Authors: Llusià J1,2, Filella I1,2, Sardans J1,2
Reviewer: Peñuelas J1,2
Measurement unit: µg g-1dm h-1, nmol m-2 s-1 or µg m-2 h-1 (GC-MS, PTR-MS); Measurement scale: site, plot, plant, leaf; Equipment costs: €€€; Running costs: €€€; Installation effort: high; Maintenance effort: high; Knowledge need: high; Measurement mode: manual
Biogenic volatile organic compounds (BVOCs) are a very large variety of molecules including isoprene, terpenes, alkanes, alkenes, alcohols, esters, carbonyls, and acids (Peñuelas & Llusià, 2003). The main producers and emitters of BVOCs are plants, which synthesise them in many different tissues by mean of different physiological processes. Among BVOCs, isoprene, monoterpenes, and sesquiterpenes are synthesised and emitted by several plant species. These volatile isoprenoid compounds have many protective and ecological functions and have important effects on the photochemistry and radiative properties of the atmosphere (Zimmerman et al., 1978; Kavouras et al., 1998; Peñuelas & Llusià, 2003; Owen & Peñuelas, 2005). Consequently, there is great interest in determining the emission capacities of the different species and how environmental factors and especially climate change affect the volatile isoprenoid emissions (Peñuelas & Llusià, 2001). Furthermore, BVOCs affect the chemical and physical properties of the atmosphere and therefore they are considered promoters of climate change (Fehsenfeld et al., 1992; Singh & Zimmerman, 1992; Kesselmeier & Staudt, 1999; Peñuelas & Llusià, 2001, 2003).
5.14.1 What and how to measure?
Plant BVOC emissions can be measured with different types of chambers measuring gas-exchange. Air samples are collected using an air-sampling pump that direct the air to a stainless steel tube filled with adsorbents. To measure BVOC contents in plant tissues, liquid nitrogen is used to maintain the terpenes unaltered and to crush the leaves. Pulverised leaves are then submerged in organic solvent to extract terpenes. Terpene content is calculated per dry weight, after drying plant material until constant weight.
After sampling, BVOC determination is generally performed by using gas chromatography-mass spectrometry (GC-MS) or by a proton-transfer-reaction mass spectrometer (PTR-MS) system. PTR-MS provides continuous monitoring of BVOCs and it is fast enough to be used in eddy covariance towers to measure BVOC exchange at the ecosystem level, but does not allow speciation of the thousands of BVOCs: for example, it does not distinguish the different monoterpenes that have the same mass. The PTR-MS system and its use in BVOC analysis have been described by Lindinger et al. (1998).
Sampling, preparation, and analysis of terpene content and emission
A gas-exchange system is frequently used for sampling plant volatile emissions. Air exiting the cuvette is pumped through a stainless steel tube filled with adsorbents. Air samples are collected using an air-sampling pump. The flow is measured with a flowmeter. Prior to use, these tubes are conditioned. Emission rate calculations are made on a mass balance basis and by subtracting the control samples without plants from the samples with plants (Llusia et al., 2012).
To measure BVOC contents, plant organs are submerged in liquid nitrogen immediately after sampling and transported to the laboratory. In the laboratory, samples are stored at -20 °C prior to BVOC extraction and analysis. The BVOC extractions are conducted by submerging 1–2 leaves or flowers in liquid nitrogen in a Teflon tube and crushing them with a Teflon pestle. Leaves are thereafter pulverised, and 2 ml of pentane is added to the extract. Before analysis, extracted plant material with pentane is centrifuged at 10,000 rpm for 5 min. The extracts are finally concentrated up to 200 ml with a stream of nitrogen. BVOC content is then calculated per dry weight basis, after drying plant material at a maximum of 70 °C for 72h to constant weight (Llusia et al., 2006; Pérez-Harguindeguy et al., 2013). The temperature and time for drying depend on the study question, how many samples are dried, the size, thickness and type of the plant material (e.g. large, fleshy or succulent leaves need more time) see protocol 2.1.1 Aboveground plant biomass for more details on the drying. BVOC determination is generally performed using a GC-MS system. Tubes with trapped emitted monoterpenes are inserted in the injector and desorbed into a chromatographic column. The injector is connected to a gas chromatograph with a mass spectrometer detector. A full-scan method is used in the chromatographic analyses. The desorbed samples are injected into a capillary column. The identification of monoterpenes is conducted by GC-MS and compared with standards from Fluka (Buchs, Switzerland), literature spectra, and GCD Chemstation G1074A HP. Internal standard dodecane that does not mask any terpene, together with frequent calibration with common terpene standards can be used for quantification. Blank samples of air without plants in the cuvette are necessary if clean air input is not used.
To monitor BVOC emissions continuously in time, PTR-MS can be used. The PTR-MS system and its use in BVOC analysis is described in detail by Lindinger et al. (1998). Intact leaves (or the plant organ of study) are clamped in a leaf cuvette. All tubing used is made of inert polytetrafluoroethylene (PTFE). Part of the air exiting the leaf cuvette flows through a T-system (T-shaped tube) to the PTR inlet. For volatile determination and quantification, both the air entering and exiting the leaf cuvette is analysed by PTR-MS and monitored with flow meters. The difference between the concentration of BVOCs before and after passing through the cuvette, along with the flow rates, are used to calculate the BVOC exchange (Peñuelas et al., 2007). The quantification of volatiles is based on the use of calibration standards. PTR-MS is also used in eddy covariance studies to measure BVOCs exchange at the ecosystem level.
Where to start
Lindinger et al. (1998), Llusia et al. (2006, 2012), Peñuelas et al. (2007)
5.14.2 Special cases, emerging issues, and challenges
Species that store BVOCs
Some species not only produce and emit terpenes but also store them. The production of terpenoids in terpene-storing plants is highly influenced by abiotic factors (Letchamo et al., 1994; Llusià & Peñuelas, 1998, 2000). Terpene emission rates in terpene-storing plants are not necessarily determined by terpene content, but the patterns of terpene emission from plants that store terpenes in specialised structures may be different from those of plants not having specialised structures for their storage (Lerdau et al., 1995; Seufert et al., 1995; Loreto et al., 1996; Peñuelas & Llusià, 2001).
BVOCs uptake
In some cases, there are not emissions but uptake of BVOCs, posing an interesting question of impacts of those uptakes. Furthermore, several experiments can be conducted fumigating plants and soils with BVOCs to study the physiological reactions of plants, microbes, and animals.
Soil BVOC exchange
Soil BVOC exchanges (Peñuelas et al., 2014) can be measured in situ using a flow-through chamber method. A vented soil chamber system is used with PVC collars installed permanently 3–4 cm into the soil. The collars are covered by a PVC lid with two outlets. Air samples from soil are collected using the same method as for plants described above. Soil measurements are measured in situ using a flow-through chamber method. Soil VOCs are sampled and the flow is regulated with a peristaltic pump. The flow adjustment is determined with a flow-meter (Asensio et al., 2008).
Improved continuous and fast measurements of BVOC exchange with PTR-TOF-MS
The conventional PTR-MS (with the analyses performed with a quadrupole mass detector) offers a high temporal resolution, but does not allow the distinguishing of compounds with the same mass, which constitutes an important analytical limitation (Müller et al., 2010). PTR-“time-of-flight”-mass spectrometry (PTR-TOF-MS) couples high sensitivity with high mass resolution, for instantaneous real-time detection of multiple emitted VOCs with unambiguous identification of compounds (Brilli et al., 2014).
SPME in dynamic systems
Another interesting BVOC sampling technique is the use, especially in floral studies, of spme (solid phase micro extraction) columns, i.e. a solid phase extraction sampling technique that involves the use of a fibre coated with an extracting phase (Courtois et al., 2009). Its use is very widespread to analyse mixtures of VOCs in both gaseous and liquid media (headspace), provided they are static systems. Its possible use in dynamic systems is a challenge that would greatly facilitate the study of BVOC emissions in gas-exchange systems.
BVOCs and climate change
Recent data intriguingly link BVOCs with climate. BVOC emissions increase with warming and with most of the other components of the current global environmental change. This increase, apart from influencing the oxidising potential of the troposphere, might produce both negative and positive feedbacks on warming depending on the spatial scales. Until recently, the short lifetime of BVOCs was thought to preclude them from having a significant direct influence on climate. However, there is emerging evidence that this influence might be important at different spatial scales, from local to regional and global, through aerosol formation and direct and indirect greenhouse effects (Peñuelas & Llusia, 2003; Claeys et al., 2004). BVOCs generate large quantities of organic aerosols (Laaksonen et al., 2008; Jiang et al., 2009; Spracklen et al., 2010) that could affect climate by forming cloud condensation nuclei. The result should be a net cooling of Earth’s surface during the day because of radiation interception. Furthermore, the aerosols also diffuse the light received by the canopy increasing CO2 fixation. However, the BVOCs also increase ozone production and the atmospheric lifetime of methane, thus enhancing the greenhouse effect of these gases. Whether increased BVOC emissions will cool or warm the climate depends on the relative weights of the negative (increased albedo and CO2 fixation) and positive (increased greenhouse action) feedbacks (Claeys et al., 2004).
5.14.3. References
Theory, significance, and large datasets
BEMA (1997), Kesselmeier & Staudt (1999), Laothawornkitkul et al. (2009), Peñuelas et al. (1995, 2013)
More on methods and existing protocols
Hewitt et al. (2003), Ormeño et al. (2011), Watson et al. (2001)
All references
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BEMA, (1997). BEMA: A European Commission project on biogenic emissions in the Mediterranean area. Atmospheric Environment, 31, 1-256.
Brilli, F., Gioli, B., Ciccioli, P., Zona, D., Loreto, F., Janssens, I. A., & Ceulemans, R. (2014). Proton Transfer Reaction Time-of-Flight Mass Spectrometric (PTR-TOF-MS) determination of volatile organic compounds (VOCs) emitted from a biomass fire developed under stable nocturnal conditions. Atmospheric Environment, 97, 54-67.
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Authors: Llusià J1,2, Filella I1,2, Sardans J1,2
Reviewer: Peñuelas J1,2
Affiliations
1 CSIC Global Ecology Unit, CREAF-CSIC-UAB, Barcelona, Spain
2 CREAF, Barcelona, Spain