Volatiles released by living trees

In the discussion of greenhouse gases and their impact on global climate changes (Hüttermann and Metzger 2007; Majcherczyk and Hüttermann 2007), there is an increasing interest in the complex chemistry of the troposphere. The dynamics of the global atmospheric chemistry through climate forcing is triggered by VOCs (Holopainen 2004; Dindorf et al. 2005). Beside VOCs of anthropogenic origin, especially VOCs emis-sions from forests which are covering ca. 30% of landmass (FAO 2006) are sources affecting the system. The quantities of volatiles of natural origin (NVOC) released above the main landmasses as arable land and forests exceed by far the quantities from an-thropogenic sources. Due to their dominance, reactivity and physical properties, they are classified as VVOCs (very volatile organic compounds like methane), reactive VOCs (isoprene and terpenes) and non-reactive VOCs (Guenther et al. 1995).

VOC-emissions by plants are unavoidable due to their metabolic activities (Peñuelas and Llusià 2004). A dominant reactive VOC released by forests for example is isoprene, which is widespread but not generally present throughout the plant kingdom (Harley et al.

1999, Owen and Peñuelas 2005). Isoprene is discussed to play an important role in tropospheric chemistry (Fehsenfeld et al. 1992; Lerdau et al. 1997). Similar to terpenes, its reactivity influences the atmospheric dynamics of ozone, formation and deposition of or-ganic nitrates and oror-ganic acids (Harley et al. 1999). Due to this importance in atmosphe-ric processes, algorithms were developed describing the dependence of isoprene and terpene emissions of plants on light and temperature (Dindorf et al. 2005). Further factors as drought, diurnal and seasonal variation or growth conditions were discussed as para-meters influencing the VOC emissions of plants (Dudt and Shure 1994; Staudt et al. 2001, 2003). However, there are undisputable many additional internal (e.g. genetic, biochemical) and external (e.g. interaction with fungi and insects) factors that affect the presence (Lit-vak and Monson 1998) and emission of different VOCs by trees and other plants (Apel et al. 1999; Peñuelas and Llusià 2001; Schütz et al. 2004) which are not yet covered by known algorithms.

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Most trees are grouped, due to their affinity, in coniferous and broadleaved species.

This is also reflected in their VOCs composition: VOCs differ highly from coniferous to broadleaved woodlands. Regarding coniferous trees, VOC-research is almost exclusively done in the family of Pinaceae, e.g. Pinus, Picea, Larix, Abies, Tsuga, and Cedrus (Hayward et al. 2004; Lee et al. 2005). Broadleaved species were examined on a somewhat broader scale comprising Fagales (Betula, Fagus, Quercus), Sapindales (Acer, Castanea) and e.g. Salicaceae (Salix, Populus) (Pasteels and Rowellrahier 1992; Tollsten and Müller 1996; Hakola et al.

2001; Paczkovska et al. 2006). Further genera such as Eucalyptus (Guenther et al. 1993; Zini et al. 2002) are characterised and several comparative studies screened plant species for single VOCs only (Owen et al. 1997). Plant VOCs are mostly alkanes/alkenes, aromatic hydrocarbons, alcohols, phenolics, terpenes, esters, aldehydes and ketones (Kesselmeier and Staudt 1999). However, due to technical restrictions, the analytical window covers currently only compounds with boiling points between 60°C to 250°C at atmospheric pressure, and of intermediate to high thermal stability (Schütz 2001).

General processes in plant cells, as the lipoxygenase (LOX)-pathway (Feussner and Wasternack 2002) are responsible for the release of generalistic VOCs as the so called

“green leave volatiles” (GLV). Mainly alcohols, aldehydes of linear six carbon chains and their derivatives such as (Z)-3-hexen-1-ol, (Z)-3-hexen-1-yl-acetate, hexan-1-ol, and (E)-2-hexenal belong to this group (Visser 1979). Whereas the name GLV implies the paradigm that only leaves (not needles) are releasing these compounds, it was proven that coniferous trees release these compounds, too, but only in minute amounts (Schütz et al.

2004). GLV are released in low rates from nearly every plant species (Hatanaka 1993) and show a typical increase on mechanical wounding (de Bruxelles and Roberts 2001; Mithöfer et al. 2005) of any type of plant tissue, be it leaves, needles, stems or roots (Matsui 2006).

Especially young developing leaves and damaged leaves - and leaves are damaged by wind or insects in a forest all the time - release increased rates of GLV. With regard to the func-tion of trace compounds with low emission rates as carrier of informafunc-tion (“info-chemicals”), these minor components must however not be neglected (Schütz 2001; Schütz et al. 2004). GLV are known to play an important role in insect attraction and aggregation (Visser 1979; Schütz et al. 1997, 2004; Ruther 2000) or insect repellence

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(Huber and Borden 2001; Zhang and Schlyter 2004) and even in signalling between plant individuals, known as the phenomenon of “talking trees” (Tscharntke et al. 2001; Arimura et al. 2002, Engelberth et al. 2004; Farag et al. 2005). All this points out a complex interactive defence system in plants in which the VOCs play the role of a language. VOCs carry information about the constitutive or induced defence status of the plant, whether it is mechanically wounded, attacked by insects or micro-organisms (Schütz et al.

1997; Schütz 2001; Holopainen 2004: Weissbecker et al. 2004; Holighaus and Schütz 2006;

Johne et al. 2006a, b; Paczkovska et al. 2006).

Isoprenoids are characteristic defence chemicals of conifers and are produced through the mevalonate (MEV) or methyl-erythritol-diphosphate (MEP) pathways (Keeling and Bohlmann 2006). They are highly variable in structure (>30,000 terpenes are known) and occur in trees as isoprene (C5), monoterpenes (C10), sesquiterpenes (C15) and diterpenes (C20) (Sharkey and Singsaas 1995; Phillips and Croteau 1999; Trapp and Croteau 2001).

Following just the name of a compound, for instance, α-pinene should not be mistaken in that it is exclusively released by coniferous trees like Pinus spp. For example, European beech (Fagus sylvatica, Fagaceae) seems to be a much stronger monoterpene emitter than expected. The monoterpenes of this species, studied by Dindorf et al. (2005) and Moukhtar et al. (2005), are dominated by sabinene with more than 90% of the daily terpene emission, but the typical coniferous volatiles α-pinene and β-pinene were also found in the VOC pattern of beech trees. This holds also true for Quercus suber, the cork oak (Pio et al. 2005). α-Pinene, sabinene, β-pinene and limonene were the main com-pounds (80%) among the released terpene fraction from the oak.

Within taxonomic groups of lower plants, the VOC patterns are more alike, based on a more similar biochemistry of secondary plant compounds (Asakawa 2004). This relation-ship is treated in the scientific field of chemotaxonomy (Harborne and Turner 1984).

However, variability of VOC patterns can be high, notwithstanding the degree of relation-ship. The Southern beech Nothofagus dombeyi releases α-pinene in considerable amounts, whereas five other species of Nothofagus do not at all (Quiroz et al. 1999). A similar variability was shown by Harley et al. (1999) for isoprene emission of several woody and herbaceous plant species of Northern America.

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