Secondary Pollen Presentation in Angiosperms and Its Biological Significance

Abstract

Secondary pollen presentation is the developmental relocation of pollen from the anthers onto another floral organ which then functions as the pollen presenting organ for pollination. Nine different types have been identified in sixteen angiosperm families according to which organ is used for presentation, whether the pollen is exposed or concealed within a structure and how pollen is loaded onto the presenting surface: (1) Enveloping bloom presenters (Araceae); (2) Perianth presenters with exposed pollen presentation (Epacridaceae); (3) Androecial presenters (Santalaceae); (4) Terminal stylar presenters with passive pollen placement and concealed stigmas (Rubiaceae and Proteaceae); (5) Terminal stylar presenters with passive pollen placement and sub-terminal stigmas (Marantaceae and Polygalaceae); (6) Terminal stylar presenters with active pollen placement (Asteraceae, Calyceraceae and Lobeliaceae); (7) Sub-terminal stylar presenters (Campanulaceae, Cannaceae, Fabaceae and Myrtaceae); (8) Exposed stigmatic presenters (Rubiaceae); (9) Indusial stigmatic presenters (Goodeniaceae and Brunoniaceae). Secondary pollen presentation occurs in three monocotyledon and thirteen dicotyledon families. The presentation types appear to have been independently derived indicating that secondary pollen presentation is a character with a selective advantage.

In all but the enveloping bloom type of secondary pollen presentation, developmental relocation of pollen requires simultaneous, introrse anther dehiscence and a close association of the presenting organ to the anthers prior to anthesis.

The various secondary pollen presentation systems may be modified to promote xenogamy or autogamy and this can even change during anthesis. Most plants which have secondary pollen presentation, display reduced herkogamy within the flower to facilitate pollination. Increased risk of self-pollination due to this may be overcome through dichogamy, herkogamy within inflorescences, dry stigmas, self-incompatibility systems and passive or active control over pollinator behaviour. Enhanced male function of the flowers of secondary pollen presenting plants is also evident through extension of the male phase by the protection, controlled release and precise placement and receipt of pollen. Plants displaying secondary pollen presentation are almost always protandrous.