Extrafloral nectary phenotypic plasticity is damage- and resource-dependent inVicia faba

Abstract

Phenotypic plasticity enables many damaged plants to increase nectar secretion rates from extrafloral nectaries (EFNs), or in the case of broad bean, Vicia faba L., to produce additional EFNs, to attract natural enemies of herbivores. While plants benefit greatly from these defensive mutualisms, the costs of producing EFNs are largely unknown. We hypothesized that if EFN production is costly, then damaged plants with high resource levels would be able to produce more EFNs than plants that are resource-limited. Here, we show that this indirect inducible defence does follow this general pattern. Vicia faba enriched with 6 or 12 g of 14 : 14 : 14 NPK fertilizer increased EFN numbers after leaf damage by 46 and 60%, respectively, compared with nutrient-poor plants. Thus, EFN production is both damage- and resource-dependent. Analogous to direct defences, production of EFNs may limit the overall loss of leaf tissue when risk of herbivory increases.

<i>Rhizobium</i> alters inducible defenses in broad bean, <i>Vicia faba</i>

“…This might seem contrary to prior experiments, where damaged plants produce more EFNs compared to undamaged plants [17,20,22]. It must be noted, however, that EFN induction has also been shown to be resource dependent [20].…”

“…While each broad bean leaf pair generally has 0 or 2 EFNs on the associated stipules, broad beans can alter the total number of EFNs on a plant by clustering the stipules, and hence the EFNs, on the apical meristem prior to the leaves unfolding [20]. After recording these traits, leaf damage treatments were administered to the appropriate plants.…”

“…In broad bean, EFNs are located on the stipules at the base of each leaf petiole [17,18]. Attractive to ants [18,19], broad bean plants increase EFN numbers when herbivory increases [17,20]. As broad bean EFNs are visually conspicuous it may be adaptive to produce a more striking visual display rather than to augment nectar from existing nectaries [17].…”

“…The induction of extrafloral nectar [13,16,21] and nectaries [17,20,22] are believed to be costly as, in several cases, these responses are resource dependent [14,20]. Resultantly, many plants have evolved phenotypically plastic responses to augment these defensive traits only when risk of herbivory increases [23,24].…”

Testing for mycorrhizal fungi-plant-ant indirect effects

“…Yet, in an earlier study that used the same batch of inoculant, we showed the opposite effect Á a reduction in the construction of EF nectaries in M' plants (Laird and Addicott 2007). Therefore, EF nectary production in V. faba appears to be highly context-dependent (Mondor et al 2006;Laird and Addicott 2007). Specifically, the variable plant response in EF nectary production reflects variable responses in plant growth.…”