Do extrafloral nectaries present a defensive role against herbivores in two species of the family Bignoniaceae in a Neotropical savannas?

Abstract

Despite the general belief that the interaction between extrafloral nectaries (EFNs) and ants is mutualistic, the defensive function of EFNs has been poorly documented in South American savannas. In this article, we evaluate the potential impact of EFNs (benefits and costs) on two species of plants from the dry areas of Central Brazil, Anemopaegma album and Anemopaegma scabriusculum (Bignoniaceae). In particular, we characterize the composition of substances secreted by the EFNs, test whether EFNs attract ants, and whether ants actually present a defensive role, leading to reduced herbivory and increased plant fitness. Histochemical analyses indicated that EFNs from both species of Anemopaegma secrete an exudate that is composed of sugars, and potentially lipids and proteins. Furthermore, EFNs from both species were shown to present a significant role in ant attraction. However, contrary to common expectations, ants were not found to protect plants against herbivore attack. No effect was found between ant visitation and flower or fruit production in A. album, while the presence of ants led to a significant decrease in flower production in A. scabriusculum. These results suggest that EFNs might present a similar cost and benefit in A. album, and a higher cost than benefit in A. scabriusculum. Since the ancestor of Anemopaegma occupied humid forests and already presented EFNs that were maintained in subsequent lineages that occupied savannas, we suggest that phylogenetic inertia might explain the presence of EFNs in the species of Anemopaegma in which EFNs lack a defensive function.

Presumed domatia are actually extrafloral nectaries on leaves of Anacardium humile (Anacardiaceae)

“…Given this, the ants could be collecting nectar since extrafloral nectaries on the axil of leaf veins have been reported for other families, e.g., Fabaceae and Bignoniaceae (Díaz-Castelazo et al 2005;Nogueira et al 2012). Extrafloral nectaries are specialised tissues on vegetative structures that secrete nectar (Durkee 1982;Elias 1983), which is a source of nutrients for some organisms (Ruhren & Handel 1999;Melo et al 2010;Marazzi et al 2013) since it is rich in sugars, amino acids, vitamins, water and other organic compounds (Bentley 1977;Baker 1978;Durkee 1982;Nicolson & Thornburg 2007).…”

Evolution of extrafloral nectaries: adaptive process and selective regime changes from forest to savanna

“…In some ecological conditions such as the savanna environments, the occurrence of highly efficient herbivores (harmful to plants and indifferent to ants), the occupation of EFNs by nonprotective ants (e.g. Nogueira et al ., ) or the lack of ants (e.g. Moraes & Vasconcelos, ) may lead to a high cost for the production of EFNs, with the strength of selection against EFNs depending on the cost associated with these structures.…”

Evolutionary dynamics of interactions between plants and their enemies: comparison of herbivorous insects and pathogens

“…Several studies examined physical traits, such as leaf color or other morphological characters, but most of them focused on plant chemical defenses against herbivory. A few studies examined plant traits (e.g., extrafloral nectaries) that reduce herbivory by enhancing the local abundance of natural enemies …”