Changes in the Cytokinins of Radish Roots during Maturation

The cytokinins of developing radish roots were extracted, partially purified, and separated by thin-layer chromatography into three distinct bands of activity. One band was identified chromatographically as zeatin ribonucleotide and another was indistinguishable from a mixture of zeatin and zeatin ribonucleoside. The third band was not identified, but it was not a derivative of zeatin or of isopentenyladenine. The unidentified cytokinin had physiological properties quite different from those of the zeatin derivatives. The zeatin-based cytokinins increased in radish roots with the onset of cambial activity, and reapplication of these cytokinins to cultured primary roots stimulated cambial activity. The unidentified cytokinin became abundant only after extensive secondary thickening had occurred, and it was localized almost entirely in the xylem. It did not stimulate cambial activity in cultured roots. The evidence indicates that zeatin and its derivatives regulate cambial activity in radish, and that the unidentified cytokinin may be synthesized in the roots and transported to the shoot.