Exchange diffusion of alkali ions through the apoplast of the potato (Soianum tuberosum L.) storage parenchyma

Abstract

Interdiffusion coefficients of K and Na for tangential transport through cell walls were found to be independent of ion strength and only moderately (c. 50%) lower than values known for diffusion in water. Although at low salt concentrations alkali ion exchange diffusion on the cell wall pathway is facilitated by accumulation of diffusing species in the Donnan space, at all concentrations the resistance of the whole tissue for apoplastic cation exchange diffusion remained significantly higher than that of unstirred liquid of equal thickness, as the area fraction of the cell wall pathway in the parenchyma is small.

Apoplasmic and Protoplasmic Water Transport through the Parenchyma of the Potato Storage Organ

“…The often-used term "diffusivity" for LpCWsPeC is misleading. In remarkable contrast to the diffusion coefficients of the cell wall phase for water and low-molecular-mass solutes, which are a little smaller than those of nonstirred water (Michael and Ehwald, 1996), LpCWsPeC of the cell wall is at least 4 orders of magnitude smaller than that of liquid-filled intercellular channels. This follows from Poiseuille's law: LpSPeC = constant x w 2 if the effective channel width, w, in each system is considered.…”

“…The thickness of the dehydrated cell wall measured in ultrathin slices by electron microscopy is smaller than that of the swollen wall in vivo. To get a realistic estimate of the area fraction of the cell walls between protoplasts, we determined the apoplasmic area fraction in the cross-section of the living tissue that is available for diffusion of amethylglucoside (compare Richter and Ehwald, 1983;Michael and Ehwald, 1996). The time course of diffusion of a-methylglucoside through a 3-mm-thick living parenchyma slice is shown in Figure 5.…”

“…AI1 experimental solutions contained 0.1 mM CaC1, and 0.1 g / L penicillin V (basic solution). A tight seal between the tube and the tissue slice was obtained in a manner similar to that described by Michael and Ehwald (1996). Tissue cylinders 12.5 mm in diameter were prepared from the central core of the tuber using a sharpened cork borer and sliced with a hand microtome and a razor blade.…”

“…Although diffusion coefficients for sugars and salts in the parenchymal apoplasts of sugar beet and storage organs of potato are not less than 30% of those in nonstirred solutions, the permeation coefficient of parenchyma slices for these low-molecular-mass osmotica (referred to as the total area) is smaller by about 2 orders of magnitude than that of a nonstirred liquid layer of equal thickness (Richter and Ehwald, 1983;Michael and Ehwald, 1996). The resistance of the living parenchyma slices to solute diffusion is large because of the small area fraction 1985; Dainty, 1985;Passioura, 1988;Steudle, 1993;Canny, 1995) have considered that the small area fraction and length Of the wall bypass are reaSon to doubt a general dominante Of this route for water transport through parenChYma tissue.…”

Mannitol permeation and radial flow of water in maize roots

“…, 1991; Woehlecke & Ehwald, 1995), and hence is much larger than the viscosity diameter of hydrated ions and other small diffusing solute species. Therefore, in the water phase of the parenchyma cell walls, the diffusion coefficients of sugars and electrolytes are only moderately reduced compared with those of unstirred water (Richter & Ehwald, 1983; Michael & Ehwald, 1996). There is much evidence to suggest that even the suberized and lignified radial walls of the endodermis, hypodermis or exodermis do not form an absolute barrier for small solutes (e.g.…”