Production of extrafloral nectar in the Neotropical shrub Turnera subulata mediated by biotic and abiotic factors

Abstract

Interactions among plants secreting extrafloral nectar (EFN) and arthropods, mainly ants, have traditionally been considered as a classic example of mutualism. However, the net balance of these interactions can be highly variable according to biotic and abiotic factors. In the present study, we investigated whether the production of EFN in the Neotropical shrub Turnera subulata (Turneraceae) is induced by ecological context (i.e. biotic and abiotic factors). For this, we tested the hypotheses that the production of EFN in T. subulata plants is increased according to the biotic (phonological stage, herbivore and its simulation, presence of associated ants) and abiotic factors (water stress) in order to protect the most valuable and vulnerable parts. Manipulative experiments were carried out under greenhouse conditions. First, the production of EFN was analyzed in plants of T. subulata in vegetative and reproductive stages with and without water stress. Then, EFN production in response to herbivory and presence of associated ants were tested in plants in the reproductive with and without water stress. EFN volume was estimated with microcapillaries tubes. In general, plants under water stress produced less EFN than plants without stress. The production of EFN was increased in the reproductive stage and under herbivory. Plants under herbivory and with water stress produced a higher amount of EFN only when experience damages caused by the specialist herbivore. On the other hand, plants did not respond to the removal of EFN by ants or manually. Our results demonstrate that T. subulata can modulate the production of EFN according to the ecological context. The plastic response in the production of EFN by T. subulata seems to minimize the costs in defense.