Capitulum compartmentalization in Leucheria (Nassauvieae): Insights into the evolution of Asteraceae inflorescence

Abstract

An unusual feature occurs in the capitula of Leucheria (Nassauvieae, Asteraceae), some phyllaries of the involucre are twisted and/or rotated. The different spatial orientations of these phyllaries, called here paleaceous phyllaries, divide the capitulum into compartments enclosing florets and fruits. Because the internal morphology of the phyllaries could play a role in the compartmentalization, the anatomy of the phyllaries together with their different spatial arrangements was analyzed in the 49 species of Leucheria. We found differences between the abaxial and adaxial epidermis, bulliform-like cells in the epidermis, and a callus at the base of the phyllaries. Cohesion, swelling, and turgor mechanisms in tissues are proposed to play a role in the positional changes of the phyllaries. The phyllaries bend, twist, rotate, and spread down and, excepting the latter, all are involved in capitula compartmentalization. The paleaceous phyllaries are already present in buds and continue during the whole plant life. Our results support previous phylogenetic hypotheses that suggest that Leucheria achieved a regular capitulum but maintained the remnants of an ancestor's pseudocephalium.