Endogenous Indole-3-acetic Acid and Trans-zeatin Ribosides in Relation to Axillary Bud Formation in Standard Chrysanthemum

The lateral buds of non-branching chrysanthemum cultivars are affected by temperature and exogenous plant growth regulator (PGR) treatment. The number of axillary buds, endogenous indole-3-acetic acid (IAA) and t-zeatin riboside (t-ZR) concentrations by planting date and PGR treatments were investigated in three genotypes. Two non-branching genotypes, 'Iwanohakusen' and '01B1-8' and branching type 'Jinba' were planted on May 2 and June 29. 'Jinba' always bore axillary buds and '01B1-8' showed stronger non-branching traits than 'Iwanohakusen'. Only 22.9% viable buds developed in the axils of '01B1-8' whereas 68.7% developed in 'Iwanohakusen' on the May 2 planting date group. When planting was delayed from May 2 to June 29, the number of axillary buds decreased in both non-branching genotypes. Endogenous IAA concentrations remained unchanged and t-ZR concentrations decreased in all the three genotypes when the planting date was delayed. This reduced t-ZR level corresponds to the increased ratio of IAA/t-ZR. The number of viable axillary buds increased from 21.7% to 50.1% upon ethephon treatment and to 30.3% by synthetic cytokinin 6-benzylaminopurine (BAP) treatment in '01B1-8'. Viable axillary buds of '01B1-22' increased from 17.1% to 25.1% and 29.1% respectively. Ethephon and BAP application decreased endogenous IAA contents and increased t-ZR contents. Elevated concentrations of t-ZR rather than IAA probably account for axillary bud formation in non-branching chrysanthemum. Just as the non-branching genotypes, the branching type cultivar 'Jinba' showed similar changes in IAA and t-ZR contents according to planting date and exogenous PGR treatments. These results showed that genotypic difference of branching patterns is not a result of concentration-dependent reaction.