Does leaf morphoanatomy corroborate systematics and biogeographic events in the Paleotropical genus Acridocarpus (Malpighiaceae)?

Abstract

The Paleotropical genus Acridocarpus stands out among the lineages recovered in the most recent phylogeny of the Malpighiaceae. Its 30 species occur disjunctively in Africa and only one species in New Caledonia, exhibiting a coincidence between the period of dispersion/diversification with the increase of arid areas in Africa. This study aims to investigate whether the anatomical leaf of the Acridocarpus can support the hypothesis of diversification of the genus in response to environmental, besides the contribution to systematic approaches. Leaves from 21 species were submitted to the usual methodologies for analysis under light microscope. Coded anatomical characters were also mapped in the phylogenetic tree of Acridorcarpus. Ericoid leaves, compact mesophyll, adaxial epidermis with elongated cells, epidermal cells with thick outer periclinal walls, were found in most species. These xeromorphic features would be correlated to the arid environments where the species occurs. The presence, distribution, and position of nectaries on the leaf, the contour of both petiole and midrib, the conformation of both petiole and midrib vascular system, the presence of sheath extending laterally between the palisade and spongy parenchyma, the presence and location of druse crystal in the petiole and mesophyll, the presence of stomatal ridges and distribution of stomata on the blade were recognized as useful characters for the taxonomy that contribute to the interspecific distinction. The lateral extension of the bundle sheath in the mesophyll was interpreted as a synapomorphy for the genus and amphistomatic leaves were considered an autapomorphy in A. spectibilis. These findings allowed the construction of an identification key and may contribute to new evolutionary interpretations in Malpighiaceae.