Abstract: Flowering onset in species with wide geographic distribution often correlates with latitude. The variation of flowering with latitude has been interpreted in the context of local adaptation. With the current global climate change and the observation that flowering onset has advanced substantially in the last 40 years, there has been renovated interest in discovering the ecological and genetic mechanisms that control flowering onset. This is important since it remains to discover if advances in flowering onset represent an adaptive response to climate change. We studied flowering onset in 30 populations of pale blue flax Linum bienne (the wild crop relative of cultivated flax) from SW Spain to N England, with the goal to (i) describe flowering onset under control conditions, (ii) determine if the local climate predicts flowering onset better than latitude and (iii) quantify the importance of vernalization to advance flowering onset. Flowering was positively correlated with latitude, but the local climate better predicted days to flowering onset. Vernalisation reduced the number of days to flowering in northern populations, and southern populations showed a milder response. The results indicate that L. bienne has evolved two strategies along its geographic range. Early flowering helps avoid summer drought within the Mediterranean range, favoring a fast and annual strategy. Towards the northern range, the late flowering and vernalization requirements suggest that plants follow an annual or biannual strategy. Taken together, the results suggest that population differentiation in flowering onset evolved in response to local climatic conditions, but populations display substantial variation which might allow providing an adaptive response to climate change.
