Broad bean cultivars increase extrafloral nectary numbers, but not extrafloral nectar, in response to leaf damage

Abstract

Phenotypic plasticity allows organisms to maximize fitness, by optimizing the expression of costly defensive traits. Broad bean, Vicia faba L. “Broad Windsor”, produces increased numbers of extrafloral nectaries (EFNs) in response to leaf damage to attract mutualistic partners and reduce herbivory. It is currently unknown, however, whether EFN induction is cultivar-specific or is a more general phenomenon. It has also not been determined whether broad beans increase nectar secretion rates in conjunction with EFN induction. We hypothesized that: a) as all broad beans have conspicuous EFNs, all cultivars should produce additional EFNs in response to leaf damage, and b) overall nectar secretion rates should increase with EFN numbers, to attract additional mutualists. We tested our hypothesis by subjecting three broad bean cultivars, Vicia faba L. “Broad Windsor”, “Stereo”, and “Witkiem” to mechanical leaf damage. The degree of change in plant traits associated with growth, in addition to EFN induction, was assessed 1 week after leaf damage. Extrafloral nectar volumes were also assessed, every 24 hours, pre- and post-leaf damage. We confirmed our first, but rejected our second, hypothesis. All cultivars produced additional EFNs, but none increased extrafloral nectar volumes, when experiencing leaf damage. Further experimentation is required to determine if energetic tradeoffs limit multiple forms of defense (i.e., EFN vs. nectar induction), or if this alternative strategy is adaptive for attracting and retaining mutualists. Understanding the costs and benefits of EFN vs. nectar induction will provide insight into the evolution of defensive mutualisms between plants and predatory arthropods.