Extrafloral nectaries and plant–insect interactions in Passiflora L. (Passifloraceae)

Abstract

Petiolar, marginal, and laminar leaf glands are commonly described in Passiflora as extrafloral nectaries (EFNs). However, secretory structures at this same position in some representatives of the genus have been misidentified as EFNs, since they release exudates other than nectar. In view of the inconsistencies in the classification of EFNs in Passiflora as well as in other botanical groups, we aimed to determine whether the foliar glands in Amazonian Passiflora acuminata DC., P. glandulosa Cav., P. nitida Kunth, P. tholozanii Sacco, and P. vespertilio L., are EFNs and to investigate the plant–insect interactions in these species by performing a survey of their visitors. Glands were evaluated by means of standard protocols in light and scanning electron microscopies. Additionally, glucostrips were applied over the profuse secretion to the presence of glucose. The glucostrip test showed positive results in all species, which enabled the classification of all glands as EFNs. Ants collecting nectar and patrolling the leaf surface belong to Ectatomma, Crematogaster, and Camponotus. Petiolar EFNs are present in P. acuminata, P. glandulosa, P. nitida, and P. tholozanii. Laminar EFNs usually described as ocelli are present only in P. vespertilio and P. glandulosa. Regardless of the morphology, the EFNs have a multiple or biseriate epidermis, a nectariferous parenchyma consisting of voluminous, thin-walled cells whose vacuoles contain dense contents, plus the occurrence of druse and a predominantly phloematic vascularization. The structure of the nectaries is consistent within Passiflora; other characteristics such as the presence of sugar in the secretion, the visitation by ants and butterflies, indicate the importance of the ecological interactions between EFNs and their visitors.