Localized chilling of crowns promotes floral bud differentiation in strawberry transplants in a closed transplant production system

A stable supply of transplants with floral buds is required to improve the initial yield of the June-bearing cultivars of strawberry (<i>Fragaria</i> × <i>ananassa</i> Duch.). A closed transplant production system (CTPS) enables year-round production to meet the demands for the year-round production of strawberries in plant factories. In this study, we evaluated the performance of a novel method involving the localized chilling of strawberry crowns using silicone tubes containing circulated chilled water at different temperatures (10, 15, or 20°C) at the nighttime and different chilling regimes (daytime, nighttime, or entire day) under high air temperature conditions in a CTPS in terms of floral bud differentiation. We observed that 4 weeks of localized chilling at 10 or 15^oC during the nighttime under the air temperature of 25/20°C (photo-/dark periods) and a photoperiod of 10 h promoted floral bud differentiation, whereas 6 weeks of localized chilling under the same conditions inhibited differentiation. Moreover, 4 weeks of localized chilling at 5^oC during the daytime or entire day under the elevated air temperatures of 28/21°C and an extended photoperiod of 14 h promoted floral bud differentiation, and 6 weeks of localized chilling during the entire day under the same conditions further promoted bud differentiation compared with that in the control. Plant growth was generally unaffected by the localized chilling of the crowns. The results indicate that to cope with the impacts of elevated air temperature and photoperiod conditions, the continuous localized chilling of crowns at 5^oC during the entire day for 6 weeks must be used to achieve optimal bud differentiation. These findings suggest the effectiveness of the localized chilling of the crowns for floral bud differentiation in strawberry in CTPSs, without disrupting the high-air temperature and long-day conditions required for vegetative growth.