Leaf morphology and anatomy of Schlechtendalia luzulifolia, a basal member of subfamily Barnadesioideae (Asteraceae)

Based on molecular phylogenetic studies, Barnadesioideae have been proposed to be the basal subfamily of Asteraceae. This is a complex of 10 genera and 87 species distributed primarily along the Andean mountains, Patagonia, and into southern Brazil and Uruguay. Phylogenetic analyses have recovered all genera as monophyletic groups and have provided insights to their inter-relationships. Four generic clades have been substantiated: (1) Chuquiraga, Doniophyton, and Duseniella; (2) Dasyphyllum; (3) Barnadesia and Huarpea; and (4) Archidasyphyllum, Arnaldoa, and Fulcaldea. The remaining genus, the monospecific Schlechtendalia, has been an outlier in the subfamily, with some previous analyses recovering it as basal for the entire subfamily, and others showing it as sister to Barnadesia and Huarpea (with weak support) as well as to other genera. Recent massive sampling of loci has confirmed Schlechtendalia as the sister genus for the subfamily. Schlechtendalia luzulifolia has morphology atypical for Asteraceae. The capitula are loose aggregations of florets, and the leaves are long and strap shaped, more reminiscent of monocots. Morphological and anatomical investigations of the leaves reveal long, laminar blades with parallelodromous vascularization. The vesture is often with ‘barnadesioid trichomes’, especially towards the base of the plant, plus additional uniseriate trichomes consisting of 3 to many cells, newly reported for the subfamily. Some glandular trichomes with 2-4 short cells also occur. The transverse anatomy of the leaves reveals a single epidermal layer on both surfaces, which also contain the stomata (the leaf being amphistomatic). The mesophyll is undifferentiated; the vascular traces are surrounded by sclerenchyma that not only encircles the traces but also extends towards the epidermis and connects with it. The morphology and anatomy of the leaves of Schlechtendalia are divergent in comparison with other genera of the subfamily. Chuquiraga, Doniophyton, and Huarpea have leaf adaptations for survival in xeric habitats, such as dense pubescence, grey surfaces, and revolute margins. Schlechtendalia, in contrast, is adapted to a more mesic environment, especially near the Atlantic Ocean and along the Uruguay and La Plata rivers. The leaves are oriented upright, which correlates with undifferentiated mesophyll and stomata on both epidermal layers. The stem is an underground rhizome, an adaptation that permits survival during seasonal drought in the austral summer in Uruguay and adjacent regions. It is hypothesized that Schlechtendalia may have become adapted to more mesic environments in the Miocene prior to the rise of the Andes and development of the modern arid environments, into which many of the other genera of the subfamily subsequently radiated.