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1 CREAF (Centre de Recerca Ecològica i Aplicacions Forestals), Facultat de Ciències, Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona), Catalonia, Spain
Received for publication October 20, 1998. Accepted for publication April 13, 1999.
ABSTRACT
The seasonal pattern of terpene content and emission by seven Mediterranean woody species was studied under field conditions. Emission rates were normalized at 30°C and 1000 µmol·m·s PFD (photosynthetic photon flux density). Bupleurum fruticosum, Pinus halepensis, and Cistus albidus stored large amounts of terpenes (0.01–1.77% [dry matter]) with maximum values in autumn and minimum values in spring. They emitted large amounts of terpenes (2–40 µg·g DM·h), but with no clear seasonal trend except for Cistus albidus, which had maximum values in spring and minimum values in autumn. The nonstoring species Arbutus unedo, Erica arborea, Quercus coccifera and Quercus ilex also emitted large amounts of terpenes (0–40 µg·g DM·h) and also tended to present maximum emission rates in spring, although this trend was significant only for A. unedo. At the seasonal scale, emission rates did not follow changes in photosynthetic rates; instead, they mostly followed changes in temperature. From autumn to spring, the least volatile monoterpenes such as limonene were emitted at highest rates, whereas the most volatile monoterpenes such as 
-pinene and ß-pinene were the most emitted in summer. The monoterpene emission rates represented a greater percentage of the photosynthetic carbon fixation in summer (from 0.51% in Arbutus unedo to 5.64% in Quercus coccifera) than in the rest of the seasons. All these seasonality trends must be considered when inventorying and modeling annual emission rates in Mediterranean ecosystems.
Key Words: -pinene • limonene • Mediterranean woody species • photosynthetic rate • relative humidity • seasonality • temperature • terpene emission and storage • volatility
Much of the volatile organic compounds (VOCs) in the atmosphere comes from plants (Simpson et al., 1995
; Seufert, 1997
). These VOCs have an important role in atmospheric chemistry (Singh and Zimmerman, 1992
; Lerdau and Peñuelas, 1993
) particularly in the development of aerosols (Andreae and Crutzen, 1997
) and in ecological relationships with other plants and animals (Takabayashi, Dicke, and Posthumus, 1994
; Peñuelas, Llusià, and Estiarte, 1995
; Llusià, Estiarte, and Peñuelas, 1997
). Their production and emission rates are conditioned by biotic and abiotic factors (Langenheim, 1994
; Takabayashi, Dicke, and Posthumus, 1994
; Peñuelas and Llusià, 1998
). Among the abiotic factors, temperature (Seufert, 1997
; Peñuelas and Llusià, 1999b
), irradiance (Staudt and Seufert, 1995
; Loreto et al., 1996
; Seufert, 1997
), and water availability (Bertin and Staudt, 1996
; Peñuelas and Llusià, 1997
; Llusià and Peñuelas, 1998
; Peñuelas and Llusià, 1999b
) are important. They affect fluxes by altering the vapor pressure of the VOCs or by changing the diffusive resistance of the leaves, the two factors governing emissions. The abovementioned abiotic factors need to be specially considered in Mediterranean conditions, which are characterized by a marked seasonality and a long dry summer with low precipitation coinciding with high irradiance and high temperature (Di Castri, 1973
). These factors may both favor and/or hinder emissions by a vegetation specially rich in terpenes (Seufert et al., 1995
).
Not all plants that emit terpenes necessarily store them in specialized structures (Staudt et al., 1993
; Seufert et al., 1995
; Loreto et al., 1996
; Llusià and Peñuelas, 1998
). The pattern of terpene emission from plants that do not store terpenes in specialized structures may be different from that of plants having specialized structures for their storage (Lerdau, 1991
; Lerdau et al., 1995
; Seufert et al., 1995
; Loreto et al., 1996
). In terpene-storing species, pool size in resin ducts and internal or external glands affects the emission rates, and it can be expected that the short-term response of monoterpene emission rates to photosynthetic photon flux density (PFD) and photosynthetic rates could be stronger and faster in nonstoring species than in storing species (Staudt and Seufert, 1995
). In fact, both light and temperature are known to be involved in the short-term control of Q. ilex emissions (Staudt and Seufert, 1995
; Kesselmeier et al., 1996
; Loreto et al., 1996
), but less is known about what happens at the longer term seasonal scale.
In this work we studied and compared the seasonal patterns of terpene content and emission rates of three terpene-storing (Bupleurum fruticosum, Cistus albidus, and Pinus halepensis) and four nonstoring (Arbutus unedo, Erica arborea, Quercus coccifera, and Quercus ilex) woody Mediterranean plant species in field conditions during the course of a year in Collserola, a natural park of Barcelona (Catalonia, northeast Spain). To study the seasonal trends we normalized emission rates at 30°C and at 1000 µmol·m-2·s-1 PFD by using the algorithms developed by Tingey et al. (1980)
for emissions from terpene pools and Guenther et al. (1993, 1995)
for emissions under control of temperature and light. We also studied the possible different seasonal emission patterns of individual terpenes depending on their different volatility, as suggested by our previous work on summer terpene emissions by Quercus ilex (Peñuelas and Llusià, 1999b
), and emission rate relationships with seasonal changes in temperature, relative humidity, and photosynthetic rates.
MATERIALS AND METHODS
Experimental set-up, site conditions, and species studied
The experiment was performed in a maquis located at 350 m altitude in Barcelona, Collserola natural park, (central Catalonia, northeast Spain, 41°27' N, 2°7.7' E). The dominant species are 1–3 m tall Arbutus unedo L., Bupleurum fruticosum L., Cistus albidus L., Erica arborea L., Pinus halepensis L., Quercus coccifera L., and Quercus ilex. The climate is typical Mediterranean, with cool winters and warm, dry summers, with a mean annual temperature of 14.5°C and a mean annual precipitation of 610 mm.
The measurements were conducted on 6 –7 November 1996, 11–12 February 24–25 April, and 25–26 July 1997. Measurements were made on the last 8-cm of twigs with several (4–6 for most species) sun-exposed, apparently healthy current-year leaves, facing southward at 1.5–2 m aboveground. We sampled three twigs (from different plants) per species. The twigs included fully current-year extended leaves but also young developing once after they were initiated in spring. At all dates, leaf gas exchange, air temperature, and relative humidity were measured between 0815 and 1130 (solar time). Measurements were conducted in sunny days and PFD values inside the measuring chamber ranged between 900 and 1400 µmol·m-2·s-1. Immediately after gas exchange and terpene sampling, the leaves were cut and stored in a portable fridge at 4°C. Leaf area was measured in the laboratory using a LICOR LI-3100 area meter (LI-COR, Lincoln, Nebraska).
Gas exchange and terpenes sampling and analysis
Measurements of CO2 and H2O exchange and terpene sampling were conducted simultaneously. An IRGA porometer (LCA-4, ADC, Hoddeson, Hertfordshire, UK) was used for determination of CO2 and H2O exchange. A terminal twig with intact leaves (the last 8 cm) was clamped in a PLC-2 ADC cuvette of 90 cm3 connected to the ADC-LCA-4 porometer. Air coming out of the cuvette flowed through a T system to a glass tube (11.5 cm long and 0.4 cm internal diameter) manually filled with terpene adsorbents Carbotrap C (300 mg), Carbotrap B (200 mg), and Carbosieve S-III (125 mg) (Supelco, Bellefonte, Pennsylvania) separated by plugs of quartz wool. The hydrophobic properties of the tubes were supposed to minimize sample displacement by water. In these tubes terpenes did not suffer chemical transformations as checked with standards (-pinene, ß-pinene, camphene, myrcene, p-cymene, limonene, sabinene, camphor, and dodecane). Prior to use, these tubes were conditioned for 3 min at 350°C with a stream of purified helium. The sampling time was 5 min, and the flow varied between 100 and 200 mL/min depending on the glass tube adsorbent and quartz wool packing. The flow for each tube was determined with a bubble flowmeter. The trapping and desorption efficiency of liquid and volatilized standards such as -pinene, ß-pinene, or limonene was practically 100%. In order to eliminate the problem of memory effect of previous samples, blanks of 5-min air sampling without clamping twigs were carried out immediately before and after each measurement. The glass tubes were stored in a portable fridge at 4°C and taken to the laboratory. There, glass tubes were stored at -30°C until analysis (within 24–48 h). There were no observable changes in terpene concentrations after storage of the tubes as checked by analyzing replicate samples immediately and after 48-h storage. Emission rate calculations were made on a mass balance basis and by subtracting the control samples without twigs from the samples with twigs.
For extraction of leaf terpenes, the same leaves used to measure terpene emission were cut, submerged in liquid nitrogen, and taken to the laboratory where they were homogenized in Teflon tubes with pentane as the extractant with a Teflon embolus under liquid nitrogen. A nonterpenoid volatile internal standard, dodecane, was used to avoid interference with terpenes. It was added to the pentane extraction procedure before grinding in order to quantify recovery (it was >90%). The extract was stored at -30°C until analysis (within 48–72 h). The pentane extract (3 µL) was then injected directly to the GC-MS. The leaf pellet dry mass was determined after drying at 60°C until mass constancy.
Terpene analyses were conducted in a GC-MS (Hewlett Packard HP59822B, Palo Alto, California). Trapped emitted monoterpenes were desorbed (Thermal Desorption Unit, Model 890/891; Supelco, INC, Bellefonte, Pennsylvania) at 320°C during 3 min and injected into a 30 m x 0.25 mm x 0.25 mm film thickness capillary column (Supelco HP-5, crosslinked 5% pH Me silicone). After sample injection, the initial temperature (46°C) was increased at 30°C/min up to 70°C, and thereafter at 10°C/min up to 150°C, which was maintained for another 5 min. Helium flow was 1 mL/min. The identification of monoterpenes was conducted by GC-mass spectroscopy and comparison with standards from Fluka (Chemie AG, Buchs, Switzerland), literature spectra, and GCD Chemstation G1074A HP. Internal standard dodecane, which did not mask any terpene, together with frequent calibration with common terpene standards (-pinene, ß-pinene, 3-carene, ß-myrcene, p-cymene, limonene, and sabinene) once every three analyses were used for quantification. Terpene calibration curves (N = 5 different terpene concentrations) were always highly significant (r2 > 0.97) in the relationship between signal and terpene concentration. The most abundant terpenes had very similar sensitivity (differences were 5%). For most of the measurements, these techniques showed a good reproducibility in the measured emission rates and relative composition of terpenes produced by different leaves of the same species. Repeated measurements of different leaves of the same plant showed the same individual terpenes and usually less than 10% variation of mean values.
The algorithms and coefficients of Tingey et al. (1980)
and Guenther et al. (1993, 1995)
were used to normalize terpene emission rates at 30°C and 1000 µmol·m-2·s-1 PFD. For terpene-storing species we used the algorithm
 |
 |
= 0.0027, cL1 = 1.066, cT1 = 95 000 J/mol, cT2 = 230 000 J/mol, and TM = 314 K.
Statistical analyses
Statistical analyses of variance (ANOVAs), and regression were conducted using STATISTICA v. 5.0 for Windows (StatSoft, Inc. Tulsa, Oklahoma, 1996). Three plants per species were measured each season. They were not the same in each season. ANOVAs with season as independent factor were conducted for all studied species together and for each one separately and also for total terpenes and for each one separately. Data were log transformed when necessary to accomplish normal distribution requirements.
RESULTS
Seasonal storage of terpenes
Bupleurum fruticosum, Pinus halepensis, and Cistus albidus stored large amounts of terpenes with maximum values in autumn and minimum values in spring both for total terpenes (Fig. 1) and most individual terpenes (Fig. 2).
|
|
-pinene and ß-pinene (Fig. 2). In Cistus albidus the maximal terpene storage was only 0.25 mg/g DM and the minimum 0.1 mg/g DM (Fig. 1). Its most abundant terpene was caryophyllene (Fig. 2). Finally, in Pinus halepensis the maximum terpene storage was 10 mg/g DM and the minimum 3.6 mg/g DM (Fig. 1). Its most abundant monoterpene was -pinene, closely followed by ß-myrcene and ß-pinene and the sesquiterpenes caryophyllene and -caryopyllene (Fig. 2).
Seasonal emission of terpenes
Terpene storing plants
Bupleurum fruticosum, Cistus albidus, and Pinus halepensis emitted large amounts of terpenes (maxima of 45, 32.8, and 11 µg terpenes·g DM·h-1, respectively). Emitted terpenes were mostly the same ones as stored terpenes except for C. albidus, which emitted a greater number of detectable terpenes than it stored (Fig. 2). For C. albidus, a significant seasonal effect (P < 0.01, ANOVA) was found with maximum rates in spring and minimum rates in autumn. Bupleurum fruticosum and Pinus halepensis had no clear seasonal trend (Fig. 1).
Bupleurm fruticosum had its maximum emission rates in summer, and its minimum ones (11 µg monoterpenes·g DM-1·h-1) in winter (Fig. 1). In summer, -pinene was the terpene most emitted followed by ß-pinene and limonene, whereas limonene was the most emitted in spring, autumn, and winter. Cistus albidus had its maximum emission rates in spring and its minimum ones (2 µg monoterpenes·g DM-1·h-1) in autumn (Fig. 1). Caryophyllene and limonene were the terpenes most emitted (Fig. 2). Pinus halepensis had its maximum emission rates in autumn and its minimum ones (2.4 µg monoterpenes·g DM-1·h-1) in winter (Fig. 1). In summer, 
3-carene was the monoterpene most emitted, followed by ß-myrcene and -pinene. In the other seasons, limonene was the monoterpene most emitted.
Terpene nonstoring plants
The other four species studied, Arbutus unedo, Erica arborea, Quercus coccifera, and Quercus ilex did not store detectable amounts of terpenes in specialized structures, but emitted significant amounts (Fig. 3), with maximum rates in spring for A. unedo (43 µg·g DM-1 h-1), E. arborea (17.5 µg·g DM-1·h-1) and Q. ilex (11 µg·g DM-1·h-1) and in autumn for Q. coccifera (5.5 µg·g DM-1·h-1).
|
3-carene and limonene were the monoterpenes most emitted by A. unedo, -pinene, ß-pinene, ß-myrcene and limonene were similarly emitted by E. arborea, -pinene was the most emitted by Q. coccifera, and limonene the most emitted by Q. ilex, closely followed by - and ß-pinene (Fig. 4). In autumn, ß-myrcene was the most emitted by E. arborea, limonene the most emitted monoterpene by Q. coccifera, and -pinene the most emitted by Q. ilex (Fig. 4). In winter ß-myrcene was the most emitted by A. unedo, (+)2-carene the terpene most emitted by E. arborea, limonene the most emitted by Q. coccifera, and - and ß-pinene the ones most emitted by Q. ilex. Minimum emission rates were found in autumn except for Q. coccifera, which had them in spring (Fig. 3). However, a clearly significant seasonal effect (P < 0.01, ANOVA) was only found in A. unedo (Fig. 3). Most volatile monoterpenes had their maximum emission rates in summer, whereas least volatile monoterpenes had them in spring. There was thus no significantly different seasonal pattern of emission between storing and nonstoring species.
|
-pinene, ß-pinene, myrcene, and 3-carene were log linearly related with temperature, whereas the emission rates of the least volatile, phellandrene and limonene, were not (Fig. 5). The slopes of their log-linear relationships presented a decreasing trend with decreasing volatility except for 3-carene (Fig. 5).
|
|
In general, the stored and emitted amounts of terpenes for the different species are in the range previously reported in the literature (Owen et al., 1997
; Staudt et al., 1997
). However, for some species they are much larger than previously reported. For example, Owen et al. (1997)
found only 0.114 µg·g DM-1·h-1 for A. unedo. The amounts reported here also agree with our previous results of content ranging from 1.5 to 3 mg/g DM-1 and emission rates ranging from 4 to 86 µg·g DM-1·h-1 in these and other woody Mediterranean species in a study we conducted on potted Mediterranean woody plants in spring-summer period (Llusià and Peñuelas, 1998
). All these results show an outstanding emission of limonene by these woody species in contrast with results from other Mediterranean regions such as southern France or northern Italy (Seufert, 1997
), indicating possible important population-level differences in terpene emission.
Although there were slight differences among species, storage had its maximum in the autumn-winter period, whereas emission patterns differed among species, but maximum values were usually found in the spring-summer period (Figs. 1–4). These results expressed after normalizing emission to standard temperature and PFD conditions (30°C, 1000 µmol·m-2·s-1 PFD) add new evidence on the importance of seasonal patterns in terpene storage and emission in Mediterranean plants (Peñuelas and Llusià, 1997
; Seufert, 1997
; Staudt et al., 1997
). A seasonal induction of monoterpene emissions could be linked to leaf development, which entails changes in hardening of the cuticle and in dilution of terpene concentrations, and therefore may alter both diffusive resistance and terpene vapor pressure.
Dement, Tyson, and Mooney (1975)
and Tingey et al. (1980)
found that monoterpene emissions by storing species such as Salvia mellifera or pines were controlled by pool size, vapor pressure of terpenes, and emission pathway. This suggested that only monoterpenes with a sufficient pool size and with appreciable vapor pressures at ambient temperatures would be emitted in significant amounts to the atmosphere. However, there were no clear differences in seasonal patterns of emission between terpene-storing species and nonstoring species. This does not preclude slight differences at the daily scale, where emission of storing species seems to depend more on temperature, whereas emission of nonstoring species seems to depend more on PFD and photosynthetic rates (Peñuelas and Llusià, 1999a
) that alter the production of monoterpenes and so affect their vapor pressure according to a concentration-dependent relationship.
These results show larger emission rates of most volatile terpenes such as -pinene and ß-pinene in summer and larger emission rates of least volatile terpenes such as limonene in spring. Limonene was also reported to be emitted more in spring than in summer in Pinus pinea (Staudt et al., 1997
). This is in agreement with our previous results showing that in the annual field mediterranean temperature range, the most volatile terpenes are more responsive on temperature than least volatile terpenes, and that the latter are more responsive to PFD and photosynthetic rates (Peñuelas and Llusià, 1999a
). Over this temperature range, the terpenes with higher vapor pressure were in a steeper part of their exponential relationship than those with lower vapor pressures.
In this study, log-linear increases of emission rates with temperature presented similar relationships and slopes to those reported by other authors both for individual monoterpenes and for the sum of all monoterpenes and as expected from temperature effects on terpenes vapor pressure (Tingey et al., 1980
). Most species had their maximum emission in spring. They had a decrease in their emission rates in summer in spite of the higher temperatures. These reductions of emission rates in summer may be due to high temperatures above the optimum as assumed by the light and temperature algorithm of Guenther et al. (1993)
. Moreover, Staudt and Seufert (1995)
have found this optimum to be shifted from 40°C to 35°C with a sharp negative effect of temperature above 35°C in Q. ilex, and summer temperatures were usually above this threshold in our study. The summer reductions may also be linked to the low physiological activity due to water limitation (Larcher, 1995
) and to increased vapor pressure deficit (VPD). The lack of carbon substrate and/or ATP and the decreased permeability (increased terpene diffusive resistance) of the cuticle to gas exchange may explain this effect of water limitation and high VPD (Bertin and Staudt, 1996
; Llusià and Peñuelas, 1998
; Peñuelas and Llusià, 1999a, b
).
Contrary to short-term daily patterns (Loreto et al., 1996
; Peñuelas and Llusià, 1999a, b
), photosynthetic rates were not related with terpene emission rates when considering the seasonal changes throughout the whole year. The increase of percentage of C emitted relative to C fixed from autumn to summer agrees with results of Sharkey and Loreto (1993)
, which showed that in kudzu [Pueraria lobata (Willd) Ohwi.] the percentage of C fixed in photosynthesis that was reemitted as isoprene increased after increased PFD and temperature. These authors speculate that isoprene emission may help plants to cope with stressful conditions. The high percentages found here in summer, from 0.51% in A. unedo to 5.64% in Q. coccifera, also suggest a possible role in coping with stressful conditions produced by elevated temperatures and irradiances. In Pinus pinea Staudt et al. (1997)
also found that whereas emitted relative to assimilated C ranged from 0.10 to 0.26% in May and October, it ranged from 2.5 to 7.6% in August. Moreover, other reports show that between 5 and 40% of fixed carbon may be allocated into the biosynthesis of essential oils in extreme conditions (Ross and Sombrero, 1991
).
Monoterpene emissions do not seem to be limited by stomatal resistance or stomatal opening (Bertin and Staudt, 1996
; Loreto et al., 1996
) because the intercellular air spaces of the leaf are not saturated (Fall and Monson, 1992
). In stressful conditions, when stomata are closed, the higher diffusive resistance is compensated for by an increase of the intercellular monoterpene concentration until a new equilibrium is reached, where the new efflux returns to its previous rate (Tingey et al., 1980
; Bertin and Staudt, 1996
).
Any parameter studied alone is insufficient to understand controls over terpene vapor pressure and diffusive resistance and thus changes in emissions. Emissions did not fully follow content amount, nor temperature, nor relative humidity, nor photosynthetic rates, suggesting that a combination of these factors and other internal (enzyme activity or leaf development for example) or external factors (long-term drought exposure that limits substrate availability, for example) could be involved in the emission control. The relative importance of all these factors is very difficult to evaluate from our data since most of these factors covary and are intimately related. However, the seasonality in terpene content and emission shows that models based on short-term response of emissions to temperature, relative humidity, and/or photosynthetic rates cannot always be used alone at an annual scale. The results presented here may allow the improvement of such models and the knowledge of biogenic gas emissions by the particular vegetation of this region (Seufert et al., 1995
; Seufert, 1997
). The improvement in knowledge of seasonal emission rates might also help to predict the behavior of ozone and aerosol formation, especially relevant in the Mediterranean area, where biogenic emissions are now recognized as one of the major sources of tropospheric ozone in Europe (Seufert, 1997
). It might also help in the study of flammability of Mediterranean vegetation, also an especially relevant issue in Mediterranean ecology (Terradas, 1996
).
FOOTNOTES
1 The authors acknowledge grant CL197-0344 from the CICYT (Spain), several grants from the CIRIT (Catalonia), a F.P.I. (Spain) fellowship to J.LL, and CARBUROS METÁLICOS, SA for continuous support to this research. 
2
Author for correspondence, current address: CREAF (Centre de Recerca Ecològica i Aplicacions Forestals), Facultat de Ciències, Universitat Autònoma de Barcelona, 08193 Bellaterra (Barcelona), Catalonia, Spain (Tel.: 34.93.5812934, FAX: 34.93.5811312, e-mail: j.Llusia{at}creaf.uab.es
).
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Arbutus unedo; • Bupleurum fruticosum; 
Cistus albidus; 
Erica arborea; 
Pinus halepensis; 
Quercus coccifera; 
Quercus ilex.