Cytokinin metabolism in Narcissus bulbs:: chilling promotes acetylation of zeatin riboside

The levels of the cytokinins zeatin, dihydrozeatin and N-6-(isopent-2-enyl) adenine and the corresponding 9-ribosides were determined in whole Narcissus pseudonarcissus L. bulbs after onset of summer dormancy, and after storage of the dormant bulbs at 4 or 15 degreesC under moist conditions. The cytokinin level was increased at both temperatures; the dominant cytokinin at 15 degreesC was zeatin riboside but at 4 degreesC the free base, zeatin, was the principal cytokinin. The metabolism of H-3-labelled zeatin, zeatin riboside and 6-benzylaminopurine was studied in scales and base plates excised from bulbs previously held at 4 or 15 degreesC. The chilling treatment promoted conversion of [H-3] zeatin riboside to O-acetylzeatin riboside ( an unusual cytokinin metabolite) in the excised base plate and inhibited 9-riboside formation from 6-benzylaminopurine. Chilling also promoted formation of O-acetylzeatin riboside from [H-3] zeatin in excised scale tissue. Endogenous O-acetylzeatin and O-acetylzeatin riboside were concentrated in the base plate. Relative to zeatin riboside, O-acetylzeatin riboside was degraded to adenosine and related compounds less rapidly in the leaf cluster. Since acetylation of the zeatin side chain enhances activity in cytokinin bioassays, O-acetylzeatin riboside may play a role in leaf development following chilling of Narcissus bulbs.