Protandry promotes male pollination success in a moth-pollinated orchid

Summary

• 1
  Temporal separation of male and female phases in hermaphrodite flowers (dichogamy) is proposed to reduce self-pollination, both within and among flowers. Darwin and later workers suggested that protandry (the most common form of dichogamy, in which the male phase precedes the female phase) should be most effective in reducing geitonogamous (between-flower) self-pollination when pollinators forage upwards from older female-phase flowers to younger male-phase flowers on vertical inflorescences.
• 2
  We tested this hypothesis by manipulating the extent of protandry in artificial inflorescences of the orchid Satyrium longicauda Lindl. and using stained pollen to quantify self-pollination and pollen export.
• 3
  Upper flowers of non-protandrous inflorescences received more self-pollen through geitonogamy than lower flowers, unlike protandrous inflorescences. Protandry reduced absolute levels of self-pollination, as the amount of removed pollen involved in self-pollination was three times greater in non-protandrous than in protandrous inflorescences. This high level of self-pollination reduced the pollen available for export, as the ratio of pollen export to self-pollination declined with increasing self-pollination, indicating the occurrence of pollen discounting.
• 4
  This study represents the first direct measurement of the effects of protandry on the pollination process, and indicates that the evolution of protandry in plants could be driven strongly by the consequences of this trait for male mating success.