Promotion of flowering by photoperiod treatment in six strawberry (Fragaria X ananassa Duch.) cultivars with different everbearing patterns

The effects of different photoperiod treatments (12, 16, 20, or 24 h) and their duration (1, 2, 3, or 4 weeks) on the percentage of flowering plants and the number of inflorescences were investigated in the Summer in everbearing (EB) strawberry (Fragaria×ananassa Duch.) cultivars with various EB strengths in a field in northern Japan. For ‘Tochihitomi’, a strong EB cultivar, inflorescences emerged in all control plants grown under the natural photoperiod (≤ 14.5 h), and the number of inflorescences was not affected by any photoperiod treatment. In cultivars of intermediate EB strength (i.e., ‘Summer-drop’, ‘Summer-tiara’, ‘Miyazaki-natsuharuka’, and ‘Summer-fairy’), flowering was < 100% under a 12-h photoperiod. The number of inflorescences was higher under the 24-h photoperiod than in control plants, with the effect of the duration of the photoperiod dependent on cultivar. For ‘Natsuakari’, a weak EB, 0 - 20% of plants flowered under the 12-h photoperiod, and 0% under the natural photoperiod. However, flower buds were induced in all plants treated with the 16-h or 20-h photoperiods for 2 weeks, or with the 24-h photoperiod for 1 week. Furthermore, the 16-h (2 weeks) and 20-h and 24-h (1 week) photoperiod treatments induced significantly more inflorescences than in control plants. Although the characteristics of flowering of EB strawberry plants differed between cultivars, we suggest that flowering can be promoted by a 24-h photoperiod applied for 1-4 weeks in cultivars of weak or intermediate EB strength.