Influence of the Counter-ion on the Absorption Isotherm for Chloride at Low Temperature

Effect of poly-L-lysine on potassium fluxes in red beet tissue

Poly-L-lysine concentrations (10(-6) M) which cause slight leakage of pigment from beet cells completely disrupt the kinetics of(*)K (labeled) absorption at 25°C in the range 0.01 to 50MM KCl. Lower concentrations of polylysine (10(-7) to 10(-9) M) interfere with potassium fluxes at both cell membranes, initially increasing efflux across the plasma membrane and decreasing the capacity of the cytoplasm to retain ions during flux experiments at 2°C. At 25°C, these concentrations of polylysine increase(*)K (labeled) absorption from 0.2MM KCl, but not from 10MM KCl. These responses are discussed in relation to ion transport via the three-compartment in-series model proposed for plant cells. Particular emphasis is placed on the role of the plasma membrane in K transport from solutions of low concentration.

Magnesium transporters, MGT2/MRS2-1 and MGT3/MRS2-5, are important for magnesium partitioning within Arabidopsis thaliana mesophyll vacuoles

• Magnesium accumulates at high concentrations in dicotyledonous leaves but it is not known in which leaf cell types it accumulates, by what mechanism this occurs and the role it plays when stored in the vacuoles of these cell types. • Cell-specific vacuolar elemental profiles from Arabidopsis thaliana (Arabidopsis) leaves were analysed by X-ray microanalysis under standard and serpentine hydroponic growth conditions and correlated with the cell-specific complement of magnesium transporters identified through microarray analysis and quantitative polymerase chain reaction (qPCR). • Mesophyll cells accumulate the highest vacuolar concentration of magnesium in Arabidopsis leaves and are enriched for members of the MGT/MRS2 family of magnesium transporters. Specifically, AtMGT2/AtMRS2-1 and AtMGT3/AtMRS2-5 were shown to be targeted to the tonoplast and corresponding T-DNA insertion lines had perturbed mesophyll-specific vacuolar magnesium accumulation under serpentine conditions. Furthermore, transcript abundance of these genes was correlated with the accumulation of magnesium under serpentine conditions, in a low calcium-accumulating mutant and across 23 Arabidopsis ecotypes varying in their leaf magnesium concentrations. • We implicate magnesium as a key osmoticum required to maintain growth in low calcium concentrations in Arabidopsis. Furthermore, two tonoplast-targeted members of the MGT/MRS2 family are shown to contribute to this mechanism under serpentine conditions.

Chloride accumulation by mung bean root tips: a low affinity active transport system at the plasmalemma

Net uptake of Cl(-) into root tips of mung bean (Phaseolus aureus) increases steadily with increasing external concentrations from 1 to 60 mm. Membrane potentials were measured to determine the equilibrium concentration of Cl(-) in the tissue which could be due to diffusion. This concentration was readily exceeded in both the relatively nonvacuolate tips (0 to 1 mm) and the vacuolate, mature upper sectons (1 to 11 mm) of the roots. The activity coefficient of both cytoplasmic and vacuolar Cl(-), measured with Cl(-) sensitive microelectrodes, was approximately the same as that of a pure KCl solution of the same concentration. It is concluded that the "second mechanism" of ion uptake involves a large increase in the rate of active transport at the plasmalemma as the external concentration is increased above 1 mm.

Simulation of Cl Uptake by Low-salt Barley Roots as a Test of Models of Salt Uptake

Computing techniques are used to simulate the course of uptake of K(+), Na(+), and Cl(-) by low-salt roots. Measurements of the fluxes of these ions in high-salt roots are used to calculate membrane permeabilities, which are then used to calculate cell uptake. In this way it is possible to test the predictive value of different models for the location of sites of salt uptake in the cell.On the basis of the permeability data used, it is suggested that anion transport at the plasmalemma must be larger than that provided by Mechanism I and that there is a need for transport of both cations and anions at the tonoplast. To account for the observation that the level of salt at equilibrium in the tissue is very nearly independent of concentration, it is suggested that there must be some feedback from vacuolar content to rate of transport at the tonoplast.

Water dependent and water independent fluxes of potassium in exuding root systems of Ricinus communis

The flux of water, [Formula: see text], to the xylem of exuding root systems of Ricinus communis was controlled using a range of mannitol concentrations permitting the influence of this water flux on the potassium flux, f K, to be studied. The relationship between [Formula: see text] and f K thus obtained was investigated, for a number of external concentrations of potassium, Cm, supplied as potassium nitrate. An analysis of these data indicated the presence of a water dependent and a water independent f K both of which varied with Cm. The water dependent f K shows a parabolic relationship with Cm for Cm values <1 mM followed by a sharp inflection and decline at higher Cm values whereas the water independent f K shows an hyperbolic relationship over the same range of Cm values.Uptake of potassium by exuding root systems was measured and shown to be dependent on the solute potential of the medium. The uptake was also shown to exhibit a dual absorption isotherm the kinetics of which indicate a low Km system (system 1) and a high Km system (system 2). The Km value obtained for system 1 is very similar to that obtained for the water independent f K. It is postulated that the water independent f K is contributed by that portion of f K arriving in the stele via the cortical symplast and is directly dependent on Cm. The water dependent f K is contributed by those ions moved across the root in response to centripetal water movement through the cortical cell walls.

[The kinetics of ion uptake by young and old branches of mnium cuspidatum]

Isotherms of K(Rb)-, Cl- and SO4-uptake by young and old branches of the moss Mnium cuspidatum were investigated. Old moss gametophytes from the 1966 vegetation period were collected in the forests surrounding Darmstadt from February to mid-April 1967 and from the 1967 season in late September 1967. Young plants were sampled from mid-April to the end of May 1967 and they were also grown by water culture of old plants.Both young and old branches have hyperbolic isotherms of ion uptake in the low concentration range (0-0.5 mM) (Fig. 1-3), which slightly differ in K mand V max (Table). Isotherms in the high range (1-10 mM), however, are drastically different, changing from linear or exponential with young moss branches to hyperbolic with old gametophytes (Figs. 1-3).The linear or exponential high-range isotherm obtained with young moss plants is compared with other examples reported in the literature (Fig. 4). As the leaflets of the moss plants, which constitute 2/3 of the fresh weight of the material used in the experiments, have well developed vacuoles, the correlation between hyperbolic isotherms and vacuolation does not apply here (Fig. 4a, TORII and LATIES, 1966).The change in shape of the high-range moss isotherm with age resembles the change from exponential to hyperbolic kinetics in isolated potato discs during washing (Fig. 4b, LATIES, MACDONALD and DAINTY, 1964). The events triggered by isolation of potato discs from the interior of the tuber may be similar to the changes in the moss material under the control of the terminal bud, which is only active in the young branches.The suggested influence of the active terminal bud of young moss plants on the ion absorption process of cells in the tissue may be related to effects of growth substances on translocation reported in the literature and may point to a direct effect of these regulatory systems on membrane function.In this respect the comparison of corn root stele and cortex is of interest. Isolated steles, both freshly isolated and after washing, have exponential isotherms in the high range (Fig. 4c), whereas cortex displays a hyperbolic isotherm which changes little with ageing (LüTTGE and LATIES, 1967). In contrast to the case in potato and moss materials, this phenomenon is not simply due to ageing but involves morphogenetic differences.Temperature is another factor which influences the shape of the high range isotherm. All examples discussed so far refer to experiments at room temperature. At low temperatures high-range isotherms for proximal root tissue or aged potato discs have an exponential shape (TORII and LATIES, 1966; LATIES, MACDONALD and DAINTY, 1964). It thus appears that the exponential isotherm of young moss branches indicates that as in freshly isolated potato discs or in corn root stele the metabolic high-range uptake system is not developed.

[Ultrastructure of the leaf cells of young and old branches of Mnium and its relation to ion uptake]

Differences in the ultrastructure of the leaf cells of young and old branches of Mnium are demonstrated. The appearance of the cytoplasm and the endoplasmatic reticulum membranes differs with the age of the leaves. In the young leaves numerous vesicles are found within the tonoplast. Quite pronounced are changes in plastid structure. Compared with those of the young leaves the chloroplasts of the old leaf cells have many more thylacoid membranes which form large grana. The changes in ultrastructure coincide with differences in transport phenomena reported earlier. A few speculations on the possible significance of this correlation are presented.

Interpretation of the dual isotherm for ion absorption in beet tissue

Beet discs aged in 0.5 mM CaSO(4) develop a capacity to absorb K(+) and Cl(-) from solutions of low concentration. The initial influx of these ions is described by a hyperbolic relationship with concentration in the range 0.01 to 0.5 mM KCl, which is identical with the system 1 absorption isotherm found in other tissues. A second hyperbolic isotherm, attributable to system 2, is found at higher concentrations (1-50 mM KCl).When the transport of labeled ion to the vacuole is studied by wash-exchanging the bulk of the cytoplasmic label following the absorption period, it is noted that in the range of system 1, isotope influx to the vacuole increases with time as the concentration of labeled ions in the cytoplasm increases, while in the range of system 2, influx to the vacuole is constant from the beginning. Diminution of the cytoplasmic specific activity during radio-isotope absorption by prefilling the cytoplasm with the analogous unlabeled salt, markedly reduces subsequent radioisotope uptake to the vacuole only in the range of system 1. These experiments suggest that the cytoplasm serves as a mixing chamber, and that the plasma membrane controls ion uptake to the tissue at low concentrations, indicating that the system 1 isotherm reflects ion movement into the cytoplasm through the plasma membrane. Flux experiments support this conclusion, showing that development with age of the system 1 isotherm corresponds to a quantitatively similar increase in plasma membrane influx in 0.2 mM KCl.At higher concentrations the outer membrane no longer rate-limits entry of ions to the vacuole. Isotope influx under these conditions, described by the system 2 isotherm, presumably reflects movement across the tonoplast.

Absorption and long distance transport by isolated stele of maize roots in relation to the dual mechanisms of ion absorption

Ion absorption and transport by intact roots, isolated cortex and isolated stele were compared shortly after tissue isolation and after aging. Absorption isotherms in the low and in the high concentration range show that in stripped-stele, which absorbs at a very low rate immediately after isolation, the capacity of system 1 but not system 2 is built up with aging. In agreement with this result analysis of individual fluxes across plasmamembrane and tonoplast reveals that only the influx from the medium into the cytoplasm increases considerably with aging of stele. Changes observed in aging excised roots and in isolated cortex are much less significant. In spite of the increase of absorption with aging by isolated stele, long distance transport, which is essentially passive through freshly stripped stele, decreases with aging. The reported results reflect the marked permeability of the plasmamembrane of fresh isolated stele, and demonstrate the importance of the cortex as a tissue "collecting" ions for long distance transport. New evidence for the theory of symplasmatic transport of ions into the xylem vessels is thus provided.

Mineral ion contents and cell transmembrane electropotentials of pea and oat seedling tissue

The relationships of concentration gradients to electropotential gradients resulting from passive diffusion processes, after equilibration, are described by the Nernst equation. The primary criterion for the hypothesis that any given ion is actively transported is to establish that it is not diffusing passively. A test was made of how closely the Nernst equation describes the electrochemical equilibrium in seedling tissues. Segments of roots and epicotyl internodes of pea (Pisum sativum var. Alaska) and of roots and coleoptiles of oat (Avena sativa var. Victory) seedlings were immersed and shaken in defined nutrient solutions containing eight major nutrients (K(+), Na(+), Ca(2+), Mg(2+), Cl(-), NO(3) (-), H(2)PO(4) (-) and SO(4) (2-)) at 1-fold and 10-fold concentrations. The tissue content of each ion was assayed at 0, 8, 24, and 48 hours. A near-equilibrium condition was approached by roots for most ions; however, the segments of shoot tissue generally continued to show a net accumulation of some ions, mainly K(+) and NO(3) (-). Only K(+) approached a reasonable fit to the Nernst equation and this was true for the 1-fold concentration but not the 10-fold. The data suggest that for Na(+), Mg(2+), and Ca(2+) the electrochemical gradient is from the external solution to the cell interior; thus passive diffusion should be in an inward direction. Consequently, some mechanism must exist in plant tissue either to exclude these cations or to extrude them (e.g., by an active efflux pump). For each of the anions the electrochemical gradient is from the tissue to the solution; thus an active influx pump for anions seems required. Root segments approach ionic equilibrium with the solution concentration in which the seedlings were grown. Segments of shoot tissue, however, are far removed from such equilibration. Thus in the intact seedling the extracellular (wall space) fluid must be very different from that of the nutrient solution bathing the segments; it would appear that the root is the site of regulation of ion uptake in the intact plant although other correlative mechanisms may be involved.

Regulation of betacyanin efflux from beet root by poly-L-lysine, ca-ion and other substances

Poly-l-lysine, poly-alpha, gamma-diaminobutyric acid and basic proteins cause efflux of betacyanin from beet root tissues to varying degrees. Membrane activities fall in the order: polylysine > poly-alpha, gamma-diaminobutyric acid > polyarginine (protamine), suggesting the importance of steric factors in side-chain to backbone relations. It was also observed that homopolymer activity > heteropolymer activity, using ribonuclease and lysozyme as examples of the latter. Among polylysines, there appears to be an optimal chain length at a molecular weight equal to 50,000. Lowered activity of larger polymers is interpreted in terms of a diffusion barrier, the cell wall.Polylysine and Ca(++) exhibit competitive kinetics, and Ca(++) otherwise is far more active than other cations. It is assumed that polylysine displaces Ca(++) from anionic centers on the membrane, but cannot confer equivalent dimensional stability, rendering the membrane leaky. The possible role of cationic shielding in ionic stabilization of the membrane was also considered. The order of divalent ion activity against polylysine was Ca(++) > Sr(++) > Mg(++), suggesting again a specific size-fit relationship.

Wounding Response in Relation to Polar Transport of Radiocalcium in Isolated Root Segments of Zea mays

A perfusion bridge technique is described which permits the continuous collection of exudations from both ends of corn root segments. By exposing the central portion of the segments to radiocalcium, the amounts and rates of tracer movement in either direction may be determined. Typically, a peak in both acropetal and basipetal transport occurs at about 90 minutes after exposure to tracer. This transport peak is followed by a sharp decline to relatively low transport rates. Thereafter the 2 perfusates from opposite ends of a segment pair show significant differences. The acropetal increments decrease somewhat erratically to 0 at 10 to 12 hours, while the basipetal increments steadily increase to a steady-state value which remains constant from 8 to 24 hours. After a segment pair has reached steady-state polar transport, a fresh cut on the apical ends causes the resumption of acropetal transport. Such response suggests that polar transport in these root segments is at least partially a wound response. A possible explanation of the complex transport behavior is advanced.

The influence of the intracellular potential on potassium uptake by beetroot tissue

Intracellular potentials were measured in beetroot tissue during the steady-state uptake of K(+) from various solutions. In solutions containing bicarbonate, the membrane potential becomes up to 70 mv more negative than the estimated equilibrium potential for K(+). The uptake of K(+) from such solutions is correlated with variations in the potential, both when the bicarbonate concentration is changed and also when the metabolic activity of the tissue is changed by washing in water for various periods. However, the estimated permeability to K(+) varies from 0.4 x 10(-7) to 1.5 x 10(-7) cm.sec(-1). It is postulated that the change of potential arises from the metabolic transport of HCO(3) (-) into the cell or H(+) outwards, and that the associated uptake of K(+) is partly or entirely by passive diffusion across the cell membrane. In contrast, K(+) uptake from KCl solutions is not accompanied by any significant change in the membrane potential, which remains relatively close to the K(+) equilibrium potential. In solutions containing both KHCO(3) and KCl, it appears that an amount of K(+) equal to the influx of Cl(-) is taken up independently of the potential, while the component of K(+) uptake which is not balanced by Cl(-) uptake is related to the potential in the manner described. These results suggest that K(+) uptake is linked to Cl(-) uptake in an electrically neutral active transport process.