Kinetic Studies of Anion Absorption by Potato Slices at 0 C

Abstract

Generally speaking, the time course of ion uptake by plant tissue is characterised by two distinct phases, a relatively short initial period of rapid uptake followed by a less rapid but prolonged period of absorption. Frequently this basic absorption pattern still holds after discounting the ions present in the water free space (WFS), i.e., that part of the apparent free space (AFS) which is in effect an extension of the medium. The term, AFS, refers to the fraction of the tissue volume into which the external solution appears to diffuse readily and equals the sum of the WFS, in which both ions of a permeating salt are freely diffusible, and the DFS in which ions (mostly cations) are held in exchangeable form by the fixed charges of the tissue. With cation uptake the absorption shoulder can be attributed to adsorption exchange on fixed negative groups (mainly free carboxyl groups) on the cell wall (See Laties, 8). A two-component anion absorption pattern is less readily observed and is more difficult to interpret. Since the cell wall with its associated DFS is predominantly negatively charged, anion adsorption to exchange surfaces is thought to be negligible, i.e. when using mobile anions the DFS assumes a zero value. Even so an anion absorption shoulder has been observed in wheat roots and potato disks by Lundegardh (10, 11, 12), who attributed the initial rapid uptake to anion adsorption sites, and in potato disks by Laties (9), who suggested that the shoulder represented latent carrier potential attending the transfer of potato slices from room temperature to 0 C. The rationale offered was that the concentration of carrier (using the term in an operational sense) present in the tissue at any given moment is a function of temperature, and that the carrier level characteristic of the steady state at 25 C is measureably higher than that at 0 C, so that on transferring tissue from 25 C to 0 C there is a transistory excess of carrier for the 0 C steady state which leads to a more rapid absorption initially. The absorption shoulder reported by Lundegardh (10, 11, 12) is ambiguous in nature. In some experiments the technique involved a switch to a lower temperature, but it may well be that for the most part his absorption shoulders are attributable to the fact that uptake was measured by determining the salt concentration remaining in the experimental solution, and, therefore, his absorption values would include free space uptake, since a pre-absorption rinse with the test solution of only seconds duration (10) would not fill the free space. That his absorption shoulders were not under metabolic control would seem to be borne out by the fact that they varied in magnitude almost linearly with concentration yielding values unreasonably high and independent both of temperature and ion species. Only the cyanide results reported would seem to be at variance with this interpretation and their significance cannot be assessed without a clearer understanding of the separate effects of cyanide on AFS penetration and phase I uptake. The absorption shoulder observed by Laties (9), however, is directly related to a lowering of the temperature and, thus, represents a special phenomenon of particular interest since it may conceivably illumine an aspect relating to the mechanism or geometry of absorption not readily detectable otherwise. This study was an attempt to characterise further the anion absorption shoulder in potato tissue and to determine in particular whether the carrier potential as manifested by the shoulder is specific with respect to a given ion or whether ions presumed to be transported by different carriers utilize the latent transport capacity in a mutually exclusive manner.