The Metabolism and Translocation of Zeatin in Intact Radish Seedlings1

After the roots of intact radish seedlings had taken up [3H]zeatin for 1 h, the seedlings were transferred to nutrient lacking zeatin and extracted at intervals. After 23 h in the absence of zeatin, 6 per cent of the radioactivity extracted per seedling was recovered from the de-ribbed cotyledon laminae, 4 per cent from the hypocotyls, and 87 per cent from the roots. Per unit weight of tissue, the radioactivity extracted from the root was about 40 times that recovered from any other region. Zeatin was rapidly metabolized by the root tissue, and 4 to 9 h after transfer of the seedlings to nutrient lacking zeatin, accounted for a negligible proportion of the radioactivity. Initially zeatin riboside 5'-monophosphate was the principal root metabolite, but after 9 h, 7-glucosylzeatin (raphanatin) was the dominant metabolite. Conversion of zeatin to dihydrozeatin was not detected. Raphanatin was also the major metabolite in the cotyledon laminae where some free zeatin was detectable. The principal metabolites in hypocotyl extracts were AMP and zeatin riboside 5'-monophosphate but zeatin riboside was the only significant source of radioactivity in the xylem sap. When [3H]zeatin was applied directly to cotyledon laminae, 99 per cent of the radioactivity was localized in the treated laminae; however traces of zeatin were detected in the roots. In radish seedlings, zeatin riboside appears to be the translocational form of zeatin, while raphanatin may be a storage form.