Changes in Levels of Butanol- and Water-Soluble Cytokinins during the Life Cycle of Potato Tubers

The nature of the water-soluble cytokinin in potato tubers (Solanum tuberosum L.) and the changes in the levels of both butanol- and water-soluble cytokinins during their life cycle were examined. The main cytokinin detected in the water-soluble fraction had the same chromatographic behavior and susceptibility to enzymes as zeatin ribotide. The level of butanol-soluble cytokinin in elongating stolon tips was low, while that of water-soluble cytokinin was extremely high. Upon swelling of the stolon tips, the former increased greatly as the latter decreased. The results suggest that the increased butanol-soluble cytokinin is responsible for the subsequent vigorous thickening growth of the stolons to form tubers and that the water-soluble cytokinin is a temporary storage form.