Herbivore-induced plant volatile (HIPVs) mediated eavesdropping is a well-documented phenomenon. As publically available signals, HIPVs indicate the type and degree of herbivory a neighboring plant is experiencing. Therefore, an eavesdropping plant initiates an enhanced induction profile upon herbivory compared to a naïve individual via Defense Priming (DP). Since DP is an inducible phenomenon, HIPV-exposure alone is expected to incur fitness costs. Additionally, since volatile duration indicates the amount of damage, HIPV-mediated fitness effects maybe correlated with HIPV duration and affect induced defense.
To determine the relationship between DP and fitness costs, Phaseolus lunatus (Lima bean) plants were grown in a common garden and exposed to varying durations of the green-leaf volatile cis-3-hexenyl acetate (z3HAC). z3HAC was chosen because it is a ubiquitous C6 fatty-acid derivative that is released immediately with mechanical and herbivore damage. Proximate fitness measures, such as bud and flower production, were monitored throughout the field season while Defense Priming was quantified by measuring cyanogenic potential after an oral secretion plus mechanical wounding treatment. Mature leaves were artificially damaged 20% before applying a Spodoptera exigua elicitation treatment and collected 24 hours after damage.
Results/Conclusions
In this study, we found no direct correlation between Defense Priming and fitness effects under ecologically realistic volatile exposure regimes. Short exposures to z3HAC resulted in enhanced cyanogenic potential while a continuous exposure had similar cyanide levels relative to controls. Conversely, plants exposed to a continuous exposure had increased fitness relative to controls while pulsed and transient plants were unaffected. Continuous exposure plants had increased bud and floral output along with increased biomass while transient and pulsed treatment plants were not discernable from controls.
These results have significant implication for defining DP and understanding why this phenomenon is observed. Since fitness effects were independent from cyanogenic potential, DP may be independent of fitness costs. Therefore, growth-defense tradeoffs are more nuanced between different inducible defense mechanisms within a plant. To gain a comprehensive understanding of DP mechanisms, future work for defining the relationship between fitness and DP must investigate additional inducible defense mechanisms and environmental factors dictating HIPV source and identity.