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.

The Free Space of Citrus Leaf Slices

Measurements of 36Cl and 22Na efflux have been used to estimate water free space and Donnan free space in Citrus (orange) leaf slices. The water free space within the slices amounts to about 0.025 ml/g fresh weight, suggesting that there is little infiltration of bathing solution into intercellular air spaces. The exchangeable cations of the Donnan free space within the slices total 20-25 μ-equiv/g fresh weight, and these values appear to reflect the exchange properties of the cell walls. The role of the free space as a 'reservoir' for ions in the intact leaf is discussed.

Free Space Characteristics of Barley Leaf Slices

Measurements are described of free space content of barley leaf slices. It is shown that the leaf slices contain a Donnan free space of about 3 µ-equiv/g fresh weight of tissue at a concentration of 300 mN, together with a water free space occupying 0.21 ml/g fresh weight. The Donnan free space is shown to be located in cell walls, as in other tissues (beet discs, barley roots) but the water free space is largely due to cut or damaged cells, injected intercellular spaces, and surface films of solution. The results are discussed in relation to free space of intact leaves.

Salt relations of woody tissues. II. Experiments with lengths of wood and cut leafy shoots

The relationship between the concentration of a cation in the solution flowing from a length of wood from which the bark has been removed and time is compared with that expected on the assumption that the velocity of flow is uniform and that the constants for interchange between water free space (WFS) and Donnan free space (DFS) in the wood, established previously, apply. The difference is attributed to the variation of velocity of flow with diameter of the elements in the wood and resistance in cross-walls. Measurements of uptake of cations and anions into cut shoots show that the bulk of the supply to the bark is by way of the wood although there appears to be some movement other than diffusion in the bark. The relationship between the concentration of an ion and distance from the base is considered and an exponential fall is shown to be fortuitous.

Salt relations of woody tissues. - I. Experiments with disks of wood

Solutions of electrolytes when passing through the xylem undergo change in composition through interchange of ions with the walls of the cells, living and dead, and with the contents of the living cells. This interchange has been investigated in thin transverse sections of wood (mainly yew) in solutions labelled with radioactive isotopes (mainly 42 K and 82 Br). The uptake is assumed to proceed from the external solution via the ‘water free space’ (WFS) to the ‘Donnan free space’ (DFS) and the vacuoles of the living cells; for washing out the reverse. Of the total volume of yew wood about 37% is solids, about 4% is living cells, about 55% is WFS leaving about 4% DFS. The concentration of weak non-diffusible acid in the latter is about 0.8 equiv. I. -1 and its pK between 2 and 3. The velocity constant for the loss of 42 K from WFS to DFS is about 10 -2 s -1 when the wood is in equilibrium with 20 mM KCl. It is greater when the concentration is smaller and the potential difference between WFS and DFS is greater. The Q 10 of this constant is about 1.9. The efflux of potassium from the living cells is about 0.2 pequiv. cm -2 s -1 .

The Location of the Donnan Free Space in Disks of Beetroot Tissue

This paper describes experiments which show that the cell walls of beetroot tissue contain sufficient cation�exchange sites to account for at least 95% of the Donnan free space (D.F.S.) as measured by Briggs, Hope, and Pitman (1958). The contribution of the cytoplasm to the D.F.S. in their measurements was therefore less than 5%. The exchange sites in the D.F.S. of the tissue and in the cell walls have the same pKa of about 2�8, and are considered to be due to bound Ilronic acids.

The Electric Double Layer and The Donnan Equilibrium in Relation to Plant Cell Walls

space in plant tissues into "water free space" (W.F.S.) and "Donnan free space" (D.F.S.) is examined in systems which contain electrically charged surfaces separated by various distances. It is suggested that plant cell walls should be described in terms of a system of electric double layers and not by classical Donnan equations. An approximate theory is presented which resl1lts in an expression for the equivalent width of D.F.S. in terms of the external concentration but which is independent of the surface charge density.