Search

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., (© 2023 The Ecological Society of America.)

1. Ants are recognized as induced biotic defence in mutualistic relationships with myrmecophytic plants. Pseudomyrmex concolor is an aggressive Amazonian ant that works as an anti‐herbivory agent for its host plant Tachigali myrmecophila. However, it is not known how this ant maintains the plant on low levels of herbivory. 2. Tachigali myrmecophila provides no food for P. concolor making this model appropriate to test the hypothesis that ants associated with myrmecophytes act as induced biotic defence by herbivory. Based on Optimal Defence Theory, we tested whether the response to herbivory and the ant daily activity rhythm is more frequent in most valuable parts of the plant, such as young leaves. 3. Ants were observed in young and mature leaves exposed to herbivory by experimental manipulation. Results suggest that ants act as induced biotic defence in T. myrmecophila: number of ants was significantly higher in manipulated leaflets, as well as the daily activity rhythm was increased in young leaves. The time to detect herbivory in young and mature leaves did not differ. However, leaf extracts signs were identified more quickly by the ants. 4. Induced ant responses in the ant–myrmecophytic plants system can be considered constitutive defence because ants are present throughout the development of their host plant from seedling to adulthood, responding aggressively to any intruder. In the Pseudomyrmex–Tachigali system, the presence of ants on the plants is, per se, a constitutive defence. However, the patrolling activity is an induced response, being the result of several plant‐related cues (chemical and physical) associated with herbivory activity. Pseudomyrmex concolor is an aggressive Amazonian ant that works as an anti‐herbivory agent for its host plant Tachigali myrmecophila. However, it is not known how this ant maintains the plant at low levels of herbivory.Based on Optimal Defence Theory, we tested whether the response to herbivory and the ant daily activity rhythm is more frequent in most valuable parts of the plant, such as young leaves.In the Pseudomyrmex–Tachigali system, the presence of ants on the plants is, per se, a constitutive defence, yet the patrolling activity is an induced response, being the result of several plant‐related cues (chemical and physical) associated with herbivore activity. [ABSTRACT FROM AUTHOR]

1. Induced chemical responses to herbivory have been widely documented, however less attention has been given to the induced morphological defences, particularly those that mediate protection mutualisms. One of the most widely studied mutualisms is the ant–plant protection mutualism, in which ants and other predatory arthropods defend plants against herbivores in exchange for food or housing. 2. Chemical and morphological traits that mediate this protection mutualism attract beneficial partners to plants. Thus induction of traits can lead to greater protection by mutualists when herbivores are present. In this study we examined whether extrafloral nectaries (EFNs) are induced in response to simulated herbivory in Prunus avium. 3. In a greenhouse experiment, continuous damage to seedlings was simulated in two treatments intended to mimic different types of herbivores: (i) 50% of the area of each leaf was removed using a paper hole punch (e.g. insect herbivore simulation), and (ii) 50% of the area of each leaf was removed using scissors (e.g. browsing mammal simulation). Seedlings in the control group were not damaged. 4. Post-treatment, damaged plants produced significantly more EFNs per leaf on pre-existing leaves, as well as those that emerged following the onset of damage, than plants in the control group. Regardless of treatment, leaves emerging earlier supported fewer EFNs than leaves emerging later in the experiment. 5. Allocation to defence by seedlings, which have limited resources available, suggests the protection mutualism could be important to plant fitness in the field. Further study of patterns of EFN induction in P. avium seedlings is needed to better understand the significance of this induced response. [ABSTRACT FROM AUTHOR]

Many plants offer rewards to attract ants that provide indirect defense against herbivores. These rewards include nectar and swollen thorn domatia, and their production can be inducible. Theory predicts that costly rewards should be . relaxed if the risk of herbivory is reduced, although this has not been previously demonstrated. Acacia drepanolobium trees in East Africa produce two ant rewards, extrafloral nectar and swollen thorns. We compared the rewards offered by trees experimentally protected from mammalian browsing for seven years with those offered by unprotected trees. Protected trees produced 25% fewer nectaries per leaf and 25% fewer swollen thorns than did trees in browsed plots. Relaxation of indirect defenses when trees are protected from herbivores is consistent with the dual hypotheses that inducible defenses are dynamic and that plants can save costs by relaxing these defenses when they are not needed. Among trees exposed to mammalian herbivores, trees that were browsed within roughly the past year provided rewards for ants similar to those provided by trees exposed to herbivores but not browsed within the past year. This result suggests that relaxation of indirect defenses only occurs following more than one year of protection from mammalian herbivory. [ABSTRACT FROM AUTHOR]

We compare the dynamic nature of anti-herbivore defense in ant-plants to other plant-herbivore systems. Previous studies have neglected the dynamic nature of defense in ant-plants. Changes in environmental cues can trigger modifications in a plantis defensive strategy (induced responses). In ant-plants, many cues and signals controlled by theplant and the environment can enhance defense by influencing recruitment, patrolling, and persistence of ants. Leaf damage may trigger plant signals such as the release of plant sap or green leaf volatiles that attract ant defenders. Similarly, disturbance and mammalian breath may themselves induce ants to recruit. We also review studies thathave looked for changes in extrafloral nectar production induced by herbivory. Induced responses to herbivory that stimulate changes in nectar volume, sugar concentration, and amino acid concentration of extrafloral nectar have been reported. This should increase patrolling by defending ants. We propose that non-nectary mediated ant recruitment should also occur, but may be restricted to tightly linked ant-plant systems. We suggest that induced responses to herbivory are commonin ant-plants. [ABSTRACT FROM AUTHOR]