The extrafloral nectary traits of woody plants in Brazil’s Caatinga: describing an ecological spectrum

Abstract

Plants use extrafloral nectaries (EFNs) as indirect defence mechanisms against herbivores. These structures contain nectar that is offered to ants in exchange for their protection. EFNs display pronounced functional variation, but it is unknown how this variation comes together in phenotypes. Here, we characterized the main functional traits of EFNs and investigated the associations among them. This work was conducted at a study site in the Caatinga that hosts 14 species of woody plants. We characterized the following functional traits: EFN type, position, arrangement, size, and reducing-sugar level. We observed a marked degree of trait variation (~30%) that was manifested in species-specific trait combinations, giving rise to an ‘ecological spectrum’. At one end were Fabaceae species with large, vascularized EFNs that occur individually on the leaf petiole and/or rachis and that produce high levels of reducing sugars. At the other end were Euphorbiaceae species with small, nonvascularized EFNs that are generally grouped on the leaf blade and that produce low levels of reducing sugars. Despite its limited geographical and phylogenetic scale, this study represents an important first step in describing an ecological spectrum that can inform our understanding of the ecological interactions and evolutionary history of this functionally relevant group of plants.