Cytokinins in photoperiodic induction of flowering in Chenopodium species.
Changes in cytokinin (zeatin, zeatin riboside, isopentenyladenine [2iP] and isopentenyladenosine) levels were determined under light regimes which are inductive and non-inductive for flowering in leaves, stems, roots and apical parts of short-day C. rubrum and long-day C. murale. In leaves, stems and roots of both plant species, the level of cytokinins (zeatin and zeatin riboside in C. rubrum and all 4 cytokinins in C. murale) decreased by about 50% during the dark period and increased again during the subsequent light period. No significant changes in cytokinin levels were observed in continuous light. In apical parts of C. rubrum, the level of zeatin, zeatin riboside and isopentenyladenine was dramatically increased (by 400-500%) at the end of the dark period and decreased to approx. the original value during the following light period; no changes were observed in continuous light. In apical parts of C. murale, the level of cytokinins doubled during a floral induction period of 10 d of continuous light. A red light break (R) which prevented flowering in C. rubrum (15 min at the 6th hour of darkness) had no significant effect on cytokinin levels in leaves at the end of the dark period. Cytokinin levels increased 1 h after R and decreased again rapidly. On the other hand, the increase of cytokinin level in the apical parts of C. rubrum was largely prevented by the R break. These effects of R on cytokinin levels were not reverted by far-red light (FR), while the effect on flowering was reverted. It was concluded that there was no correlation between changes in cytokinin levels in leaves, stems and roots and photoperiodic flower induction since both species (which represent different photoperiodic types) showed similar changes under the same light regime. The increase of cytokinin levels in apical parts of both photoperiodic species during floral induction suggests a role for cytokinins in apex evocation (e.g increased cell division and branching).