Styles of Asteraceae: A synopsis of their morphological and functional diversity

Claudia Erbar

Centre for Organismal Studies (COS), Faculty of Biosciences, Heidelberg University, GERMANY

https://orcid.org/0000-0003-3562-748X

Peter Leins

Centre for Organismal Studies (COS), Faculty of Biosciences, Heidelberg University, GERMANY

DOI: http://dx.doi.org/10.53875/capitulum.03.2.01

ABSTRACT:  The style of an individual flower of the Asteraceae (or Compositae) is one of the most important floral organs in two respects. 1. It is important for the systematics of the family: its characters have significantly contributed to its past and present classification. 2. The styles in their different forms are essential components in the mechanisms of secondary pollen presentation. The latter ensures an optimization of pollination by pollen portioning, a widespread phenomenon in angiosperms. After a detailed study of style morphology (by SEM and sectioning) of more than 580 species of 346 genera covering all (presently accepted) 44 tribes of the Asteraceae, a total of 49 style types are established. Bringing together both morphology and function, the style types represent eight possibilities of secondary pollen presentation, which can be subsumed into four main functional categories: deposition, brushing, pump, and pump and brushing combined. From style characters and the position of the style tip within the anther tube shortly before anthesis, it is now in most cases easy to predict the mechanism of secondary pollen presentation. Pollen protection and pollen portioning are the two advantages achieved by secondary pollen presentation. Pollen portioning, the economical use of pollen grains, is one of the most important factors in the context of optimization of pollination. Particularly the pump mechanism seems to be an optimised mechanism in terms of pollen protection and refined pollen portioning (small pollen portions exuding from the five slits between the connective appendages (when looking from above, the "five-pointed star" is observable).

Keywords: Compositae, secondary pollen presentation.