Relationships Between Flower Bud Formation and Endogenous Growth Regulators in Japanese Pear Cultivars (Pyrus serotina Rehd.)

In order to clarify the mechanism of flower bud formation in Japanese pear (Pyrus serotina Rehd.), the relationsips between the flower initiation on an extention shoot and the nutrient elements and endogenous growth regulators in various portion of the shoot of cvs. ‘Shinsui’ and ‘Hosui’, were investigated. ‘Shinsui’ is characterized by having few flower buds on extention shoots, while ‘Hosui’ has many flower buds.1. The shoot of ‘Hosui’ ceased growing on June 30, while that of ‘Shinsui’ ceased on July 20, twenty days later than in ‘Hosui’.2. In axillary buds of ‘Hosui’ there was a rapid increase in the number of nodes and in flower bud differentiation and development immediately after shoot growth ceased. On the other hand, in axillary buds of ‘Shinsui’ the first visible signs of flower initiation were found on July 30, twenty days later than in ‘Hosui’. Thereafter differentiation and development did not occur. The final percentage of flower bud formation was 15.5% in ‘Shinsui’ and 79.0% in ‘Hosui’.3. There was no marked difference in total nitrogen content of axillary buds between the two cultivars, but the total sugar content, starch content and C/N ratio were higher in ‘Shinsui’ than in ‘Hosui’.4. IAA and gibberellin contents in the shoot tip of ‘Shinsui’ were higher than those of ‘Hosui’ during the period of vigorous shoot growth.5. Throughout the growing season both gibberellin and ABA contents in the axillary buds of ‘Hosui’ were lower than those of ‘Shinsui’, while the cytokinin content in the axillarybuds of ‘Hosui’ was higher than that of ‘Shinsui’.From these results it was concluded that flower initiation in Japanese pear is determined by a rapid increase in the number of nodes followed by formation of twelve scales and is closely related to endogenous growth regulators, in which gibberellin and cytokinin play important roles.