Time course of inducibility of indirect responses in an ant‐defended plant

Abstract

Plants have evolved inducible defenses that allow them to minimize costs associated with the production of constitutive defenses when herbivores are not present. However, as a consequence, some plants might experience a period of vulnerability between damage and the onset of defense and/or between the cessation of damage and relaxation of defense. Few studies have examined the time course in the inducible protective mutualism between ants and extrafloral nectary (EFN)‐bearing plants. None has compared the inducibility of EFNs on vegetative versus reproductive parts or in response to different levels of herbivore damage. Here, we disentangle the inducibility process by evaluating extrafloral nectar production and ant attendance over time, the time course of inducibility on different plant parts, and the time course of inducibility in response to different levels of foliar damage in a Brazilian tree, Qualea multiflora (Vochysiaceae). Using simulated herbivory on leaves and flowers, we found that (a) the production of extrafloral nectar from foliar and floral EFNs, as well as ant attendance, exhibited a lag between the moment of damage and the peak of response, followed by a response peak (usually 24 h after damage) at which the defense remains at its maximum level, then declines to prestimulus levels; (b) the time course of inducibility and the peak activity did not differ between EFNs located in vegetative versus reproductive parts, except for sugar concentration, which was higher in EFNs on vegetative parts; and (c) the time course of inducibility of foliar EFNs depended on damage level. Although considered a cost‐saving strategy, inducible defenses can be disadvantageous since they can leave plants vulnerable to attack for extended periods. Our results illuminate the dynamics of the induced response and the underlying mechanisms that might mediate it, ultimately providing new insights into defense strategies employed by plants.

Emerging Trends in Ant–Pollinator Conflict in Extrafloral Nectary-Bearing Plants

“…The presence of EFNs near to or associated with the reproductive parts could, to some degree, attract ants, which protect the plant from the attack of florivores, [ 59 , 60 ] and/or distract ants so that they do not reach the flower buds and flowers ( Figure 1 E–G), resulting in potential negative effects on the pollinator behavior and plant fitness [ 30 , 32 , 61 ]. The dynamics of extrafloral nectar production (constitutive or induced) and the quantity and quality of nectar produced are important factors for regulating ant attendance and aggressiveness [ 41 , 60 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 ]. Future studies should evaluate each EFN-bearing plant as an isolated system, mesuaring the herbivory rates and the impacts of ants on pollinator deterrence when manipulating the presence of ants, pollinators, florivores, and EFNs.…”

“…Thus, the abundance of ants close to the reproductive parts would be low, potentially decreasing their impacts on pollination processes. Therefore, bottom-up control from plants on ant foraging behavior [ 62 , 63 , 64 , 65 , 67 ] might determine the balance of the potential trade-off between protection against herbivores and the deterrence of pollinators.…”

“…The production of extrafloral nectar in the reproductive parts can be induced rather than constitutive, attracting ants only after attack or risk of attack [60,65,89] Narrow tubes of nectar Narrow tubes of nectar, as well as flowers with short, narrow corollas, prevent ants from reaching the nectar [58,85] Repellent nectar Floral nectar that is toxic or repellent to ants [58,79] Repellent pollen Toxic or repellent pollen to ants [56,90] Trichomes…”