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.
Highlights
When herbivory or predation risk increases, some plants and animals have the ability to alter their phenoltype to decrease risk of further attack [1,2,3,4]
If herbivory or predation is predictable and recurrent, constitutive defenses would be selected for, as the costs of developing and expressing defensive traits would be offset by the benefits of increased survival and reproduction [2,8,9]
Plants that suffered leaf damage produced more extrafloral nectaries (EFNs) than undamaged plants (F1.161 = 34.44, P < 0.0001), and this effect was consistent across cultivars, as demonstrated by no significant interaction (F2.161 = 2.43, P = 0.092; Figure 1)
Summary
When herbivory or predation risk increases, some plants and animals have the ability to alter their phenoltype to decrease risk of further attack [1,2,3,4]. This plasticity enables organisms to express costly traits when required, and to reduce or eliminate these costs when such phenotypic expression is not necessary [5,6,7]. Increased plant fitness could maintain the expression of these sugar-producing organs, despite the initial selective pressures resulting in their evolution
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