Potassium-Ammonium Uptake Interactions in Tobacco Seedlings

Short-term (< 12 h) uptake experiments were conducted with6–7-week-old tobacco (Nicotiana tabacum L. cv. Ky 14)seedlings to determine absorption interactions between K⁺ andNH₄⁺. At equal solution concentrations (0.5 mol m^–3) netK⁺ uptake was inhibited 30–35% by NH₄⁺ and NH₄⁺ uptakewas decreased 9–24%. Removal of NH₄⁺ resulted in completerecovery in K⁺ uptake rate, but NH⁴⁺ uptake rate did not recoverwhen K⁺ was removed. In both cases, inhibition of the uptakerate of one cation saturated as the concentration of the othercation was increased up to 0.5 mol m^–3. The relative effectof K⁺-NH₄⁺ interactions was not altered when Cl- was replacedwith SO₄^2–, but the magnitudes of the uptake rates wereless in the absence of Cl-. The V_max for NH₄⁺ uptake was reducedfrom 128 to 105 µmol g^–1 dry wt. h^–1 in thepresence of 0.5 mol m^–3 K⁺ and the K_m for NH₄⁺ doubledfrom 12 to 27 mmol m^–3 in the presence of K⁺. The resultsof these K⁺-NH₄⁺ experiments are interpreted as mixed-noncompetitiveinteractions. However, an enhanced efflux of K⁺ coupled to NH₄⁺influx via an antiporter cannot be ruled out as contributingto the decrease in net K⁺ uptake. Key words: Nicotiana tabacum, K⁺, NH₄⁺, Uptake interactions     

Ca2+ Regulation of Outward Rectifying K+ Channel in the Plasma Membrane of Tobacco Cultured Cells in Suspension: a Role of the K +Channel in Mitigation of Salt-Stress Effects by External Ca2+

The patch-clamp technique was used to study effect of the Ca²⁺on K⁺ channels in the plasma membrane of protoplasts isolatedfrom tobacco (Nicotiana tabacum L., cv. Bright Yellow) culturedcells in suspension. The outward rectifying whole-cell K⁺ currentswere not affected by in-tracellular Ca²⁺, but they were reducedwith increasing extracellular Ca²⁺. Neither extracellular norintracellular Ca²⁺ affected the permeability ratios (p_K+/P_Na+)of the plasma membrane. These results suggest that the inhibitionof outward-rectifying K⁺ channels by extracellular Ca²⁺may partiallycontribute towards the mitigation of detrimental effects ofsalinity on growth by extracellular Ca²⁺. (Received January 19, 1998; Accepted July 30, 1998)     

Effect of Root Zone Temperature on the Mineral Composition of Xylem Sap and Plasma Membrane K+-Mg++-ATPase Activity of Grafted-Cucumber and -Figleaf Gourd Root Systems

Cucumber (Cucumis sativus L.) seedlings were grafted onto cucumber-(CG) or figleaf gourd- (FG, Cucurbita ficifolia Bouché)seedlings in order to determine the effect of solution temperature(12, 22, and 32°C) on the mineral composition of xylem sapand the plasma membrane K⁺-Mg⁺⁺-ATPase activities of the roots.Low solution temperature (12°C) lowered the concentrationof NO^–₃ and H₂PO^–₄ in xylem sap of CG plants butnot of FG plants. Concentrations of K⁺, Ca⁺⁺ and Mg⁺⁺ in xylemsap were less affected than anions by solution temperature.The plasma membrane of FG plants grown in 12°C solutiontemperature showed the highest K⁺- Mg⁺⁺-ATPase activity at allATP concentrations up to 3 mM and at low reaction temperatureup to 12°C, indicating resistance of figleaf gourd to lowroot temperature. (Received December 27, 1994; Accepted March 10, 1995)     

The involvement of ethylene in in vitro maturation of mid-binucleate pollen of Nicotiana tabacum

The role of ethylene during in vitro maturation of Nicotianatabacum pollen from the mld-binucleate (MB) stage was analysedby the addition of aminooxyacetic acid (AOA), aminoethoxyvinylglycine(AVG), CoCl₂ and AgNO₃ to the maturation medium (AMGLu). Anincrease in ethylene production was obtained in both isolatedpollen and pollen surrounded by sporophytic tissue during insitu maturation. in vitro maturation of pollen was inhibitedby AOA and AVG; ACC and ethrel were able to overcome this inhibitoryeffect. Cyclohexylamine (CHA) reverted the inhibition provokedby both Ag⁺ and Co²⁺ The results reported in this paper indicatethat ethylene is one of the factors implicated in in vitro maturationof MB pollen of Nicotiana tabacum. Key words: Nicotiana tabacum, maturation, germination, pollen, ethylene     

The Effect of Salinity Changes Upon the Physiology of Eulittoral Green Macroalgae from Antarctica and Southern Chile: II INTRACELLULAR INORGANIC IONS AND ORGANIC COMPOUNDS

The effects of hypo- and hypersaline treatments ranging from7–68% on the intracellular inorganic ion and organic soluteconcentrations were determined in the eulittoral green macroalgaeUlothrix implexa, Ulothrix subflaccida, Enteromorpha bulbosa,Acrosiphonia arcta, and Ulva rigida from Antarctica and SouthernChile. The main inorganic cations were K⁺, Na⁺, and Mg²⁺ inall species. The major osmolyte in E. bulbosa, A. arcta, andU. rigida was K⁺ at increasing salinities. In both Ulothrixspecies, however, K⁺ levels declined during hypersaline stressand Na⁺ concentrations rose significantly. The main inorganicanions were Cl^-, SO²_4-, and PO³_4- in all algae, while E. bulbosaand U. rigida also contained NO⁺₃. A. arcta showed an extremelyhigh SO^2-₄ content. The organic solutes proline, sucrose, andß-dimethylsulphoniopropionate (DMSP) played an importantrole in osmotic acclimation. The occurrence of three organicosmolytes suggests an additional function of these solutes ascryoprotectants in the cold-water macroalgae investigated.     

Solute Leakage from Solanum nigrum L. Seeds Exposed to High Temperatures during Imbibition

Exposure of Solanum nigrum L. seeds to high temperatures duringimbibition affected their leakage pattern: (1) The rate of leakageof total electrolytes was markedly increased with elevationof temperature. The increase was highest during the first 3h of imbibition but with a reduced rate thereafter. (2) Leakageof Na⁺ was almost complete after 6 h of imbibition at both temperatures,but much more Na⁺ leaked out at 50?C than at 25?C. (3) A markedincrease in leakage of K⁺ occurred after 24 h of exposure to50?C so that after 96 h three times more K⁺ leaked out at 50?Cthan at 25?C. (4) After 6 h of imbibition Ca11 and Mg⁺⁺ continuedto leak out at 25?C and at 50?C at a similar rate. (5) Imbibitionat an elevated temperature induced a marked increase in theleakage of both nucleic acids and proteins. (6) Malate dehydrogenasewas not detected in the leachate at 25?C, but was found after48 h at 50?C. It is assumed that this enzyme was of cytoplasmicorigin, indicating heat damage to membranes. The possible roleof the above phenomena in the loss of viability of the seedsdue to exposure to high temperature during imbibition is discussed. Key words: Leakage, Germination, S. nigrum     

Mechanisms of solute efflux from seed coats: whole-cell K+ currents in transfer cell protoplasts derived from coats of developing seeds of Vicia faba L.

In developing seed ofVicia faba L., solutes imported throughthe phloem of the coats move symplastically from the sieve elementsto a specialized set of cells (the thin-walled parenchyma transfercells) for release to the seed apoplast. Potassium (K⁺) is thepredominant cation released from the seed coats. To elucidatethe mechanisms of K⁺ efflux from seed coat to seed apoplast,whole-cell currents across the plasma membranes of protoplastsof thin-walled parenchyma transfer cells were measured usingthe whole-cell patch-clamp technique. Membrane depolarizationelicited a time-dependent and an instantaneous outward current.The reversal potential (E_R of the time-dependent outward currentwas close to the potassium equilibrium potential (E_K and itshifted in the same direction as E_K upon changing the externalK⁺ concentration, indicating that this current was largely carriedby an efflux of K⁺. The activation of the time-dependent outwardK⁺ current could be well fitted by two exponential componentsplus a constant. The instantaneous outward current could alsobe carried by K⁺ efflux as suggested by ion substitution experiments.These K⁺ outward rectifier currents elicited by membrane depolarizationare probably too small to represent the mechanism for the normalK⁺ efflux from seed coat cells. Membrane hyperpolarization morenegative than –80 mV activated a time-dependent inwardcurrent. K⁺ influx was responsible for the inward current asthe current reversed at membrane voltage close to E_K and shiftedin the same direction as E_K when external [K⁺] was varied. Activationof this K⁺inward rectifier current was well fitted with twoexponential components plus a constant. A regulating functionfor this current is suggested. Key words: Potassium outward rectifier, potassium inward rectifier, transfer cell protoplast, seed coat, Vicia faba L     

Movement of Ions and Electrogenesis in Higher Plant Cells

During the past 10 years considerable information has accumulatedon the electrochemical relationships of higher plant cells duringtransport of mineral ions. Using the Nernst equation as a criterion,none of eight ions (K⁺, Na⁺, Ca⁺⁺, Mg⁺⁺, NO₃^–, Cl^–,H₂PO₄^–, and SO₄^–) is in a passive equilibrium. Na⁺,Ca⁺⁺, and Mg⁺⁺ are subject to an exclusion mechanism, and allof the anions appear to be pumped inwardly. K⁺ apparently approachesan electrochemical balance under certain conditions but probablyis actively accumulated. Compartmental analyses giving estimatesof amounts in the cytoplasm and vacuole and of unidirectionalfluxes permit application of the Ussing flux-ratio equation.The criterion in oat coleoptile cells suggests that at the plasmalemmaNa⁺ is pumped out while K⁺ and Cl^– are pumped in. K⁺ andCl^– appear to be coupled in active transport across thetonoplast into the vacuole. Good evidence has been found thatthe cell's electropotential arises from an electrogenic pump:CN^– (cyanide) and DNP (dinitrophenol) reversibly blockthe potential and ionic transport; cell potentials are higherthan can be accounted for by diffusion; the responses of respirationand potential to the concentration of CN^– are nearly parallel;and CN^– inhibited tissue approaches a fit to the Goldmanconstant field equation. Future objectives should be identificationof the ion, or ions, subject to the electrogenic pump and discoveryof the immediate energy source.     

Effect of Abscisic Acid on the K+ Efflux and Membrane Potential of Nicotiana tabacum L. Leaf Cells

K⁺ efflux from tobacco (Nicotiana tabacum L, cv. Samsun NN)leaf discs into the external medium was increased and the membranepotential (Em) changed in the positive direction with a changein pH from 8.0 to 4.0. E_m was affected by the external concentrationof KCl, greatly decreasing with a change in concentration from1 mM to 100 mM. The equilibrium potential of the membrane forK⁺ (E_k) was decreased in a Nernst fashion with increasing externalconcentrations of KCl. E_k is more positive than E_m above ca.50 µM KCl. Most of the experiments were carried out underconditions in which the difference between the electrochemicalpotential for K⁺ on the inside to the outside of the cell (µ_kis positive. Thus, K⁺ may passively flow to the outside of thecells accompanied by the depolarization of the membrane. Abscisic acid (ABA) increased the K⁺ efflux under conditionsof passive transport. K⁺ efflux was accelerated with an increasingconcentration of ABA, being maximal at 10^–4 M–10^–3M. This acceleration was due to the enhancement of the potassiummotive force (µ_k/F) which is the force causing the netpassive transport of K⁺. The membrane potential was decreasedfrom –205 mV to –170 mV by 2 x 10^–4 M ABAwithin 10 min. The depolarization was not transient, being lostfor at least 3 hr. These results show that ABA accelerated passive K⁺ efflux, whichaccompanied depolarization of the membrane. (Received June 22, 1981; Accepted August 24, 1981)     

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.     

Efficiency of Potassium Utilization by Barley Varieties: The Role of Subcellular Compartmentation

Memon, A. R., Saccomani, M. and Glass, A. D. M. 1985. Efficiencyof potassium utilization by barley varieties: The role of subcellularcompartmentation.?J. exp. Bot. 36: 1860–1876. The subcellulardistributions of K⁺ in roots of three barley (Hordeum vulgareL.) varieties, grown at 10 and 100 mmol m^–3 external K⁺([K⁺]_o) were estimated by compartmental analyses. In general,increased [K⁺]_o caused a 2–3 fold increase in vacuolar[K⁺], but cytoplasmic [K⁺] increased only slightly. Nevertheless,the three varieties, which had been selected for study on thebasis of their different rates of K⁺ utilization, showed distinctdifferences in the allocation of K⁺ between cytoplasm and vacuole.At 10 mmol m^–3 [K⁺]_o var. Betzes exhibited typical K⁺deficiency symptoms while var. Fergus and var. Compana did not,even though Betzes had higher [K⁺] in shoots and roots. Theinefficient utilization of K⁺ in this variety appears to beassociated with a failure to mobilize vacuolar K⁺ into the cytoplasmiccompartment (the ratio of vacuolar: cytoplasmic K⁺ contentsfor Betzes was 4.1 compared to 2.7 and 2.5, respectively, forFergus and Compana). Fergus and Betzes, which demonstrate pronouncedgrowth responses to increased [K⁺]₀ between 10 and 100 mmolm^–3, showed significant increases of cytoplasmic [K⁺]in this range of [K⁺]_o. By contrast, cytoplasmic [K⁺] in Compana,a variety whose growth is not stimulated by increased [K⁺]₀(from 10 to 100 mmol m^–3) showed virtually no increase.It is suggested that the efficiency of K⁺ utilization and thegrowth response to [K⁺]₀ in these varieties are functions ofthe subcellular distribution of this ion between cytoplasm andvacuole. Key words: Barley varieties, K⁺ subcellular compartmentation, utilization efficiency     

Effect of Potassium on Sucrose and Amino Acid Release from the Seed Coat of Developing Seeds of Pisum sativum

After removal of the embryo from developing seeds of Pisum sativum,the ‘empty’ ovules (seed coats without enclosedembryo) were filled with a solution (pH 5.5) containing mannitol(usually 400 mM) to which various salts were added. A solutioncontaining two isotopes ((a) [²H]-sucrose/[–¹⁴C]aminoisobutyricacid (AIB) or (b) [³H]valine/[₁₄C]asparagine mixture) was administeredto the plant via the petiole subtending the fruiting node, and[²H]solute and [¹⁴C]solute unloading from the seed coat wasmeasured, in pulse-labelling experiments of about 5 h. The presenceof 25 or 50 mM K⁺ in the ‘empty’ ovule enhancedthe release of sucrose from the seed coat particularly duringthe first hours of the experiment, but the stimulating effectof K⁺ on the release of labelled solutes derived from aminoacids was much smaller. The presence of 25 mM CaCl₂ did notaffect the release of sucrose or amino acids from the seed coat.The effect of K⁺ on sucrose and amino acid release is explainedas an inhibition of sucrose and amino acid resorption from theseed coat apoplast into seed coat cells, after unloading fromthe seed coat unloading sites. It is suggested that amino acidrelease is much less affected by K⁺ than sucrose release, becausefar less resorption of amino acids by seed coat parenchyma cellstakes place during amino acid transport into the seed coat cavity. Pisum sativum, pea, assimilate transport, assimilate unloading, seed-coat exudate, seed development, sucrose resorption, surgical treatment     

Interaction of Extracellular Potassium and Cesium with the Kinetics of Inward Rectifying K+ Channels in the Plasma Membrane of Mesophyll Protoplasts of Avena sativa

Using the patch-clamp technique the kinetics of whole-cell andsingle channel inwardly rectifying K⁺ currents were measuredin enzymatically-isolated protoplasts from Avena sativa mesophyllleaf cells. The hyperpolarization-activated whole-cell currenthad an initial K⁺ component (I_KI) and a time-dependent K⁺ componentwhich reaches steady state (I_KSS) within 500 ms. After an initialdelay, the activation of IKss and the deactivation of the tailK⁺ current (I_KT) followed an exponential time course. The time-constantsof activation (     

Enzymic mechanism of starch breakdown in germinating rice seeds V. Sucrose phosphate synthetase in the scutellum

Some enzymic Properties of a partially purified preparationof sucrose phosphate synthetase (E.C.2.4.1.14) from germinatingrice seed scutella were studied. Examination of the reactionkinetics revealed that the rate of synthesis of sucrose phosphatefollows the Michaelis-Menten equation at an optimum PH of 7.5,having K_m of 25 mM for UDP-glucose, and of 4.9 mM for fructose6-phosphate. UDP inhibited the enzyme reaction competitively;K₁ of 3.3 mM. Fe⁺⁺ and Fe⁺⁺⁺ activated the enzyme reaction about2-fold; K_a, 0.3 mM and 2.0 mM, respectively. Co⁺⁺, Co(NH₃)₆⁺⁺⁺,Mg⁺⁺ and Mn⁺⁺ also activated the enzyme reaction. At high concentrationK⁺ activated the enzyme reaction with the maximum activationof 24% at 400 mM. The molecular weight and S_20,w value of theenzyme were determined as 4.5 ? 10⁵ and 10.4S, respectively. ¹Part IV of this series is Ref. (5). ²California Foundation for Biochemical Research Fellow (1973). (Received December 20, 1973; )     

Drought Tolerance in Two Mosses: Correlated with Enzymatic Defence Against Lipid Peroxidation

Drought-induced changes in the activities of superoxide dismutase(SOD) and catalase, level of lipid peroxidation, and membranepermeability (solute leakage) have been studied in two mosses,the drought-tolerant Tortula ruralis and the drought-sensitiveCratoneuron filicinum. In T. ruralis the activities of SOD andcatalase increase during slow drying. The level of lipid peroxidationconsequently declines. On subsequent rehydration the enzymeactivities decline and the level of lipid peroxidation risesgradually to normal levels. The leakage of preloaded ⁸⁶Rb onrehydration of slowly dried T. ruralis is similar to that inturgid moss, i.e. leakage of about 20% of tissue ⁸⁶Rb. WhenT. ruralis is subjected to rapid drying there is no change inthe enzyme activities or in lipid peroxidation. However, whenthis moss is rehydrated there is a large immediate increasein lipid peroxidation. Half of the tissue ⁸⁶Rb is leaked intothe bathing medium during the first hour of rehydration. Butwithin the next hour, when SOD and catalase activities haveincreased to high levels, lipid peroxidation quickly declinesto a level lower than that in the turgid control moss, and the⁸⁶Rb leaked earlier is partly reabsorbed indicating that membranerepair is well underway. On prolonged rehydration the enzymeactivities decline and the level of lipid peroxidation risesgradually to reach normal levels found in control turgid moss.In the case of drought-sensitive C. filicinum the activitiesof SOD and catalase decline during drying as well as duringsubsequent rehydration. There is a rapid increase in lipid peroxidationduring rehydration and most of the preloaded ⁸⁶Rb leaks intothe bathing medium irreversibly. The changes in lipid peroxidationduring drying and subsequent rehydration of both the mossesappear to coincide in time with the reported changes in O₂ uptake,indicating that the drought-induced membrane damage may be dueto free radical-induced lipid peroxidation which is known torequire active O₂ uptake. Furthermore, there appears to be agood correlation between an ability of the tissue to controllipid peroxidation and its ability to retain solutes. It issuggested that ability of plant tissues to mobilize enzymaticdefence against uncontrolled lipid peroxidation may be an importantfacet of their drought tolerance.     

Lamprothamnium, a Euryhaline Charophyte: I. OSMOTIC RELATIONS AND MEMBRANE POTENTIAL AT STEADY STATE

The charophyte Lamprothamnium papulosum (Wallr.) J. Gr. is foundat salinities varying from nearly fresh water to twice thatof sea water. It can maintain its turgor constant at 302 mosmolkg^–1 (0.73 MPa) when exposed to external osmotic pressuresof 550 to 1350 mosmol kg^–1 (1.3–3.3 MPa). Turgorshows a tendency to rise slightly at lower osmotic pressure(388 mosmol kg^–1 of turgor at 150 mosmol kg^–1 externalosmolality). K⁺ and Cl^– are the main solutes in the vacuole,and are most important in controlling internal osmotic pressure.Mg²⁺, Ca²⁺, and SO^2–₄ are present in significant amountsbut their concentrations do not change with changes in externalsalinity. Na⁺ is present in lower concentration than K⁺, andplays a minor role in regulating turgor. Sucrose is presentin significant concentrations, but changes little with changesin salinity. Two enzymes involved in sucrose metabolism, sucrosephosphate synthetase (EC 2.4.1.14 [EC] ), and sucrose synthetase (EC2.4.1.13 [EC] ) are active in whole cell extracts of Lamprothamnium.As in the fresh water charophytes, Lamprothamnium membrane potentialmay be depolarized (close to E_K) or hyperpolarized, and presumablyof electrogenic origin. Both types of potential are found atall salinities tested.     

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 cations on the sugar receptor-site of the blowfly, Phormia

1) Ca^{+ +} (1 to 10 mM) lowered the binding affinity of sugarreceptor-site for sucrose in the labellar sugar receptor ofthe blowfly, Phormia regina, without changing the maximum-responseamplitude. It also elevated the values of the Hill coefficient(n_H) in some degrees. 2) Other divalent cations such as Mg^{+ +}, Ba^{+ +} or Cd^{+ +} alsoshowed almost the same property as above. The sequence of theeffect is as follows: Ba^{+ +}, Mg^{+ +} x Ca^{+ +} x Cd^{+ +}. Trivalentcation, La^{+ + +} (1 mM), changed the value of n_H from 1 (La^{++ +}-free) to 2. 3) On the contrary, the action of monovalent cations such asK⁺ or Na⁺, of which ionic strength was made the same as thatof the divalents hardly suppressed the response. 4) The results obtained do not support the hypothesis, at leaston the sugar receptor of the fly, that the receptor potentialis attributable to a change of the surface potential (zeta potential)as is proposed for the frog sugar receptor.     

Regulation of K+ Influx in Barley: Evidence for a Direct Control of Influx by K+ Concentration of Root Cells

Siddiqi, M. Y. and Glass, A. D. M. 1987. Regulation of K⁺ influxin barley: Evidence for a direct control of influx by K⁺ concentrationof root cells.—J. exp. Bot. 38: 935–947. The kinetics of K⁺ (⁸⁶Rb⁺) influx into intact roots of barley(Hordeum vulgare L. cv. Fergus) seedlings having different combinationsof root and shoot [K⁺], different growth rates and differentroot:shoot weight ratios were studied. K⁺ influx was stronglycorrelated with root [K⁺]; shoot [K⁺], growth rates, and root:shoot ratios appeared to have little effect on K⁺ influx. Adetailed study showed that both V_max and K_m for K⁺ influx wereaffected by root [K⁺] but not by shoot [K⁺]. We have suggestedthat factors such as growth rates and root: shoot ratio mayaffect K⁺ influx indirectly primarily via their influence onroot factors such as root [K⁺]. We have reiterated that othertypes of kinetic control, e.g. increased or decreased synthesisof ‘carrier systems’, may operate in addition todirect (allosteric?) control of K⁺ influx by root [K⁺]. Thenegative feedback signal from root [K⁺] appeared to be the primeeffector in the regulation of K⁺ influx. Key words: Barley, K⁺ influx     

Characterization of Two Proton Transport Systems in the Tonoplast of Plasmalemma-Permeabilized Nitella Cells

ATP-dependent and PP_i-dependent H⁺-transport systems of thetonoplast were characterized in plasmalemma-permeabilized Nitellacells, where direct access to the protoplasmic surface of thetonoplast was possible. Since H⁺ transport across the tonoplastcan be measured in situ, the identity of the membrane responsiblefor H⁺ pumping is unequivocal. H⁺ transport was evaluated bythe accumulation of neutral red. While both transport systemswere obligately dependent on Mg²⁺, the two transport systemsshowed completely different sensitivity to NO₃^– and K⁺,suggesting the presence of two types of H⁺-pumps in Nitellatonoplast. NO₃^– applied to the protoplasmic surface, completelyand reversibly inhibited ATP-dependent transport but had noeffect on PP_i-dependent transport. By contrast, NO₃^– appliedinto the vacuole by the vacuolar perfusion technique did notinhibit ATP-dependent or PP_i-dependent H⁺ transport. Replacementof K⁺ with the organic cation, BTP, inhibited PP_i-dependenttransport but not the ATP-dependent one, indicating that PP_i-dependenttransport is K⁺ dependent. The sensitivities of the H⁺ transportsystems found in the tonoplast of Nitella are quite similarto those of higher plant tonoplasts. ¹ Present address: Department of Botany, Faculty of Science,University of Tokyo, Hongo, Tokyo 113, Japan. (Received February 21, 1987; Accepted May 27, 1987)