Dichogamy variation in Gentiana lutea L.

Dichogamy is the temporal separation of the presentation of male and female function within a plant. During last decades there has been increasing interest on the selective forces responsible for this trait and on its significance. Anyhow, the type of dichogamy (protandry vs. protogyny) is normally invariant within species. Here we report an unusual case of dichogamy variation in the species Gentiana lutea L.. We discuss the observed pattern on the basis of up-to-date hypotheses on dichogamy evolution. Floral anthesis was studied in three wild populations of G. lutea, belonging to subspecies lutea, subsp. symphyandra, and subsp. vardjanii. The morphology of floral structures, the degree of anthers dehiscence, pollen viability and stigma receptivity were monitored during flower lifespan, identifying different flower developmental stages for each subspecies. Gentiana lutea shows asynchronous dichogamy within the inflorescence. Considering intrafloral dichogamy, our results point out a striking variation among subspecies. G. lutea subsp. lutea looks functionally adichogamous, with a slight tendence to protogyny. Incomplete protogyny characterizes G. lutea subsp. vardjanii, while G. lutea subsp. symphyandra can be considered fully protandrous. Low levels of autocompatibility have been detected in all studied populations: this would contribute to reduce the success of interfloral self-pollination within the raceme of pseudo-whorls, where flowers of various ages are in close contact. Inflorescence architecture is somehow different among subspecies: more specifically, pseudo-whorls of flowers are more compact in G. lutea subsp. vardjanii, while they are less dense in subspecies lutea and symphyandra. The temporal distance between stamens and pistil maturation seems to be related to the degree of compactness of floral whorls: this interval is in fact longer in subsp. vardjanii, shorter in subsp. symphyandra, and almost null in G. lutea subsp. lutea. These findings, in accordance with recent theories, would suggest that avoidance of self-fertilization is not the unique or most important force in the evolution of dichogamy, which seems more likely advantageous in avoiding interference between male and female sexual functions.