Floral initiation in Rudbeckia hirta: limited inductive photoperiod, polyamines and cytokinins

This study examined floral initiation in Rudbeckia hirta at the biochemical, cellular, and whole plant levels. Histological and histochemical examination of floral initiation revealed that the pattern of initiation followed closely that described in other species. The primary difference was in the length of time over which initiation and differentiation occurred. When subjected to limited inductive photoperiods, R. hirta responded with a delay in flowering if the plants were returned to short days (SD) before bract initiation. Increased exposure to long days (LD) increased stem height and enhanced floral development. A limited induction period of at least 8 LD allowed enough of the floral stimulus to be translocated to the meristem to cause no interruption in development even upon return to non-inductive conditions. An inhibition of development occurred only when plants were returned to SD before periclinal divisions in the pith rib meristem commenced after approximately 8 LD. Axillary bud development and final plant height were dependent on the number of inductive LD received.

Polyamines have been linked to floral initiation and, in this study, were strongly correlated to the stage of floral initiation. As initiation progressed, the observed increases in putrescine and spermidine were followed by a decrease after 16 LD, the observed onset of floral development. This was contrary to that previously observed in SD plants but followed a pattern similar to that reported for cytokinin behavior.

Exogenous cytokinins have been used to stimulate floral initiation in several species but Rudbeckia hirta did not respond to benzyladenine (BA) applied at the onset of LD. Floral initiation has been found to begin after six to eight LD and, in most species, BA was most effective when applied during initiation. In an attempt to increase uptake, BA was dissolved in dimethyl sulfoxide (DMSO). This did not enhance the effects of BA and, in fact, DMSO was found to be toxic at concentrations of 25% or more.