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

Abstract

The nature of the cytokinins present in mature, expanded leaves of alder (Alnus glutinosa (L.) Gaertn.) was investigated using a chromatographic system capable of partially resolving zeatin from (±)-dihydrozeatin. A zeatin-like cytokinin, present as both a ‘free’ form and as a polar conjugate, postulated to the the O-β-D-glucoside of zeatin, accounted for the bulk of the cytokinin activity (detected by the soybean callus assay). Studies on the fate of [8-¹⁴C]zeatin supplied to detached alder leaves indicated this cytokinin to undergo extensive metabolism. An appreciable proportion of the radioactivity was incorporated into 80% methanol-insoluble and soluble acidic/neutral fractions, while adenosine- and adenine-like peaks were prominent metabolites in the basic n-butanol-soluble fraction. Small amounts of glucosides, with the properties of both zeatin-O-β-D-glucoside and dihydrozeatin-O-β-D-glucoside were formed, the latter 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, continuous and substantial conversion to its putative O-β-D-glucoside and cytokinin activity was consequently conserved while, in contrast, leaves supplied with zeatin exhibited a progressive loss of cytokinin activity and, in agreement with the radioactivity experiments, produced only a small amount of activity attributable to the putative O-glucosides. The significance of the observed cytokinin metabolism is discussed in relation to the endogenous cytokinin status of the leaf.