J Exp Bot. 62(15): 5397–5404. (Nov 2011)
Temperature rather than photoperiod controls growth cessation and dormancy in Sorbus species
Ola M. Heide

Environmental regulation of growth and dormancy of four Sorbus genotypes was studied in controlled environments. Emphasis was placed on assessment of the presence and nature of the deficient photoperiodic dormancy regulation system that has previously been reported for some woody Rosaceae species. Two genotypes of Sorbus aucuparia L. maintained indeterminate growth for 8 weeks and 9 weeks at temperatures of 15°C and 21°C in both 20 h and 10 h photoperiods, while at 9°C, in the same photoperiodic conditions, they immediately ceased growing. At the higher temperatures, initiation of new leaves (nodes) was unaffected by photoperiod, while internode elongation was significantly enhanced by long days (LD). However, even after prolonged exposure to 9°C, most plants resumed growth when moved to high temperature and LD, indicating a shallow state of dormancy. Seedlings of Sorbus intermedia (J. F. Ehrh.) Pers. and micro-propagated plantlets of S. commixta Hedl. ‘Dodong’ were also unaffected by photoperiod during primary growth, but failed to elongate and gradually became dormant regardless of temperature and day-length conditions. However, after chilling and breaking of dormancy, the plants elongated vigorously but changed to a determinate mode of growth. Furthermore, a temperature of 9°C was found to be fully effective for breaking dormancy in S. intermedia plants. It is concluded that deficient photoperiodic dormancy control seems widespread in the Rosaceae and that, in such plants, both dormancy induction and release is brought about by low temperature. The potential impacts of climate change on such trees are discussed.

Dormancy is not an all or none situation, but a quantitative condition that is gradually established and lost (Thomas and Vince-Prue, 1997; Junttila, 2007). The results with S. aucuparia shown in Fig. 5, demonstrate that the plants did not enter a deep state of dormancy, even after extended exposure to low temperature. However, realizing that a temperature of 9°C was also fully effective in breaking dormancy in Sorbus plants (Fig. 6; Table 4), this is actually what would be expected. Paradoxically, low growth temperature seems to have dual and opposing effects, not only inducing dormancy, but also continuously negating the dormancy-inducing effect. That the depth of dormancy is affected by the temperature conditions during dormancy induction, is not specific for photoperiod-insensitive species such as Malus and Sorbus, but is reported for a range of boreal trees with the normal photoperiod-induced dormancy, such as Betula, Alnus, Acer, Picea, and others (Heide, 2003, and references therein). In all these trees, the depth and duration of dormancy increased with increasing temperature during SD dormancy induction. Parallel responses were even found in both cultivated and wild strawberries (Sønsteby and Heide, 2006, 2011), in which relatively high temperatures are required for SD induction of dormancy. In all these cases it seems that the dormancy-inducing effect of SD is continuously negated by low temperature, thereby establishing a relatively shallow state of dormancy (Sønsteby and Heide, 2011).