The interaction between plants and herbivores is often mediated by visual or olfactory cues. Among the olfactory cues, the emission of constitutive biogenic volatile organic compounds (BVOCs) can play a major direct role in attracting or deterring herbivores (Visser 1986; Dicke, Agrawal & Bruin 2003). In response to the attack by herbivores, BVOC emission can also be induced. Numerous studies demonstrated that induced BVOCs act as infochemicals, allowing plant communication with other organisms, and indirect defence against herbivores (Dicke & Sabelis 1988). Induced BVOCs are often the main components of the bouquet that recruits parasitoids or predators of the herbivores (reviewed by Baldwin et al. 2006; Gershenzon & Dudareva 2007). These BVOCs are frequently induced by chemicals released by the herbivore (e.g. Mattiacci, Dicke & Posthumus 1995; Mithöfer & Boland 2008) and the induction is often systemic (i.e. it is not limited to the attacked leaves, but is spread to the entire plant) (e.g. Röse et al. 1996). The spectrum of induced BVOCs is often different from that of constitutive BVOCs (Pare' & Tumlinson 1999) and can be also different from the BVOC induced by mechanical stresses.
Two classes of induced BVOCs have been generally separated. BVOCs emitted immediately (within few minutes) after wounding are characterized by products of membrane breakdown, under the action of lipoxygenase and hydroperoxide lyase (Hatanaka 1993). This family of C-6 compounds is often referred to as ‘green leaf volatiles’ for its typical fragrance. Green leaf volatiles are not specific to herbivore feeding because their emission is also induced by mechanical wounding (Röse et al. 1996). However, they can prime inducible defence responses and the release of other classes of volatiles (Farag et al. 2005; Ruther & Kleier 2005).
A second class of BVOCs is emitted hours to days after herbivore feeding. Phenolic compounds (Mumm & Hilker 2006), methyl salicylate, nitriles and indoles (Van Poecke, Posthumus & Dicke 2001) and methyl jasmonate (Pare' & Tumlinson 1999; Orozco-Cárdenas, Narváez-Vásquez & Ryan 2001) are often components of this blend, and may have a key role in the activation of defensive pathways. Monoterpenes and sesquiterpenes (Kessler & Baldwin 2001), and in particular acyclic isoprenoids [e.g. (E)-β-ocimene, (E)-β-farnesene, linalool] (Röse et al. 1996; De Moraes et al. 1998; Heil & Silva Bueno 2007) are also main constituents of induced BVOCs. Delay with respect to herbivore feeding reflects gene induction (Arimura et al. 2000) and terpene synthases formation (Bohlmann et al. 1998; Arimura, Huber & Bohlmann 2004a). Induced isoprenoids are perhaps the most powerful mediators of plant herbivore interactions. As recently, exhaustively reviewed (Gershenzon & Dudareva 2007), volatile isoprenoids are toxic, may deter a large number of pathogens and herbivores or may attract numerous enemies of the herbivores.
Isoprene (2-methyl-1,3-butadiene) is the simplest isoprenoid, and accounts for more than 50% of the biogenic emission of isoprenoids (Guenther et al. 1995). Poplars are large emitters of isoprene with rates often exceeding 5–10% of the carbon fixed photosynthetically. The emission of isoprene is widespread in North American and European poplar species, with no reported exceptions (Kesselmeier & Staudt 1999). The role of isoprene has been clearly related to protection against environmental stresses (Sharkey & Yeh 2001), while no clear impact on plant–herbivore interactions is known. Preliminary results show that isoprene emission may deter caterpillars from feeding on Nicotiana transgenic leaves (Laothawornkitkul et al. 2008), and may also repel parasitic wasp (Loivamäki et al. 2008). A typical feature of isoprene-emitting plants is that they do not generally emit substantial amounts of other isoprenoids, namely monoterpenes (Kesselmeier & Staudt 1999). Accordingly, no healthy leaves of poplar species have been reported to constitutively emit a substantial amount of monoterpenes; minor emissions of monoterpenes having been only measured in young aspen (Populus tremula) leaves (Hakola, Rinne & Laurila 1998). On the contrary, the induced emission of monoterpenes, sesquiterpenes and homoterpenes has been reported from leaves of the hybrid Populus trichocarpa × deltoides, following the attack by forest tent caterpillars (Arimura et al. 2004a), and from leaves of the hybrid P. tremula × tremuloides, following the attack by Phyllobius piri or Epirrita autumnata (Blande et al. 2007).
Chrysomela (syn: Melasoma) spp. are among the main herbivores of poplars worldwide. Both larvae and adult beetles attack the leaves. Typical of some Chrysomela species is the production of volatiles that deter predatory attacks (Pasteels, Duffey & Rowell-Rahier 1990). Both emission of iridoids and sequestration of the phenol glucoside salicin from plants and subsequent release of salicylaldehyde are reported (Burse et al. 2007). The high foliar damage and production losses caused by this insect have driven research on BVOC-mediated poplar–Chrysomela interactions, aiming at a biological control of the infestations. Kendrick & Raffa (2006) found that Chrysomela scripta adults are attracted by host plants, as also observed for other chrysomelid beetles (Tansey et al. 2005). Behavioural assays suggested that the attractive cue was olfactory, but did not reveal the exact nature of the attractant. The observation that Chrysomela adults preferentially land and feed on young leaves, a choice that is not associated to differences in anatomical, morphological and biochemical traits (Harrell et al. 1981; Fernandez & Hilker 2007), led us to investigate whether the pattern of isoprenoids emitted by young and adult leaves plays a role in insect attraction. We tested the hypotheses that young and adult leaves emit different isoprenoids, and that volatiles orient Chrysomela insects in their choice of young poplar leaves.