Leaf anatomy of species from the Pombalia lanata complex (Violaceae) with implications for taxonomy and ecology

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As traditionally circumscribed, Hybanthus Jacq. was the third-largest Violaceae genus, and the circumscription of its 9-10 lineages has been very problematic, particularly Pombalia Vand.. This largest and most heterogeneous segregate genus includes a phylogenetically strongly supported subclade exclusive of the Brazilian Cerrado known as Pombalia lanata complex comprising six species. Though this complex is readily distinct due to its adaptations to the Cerrado environment, the often sympatric species exhibit a high degree of superposition in its features, hampering their precise identification. Here, we aim to verify the taxonomic value of leaf anatomy for the species of the Pombalia lanata complex, comparing them to its sister group Pombalia calceolaria. An additional goal was to identify characters related to the adaptation of these species to the constraining environmental conditions of the Cerrado. Expanded leaves (field-collected and herbarium materials) were processed using light and scanning electron microscopy and histochemistry. The species present diminutive and densely hairy leaves, with simple or branched (stellate) trichomes, colleters, uniseriate and mucilaginous epidermis and prismatic calcium oxalate crystals. Among these characters, the distribution and morphotype of tector trichomes, and presence of marginal colleters are potentially useful in distinguishing the species of the complex. The remaining anatomic characters are mostly effective in circumscribing the complex as a whole, and reinforce the proximity to its sister group P. calceolaria. Moreover, such characteristics reflect adaptative strategies to the unique Cerrado conditions of high temperatures and solar irradiance, severe seasonal water shortfall, low humidity, and nutrient-poor soils with high aluminum content. This is the first description of colleters in Violaceae, opening perspectives in studies with secretory structures in the group. Our work highlights the potential of anatomy in taxonomic studies, which can be applied to Pombalia s. str. and the remaining lineages of Hybanthus s.l.