Pericarp ontogenesis in wild species of Arachis L. (Leguminosae; Papilionoideae)

The genus Arachis encompasses 80 validly described species and the most striking characteristics is the geocarpic fruit, which occurs in every species of the genus. Taking into consideration that much of the current knowledge about this subject is based on the anatomy and morphology of A. hypogaea, a cultivated species with fruits that were modified during domestication, the objective of this study was to learn more about the development of the geocarpic fruits of wild species of Arachis (A. paraguariensis, A. pintoi, A. stenosperma, and A. tuberosa), and to provide characters that can be used in future phylogenetic works about this group. Buds, flowers and fruits at different stages of development were collected and processed according to standard methods used for light field, interferential contrast, and scanning electron microscopy. Although the geocarpic fruits of the wild species share many adaptations, this study identified potentially diagnostic infrageneric characters, such as the type of trichomes on the pericarp, the presence of sclerenchyma caps in the bundles of the pericarp of the seed chamber of the fruit and isthmus, the presence of tracheoid cells in the pith of the isthmus, the location of crystalliferous cells, the presence of tannins in the cells of mesocarp outer layers, and the presence of projections in the sclerenchyma tissue. In addition, this article brings some inferences about functional characteristics of this peculiar geocarpic fruit.