The Growth and Intracellular Ionic Composition of Dunaliella tertiolecta in Magnesium-Rich Media

Dunaliella tertiolecta grew in a medium that contained MgSO₄(MgSO₄ medium), while this alga did not grow at all in mediathat contained MgCl₂ or Mg(NO₃)₂. The growth in the MgSO₄mediumwas inhibited in comparison with that in media that containedsodium salts, such as NaCl, NaNO₃, and Na₂SO₄. The energy chargeobtained from measurements of levels of adenine nucleotidesby HPLC were almost constant in Na- and Mg-containing media(about 0.87), indicating that the failure of growth in MgCl₂medium and Mg(NO₃)₂ medium was not directly related to.changesin the energy metabolism. K⁺ and Mg²⁺ were the dominant intracellularcations not only in Na-containing media (Na-media) but alsoin Mg-cohtaining media (Mg-media). The intracellular concentrationof Ca²⁺ was lower in Mg-media (1.6 mM) than that in Na-media(6mM). The concentrations of HPO₄^2– in cells incubatedin Mg-media were lower (less than 60 mM) than those in Na-media(greater than 110 mM). By contrast, the intracellular concentrationof SO₄^2– was higher in a MgSO₄ medium (26 mM) than thatin a Na₂SO₄ medium (4 mM) which, at least, compensated by 40%for the decrease in HPO₄^2–. The ability to grow in a MgSO₄medium may be related to the high intracellular concentrationof SO₄^2–. (Received September 20, 1990; Accepted March 22, 1991)     

Accumulation of phosphorus, magnesium and potassium in developing rice grains: followed by electron microprobe X-ray analysis focusing on the aleurone layer

The levels of P, Mg and K and their distributions in rice grainswere followed during the ripening period. These elements wererapidly translocated into the grains between the 10th and 20thday after flowering and thence their amounts remained constantuntil maturation. The ratio of the electric charges K⁺+Mg²⁺to –PO₄^2– (monoester-P) became 1 with maturation. Electron microprobe X-ray analysis of transverse sections ofgrains indicated that P and Mg began to concentrate in the aleuronelayer about the 12 th day after flowering. The accumulationprocesses and the distribution patterns of P and Mg were verysimilar throughout the ripening period. In contrast, K was notconcentrated in the aleurone layer until the 19th day and itsdistribution pattern differed from that of P and Mg. (Received July 14, 1978; )     

Unequal distribution of potassium and anions within the Phaseolus pulvinus during circadian leaf movement

Ion and saccharide concentrations in the upper and lower partsof the laminar pulvinus of the primary leaf of Phaseolus vulgariswere measured in relation to the circadian movement. Concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^–, organic acid,NO₃^–, H₂PO₄^–, fructose and fructose-yielding saccharidesin the pulvinus were 75–120, 0.3–0.7, 5–8,6–12, 40–60, 60–73, 19–35, 2–9and 1–5 mM, respectively, and the osmotic pressure ofthe pulvinus was considered to be due to these ions. The cell volume in the expanding part was larger than that inthe contracting part. The change of the cell volume alteredthe molar concentration in the cell sap and therefore the amountof solutes actually transported from the upper to the lowerpart and vice versa was estimated from the concentration expressedin moles per gram of dry weight. Results showed that K⁺, Cl^–, organic acid (or H⁺) andNO₃^– moved from the upper to lower parts or vice versain the pulvinus in relation to its deformation, keeping theelectroneutrality among those ions, whereas C_a2⁺ and Mg²⁺ didnot move. The difference in the K⁺ concentration between theupper and lower parts when the leaf was up or down amountedto 30% of the whole osmotic pressure. This lead to the conclusionthat the endogenous clock-controlled unequal distribution ofK⁺, Cl^–, organic acid (or H⁺) and NO₃^– in the pulvinuscould be the force for the circadian leaf movement. (Received August 7, 1979; )     

Diurnal K+ and Anion Transport in Phaseolus Pulvinus

Diurnal movement of Phaseolus leaf is caused by deformationof the laminar pulvinus located at the joint of the leaf bladeand the petiole. The plants were cultured in solutions withvarious ion compositions, and changes of K⁺, Na⁺, Ca²⁺, Mg²⁺,Cl^–, NO₃– and P₁ concentrations both in the upperand lower parts of the laminar pulvinus were measured. Culturein 10 mM KCl solution caused an increase in K⁺ and Cl^–concentrations both in the upper and lower parts without anysignificant change in the concentration of NO₃^–; culturein 10 mM KNO₃ solution caused an increase in K⁺ and NO₃^–concentration without any significant change in the concentrationof Cl^–; and culture in 10 mM KH₂PO₄ solution caused anincrease in K⁺ and P₁ concentrations without any significantchange in the concentrations of NO₃- and Cl^–. K⁺ moved from the upper to lower parts or from the lower toupper parts diurnally in all plants cultured in any solutionmentioned above. The main inorganic anion that accompanied thisK⁺ movement was Cl^– in KCl solution, and NO₃^– inKNO₃ solution. When the seedlings were cultured in distilledwater or in KH₂PO₄ solution, neither Cl^– NO₃^– norP₁ accompanied this K⁺ movement. In these cases, mainly H⁺ and/ororganic anions are supposed to move in exchange for and/or incombination with K⁺ movement. (Received November 8, 1982; Accepted June 13, 1983)     

External Potassium Supply is not Required for Root Growth in Saline Conditions: Experiments with Ricinus communis L. Grown in a Reciprocal Split-Root System

Ricinus communis L. (castor bean) plants were grown in the absence(control) and in the presence of 100molm^–3NaCl with areciprocal split-root system, in which K⁺ was supplied to oneand NO₃^– to the other part of the root system. In theseplants shoot and, to a lesser extent, total root growth wereinhibited compared to plants with non-split roots. Without andwith NaCl, growth of roots receiving NO₃^– but noK⁺ (‘minusK/plus N-roots’) was substantially more vigorous thanunder the reverse conditions (‘plus K/minus N-roots¹).100mol m^–3 NaCl inhibited growth of minus K/plus N-roots¹to the same extent as that of non-split roots, indicating thatexternally supplied K⁺ was not required for root growth undersaline conditions. In growth media without added K⁺ the rootdepleted the external low K ⁺ levels resulting from chemicalsdown to a minimum value C_mln (1.0 to 1.4 mmol m^–3); inthe presence of 100 mol m^–3 NaCl, C_min, was higher (10–18mmol m^–3) and resulted from an initial net loss of K ⁺.C_min, was pH-dependent The distribution of K⁺, Na⁺ and Mg²⁺along the root was measured. In meristematic root tissues, K⁺ concentrations were scarcely affected by external K⁺ or byNaCl, where Na ⁺ concentrations were low, but somewhat elevatedat low external K+ and/or high NaCl. In differentiated, vacuolatedtissues K ⁺ concentrations were low and Na⁺ concentrations high,if K ⁺ was not supplied externally and/or NaCl was present.The longitudinal distribution of ions within the root was usedto estimate cytoplasmic and vacuolar ion concentrations. Thesedata showed a narrow homoeostasis of cytoplasmic K+ concentrations(100–140 mol m^–3) independent of external K ⁺ supplyeven in the presence of 100 mol m ^–3 NaCl. CytoplasmicNa ⁺ concentrations were maintained at remarkably low levels.Hence, external K⁺ concentrations above C_min, were not requiredfor maintaining K/Na selectivity, i.e. for controlling Na⁺ entry.The results are discussed with regard to mechanisms of K/Naselectivity and to the importance of phloem import of K⁺ forsalt tolerance of roots and for cytoplasmic K⁺ homoeostasis. Key words: Ricinus communis, nitrate, potassium, root (split-root), salt tolerance, phloem transport     

The Effects of Salinity upon Ion Content and Ion Transport of the Marine RedAlga Porphyra purpurea(Roth) C.Ag.

Ion contents and concentrations (K⁺, Na⁺, Cl^–, Ca²⁺, Mg²⁺,SO^2–₄, NO^–₃, HPOJ^2–₄, amino and organic acids)of P. purpurea have been studied in relation to salinity variation.Cells were shown to accumulate large amounts of K⁺ and Cl^–against their respective gradients of electrochemical potentialin all dilute and concentrated seawater media. Active influxof SOJ^2–₄, NO^–₃, and HPOJ^2–₄ is also suggested,while Na⁺ is actively excluded from cells under hyposaline andhypersaline conditions. The relative proportions of individualcomponents of the internal osmotic potential were found to changeaccording to the external salt concentration. KCL forms themajor fraction of _j} in concentrated seawater media while K⁺-aminoacids form the major fraction in dilute seawaters. Other intracellularsolutes comprise less than 15% oft_j, in all media. Unidirectional fluxes of K⁺ and Cl^– were studied by radioisotopicmeans. Fluxes of K⁺ and Cl^– are reduced in hyposalinemedia, as is absolute KCL content per cell. Intracelhilar KCLcontent was also found to be markedly dependent upon externalK⁺ concentration, rather than water potential. Changes in KC1levels induced by salinity variation occur over a 6 h period.     

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)     

Application of 39K NMR Spectroscopy to Potassium Transport in Mung Bean Root Tips

The intracellular K⁺ concentration and its change in mung bean[Vigna mungo (L.) Hepper] root tips were investigated non-invasivelywith ³⁹K nuclear magnetic resonance spectroscopy using a membraneimpermeable shift reagent, dysprosium (III) tripolyphosphate[Dy(PPP_i)^7–₂]. The K⁺ resonance was shifted to highermagnetic field in proportion to the concentration of the shiftreagent. In addition to a reference capillary peak for measuringthe K⁺ concentration, two well-resolved peaks (intra- and extracellularK⁺ resonances) were observed in the 39K NMR spectra of mungbean root tips. The intracellular K⁺ concentration was determinedto be 41 mM, which was similar to the value obtained by flamephotometry. When 20 mM KCl was added to the external medium,the intensity of the intracellular K⁺ resonance gradually increasedand the net K⁺ uptake rate was calculated to be 4.1 micromolesper gram fresh weight per hour. After removal of KCl from theperfusion medium, the intracellular K⁺ concentration considerablydecreased. With ³¹P NMR method, 2.5 mM Dy(PPP_j)^7–1₂ and20 mM KCl had little effect on the ATP level in the cells. Wehave indicated that the ³⁹K NMR method can be used to determinethe K⁺ levels and net fluxes of the K⁺ transport in perfusedroot tips successively. (Received April 6, 1988; Accepted September 29, 1988)     

Effect of Ions on the Orientation of Cortical Microtubules in Spirogyra Cells

Effects of ions on the orientation of cortical micro-lubules(MTs) in Spirogyra cells were studied. After depo-lymerizalionwith amiprophos-methyl (APM), MTs were allowed to reorganizein NaCI solutions of various concentrations. As the concentrationof NaCI increased, the frequency of cells that had oblique MTsincreased. When cells in NaCI solution were transferred intoartificial pond water (APW) and incubated for 6 h, all the MTschanged to become transverse to the longitudinal axis of thecell. KC1 and MgCl₂ also had effects on the orientation of MTs.However, NH₄Cl, CaCl₂;, CoCl₂, and Co(NO₃)₂ did not show anyeffect. These results suggest that Na⁺, K⁺, and Mg²⁺have effectson MT orientation and that NH⁺₄, Ca²⁺, Co²⁺, Cl^–, andNO^–₃ have little effect. When MTs were reorganized ineither NaCl or KCl solutions, all the oblique MTs were organizedinto an S-helix. In contrast, some of the oblique MTs were foundas a Z-helix in the cells incubated in MgCl₂ or mannitol solutions.These results suggest that effects of Na⁺ and K⁺ on the orientationof MTs are not the same as those of Mg²⁺ and mannitol. Theseresults provide the first evidence that ions are involved inthe orientation of MTs in algae. (Received January 27, 1998; Accepted August 10, 1998)     

Ontogenetic Changes in the Chemical Composition of Ricinus communis Grown with NO-3 or NH+4 as N Source

Ricinus communis L. var. Gibsonii was grown in Long Ashton nutrientmedium with either 12mol m^–3 NO^–₃ or 8.0 mol m^–3NH⁺₄ as N source. Two plants from each N treatment were harvestedtwice a week and analysed for C, N, P, S, NO^–₃, SO^2–₄Cl^–K⁺Na⁺, Ca²⁺ Mg²⁺ and ash alkalinity. Statistical analysis of thedata showed that the effect of age and N source was differentfor the chemical variables analysed. Thus [Na⁺] was unaffectedby age or N source, and for both N sources [Mg²⁺] started atthe same level and decreased at the same rate as the plantsmatured. With NH⁺₄ as N source, [SO^2–₄] was higher thanwith NO^–₃, but did not alter with age. The concentrations,in mmol g^–1 dry wt, of C, organic N, K⁺ and Ca²⁺ weredifferent for the two N sources, but the levels of these variablesaltered with age in the same way for both N sources; i.e. therewas no age x N interaction. In the case of P, NO^–₃, Cl^– and COO^–, however,age-related variations were different for the two N sources.It is concluded, inter alia, that [Na⁺] is determined by external[Na⁺] alone, and that K⁺, Ca²⁺ and Cl^– are the inorganicions actively involved in charge balance during ion uptake bythe roots. Key words: Ontogeny, Chemical composition, Plant nutrition     

Osmotic and Ionic Regulation in Chara L-cell Fragments

Ion absorption from rather complicated artificial pond water(APW) by cell fragments having a lower osmotic pressure thanthe intact internodal cell (L-cell fragments) was studied. L-cellfragments were prepared by taking advantage of trans cellularosmosis and ligating the cell with thread. The results wereas follows: (1) L-cell fragments absorbed more K⁺ than Na⁺ fromaKCL + NaCl mixture in the presence of Ca²⁺, Mg²⁺ and SO₂₄ inthe light; (2) the influx of KCI was larger than that of KNO₃;(3) the amount of positive charge carried by K⁺, Na⁺ and Mg²⁺across the cell membrane balanced well with the amount of negativecharge carried by Cl^– in Cl^–-containing and NO₃^–-free APW; (4) no conclusion could be made as to whether ornot the rule of electro neutrality held for the K⁺, Na⁺, Ca²⁺and NO₃^– fluxes across the cell membrane, because dilutedKNO₃ is unstable; (5) L-cell fragments in KCl-containing APWsurvived longer than those in KNO₃-containing and Cl^–-free APW; (6) after incubation in KNO₃-containing and Cl^–-freeAPW, L-cell fragments absorbed a great amount of KCI immediatelyafter being transferred to KCl-containing and NO₃^– -freeAPW; and (7) lowering the turgor pressure of the intact cellby raising the external osmotic pressure did not induce ionflux into the cell. Thus, we concluded that the L-cell fragmentsabsorbed ions from the external solution not because of theirlower turgor pressure, but because of the diluted ion concentrationof the cytoplasm and the vacuole. The electroneutrality ruleheld, at least, for K⁺, Na⁺, Mg²⁺ and Cl^– influxes acrossthe cell membrane inthe KCl-containing and NO₃^–-free APW.These results were analyzed on the basis of an extended poremodel which presumed the existence of ATP-dependent processesin the membrane, and suggested that K⁺, Na⁺ and Mg₂₊ inflowsinto an L-cell fragment are likely to be induced by active Cl^–inflow. (Received May 18, 1987; Accepted September 29, 1987)     

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     

Extracellular Potassium Activity in Attached Leaves and its Relation to Stomatal Function

Apoplastic potassium activities (a_k) in leaves of Commelinacommunis L., Vicia faba L. and Pisum sativum L. var. argenteumwere recorded with neutral-carrier-based, potassium-sensitivemicro-electrodes. Measurements were carried out in 0.3–1.4nl volumes contiguous with the extracellular space of attachedleaves and were held for periods of 7–68 min. Mean steady-statea_K values recorded from all three species were below 50 µM.Similar potassium activities were attained, regardless of theinitial values obtained after washing with distilled water orpotassium additions, and the activities recorded showed onlyminimal dependence on the relative vapour pressure difference.Tissue capacity for K⁺ absorption was increased 15–30-foldin the presence of added Ca²⁺. By contrast, cyanide reducedboth the initial rate of potassium absorption by the tissuesand their apparent capacity for the cation. These observationsindicate that the free potassium pool in the leaf apoplast issignificantly smaller than has previously been assumed. Theresults contradict the notion that high concentrations of potassiumaccumulate locally as a result of transpiration, and may indicatethe presence in the leaf tissues of potassium transport activitysensitive to Ca²⁺ and dependent on metabolism. Key words: Apoplastic potassium activity, Transpiration/K⁺ transport, Guard cells     

Studies in the Nutrition of Apple Rootstocks: IV. The Effect on Growth and Mineral Coomposition of Supplying Magnesium through the Leaves

One-year rooted shoots of M.VII apple rootstock were grown fora single season by spraying their roots continuously with nutrientsolutions containing either < 3 ppmMg(Mg₍₀₎) or 45 ppm(Mg₍₀₎)to give, respectively, potentially very deficient or healthyplants. The new shoots of half the plants in each of these treatmentswere dipped periodically in a 2 per cent solution of MgSO₄.₇H₂O plus ‘wetter’. Mg₍₀₎ undipped plants developed severe symptoms of Mg deficiency,growth was poor, and the shoot/root dry-weight increment ratiowas high; none of these characteristics was found in Mg₍₀₎ dippedplants, whose growth was not appreciably less than that of Mg₍₂₎undipped controls. There was little translocation of Mg from leaves to roots: theconcentration of Mg in roots of Mg(0) dipped plants was as lowas that of the undipped. The large accumulation of Mn and, toa less extent, of Fe in Mg₍₀₎ dipped roots was not apparentlydetrimental to growth. Growth of Mg⁽²⁾ dipped plants was similar to that of the undipped. Dipping had little effect on the chemical composition of leaves,except to raise the concentration of Mg.     

The Role of the Stem in the Partitioning of Na+ and K+ in Salt-Treated Barley

Hordeum vulgare cv. California Mariout was grown for 50 d insand culture at 100 mol m^–3 NaCl. Xylem sap was collectedthrough incisions at the base of individual leaves along thestem axis by applying pressure to the root system. K⁺ concentrationsin the xylem sap reaching individual leaves increased towardsthe apex, while concentrations of Na⁺, NO₃^–, and Cl^–declined. Phloem exudate was obtained by collecting into Li₂EDTAfrom the base of excised leaves. K/Na ratios of phloem exudatesincreased from older to younger leaves. K/Na ratios in xylem sap and phloem exudate were combined withchanges in ion content between two harvests (38 and 45 d aftergermination) and the direction of phloem export from individualleaves, to construct an empirical model of K⁺ and Na⁺ net flowswithin the xylem and phloem of the whole plant. This model indicatesthat in old leaves, phloem export of K⁺ greatly exceeded xylemimport. In contrast, Na⁺ export was small compared to importand Na⁺ once imported was retained within the leaf. The direction of export strongly depended on leaf age. Old,basal leaves preferentially supplied the root, and most of theK⁺ retranslocated to the roots was transferred to the xylemand subsequently became available to the shoot. Upper leavesexported to the apex. Young organs were supplied by xylem andphloem, with the xylem preferentially delivering Na⁺ , and thephloem most of the K⁺ . For the young ear, which was still coveredby the sheath of the flag leaf, our calculation predicts phloemimport of ions to such an extent that the surplus must havebeen removed by an outward flow in the xylem. Within the culm,indications for specific transfers of K⁺ and Na⁺ between xylemand phloem and release or absorption of these ions by the tissuewere obtained. The sum of these processes in stem internodes and leaves ledto a non-uniform distribution of Na⁺ and K⁺ within the shoot,Na⁺ being retained in old leaves and basal stem internodes,and K⁺ being available for growth and expansion of young tissues. Key words: Hordeum vulgare L., K⁺, Na⁺, stem, salt stress     

Ion Composition of the Chara Internode

Ion compositions of the cytoplasm and the vacuole of Chara australiswere analyzed according to Kishimoto and Tazawa (1964) and Kiyosawa(1979a). The ions in the cytoplasm and the vacuole analyzedwere K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^–, NO₃^– and H₂PO₄^–.Assuming that the volume of the cytoplasm V_p is 10% of thatof the whole cell V, the concentrations of K⁺, Na⁺, Ca²⁺, Mg²⁺,Cl^–, NO₃^– and H₂PO₄^– in the cytoplasm averaged70, 15, 13, 4.6, 31, 2.2 and 16 mM, respectively. If the volumeof the cytoplasm was assumed to be 5% of that of the whole cell,their averaged concentrations were 139, 31, 25, 9.2, 62, 4.4and 33 mM, respectively. The averaged ion compositions of thecell sap were K⁺, 111; Na⁺, 47; Ca²⁺, 4.4; Mg²⁺, 8.9; Cl^–,91; NO^–₃, 3.3 and H₂PO₄^–, 6.0 mM. These values,taking the concentrations and the charges of the protein (Kiyosawa1979b) and amino acids (Sakano and Tazawa 1984) into accountand assuming the presence of some uni- or oligovalent anionsand/or small nonelectrolyte molecules, could explain fairlywell both the electroneutrality and the osmotic pressure ofthe cell, except when V_p/V = 5%. (Received May 18, 1987; Accepted September 29, 1987)     

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.     

Interrelationships between Potassium, Calcium, and Magnesium Nutrition of Zea mays L.

The uptake of potassium, calcium, and magnesium ions by maizeand the interrelationships among the cations have been investigatedat 48 K: Ca: Mg ratios in culture solutions. Calcium was foundto stimulate K⁺ and Mg⁺⁺ uptake at certain cation ratios butinhibit it at others. Potassium did the same for Ca⁺⁺ uptake,and Mg⁺⁺ for Ca⁺⁺ and K⁺. The uptake of Mg⁺⁺ was generally enhancedby K⁺. The sum of the cations in the plants expressed in meqwas fairly constant for treatments of the same K⁺ concentrationat the low to moderate levels of K⁺, but at considerably higher(> 24 meq l^–1) K⁺ levels the constancy was not dependenton K⁺ concentration. Potassium depressed, but Mg⁺⁺ stimulatedphosphorus accumulation. Calcium stimulated phosphate absorptionat certain cation ratios but had no effect at others. The plantyield increased with increasing K⁺ up to 24 meq l^–1 ofK⁺ after which the yield tended to fall with further increasein K⁺. The yield was also increased by Ca⁺⁺. Magnesium increasedthe yield at certain cation ratios and either depressed it orwas without effect at others.     

Inhibition of Nitrate Assimilation in Roots in the Presence of Ammonium: The Moderating Influence of Potassium

Experiments were conducted to investigate the effect of concentrationof NH₄⁺ in nutrient solution on root assimilation of NO₃^–and to determine whether the NH₄⁺NO₃^– interaction wasmodified in the presence of K⁺. Dark-grown, detopped corn seedlings(cv. Pioneer 3369A) were exposed for 8 h to 0.15 mM Ca(NO₃)₂and varying concentrations of (NH₄)₂SO₄ in the absence or presenceof 0.15 mM K₂SO₄. The accelerated phase of NO₃^– uptakeappeared most sensitive to restriction by additions of 0.15mM (NH₄)₂SO₄. In the absence of K⁺, the restriction increasedonly slightly even when solution (NH₄)₂SO₄, was increased from0.15 mM to 12.5 mM which was accompanied by an increase of NH₄⁺in the tissue from about 7.0 to 35 µmol g^–1 fr.wt. of root. Increasing concentrations of solution NH₄⁺ progressivelyinhibited net K+ uptake. At the highest solution NH₄⁺ concentrations,there was an initial net efflux of K⁺ and no net influx occurredduring the treatment period. The severity of the NH₄)SO₄ restrictionof NO₃^– uptake was moderated considerably in the presenceof K⁺ as long as a net influx of K⁺ occurred. However, net influxof K⁺ was not associated with alteration of NH₄⁺ uptake, assimilation,or accumulation in the root tissue. The lack of correlationbetween the severity of restriction of NO₃^– uptake andendogenous NHJ suggested the restriction resulted from an effectexerted by exogenous NH₄⁺ which tended to saturate at lowersolution NHJ concentrations or by inhibitory factors generatedduring assimilation of NH₄⁺. Several mechanisms were postulatedto account for the moderating influence of K⁺. In all experiments,root NO₃^– reduction was restricted by the presence ofambient NH₄⁺. The quantitative decreases in reduction tendedto be less than decreases in NO₃^– uptake and therefore,could result from inhibition solely of uptake with subsequentlimitation in availability of substrate for the reduction process,but the possibility of a direct effect on reduction could notbe excluded.     

Solute Accumulation In Plant Cells IV. Effects of Ammonium lons on Growth and Solute Content

Procedures previously described were used to study growth andsolute content of aseptically cultured carrot explants as affectedby supplementary salts in the medium. The salts chosen (KC1,KNO₃, NH₄,Cl, and NH⁴,NO₃) contrasted, with appropriate controls,the effects due to nitrate and ammonium. Growth was measuredin terms of fresh weight, the number and average size of cells:solute concentrations were recorded for total solutes, sugars,soluble nitrogen compounds, and the electrolytes K⁺, Na⁺, C1^–,NO^–₃, and organic acids. The time-response curves of thecultures were traced at a fixed concentration of the added saltsand the effects due to the concentration of the supplementarysalts were tested after a fixed time period, For the same nitrogensource the concentrations of metabolites and solutes in cellswere very similar despite some clonal differences in their growth.When cells in a nitrate medium were small and dividing, thecultures had a low osmotic value, contained K⁺ as the principalcation balanced by organic acid, had relatively low sugar content,and their enriched total nitrogen content emphasized proteinrather than soluble nitrogen compounds. Later, as the cellsbecame older and larger, salts (K⁺, organic anions, Cl^–)contributed substantially to their increased osmotic value butthey accumulated sugar as their main, osmotically active solute,and the ratio of soluble to protein nitrogen declined as proteinsynthesis progressed. The extra nitrogen supplied by the additionalpotassium nitrate contributed more to protein and caused potassium,organic acids, and sugars to accumulate to higher levela. Supplementaryammonium salts required that more sugar be metabolized to organicnitrogen compounds (e.g. glutamine), contributed more to solublethan to protein nitrogen, and sharply reduced. both the osmoticvalue of the cells and the potassium linked to organic anions.The selectivity of the growing cells for K⁺ over Na⁺ and theirdiscrimination. between alkali cations (Ka⁺+Na⁺) and halides(C1^–) were relaxed in the presence of ammonia. Attentionis drawn to the implications of these results for the accumulationof solutes, organic and inorganic, by dividing and enlargingcells.