Extrafloral nectary phenotypic plasticity is damage- and resource-dependent in Vicia faba

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.