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The cytokinin, N6-(A2-isopentenyl)adenine, is found to be at least 3.3 times as active as N6-(A2-isopentenyl)adenosine in promoting the growth of cytokinin-requiring tobacco (Nicotiana tabacum) callus. Absorption rates of N6-(A2-isopentenyl)adenine and N6-(A2-isopentenyl)adenosine by tobacco cells in liquid suspension do not differ significantly. In these cells, N6-(A2-isopentenyl)adenosine-5'-monophosphate, di-, and triphosphate are synthesized in both cases, but 7-glucosylation occurs significantly only with N6-(A2-isopentenyl)adenine, protecting thereby its N6-isopentenyl side chain from cleavage. Degradation by N6-side chain removal appears to be intense, leading to the formation of adenine, adenosine, and adenylic nucleotides. Thus, it is suggested that N6-(A2-isopentenyl)adenine-7-glucoside is a protected or storage form of the cytokinin which could account for the higher biological activity of N6-(A2-isopentenyl)adenine than of N6-(A2-_sopentenyl)adenosine.In the last years, metabolism of exogenously supplied cytokinins has been studied in various plant tissues and plant cells. Since the reports of the formation of the unusual 7-glucosylbenzyladenine in various plant tissues (1) and 7-glucosylzeatin in radish cotyledons (19) as major and highly active (4, 18) cytokinin metabolites of the supplied cytokinin, attention has been focussed on cytokinin glucosides (7,10,13,20,26).However, cytokinin glucosides are not the only cytokinin metabolites synthesized from exogenously supplied cytokinin that keep the intact cytokinin base moiety. In most of the studies cited above and in other studies (2, 24), cytokinin riboside and cytokinin riboside-5'-monophosphate were also found to be synthesized. In a previous communication (12), we established the formation of cytokinin riboside-5'-di and triphosphates in tobacco and Acer cells.So far, the only correlation that could be established between cytokinin metabolism and cytokinin activity were based on the cytokinin-requiring growth of plant tissues grown as callus and cytokinin metabolites that were present in this callus. Fox et al.(5) found that 7-glucosylbenzyladenine was the only cytokinin molecule that persisted during the active growth of cytokininrequiring soybean callus and tobacco KX4 callus, suggesting that 7-glucosylbenzyladenine could be the "physiologically active form of the cytokinin" (4). Horgan (7) showed that 7-glucosylzeatin was not formed in cytokinin-requiring soybean callus, 6(4-0-,8-D-glucosyl-3-methyltrans-2-butenylamino) purine being the major metabolite present 24 hr after the zeatin had been sup-

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Uptake and degradation of the cytokinin, N-6A2-isopentenyl) adenosine, were studied in tobacco cells grown as cell suspensions. Degradation occurs by cleavage of the isopentenyl chain which gives adenyllc products.Rate of N'(A2-isopentenyl)l8_l4Cladenosine degradation increases severalfold after a 3-to 4-hour delay when ceUs have been exposed to a cytokini.Consequently, only rates of N-_A2-isopentenyl)adenosine degradation measured during the first 3 hours of incubation with 18-4Cl-N-6(A2-isopentenyl)adenosine are representative of the intrinsic in vivo cytokinin degradatve lctivity of tobacco cells. Within these limits, it appears that cytokinin degradative activity is high in cytokinin-autonomous tobacco cells, as indicated by the half life of the supplied N6(A2 isopntenyl adenosine (about 3 hours) when it is supplied at the physiological concentration of 0.2 micromolar. This cytokiin degradative activity appears to be under the control of cytokinins themselves because N6-(A2-isopentenyl)adenosine degradative activity is increased several-fold folowing a 3-to 4-hour delay after these cells have been exposed to a cytokinin.Studies of cytokinin metabolism in plant cells, tissues and organs have established that adenine and/or adenosine and/or adenylic nucleotides are catabolic products of zeatin (12), BA (3), i6Ade' or i6Ado (8, 10, 11). These observations indicate that cytokinin degradation occurs by side-chain cleavage in a manner specific for plants because in animal systems i6Ado is shown to be converted to inosine (5, 6).Presumably, irreversible inactivation of cytokinins by degradation plays an important role in the expression of their biologic activities in plants, but to our knowledge, no quantitative study of cytokinin degradation in plants has been reported. Paces et al. (11) detected an enzymic activity in crude extracts of tobacco tissues which catalyzed conversion of i6Ado to adenosine. Whitty and Hall (14) partially purified such an enzymic activity from corn kernels which they termed cytokinin oxidase. This enzymic activity removed the A2 -isopentenyl chain as the aldehyde from either N6-(A2-isopentenyl)-substituted adenine or adenosine (1). It is not known, however, if degradation of cytokinin in vivo occurs specifically at the level of the base, riboside or ribotide(s). Occurrence of cytokinin activity in cytokinin-autonomous plant tissues grown in vitro has been demonstrated (2, 4, 13). Cytokinin-autonomous 'Abbreviations: i6Ade: N6-(W'-isopentenyl)adenine; i6Ado: N6-(A'-isopentenyl)adenosine; i6AMP, i6ADP, i6ATP: N6-(A2-isopentenyl)adenosine-5'-mono-, di-, and triphosphate; Ado: adenosine; Ade: adenine. cells apparently produce enough cytokinins to meet their own requirements for cell division, either by increased cytokinin biosynthesis or by reduced cytokinin degradation, as compared to cytokinin-requiring cells. In a previous paper (10), we reported that i6Ado is rapidly degraded in cytokinin-requiring tobacco cells. To evaluate the second hypothesis, we further studied the degrada...