Jasmonic acid (JA) is a naturally occurring signalling compound found in higher plants that plays a key role in both stress and development (Creelman & Mullet, 1997; Gfeller & Farmer, 2004). Jasmonic acid and its methyl ester, methyl jasmonate (MeJA), collectively referred as jasmonates, constitute a major signalling mechanism in stress-induced gene expression (Wasternack & Parthier, 1997). Endogenous levels of JA increase in plants in response to a wide range of biotic and abiotic stresses such as water deficit (Creelman & Mullet, 1997; Gao et al., 2004); salinity (Mopper et al., 2004); low temperatures (Kondo et al., 2004); and ozone (Koch et al., 2000; Rao et al., 2000), and it has been suggested that jasmonates could mediate defensive responses to these environmental stresses (Wilen et al., 1994; Tsonev et al., 1998; Mackerness et al., 1999). Although natural seasonal cycles and/or environmental stress induce more significant changes in JA than herbivory (Mopper et al., 2004), JA is best known for its role in response to herbivory and mechanical wounding (Baldwin, 1998; Reymond et al., 2004).
Jasmonates have also been associated with the increased production of volatile organic compounds (Hopke et al., 1994; Boland et al., 1995; Rodriguez-Saona et al., 2001; van Poecke & Dicke, 2004). Increases in the biosynthesis of isoprene from recently fixed carbon caused by exogenous JA have been also found (Ferrieri et al., 2005). Application of exogenous JA has been used to stimulate induced plant resistance without damaging the plant (Thaler et al., 1996), particularly in studies of herbivory: JA induces systemic accumulation of compounds with antiherbivore properties and elicits volatile emissions that attract the natural enemies of herbivores (Thaler et al., 2001). The application of JA has also been described as inducing the release of volatile sesquiterpenes in Zea mays (Schmelz et al., 2001) and increasing the emission of volatiles in Lima bean (Heil, 2004). The exogenous application of the methyl ester, MeJa, has also been described as triggering a twofold increase in monoterpene and sesquiterpene accumulation in needles and a fivefold increase in total terpene emissions in the foliage of Norway spruce (Martin et al., 2003), as well as dramatic increases in terpenoid emissions in Nicotiana attenuata (Halitschke et al., 2000).
Proton transfer reaction mass spectrometry (PTR-MS) has emerged as a useful tool that permits us to monitor online, almost simultaneously, a large number of different volatile organic compound (VOC) species with a fast time response (< 1 s) and with a low detection limit (parts per trillion by volume). By comparing PTR-MS and GC-PTR-MS measurements, Warneke et al. (2003) showed that PTR-MS accurately measured some VOCs (isoprene, monoterpenes, methanol, acetonitrile, acetaldehyde, acetone, benzene, toluene and higher aromatic VOCs). However, GC-MS is also needed to identify different compounds with the same mass (e.g. the various monoterpenes) that PTR-MS cannot separate.
The holm oak (Quercus ilex L.) is one of the dominant Mediterranean forest species. No specialized storage structures for monoterpenes have been found in its leaves or bark, and emissions appear to be mainly influenced by temperature and light, although water availability is also influential because of the dependence of terpene production on metabolites originating in the photosynthetic processes (Staudt & Seufert, 1995; Loreto et al., 1996; Bertin et al., 1997; Owen et al., 1997; Llusià & Peñuelas, 1999, 2000; Peñuelas & Llusià, 1999). The emissions of VOCs other than terpenoids have also been studied in Q. ilex (Kesselmeier et al., 1997; Holzinger et al., 2000; Peñuelas & Llusià, 2001), although no study that we are aware of has focused on trees’ VOC emissions as responses to JA.
We compared emissions from Q. ilex leaves sprayed with JA (JA-S) with those from control leaves by means of PTR-MS and GC-MS analyses. Our aim was to study the online dynamics of the effect of JA on emissions of terpenes and other VOCs in this common Mediterranean species, which is often subject to stressful conditions associated with high temperatures, high irradiance and low water availability in summer (Peñuelas et al., 1998), and high irradiance and relatively low temperatures in winter (Oliveira & Peñuelas, 2000, 2001). In order to study the role of photosynthesis and conductance in these emissions, we submitted the measured leaves to a dark–light transition during emission monitoring and conducted CO2 response curves of JA-S and control leaves.