Types, Formation, and Metabolism of Cytokinins in Leaves of Alnus Glutinosa (L.) Gaertn.

The nature of the cytokinins present in mature, expanded leavesof alder (Alnus glutinosa (L.) Gaertn.) was investigated usinga chromatographic system capable of partially resolving zeatinfrom (?)-dihydrozeatin. A zeatin-like cytokinin, present asboth a ‘free’ form and as a polar conjugate, postulatedto the the O-ß-D-glucoside of zeatin, accounted forthe bulk of the cytokinin activity (detected by the soybeancallus assay). Studies on the fate of 8-¹⁴C]zeatin supplied to detached alderleaves indicated this cytokinin to undergo extensive metabolism.An appreciable proportion of the radioactivity was incorporatedinto 80% methanol-insoluble and soluble acidic/neutral fractions,while adenosine- and adenine-like peaks were prominent metabolitesin the basic n-butanol-soluble fraction. Small amounts of glucosides,with the properties of both zeatin-O-ß-D-glucosideand dihydrozeatin-O-ß-D-glucoside were formed, thelatter becoming the most prominent with time. The ability of alder leaves to form glucoside directly from(?)-dihydrozeatin was assessed using the soybean callus assay.(?)-Dihydrozeatin was subject to a relatively rapid, continuousand substantial conversion to its putative O-ß-D-glucosideand cytokinin activity was consequently conserved while, incontrast, leaves supplied with zeatin exhibited a progressiveloss of cytokinin activity and, in agreement with the radioactivityexperiments, produced only a small amount of activity attributableto the putative O-glucosides. The significance of the observed cytokinin metabolism is discussedin relation to the endogenous cytokinin status of the leaf.