Selection of Hydrangea (Hydrangea spp.) Cultivars with High Potential for Unseasonable Flower Bud Production in Japan

摘要

Hydrangea (Hydrangea spp.) flower buds usually differentiate from the end of summer through autumn, but some cultivars can also produce flower buds in spring. In the present study, we selected cultivars with high potential for such unseasonable flower bud production in Japan by evaluating the flower bud production on pinch-treatment-induced axillary shoots. We also examined the pinching time and the difference between the two shoot types: previously formed shoots that developed from buds formed in the previous autumn and basal shoots that formed in the current spring. We found that 'Christmas', 'Endless Summer', and 'Rosea' had the highest frequencies of unseasonable flower bud production on the axillary shoots of previously formed shoots (90.0%, 80.0%, and 90.0%, respectively) and basal shoots (90.0% for all three cultivars). When the buds were pinched in mid-April, the axillary shoots that formed on both shoot types flowered from early July to mid-August, which was approximately 1 month later than for seasonably produced flower buds in the same cultivars. In 2011, 98.0%, 90.5%, and 84.0% of the axillary shoots that developed on previously formed shoots flowered in 'Christmas', 'Endless Summer', and 'Rosea', respectively, while in 2012, 63.2%, 54.9%, and 75.3% flowered, respectively. Similar values were seen for the basal shoots, with the exception of 'Christmas' in 2011 and 'Endless Summer' in 2012, which both had lower flowering rates than previously formed shoots. In 'Rosea', flower bud differentiation started just after the pinch treatment, and the frequency of flower bud production decreased when the pinch treatment was conducted after late April. Thus, 'Christmas', 'Endless Summer', and 'Rosea' were considered to have high potential for unseasonable flowering following pinch treatment in early spring. However, unseasonable flower bud production may often be suppressed in pinched basal shoots.