Micropropagation and medicinal properties of Barleria greenii

four main categories. These include: (i) their interconversion, (ii) hydroxylation, (iii)
conjugation, and (iv) degradation.
According to LETHAM and PALNI (1983), exogenous cytokinin bases are
converted by plant tissues into various types of metabolites. Such metabolites
include products of ring substitution (ribosides, nucleotides, N-glucosides), and
products of isoprenoid side chain cleavage (adenine, adenosine, adenosine-5'-
monophosphate) and substitution (O-glucosides) (LETHAM and PALNI, 1983).
The authors suggested that these metabolites could be active forms of cytokinin,
translocation forms, storage forms (releasing free cytokinin when needed), or
detoxification products (formed when exogenous levels become too high and
toxic), amongst others. According to STRNAD (1997), there is indirect evidence
suggesting that the free cytokinin bases are most likely the biologically active
forms of cytokinins. That being the case, the interconversion of cytokinin bases,
nucleosides and nucleotides is an important process in the regulation of cytokinin
activity (CHEN, 1997; BAJGUZ and PIOTROWSKA, 2009). The stereo- and/or
regiospecific hydroxylation of isoprenoid and aromatic cytokinins, respectively, is
another factor that could regulate cytokinin activity (KAMĺNEK et al., 1979;
KAMĺNEK et al., 1987a; STRNAD, 1997). KAMĺNEK et al. (1987a) observed that
the hydroxylation of the phenyl ring of N 6 -benzyladenosine in meta-position
increased cytokinin activity in tobacco callus and wheat leaf chlorophyll retention
bioassays. On the other hand, they noted that the hydroxylation of the phenyl ring
in ortho- and para-positions significantly decreased cytokinin activity, suggesting a
possible regulation of cytokinin biological activity by a position specific
hydroxylation of the side chain.
25