Abscisic-acid-induced turion formation in Spirodela polyrrhiza L. IV. Comparative ion flux characteristics of the turion and the vegetative frond and the effect of ABA during early turion development

Abstract Using the method of compartmental analysis, the ion fluxes and compartment concentrations of Ca²⁺, K⁺ and Cl^- have been compared in the untreated vegetative frond and the abscisic acid (ABA) induced turion of Spirodela polyrrhiza. The ABA-induced turion is characterized by reduced Ca²⁺ exchange across the tonoplast and low vacuolar Ca²⁺ concentration relative to the vegetative frond. In addition the turion exhibits a higher plasmalemma flux with a correspondingly high Ca²⁺ concentration in the cytoplasm. The concentration of K⁺ and Cl^- is much lower in the cytoplasm of the ABA-induced turion than in the vegetative frond with the influx/efflux ratio at both the plasmalemma and the tonoplast being less than 1, a finding exhibited also in dormant storage tissue. Treatment of vegetative fronds with ABA for 18 h resulted in a reduced K⁺ plasmalemma efflux relative to untreated vegetative fronds and a concomitant increase in the cytoplasmic concentration. There was no rapid effect of ABA on Ca²⁺, K⁺ or Cl^- fluxes through either membrane. These results are consistent with the notion that drastic changes in ion fluxes and concentrations in the turion are a secondary consequence of ABA-induced development, possibly due to prior regulation by ABA of enzymes inherent to processes involved in membrane transport.     

Evidence for differential action of indoleacetic acid upon ion fluxes in single cells of Petroselinum sativum

Summary The apparent influx of ³⁶Cl^- and ⁸⁶Rb⁺/K⁺ into cells from the higher plant Petroselinum sativum has been measured during the presence and absence in the culture medium of indolacetic acid (IAA) which is an essential auxin of these cells. While 10^-5 M IAA did not significantly affect the influx of ⁸⁶Rb⁺/K⁺, it substantially reduced that of ³⁶Cl^-, i.e. by a factor 0.25 within 30 min. This differential action of IAA, which holds for a reasonable range of external pH, is assumed to bear on current hypotheses that the primary events of auxin action involve plasmalemma functions.     

Ion fluxes and phytochrome protons in mung bean hypocotyl segments: I. Fluxes of potassium

K⁺ [⁸⁶Rb⁺] uptake by Phaseolus aureus Roxb. hypocotyl segments cut immediately below the hook is inhibited by the active form of phytochrome (Pfr). Short load-short wash experiments indicate that the inhibition of uptake occurs across the plasmalemma. A maximal inhibition of short term uptake occurs in 10 to 50 millimolar KCI. Low temperature had only a small effect on influx and the inhibition of influx from 50 millimolar KCI. A consideration of the electrochemical gradient for K⁺ suggests that passive K⁺ fluxes may predominate under these conditions. Red light induces small depolarizations of membrane potential in subhook cells. Far red light antagonizes this effect. Pfr inhibits efflux of K⁺[⁸⁶Rb⁺] from subhook segments. This effect is also relatively insensitive to low temperature. This inhibition of efflux may reflect inhibition of a K⁺ -K⁺ exchange process, or reduced passive permeability of the plasmalemma to K⁺. In contrast, Pfr enhances short term uptake of K⁺[⁸⁶Rb⁺] in apical hypocotyl hook segments of Phaseolus aureus Roxb. Short load-short wash experiments indicate that fluxes across the plasmalemma are modified by Pfr. A maximal enhancement of short term influx occurs in 50 millimolar KCI. Influx and the red light enhancement of influx from 50 millimolar KCI are relatively insensitive to low temperature. Pfr also enhances efflux of K⁺[⁸⁶Rb⁺] from preloaded apical hook segments. This increased influx may reflect enhancement of a K⁺ -K⁺ exchange process or increased passive permeability of the plasmalemma to K⁺.     

Fluxes and compartmentation of K+, Na+ and Cl−, and action of auxins in suspension-cultured Petroselinum cells

Transport of ⁸⁶Rb⁺/K⁺, ²²Na⁺, ³⁶Cl^?, and [³H]indole acetic acid (IAA) has been studied on suspension-cultured cells of the parsley, Petroselinum crispum (Mill) Nym. By compartmental analysis two intracellular compartments of K⁺, Na⁺, and Cl^? have been identified and ascribed to the cytoplasm and vacuole; half-times of exchange were around 200 s and 5 h, respectively. According to the Ussing-Teorell flux equation, active transport is required for the influx into the cytoplasm at the plasmalemma (K⁺, Cl^?) and the tonoplast (K⁺, Na⁺, Cl^?). The plasmalemma permeability pattern, P_K:P_Na:P_Cl=1.00:0.24:0.38, features an increased chloride permeability compared with cells from higher plant tissues. IAA uptake showed an exponential timecourse, was half-maximal after 10 min, and a linear function of the IAA concentration from 10^?9 to 10^?5 M. IAA and 2,4-dichlorophenoxy acetic acid reduce the apparent influx of K⁺, Na⁺, Cl^? during the initial 30 min after addition and subsequently accelerate both in- and efflux of these ions. We discuss that auxins could affect the ion fluxes in a complex way, e.g. by protonophorous activity and by control of the hypothetical proton pump.     

Identification and reconstitution of a Na/K/Cl cotransporter and K channel from luminal membranes of renal red outer medulla

Electrophysiological studies on renal thick ascending limb segments indicate the involvement of a luminal Na⁺/K⁺/Cl⁻ cotransport system and a K⁺ channel in transepithelial salt transport. Sodium reabsorption across this segment is blocked by the diuretics furosemide and bumetanide. The object of our study has been to identify in intact membranes and reconstitute into phospholipid vesicles the Na⁺/K⁺/Cl⁻ cotransporter and K⁺ channel, as an essential first step towards purification of the proteins involved and characterization of their roles in the regulation of transepithelial salt transport. Measurements of ⁸⁶Rb⁺ uptake into membrane vesicles against large opposing KCl gradients greatly magnify the ratio of specific compared to non-specific isotope flux pathways. Using this sensitive procedure, it has proved possible to demonstrate in crude microsomal vesicle preparations from rabbit renal outer medulla two ⁸⁶Rb⁺ fluxes. (A) A furosemide-inhibited ⁸⁶Rb⁺ flux in the absence of Na⁺ (K⁺-K⁺ exchange). This flux is stimulated by an inward Na⁺ gradient (Na⁺/K⁺ cotransport) and is inhibited also by bumetanide. (B) A Ba²⁺-inhibited ⁸⁶Rb⁺ flux, through the K⁺ channel. Luminal membranes containing the Na⁺/K⁺/Cl⁻ cotransporter and K⁺ channels, and basolateral membranes containing the Na⁺/K⁺ pumps were separated from the bulk of contaminant protein by metrizamide density gradient centrifugation. The Na⁺/K⁺/Cl⁻ cotransporter and K⁺ channel were reconstituted in a functional state by solubilizing both luminal membranes and soybean phospholipid with octyl glucoside, and then removing detergent on a Sephadex column.     

The Na+,K+,2Cl−-cotransport system in HeLa cells: Aspects of its physiological regulation

We have previously reported on the biochemical properties of a Na⁺,K⁺,2Cl^?-cotransport in HeLa cells and here we deal with aspects of its physiological regulation. Na⁺,K⁺,2Cl^?-cotransport in HeLa cells was studied by ⁸⁶Rb⁺ influx and ⁸⁶Rb⁺/²²Na⁺ efflux measurements. The effects of rat atrial natriuretic peptide (ANP), isoproterenol, and amino acids on ⁸⁶Rb⁺ flux, mediated by the bumet-anide-sensitive Na⁺, K⁺, 2Cl^?-cotransport system and the ouabain-sensitive Na⁺/K⁺-pump, were investigated. ANP reduced bumetanide-sensitive ⁸⁶Rb⁺ influx under isotonic as well as under hypertonic conditions. Similar decrease of bumetanide-sensitive ⁸⁶Rb⁺ influx was observed in the presence of 8-bromo-cGMP, while neither isoproterenol as a β-receptor agonist nor 8-bromo-cAMP-could alter bumetanide-sensitive ⁸⁶Rb⁺ influx. Furthermore, efflux of ⁸⁶Rb⁺ and ²²Na⁺ was greatly reduced in the presence of bumetanide and ANP. Together with our recent findings, showing functionally active, high affinity receptors for ANP on HeLa cells (Kort and Koch, Biochim. Biophys. Res. Commun. 168:148–154, 1990), this study indicates that ANP participates in the regulation of the Na⁺, K⁺, 2Cl^?-cotransport system in HeLa cells. Further measurements revealed that amino acids as present in the growth medium (Joklik's minimal essential medium) and the amino acid derivative α-methyl-aminoisobutyric acid (metAlB, 1 and 5 mM, respectively) also reduced Na⁺, K⁺, 2Cl^?-cotransport-mediated ⁸⁶Rb⁺ uptake and diminished the stimulatory effect of hypertonicity on the cotransporter. In addition, the Na⁺/K⁺-pump was markedly stimulated in the presence of amino acids, while neither ANP and 8-Br-cGMP nor isoproterenol and 8-Br-cAMP had a significant effect on the activity of the Na⁺/K⁺-pump.     

Changes in K<Superscript>+</Superscript>, Na<Superscript>+</Superscript>, Cl<Superscript>−</Superscript> balance and K<Superscript>+</Superscript>, Cl<Superscript>−</Superscript> fluxes in U937 cells induced to apoptosis by staurosporine: On cell dehydration in apoptosis

The K⁺, Na⁺, and Cl⁻ balance and K⁺ (Rb⁺) and ³⁶Cl⁻ fluxes in U937 cells induced to apoptosis by 0.2 or 1 μM staurosporine were studied using flame emission and radioisotope techniques. It is found that two-thirds of the total decrease in the amount of intracellular osmolytes in apoptotic cells is accounted for by monovalent ions and one-third consists of other intracellular osmolytes. A decrease in the amount of monovalent ions results from a decrease in the amount of K⁺ and Cl⁻ and an increase in the Na⁺ content. The rate of ³⁶Cl⁻, Rb⁺ (K⁺), and ²²Na⁺ equilibration between cells and the medium was found to significantly exceed the rate of apoptotic change in the cellular ion content, which indicates that unidirectional influxes and effluxes during apoptosis may be considered as being in near balance. The drift of the ion flux balance in apoptosis caused by 0.2 μM staurosporine was found to be associated with the increased ouabain-resistant Rb⁺ (K⁺) channel influx and insignificantly altered the ouabain-sensitive pump influx. Severe apoptosis induced by 1 μM staurosporine is associated with reduced pump fluxes and slightly changed channel Rb⁺ (K⁺) fluxes. In apoptotic cells, the 1.4–1.8-fold decreased Cl⁻ level is accompanied by a 1.2–1.6-fold decreased flux.     

Interactions between K+ and NH4+: effects on ion uptake by rice roots

Interactive effects of K⁺ and N (principally NH₄⁺) on plant growth and ion uptake were investigated using hydroponically grown rice (Oryza sativa L. cv. M202) seedlings by varying the availability of NH₄⁺ or NO₃^? and K⁺ during an 18d growth period, a 3d pretreatment period and during flux measurements. Plants grew best in media containing 100 mmol m^?3 NH₄⁺ and 200mmolm^?3 K⁺ (N100/K200), followed by N2/K200 < N100/K2 < N2/K2. ⁸⁶Rb⁺(K⁺) fluxes were increased by exposure to N during the 18 d growth period and the 3 d of pretreatment, but decreased by the presence of NH₄⁺ during flux measurements. This inhibition was a function of prior N/K provision and the [NH₄⁺]₀ present during flux determinations. NH₄⁺ was least inhibitory to 86Rb⁺(K⁺) influx in high-N/low-K plants. Pretreatments with K⁺ failed to stimulate NH₄⁺ uptake, and the presence of K⁺ in the uptake solutions reduced NH₄⁺ fluxes only in high-N/low-K plants.     

The ionic relations of Porphyra purpurea(Roth) C.Ag. (Rhodophyta, Bangiales)

Abstract Radioisotope equilibration techniques have been used to determine the intracellular concentration of K⁺, Na⁺ and Cl_?, together with the unidirectional ion fluxes across the plasmalemma of Porphyra purpurea. Influx and efflux of ⁴²K⁺, ²⁴Na⁺ and ³⁶C1^? are biphasic, the rapid, initial uptake and loss of tracer from individual thalli being attributable to desorption from extracellular regions. Cellular fluxes are slower and monophasic, cells discriminating in favour of K⁺ and Cl^? and against Na⁺. A comparison between the equilibrium potential of individual ion species and the measured membrane potential demonstrates that there is an active component of K⁺ and Cl^? influx and Na⁺ efflux. ‘Active’ uptake and ‘passive’ loss of K⁺ and Cl^? are reduced when plants are kept in darkness, suggesting that a fraction of the transport of K⁺ and Cl^? may be due to ‘exchange diffusion’ (K⁺/K⁺ and Cl^?/Cl^?antiport).     

Evidence for the Induction of Repetitive Action Potentials in Synaptosomes by K+-Channel Inhibitors: An Analysis of Plasma Membrane Ion Fluxes

Abstract: The effects of four K⁺-channel inhibitors on synaptosomal free Ca²⁺ concentrations and ⁸⁶Rb⁺ fluxes are analysed. 4-Aminopyridine, α-dendrotoxin, charybdotoxin, and tetraethylammonium all increase the free Ca²⁺ concentration, although their potencies differ widely. In each case, the elevation in free Ca²⁺ concentration is reversed by the subsequent addition of tetrodotoxin. The transient ⁸⁶Rb⁺ efflux from preequilibrated synaptosomes induced with high concentrations of veratridine is partially inhibited by 4-aminopyridine and α-dendrotoxin. In contrast, when 4-aminopyridine or α-dendrotoxin is added to polarized synaptosomes, an enhanced⁸⁶Rb⁺ flux is seen, both for uptake and for efflux with no change in the total ⁸⁶Rb⁺/K⁺ content of the synaptosomes and with only a slight time-averaged plasma membrane depolarization (6.4 and 3.3 mV, respectively). The enhancements of flux by 4-aminopyridine or α-dendrotoxin are sensitive to ouabain and/or to tetrodotoxin. Furthermore, these flux changes show the same concentration dependencies as the blocked component of veratridine-stimulated ⁸⁶Rb⁺ efflux, the elevation of free Ca²⁺ concentration, and the facilitation of glutamate exocytosis that are elicited by 4-aminopyridine or α-dendrotoxin. It is concluded that these findings support the proposal of spontaneous, repetitive firing of synaptosomes evoked by K⁺-channel inhibitors and that the enhanced ⁸⁶Rb⁺ flux is a consequence of the activity of 4-aminopyridine- and α-dendrotoxin-insensitive K⁺ channels during these action potentials.     

Mechanism of Na+/proline symport inEscherichia coli: Reappraisal of the effect of cation binding to the Na+/proline symport carrier

Summary Recently we proposed that cytoplasmic acidification of low K⁺ (LK) sheep erythrocytes may stimulate ouabain-resistant Cl^–-dependent K⁺ flux (K⁺Cl^– cotransport), also known to be activated by cell swelling, treatment with N-ethylmaleimide (NEM), or removal of cellular bivalent cations. Here we studied the dependence of K⁺ transport on intracellular and extracellular pH (pH _i , pH _o ) varied either simultaneously or independently using the Cl^–/HCO ₃ ^– exchange inhibitor 4,4, diisothiocyanatostilbene-3,2-disulfonic acid (DIDS). In both control and NEM-treated LK cells volumes were kept near normal by varying extracellular sucrose. Using DIDS as an effective pH clamp, both K⁺ efflux and influx of Rb⁺ used as K⁺ congener were strongly activated at acid pH _i and alkaline pH _o . A small stimulation of K⁺ (Rb⁺) flux was also seen at acid pH _i in the absence of DIDS, i.e., when pH _i pH _o . Anti-L _l serum, known to inhibit K⁺Cl^– cotransport, prevented the pH _i -stimulated K⁺ (Rb⁺) fluxes. Subsequent to NEM treatment at pH 6, K⁺ (Rb⁺) fluxes were activated only by raising pH, and thus were similar to the pH activation profile of K⁺ (Rb⁺) fluxes in DIDS-treated cells with pH _o varied at constant physiologic pH _i . Anti-L _l , which inhibited NEM-stimulated K⁺ (Rb⁺) fluxes, failed to do so in NEM-plus DIDS-treated cells. Thus, NEM treatment interferes with the internal but not with the external pH-sensitive site.     

Thiol-dependent passive K+Cl− transport in sheep red blood cells: VI. Functional heterogeneity and immunologic identity with volume-stimulated K+(Rb+) fluxes

Summary Ouabain-resistant (OR), volume-or N-ethylmaleimide (NEM)-stimulated K⁺(Rb⁺)Cl^– fluxes were measured in low-K⁺ sheep red cells and found to be functionally separate but immunologically similar. In anisosmotic solutions both K⁺ effluxes and Rb⁺ influxes of NEM-treated and control cells were additive. In contrast to the NEM-stimulated K⁺Cl^– flux, metabolic depletion did not reduce K⁺Cl^– flux of normal or swollen cells. The anion preference of OR K⁺ efflux in NEM-treated cells was Br^–>Cl^–>HCO ₃ ^– =F^–I^–=NO ₃ ^– =CNS^–, and hence consistent with a reported Br^–>Cl^–>NO ₃ ^– sequence of the volume-dependent K⁺Cl^– transport. Alloimmune anti-L_l antibodies known to decrease passive K⁺ fluxes in low K⁺ cells reduced by 51% both volume-and NEM-stimulated, furosemidesensitive Rb⁺Cl^– fluxes suggesting their immunologic identity, a conclusion also supported by anti-L₁ absorption studies. Since pretreatment with anti-L₁ prevented the activation of Rb⁺ influx by NEM, and the impermeant glutathionmaleimide-I did not stimulate Rb⁺Cl^– influx, the NEM reactive SH groups must be located apart from the L₁ antigen either within the membrane or on its cytoplasmic face. A model is proposed consisting of a K⁺Cl^– transport path(s) regulated by a protein with two functional subunits or domains; a chemically (C _s) and a volume (V _s)-stimulated domain, both interfacing with the L₁ surface antigen. Attachment of alloanti-L₁ from the outside reduces K⁺Cl^– transport stimulated throughC _s by NEM orV _s by cell swelling.     

Pig reticulocytes. V. Development of Rb+ influx during in vitro maturation

Influx of the K⁺ analogue Rb⁺ was measured through the ouabain-sensitive Na⁺/K⁺ pump and the ouabain-insensitive “leak” pathways in Cl^? or NO in mature red cells from adult pigs and in reticulocytes naturally occurring in 7-day-old piglets. In reticulocytes, Rb⁺ influxes by the two pathways were of about equal magnitude in Cl^? (13 and 10 mmoles/liter cells × hr) and at least 25-fold larger than in mature red cells (0.5 and 0.4 mmoles/liter cells × hr). In Na ⁺ media, a portion of the ouabain-insensitive “leak” flux of Rb⁺ was Cl^? dependent (Rb⁺Cl^? transport) as NO replacement reduced Rb⁺ influx by 90% in reticulocytes and by 40% in mature red cells. The sulfhydryl reagent N-ethylmaleimide (NEM) stimulated Rb⁺Cl^? transport about twofold in reticulocytes and up to 13-fold in mature red cells. When reticulocytes matured to erythrocytes during in vitro incubation, about 90% of both ouabain-sensitive Rb⁺ pump and ouabain-insensitive Rb⁺Cl^? influx were lost. In contrast, the NEM-stimulated Rb⁺Cl^? transport changed much less throughout this period, suggesting an entity operationally but not necessarily structrually distinct from the basal Rb⁺Cl^? transport. Although the experimental variability precluded a full assessment of significant changes in the small Na⁺/K⁺(Rb⁺) pump and Rb⁺Cl^? fluxes in mature pig red cells kept for the same time period in vitro, Rb⁺ flux changes in reticulocytes appear to be maturational in nature, reflecting parallel activity transitions of Na⁺/K⁺ pump and Cl^?-dependent K⁺ fluxes in vivo.     

Functional Reconstitution of a Prokaryotic K+ Channel

SliK, a K⁺ channel encoded by the Streptomyces KcsA gene, was expressed, purified, and reconstituted in liposomes. A concentrative ⁸⁶Rb⁺ flux assay was used to assess the ion transport properties of SliK. SliK-mediated ionic flux shows strong selectivity for K⁺ over Na⁺ and is inhibited by micromolar concentrations of Ba²⁺, mirroring the basic permeation characteristic of eukaryotic K⁺ channels studied by electrophysiological methods. ⁸⁶Rb⁺ uptake kinetics and equilibrium measurements also demonstrate that the purified protein is fully active.     

86Rb+ fluxes in Chinese hamster ovary cells as a function of membrane cholesterol content

Steady-state fluxes of ⁸⁶Rb⁺ (as a tracer for K⁺) were measured in Chinese hamster ovary cells (CHO-K1) and a mutant (CR1) defective in the regulation of cholesterol biosynthesis; the membrane cholesterol content of this mutant was varied by growing it on a range of cholesterol supplements to lipid-free medium (Sinensky, M. (1978) Proc. Natl. Acad. Sci. U.S. 75, 1247–1249).Analogous to previous findings in ascites tumor cells, ⁸⁶Rb⁺ influx in the parent strain was differentiated into a ouabain-inhibitable ‘pump’ flux, furosemide-sensitive, chloride-dependent exchange diffusion, and a residual ‘leak’ flux.On the basis of this flux characterization, ⁸⁶Rb⁺ pump and leak fluxes were measured in the mutant as a function of membrane cholesterol content. Pump and leak fluxes, when expressed per ml cell water, were independent of the cholesterol content of the mutant. Moreover, ⁸⁶Rb⁺ fluxes in the mutant were equal to those in the parent strain. Our data imply that the flux behavior of K⁺ in the steady state is independent of the ordering of membrane lipid acyl chains.     

Rb fluxes in Chinese hamster ovary cells as a function of membrane cholesterol content

Steady-state fluxes of ⁸⁶Rb⁺ (as a tracer for K⁺) were measured in Chinese hamster ovary cells (CHO-K1) and a mutant (CR1) defective in the regulation of cholesterol biosynthesis; the membrane cholesterol content of this mutant was varied by growing it on a range of cholesterol supplements to lipid-free medium (Sinensky, M. (1978) Proc. Natl. Acad. Sci. U.S. 75, 1247–1249).Analogous to previous findings in ascites tumor cells, ⁸⁶Rb⁺ influx in the parent strain was differentiated into a ouabain-inhibitable ‘pump’ flux, furosemide-sensitive, chloride-dependent exchange diffusion, and a residual ‘leak’ flux.On the basis of this flux characterization, ⁸⁶Rb⁺ pump and leak fluxes were measured in the mutant as a function of membrane cholesterol content. Pump and leak fluxes, when expressed per ml cell water, were independent of the cholesterol content of the mutant. Moreover, ⁸⁶Rb⁺ fluxes in the mutant were equal to those in the parent strain. Our data imply that the flux behavior of K⁺ in the steady state is independent of the ordering of membrane lipid acyl chains.     

Control of Ion Fluxes in Stomatal Guard Cells

Opening and closing of the stomatal pore is associated with very large changes in K-salt accumulation in stomatal guard cells. This review discusses the ionic relations of guard cells in relation to the general pattern of transport processes in plant cells, in plasmalemma and tonoplast, involving primary active transport of protons, proton-linked secondary active transport, and a number of gated ion channels. The evidence available suggests that the initiation of stomatal opening is regulated through the uptake mechanisms, whereas initiation of stomatal closing is regulated by control of ion efflux at the plasmalemma, and of fluxes to and from the vacuole. In response to a closing signal there are large transient increases in efflux of both Cl^? (or Br^?) and Rb⁺ (K⁺) at the plasmalemma, with also a probable increase in anion flux from vacuole to cytoplasm and decrease in anion flux from cytoplasm to vacuole. A speculative hypothetical sequence of events is discussed, by which the primary response to a closing signal is an increase in Ca²⁺ influx at the plasmalemma, producing depolarisation and increase in cytoplasmic Ca²⁺. The consequent opening of Ca²⁺-sensitive Cl^? channels, and voltage-sensitive K⁺ channels (also Ca²⁺-sensitive?) in the plasmalemma, and of a Ca²⁺-sensitive nonspecific channel in the tonoplast, could produce the flux effects identified by the tracer work; this speculation is also consistent with the Ca²⁺-sensitivity of the response to closing signals and with evidence from patch clamping that such channels exist in at least some plant cells, though not yet all shown in guard cells.     

Electrochemical gradients and k and cl fluxes in excised corn roots

The compartmental analysis method was used to estimate the K⁺ and Cl⁻ fluxes for cells of excised roots of Zea mays L. cv. Golden Bantam. When the measured fluxes are compared to those calculated with the Ussing-Teorell flux-ratio equation, an active inward transport of Cl⁻ across the plasmalemma is indicated; the plasmalemma K⁺ fluxes are not far different from those predicted for passive diffusion, although an active inward transport cannot be precluded. Whether fluxes across the tonoplast are active or passive depends upon the vacuolar potential which is unknown. Assuming no electropotential gradient, the tracer flux ratios are fairly close to those predicted for passive movement. However, if the vacuole is positive by about 10 millivolts relative to the cytoplasm, the data suggest active inward transport for K⁺ and outward transport for Cl⁻.     

Rubidium transport in the cyanobacterium Synechococcus R-2 (Anacystis nidulans, S. leopoliensis) PCC 7942

Synechococcus R-2 is a unicellular blue-green alga. The cells will grow on Rb⁺ as a substitute for K⁺ but at a slower rate (t₂～ 15 h versus 12 h). Potassium is not, strictly speaking, an essential element for Synechococcus. Rubidium duxes (using ⁸⁶Rb⁺) are much slower than those of potassium, about 1 nmol m^?2 s^?1 in the light (0.35 mol m^?3 Rb⁺). ⁸⁶Rb⁺ fluxes in the dark are about 0.1 nmol m^?2 s^?1. These fluxes are very slow compared to those of Na⁺ and other ions. Isotopic influx of Rb⁺ can supply sufficient Rb⁺ to keep up with the demands for growth, but the net dux needed to keep up with growth in the light is a large proportion of the total observed dux. Kinetic studies of Rb⁺ uptake versus [Rb⁺] show two uptake phases consistent with a high-affinity and a low-affinity system. Both systems appear to be light-activated. Transport of Rb⁺ appears to be passive at pH_o 10 in the light and dark. There is no case for active transport of Rb⁺ at pH_o 7.5 in the light, but a marginal case for active uptake in the dark (about 3 kJ mol^?1). There is only a small effect of Na⁺ upon Rb⁺ transport. ⁸⁶Rb⁺ should not be used in place of ⁴²K⁺ in K⁺ nutrition studies as the details of Rb⁺ transport are different to those of K⁺ transport.     

Wirkung von CCCP und UO 2 ++ auf die Ionenfluxe in Wurzeln

Summary Using the device shown in Fig. 1 of a previous paper (Weigl, 1969) efflux of Rb⁺ from preloaded corn roots across the plasmalemma of root cortex cells was stimulated by addition of 5×10^-4 M UO ₂ ⁺⁺ , whereas the flux of Rb⁺ into the xylem was inhibited by addition of 5×10^-4 M UO ₂ ⁺⁺ . Efflux of Cl^- across the plasmalemma was stimulated by 10^-5 M CCCP, the flux of Cl^- via the xylem was inhibited by 10^-5 M CCCP. The problem of interpreting efflux experiments with roots is stated and discussed.