Scientia Horticulturae 115(3): 309-314 (Feb 2008)
Interaction of photoperiod and temperature in the control of growth and dormancy of Prunus species
Ola M. Heide

Abstract
Growth and dormancy induction of seedlings or micropropagated plants of three Prunus species were studied under controlled environment conditions. All the species tested, P. cerasus L. and P. insititia L. (two cultivars each), and P. avium L. were insensitive to photoperiod at high temperature and maintained continuous growth in both 10 and 24-h photoperiods at 21°C. At lower temperatures, however, growth was controlled by the interaction of photoperiod and temperature, the species and cultivars varying somewhat in their responses. At 9°C growth cessation was induced regardless of day-length conditions in the plum rootstocks ‘St. Julien A’ and ‘Weito’ as well as in the sour cherry rootstock ‘Weiroot’, whereas in the sour cherry rootstock ‘Gisela 5’ growth cessation took place in short day (SD) only. At intermediate temperatures (12 and 15°C) growth cessation occurred in SD only in both sour cherry cultivars. In P. avium seedlings on the other hand, growth cessation in SD was only induced at 9°C, continuous but reduced growth taking place also in SD at all higher temperatures. Growth rates increased progressively with increasing temperature under long day (LD) conditions in all species, and this was associated with increased internode length in LD compared with SD conditions. Production of new leaves was unaffected by photoperiod at high temperature, but was higher in LD than in SD at lower temperatures. After growth cessation at low temperature the plants developed winter buds and became dormant also in LD conditions. These results demonstrate that, like several species of the Pomoidae subfamily of the Rosaceae, these Prunus species are insensitive to short photoperiods at relatively high temperatures. However, the photoperiodic response of the Prunus species is highly temperature dependent, and the transition temperatures for shifts in the photoperiodic response mode vary among the species.


4. Discussion

The results of these experiments demonstrate a pronounced interaction of photoperiod and temperature in the regulation of growth and growth cessation in all the tested Prunus species. At high temperature (21°C) they responded with continuous growth regardless of photoperiodic conditions in much the same way as Malus and Pyrus (Nitsch, 1957; Heide and Prestrud, 2005). However, unlike these genera, most of the Prunus species were clearly sensitive to daylength at intermediate temperatures (12 and 15°C), and ceased growing after a few weeks in SD conditions (Figs. 1 and 3). At low temperature (9°C) on the other hand, the Prunus species varied in their response to photoperiod. While the wild sweet cherry and the sour cherry 'Gisela 5' required the combination of low temperature and SD for growth cessation and formation of winter buds, the other Prunus cultivars ceased growing and formed winter buds at 9°C in both SD and LD conditions in much the same way as Malus and Pyrus (Figs. 1 and 3).

Such photoperiod x temperature interactions are well known in the control of flowering (Thomas and Vince-Prue, 1997), and the molecular mechanisms controlling such interactions are now beginning to be understood (Halliday et al., 2003; Benedict et al., 2006). It has also recently been demonstrated that, in aspen trees, the same molecular mechanisms control SD induction of both flowering and growth cessation (Böhlenius et al., 2006). Although the present investigation was not intended to study such molecular transduction processes, the results provide circumstantial evidence for temperature "gating" of phytochrome controlled photoperiodic processes as suggested by Halliday et al. (2003).

Dormancy is a quantitative condition that is gradually attained and overcome. It has been demonstrated in both conifers (Heide, 1974) and deciduous trees (Junttila, 1976) that during early stages of SD-induced growth cessation, a shift back to LD is all that is needed for growth resumption. Even with as many as 32 SD cycles, growth resumption took place in subsequent LD in Salix (Junttila, 1976). Furthermore, buds at different positions on the annual shoot often exhibit different dormancy states. For these reasons a precise time of dormancy attainment cannot be determined. However, after growth cessation, the Prunus species formed terminal winter buds, and a relatively deep state of dormancy was demonstrated after 8 weeks of low temperature treatment in Exp. 1, in much the same way as previously demonstrated for Malus, which under the same conditions required about 1000h of chilling for growth resumption (Heide and Prestrud, 2005). There is thus little doubt that the described growth cessation in the Prunus species also led to bud dormancy.

The pronounced interaction of photoperiod and temperature that was demonstrated in the Prunus species, suggests that these species actually have a dual dormancy induction control system, securing timely growth cessation and dormancy induction in response to the progressive lowering of daylength and temperature in the autumn. Thus, under unusually mild autumn conditions the Prunus species will still be induced to growth cessation by the rapidly decreasing photoperiod around fall equinox. Such a dual control mechanism would make these species especially well adapted to avoiding the possible negative effects on winter bud preparations by the predicted and ongoing global warming (Serreze et al., 2000). An exception to this behaviour was seen in P. avium, which maintained an active (but reduced) growth in SD at intermediate temperatures and ceased growth in SD at low (9°C) temperature only.

A similar temperature x photoperiod interaction was reported for raspberry (Rubus idaeus), another member of the Rubus (Williams, 1959). Thus, raspberry plants grew continuously in both 9 and 14-h photoperiod at 21°C, at 10°C they ceased growing in both daylengths, whereas at 15°C growth cessation took place in SD only. Nestby (1986) demonstrated that even under natural continuous light conditions at high latitudes, raspberry plants ceased growing and became dormant at low summer temperatures of about 10°C. Growth cessation and dormancy induction by low temperature thus seems to be rather widespread within the Rubus. However, in Prunus species the photoperiodic response is highly temperature dependent, and the transition temperatures for shifts in the photoperiodic response mode vary among species and cultivars.