Plant material and collection of volatiles.

Four potted indoor ornamental plant species were purchased from a commercial market (Spathiphyllum wallisii Regel, Sansevieria trifasciata Prain, Ficus benjamina L., and Chrysalidocarpus lutescens Wendl.) and acclimatized for 2 weeks to indoor conditions, i.e., 25 ± 1 °C, 50% relative humidity, and 5.45 μmol·m⁻²·s⁻¹ photosynthetically active radiation. S. wallisii was in flower during the test period. The dry weights of the aerial part in S. wallisii, S. trifasciata, F. benjamina, and C. lutescens were 16.7, 61.4, 43.1, and 34.3 g, respectively. Individual plants planted in 15-cm-diameter pots were placed in a meticulously cleaned 39-L glass container with inlet and outlet ports. The inlet port was connected to a charcoal filter (Alltech Assoc. Inc., Deerfield, IL) [Pyrex glass tube (10 cm × 1 cm i.d.) with 7 cm of 2.5 g of charcoal] to supply purified air and the outlet port was connected to a sorbent trap (10 cm long, 6 mm o.d., 4 mm i.d. stainless steel sorbent trap; Scientific Instrument Services, Inc., Ringoes, NJ) containing 150 mg of 60/80 mesh Tenax-TA^® (Alltech Assoc. Inc.) followed by a portable air sampling pump (LFS-113DC; Sensidyne Inc., Clearwater, FL). Volatile compounds emanating from the plant were collected for 10 h by flowing purified air into the container at 20 mL/min. Samples were also collected from empty chambers to verify the absence of contaminants. Samples were collected during both the day and night (12-h photoperiod). The rates of emanation for the individual volatiles are relative and do not represent absolute amounts. Collection began 2 h after the plant was placed in the container. Volatiles from the plastic pot with the media and the plastic pot alone were similarly assessed to determine the source of the individual compounds, i.e., plant, media/micro-organisms, and pot.

Short-path thermal desorption.

An automated short path thermal desorption system (TD-5; Scientific Instrument Services) mounted on the injection port of the gas chromatograph–mass spectrometer (GC-MS) (6890N/5973; Agilent, Palo Alto, CA) desorbed the volatiles from the Tenax trap at 250 °C for 5 min. The volatiles were collected on the first 4 cm of the capillary column using a cryofocus trap (SIS 2” Cryo-Trap; Scientific Instrument Services) cooling to –40 °C with liquid CO₂, and subsequently released by rapidly heating the trap to 200 °C.

Separation, identification, and quantification of volatiles.

Volatiles were identified and quantified using a GC-MS equipped with a 30-m length × 0.25-mm i.d., 0.25-μm film thickness of 5% phenyl methyl siloxane, fused silica capillary column (HP-5MS; Agilent). The injection port temperature was 225 °C with a split ratio of 5:1. Helium was used as the carrier gas at a flow rate of 1.0 mL/min. The column temperature was held at 40 °C for 1 min and then programmed 5 °C/min to 280 °C and held for 5 min. MS conditions were: ion source 230 °C, electron energy 70 eV, multiplier voltage 1247 V, GC-MS interface zone 280 °C, and a scan range of 35 to 350 mass units. The volatiles were identified based on comparison of their mass spectra and relative abundances with NIST 02 and Wiley 7 spectral libraries. The concentrations of individual volatiles were expressed as δ-carvone equivalents and are considered relative because recovery and calibration factors were not determined. The internal standard was introduced by placing 5 mL of δ-carvone in a sealed 1-L Erlenmeyer flask. After 24 h, 50 mL of air saturated with δ-carvone was removed and injected into the glass container holding the test plant at the beginning of volatile collection. The concentration of the internal standard in the trapped sample was determined using a concentration range of authentic standards in hexane directly injected into the GC.

Volatile organic compounds emanating from individual plants.

A total of 23, 12, 13, and 16 volatile organic compounds were identified from S. wallisii, S. trifasciata, F. benjamina, and C. lutescens, respectively (Table 1). The volatiles were comprised of terpenoids, alcohols, ketones, and esters (Fig. 1). Based on the relative proportion of the primary classes, terpenoids were quantitatively dominant in S. wallisii (97.8%), F. benjamina (79.0%), and C. lutescens (66.9%); however, esters (34.6%) were quantitatively dominant in S. trifasciata followed by ketones (26.9%), alcohols (26.6%), and terpenoids (11.9%). Certain compounds were found in all four species, i.e., butyl butyrate, methyl salicylate, and isopropyl myristate.

Table 1.Relative concentration of volatile organic compounds emanating from four species of indoor ornamental plants (Spathiphyllum wallisii, Sansevieria trifasciata, Ficus benjamina, and Chrysalidocarpus lutescens) during the day and night.

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The phenolic compound methyl salicylate, known to emanate from a number of species, comprised 0.2%, 17.8%, 7.8%, and 6.9% in the volatiles of S. wallisii, S. trifasciata, F. benjamina, and C. lutescens, respectively. The compound acts in signal transduction and plant defense; it activates resistance in the healthy tissues of damaged and neighboring plants against pathogens. Methyl salicylate is also thought to attract predators of certain herbivores (Holopainen, 2004). Many of the volatile compounds emanating from the test species differed. There was a wide range of terpenoid compounds (Table 1), some of which have been implicated in survival and quality attributes (e.g., flavor, fragrance) or noted for their pharmacological properties (e.g., anticarcinogenic, antimicrobiological) and ecological significance (e.g., pollinator attraction, defense) (Aharoni et al., 2005).

The specific terpene synthesized varied among species (Owen and Peñuelas, 2005). Fifteen terpenoids were identified from S. wallisii of which four were monoterpenes [e.g., (Z)-β-ocimene, (Z)-linalool oxide, (E)-linalool oxide, linalool], 10 sesquiterpenes [e.g., α-farnesene, (Z)-β-farnesene, farnesal, (3Z,6E)-α-farnesene, (E)-β-farnesene, (+)-β-costol, sesquirosefuran, (+)-cycloisosativene, santalol, caryophyllene], and one homoterpene (e.g., (E)-4,8-dimethyl-1,3,7-nonatriene] (Table 1). Alpha-farnesene (90.3%) was quantitatively the dominant volatile present followed by (Z)-β-farnesene (1.4%), (+)-β-costol (1.4%), and farnesal (1.1%). Alpha-farnesene, a component of the surface coating of apple, is known to be induced in plants damaged by herbivores (Holopainen, 2004). An oxygenated product (farnesal) and isomers thereof [i.e., (Z)-β-farnesene, (E)-β-farnesene, and (3Z,6E)-α-farnesene], present in S. wallisii (Table 1), are known to function as insect pheremones as does α-farnesene (Dawson et al., 1982). Santalol, present in S. wallisii, has been reported to have a relaxing and sedative effect in humans (Hongratanaworakit et al., 2004). Several monoterpenes identified in S. wallisii have been reported as major compounds in a variety of species, e.g., linalool in lavender and jasmine tea (Baser et al., 2005; Ito et al., 2002), linalool, and its derivatives [(Z)-linalool oxide and (E)-linalool oxide] in carnation flowers (Lavy et al., 2002). Linalool is known to have a sedative effect on autonomic nerve activity and mood and antifungal activity (D'Auria et al., 2005). Substantially fewer terpenoids (i.e., two, eight, and eight) emanated from S. trifasciata, F. benjamina, and C. lutescens, which were not in flower and quantitatively emitted fewer volatiles than S. wallisii. Germacrene D and E-farnesene epoxide were found only in F. benjamina and C. lutescens; α-cubebene, β-cubebene, and (-)-alloaromadendrene in F. benjamina; and limonene in S. trifasciata.

Most terpenoids from the four species were sesquiterpenes rather than monoterpenes. Generally, monoterpenes are synthesized in the plastids by way of the methylerythritol phosphate (MEP) pathway, whereas sesquiterpenes are synthesized in the cytosol through the mevalonate (MVA) pathway (Schwab et al., 2008). Mono- and sesquiterpenes are generally sequestered in specialized structures such as ducts or glandular storage cavities and their emanation appears to be developmentally regulated (Holopainen, 2004). For example, in S. wallisii, the number of terpenes increased markedly on flowering (data not shown). Therefore, variation in the biosynthesis and emission of monoterpenes and sesquiterpenes depends on the species and developmental stage. In addition to the mono- and sesquiterpenes, a C₁₁ homoterpene [(E)-4,8-dimethyl-1,3,7-nonatriene] was emitted from S. wallisii, F. benjamina, and C. lutescens. It has been shown to be synthesized in response to herbivore (maize) and ozone damage (lima bean), acting as a predator attractant and in defense against phytotoxic ozone, respectively (Vuorinen et al., 2004). Blending (E)-4,8-dimethyl-1,3,7-nonatriene with (E)-β-farnesene and β-caryophyllene attracts grapevine moth females for egg laying on flower buds through an antennal response (Tasin et al., 2007).

Volatile organic compounds emanating from micro-organisms/media and plastic pot.

Certain volatiles [e.g., cyclohexanol, 2-ethyl-1-pentanol, heptanol, 2-chlorobenzo-nitrile, (E)-2-decenal, 2,4,6-trimethyloctane, 2-undecanone, 2-ethyl-2-methyltridecanol, 2,3-dihydro-4- propyl-1H-indene, 2-undecenal, tetradecyloxirane, 1-ethyl-3,5-dimethylbenzene] emanated from the growing media or micro-organisms therein (Table 2). Although micro-organisms in the media have been shown to be important in the removal of volatile air pollutants, they also release volatiles into the atmosphere, some of which are biologically active. For example, 2- chlorobenzonitrile has been shown to stimulate root formation in bean seedlings and 1-ethyl-3,5-dimethylbenzene is considered an allelopathic substance found in rice root exudate (Kim and Kim, 2000). Therefore, the media/micro-organisms are not only involved in removing indoor pollutants, but also release volatiles into the air.

Table 2.Relative concentration of volatile organic compounds emanating from the media/micro-organisms, plastic pot, and pesticides of four species of potted indoor ornamental plants (Spathiphyllum wallisii, Sansevieria trifasciata, Ficus benjamina, and Chrysalidocarpus lutescens).

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Eleven volatile compounds originated from the plastic pots (Table 2). The silicone compounds hexamethyl cyclotrisiloxane, octamethyl cyclotetrasiloxane, and 2,4-bis(trimethylsiloxy) benzaldehyde were quantitatively the most abundant emitted. Several of the compounds are known to have undesirable effects on animals. For example, low-level octamethyl cyclotetrasiloxane exposure has been shown to cause altered immune responses in animals. One of the compounds, 2-ethyl-1-hexanol in the 2 to 32 μg·m⁻³ concentration range, is associated with “sick building syndrome,” which can cause asthma in humans (Norbäck et al., 2000). 2-(2-Methoxyethoxy)ethanol, 2-ethylhexyl salicylate, and homosalate are ingredients of pesticides that were apparently applied to several of the species during the production phase.

Difference in emission between day and night.

The volatiles from individual plants were collected for 10 h during the day and again at night. The total amounts during the day were 61.5, 0.4, 1.5, and 1.2 ng·100 g⁻¹ dw/hour from S. wallisii, S. trifasciata, F. benjamina, and C. lutescens, respectively. At night, the volatile emission rate was substantially reduced (e.g., by 30.1%, 69.5%, 73.7%, and 63.1%, respectively) and the total number of volatiles was also reduced to five, seven, four, and six, respectively (Table 1; Fig. 2). In addition to the total amounts, all classes (i.e., alcohols, esters, ketones, and terpenoids) were substantially higher during the day for each of the species (Table 1; Fig. 2). Past research has found that rose flowers also emitted a greater amount of two terpenoid compounds (germacrene D and geranyl acetate) during the first 3 h of the light period (Hendel-Rahmanim et al., 2007). Plant volatile emissions are regulated by their rate of synthesis and release (Dudareva et al., 2004). The biosynthesis of volatile compounds in the epidermal cells of plant tissues, secretory structures, and glandular trichomes depends on multiple biochemical pathways [e.g., terpenoids by MVA and MEP pathways; volatile fatty acid derivatives (e.g., alcohols, ketones, esters) by α-oxidation, β-oxidation, and lipoxygenase pathways] involving a number enzymes (Schwab et al., 2008). Emanation is affected by the physical properties of the individual compounds and the characteristics of their site of accumulation (e.g., cellular and intracellular membranes, secretory compartment) (Dudareva et al., 2004). The rate of biosynthesis and release is greatly influenced by abiotic factors such as light (Dudareva et al., 2004; Sharkey and Yeh, 2001). Light decreases the diffusion resistance of stomata during the day and the availability of glyceraldehyde-3-phosphate, a precursor of terpenes formed during photosynthesis (Niinemets et al., 2004). There are a number of reports indicating the effect of light on monoterpene synthesis and emission (Kesselmeier and Staudt, 1999; Logan et al., 2000; Loreto et al., 1996; Staudt and Bertin, 1998; Yokouchi and Ambe, 1984).

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Although indoor ornamental plants have been shown to remove certain volatile air pollutants from the air (Yoo et al., 2006), they also emit a diverse cross-section of compounds, some of which are known to be biologically active. The longevity and fate of these compounds have not been adequately studied. Also, the positive or negative impact of these compounds on humans balanced with the ability of plants to remove other volatile organic compounds is unknown.