The Exchangeability of Potassium and Bromide Ions in Cells of Red Beetroot Tissue

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Abstract
The exchangeability of potassium and bromide ions accumulated by cells of red beetroot tissue has been examined under various experimental conditions by means of radioactive tracers. It is established that the cells contain a certain amount of easily exchanged ions which is largely independent of the salt content of the tissue. The remainder of the salt does not exchange appreciably within 24 hours either during absorption or when accumulation is stopped by low temperature, potassium cyanide, high internal salt content, or by an insufficient preliminary washing of the material. An hypothesis is proposed that the easily exchanged ions are distributed throughout the intercellular spaces, cell walls, and in parts of the protoplasm, whilst those which do not readily exchange may be situated in the cell vacuoles, or else strongly associated with protoplasmic constituents. The results suggest that a considerable barrier to the free diffusion and exchange of ions is located in the region of the tonoplast of a plant cell, and the metabolic transport of ions across this membrane is discussed. An examination of the changes which occur in the amounts of readily exchanged potassium during the washing of freshly cut beet disks in aerated distilled water indicates that the protoplasm probably acquires a capacity to fix ions more strongly as a result of this treatment. This supports the view that an effect of washing on the capacity of cells to absorb salts metabolically is to increase the number of ‘absorption centres’ involved.