|Authors: ||A. Sønsteby, O.M. Heide|
|Keywords: ||dormancy, Rubus idaeus, temperature, photoperiod, biennial-fruiting, primocane-fruiting, plant architecture, growth cessation, flowering, raspberry|
In red raspberry (Rubus idaeus L.) two groups of cultivars with contrasting life cycles are commonly recognised.
While low temperature and/or short days are required for flowering in biennial-fruiting cultivars, the annual-fruiting cultivars flower freely at temperatures as high as 30°C. A notable exception is ‘Autumn Treasure’ in which flowering was delayed and reduced by temperatures above 20°C. Also, at least in some annual-fruiting cultivars, flowering is advanced by long-day conditions.
In biennial-fruiting cultivars, floral initiation is accompanied by growth cessation and bud dormancy; and, since dormancy release requires exposure to low temperatures or winter conditions, this confers a 2-year life cycle upon the shoots.
Shoots originating from root adventitious buds in biennial cultivars are juvenile and must develop 15-20 leaves before they can be induced to flower.
In contrast, the juvenile phase is absent in annual-fruiting cultivars which respond to a vernalization treatment already at the 5-leaf stage.
In these cultivars, floral initiation starts at the tip and is then progressing basipetally, but only some buds on the cane will sprout and flower in the current season, while the remaining buds become dormant.
In most cultivars, the number and proportion of annual-flowering buds increase with increasing temperature, being much larger at temperatures above, than below 20°C. We conclude that the distinctive physiological characteristics of annual-fruiting raspberries are the ability to initiate flower buds under the high temperature and long-day conditions of summer and the absence of dormancy in a certain proportion of the initiated buds, leading to flowering in the first year.
Practical applications of these findings are discussed.
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