CYTOKININ BIOCHEMISTRY IN RELATION TO LEAF SENESCENCE .5. ENDOGENOUS CYTOKININ LEVELS AND METABOLISM OF ZEATIN RIBOSIDE IN LEAF-DISKS FROM GREEN AND SENESCENT TOBACCO (NICOTIANA-RUSTICA) LEAVES

The cytokinin (bases and ribosides) content of leaf discs derived from green and senescent leaves of Nicotiana rustica was analyzed by radioimmunoassay. Zeatin was identified as the major base and dihydrozeatin riboside as the main riboside. Leaf discs from expanded green (presenescent) leaves contained three times higher zeatin levels than those from partly-senescent leaves. The uptake and metabolic fate of [H-3]-zeatin riboside supplied to discs from tobacco leaves of different maturities was also investigated. The uptake of zeatin riboside declined with an increase in leaf maturity and senescence. Zeatin riboside was rapidly inactivated mainly by side-chain cleavage. Leaf discs excised from upper green leaves metabolized zeatin riboside into inactive compounds more rapidly than did the basal senescent leaves. It is suggested that higher cytokinin levels in discs from green leaves (in comparison to senescent leaves) may result from higher uptake of xylem cytokinins and/or in situ biosynthesis of cytokinins, rather than a more rapid degradation of zeatin-type cytokinins.