Role of Benzyladenine in Flower Bud Regeneration in vitro: Timing of Hormone Action

Abstract

Benzyladenine was used to induce flower buds on pedicel explants of tobacco ( Nicotiana tabacum L.). The first buds appeared after 7 days and development was completed in 14 days. At the optimal concentration of 1 μmol L -1 , exposure to the hormone for only 2 days was sufficient to induce the same number of buds as were formed upon continuous incubation at the same concentration. The role of benzyladenine in flower bud formation is thus restricted to the induction phase of development unless a physiologically active concentration is maintained within the tissue after removal of the cytokinin from the medium. During the 2-day initiation period the concentration of free benzyladenine in the tissue was 0.5 μmol kg -1 , which is 50% lower than the medium concentration. More than 90% of the total benzyladenine in the tissue was bound in various conjugates. When explants that had been incubated on 1 μmol L -1 3 [H]benzyladenine were shifted to a medium without radioactivity, the internal concentration of free radiolabeled benzyladenine dropped 50-fold within 10 h irrespective of the cytokinin concentration in the chase medium. This reduction was a combined effect of conjugation and leakage of benzyladenine from the tissue. The results indicate that in explants induced to form flower buds by a 2-day exposure to l μmol L -1 benzyladenine, the hormone concentration rapidly drops to approximately 0.01 μmol kg -1 upon transfer of the tissue to hormone-free medium. This concentration presumably is too low to be physiologically active.