Functional biogeography of the thermal thresholds for post-dispersal embryo growth in Conopodium majus

Plant regeneration by seeds is driven by a set of physiological traits, many of which show functional intraspecific variation along biogeographical gradients. In many species, germination phenology depends on a germination delay imposed by the need for post-dispersal embryo growth (a.k.a. morphological dormancy). Such growth occurs as a function of environmental temperatures and shows base, optimum and ceiling temperatures (i.e. cardinal temperatures or thermal thresholds). However, the biogeographical variation in such thresholds has not been tested.

We used a thermal time approach and field experiments to assess intraspecific variation at the continental scale in the embryo growth thermal thresholds of the geophyte Conopodium majus (Apiaceae) across its distribution from the Iberian Peninsula to Scandinavia.

Thermal thresholds for embryo growth varied across the latitudinal gradient, with estimated optimum temperatures between 2.5 and 5.2 °C, ceiling temperatures between 12 and 20.5 °C, and base temperatures between -6.6 and -2.7 °C. Germination in the field peaked in January and February. The limiting factor for embryo growth was the ceiling temperature, which was negatively correlated with latitude and the bioclimatic environment of each population. In contrast, the optimal and base temperature were independent of local climate.

These results indicate that thermal thresholds for embryo growth are functional ecophysiological traits that drive seed germination phenology and seed responses to the soil climatic environment. Therefore, post-dispersal embryo growth can be a key trait impacting climate change effects on phenology and species distributions.