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Publication Title | Extracting and trapping biogenic volatile organic compounds stored in plant species

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Trends Trends in Analytical Chemistry, Vol. 30, No. 7, 2011

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Extracting and trapping biogenic volatile organic compounds stored in plant species

Elena Ormen ̃ o, Allen Goldstein, U ̈ lo Niinemets

Biogenic volatile organic compounds (BVOCs), released by practically all plants, have important atmospheric and ecological consequences. Because BVOC-emission measurements are especially tedious, complex and extremely variable between species, two approaches have been used in scientific studies to try to estimate BVOC-emission types and rates from plant species. The first, which has known little success, involves grouping species according to plant-taxonomy criteria (typically, genus and family). The second involves studying the correlation between BVOC content and emission (i.e. how leaf content could be used to estimate emissions). The latter strategy has provided controversial results, partly because BVOCs are amazingly chemically diverse, and, as a result, techniques used to study plant BVOC content, which we review, cannot be equally adequate for all analytes.

In order to choose an adequate technique, two patterns must be distinguished. Specifically stored compounds – mainly monoterpenes and sesquiterpenes that dominate the essential oil obtained from a plant – are permanently and massively present in specific storage structures (e.g., secretory cavities, trichomes) of the order of lg/g–mg/g and usually allow emissions to occur during stress periods when terpenes are weakly synthesized. These BVOCs can be studied directly through traditional extraction techniques (e.g., hydrodistillation) and novel techniques (e.g., application of microwaves and ultrasound), and indirectly by trapping techniques involving the collection, within adsorbent material, of BVOCs present in the headspace of a plant.

Non-specifically stored compounds (e.g., isoprene, 2-methyl-3-buten-2-ol, and, in species without storage structures, monoterpenes and sesquiterpenes) can only be temporarily accumulated in leaf aqueous and lipid phases in small concentrations of the order of ng/g. As a result, studying their concentration in leaves requires the use of trapping techniques, more sensitive to trace amounts. Unlike for specifically stored BVOCs, knowledge of the concentration of non-specifically stored BVOCs cannot provide any information regarding the emission potential of a species but, instead, provides crucial information to understand why BVOC emissions may be uncoupled from the physiological processes that drive their synthesis.

We describe both extracting and trapping techniques and discuss them in terms of the technical choices that may cause losses of thermolabile constituents, chemical transformations, different volatile recoveries and suitability to represent plant content of BVOCs faithfully. The second part of this review addresses technical shortcomings and biological and environmental factors that may alter the correlations between BVOC content and emission from plants.

a 2011 Elsevier Ltd. All rights reserved.

Keywords: Biogenic volatile organic compound; BVOC content; BVOC emission; Hydrodistillation; Leaf BVOC; Microwave-assisted extraction;

Solid-phase microextraction; Storage structure; Terpene

Abbreviations: DHT, Dynamic headspace trapping; HD, Hydrodistillation; MAHD, Microwave-assisted hydrodistillation; MASE, Microwave- assisted solvent extraction; PSE, Pressurized solvent extraction; SDE, Simultaneous distillation solvent extraction; SFE, Supercritical fluid extraction; SFME, Solvent-free microwave extraction; SWE, Subcritical water extraction; UAE, Ultrasound-assisted extraction

Elena Ormen ̃o*

Institut Me ́diterrane ́en dÕEcologie et Pale ́oe ́cologie, Institut Ecologie et Environnement, Centre National de la Recherche Scientifique (INNE-CNRS), Aix-Marseille University, 13331 Marseille Cedex 03, France Allen Goldstein Division of Ecosystem Sciences, Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720, USA U ̈ lo Niinemet Department of Plant Physiology, Estonian University of Life Sciences, Kreutzwaldi 1, 51014 Tartu, Estonia

*Corresponding author. Tel.: +33 (0) 4 13 55 12 26; Fax: +33 (0) 4 13 55 11 51; E-mail: elena.ormeno@univ-provence.fr

1. Introduction

Plants release to the atmosphere impor- tant amounts of biogenic volatile organic compounds (BVOCs) that account for up to 30% of the photosynthetically fixed carbon under stress conditions [1]. These metabolites may act as plant defenses as they repel herbivores and facilitate the foraging behavior of natural enemies of herbivores, and protect leaf cells from a variety of abiotic stresses [2]. Likewise,

978 0165-9936/$ - see front matter a 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2011.04.006

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