Underground system of geoxylic species of Homalolepis Turcz. (Simaroubaceae, Sapindales) from the Brazilian Cerrado

In the prevailing savanna formations of the Cerrado biome, several plants show special morphological and anatomical features, such as cork bark, a thickened underground system and bud-forming underground structure. These traits have been considered to have evolved in distinct lineages as adaptations to drought, fire, or nutrient-deficient soils. Among the 28 recognized species of Homalolepis Turcz. (Simaroubaceae), 20 species occur in drier habitats, as semiarid Caatinga, Cerrado and Restinga. Due to different habitats, a variety of either small geophytic shrubs or large trees can be found in this family. The geoxylic species are characterized by a woody underground perennial axis and a less persistent aerial system that can regrow. However, studies including its morphology and anatomy are lacking in the literature. In the present study, morphology and anatomy of the underground system of five geoxylic species of Homalolepis occurring in the Brazilian Cerrado were investigated. Our results show that all species have xylopodia (upper region) associated with tuberous roots (lower region), and possibly have a sympodial branching from the news shoots, originating from the axillary buds, which were covered by cataphylls. The gemmiferous potential of the xylopodia associated to its tissue parenchyma, with nutrient reserve (starch), and tuberous roots, with wide storage xylematic parenchyma tissue, are important features in the regeneration of aerial organs after drought and fire occurrence. This study then evaluates the possible relation between characteristics of the underground systems found within two taxonomic sections of Homalolepis, as well to the evolution of the geoxylic life-form in the Cerrado lineages of the genus.