Control of growth cessation and floral initiation in red raspberry (Rubus idaeus L.) cultivars of diverse origin in controlled and natural environments

The aim of the investigation was to assess and compare the environmental limits for growth cessation and floral initiation in a range of new and established biennial-fruiting red raspberry (Rubus idaeus L.) cultivars of diverse origin under phytotron and field conditions. The results confirmed that growth cessation and floral initiation in biennial-fruiting red raspberry are jointly controlled by the interaction of low temperature and short days (SD). When transferred from non-inductive high temperature and long day (LD) conditions to naturally decreasing autumn daylengths at varying phytotron temperatures on 18 August, growth immediately levelled off and ceased completely within 2 weeks in all cultivars at 9 °C. Serial dissections of lateral buds revealed that floral initiation simultaneously took place. At 15 °C on the other hand, the plants continued growing and remained vegetative until around 15 September when the daylength had decreased to approximately 13 h. The change to 9 °C resulted in an immediate but short-lasting floral induction response that did not bring about initiation in buds situated near the base of the canes, as was the case at 15 °C. At 18 °C, marginal floral induction took place only in the cultivars 'Glen Ample', 'Balder' and 'Vene', even at photoperiods down to 10 h, whereas at 21 °C, all cultivars grew vegetatively regardless of daylength conditions. However, exceptions were some plants of 'Vene' and 'Anitra' that initiated terminal flowers at 18 and 21 °C and flowered directly without chilling (so-called tip flowering). Although some cultivars of Northern origin ceased growing and initiated floral primordia somewhat earlier (at longer photoperiods) than those of more southerly origin, the differences were relatively minor and not consistent (no latitudinal cline). Results obtained in the field under decreasing autumn temperature and daylength conditions agreed closely with the results in the phytotron. We therefore conclude that results obtained with raspberry in properly controlled daylight phytotron experiments are generally applicable to field conditions.