Exchangeable Ions in Beet Disks at Low Temperature

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Abstract
The relatively rapid passage of ions into the ‘Free Space’ of disks of beetroot has been studied using radioactive tracers while active accumulation was reduced to a negligible proportion by using ‘salt saturated’ tissue at 20°C. The experiments confirm the suggestion made previously that the free space can be treated as if made up of two main components, the ‘Water Free Space’ (W.F.S.), where the concentrations of anion and cation quickly become equal to those of the external solution, and the ‘Donnan Free Space’ (D.F.S.), containing a high concentration of non-diffusible anions. In disks pretreated with solutions of RbI to remove all other mobile ions from the free space the amount of exchangeable I and Rb was measured by the uptake of I131 and Rb86 at various external concentrations. The excess of cations over anions (the ‘extra exchangeable Rb‘) was used as an estimate of the amount of non-diffusible anions in the D.F.S. This was approximately constant at 10–13 m.equiv./kg. A gradual rise in the extra exchangeable Rb as the external concentration rose from 1 to 20 m.equiv./l. has been explained as consistent with the Donnan anions having arisen from weak acids with a pK of about 3. The volume of the D.F.S. was estimated from the amount of extra exchangeable Rb in disks which had previously been treated so that the counterions in the D.F.S. were exclusively Ca, and which were subsequently brought to equilibrium with various concentrations of RbBr. The mean volume from four experiments was 2·1 percent. so that the concentration of non-diffusible anions in the D.F.S. was 560 m.equiv./l. In consequence the fraction of the exchangeable anion in the Donnan part of the free space is negligible and so the amount in the free space divided by the external concentration gives an estimate of the volume of W.F.S. as 200 ml./kg. The results are compared with earlier estimates of the non-diffusible anions concentration made by different means. In considering the location of the D.F.S. in the tissue, account must be taken of the fact that the area of the cell protoplasts considered as smooth spheres is much too small to contain the number of immobile anions present (c. 12 m.equiv./kg) since there would be less than 1 A˚2 for each ion. For reasons given, the D.F.S. is thought to be mainly in the cell cytoplasm, a layer 1 micron thick in cells of diameter 120 microns contributing the required volume.

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