Journal of Experimental Botany, 64(14): 4132-4133 (8 August, 2013)
The regulation of seasonal flowering in the Rosaceae
Takeshi Kurokura, Naozumi Mimida, Nicholas H. Battey and Timo Hytönen

Environmental regulation of flowering in Rosaceae

Regulation of flowering by photoperiod

Strawberry and raspberry can be placed into two groups according to their flowering habits and sensitivity to photoperiod (Carew et al., 2000; Hytönen and Elomaa 2011). June-bearing strawberry cultivars and biennial (‘floricane’) raspberries are facultative short-day (SD) plants, in which flowering is regulated by the interaction of photoperiod and temperature (see, for example, Heide, 1977; Carew et al., 2000; Heide and Sønsteby, 2007). In June-bearing strawberries, there is a large variation in the photoperiodic responses. Typically, the longest photoperiod in which flower induction occurs (i.e. the critical photoperiod) is relatively long, between 12h and 16h, and the number of SD cycles needed for flower induction varies between seven and 35 (Heide, 1977; Guttridge, 1985; Heide and Sønsteby, 2007). In biennial raspberry, a critical photoperiod of ~15h and acceleration of flower initiation by a shorter photoperiod have been reported (Sønsteby and Heide, 2008).

The terminology used in the literature on the photoperiodic responses of perpetual flowering strawberry cultivars is rather confusing. Some reports classify two types of perpetual flowering habit; one is ‘everbearing’ in which flower initiation takes place mainly under long days (LDs), and another is ‘day-neutral’ which flower at the same rate irrespective of photoperiod (reviewed by Stewart and Folta, 2010; Heide et al., 2013). It has been reported that LDs advance flower initiation in perpetual flowering Fragraria vesca and cultivated strawberry, especially under higher temperatures (Sønsteby and Heide, 2007; Mouhu et al., 2009). However, after flower induction, perpetual flowering F. vesca continuously produces new inflorescences irrespective of photoperiod (Koskela et al., 2012). Therefore, ‘everbearing’ and ‘day-neutral’ can be used to describe the same plant at different growth stages. Similar to perpetual flowering strawberries, a few studies on annual (‘primocane’) raspberries suggest that they are also LD plants (Carew et al., 2003; Sønsteby and Heide, 2009).

The photoperiodic regulation of flowering in roses and apples is not well understood, although they are generally considered as day-neutral plants (Zieslin and Moe, 1985; Wilkie et al., 2008). However, careful analysis in controlled conditions should be carried out, since a few studies suggest that photoperiod may affect flower induction in both species. Moe (1972) reported that LDs advanced flower initiation in some perpetual flowering rose cultivars. Also in apple cv. Jonathan, the increase of photoperiod from 8h to 12 or 14h increased flower bud formation, whereas a 16h photoperiod repressed flower induction compared with shorter photoperiods (Stahly and Piringer, 1962).

Regulation of flowering by temperature

As mentioned in the previous section, an interaction between photoperiod and temperature determines flower induction in both strawberries and raspberries. In fact, the temperature effect is dominant over photoperiod, and flower induction only occurs within a certain permissive temperature range. In seasonal flowering F. vesca as well as biennial raspberries, temperatures above ~18°C inhibit flower induction, SD conditions are required at intermediate temperatures, whereas at cool temperatures of ~10°C plants become day-neutral (Heide and Sønsteby, 2007; Sønsteby and Heide, 2008). Similar trends have been shown in seasonal flowering octoploid strawberries, although the effect of temperature seems to be quantitative and there is variation in the photoperiod-insensitive temperature range between cultivars (Heide, 1977; Bradford et al., 2010). It was also found that chilling temperatures <6°C repress flowering (Verheul et al., 2006). Additionally, long exposure to chilling contributes to seasonality of flowering, since it changes the physiological status of the plants so that further flower initiation is prevented even under an inductive photoperiod in the spring (Guttridge, 1985).

In perpetual flowering strawberries, the effect of photoperiod on flower induction also depends on temperature. They have an obligatory LD requirement for flower induction at high temperature of ~27°C. However, at lower temperatures, the effect of photoperiod becomes quantitative, and, at 9°C, plants are almost day-neutral (Bradford et al., 2010). Similar to perpetual flowering strawberries, high temperature has been shown to promote flowering in annual raspberries under LDs (Carew et al., 2003; Sønsteby and Heide, 2009).

Temperature may affect flower induction in apple. An increase in temperature from 13°C to 20°C for 6–7 weeks after bloom enhanced flower bud formation, whereas a higher temperature of 24°C was found to delay floral development (Tromp, 1980; Zhu et al., 1997). Temperature also controls dormancy in apple and pear: low temperatures induce dormancy, with no effect of photoperiod (SDs or LDs); lack of sufficient chilling during winter can delay release from dormancy and consequently flowering time (Heide and Prestrud, 2005). Prunus species show a pronounced photoperiod/ temperature interaction in the control of growth cessation and dormancy (Heide, 2008), and, in raspberry, a photoperiod–temperature interaction in the control of dormancy onset was reported by Williams (1959). In seasonal flowering R. x wichurana, low temperature (vernalization) during winter is probably needed before flowers can be initiated in the spring (Foucher et al., 2008).