Relative Uptake and Translocation of Potassium and Cesium in Barley.

The ability of plants to absorb ions selectively is a major reason for the preference for chemical rather than physical theories of ion uptake (11), yet in spite of this, comparatively little attention has been paid to this property. Thus far, selectivity between alkali cations has been examined in excised barley roots (6) in beet discs (12) and in whole plants (1,4). In recent years the relative absorption of potassium and cesium has become of practical interest because of the position of Cs137 as one of the principal long-lived fission products from nuclear weapons. While cesium behaves in a qualitatively similar fashion to potassium in absorption by excised roots (2. 6) and by whole plants (4, 9) and also in redestribution through plants (10), simultaneous determinations of the rates at which potassium and cesium are absorbed have rarely been made. The experiments reported here were designed to examine the relative rates at which cesium and potassium were absorbed by roots and were transferred to the shoots of young barley plants. It was desired in particular to examine the effect of varying the external concentration of potassium. It was therefore not possible to grow plants for long periods of time since the experimental treatments would lead to differences in growth rate. Further, the interpretation of results over long periods may be complicated by differential redistribution of potassium and cesium during growth. For these reasons a short experimental period was adopted. From a practical point of view interest lies in the behavior of carrier-free Cs'37 rather than of macro amounts of cesium. However. it is not legitimate to compare the absorption of Cs'37 and stable potassium over short periods since, whereas a gross uptake of the ra(lioactive Cs'37 is observed, the uptake of stable potassium is nieasured as a net result of an influx and an efflux of the element into and out of the tissue. In the extreme case, no uptake of stable potassium might be measured whereas a radioactive isotope of potassium might be taken up to a considerable degree by