First record of a natural hexaploid population for Valeriana officinalis: genome size is confirmed to be a suitable indicator of ploidy level in the species

A recent study of genome size in Valeriana officinalis has established for a Pyrenean population a C-value divergent from those assessed in the currently known diploid, tetraploid and octoploid cytotypes, which proportionally indicates a hexaploid cytotype. If confirmed, this would constitute the first record of a natural hexaploid population for Valeriana officinalis. If not, these data would diminish the present strong support for accurate deduction of ploidy levels from genome sizes within the species. In order to discern between these two alternative interpretations, chromosome counts were carried out in the population of interest. Counting reveals 2n = 42, that is to say, the hexaploid cytotype. This result completes the polyploid series of V. officinalis and confirms the suitability of nuclear DNA amount for inferring ploidy level in the species. Such a finding raises the question of the origin and the extreme rarity of the hexaploid cytotype in V. officinalis. Furthermore, given that ploidy level impacts upon many plant features, including the chemical compound profile, the hexaploid population might constitute a new chemotype for this medicinal species.