Exchangeable Ions in Beet Disks at Low Temperature

The relatively rapid passage of ions into the ‘Free Space’of disks of beetroot has been studied using radioactive tracerswhile active accumulation was reduced to a negligible proportionby using ‘salt saturated’ tissue at 20°C. The experiments confirm the suggestion made previously thatthe free space can be treated as if made up of two main components,the ‘Water Free Space’ (W.F.S.), where the concentrationsof anion and cation quickly become equal to those of the externalsolution, 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 othermobile ions from the free space the amount of exchangeable Iand Rb was measured by the uptake of I¹³¹ and Rb⁸⁶ at variousexternal concentrations. The excess of cations over anions (the‘extra exchangeable Rb‘) was used as an estimateof the amount of non-diffusible anions in the D.F.S. This wasapproximately constant at 10–13 m.equiv./kg. A gradualrise in the extra exchangeable Rb as the external concentrationrose from 1 to 20 m.equiv./l. has been explained as consistentwith the Donnan anions having arisen from weak acids with apK of about 3. The volume of the D.F.S. was estimated from the amount of extraexchangeable Rb in disks which had previously been treated sothat the counterions in the D.F.S. were exclusively Ca, andwhich were subsequently brought to equilibrium with variousconcentrations of RbBr. The mean volume from four experimentswas 2·1 percent. so that the concentration of non-diffusibleanions in the D.F.S. was 560 m.equiv./l. In consequence thefraction of the exchangeable anion in the Donnan part of thefree space is negligible and so the amount in the free spacedivided by the external concentration gives an estimate of thevolume of W.F.S. as 200 ml./kg. The results are compared with earlier estimates of the non-diffusibleanions concentration made by different means. In consideringthe location of the D.F.S. in the tissue, account must be takenof the fact that the area of the cell protoplasts consideredas smooth spheres is much too small to contain the number ofimmobile anions present (c. 12 m.equiv./kg) since there wouldbe less than 1 A{ring}² for each ion. For reasons given, theD.F.S. is thought to be mainly in the cell cytoplasm, a layer1 micron thick in cells of diameter 120 microns contributingthe required volume.