How does micromorphology reflect taxonomy within the Xeranthemum group (Cardueae-Asteraceae)?

Abstract

Comparative micromorphological analyses were conducted on five members of the Xeranthemum group, both perennial (Amphoricarpos exsul and Shangwua masarica) and annual (Chardinia orientalis, Siebera pungens and Xeranthemum inapertum), using scanning electron microscopy (SEM) and SEM coupled with an energy-dispersive spectrometer (SEM-EDS) in order to evaluate whether micromorphological characters correlate with current phylogenies. “Puzzle-like” epidermal cells at the adaxial leaf surface with smooth outer periclinal walls and slightly sinuate anticlinal walls, and lack of weddellite crystals on the involucral bracts are shared by A. exsul and Sh. masarica and Sh. denticulata, while sinuate anticlinal adaxial leaf epidermal cell walls, weddellite crystals on the involucral bracts and an aristate-paleaceous pappus link together Ch. orientalis, S. pungens and X. inapertum. The following traits are probably species-specific: a glabrous adaxial leaf epidermis and cypsela surface, and a plumose-setose pappus in Sh. masarica; a barbate-aristate pappus in A. exsul; irregular shape of cells of the adaxial leaf epidermis with cuticular corrugations, and a papillose cypsela surface in Ch. orientalis; channeled anticlinal borders of adaxial leaf epidermal cells, and wax deposition on the surface of involucral bracts in X. inapertum; and strongly ribbed thickenings of outer periclinal adaxial leaf epidermal cell walls and involucral bracts with a whitish patch of appressed trichomes on their central surface in S. pungens. All of the examined species exhibit vermiform trichomes on the stems and on the abaxial leaf surfaces, adaxial detachment area of cypselae, an asymmetrical carpopodium and direct insertion of the pappus to the pericarp. Our results revealed valuable qualitative characters that facilitate species identification and contribute to a better understanding of relationships between genera. Optimization of these traits onto phylogenetic analyses suggests that some characters found on leaves and involucral bracts are likely due to adaptations of perennial ancestors to xeric habitats.