Transport and metabolism of 8(14C)t-Zeatin.
A review of the literature established that there were areas of cytokinin transport which needed further investigation, in order to determine the function and/or mode of action of cytokinins during certain stages of plant development. Radioactive zeatin was applied to plant systems suitable for determining more about specific problems of cytokinin transport. The metabolism of the radioactive zeatin was monitored in relation to transport. The metabolism in, and possible export of, radioactive zeatin out of immature, mature and senescing Ginkgo biloba leaves was monitored using explants. The results showed that approximately the same percentage radioactivity was exported from the leaves at all three stages of their development. This indicates that these deciduous leaves could potentially export cytokinins, but the results were not regarded as being significant. Cytokinins would not have been expected to be transported out of expanding leaves, which rapidly utilize cytokinins, and in comparison greater cytokinin export would have been expected to occur from senescing leaves, but this did not occur. The resuIts could indicate that cytokinin glucosides in deciduous leaves are primarily inactivation products rather than storage compounds. The metabolism and transport of radioactive zeatin, applied to the leaves of Citrus sinesis trees, was monitored during the flush of new growth following a dormant period. Some of the radioactive zeatin applied to these leaves appeared to be utilized in the new shoot growth. This could imply that accumulated cytokinin glucosides in these evergreen leaves are exported out of the leaves and reutilized, and thus fulfil a storage function; although the extent to which export occurred as opposed to interconversion and/or catabolic metabolism in the leaves could not be determined. The distribution of radioactive zeatin applied to the xylem and phloem of Phaseolus vulgaris plants at three stages of development was also monitored. There appeared to be preferential transport of cytokinins in the transpiration stream, that is, applied to the xylem. The vegetative apices, buds and flowers appeared to be the major sinks for radioactive compounds in the vegetative and flowering plants, and the leaves were the most important sink in the fruiting plants. Radioactive zeatin in the phloem appeared to move passively with the assimilate stream. The transport of radioactive zeatin applied to the primary leaves appeared to confirm that cytokinins in the phloem were transported along with the assimilate stream. The results also indicated that the accumulated cytokinin glucosides in these annual leaves were primarily inactivation products, involved in regulating leaf metabolism, although some re-utilization of these cytokinins could potentially take place. could have reflected normal cytokinin transport occurring in the tissues. The main metabolic pathway, oxidation, did not, however, appear to be the main metabolic pathway in the plant tissues and may have been the result of the exogenous application of zeatin. The fact that metabolism did not appear to be the normal metabolism of the tissues, could potentially have affected transport.