Characterizing the Vernalization Response of Campanula `Birch Hybrid' to Temperatures Between -2.5 to 20 °C

Abstract

Campanula `Birch Hybrid' has an obligate vernalization requirement, though little is known about the vernalization response as a function of temperature and duration. The objective of this study was to characterize the qualitative and quantitative effects of exposure to -2.5 to 20 °C on C. `Birch Hybrid' flowering. Plugs were bulked at 20 °C for 4 weeks and then transferred to -2.5, 0, 2.5, 5, 7.5, 10, 12.5, 15, 17.5 or 20 °C for 0, 3, 5, 7, 9, or 12 weeks. Plugs were then potted and grown at 20 °C under 16-h photoperiod. Nine plants were used per treatment. Date of first open flower and the number of open flowers and flowering nodes 7 d later were recorded. No plants flowered after 0 or 3-week treatments. One plant held at 20 °C flowered and no plants flowered after exposure to 17.5 °C. After 5 weeks at 0 to 7.5 °C, 100% of plants flowered with the fastest flowering after 2.5 to 7.5 °C. The number of flowering nodes and open flowers were similar for plants held at -2.5 to 10 °C for 5 weeks. All plants flowered following 7 weeks at -2.5 to 12.5 °C, though flowering was quickest after exposure to 2.5 to 7.5 °C. After 7 weeks, plants held at -2.5 to 10 °C produced similar number of flowering nodes and open flowers. Following 9 weeks, all plants at -2.5 to 12.5 °C flowered and 2.5 to 7.5 °C treated plants flowered first. The number of flowering nodes was uniform across -2.5 to 12.5 °C and the highest number of flowers was produced at 12.5 °C. All plants held at -2.5 °C died after 12 weeks. After 12 weeks, all plants flowered following 0 to 15 °C. However, following 15 °C, plants produced fewer flowers and flowering nodes. Overall, the optimal vernalization response was between 0 to 7.5 °C.