Accurate position exchange of stamen and stigma resolves the herkogamy dilemma in a protandrous plant, Ajuga decumbens (Labiatae)

Spatial separation of stamen and stigma (herkogamy) is an effective way for angiosperm species to reduce sexual interference and prevent self-pollination. It can however also reduce the possibility of pollinators contacting both sexual organs. For, example, pollen may be deposited on a site of the pollinator’s body that will not come into contact with the stigma. An ideal floral mechanism to resolve this herkogamy dilemma is for male and female functions to separate in time (dichogamy) and moreover, for stamen and stigma to sequentially occupy the same position for pollination at male and female phases. This mechanism of movement herkogamy could be achieved by successive stamen movement or by elongation of the style.

The floral display of Ajuga decumbens in natural population, and a carpenter bee was flying to the flower. Image credit: Ye et al.

In a recent study published in AoBP, Ye et al. document a floral mechanism of movement herkogamy in a protandrous herb, Ajuga decumbens, where the stamen and stigma of a flower exchanged their position by movement in opposite directions with floral development. The movement of the stamen and stigma in opposite direction resulted in position exchange at male and female floral phases. The interactions of dynamic herkogamy and dichogamy not only completely avoided sexual interferences, but also ideally maintained pollination accuracy. This novel floral mechanism contributes significantly to our understanding of the evolution of herkogamy.