Endogenous cytokinin accumulation and cytokinin oxidase activity during shoot organogenesis of Petunia hybrida

Changes in endogenous cytokinin content and cytokinin oxidase activity were characterized in leaf explants from two Petunia hybrida Vilm. genetic line s which differed in their shoot organogenic response to exogenous N-6-benzy ladenine (BA), Endogenous cytokinin content in leaf explants of the highly shoot organogenic line, St40, increased 1.7-fold during the shoot induction phase (days 6-10) and had an additional 2.6-fold cytokinin increase correl ated with the shift from induction to the shoot development phase. The cyto kinins isopentenyl adenine (iP) and isopentenyl adenosine (iPAR) increased, while the cytokinins zeatin, zeatin riboside and dihydrozeatin remained at consistently low levels. In contrast, isoprenoid cytokinins did not accumu late in petunia TLV1 leaf explants which were incapable of shoot induction during 12 days of culture with BA, Cytokinin oxidase activity continuously increased in leaf explants of both petunia genotypes in response to BA, wit h a larger increase in St40, These results suggest that the differences in organogenic response in the two petunia genotypes may be the result of diff erences in BA uptake and metabolism which subsequently affects the accumula tion of isoprenoid cytokinins and the activity of cytokinin oxidase in the early stages of shoot development.