The effect of ozone on the emission of carbonyls from leaves of adult Fagus sylvatica

Plant, Cell and Environment

(2005)

28

, 603–611

© 2005 Blackwell Publishing Ltd

603

Blackwell Science, LtdOxford, UKPCEPlant, Cell and Environment0016-8025Blackwell Science Ltd 2005? 2005

28?603611Original Article

Carbonyl exchange by adult beechC. Cojocariu

et al.

Correspondence: Dr Jürgen Kreuzwieser. Fax:

+

49 0761203 8302; e-mail: [email protected]

The effect of ozone on the emission of carbonyls from leaves of adult

Fagus sylvatica

CRISTIAN COJOCARIU

1

, PETER ESCHER

1

, KARL-HEINZ HÄBERLE

2

, RAINER MATYSSEK

2

, HEINZ RENNENBERG

1

& JÜRGEN KREUZWIESER

1

1

Institute of Forest Botany and Tree Physiology, Albert-Ludwigs-Universität Freiburg, Georges-Köhler-Allee, Geb. 053/054, D-79110 Freiburg i. Br., Germany and

2

Department of Ecology/Ecophysiology of Plants, Technische Universität München, Am Hochanger 13, D-85354 Freising, Germany

ABSTRACT

Under the site conditions of a temperate forest, theexchange of short-chained oxygenated carbonyls (alde-hydes, ketones) was assessed from leaves of adult Europeanbeech trees. The crowns of the trees were either exposed toan elevated O

3

regime as released by a free-air fumigationsystem (2 ¥¥¥¥

O

3

) or to the unchanged O

3

regime at the site(1 ¥¥¥¥

O

3

, ‘control’). Daily fluctuations of oxygenated carbo-nyls were quantified in relation to environmental and phys-iological factors. In particular, the effect of O

3

on carbonylexchange was studied. Measurements of leaf gas exchangewere performed with a dynamic cuvette system, and carbo-nyl fluxes were determined using 2,4-dinitrophenylhydra-zine (DNPH)-coated silica gel cartridges. Leaves mainlyemitted acetaldehyde, formaldehyde and acetone. Acetal-dehyde dominated the emissions, amounting up to100 nmol m----

2

min----

1

, followed by formaldehyde (approxi-mately 80 nmol m----

2

min----

1

) and acetone (approximately60 nmol m----

2

min----

1

). Carbonyl emissions were highest dur-ing midday and significantly lowered at night, irrespectiveof the O

3

exposure regime. Trees exposed to 2 ¥¥¥¥

O

3

emittedacetaldehyde and acetone at enhanced rates. The findingsare of particular significance for future climate change sce-narios that assume increased O

3

levels.

Key-words

:

Fagus sylvatica

; beech; carbonyl emission;tropospheric ozone.

INTRODUCTION

Vegetation contributes about 90% to the global emissionof volatile organic compounds (VOC) into the atmosphere(Kesselmeier & Staudt 1999). Particularly trees emit a largequantity and variety of different hydrocarbons includingnumerous oxygenated compounds, primarily aldehydes,ketones, organic acids and alcohols (Graedel 1979; Fehsen-feld

et al

. 1992). Among oxygenated VOCs, short-chainedcarbonyls play an important role in atmospheric chemistry

as they strongly affect the oxidative capacity and the pro-duction of tropospheric O

3

and peroxyacylnitrates (PANs)that both perturb plant growth and human health (Kotzias,Konidari & Sparta 1997; Sakaki 1998). VOC emission ratescan vary considerably depending on plant species and envi-ronmental conditions such as temperature, solar radiationand other stress (wounding, drought, impact of SO

2

or O

3

;Kimmerer & Kozlowski 1982; Kesselmeier & Staudt 1999).The majority of studies in Europe on the emission of VOCincluding carbonyls have been performed with Mediterra-nean tree species (e.g. Kesselmeier

et al

. 1997). However,to improve regional models of VOC exchange, it is impor-tant to clarify the involvement of dominating tree species.For this purpose, knowledge on VOC exchange from spe-cies of temperate forest ecosystems is urgently required, aslittle is known about the carbonyl emission by CentralEuropean key species. One of these species is Europeanbeech (

Fagus sylvatica

L.) that dominates the potential nat-ural vegetation (Ellenberg 1986) and will preferentially becultivated in the future (e.g. Hahn 1995; Schraml & Volz2004). It is therefore necessary to quantify the carbonylemissions from this tree species and to elucidate environ-mental and physiological influences.

Global climate change scenarios assume increasingatmospheric concentrations of trace gases such as CO

2

, CH

4

and N

2

O to cause increase in temperature and changes inprecipitation patterns and other meteorological parameters(Watson

et al

. 1990; Bretherton, Bryan & Woods 1990;Santer

et al

. 1996; Mickley

et al

. 2004). In addition, anincreased tropospheric O

3

concentrations towards 50% isprognosticated by the year 2100 (Fowler

et al

. 1999). Eventoday about 25% of the global forests are at risk of beingexposed to O

3

levels

>

60 p.p.b. during the growing seasonscausing negative effects, for example, on growth. Labora-tory studies of Kimmerer & Kozlowski (1982) have shownthat oxidative stress including elevated O

3

levels can triggerthe emission of VOC including acetaldehyde. In accor-dance, increased monoterpene emissions were found inScots pine (Heiden

et al

. 1999) and

Quercus ilex

(Loreto

et al

. 2004) due to elevated O

3

. Hence, the expectedincrease in tropospheric O

3

levels along with higher airtemperatures will likely raise VOC emissions.

604

C. Cojocariu

et al

.

© 2005 Blackwell Publishing Ltd,

Plant, Cell and Environment,

28,

603–611

The present study pursued two aspects: First, the spec-trum of carbonyl compounds exchanged between leaves ofadult European beech trees and the atmosphere was to beelucidated under the natural site conditions of a temperateforest, including factors that influence such exchange. Sec-ond, the particular influence of increased future O

3

levelswas assessed, making use of a free-air O

3

exposure system(Werner & Fabian 2002; Nunn

et al

. 2002) that wasemployed within the stand canopy.

MATERIALS AND METHODS

Field site description

Carbonyl emissions from leaves were measured duringAugust 2001 and 2002 in about 27-m-high trees of Euro-pean beech (

Fagus sylvatica

) that grew together with Nor-way spruce (

Picea abies

) in a mixed 55–60-year-old-standat ‘Kranzberger Forst’ near Freising (48

∞

25

¢

08

¢¢

N,11

∞

39

¢

41

¢¢

E, 485 m altitude; Pretzsch, Kahn & Grote1998). The beech trees were spread as dense groups of upto 50 individuals across the stand, which was dominated byspruce and formed a closed canopy. Scaffolding and aresearch crane (Matyssek & Häberle 2002) allowed accessto sun and shade crowns of beech and spruce trees. Theentire crowns of five spruce and beech trees each (totalvolume of about 2000 m

3

) were experimentally exposed toan enhanced O

3

regime by means of a free-air O

3

canopyexposure system (Werner & Fabian 2002; Nunn

et al

. 2002).O

3

was generated from oxygen-enriched air and spreadthrough a tubing system across the adjacent tree crowns.The experimental O

3

levels (2

¥

O

3

regime, through dou-bling of the unchanged ambient O

3

levels, 1

¥

O

3

, at thesite) were controlled on a half-hourly basis by a computer-ized feedback system as based on online O

3

analysis at sixsampling positions inside and outside the fumigation zone.Experimental maximum levels were restricted to 150 p.p.b.O

3

to prevent the risk of acute O

3

injury (cf. Reich 1987;Matyssek & Sandermann 2003). A corresponding neigh-bouring group of trees under the unchanged ambient O

3

concentrations of the forest site (1

¥

O

3

regime) served ascontrol (Nunn

et al

. 2002). The free-air O

3

fumigation hadbeen running throughout growing seasons since May 2000(for O

3

levels see Werner & Fabian 2002).

Gas-exchange measurements

Carbonyl emissions as well as the rates of photosynthesisand transpiration were determined on sun-exposed leavesof eight beech trees situated in the vicinity of the scaffold-ing at a height of about 27 m. Gas exchange measurementswere performed using the dynamic flow-through systemdescribed by Cojocariu, Kreuzwieser & Rennenberg(2004). The measuring system consisted of two identicalcuvettes of 0.5 L volume constructed of chemically inertperfluorethylenpropylen (FEP) plates and foils (200

m

m;Dyneon GmbH, Burgkirchen, Germany). About four tofive beech leaves were placed into one of the cuvettes

(‘plant cuvette’), whereas the other was kept empty andused as control. During the measurements the cuvetteswere flushed with ambient air at flow rates of 2–4 L min

-

1

(MAS, Kobold, Germany). Photosynthetic photon fluxdensity (PPFD; LI-190SA; Li-Cor Inc., Lincoln, NE, USA)was measured continuously outside the plant cuvette at leaflevel whereas air temperature and relative humidity (1400–104; Walz, Effeltrich, Germany) were determined in thecontrol cuvette. The concentrations of CO

2

, H

2

O (Li-6262:Li-Cor Inc.) and carbonyls were measured at the outlets ofboth cuvettes. Data were stored on a logbook (LogBook

®

Stand-Alone Data Acquisition system; IOtech

®

; SpectraComputersysteme GmbH, Echterdingen, Germany) con-nected to the system. Carbonyls were sampled for 60 minthrough 2,4-dinitrophenylhydrazine (DNPH)-coated silica-gel cartridges (Sigma, Munich, Germany) with flow rates of1 L min

-

1

. Carbonyl-hydrazones were eluted from the car-tridges with 2 mL acetonitril (ACN)/1 mL H

2

O. Aliquots of100

m

L of the eluate were injected into a high-performanceliquid chromatography system (Beckman, Munich, Ger-many). Hydrazones were separated by a reversed-phasecolumn (Octa-Decyl-Silicium column; C-18, 5

m

m,250

¥

4.6 mm; Supelco, Munich, Germany) using an ACN/water gradient (20–80% ACN, 0–20 min) at a flow rate of1 mL min

-

1

. They were detected by a diode array detector(Module 168; Beckman) at 354 nm. For identification andquantification external and internal standard solutions(Sigma-Aldrich, Munich, Germany) were used.

Fluxes of CO

2

, H

2

O and carbonyls were calculated fromthe concentration differences between plant cuvette andcontrol cuvette by accounting for the flow rates through thecuvettes and the leaf area (von Caemmerer & Farquhar1981). Projected leaf area was determined with a videocamera (TC2000; RCA, Lancaster, CA, USA) adapted toan area-meter (

D

T area meter; Delta-T Devices Ltd., Cam-bridge, UK).

Error estimation of carbonyl flux

An estimation of total errors of the carbonyl exchange rateswas performed as described by Kesselmeier

et al

. (1997)and Cojocariu

et al

. (2004) using the error propagationmethod according to Doerffel 1984) by considering theabsolute errors for the carbonyl concentration measure-ments in the cuvettes, the variation of blank cartridges, theerror for the cuvette flow (

Err

flow

) and the leaf area (

Err

LA

).According to these calculations, the total emission errorsfor acetaldehyde, formaldehyde and acetone emissionsamounted to 9.5, 7.0 and 5.2%, respectively.

It is important to mention that both, during flushing thecuvettes with ambient air as well as during sampling ofcarbonyls onto DNPH cartridges, no O

3

scrubbers wereused. In the former case this was required to have actualambient O

3

concentrations in the cuvettes as O

3

effects hadto be studied. As clearly seen from a control experiment inwhich carbonyl standards were mixed into ambient andsynthetic air and sampled at zero and elevated ozone (upto approximately 100 p.p.b) as well as at 25 and 50

∞

C, had

Carbonyl exchange by adult beech

605

© 2005 Blackwell Publishing Ltd,

Plant, Cell and Environment,

28,

603–611

no effect on the efficiency of carbonyl sampling and quan-tification (Hansel, Müller, Kreuzwieser, unpublishedresults). Considering the low isoprenoid emission potential(Tollsten & Mueller 1996; Schuh

et al

. 1997; Kahl, Hoff-mann & Klockow 1999) and the low residence time of airin the cuvette (approximately 20 s) oxidation of emittedisoprenoids resulting in the production of carbonyls seemsto be negligible.

Statistics

The data obtained were subjected either to Student’s

t

-testor to LSD under

ANOVA

(SPSS for Windows, release 10.0;SPSS Inc., Chicago, IL, USA). Significant differences (

P

-value) at a confidence level of 95% are indicated in thetables and figures. In order to identify the factors mostprobably affecting the carbonyl emissions, the dataobtained were subjected to multiple regression analysis fol-lowing a stepwise regression to identify significant param-eters. Results including

R

2

, degrees of freedom (d.f.),

N

and

F

-statistic values (see below) are given in Table 1.

RESULTS

Carbonyl exchange – spectrum of compounds and diurnal variations in trees under 1 ¥¥¥¥

O

3

Carbonyl emissions from beech leaves were dominated byacetaldehyde, along with significant amounts of formalde-hyde and acetone (Fig. 1a). The diurnal fluxes of all threecarbonyl compounds showed higher rates in the afternoonthan at night. At some nights even uptake of carbonylswas observed. Acetaldehyde was emitted with maximumrates of about 45 nmol m

-

2

min

-

1

, closely followed by

formaldehyde and acetone emissions with rates of about40 nmol m

-

2

min

-

1.

Environmental parameters and physiology

Meteorological conditions varied during the measurementson cloudy as well as sunny days and with temperaturesinside the cuvettes of up to 42 ∞C (Fig. 1b), and PPFDassessed at the leaf level outside the plant cuvette reachedup to 2000 mmol m-2 s-1 (Fig. 1c). Stomatal conductance(gH2O), rates of transpiration (E) and net-carbon assimila-tion (A) were measured simultaneously with the carbonylexchange. E and A closely followed meteorological condi-tions such as air temperature and PPFD. gH2O reached amaximum of 11 mmol m-2 s-1 (Fig. 1d). At the same time, Eamounted to around 0.8 mmol m-2 s-1 (Fig. 1e) and A to4.5 mmol m-2 s-1 (Fig. 1f). The physiological parameters ofthe trees at 2 ¥ O3 did not differ significantly from the con-trols (1 ¥ O3). As for non-fumigated trees, maximal transpi-ration rates amounted to 1.0–2.0 mmol m-2 s-1. SimilarlygH2O was in the range 5–11 mmol m-2 s-1 (Fig. 2).

Effect of the 2 ¥¥¥¥ O3 regime on carbonyl exchange

Similar to control trees, carbonyl emissions of beech leavesexposed to 2 ¥ O3 showed diurnal patterns with signifi-cantly higher emission rates in the afternoon than at night(Fig. 2a). Maximal emissions rates amounted to 90 nmolm-2 min-1 (acetaldehyde), 84 nmol m-2 min-1 (formalde-hyde) and 63 nmol m-2 min-1 (acetone) (Fig. 2). In parallel,gH2O (up to 11 mmol m-2 s-1), E (up to 2.0 mmol m-2 s-1) andA (up to 4.6 mmol m-2 s-1) were in a range (Fig. 2d–f) similarto that observed in trees under 1 ¥ O3 (Fig. 1). Further-

Table 1. Correlation analysis of the influence of environmental and physiological factors on carbonyl flux rates from F. sylvatica leaves

ParameterAcetaldehyde fluxP

Formaldehyde fluxP

Acetone flux P

T – – –RH – 0.03 –PPFD – – –A – – < 0.0004gH2O – – –E 0.02 – –O3 0.002 – 0.01Flux model = –(22 ± 10) + (0.7 ±

0.6 O3) + (21 ± 8 E)= (140 ± 28) - (1.24 ±

0.35 RH)= –(36 ± 14) + (15 ±

4 A) + (0.6 ± 0.2 O3)Statistics of estimated flux modelsN 35 35 36R2 0.70 0.13 0.56d.f. 2 1 2F-statistics 37.9 12.4 20.78Probability level < 0.0001 < 0.001 < 0.0001

Field measurements were conducted with fully sun-exposed leaves on approximately 27-m-tall, field-grown beech trees at the canopy levelin August 2002. Data were subjected to stepwise regression in order to identify the significance of factors for carbonyl fluxes. Only significantfactors were used for multiple regressions. Probability levels (P), significances from multiple regressions, and calculated flux models areindicated. PPFD, photosynthetic active photon flux density; RH, relative humidity; E, transpiration rate; A, net carbon assimilation rate;gH2O, stomatal conductance for water vapour; T, cuvette air temperature.

606 C. Cojocariu et al.

© 2005 Blackwell Publishing Ltd, Plant, Cell and Environment, 28, 603–611

more, meteorological parameters were comparable, withhighest temperatures of 37 ∞C (Fig. 2b) and PPFD of up to1700 mmol m-2 s-1 (Fig. 2c).

A comparison of carbonyl exchange between the treesunder the 1 ¥ O3 and 2 ¥ O3 regimes revealed elevated O3

levels to cause significantly higher rates of formaldehyde,acetaldehyde and acetone emissions. The lowest acetalde-hyde emission under 2 ¥ O3 was about 7.5 nmol m-2 min-1,whereas maximum rates amounted to about 116 nmolm-2 min-1. In contrast, acetaldehyde emitted by controltrees (1 ¥ O3) ranged between 0.25 (minimum) and 46 nmolm-2 min-1 (maximum). Formaldehyde emission in treesunder 2 ¥ O3 was with minimum rates of 4.31 nmol m–2

min-1 and maximum rates of 84 nmol m-2 min-1; almost 2.5times higher than emissions from control trees [1 ¥ O3:0.52 nmol m-2 min-1 (minimum); 56 nmol m-2 min-1 (maxi-mum)]. Similarly, minimum acetone emission of treesunder 2 ¥ O3 was 1.68 nmol m-2 min-1, whereas maximumemission amounted to 99 nmol m-2 min-1, compared with0.71 nmol m-2 min-1 (minimum) and 43 nmol m-2 min-1 ofcontrols (1 ¥ O3).

During the measurements, the O3 levels of the 1 ¥ O3

regime varied at the canopy level between 1.5 and 132p.p.b.v., whereas levels of the 2 ¥ O3 regime ranged between0.1 and 206 p.p.b.v. (Werner & Fabian 2002). In order totest whether O3 stimulated carbonyl emissions by directlytriggering its production, the relationship between carbonylexchange and O3 levels was examined in Fig. 3. Linearregression calculations showed acetaldehyde (P < 0.0001)(Fig. 3a) and acetone emissions (P < 0.0001; Fig. 3c) to sig-nificantly correlate in trees under 2 ¥ O3 with the O3 levelsmeasured at canopy level (25 m above-ground). In contrast,formaldehyde emissions failed to significantly correlate(P = 0.51; Fig. 3b).

Correlation of carbonyl exchange with environmental and plant physiological parameters

From linear regression analysis regarding ozone concentra-tions as the only possible factor influencing carbonylexchange rates, it cannot unambiguously be concluded on

Figure 1. Daily pattern of carbonyl exchange rates (a); cuvette air temperature (b); PPFD (c); gH2O (d); and the rates of transpiration, E (e) and net-carbon assimilation, A (f) of field-grown European beech under the 1 ¥ O3 regime. Data were recorded from a single beech tree/twig at ‘Kranzberger Forst’ on 15 August 2002 (representative recording out of two time courses from four different trees each). About 4–5 fully expanded leaves were placed into a dynamic cuvette system. Carbonyls (�, acetaldehyde; �, formaldehyde; �, acetone) were sampled on DNPH-coated silica gel cartridges for 60 min; simultaneously, gas exchange and meteorological data were recorded continuously as described in the methods section.

a d

b e

c f

Carbonyl exchange by adult beech 607


a causal relationship since ozone concentrations, for exam-ple, vary with temperature, light, etc. In order to identifyfactors controlling carbonyl emissions from leaves of adultbeech trees correlation analyses were performed, subject-ing all available data (environmental and physiological fac-tors) to multiple regression analysis (Table 1). In a firstprocedure, a stepwise analysis, parameters without anyinfluence on carbonyl exchange were identified andexcluded prior to multiple regressions. Such factors werePPFD, gH2O and air temperature. Multiple regressions withthe remaining significant factors, namely relative humidity,transpiration rate, A and O3 concentration indicated bothtranspiration rate and O3 concentration to be the mainparameters that significantly affected acetaldehydeexchange, whereas acetone emission depended on A andO3 concentration. In contrast, formaldehyde exchange wassignificantly influenced by relative humidity but not by O3

concentration.

DISCUSSION

In the present field study, the carbonyl exchange fromleaves of adult European beech trees was investigatedalong with the response in exchange to chronic O3 expo-sure. The carbonyls mainly released into the atmospherewere the short-chain compounds acetaldehyde, formalde-hyde and acetone. Other carbonyls, such as C6-compoundswhich are known to be produced in wounded leaves (Kes-selmeier & Staudt 1999) and have also been observed inbeech (Tollsten & Mueller 1996), were not emitted at sig-nificant amounts by the study trees. Carbonyls were emit-ted at significantly (P £ 0.05) higher rates during daylighthours (at the control site approx. 40–50 nmol m-2 min-1)than at night (< 1 nmol m-2 min-1; Fig. 1a). Acetaldehydedominated the carbonyl flux with highest emission rates of50 nmol m-2 min-1 at the control site during midday(Fig. 1a). Considerably lower emission rates than in the

Figure 2. Daily pattern of carbonyl exchange rates (a); cuvette air temperature (b); PPFD (c); gH2O (d); and the rates of transpiration, E (e) and net-carbon assimilation, A (f) of field-grown European beech under the 2 ¥ O3 regime. Data were recorded from a single beech tree/twig at ‘Kranzberger Forst’ on 13 August 2002 (representative recording out of two time courses from four different trees each). About 4–5 fully expanded leaves were placed into a dynamic cuvette system. Carbonyls (�, acetaldehyde; �, formaldehyde; �, acetone) were sampled on DNPH-coated silica gel cartridges for 60 min; simultaneously, gas exchange and meteorological data were recorded continuously as described in the methods section.

a d

b e

c f



present study were found by Koch (2002) working withbeech seedlings in the laboratory (approx. 2–5 nmol m-2

min-1 during the day). This difference may be due to age-specific factors, as also found by Kesselmeier (2001) study-ing Mediterranean oak species. To our knowledge, thepresent study is the first to quantify the carbonyl release ofadult European beech trees. In consistency with our study,acetaldehyde, formaldehyde and acetone were the maincarbonyls emitted, at similar rates, also by Mediterraneanoak and pine species (Kesselmeier et al. 1997; Kreuzwieseret al. 2002) as well as conifers under the temperate (Hahn,Steinbrecher & Slemr 1991; Steinbrecher et al. 1997; Cojo-cariu et al. 2004) or boreal (Janson, de Claes & Romero1999) climate of their natural habitats. Given such similar-ities, it is concluded that the emission of these compoundsmay be a general feature of trees.

The present study focused on elucidating the effect ofchronic exposure to elevated tropospheric O3 levels on thecarbonyl release of adult beech trees. Considerably higheremission rates of acetaldehyde, formaldehyde and acetone

were found in trees under the enhanced O3 regime relativeto trees exposed to 1 ¥ O3 at the forest site. Nevertheless,carbonyl emissions showed the same daily pattern in bothgroups of trees. The linear correlation of O3 concentrationswith acetaldehyde and acetone emissions (Fig. 3) suggeststhat O3 drives the enhanced emissions of these carbonylsby directly affecting their production; that is, an increase ofO3 concentrations immediately caused higher emissions ofthese compounds, whereas lower concentrations causedreduced emissions. Such a linear correlation was notobserved between O3 concentrations and formaldehydeemissions indicating that there is no direct effect of O3 onthe production of this compound. Stimulated acetaldehydeemission by O3 impact and other factors mediating oxida-tive stress has previously been described in laboratorystudies by Kimmerer & Kozlowski (1982). High acetalde-hyde emissions from the leaves of O3-exposed beech treessupport the view that O3 stress may trigger fermentationprocesses in the leaves even in the presence of oxygen aspreviously suggested by Kimmerer & Kozlowski (1982). It

Figure 3. Correlation of acetaldehyde (a), formaldehyde (b) and acetone (c) exchange from beech leaves with O3 concentration at the canopy level of ‘Kranzberger Forst’. The results of linear regression analyses (Y and R2) and the significance levels (P) are shown. Data include both measurements at under the 2 ¥ O3 (white symbols) and 1 ¥ O3 regime (black symbols).



has been hypothesized that O3 impairs mitochondrial func-tion and changes the carbon flux from the tricarboxylicacid cycle and the respiratory chain towards pyruvatedecarboxylase and ethanolic fermentation (Tadege, Brän-dle & Kuhlemeier 1998). Regarding the present study,there are no hints that other metabolic pathways known toform acetaldehyde such as the oxidation of xylem-derivedethanol (Kreuzwieser, Scheerer & Rennenberg 1999) orthe release after light–dark transitions (Holzinger et al.2000; Karl et al. 2002) contributed to the enhanced emis-sion rates at elevated O3 levels. In addition to these path-ways, leaf senescence has been suggested to causeacetaldehyde and acetone emission (de Gouw et al. 1999).Since previous studies indicate that O3 stress can acceleratefoliar senescence in many plant species including trees(Pell & Pearson 1983; Pell, Schlagnhaufer & Arteca 1997),also this possibility has to be taken into account. Nunnet al. (2002) studying the same trees observed that O3 pro-moted macroscopic symptoms (yellowish spots) indicatingcell collapse in the mesophyll and accelerated autumnalsenescence by up to 9 d under elevated O3. However, thebeech leaves examined in this study did not show any visi-ble injury, nor differences in gH2O or A between the 1 ¥ O3

and 2 ¥ O3 regimes (Figs 1 & 2). Assimilation (up to6.0 mmol m-2 s-1) was consistent with literature data frombeech seedlings (Grams et al. 1999; Bortier, de Temmer-mann & Ceulemans 2000) and well within the range ofother gas exchange assessments of both treatments on thesame beech trees at ‘Kranzberger Forst’ (Reiter, Nunn &Löw, unpublished). From the results it may therefore beconcluded that carbonyl emission is a highly sensitiveparameter of ozone stress in beech trees.

Multiple regression analyses (Table 1) revealed that, incontrast to spruce, carbonyl exchange from beech leavesappeared to be independent of temperature (Table 1),although a correlation was found in laboratory studies con-ducted on beech seedlings (data not shown). Influences byother factors may mask, in the field, effects of temperatureon carbonyl exchange. Acetaldehyde emission seemed tobe mainly influenced by transpiration rate and O3 concen-tration, whereas acetone emission depended on O3 concen-tration and A. In contrast, formaldehyde emissionsdepended on relative humidity. These results are consistentwith findings in Norway spruce (Cojocariu et al. 2004), inwhich carbonyl exchange was neither controlled by gH2O norby factors determining gH2O such as PPFD. Furthermore,previous work of Kreuzwieser et al. (2001) in which abscisicacid was applied to excised poplar leaves showed thatreduced gH2O did not affect acetaldehyde emission. Thesefindings suggest that the production rather than stomatalconductance limits emission rates of carbonyls. A similareffect was demonstrated for isoprene emission, whichremained high if stomata were closed artificially (Fall &Monson 1992). Continued production probably increasedthe concentration gradient between leaf and atmosphere,thereby enhancing the driving force to sustain high iso-prene emissions through stomata even at high stomatalresistance.

The finding that O3 considerably stimulates the emissionof carbonyls from beech leaves is of high importance forclimate change scenarios. Such scenarios predict higher tro-pospheric O3 concentrations in addition to several otherchanges such as increasing temperatures. As high tempera-ture also triggers increased carbonyl emission at least inconifers, high carbonyl emissions may be predicted underfuture environmental conditions. This is of particular signif-icance since carbonyls act as tropospheric precursors of O3

formation, and high emissions therefore could promoteincrease in O3 levels.

ACKNOWLEDGMENTS

The present work was financially supported by the Deut-sche Forschungsgemeinschaft (DFG) through SFB 607‘Growth and Parasite Defense – Competition for Resourcesin Economic Plants from Agronomy and Forestry’. Theauthors are indebted to Professor Dr P. Fabian and Dr H.Werner for ensuring the continuous operation of the free-air O3 canopy exposure system.

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Received 21 July 2004; received in revised form 1 November 2004;accepted for publication 3 November 2004

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The effect of ozone on the emission of carbonyls from leaves of adult Fagus sylvatica