Geographical variation in threshold size for flowering in Cynoglossum officinale

We investigated variation in two traits that determine generation time, cold- and size-requirement for flowering, within and among European populations for the monocarpic perennial Cynoglossum officinale. When grown in an experimental garden in Leiden, no annual individuals were found among plants originating from 22 locations; all plants were biennial under nutrient-rich growing conditions. In a controlled-environment experiment, in which plants received an artificial cold treatment, flowering probability increased gradually with plant size for plants from two natural populations, signifying a large within-population variation in threshold size for flowering. The relationships between plant size and flowering of these two groups were significantly different: plants from Holkham (England) had much higher threshold sizes than plants from Meijendel (The Netherlands). Three plant groups originating from botanic gardens showed a sharp increase in flowering probability with size, indicating less variation in threshold size. Significant differences existed among all five groups. Results indicate the possibility that natural selection acts upon threshold size for flowering in Cynoglossum officinale.