Effects of photoperiod and temperature on growth and flowering in the annual (primocane) fruiting raspberry (Rubus idaeus L.) cultivar 'Polka'

Growth and flowering of the annual-fruiting raspberry (Rubus idaeus L.) cultivar 'Polka' were studied under controlled environment conditions in order to facilitate out-of-season production. Vegetatively-propagated plants originating from adventitious root buds were used. Height growth and the rate of leaf formation increased with increasing temperature, up to a broad optimum in the mid-20 degrees C range. While elongation was consistently enhanced by long-day (LD) conditions, photoperiod had no effect on the rate of leaf formation. LD stimulation of growth thus resulted from increased internode length only. In agreement with earlier reports, it was found that, in contrast to biennial-fruiting cultivars, such annual-fruiting cultivars do not need low temperatures for flower initiation, nor do they appear to have a juvenile phase during which they are un-responsive to flower-inducing conditions. 'Polka' plants responded to inductive conditions as early as the 5-leaf stage, and flowered freely across the entire range of growth temperatures, even at 30 degrees C. Flowering was advanced and the number of flowers increased with increasing temperature, up to an optimum at 27 degrees C. Flowering was also consistently advanced and occurred at lower nodes under LD than under short-day (SD) conditions across the whole range of temperatures. Night interruption for 3 h in the middle of the night was also effective, demonstrating that this is a true photoperiodic response and not merely an effect of increased light integral in LD. It was also confirmed that a distinct vernalisation-type advancement of flowering took place when small, non-dormant plants were exposed to additional chilling at 6 degrees C for several weeks. At low temperatures, a large proportion of the lateral buds were dormant, so that, at 12 degrees C, the plants actually flowered only at their tips. Dissections also revealed that the dormant buds had initiated flowers; but, because of their dormant state, they needed several weeks of chilling before they could flower (biennial-fruiting behaviour). Both types of buds were initiated by the same environmental conditions. Practical applications of the findings are suggested.