Flowering responses of North American strawberry cultivars

Flowering time is critical in strawberry production and regulated by temperature and photoperiod. Strawberry cultivars in North America are classified as short-day (June-bearing), ever-bearing and day-neutral. However, most cultivars are classified based on field performances and/or genetic background. Therefore, precise experiments are required to find optimum conditions to promote reproductive stage under controlled environment. In this research, eight cultivars of strawberry widely cultivated in North America were subjected to varied photoperiod treatments. Shortday (SD) cultivars including 'Radiance', 'F-127', 'Shuksan' and 'Chandler', and dayneutral/ ever-bearing (DN/EB) cultivars including 'Albion', 'Portola', 'Monterey' and 'San Andreas'. SD cultivars were pretreated with 16-h photoperiod during their vegetative/propagation stage and subjected to 11-, 12-, 13- and 14-h photoperiod for 8 weeks. DN/EB cultivars were grown in pretreatments of 8- or 17-h photoperiod during their vegetative/propagation stages and subjected to 8-, 11-, 14- and 17-h photoperiod for 10 weeks. Photoperiod was created by extension lighting at 2±0.5 μmol m^-2 s^-1 (400-800 nm) inside growth chambers after 7 h of natural light in greenhouse. Daily average temperature was 17.2±1.3 and 17.3±0.4°C for SD and DN/EB cultivars, respectively. Dissections were made to classify shoot apical meristems (SAM) into different developmental stages (Indices: 0-11) after 8 or 10 weeks. SD cultivars under 11-, 12- and 13-h showed reproductive stage with average index of development from 4.0 to 7.3, while plants under 14-h were all vegetative (stage index: 0), showing critical photoperiod between 13 and 14 h. DN/EB cultivars 'San Andreas', 'Albion and 'Monterey' presented facultative long-day response with positive correlation (p<0.001) between SAM developmental indices and photoperiod after 8 weeks. 'Portola' showed non-significant influences in flower primordial development, suggesting day neutral response. Further experiments must be conducted to confirm cultivar responses and identify possible interactions between photoperiod and temperature.