Chapter Fifteen - Asking the ecosystem if herbivory-inducible plant volatiles (HIPVs) have defensive functions

Summary

Introduction

Plant volatiles (PVs) comprise cues exchanged among plants and members of their ecological communities, including other plants, microorganisms and insects. Moreover, some PVs may protect plants against oxidative and thermal damage. Volatiles that are specifically herbivory-inducible (HIPVs) can betray the location of feeding herbivores to their natural enemies, and some HIPVs may defend plants by repelling herbivores or attracting natural enemies. However, the fitness benefits of HIPVs have not been clearly demonstrated in any plant system, so it remains unclear whether they function as indirect defences (Allison & Hare, 2009; Dicke & Baldwin, 2010). Indeed, HIPVs can be detrimental to plants, causing them to be more apparent to and attract herbivores as well as non-beneficial natural enemies that may interfere with other mutualistic interactors, such as pollinators (Halitschke et al., 2008; Kessler & Halitschke, 2009). And it is not clear whether the carnivores found in native plant populations can cope with variability in HIPV emissions. Within single populations of a species, there can be significant variation in the production of PVs among individuals (e.g. Skoula et al., 2000; Delphia et al., 2009), and also after herbivore attack (Schuman et al., 2009), raising the question of whether HIPVs are reliable indicators of herbivory. Do natural enemies learn which compounds are relevant in each population, or are they innately programmed to respond to certain HIPVs? Do plants that emit different or greater amounts of HIPVs than their neighbours risk making themselves more apparent to herbivores and other detrimental visitors, or benefit from greater apparency to beneficial natural enemies? The best way to answer these questions is to ask the ecosystem in which the plant evolved; however, PV research has a history of anthropomorphic metaphors and utilitarian motivations which we suggest may prevent researchers from placing their experiments in the proper ecological context. This chapter will describe an approach which attempts to ‘phytopomorphise’ the researcher by using field experiments with wild-type (WT) and appropriate transformed lines of the wild tobacco Nicotiana attenuata, in its native ecosystem.