of E. These results imply thatthe linkage between K⁺ and H⁺ movements is probably throughE. Key words: Proton efflux, membrane electrical potential difference, Trifolium repens     

Efficiency of K+ Utilization by Barley Varieties: Activation of Pyruvate Kinase

Barley (Hordeum vulgare L.) varieties differed in their raponseto [K⁺]₀, in terms of their utilization efficiencies (UE = freshweight. concentration of [K⁺]₁^–1). At low [K⁺]₀, Compana,an efficient-non-responder demonstrated superior utilizationof absorbed K⁺. On the other hand, at high [K⁺]₀, Fergus (anefficient responder) and BT 334 (an inefficient responder) hadhigher UE values for K⁺ than Compana which performed poorlyat this [K⁺]₀. Kinetic parameters for K⁺ activation of the enzyme pyruvatekinase from 12 barley varieties, representing a range of UEvalues, were determined. Varieties showed substantial differencesin their V_max values (P<0·01). Compana, an efficientvariety, had the highest V_max (31 µmol g^–1 freshwt. h^–1) which was about 50% higher than that of Mingo,an inefficient variety. By contrast, K_m values for the enzymeswere not significantly different among varieties The mean valuesfor all varieties (3·9±0·15 mol m^–3K⁺) is far below the estimated cytoplasmic [K⁺] (100-200 molm^–3). It is, therefore, unlikely that differences in theutilization of K⁺ by these varieties can be explained on thebasis of differential requirements for (K⁺) activation of theseenzymes. Alternative possibilities for differences in the utilizationof K⁺ are discussed. Key words: K⁺ utilization efficiency, Pyruvate kinase, Barley varieties     

Nitrogen-13 Studies of Nitrate Fluxes in Barley Roots: II. EFFECT OF PLANT N-STATUS ON THE KINETIC PARAMETERS OF NITRATE INFLUX

Influx of nitrate into the roots of intact barley plants wasfollowed over periods of 1–15 min using nitrogen-13 asa tracer. Based on measurements taken over 15 min from a rangeof external nitrate concentrations (0·2–250 mmolm^–3), the kinetic parameters of influx, I_max and K_m, werecalculated. Compared with plants grown in the presence of nitrate throughout,plants that had been starved of N for 3 d showed a significantlygreater value ofI_max for ¹³N-nitrate influx (by a factor of1·4–1·8), but a similar value of K_m (12–14mmol m^–3). Pre-treating N-starved plants with nitratefor about 5 h further increased the subsequent rate of ¹³N-nitrateinflux, but had little effect in the unstarved controls. Allowingfor this induction of additional nitrate transport, the differencein rates of nitrate influx in control and N-starved plants wassufficient to account for the previously-observed differencein net uptake by the two groups of plants. In barley plants grown without any exposure to nitrate, butwith ammonium as N-source, both I_max and K_m for subsequent ¹³N-nitrateinflux were significantly decreased (by about one-half) comparedwith the corresponding nitrate-grown controls. The importance of changes in the rate of influx in the regulationof net uptake of nitrate is discussed. Key words: Ion transport, nitrate, influx, kinetic parameters, N-deficiency     

Potassium Transport Across the Membranes of Chara: I. THE RELATIONSHIP BETWEEN RADIOACTIVE TRACER INFLUX AND ELECTRICAL CONDUCTANCE

Smith, J. R., Smith, F. A. and Walker, N. A. 1987. Potassiumtransport across the membrane of Chara. I. The relationshipbetween radioactive tracer influx and electrical conductance.—J.exp. Bot. 38:731–751. The ⁴²K influx () and the electrical conductance (G_m) were measured simultaneously for the ‘membrane’of internodal cells of Chara australis as a function of theexternal [KCl] (K?. In bathing solutions of pH = 5?0, progressively increased from 20?5to 430?60 nmol m^–2 s^–1 and G_m increased from 0?36?0?02to 3?8?0?8 S m^–2 when K? was increased from 0?1 to 10mol m^–3. The resting membrane potential difference (p.d.)was approximately -135 mV for low K? and approached the expectedNernst equilibrium p.d. for K⁺ ions when K? > 1?0 mol m^–3.Measurements of ³⁶Cl influx suggested that the ⁴²K influx waspredominantly electrogenic. The equivalent Goldman permeabilityto K⁺ ions (P_k) was approximately 20–30 nm s^–1 anddid not vary significantly with increasing K?. The equivalentconductance attributable to the electrogenic transport of K⁺ ions was calculated from assuming passive, independent diffusionof K⁺ ions and the ratio was found to be typically close to one. It was also found that themagnitudes of and G_m measuredsimultaneously for each individual cell were also well correlatedfor K? 1?0 mol m^–3, and that the slope of the line ofbest fit was close to one. For each K? it was found that theconductance not attributable to K⁺ translocation and presumablyassociated primarily with the transport of protons or theirequivalents was typically 0?2–0?5 Sm^–2. For K? >1?0 mol m^–3 the results indicated that the transport ofK⁺ ions was essentially independent, i.e. there was no evidencefor flux interactions. The results also indicated that the equivalentconductance derived from the measured ⁴²K influx could usefullyindicate the fraction of the electrical conductance attributableto the translocation of K⁺ ions. Key words: Potassium, conductance, influx     

Rubidium Transport in Membrane Vesicles from the Halophyte Suaeda maritima

The uptake and efflux of Rb⁺ by membrane vesicles isolated fromshoots of the halophyte Suaeda maritima have been investigated.Uptake came to an apparent equilibrium after 1 h and the initialrate of uptake was considerably slower than that reported forbacterial membrane vesicles Additions of ATP reduced both Rb⁺uptake and the half-time for loss in efflux experiments, althoughthis effect was not specific for ATP and probably was not associatedwith energy transfer The permeability coefficient for Rb⁺ wascalculated to be between 0 1 and 0 3 x 10^–2 cm s^–1.The value of membrane vesicles in ion transport studies in plantsis discussed. Suaeda maritima, seablite, halophyte, membrane vesicles, ion transport, rubidium     

Ion Transport in Suaeda maritima: Its Relation to Growth and Implications for the Pathway of Radial Transport of Ions across the Root

The ion relations of the halophytc Suaeda maritima are described.When plants grew in 340 mol m^–3 sodium chloride (—1•76MPa) leaf solute potentials decreased, and were sustained around—2•5 MPa Inorganic ion concentration (mostly of sodiumchloride) accounted for this. Comparable shoot ion concentrationsof potassium, nitrate and sulphate occurred when plants grewon different salinity types characterized by these ions. Netsodium transport and shoot sodium concentration increased dramaticallywith increases in external sodium chloride concentration upto 85 mol m^–3; thereafter, further increases in externalsodium chloride concentration had relatively little effect uponeither shoot sodium concentration or upon net transport of sodiumto the shoot. The net transport of sodium plus potassium onlydoubled when the external concentration of sodium plus potassiumincreased from 24 to 687 mol m^–3 Shoot ion concentrationswere remarkably constant with time, external concentration andsalinity type. The membrane flux rates and symplasmic ion concentrations neededto sustain the observed net transport of sodium (of some 10mmol g^–1 dry wt. of roots d^–1) are calculated fromanatomical and stereological data for the root system of thisspecies. The minimum net sodium chloride flux to load the symplasmwould be 260 nmol m^–2s^–1 if the whole cortical andepidermal plasmalemmal surface area were used uniformly, butthe flux rate required would be 3000 nmol m^–2s^–1if uptake took place only at the root surface. A flux rate ofat least 1000 nmol m^–2s^–1 is needed between symplasmand xylem. The symplasmic concentration of NaCl would be atleast 80 mol m^–3. It is argued (1), that both symplasmicand xylem loading are likely to be passive processes mediatedby ion channels rather than active carriers, (2), that net iontransport at 340 mol m^–3 sodium chloride is close to themaximum which is physiologically sustainable and (3), that growthof this halophyte is limited by NaCl supply from the root. Key words: Suaeda maritima, halophyte, salinity, roots, radial ion transport     

Modest dietary K+ restriction provokes insulin resistance of cellular K+ uptake and phosphorylation of renal outer medulla K+ channel without fall in plasma K+ concentration

Extracellular K⁺ concentration ([K⁺]) is closely regulated by the concerted regulatory responses of kidney and muscle. In this study, we aimed to define the responses activated when dietary K⁺ was moderately reduced from a control diet (1.0% K⁺) to a 0.33% K⁺ diet for 15 days. Although body weight and baseline plasma [K⁺] (4.0 mM) were not reduced in the 0.33% K⁺ group, regulatory responses to conserve plasma [K⁺] were evident in both muscle and kidney. Insulin-stimulated clearance of K⁺ from the plasma was estimated in vivo in conscious rats with the use of tail venous and arterial cannulas. During infusion of insulin·(50 mU·kg^–1·min^–1), plasma [K⁺] level fell to 3.2 ± 0.1 mM in the 1.0% K⁺ diet group and to only 3.47 ± 0.07 mM in the 0.33% K⁺ diet group (P < 0.01) with no reduction in urinary K⁺ excretion, which is evidence of insulin resistance to cellular K⁺ uptake. Insulin-stimulated cellular K⁺ uptake was quantitated by measuring the K⁺ infusion rate necessary to clamp plasma K⁺ at baseline (in µmol·kg^–1·min^–1) during 5 mU of insulin·kg^–1·min^–1 infusion: 9.7 ± 1.5 in 1% K⁺ diet was blunted to 5.2 ± 1.7 in the 0.33% K⁺ diet group (P < 0.001). Muscle [K⁺] and Na⁺-K⁺-ATPase activity and abundance were unchanged during the 0.33% K⁺ diet. Renal excretion, which was measured overnight in metabolic cages, was reduced by 80%, from 117.6 ± 10.5 µmol/h/animal (1% K⁺ diet) to 24.2 ± 1.7 µmol/h/animal (0.33% K⁺ diet) (P < 0.001). There was no significant change in total abundance of key renal K⁺ transporters, but 50% increases in both renal PTK cSrc abundance and ROMK phosphorylation in the 0.33% K⁺ vs. 1% K⁺ diet group, previously established to be associated with internalization of ROMK. These results indicate that plasma [K⁺] can be maintained during modest K⁺ restriction due to a decrease in insulin-stimulated cellular K⁺ uptake as well as renal K⁺ conservation mediated by inactivation of ROMK, both without a detectable change in plasma [K⁺]. The error signals inciting and maintaining these responses remain to be identified. potassium homeostasis; Na⁺-K⁺-ATPase; H⁺-K⁺-ATPase; protein tyrosine kinase; cSrc     

Red and blue light-stimulated proton efflux by epidermal leaf cells of the Argenteum mutant of Pisum sativum

Light stimulates leaf expansion in dicotyledons by increasingapoplastic acidification, cell wall loosening and solute accumulationfor turgor maintenance. Red and blue light enhance growth viadifferent photo-systems, but the cellular location and modesof action of these systems is not known. Here, the effect of red and blue light was studied on transportprocesses in epidermal cells of expanding leaves of the Argenteummutant of Pisum satlvum. Both red and blue light caused extraceiiuiaracidification by isolated epidermal tissue, which was stimulatedby extracellular K⁺ and inhibited by DCCD at 0.1 mol m^–3.Acidification induced by red compared with blue light showeddifferent saturating kinetics in fluence rate-response curves.Under near saturating light conditions the effects of red andblue light were additive. The red light-induced acidificationwas inhibited by far-red light while the blue light-inducedacidification was not. Light caused a hyperpoianzation of themembrane potential in epidermal strips, and stimulated ⁸⁶Rb⁺uptake by epidermal protoplasts. These results show that phytochromeand an additional blue light-photoreceptor function in isolatedepidermal cells to promote proton efflux, hyperpolarization,and cation uptake. Key words: Pisum sativum, light-induced acidification, ion transport, epidermis, photoreceptor     

Phosphate Uptake in the Cyanobacterium Synechococcus R-2 PCC 7942

Phosphate uptake rates in Synechococcus R-2 in BG-11 media (anitrate-based medium, not phosphate limited) were measured usingcells grown semi-continuously and in continuous culture. Netuptake of phosphate is proportional to external concentration.Growing cells at pH_o 10 have a net uptake rate of about 600pmol m^–2 s^–1 phosphate, but the isotopic flux for³²P phosphate was about 4 nmol m^–2 s^–1. There appearsto be a constitutive over-capacity for phosphate uptake. TheK_m and V_max, of the saturable component were not significantlydifferent at pH_o 7.5 and 10, hence the transport system probablyrecognizes both H₂PO^–₄and HPO^2–₄. The intracellularinorganic phosphate concentration is about 3 to 10 mol m^–3,but there is an intracellular polyphosphate store of about 400mol m^–3. Intracellular inorganic phosphate is 25 to 50kJ mol^–1 from electrochemical equilibrium in both thelight and dark and at pH_o 7.5 and 10. Phosphate uptake is veryslow in the dark ( 100 pmol m^–2 s^–1) and is light-activated(pH_o 7.51.3 nmol m^–2 s^–1, pH_o 10600 pmol m^–2s^–1). Uptake has an irreversible requirement for Mg²⁺in the medium. Uptake in the light is strongly Na⁺-dependent.Phosphate uptake was negatively electrogenic (net negative chargetaken up when transporting phosphate) at pH_o 7.5, but positivelyelectrogenic at pH_o 10. This seems to exclude a sodium motiveforce driven mechanism. An ATP-driven phosphate uptake mechanismneeds to have a stoichiometry of one phosphate taken up perATP (1 PO_{4 in}/ATP) to be thermodynamically possible under allthe conditions tested in the present study. (Received June 16, 1997; Accepted September 4, 1997)     

Responses by Coleoptiles of Intact Rice Seedlings to Anoxia: K+ Net Uptake from the External Solution and Translocation from the Caryopses

This study evaluated the effects of anoxia on K⁺ uptake andtranslocation in 3–4-d-old, intact, rice seedlings (Oryzasativa L. cv. Calrose). Rates of net K⁺ uptake from the mediumover 24 h by coleoptiles of anoxic seedlings were inhibitedby 83–91 %, when compared with rates in aerated seedlings.Similar uptake rates, and degree of inhibition due to anoxia,were found for Rb⁺ when supplied over 1·5–2 h,starting 22 h after imposing anoxia. The Rb⁺ uptake indicatedthat intact coleoptiles take up ions directly from the externalsolution. Monovalent cation (K⁺ and Rb⁺) net uptake from thesolution was inhibited by anoxia to the same degree for thecoleoptiles of intact seedlings and for coleoptiles excised,‘aged’, and supplied with exogenous glucose. Transportof endogenous K⁺ from caryopses to coleoptiles was inhibitedless by anoxia than net K⁺ uptake from the solution, the inhibitionbeing 55 % rather than 87 %. Despite these inhibitions,osmotic pressures of sap (_sap) expressed from coleoptiles ofseedlings exposed to 48 h of anoxia, with or without exogenousK⁺, were 0·66 ± 0·03 MPa; however,the contributions of K⁺ to _sap were 23 and 16 %, respectively.After 24 h of anoxia, the K⁺ concentrations in the basal10 mm of the coleoptiles of seedlings with or without exogenousK⁺, were similar to those in aerated seedlings with exogenousK⁺. In contrast, K⁺ concentrations had decreased in aeratedseedlings without exogenous K⁺, presumably due to ‘dilution’by growth; fresh weight gains of the coleoptile being 3·6-to 4·7-fold greater in aerated than in anoxic seedlings.Deposition rates of K⁺ along the axes of the coleoptiles werecalculated for the anoxic seedlings only, for which we assessedthe elongation zone to be only the basal 4 mm. K⁺ depositionin the basal 6 mm was similar for seedlings with or withoutexogenous K⁺, at 0·6–0·87 µmolg^–1 f. wt h^–1. Deposition rates in zones above6 mm from the base were greater for seedlings with, thanwithout, exogenous K⁺; the latter were sometimes negative. Weconclude that for the coleoptiles of rice seedlings, anoxiainhibits net K⁺ uptake from the external solution to a muchlarger extent than K⁺ translocation from the caryopses. Furthermore,K⁺ concentrations in the elongation zone of the coleoptilesof anoxic seedlings were maintained to a remarkable degree,contributing to maintenance of _sap in cells of these elongatingtissues.     

Effects of Cations on the Cytoplasmic pH of Chara corallina

Smith, F. A. and Gibson, J.–L. 1985. Effects of cationson the cytoplasmic pH of Chara corallina.—J.exp. Bot.36: 1331–1340 Removal of external Ca²⁺ from cells of Chara corallina lowersthe cytoplasmic pH, as determined by the intracellular distributionof the weak acid 5,5–dimethyloxazolidine2–,4–dione(DM0), when the external pH is below about 60. This effect isreversed, at least partially, by addition of the following cationsto Ca²⁺-free solutions: tetraethylammonium (TEA⁺) and Na⁺ at5 or 10 mol m^-3, Li⁺ and Cs⁺ (10 mol m^-3), or Mg²⁺, Mn²⁺ andLa³⁺ (02 or 05 mol m^-3). Under the same conditions, increasesin pH sometimes, but not always, occur in the presence of 10mol m^-3 K⁺ or Rb⁺ The results are discussed in relation to the major transportprocesses that determine pH and the electric potential differenceacross the plasma membrane, namely fluxes of H⁺ and of K⁺. Thesimplest explanation of the effects of the various cations testedin this study is that they primarily affect pHi_c via changesin influx of H⁺ but direct effects on the H⁺ pump or on K⁺ fluxesmay also be involved Key words: Chara corallina, cytoplasmic pH, cations, H⁺transport