Emission of monoterpenes and isoprene from a Mediterranean oak species Quercus ilex L. measured within the BEMA (Biogenic Emissions in the Mediterranean Area) project

Pergamc,n

Atmospheric Environment Vol. 30, Nos 10/11, pp. 1841-1850, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 1352-2310/96 $15.00 + 0.00

1352-2310(95) 00376-2

EMISSION OF MONOTERPENES AND ISOPRENE FROM A M E D I T E R R A N E A N O A K SPECIES QUERCUS ILEX L. M E A S U R E D W I T H I N T H E BEMA ( B I O G E N I C E M I S S I O N S I N T H E M E D I T E R R A N E A N AREA) P R O J E C T J. KESSELMEIER,* L. SCH~FER,* P. CICCIOLI,# E. BRANCALEONI,t A. CECINATO, t M. FRATTONI,I" P. FOSTER,:~ V. JACOB,$ J. DENIS,~ J. L. FUGIT,§ L. DUTAUR§ and L. TORRES§ *Biogeochernistry Department, Max Planck Institute for Chemistry, Johann-Joachim Becher Weg 27, D-55128 Mainz, Germany; tlstituto sulrinquinamento Atmosferico del C.N.R., Area della Ricerca di Roma, Via Salaria km 29.300, C.P. 10, 00016 Monterotondo Scalo, Italy; :~Institut Universitaire de Technologic, GRECA, Universit~ Joseph Fourier, 1 rue Franqois-Raoult, F-38000 Grenoble, France; and §Institut National Polytechnique, Ecole Nationale Superieure de Chimie de Toulouse, Chimie Energie et Environnement, 118, Route de Narbonne, 31077 Toulouse Cedex, France (First received 21 December 1994 and in final form 1 September 1995)

Abstract--We report on some results of our studies of monoterpene and isoprene emissions and the physiological activities of an oak species (Quercus ilex L.) under the Mediterranean climatic conditions found at Castel Porziano (Rome) in June 1993. The oak species Quercus ilex L. was found to emit mainly monoterpenes in high amounts. Isoprene emissions were negligible. Diel cycles of monoterpene emissions showed correlation with light and the diel behaviour of photosynthetic CO2 assimilation, transpiration and stomatal corductance. Temperature dependence seemed to be of minor importance. Key word index: Non-methane hydrocarbons, VOC, monoterpenes, isoprene, biogenic emission, plant physiology, light, oak.

]INTRODUCTION Non-methane hydrocarbons, monoterpenes and isoprene play an important role in regulating the oxidative capacity of the atmosphere. A large number of these compounds are emitted into the atmosphere from vegetation (Warneck, 1988; Tingey et al., 1991; Lerdau, 1991). The monoterpenes are highly reactive towards OH, NO3 and 03 and rate constants for reaction with these radicals have been obtained for many common monoterpenes (Atkinson, 1989, 1991, 1994). A recent overview of product formation from gas-phase reactions is given by Hakola et al. (19c~4). Although monoterpenes may act as a sink for 03 (Kotzias et al., 1989, 1991), they may also be considered a producer of tropospheric ozone depending on the availability of NO/NOx (Lerdau, 1991; Fehsenfeld et al., 1992; Meixner, 1994). Several studies were performed on the emissions from plant species (for a recent review see Riba and Torres, 1996); however, reports on the emission pattern and quantity from Mediterranean plant species are only rarely found. These gaps in our knowledge stimulated a combined project investigating the

Biogenic Emissions in the Mediterranean Area (BEMA). Within this project some of the most important plant species were included in the investigations. One of these species was the most abundant oak species in the Mediterranean area, Quercus ilex L. As reported recently by Staudt et al. (1993) and Pio et al. (1993), this tree species has to be regarded as a possible monoterpene emitter, in contrast to other oak species emitting only isoprene (Tingey et at., 1991). Cuvette studies under controlled climatic conditions in a chamber (Staudt et al., 1993) gave a first insight into the relations between plant physiology and terpene emissions from this species. Based on this background, the emissions of monoterpenes and isoprene from Quercus ilex were studied under field conditions using dynamic cuvettes to meet the needs for the use of emission data under more natural conditions. To obtain reliable results, these experiments were performed as a close cooperation of several institutes measuring within one cuvette system with different sampling and determination techniques. We were able to combine an on-line technique combined with GC/FID, a trap technique combined with G C / F I D and a trap technique combined with GC/MS identification.

1841

J. KESSELMEIER et al.

1842 MATERIALS AND METHODS

Site description The presidential estate of Castel Porziano is a large forest area of c. 6000 hectares located 15 km southeast of the suburbs of Rome. It is comprised mostly of broad-leaved oak forest mixed with conifers, 'Mediterranean macchia', shrub areas and vegetation growing near the sandy beach. A small portion of it is also used for agricultural purposes. Pinus pinea and Quercus ilex are the dominant tree species followed by Quercus cerris, Quercus suber and Quercus robur. The forest was selected because its vegetation was quite representative of that growing in the Mediterranean region. Although the set-up for studying biogenic emission consisted of three sites, emission from Pinus pinea and Quercus ilex was measured exclusively at one site. An 8 m high scaffold was built for installing cuvettes. The scaffold was L-shaped to provide easy access to different trees, thus allowing a large selection of branches for enclosure studies. It was oriented to provide optimal light conditions to the various parts of the trees investigated. The size of the scaffold was large enough to permit the installation of all the instrumentation needed for controlling the parameters inside the cuvettes and for providing adequate air exchange through it. Small cabins were installed at the bottom of the scaffold for protecting the instrumentation used for on-line determinations. Electricity for running the entire experiment was provided by large diesel generators located more than 100 m away from the site. They were equipped with catalysts to reduce VOC emission. Tests performed by collecting aerometric samples showed that their influence was negligible. Concurrently with cuvette determinations, fluxes of both biogenic and man-made VOC were determined by gradient and eddyaccumulation methods in two sites in Castel Porziano. One of them was the pine-oak forest whereas the other one was a pseudo-steppe site located a few kilometres away from the pine-oak site. In both cases deposition of arenes was measured, indicating that transport from the urban area of Rome was the main source of man-made VOC in the pine-oak forest (Ciccioli et al., 1996). In particular, no evidence of large positive fluxes of benzene and toluene from diesel emissions was detected. Furthermore, concentrations of short-chain organic acids (formic and acetic acids) did not give reason to assume pollution from combustion processes near by (Sch/ifer et al. BEMA reports, 1993, 1994; publication in preparation). More information on the site and the campaign can be obtained from the literature (CEC, 1994). Cuvette construction and dynamics The cuvette was mounted at a height of ca. 6 m on a scaffold constructed at the southeast side of a group of oak and pine trees. The height and the light direction during the day led to a shadowing of the enclosed branch by the trees as well as by the scaffold after 12:45 h. For the light intensities under these conditions, see Fig. 1. The diel pattern of the light intensities (high from 9:00 h to 12:45 h and low from noon to the evening) gave an interesting chance to discern between light- and temperature-driven exchange processes, as the air temperatures outside and inside the cuvette did not drop immediately after shadowing. Thus, we could examine high and low light processes under similar temperature regimes. The cuvette enclosed an intact oak branch with 0.227 m 2 leaf area and 20 g leaf dry weight with 1 and 2 yr old leaves and was flushed with ambient air. It consisted of a PVC frame supporting a Teflon bag (ID 40 cm; length 60 cm; ca. 75 {) from the outside. Previous studies with a similar cuvette system (Sch~iferet al., 1992) showed that the Teflon film (FEP) is fully light permeable in the spectral range tested (300-900 nm) and shows no interference with organic acid analysis as well as reduced sulphur compound analysis (Schiller et al., 1992; Kesselmeier et al., 1993). The total air

flow through the cuvette was in the order of 20-25 drain- t. The air inside the chamber was mixed by a propeller covered with PTFE mounted at the upper face of the chamber and driven by a magnetically coupled motor situated outside the chamber. The wind speed inside an empty cuvette ranged between 0.5 and 2 m s- 1, depending on the distance from the propeller. Air samples for CO2 and trace gas measurements were taken at the inlet and the outlet of the cuvette. Exchange was calculated from the difference between the inlet and outlet concentrations. The actual experiments started with the enclosure of the oak branch; however, first measurements were done earliest 1 d after the enclosure to minimize artefacts by incubation effects.

Cuvette measurements of CO 2 exchange, transpiration and meteorological parameters The inlet and outlet of the cuvette were connected to an infrared dual channel gas analyser (LICOR 6262) for difference measurements of CO2 and water. A separate pumping device was used to supply the LICOR gas analyser with air from the cuvette inlet and outlet with a flow rate of l Emin -1. An additional gas analyser (LICOR 6252) monitored the absolute CO2 outside the cuvette. Monitoring of light, relative humidity and temperature was performed with standard sensors for air temperature, relative humidity and photosynthetic active radiation. All data were recorded as 5 min averages on a datalogger ICSI Ltd (U.K.), model 21X3. GC techniques: sampling and determination of monoterpenes and isoprene To obtain high time resolution we used an on-line GC/FID technique as described below, delivering a very dense sampling protocol. Additionally, we could include sampling by cartridges and determination by GC/FID as well as trap sampling and quantification and clear identification by GC/MS. With this procedure we were able to get a good time resolution combined with reliable quantification (consistent measurements by several groups) as well as identification. Ozone scrubbers were not used after intensive studies by the groups involved in the BEMA project during several intercomparison exercises. Laboratory experiments performed with cuvettes flushed with and without ozone have clearly shown that only low degradation of terpenes takes place on carbon adsorbents under ozone concentrations of 120 ppbv. More important is the fact that similar results were also obtained by the groups using Tenax as a trapping material. Sample recoveries obtained were independent of the trapping material used and even better than those measured by groups using ozone scrubbers. Thus, we concluded that VOC losses were mostly depending on the desorption procedure used. In particular, the removal of ozone prior to trap heating was found to be essential in preventing VOC decomposition. When Tenax was adopted, better performances were obtained by using fresh material not subjected already to a large number of collection and desorption cycles. Online GC/FID measurements The daily variations of mixing ratios and rates of terpene emissions were determined continuously by a sequential procedure using a fully automated device described in detail in a previous paper (Clement et al., 1993). The cycle included four individual steps. 1--preconcentration: The air to be analysed was sent via a light-shielded continuously flushed Teflon sampling line (200 mlmin-1; 20 m length) from the cuvette to the trap where terpenes were collected on an adsorbent. The Teflon line was checked for losses of isoprene and ct-pinene in the low ppbv range. Losses for both compounds were found to be below 3% (Fugit, 1995).

Emission of monoterpenes and isoprene 2--stabilisation: After termination of sampling and switching back to the analytical cycle (swept by He as a carrier gas), a period of "Tmin was required to ensure stabilisation of the gas flow and of the chromatographic signal. 3--thermodesorption: Rapid heating of the trap up to a temperature of 220°C transferred the desorbed compounds onto the GC column. 4~analysis: Terpenes were analysed on a Megabore Carbowax 20 M column (length 60 m, film thickness 1.2 ~m). The analysis time wa~ about 25 min. During the latter st~p, the sampling trap was cooled down to the working tempe:rature of 16°C which had to be reached prior to the beginning of the next cycle. The duration of a full preconcentration-analysis cycle was ranged between 35 and 45 min, mainly depending on the mixing ratios to be studied. The device described above is designed for the analysis of atmospheric samples with terpene mixing ratios in the range of a few pptv. The quantitative calibrations were periodically checked with permeation and/or diffusion systems used for generating terpene co~aapound mixtures as described in a previous paper (Riba et ~l., 1988).

Off-line determinations H R G C - F I D detevminations: monoterpene determination. Monoterpenes were analysed after preconcentration on stainless steel tubes packed with 200 mg Tenax TA (60-80 mesh), with a sampling flow rate of 65 ml min- 1, and a sampling time of 30 min. Desorption and analyses of adsorption tubes were carried out using a Perkin Elmer ATD 400 connected to a GC auto system directly in the Caste1 Porziano site. Adsorbed compounds were thermodesorbed (250°C) and cryofocu~ed ( - 30°C) in a glass trap filled with 20mg Tenax TA, and subsequently separated using gas chromatography (GC) after heating the trap to 300°C. The capillary column was a BP1 (50 m x 0.22 mm, 0.12 pm film thickness), and the lemperature programming was: 40°C during 5 rain then 40-160°C (3°Cmin - 1) and 160 to 200°C (10°Cmin-1). Compounds were detected with a flame ionisation detector. Some cartridges were duplicated for qualitative analyses by GC/mass spectrometry (MS). A GC/QMD 1000 in:~trument from Fison's was used. H R G C - F I D determinations: isoprene determination. Isoprene was also analysed after preconcentration on glass tubes packed with a three-sorbant bed (Carbotrap C/Carbotrab B/Carbosieve SIII, 125 mg/100 mg/125 mg) with a sampling flow rate of 50 ml min- t and a sampling time of 30 min. The compound W~LSthermodesorbed (200°C) and cryofocused ( - 150°C) in a precolumn (capillary tube) filled with A1203/KCI, and subsequently separated using capillary gas chromatography after heating the trap to 200°C. Desorption and analysis of adsorption tubes were carried out using a Chrompack thermal desorption unit connected to a Chrompack CP 9000. The capillary column was a PLOT A12Oa/Na2SO 4 (50 IrL × 0.32 ram, 10 pm particle diameter), and the temperature programming was: 40°C during 2 min then 40-180°C (5°Cmin-1). Isoprene was detected with a flame ionisation detector. Some cartridges were duplicated for qualitative analys~s by GC/MS. H R G C - M S determinations. Off-line sampling combined with HRGC-MS was used for the positive identification of VOC emitted by Quercus ilex and for the selective quantification of isoprene ant] monoterpene hydrocarbons. Compounds entering and leaving the cuvette were collected on two-stage traps consi,;ting of glass tubes (15 cm x 0.3 cm ID) filled with Carbotrap C (0.034g) and Carbotrap (0.17g) particles ranging between 20 and 40 mesh. Both adsorbents were supplied by Supelco (Bellefonte, PA, U.S.A.). Before sample collection, the traps were cleaned by passage of a stream of ultrapure helium at a flow rate of 300 ml min- ~ and under heating up to 250°C. A detailed description is given by Ciccioli et al. (1992).

1843

Since carbon as a black body is heated up by IR radiation resulting in a dramatic decrease of the adsorption capacity and the break-through volume of the traps, they had to be protected from sunlight. Therefore, after purging they were sealed with metal connectors equipped with PTFE ferrules, wrapped in aluminium foil and stored tightly in closed glass containers in the presence of active charcoal to prevent contamination of the trapping material during transport. A 250 ml glass syringe was used for drawing the air through the traps. Volumes ranging between 1 and 2 ~ were sampled at the inlet and outlet of the cuvette. VOC retained on carbon traps were transferred to the GC unit by thermal desorption. A Chrompack (Middelburg, The Netherlands) purge and trap injector was adapted for this purpose by eliminating the purging and drying units. Traps were desorbed by raising the temperature to 230°C. Desorbed VOC were cryofocused on a fused-silica liner maintained at - 1 5 0 ° C and then transferred into a 6 0 m x 0 . 3 2 m m ID capillary column (J & W-Fisons, Folsom, CA, U.S.A.) by increasing the temperature of the liner to 230°C. The column was coated with a 0.25/~m film of DB-1. The column was maintained at 5°C for 3 min, programmed to 50°C at 3°C min-1 and then to 220°C at 5°C min -1. A Model 5890 gas chromatograph (Hewlett-Packard, Palo Alto, CA, U.S.A.) connected to a quadrupole mass spectrometer (Model 5970 B) supplied by the same company was used for the analysis. Positive identification of the various components was carried out by combining the information obtained through the analysis of mass spectra with the elution sequence determined by measuring the retention indices of a large number of pure compounds.

RESULTS AND DISCUSSIONS P l a n t physiology P l a n t s or parts of plants in closed c h a m b e r s or d y n a m i c cuvettes are always u n d e r a special situation as c o m p a r e d to outside conditions. Thus, it is necessary to m o n i t o r several p a r a m e t e r s which m a y help to confirm t h a t the physiology of the enclosed p l a n t part is working as u n d e r n a t u r a l conditions. In this context c o n t i n u o u s m e a s u r e m e n t s of p h o t o s y n t h e t i c CO2 assimilation as well as H 2 0 t r a n s p i r a t i o n are needed to discuss the physiological activity of the enclosed living twig. O t h e r micrometeorological data, for example humidities a n d temperatures inside the cuvette, are used to calculate s t o m a t a l c o n d u c t a n c e a n d thus give insight into the correlations between emission rates a n d s t o m a t a l opening. Figure 1 shows the environm e n t a l p a r a m e t e r s light (measured P h o t o s y n t h e t i c Active R a d i a t i o n in /~mol p h o t o n s m - 2 s - 1; 4 0 0 - 7 0 0 nm), air a n d leaf t e m p e r a t u r e a n d relative humidity. Light was m e a s u r e d outside o n the t o p of the cuvette. T e m p e r a t u r e a n d h u m i d i t y sensors were placed inside a n d outside the cuvette. The d a t a reflect that the cuvette a n d the enclosed oak b r a n c h were in the full light until 12:45 h during the experiments. The values of the relative h u m i d i t y inside a n d outside give a first insight into the t r a n s p i r a t i o n p a t t e r n of the enclosed branch. The highest relative h u m i d i t y of the air p u m p e d into the cuvette was found d u r i n g night h o u r s a r o u n d 90%. T h e relative h u m i d i t y decreased to below 30% o n 8 J u n e d u r i n g the early afternoon. The relative h u m i d i t y inside the cuvette (measured at

1844

J. KESSELMEIER et al.

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the outlet) was lower than ambient during night and mostly higher over the day, reflecting the transpiration of the oak branch during daytime. For the determination of the photosynthetic activities we measured the mixing ratio differences between the inlet and the outlet of the cuvette (Fig. 1). In addition, we followed the ambient CO2 concentration outside the cuvette. The diel fluctuations of the ambient concentrations show CO2 values around 380 ppmv during the day and oscillations between 400 and 480 ppmv during the night, reflecting the onset of dilution processes due to turbulent transport and mixing from aloft as well as CO2 consumption by the plant community in the light. The difference measurements reflect directly the respiration and the

photosynthetic CO2 assimilation, during night and day, respectively. As branches or leaves suffering from very low CO2 concentrations may show severe nonphysiological reactions (Mansfield and Caporn, 1990), it is of interest to see that the CO2 differences between cuvette inlet and outlet during maximum photosynthetic activity did not exceed - 50 ppmv. Thus, the lowest CO2 concentrations inside the cuvette were around 330 ppmv. This is well within an acceptable range for physiological conditions avoiding artefacts by the loss of stomatal control over water transpiration. Additionally, correlations between CO2 exchange and light intensity (data not shown) indicated that the system was light saturated around 600/tmol m - 2 s - 1 (PAR). Additionally, from such data a light

Emission of monoterpenes and isoprene ~

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terion for the stomatal aperture. All data show a sharp rise in the morning and a decrease due to high temperatures as well as shading in the early afternoon. These data give reason to expect an early decrease of those trace gas emissions being under stomatal control.

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Emission of monoterpenes and isoprene In contrast to other oak species, Quercus ilex pro-

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ved to be a monoterpene emitter. The composition of the emitted species is obtained after identification by GC/MS in Fig. 3. Figure 3a shows a typical H R G C - M S profile obtained at the outlet of the cuvette where a branch of Quercus ilex was enclosed. It was obtained by recording the total ion current (TIC) from m/z 20-300. The main components identified are also listed. By subtracting the composition of the air entering the cuvette, the net emission of Quereus ilex was determined. It was found to be comprised mostly of monoterpene hydrocarbons. To quantify them, reconstructed mass chromatography carried out by plotting the ion at m/z 93 was used. Figure 3b reports the monoterpene profile obtained by treating the TIC profile of Fig. 3a. Subtraction techniques were used to distinguish coeluted components (Ciccioli et al., 1992). Examples of the approach followed are reported in Fig. 4a and b. The interference caused by 6-methyl-5-hepten-2-one on sabinene was evaluated by plotting the ion at m/z 111. By subtracting the contribution to m/z 93 arising from the fragmentation of the carbonyl component, the correct amount of sabinene was measured. A similar approach made possible the quantification of fl-phellandrene in the presence of limonene and 1,8-cineole. In this case,

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Fig. 2. Diel patterns of apparent CO2 exchange, transpiration and calculated stomatal conductance of the enclosed oak branch given as 10 min averages from 6 June to 11 June. Data on a leaf area basis.

compensation point for the CO2 exchange was found to range between 50 and 200 #mol m - 2 s - ~ (PAR). At higher light intensities, > 1 5 0 0 # m o l m - 2 s - t , we found some evidence of a decrease of the photosynthetic activity which we assumed to be a response of stomatal closure due to high temperatures around noon. This effect was observed before the cuvette was shaded, thus indicating the start of the so-called midday depression. Figure 2 shows the actual CO2 exchange data in terms of assimilation from 6 June to 11 June in comparison with the measured transpiration and the calculated stomatal conductance as a cri-

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mass chromatograms obtained by plotting the ions at m/z 68 and 154 allowed the independent quantification of the last two components. The amount of flphellandrene in the emission was then obtained by subtracting the contributions of limonene and 1,8cineole to the peak recorded at m/z 93. By using these techniques, the percent distribution of isoprene and monoterpene emission shown in Table 1 was determined. As can be seen from this table, the distribution was dominated by c~-pinene that accounted for more than 30% of the olefin emission. A low production of isoprene was measured (0.07%) although traps were collected during a time of the day when maximum photosynthetic activity occurred.

The emission rates of a-pinene and/~-pinene from the enclosed oak branch in comparison with light (PAR), leaf temperature, assimilation and transpiration over the range of 3 d are given in Fig. 5. A close relation between terpene emission and light-triggered physiological activities, i.e. assimilation and transpiration, was always found. This close correlation leads to fluctuations of the emission rates within short time intervals, as seen for example on 9 June between 11:00 and 12:00. Furthermore, the data set shows quite significant differences of the maxima emitted on cloudy and sunny days around noon (see Table 2). The emissions reached maxima between 13 and 28 (Fg/g d.w. h) on 6, 8 and 10 June (sunny days) but only

Emission of monoterpenes and isoprene Table 1. Composition of Quercus ilex emission measured from 11 a.m. to 1 p.m. Absolute (percent)

Relative (to ~-pinene)

0.07 0.15 1.84 33.46 2.70 10.13 22.92 2.25 0.79 1.93 4.56 8.54 0.54 3.32 3.52 1.30 0.40 1.66

0.20 0.46 5.49 100.00 8.06 30.28 68.49 6.74 2.37 5.77 13.64 25.51 1.61 9.92 10.51 3.88 1.21 4.96

Isoprene Tricyclene ct-Tujene a-Pinene Camphene Sabinene fl-Pinene Myrcene • -Phellandrene ct-Terpinene p-Cymene 1,8-Cineole fl-PheUandrene Limonene 7-Terpinene ~t-Terpinolene Linalool • -Terpineol

5-6 (pg/g d.w. h) on the cloudy day 9 June. The systematic decrease in emission as seen between earlier and later samples (Table 2) on sunny days is caused by the decrease of physiological activities including the be-

36

%

1847

ginning of stomatal closure due to high light and temperature. Within our cuvette, which was shaded from the early afternoon and thus allowed to discern between light and temperature effects, it is obvious that there was no simple correlation between leaf temperature and terpene emission. This is even more convincing as the diel emission pattern is confirmed by measurements of different groups, with different sampling systems and analysis techniques. The constancy of the emission rate data in a close line following principally the up and down of the assimilation rate may even be recognised as a successful intercomparison experiment under field conditions. Based on the data presented by Fig. 5 we may try to discuss the driving force for the emission of the monoterpenes. All monoterpenes behaved in the same way as ~t-pinene and fl-pinene (data not shown), showing a close relation to assimilation, light, transpiration and calculated stomatal conductance. For the partly cloudy 9 June we correlated the emission rates with several physiological parameters. Table 3 shows these correlations in terms of R 2 for the linear regression between ~-pinene emissions and terms like transpiration, PAR, stomatal conductance, assimilation and leaf temperature. The data show a quite high correlation between transpiration and pinene emission. Other parameters like PAR,

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Fig. 5. Emission of ct- and fl-pinene by an enclosed branch of Quercus ilex L. at the height of 6 m (southeast edge of the tree) during three days in June 1994. Emission rates are compared with photosynthetic active radiation (PAR), transpiration, assimilation and leaf temperature. Measurements were done by different groups as indicated: E = Ecole Nationale Superieure de Chimie de Toulouse, France; C = CNR, Istituto Inquinamento Atmosferico, Monterotondo Scalo, Italy; G = Institut Universitaire de Technologic, GRECA, Universit6 Joseph Fourier Grenoble, France. Note the different scaling for the terpene emissions on the cloudy day 9 June.

1848

J. KESSELMEIER et al. Table 2. Emission rates of monoterpenes from Quercus ilex in June 1993 on a dry weight basis (d.w.);noon data between 11:15 and 12:30on two sunny days and one cloudy day

Date

Time

PAR (#mol m - z s- 1)

6 June 6 June 8 June 8 June 9 June 9 June 10 June 10 June

11:45 12:30 12:00 12:30 11:15 11:30 11:30 12:15

1682 1678 1701 1612 246 226 1708 1597

Leaf temperature (°C)

Sum of six main monoterpenes (~g/g d.w. h)

30.0 29.6 34.8 34.8 27.4 27.9 31.8 31.5

28.0 15.7 22.8 12.7 4.7 6.2 27.8 15.9

Note that the 12:30and 12:15data on 6, 8 and 10 June are already influencedby stomatal closure due to high temperatures, whereas the low emission data on 9 June were observed on a cloudy day.

Table 3. Correlations (R2) between ct-pineneemissions and physiological parameters for the data obtained on 9 June 1994 Correlation to Transpiration PAR Stomatal conductance Assimilation Leaf temperature

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0.85 0.73 0.68 0.63 0.39

conductance and assimilation, however, are acceptable for a field experiment with mixtures of controlling factors. Leaf temperature, however, does not seem to play a significant role in the regulation of monoterpene emissions from Quercus ilex in June. This behaviour allows us to conclude that light with its effect on assimilation and stomatal behaviour is the most important factor for monoterpene emission by this oak species. This interpretation is in close agreement with Schiirmann (1993) and Steinbrecher et al. (1993), who presented evidence that the terpene emission by Picea abies is triggered not only by temperature but also by light. In the case of Quercus ilex we expect the light effect to be of even greater importance than for Picea abies. As outlined in Fig. 6 we may discern between two types of conditions to be fulfilled prior to an emission of trace gases by higher plants. Either the amount which is emitted has to be produced in close temporal relation to the emission (Type A) or there is already a pool of the trace gases inside or outside of the plant leaf tissue to be emitted (Type B) when it is just triggered by environmental parameters. This concept (Type B) is in close accordance with the model of Lerdau (1991) which represents the basic idea for a temperature-dependent parameterisation of terpene fluxes (Tingey et al., 1991; Guenter et al., 1993). For Type B the temperature should be nearly exclusively the dominant factor of influence. However, if the pool of the trace gas is inside the plant tissue, for example resin pools inside needles, then this type of emission may also even be regulated by stomatal aperture. For

Temperature Assimilation

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.

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Fig. 6. Relationships between environmental factors, physiological parameters and trace gas emission by plants discussed for plants with an actual production of the emitted trace gases mainly triggered by light (Type A) as well as for plants with storage pools mainly triggered by temperature (Type B).

Type A the light should be the dominant factor as this is the energy delivery system for the production of trace gases to be emitted. Without stored trace gases the emission should be closely linked to the production. However, the system is quite complex as light not only triggers production pathways but additionally is also one of the main regulators for stomatal aperture in addition to CO2 and the water potential inside the leaf tissue (see Kesselmeier, 1992).

CONCLUSIONS Our field data obtained with 30 yr old oak trees show high emissions of monoterpenes from Mediterranean oak species. Thus, there is increasing evidence of a species-dependent emission pattern of monoterpenes and isoprene. Even within the genus oak it is obvious that we cannot simply transfer emission pattern (and quantity) from one oak species to another. Oak species other than Quercus ilex are known to be

Emission of monoterpenes and isoprene isoprene emitters (see Tingey et al., 1991; Sanadze, 1991; Sharkey et al., 1991), whereas this Mediterranean oak species is a significant monoterpene emitter as already shown under laboratory conditions by Pio et al. (1993) and Staudt et al. (1993). We fully agree with Tingey et al. (1!991) that we have to be cautious in extrapolating from one species or genus to another. As shown by the investigations of Quercus ilex, this is true not only for the extrapolation of the a m o u n t of emission but also for the quality of the emission pattern, i.e. isoprene or monoterpenes. Whereas Tingey et al. (1980) report that monoterpene emissions from Pinus elliotii are unaffected by light intensities between 0 and 800/tmol m - 2 s - 1 and Evans et al. (1985) found only a slight increase under the influence of increasing light intensities between 400 and 1000 # m o l m -2 s - 1, Steinbrecher et al. (1993) report a significant dependence. Similarly, isoprene emissions are triggered by light intensities and even influenced by the spectral quality (Sanadze, 1991). O u r actual data giw; reason to believe that the monoterpene emissions from the green leaves of Quercus ilex are significantly influenced by light intensities. Temperature alone seems not to be the dominant factor. Thus, the light-triggered productivity can be assumed to be the backbone for this emission type. The full complex behaviour of the regulations of terpene and isoprene emissions is only understood if we know how production and emission are linked together. For this we need a better understanding of the physiological background for the emission of these trace gases as already discussed for other trace gases by Kesselmeier (1991). Acknowledgements--We acknowledge the financial support from the Commission of the European Communities (DG Xii/D-1 and JRC Environment Institute, Ispra) and of our colleagues from the University of Rome (La Sapienza), the CNR (Istituto Inquinamento Atmosferico) and the EC Joint Research Centre Ispra. In particular, we want to express our gratitude for the coordinating and logistic support provided by the colleagues from 1:heJRC (Ispra), the CNR (Rome) and the University "La Sapienza", Dipartimento Biologia Vegetale (Rome). We are grateful to the government of the Italian Republic for pelTnission to conduct our investigation on the Castel Porziano site and would like to express our special thanks to the director of Castel Porziano, Dr G. Emiliano and Ing. A. Tinelli for their help and interest. Last but not least, we thank H. Miiller and D. Weller for their enthusiastic support.

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