Studies on flowering control of dahlia. III.

Abstract

The process of flower bud formation, and the effects of photoperiod on flower initiation and development in dahlia were investigated. Especially, the difference between the optimum photoperiod for flower initiation and that for flower development was examined. Also, the difference of the lower critical photoperiod between these two stages was examined.1. Depending on the results of observations, the seven stages in development of flower bud were set up, including the vegetative stage, (1) Vegetative stage.(2) Dome forming stage.(3) Early stage of involucre and bractlet formation.Less than nine scales are formed in this stage. The inflorescence of dahlia consists of more than one hundred florets and about eight involucral scales. The involucral scale is leaf-like in form, while there is a bractlet of thin membraneous form at the each of floret. It is, however, impossible to distinguish the bractlet from the involucral scale at the early stage of flower bud differentiation. Strictly speaking, as the flower head of dahlia has generally about eight involucral scales, this stage is involucre fromation stage′, and floret fromation follows.(4) Late stage of involucre and bractlet formation.(5) Early stage of floret formation.(6) Middle stage of floret formation (petal formation stage).(7) Late stage of floret formation (petal elongation stage).2. The flower primordia initiated under any photoperiods ranging from 8 to 16 hours, while they were retarded under longer photoperiods. The lateral shoots grown under 10 hours or shorter day length had already started to initiate flower buds on the fifth day after cutting back. These facts seem to indicate that dahlia is a non-obligate short-day plant in regard to flower initiation.3. The optimum day-length for flower initiation was 10 hours or less. As the flower bud proceeded to further stages, both the optimum and the critical photoperiod for flower development became longer, and at last they reached 13 and 12 hours respectively.4. When the plants were grown under the favourable photoperiod for 40 days after the decapitation, and then were transfered to the photoperiods shorter than 12 hours, many of their flower buds did not develop normally and remained blind. The flower bud of dahlia seems to become blind, when the photoperiod is changed, within 15 days after budding, to unfavorable day-length shorter than the lower critical one.