Source-to-sink gradient of potassium in the phloem.

The potassium contents of bark strips of cassava (Manihot esculenta Crantz) and of phloem exudate of castor bean (Ricinus communis L.) were analyzed at different regions of the stem. In cassava, a peak in potassium content was observed near the first mature leaf, leveling off both above and below this point. In castor bean, only a downward decreasing gradient was observed. In both plants, the direction of the potassium gradient coincided with the presumed direction of assimilate flow.