Long-day control of flowering in everbearing strawberries By A. SØNSTEBY 1 * and O. M. HEIDE 2 1 Arable Crops Division, Section Kise, Norwegian Institute for Agricultural and Environmental Research, NO-2350 Nes Hedmark, Norway 2 Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P. O. Box 5003, NO-1432 Ås, Norway (e-mail: anita.sonsteby@bioforsk.no) (Accepted 14 August 2007) SUMMARY Photoperiod and temperature control of flowering in a number of perpetual-flowering or everbearing strawberry cultivars of widely varying pedigree has been studied in controlled environments. Flower bud initiation in the cultivars ‘Flamenco’, ‘Ridder’, ‘Rita’ and ‘Rondo’ was significantly advanced by long-day (LD) conditions at temperatures of 15°C and 21ºC; while, at 27ºC, flowering took place under LD conditions only. Some plants of the seed-propagated F 1 -hybrid ‘Elan’, raised at 21°C, also flowered under short-day (SD) conditions at 27°C, but reverted to the vegetative state after a few weeks when maintained under these conditions. When vegetative plants growing in SD at 27°C were transferred to LD conditions at the same temperature, they consistently initiated flower buds and started flowering after about 4 weeks. At such a high temperature, flowering could thus be turned on and off by switching between SD and LD conditions. This applied to all the cultivars studied. Also the cultivar ‘Everest’, which was tested only at 21°C, produced similar results. Night interruption for 2 h was effective in bringing about the LD response.At 9°C, flowering was substantially delayed, especially in ‘Flamenco’ and, at this temperature, flowering was unaffected by photoperiod. Runner formation was generally promoted by high temperature and SD conditions, but the photoperiodic effect varied between experiments. We conclude that everbearing strawberry cultivars, in general, whether of the older European-type or the modern Californian-type originating from crosses with selections of Fragaria virginiana ssp. glauca, are qualitative (obligatory) LD plants at high temperature (27°C), and quantitative LD plants at intermediate temperatures. Only at temperatures below 10°C are these cultivars day-neutral. T he physiology of flowering in cultivated strawberry (Fragaria  ananassa Duch.) has been the subject of extensive research for nearly a century. While the environmental control of flowering in single-cropping (June-bearing) strawberry cultivars is well established and documented (for reviews see Guttridge, 1985;Taylor, 2002), the situation is still rather confusing for cultivars that produce more than one crop annually. These cultivars are variably referred to as everbearers, perpetuals, rebloomers and remontants (Galletta and Bringhurst, 1990). Furthermore, modern American octoploid everbearers, mostly derived from crosses between wild Fragaria virginiana ssp. glauca Staudt selections and single-cropping Fragaria  ananassa, are frequently referred to as day-neutrals or day-neutral strawberries (Galletta et al., 1981, Galletta and Bringhurst, 1990). Thus, Durner et al. (1984) distinguished between everbearers [being older everbearers with a long-day (LD) flowering response], and day-neutrals (being modern everbearers with a day- neutral flowering response), and this terminology and classification has generally been adopted in the American literature (Galletta et al., 1981; Durner et al., 1984; Nicoll and Galletta, 1987; Durner and Poling, 1988; Galletta and Bringhurst, 1990; Sakin et al., 1997; Dale et al., 2002). Bringhurst et al. (1989) stated that they “prefer to use the term day-neutral rather than everbearer because it is more descriptive of what is really happening in strawberries”. Some workers, like Nicoll and Galletta (1987), even distinguished between what they termed “strong, intermediate and weak day- neutrals”. However, the rationale and validity of this classification may be questioned. Whereas the LD flowering response of older everbearing cultivars was clearly demonstrated by Darrow and Waldo (1934), and verified by Downs and Piringer (1955), the day-neutral flowering response of modern Californian everbearing cultivars has never been demonstrated convincingly. One complicating factor is the strong interaction of photoperiod with temperature, which has been demonstrated in floral initiation of all everbearing cultivars. Durner et al. (1984) examined the flowering response of “everbearing” (EB) and “day- neutral” (DN) cultivars over a range of temperatures (18°/14°C, 22°/18°C, 26°/22°C, or 30°/26ºC; day/night) and with two photoperiods (9 h SD, or 9 h with a 3 h night interruption = LD). Although they concluded that SD conditions promoted flowering at 18°/14°C and at 22°/18ºC in EBs, while DNs flowered independently of daylength conditions at these temperatures, the differences were small and inconsistent. However, at the higher temperatures, flowering was inhibited entirely by SD conditions in both groups. Likewise, Nishiyama and Kanahama (2000) demonstrated that floral initiation in the everbearing cultivar ‘Summerberry’ was inhibited by photoperiods of 13 h or less at 30°/25°C (day/night) temperatures, while it was promoted by photoperiods of 14 h or longer at the same temperatures. In a subsequent *Author for correspondence. Journal of Horticultural Science & Biotechnology (2007) 82 (6) 875–884