FLOWER INDUCTION AND SELECTION ON EARLINESS OF STRAWBERRY SEEDLINGS BY SHORT-DAY AND LOW NIGHT TEMPERATURE TREATMENT

A method for the selection of early flowering strawberry hybrids from a segregating population in the nursing was developed by exposing to a minimum number of cycles of 8 hr (SD) photoperiod and 12-degrees-C (low temperature) at night. The effect of sowing time, size of seedling, and size of the pot on flower induction was examined. In the process, the relationship between sensitivity to SD/low night temperature and earliness of seedling was established. 1. Seedlings induced flower by the SD/low night temperature treatment emerged flower bud 28 to 30 days after the treatment ended, whereas the uninduced seedlings required an average 86 days to emerge flower bud. The percentages of seedlings induced flower increased with an increasing number of SD/low night temperature cycles as follows: 21.6% at 10-cycles; 57.5% at 15-cycles; and over 90% at after 20 cycles. 2. Rate of seedlings induced flower in the July treatment was lower than that in the August treatment. This indicates that the natural daylength and temperature before the treatment or the natural temperature during the treatment affected on the sensitivity of seedlings to SD/low night temperature treatment. 3. The sensitivity of seedlings to SD/low night temperature increased proportionally with the increased number of true leaves and crown diameter. But seedlings with less than about 9 true leaves and smaller than 2.5 mm crown diameter were too small to almost be induced flower by the treatment. Our data indicate that sensible true leaf number and crown diameter were more than 11 leaves, including mono and di-foliate ones, and larger than 3 mm crown diameter. 4. The number of seedlings induced flower became higher as the size of pot was increased. 5. Among seedlings induced flower by the SD/low night temperature, those which emerged flower bud earlier than 'Toyonoka' consisted of 89.2% after a 20-day cycle when the experiment was begun on 20 July; 73.5% after a 10-day cycle and 92.4% after a 15-day cycle was initiated on 10 August. The sensitivity of seedlings to the SD/low night temperature was closely related to the earliness of flowering. 6. We conclude that exposure of strawberry seedlings to an 8 hr photoperiod/12-degrees-C night temperature cycles for 10 to 15 days in the nursery is an effective means to select for early flowering types. We found that the best results were obtained if the seeds are sown in pots larger than 9 cm diam. in late March and exposed to SD/low night temperature from about 10 August.