Donnan Free Space Research Articles

Calculation Procedure for Dynamic Condition of Ion Composition in Cell Walls Altered by Proton Fluxes

The calculation procedure has been developed to determine the dynamic condition of the ion composition in cell walls during proton extrusion, known as the Weak Acid Donnan Manning (WADM) model for fluxes. In this model, the walls of plant cells are considered as a system containing weak acid polymers where the interaction of ions obeys the Manning condensation theory and the Donnan theory. When protons are transported across the walls during proton fluxes, the transported proton will interact with the wall ions and alter the ion composition. This depends on the wall parameters such as the concentration of ionisable sites, the pK of the wall weak acid, and the wall linear charge density parameter. When the linear charge density parameter is still higher than the reciprocal of the highest valence of the involve cations, the transported protons during proton fluxes will decrease the wall pH, alter the ion composition in the wall Donnan free space and water free space, and release the condensed cations. Therefore some of the proton fluxes are represented by the flux of the condensed cations outside the walls. However when the linear charge density parameter is less than the reciprocal of the highest valence of the involve cations, there is no condensed cations in the walls so that the transported protons only alter the ion composition in the wall Donnan free space and water free space.

The impact of xylem cavitation on water potential isotherms measured by the pressure chamber technique in Metasequoia glyptostroboides Hu & W.C. Cheng.

Pressure-volume (PV) curve analysis is the most common and accurate way of estimating all components of the water relationships in leaves (water potential isotherms) as summarized in the Höfler diagram. PV curve analysis yields values of osmotic pressure, turgor pressure, and elastic modulus of cell walls as a function of relative water content. It allows the computation of symplasmic/apoplastic water content partitioning. For about 20 years, cavitation in xylem has been postulated as a possible source of error when estimating the above parameters, but, to the best of the authors' knowledge, no one has ever previously quantified its influence. Results in this paper provide independent estimates of osmotic pressure by PV curve analysis and by thermocouple psychrometer measurement. An anatomical evaluation was also used for the first time to compare apoplastic water fraction estimates from PV analysis with anatomical values. Conclusions include: (i) PV curve values of osmotic pressure are underestimated prior to correcting osmotic pressure for water loss by cavitation in Metasequoia glyptostroboides; (ii) psychrometer estimates of osmotic pressure obtained in tissues killed by freezing or heating agreed with PV values before correction for apoplastic water dilution; (iii) after correction for dilution effects, a solute concentration enhancement (0.27MPa or 0.11 osmolal) was revealed. The possible sources of solute enhancement were starch hydrolysis and release of ions from the Donnan free space of needle cell walls.

Computed tomographic evaluation of osmotica on shrinkage and recovery of lupin ( Lupinus angustifolius L.) and radish ( Raphanus sativus L.) roots

X-ray computed tomography (CT) may be a useful tool to evaluate in situ root response to environmental change. The effects of increasing strength of NaCl osmotic solution on the shrinkage and recovery of radish and lupin roots were examined using X-ray computed tomography (CT). The radish and lupin roots showed different patterns of response to osmotic treatments depending on the level of osmotic potential applied. For the highest osmotic potential treatment (−0.10 MPa), radish root cross-sectional areas decreased slightly (1.4%) while the lupin root cross-sectional area increased slightly (2.6%). For the lowest osmotic potential treatments (−2.0 MPa), radish roots shrank by 72.3% as compared to 54% for lupin roots. Recovery patterns of both roots were remarkably different. While radish roots exposed to the lowest osmotic treatment were able to recover all but 14% of their original volume after removal of osmotic treatments, lupin roots were fatally injured by this extreme treatment and died. The physical density of the radish and lupin roots, as reflected by X-ray CT attenuation coefficients, differed by a factor of 18. Differences in physical density reflected changes in root water content, extent and volume of Donnan free space as well as root composition. Furthermore, increases or decreases in CT Hounsfield unit values were associated with a parallel increase or decrease in root physical density which in turn reflects the direction of water movement. Data obtained in this study illustrate the dramatic variation in response in which different plant species can tolerate salinity and the usefulness of CT as a tool to study these effects.

Analysis of 13NH4+ Efflux in Spruce Roots (A Test Case for Phase Identification in Compartmental Analysis).

13NH4+-efflux analyses were conducted with roots of intact Picea glauca (Moench) Voss. seedlings at external NH4+ concentrations of 100 [mu]M and 1.5 mM. Three kinetically distinct phases were identified with half-lives of exchange of approximately 2 s, 30 s, and 14 min. The presumed identities of the subcellular compartments corresponding to these phases were confirmed by several techniques, including pretreatment of roots (a) at 75[deg]C or with SDS, (b) with [alpha]-keto-glutarate or L-methionine-DL-sulfoximine, (c) at elevated levels of Ca2+, and (d) at low pH or with Al3+ at low pH. Treatments a and b selectively influenced phase III without affecting phases I and II. Similarly, treatment c selectively perturbed phase II, and treatment d affected phases II and III. Based on these findings and the assumption of an in-series arrangement of root cell compartments, it was concluded that phase III corresponded to the cytoplasm, phase II corresponded to the Donnan free space, and phase I corresponded to a film of solution adhering to the root surface.

Compartmentation and flux characteristics of ammonium in spruce

Using 13NH4+as a tracer, compartmental analyses for NH4+were performed in non-mycorrhizal roots of intact Picea glauca (Moench) Voss. seedlings at four different concentration regimes of external NH4+([NH4+]o), i.e. 0, 10, 100, and 1500 μM. Three kinetically distinct compartments were identified, with half-lives of exchange of approximately 2 s, 30 s, and 14 min, assumed to represent surface adsorption, Donnan free space, and cytoplasm, respectively. No significant differences were found in half-lives of exchange with changes in [NH4+]o. Influx was calculated to be 0.96 μmol·g−1·h−1 in N-deprived plants (measured at 10 μM [NH4+]o), while under steady-state conditions it was 0.21 μmol·g−1h−1 at 10 μM [NH4+]o, 1.96 μmol·g−1h·−1 at 100 μM [NH4+]o, and 6.45 μmol·g−1·h−1 at 1.5 mM [NH4+]o. Efflux measured over the same range constituted approximately 9% of influx in N-deprived plants, 10% at 10 μM, 28% at 100 μM, and 35% at 1.5 mM [NH4+]o. Cytoplasmic [NH4+] was estimated at 6 m M in N-deprived plants, 2 mM at 10 μM [NH4+]o, 14 mM at 100 μM, and 33 mM at 1.5 mM. Free-space [NH4+] was 84 μM, 50 μM, 700 μM, and 8 mM, respectively. In comparison with previously published data on fluxes and compartmentation of NO3−in white-spruce seedlings, results of this study identify a pronounced physiological preference of this species for NH4+over NO3−as an inorganic N source in terms of uptake and intracellular accumulation. The significant ecological importance of this N-source preference is discussed.

Fluxes of Ca(2+), K(+) and Cl(-) across the surfaces of detached needles from Sitka spruce trees: pathways and compartmentation.

Uptake and efflux of (36)Cl(-), (45)Ca(2+) and (42)K(+) were measured in water-infiltrated detached needles from Sitka spruce (Picea sitchensis (Bong.) Carr.) trees incubated in 1 mol m(-3) KCl or CaCl(2) or 2 mol m(-3) NaCl solutions or in artificial rain water containing mmol m(-3) amounts of these ions. Surface efflux was measured separately from leakage from the cut ends of the needles. Needles loaded with (36)Cl(-) and killed in liquid N(2) before elution displayed a rapid and extensive loss of radioisotope, indicating that mesophyll cell membranes were the limiting factor for (36)Cl(-) exchange. Data for live needles revealed a novel phase of (36)Cl(-) efflux, with an exchange halftime of about 20 min, which was faster than that for either the vacuole or the cytoplasm, but much slower than that for the free space. The novel phase was interpreted as representing diffusion of Cl(-) through the predominantly negatively charged cuticle. Killing needles loaded with (45)Ca(2+) or (42)K(+) also increased efflux relative to that from live needles, but only to a limited degree, indicating that the main factor limiting cation efflux was the cuticle. During the first hours of (45)Ca(2+) uptake, the isotherms displayed a shoulder, indicating that there was a significant Donnan free space phase in the cuticle for Ca(2+). A shoulder was absent from (42)K(+) uptake isotherms because of the preferential adsorption of divalent cations on the exchange sites.

Role of calcium in aluminium toxicity

summaryAluminium is the most important growth‐limiting factor in many acid soils throughout the world. Physiological effects of Al toxicity and mechanisms of tolerance are not well understood. An initial uptake of Al is confined to the apoplasm. Aluminium complexes (whose exact identification is beyond current experimental techniques) enter the cytosol slowly and only after prolonged exposure. Electrochemical properties of the cell wall Donnan free space as well as the plasma membrane of root cells are altered by the presence of Al ions that are polyvalent cations in acidic environments. The primary Al effects are very fast (taking only seconds to several minutes to develop) and may therefore occur while Al is still in the Donnan free space and on the apoplasmic side of the plasma membrane. Resumption of root growth upon removal of Al ions supports such a claim. Aluminium affects membrane permeability for both electrolytes and non‐electrolytes; it reduces accumulation of divalent cations (especially Ca and Mg) by interfering with the membrane transport. Aluminium alters the pattern of Ca2+ fluxes across the plasma membrane, thus supposedly disturbing symplasmic Ca2+ homeostasis. Supplemental Ca2+ can greatly alleviate deleterious Al effects. Frequently observed changes in the secretory activity of root cells exposed to Al are mediated through altered cell Ca2+ homeostasis; they result in cessation of cell wall growth and stoppage of root elongation. Involvement of calmodulin in the Al‐related phenomena is suggested to be indirect, at least in the initial stages of the Al treatment when Al is likely to be confined to the apoplasm. The role of growth substances, at least auxins and cytokinins, in the Al toxicity syndrome appears to be related to the Ca‐Al interactions that may alter the pattern of auxin transport as well as cytokinin biosynthesis and transport. Disturbance of the cell Ca2+ homeostasis appears to be an important feature of ion‐related environmental stresses in general (salt, heavy metals, aluminium).

Cadmium — citric acid — xylem cell wall interactions in tomato plants

Abstract. Mutual interactions between cadmium ions, citric acid and xylem cell walls were examined. Cadmium and citric acid were measured as 115Cd and [1,5‐14C] citric acid, respectively. Xylem cell walls were obtained by bacterial degradation of tomato stem sections (Lycopersicon esculentum Mill, cv. Tiny Tim), and applied as ion‐exchange columns. The xylem column material carried 2·4 dm3 H2O kg−1 dry weight, and was temporarily capable of buffering perfusates at pH 5·7. Sorbed cadmium and citric acid were determined from H2O and HCl rinses after perfusion periods. In all experiments, total cadmium and/or citric acid recoveries were better than 98%, indicating both the effectiveness of the rinses applied and the possibility of full regeneration of the xylem column. The results indicate that the presence of 2·45 mol m−3 citric acid causes an approximately 50% reduction of adsorbed cadmium levels, irrespective of the applied total cadmium concentrations (0·04–0·4 mol m−3 Cd(NO3)2.4H2O). This reduction is probably related to a corresponding reduction to approximately 2% of the control applied free Cd2+ concentration, the latter also independent of the total cadmium concentrations. Furthermore, without inducing positively charged citrate complexes in the applied solution, the presence of cadmium resulted in increased levels of citric acid absorbed in the xylem column. The Donnan Free Space accumulation of citric acid in the presence of Cd(NO3)2.4H2O, observed in the experiments described, could be expressed by its distribution coefficient, as approximately 15 times the control accumulation. These data indicate that the xylem column may operate as a ligand exchanger, suggesting the importance of metal ions for the longitudinal and lateral movement of organic complexing compounds in the xylem.

Some aspects of the kinetics of cadmium and thallium uptake by fronds of Lemna minor L.

summaryA study of influx and accumulation of Tl and Cd by Lemna minor has revealed a number of contrasting features which arise from the basic physicochemical properties of the two elements. Short‐term influx of Cd over a range of concentration (0·1–10 üM) followed a linear trend while that of Tl over the same range produced a profile with a plateau at the higher concentrations, indicating saturation of the absorption mechanisms. Application of Ca and K antagonists aiming to indicate the nature of the absorption mechanisms did not reveal any consistent trends. Long‐term exposure of the plant to Cd and Tl produced steady‐state concentrations after 250 and 140 h respectively, by which stage Lemna had accumulated nearly twice as much Cd than Tl. It appears that a significant amount of Cd is probably in the Donnan free‐space and strongly bound to cell constituents from which the element is depurated slowly and continuously in Cd‐free medium. It is concluded that Tl is tenaciously held in the plant tissues, perhaps in the cell vacuole, as 80 % of the steady‐state concentration is retained by the plant even after 140 h in Tl‐free medium. The influence of several external physical and chemical factors further emphasizes the contrasting nature of the two elements; Tl uptake is almost entirely an active process and ion activity of Tl is relatively unaffected by changes in pH and ligand concentration. Cd uptake involves both active and passive components, ion activity is markedly reduced at low H+ion concentrations and in the presence of ligands, such as EDTA. Inhibition of uptake was found for both elements in the presence of competing ions.

Net Mg2+ uptake in relation to the amount of exchangeable Mg2+ in the Donnan free space of ryegrass roots

Ammonium acetate and BaCl2-triethanolamine were used to desorb Mg2+ from the root Donnan free space (DFS) of 23-d-old ryegrass (Lolium multiflorum Lam. cvs. Gulf and Wilo). Amounts of desorbed Mg2+ increased with the increase in Mg2+ activity of the nutrient solution. Slightly less Mg2+ was desorbed by Ba2+ than by NH4 +. Previously published data on short-term net Mg2+ uptake by intact 23-d-old ryegrass plants of the two cultivars were linearly related to the amount of exchangeable Mg+ desorbed from the root DFS (r2=0.90 and 0.81 for the desorption by NH4 + and Ba2+, respectively). A sward of Mg2+ ions attracted to the negative charges of the cell surface is suggested to represent a part of a pool of Mg2+ available for active transport through the plasmalemma.

Uptake of cations by annual ryegrass as related to cations adsorbed onto root exchange sites

Abstract Previously published results on exchange capacities for Ca2+, Mg2+, Mn2+, and K+ in the Donnan free space of roots of two ryegrass cultivars (Lolium multiflorum Lam. cv. Marshall and Wilo) grown at two Al levels in the nutrient solution (0 and 74 μM) were correlated with the average net uptakes of the same cations. For Al‐treated plants regressed separately, significant correlations r=0.906 and r=0.963 were found for Mn2+ and Ca2+/Mg2+, respectively. No significant correlations were observed for these cations in control plants. In contrast, when data of control and Al‐treated plants were combined, significant linear correlations r=0.805, r=0.924, and r=0.968 were found for Ca2+, K+, and K+/(Ca2++Mg2+)1/2, respectively. The influence of cations adsorbed onto the root exchange sites and the effect of Al on the cation uptake processes were discussed.

Aluminum and plant age effects on adsorption of cations in the Donnan free space of ryegrass roots

Four cultivars of ryegrass (Lolium multiflorum Lam. cvs. Gulf, Marshall, Urbana, and Wilo) were grown in nutrient solution (pH 4.2) at two Al levels (0 and 74 μM). Cations were desorbed from the Donnan free space of roots of 15-, 23-, and 35-day-old plants using BaCl2, BaCl2-triethanolamine, NH4OAc, and KCl. The amounts of desorbed Ca2+ and K+ decreased, while desorption of Mn2+ and Na+ increased with plant age. Differences between 15- and 35-day-old plants, but not between 15- and 23-day-old, were significant. Aluminum considerably decreased the amount of desorbed divalent cations (Ca2+, Mg2+) and increased the amount of desorbed K+ and Na+. Ability to resist these changes appeared to be one of the mechanisms determining Al tolerance of ryegrass cultivars.

Ion behavior in plant cell walls. II. Measurement of the Donnan free space, anion-exclusion space, anion-exchange capacity, and cation-exchange capacity in delignified Sphagnum russowii cell walls

Isolated delignified cell walls from Sphagnum russowii Warnsdorf were incubated in various chloride salt solutions at neutral pH (pH 7 – 8), and ion sorption was measured directly by neutron activation analysis. The anion-exchange capacity was estimated to be 63 – 66 μequiv./g dry weight of wall material in the protonated form. The volume of the anion-exclusion space was 2.63 ± 0.21 (± SD, n = 3) and 1.65 ± 0.35 (± SD, n = 2) mL/g dry weight in NaCl and CaCl2, respectively. A novel approach to measure the Donnan free space is proposed: for walls equilibrated in a salt mixture containing 10 mequiv./L NaCl and 10 mequiv./L CaCl2, the Na+ ions can be considered "uncondensed" in the Manning sense. From the Donnan relationship for Na+ and Cl− ions in the internal and external phases, the Donnan free space was calculated to be 1.77 mL/g dry weight. Titrating walls from pH 2.1 to 9.1 in the presence of 10 mequiv./L NaCl and 10 mequiv./L CaCl2 revealed a maximum cation-exchange capacity above pH 6 of ca. 1900 μequiv./g dry weight. This corresponds to a fixed anionic charge concentration in the Donnan free space of 1.1 M. Key words: ion exchange, cell wall, Donnan free space.

Ion behavior in plant cell walls. I. Characterization of the Sphagnum russowii cell wall ion exchanger

The ion-exchange properties of Sphagnum russowii cell wall material were studied in the context of the Donnan weak acid model. Titrations in the presence of Na+, Ca2+, or La3+ revealed two classes of weak acid binding sites, one with a low pK between 2 and 4 and the other with a high pK > 5. The total cation-exchange capacities of the low and high pK species were 1071 and 545 μequiv./g dry wt. wall material, respectively, and were related to the uronic, amino, and phenolic acid contents of the walls. Differences in pK estimates obtained in the presence of the different cations (pKNa > pKCa > pKLa) could not be explained by our semiarbitrary choice (1 mL/dry wt. wall material), for the effective volume (Donnan free space) bathing the exchange sites. It was concluded that valence-dependent reductions in cation activities in the wall phase are an important contributor to the differences in the pK estimates. Key words: cation exchange, cell wall, titration, Donnan model, weak acid.

Cell walls as reservoirs of potassium ions for reversible volume changes of pulvinar motor cells during rhythmic leaf movements

The laminar pulvinus of primary leaves of Phaseolus coccineus L. was investigated with respect to the total K(+) content, the apoplastic K(+) content, and the water potential of extensor and flexor sections in relation to the leaf positions in a circadian leaf-movement cycle, as well as the cation-exchange properties of isolated extensor- and flexor-cell walls. Turgid tissue showed a high total but low apoplastic K(+) content, shrunken tissue a low total but high apoplastic K(+) content. Thus, part of the K(+) transported into and out of the swelling or shrinking protoplasts is shuttled between the protoplasts and the surrounding walls, another part between different regions of the pulvinus. The K(+) fraction shuttled between protoplasts and walls was found to be 30-40% of the total transported K(+) fraction. Furthermore, 15-20% of the total K(+) content of the tissue is located in the apoplast when the apoplastic reservoir is filled, 5-10% when the apoplastic reservoir is depleted. The ion-exchange properties of walls of extensor and flexor cells appear identical in situ and in isolated preparations. The walls behave as cation exchangers of hhe weak-acid type with a strong dependence of the activity of fixed negative charges as well as of the K(+)-storing capacity on pH and [K(+)] of the equilibration solution. The high apoplastic K(+) contents of freshly cut tissues reflect the cation-storing capacity of the isolated walls. We suggest that K(+) ions of the Donnan free space are used for the reversible volume changes (mediating the leaf movement) mainly by an electrogenic proton pump which changes the pH and-or the [K(+)] in the water free space of the apoplast.

Root histology, and nutrient uptake and translocation in tissue culture plantlets and seedlings ofThuja occidentalis L.

Histological studies of the vascular connection in the root-shoot junction of tissue culture-derived plantlets and seedlings ofThuja occidentalis L. (Eastern white cedar) showed that in both plant types the xylem was well developed, smooth and not disrupted. Uptake and translocation of32P and86Rb were also similar for plantlets and seedlings. However, approximately 50% more86Rb could be desorbed from the root Donnan free space of plantlets than from seedlings. Uptake and translocation to the shoot was higher for86Rb than for32P, but no differences were found between the plant types. Tracer translocation rates to the shoot seem to be positively correlated with shoot fresh weight.

Cation exchange in isolated xylem cell walls of tomato. II. Direct determination of chemically bound and electrostatically attracted Cd2+ by γ‐γ‐directional correlation measurements

Abstract Xylem cell walls were isolated from internodes of tomato plants (Lycopersicon esculentum Mill, cv. Tiny Tim). Purification was carried out in Triton X‐100. Experiments were conducted in which the adsorption of Cd2+ (applied as 115CdCl2) in the cell walls was measured. The application of γ‐γ‐directional correlation measurements permitted the direct determination of chemically bound cell wall Cd in experiments where the total adsorption of Cd was also measured. A set of equations is presented by which the adsorption of Cd could be described, thereby considering the presence of co‐ions in the polyelectrolyte phase. In experiments with simultaneous ll5CdCl2 and K82Br applications, measurements were carried out of the presence of 82Br− in the Donnan Free Space (DFS), indicating Cl− action in the process of CdCl2 adsorption.The cell wall contents of the relevant ions were determined in such a manner that the measurements gave access to the undisturbed equilibrium concentrations. The very small Br− concentrations in the DFS indicated that, at small applied concentrations, internal free Cd may be considered to exist entirely of Cd2+. The results suggest that simultaneous measurement of internal free counter‐ion and co‐ion concentrations may be a relatively easy way to check the reliability of values for apparent formation constants in single cation adsorption experiments. From experiments in which the cell walls were incubated in solutions of varying Cd concentrations, the apparent dissociation constant (KCd2+) for the formation of R2Cd complexes could be calculated as 51±6 mol m−3. The exchange coefficient (KCd2+H+) was shown to vary with the internal free proton concentration and the applied Cd2+ concentration, due to the buffering capacity of the carboxylic acid exchanger.

Relationships between Adsorption, Chemical State and Fluxes of Cadmium Applied as Cd(NO3)2in Isolated Xylem Cell Walls of Tomato

Wolterbeek, H. Th. 1987. Relationships between adsorption, chemical state and fluxes of cadmium applied as Cd(N03)2 in isolated xylem cell walls of tomato.—J. exp. Bot. 38: 419-432. Isolated xylem cell wall pieces were applied as membranes in ion diffusion experiments. The cell walls were isolated from tomato internodes (Lycopersicon esculentum Mill, cv. Tiny Tim) and sealed in a two-compartment diffusion system. In flux and adsorption calculations, the cell wall was regarded as a leaky membrane with parallel fluxes through Donnan Free Space (DFS) and Water Free Space (WFS). During the experiments absorption into and diffusion across the walls was determined of Cd2+, applied as 115Cd(N03)2. Flux experiments with 82Br indicated that excluded volume effects and path tortuosity resulted in apparent WFS diffusion coefficients in the walls which were 0 012 times as high as in water. The free proton concentration in the DFS was shown to be related to a complex formation between fixed charges and Cd2 + . The cell wall permeability for Cd2+ and NO, varied with applied and absorbed concentrations, and the Cd2+ flux curve showed an inflexion point coinciding with a buffered degree of dissociation of fixed charges in the DFS. The necessary coupling of fluxes of opposite charges resulted in relatively high N03 and small Cd2+ permeability of the DFS for strongly diluted solutions (P = 104 m s' and I CP11 m s-1 for NOf and Cd2+ respectively). The results demonstrate the possible regulatory effects of the cell wall in processes of ion transfer from xylem vessels, or ion uptake in plant tissues. Key words—Cadmium, chemical state, DFS, WFS, ion flux, permeability, xylem cell walls, tomato, bromium, nitrate. Correspondence to: Interuniversitair Reactor Institute, Mekelweg 15,2629 JB Delft, The Netherlands.

Cation exchange in isolated xylem cell walls of tomato. I. Cd2+ and Rb+ exchange in adsorption experiments.

Purified xylem cell walls were prepared from isolated xylem bundles of tomato (an inbred line of Lycopersicon esculentum Mill, cv. Tiny Tim). Adsorption and exchange experiments were carried out with 115 Cd2+ , 82 Rb+ and 82 Br- . The application of γ-ray spectroscopy permitted the simultaneous measurement of several ions applied together. The cell-wall water volume was shown to be independent of the external pH and solution ionic strength, possibly due to the presence of lignin. The Donnan Free Space (DFS) volume could be determined as a constant 0.15 dm3 per kg cell-wall dry weight. Consequently, the total cell-wall cation exchange capacity (CEC) could be estimated based on the DFS volume, and amounted to approximately 1000 mol m-3 negative charges. The results of Cd2+ -Rb+ exchange experiments indicated an apparent CEC value of about 350-450 mol m-3 DFS, at external pH ∼ 4. These data are in agreement with earlier reports on xylem wall CEC, and indicate the weak acid characteristics of the charge groups. The rational selectivity coefficient RCdRb , of the cell wall was shown to depend on external ion fractions and ionic strength, with a maximum RCdRb of 450 at ionic Cd2+ fraction near 0.3, based at the smallest experimental ionic strength of the external solution. The adsorption of Cd2+ , applied at relatively high concentrations, was shown to be stimulated by simultaneous application of high Rb+ concentrations.

Activity of Fixed Negative Charges of Isolated Extensor Cell Walls of the Laminar Pulvinus of Primary Leaves of Phaseolus

Summary The activity of fixed charges of isolated, morphologically intact extensor cell walls of the laminar pulvinus of the primary leaves of Phaseolus coccineus L. was determined by measuring the electric potential generated by the fixed negative charges. The activity of fixed charges depends strongly on the pH and KCl concentration of the bathing solution. The maximum activity measured was approx. 570 meq 1 -1 at pH 5 in 100 mM KCl and 400 meq 1 -1 at pH 6 in 10 mM KCl, respectively. The apparent maximum activity of fixed charges in the walls of living extensor cells (250 meq 1 -1 in 10 mM KCl) is lower than in isolated cell walls. The apparent pKa value of 3.1 calculated from the titration curves of isolated extensor cell walls in both 100 mM and 10 mM KCl indicates that the carboxyl-groups of pectins are the source of the fixed charges. In isolated cell walls the mean concentration of fixed anions as calculated from the K + concentration in the walls is lower than that calculated from the wall potential. This indicates that a large fraction of the cell wall solution is unaffected by the fixed anions, i.e. may be divided into a water free space and Donnan free space. The volume of the Donnan free space is estimated to be 55 % of the total wall water or 2.9 ml g -1 dry weight. We suggest that, due to the high activity of fixed negative charges, the Donnan free space of the extensor cell wall serves as a K + reservoir which is used to change the cell volume via the pH of the apoplastic solution.