Short Main Shoot Length and Inhibition of Floral Bud Development under Red Light Can Be Recovered by Application of Gibberellin and Cytokinin

Light quality is one of the important factors controlling the morphogenesis of plants. In some species, blue light promotes shoot elongation and flowering, whereas red light inhibits these responses. In this study, we investigated the effects of light quality and application of some phytohormones on growth and floral induction in petunia Petunia hybrida 'Baccarat blue'. Petunia plants with five true leaves were exposed to white fluorescent lamps, or red or blue LED light in growth cabinets. Plant height was reduced under red light compared to blue light treatment. In addition, floral bud formation was drastically inhibited under red light. Application of GA(3) enhanced main shoot elongation and floral bud formation under red light, but did not promote further floral bud development and flowering. However, BA application increased expression of the PhFT (FT like) and PhFBP20 (SOC1 like) genes, and promoted initiation of floral buds and flowering under red light without much elongation of the main shoot. GA(1) and GA(4) contents in petunia decreased under red light compared to blue light treatment. Thus, the dwarfed plant shape under red light is linked to low content of active GA. Indeed, the expression of a PhGA20-oxidase gene under red light was low, resulting in low levels of active GA. However, in the case of petunia, GA might not have much influence on floral induction and flower bud development. On the other hand, cytokinin contents and dose response to BA application were not so much influenced by the light qualities. Thus, cytokinin seems to have an ability to promote floral bud formation through the up regulation of SOC1-like floral induction genes under red light. However, the effect of BA on floral induction may be independent from the physiological regulation systems controlled by light quality.