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Organic aerosol (OA) still remains one of the most difficult components of the atmospheric aerosols to simulate, given the multitude of its precursors, the uncertainty in its formation pathways, and the lack of measurements of its detailed composition. The LANDEX (LANDes Experiment) project, during its intensive field campaign in summer 2017, gives us the opportunity to compare biogenic secondary OA (BSOA) and its precursors and oxidants obtained within and above the Landes forest canopy to simulations performed with CHIMERE, a state-of-the-art regional chemistry transport model. The Landes forest is situated in the southwestern part of France and is one of the largest anthropized forests in Europe (1×106 ha). The majority of the forest is comprised of maritime pine trees, which are strong terpenoid emitters, providing a large potential for BSOA formation. In order to simulate OA buildup in this area, a specific model configuration setup adapted to the local peculiarities was necessary. As the forest is nonhomogeneous, with interstitial agricultural fields, high-resolution 1 km simulations over the forest area were performed. Biogenic volatile organic compound (BVOC) emissions were predicted by MEGAN, but specific land cover information needed to be used and was thus chosen from the comparison of several high-resolution land cover databases. Moreover, the tree species distribution needed to be updated for the specific conditions of the Landes forest. In order to understand the canopy effect in the forest, canopy effects on vertical diffusivity, winds, and radiation were implemented in the model in a simplified way. The refined simulations show a redistribution of BVOCs with a decrease in isoprene and an increase in terpenoid emissions with respect to the standard case, both of which are in line with observations. Corresponding changes to simulated BSOA sources are tracked. Very low nighttime ozone, sometimes near zero, remains overestimated in all simulations. This has implications for the nighttime oxidant budget, including NO3. Despite careful treatment of physical conditions, simulated BSOA is overestimated in the most refined simulation. Simulations are also compared to air quality sites surrounding the Landes forest, reporting a more realistic simulation in these stations in the most refined test case. Finally, the importance of the sea breeze system, which also impacts species concentrations inside the forest, is made evident.

Sesquiterpene cyclases (STC) catalyse the cyclization of the C15 molecule farnesyl diphosphate into a vast variety of mono- or polycyclic hydrocarbons and, for a few enzymes, oxygenated structures, with diverse stereogenic centres. The huge diversity in sesquiterpene skeleton structures in nature is primarily the result of the type of cyclization driven by the STC. Despite the phenomenal impact of fungal sesquiterpenes on the ecology of fungi and their potentials for applications, the fungal sesquiterpenome is largely untapped. The identification of fungal STC is generally based on protein sequence similarity with characterized enzymes. This approach has improved our knowledge on STC in a few fungal species, but it has limited success for the discovery of distant sequences. Besides, the tools based on secondary metabolite biosynthesis gene clusters have shown poor performance for terpene cyclases. Here, we used four sets of sequences of fungal STC that catalyse four types of cyclization, and specific amino acid motives to identify phylogenetically related sequences in the genomes of basidiomycetes fungi from the order Polyporales. We validated that four STC genes newly identified from the genome sequence of Leiotrametes menziesii, each classified in a different phylogenetic clade, catalysed a predicted cyclization of farnesyl diphosphate. We built HMM models and searched STC genes in 656 fungal genomes genomes. We identified 5605 STC genes, which were classified in one of the four clades and had a predicted cyclization mechanism. We noticed that the HMM models were more accurate for the prediction of the type of cyclization catalysed by basidiomycete STC than for ascomycete STC.

International audience; Mangroves are ecosystems interfacing terrestrial and marine environments submitted to extreme abiotic factors (e.g. anoxia, flooding, salinity) producing stress on vegetation. Due to these stresses, we hypothesized that mangroves potentially emit biogenic volatile organic compound (BVOC), particularly isoprenoids as they are defense compounds. Despite mangroves cover only about 5% of the forest areas of the world, their emissions could impact air quality at the continental-ocean interface. As a result, it is important to fill the gap in the knowledge about BVOC emissions from the canopy of the major mangrove trees. The aim of this study was thus to screen isoprenoid emissions of the mangrove species. In this study, we sampled isoprenoid emissions of 14 species foliage among the 38 core species existing in the Indo-West Pacific (IWP) and the Atlantic Est Pacific (AEP) regions. Sampling was performed using a branch-bag dynamic enclosure system and analyzed with gas chromatography coupled to mass spectrometry. Our analysis showed that mangrove tree species are very low emitters suggesting that mangrove ecosystems would not strongly influence atmospheric chemistry and air quality.

International audience

Given the importance of terpenes and fuel moisture content (FMC) on flammability, this work aims at checking how these parameters affect leaf flammability of different native and Wildland-Urban Interfaces species (Pinus halepensis, Cupressus sempervirens, Cupressocyparis leylandii, and Hesperocyparis arizonica) across seasons in the French Mediterranean region. We found that the highest terpene diversity and content seasonally varied according to the species, with diterpene content being lower in spring for C. leylandii, while monoterpene and diterpene content being higher in summer and winter, respectively, for P. halepensis. Flammability and FMC varied according to the season but the pattern differed among species. A significant correlation between the latters was rarely observed and occurred in only one season, differing among species. The correlations between flammability and terpenes were mostly highlighted using single compounds, compared to subgroups, and they presented seasonal patterns varying among species. Checking the seasonal effect of groups of terpene compounds on flammability, there were seasonal differences in these groups according to the species and the variable tested. Mostly, these significant compounds were not the most concentrated. The best flammability drivers of each model, mostly diterpenes, except for P. halepensis whose flammability was mostly drove by mono- and sesquiterpenes, changed among seasons according to the flammability variable considered. When a best driver remained the same in different seasons, its effect on flammability could be opposite. In contrast, FMC was generally not a significant explanatory parameter of leaf flammability or did not improve the fit of models.

International audience; In flammability assessment, the terpene effect is usually studied using their total or subgroup content, missing, therefore, the information that could be provided by the molecules themselves. In this study, the specific role of terpenes on leaf flammability was sought comparing different levels of terpene identification—total, subgroup (i.e. mono-, sesqui-, and diterpene), and single compound—as well as their interactions with fuel moisture content (FMC) in four species common in Mediterranean Wildland–Urban Interfaces (Pinus halepensis, Cupressocyparis leylandii, Hesperocyparis arizonica, Cupressus sempervirens). Pinus halepensis was the most flammable species (low FMC and higher sesquiterpene content but low terpene diversity) while Cupressocyparis leylandii presented the highest terpene diversity and total terpene content (higher mono- and diterpene content). Flammability was differently affected according to the terpene identification level used in the models. The effects ranged from non-significant for most species studied, using subgroup or total terpene content, to mostly significant, using single compound content. Regarding the former, the lack of significant results could be due to opposite effects of different single compounds within a terpene subgroup. For the latter, terpene molecules driving flammability and their effects (positive or negative) differed among species. A cumulative effect with FMC was also highlighted in some cases but terpenes mostly remained the main flammability drivers regardless of the species. Using the refined terpene level in modelling allowed a better understanding of the compounds’ role on flammability, which is useful in the identification of plant traits linked to flammability.

International audience; Nitrous acid (HONO) is of considerable interest because it is an important precursor of hydroxyl radicals (OH), a key species in atmospheric chemistry. HONO sources are still not well understood, and air quality models fail to predict OH as well as HONO mixing ratios. As there is little knowledge about the potential contribution of plant surfaces to HONO emission, this laboratory work investigated HONO formation by heterogeneous reaction of NO 2 on Zea mays. Experiments were carried out in a flow tube reactor; HONO, NO 2 and NO were measured online with a Long Path Absorption Photometer (LOPAP) and a NOx analyzer. Tests were performed on leaves under different conditions of relative humidity (5 to 58 %), NO 2 mixing ratio representing suburban to urban areas (10 to 80 ppbv), spectral irradiance (0 to 20 W m-2) and temperature (288 to 313 K). Additional tests on plant wax extracts from Zea mays leaves showed that this component can contribute to the observed HONO formation. Temperature and NO 2 mixing ratios were the two environmental parameters that showed substantially increased HONO emissions from Zea mays leaves. The highest HONO emission rates on Zea mays leaves were observed at 313 K for 40 ppbv of NO 2 and 40% RH and reached values of (5.6 ± 0.8) × 10 9 molecules cm-2 s-1. Assuming a mixing layer of 300 m, the HONO flux from Zea mays leaves was estimated to be 171 ± 23 pptv h-1 during summertime, which is comparable to what has been reported for soil surfaces.

International audience; Nitrous acid (HONO) photolysis is an important atmospheric reaction that leads to the formation of hydroxyl radicals (OH), the main diurnal atmospheric oxidants. The process of HONO formation remains unclear, and comparisons between field measurements and model results have highlighted the presence of unknown HONO sources. HONO production on plant surfaces was recently suggested to contribute to atmospheric HONO formation, but there is limited information on the quantification of HONO production and uptake by plants. To address this gap in the existing knowledge, the current study investigated HONO exchange on living Zea mays plants. Experiments were conducted in growth chambers under controlled experimental conditions (temperature, relative humidity, NO2 mixing ratio, light intensity, CO2 mixing ratio) at temperatures ranging between 283 and 299 K. To investigate the effect of drought on HONO plant-atmosphere exchanges, experiments were carried out on two sets of Zea mays plants exposed to two different water supply conditions during their growth: optimal watering (70% of the field capacity) and water stress (30% of the field capacity). Results indicated that the uptake of HONO by control Zea mays plants increased linearly with ambient temperature, and was correlated with CO2 assimilation for temperatures ranging from 283 to 299 K. At 299 K, HONO production on the leaves offset this uptake and Zea mays plants were a source of HONO, with a net production rate of 27 ± 7 ppt h-1. Deposition velocities were higher for HONO than CO2, suggesting a higher mesophyll resistance for CO2 than HONO. As water stress reduced the stomatal opening, it also decreased plant-atmosphere gas exchange. Thus, climate change, which may limit the availability of water, will have an impact on HONO exchange between plants and the atmosphere.

Biogenic Volatile Organic Compounds (BVOC) are largely accepted to contribute to both atmospheric chemistry and ecosystem functioning. While the forest canopy is recognized as a major source of BVOC, emissions from plant litter have scarcely been explored with just a couple of studies being focused on emission patterns over litter decomposition process. The aim of this study was to quantitatively and qualitatively characterize BVOC emissions (C1–C15) from Pinus halepensis litter, one of the major Mediterranean conifer species, over a 15‐month litter decomposition experiment. Senescent needles of P. halepensis were collected and placed in 42 litterbags where they underwent in situ decomposition. Litterbags were collected every 3 months and litter BVOC emissions were studied in vitro using both online (PTR‐ToF‐MS) and offline analyses (GC‐MS). Results showed a large diversity of BVOC (58 compounds detected), with a strong variation over time. Maximum total BVOC emissions were observed after 3 months of decomposition with 9.18 µg gDM −1 hr−1 mainly composed by terpene emissions (e.g., α‐pinene, terpinolene, β‐caryophyllene). At this stage, methanol, acetone, and acetic acid were the most important nonterpenic volatiles representing, respectively, up to 26%, 10%, and 26% of total emissions. This study gives an overview of the evolution of BVOC emissions from litter along with decomposition process and will thus contribute to better understand the dynamics and sources of BVOC emission in Mediterranean pine forests., We investigated the evolution of the BVOC emissions from the litter of Pinus halepensis during a 15‐month field decomposition experiment. High variability of emissions over decomposition was showed using online and offline measurements performed in vitro. Major compounds observed were monoterpenes, sesquiterpenes, and methanol, and their quantity and quality (terpenic composition) varied over sampling times with a high peak after 3 months of litter decomposition and, for a lesser extent, after 6 months for terpenic emission.

International audience; Leaf litter, at the interface between the soil and the atmosphere, releases Biogenic Volatile Organic Compounds (BVOC) which play an important role both in global atmospheric chemistry and plant ecology. Litter is especially important in the Mediterranean region, where it is produced in high amounts and decomposes slowly. Only leaf litter emissions of two dominant species (Pinus halepensis and Eucalyptus spp.) present in the Mediterranean region have been studied so far. The aim of this study is to characterize (quantitatively and qualitatively) the BVOC emissions (C10–C15) from litter (freshly fallen and non-decomposed leaves) under laboratory conditions. Sixteen species frequently found in the Mediterranean region were considered, nine of which possess terpene storage structures. Results show a large diversity of BVOC (87 compounds detected, terpenes, alkanes, alkenes, aldehydes, ketones, benzenoids), with terpene emission being higher than non-terpenic emission in most of the cases. Species were classified within 3 categories: negligible emitters (e.g. Acer monspessulanum and Quercus ilex, < 0.10 μg.gDM−1.h−1), low emitters (Pinus pinea, Quercus pubescens, 0.1–1.0 μg.gDM−1.h−1) and moderate emitters (Thymus vulgaris, Cotinus coggygria and Pinus halepensis between 1.5 and 4.7 μg.gDM−1.h−1). Litter that possessed terpene storage structures and known to store high terpene concentration did not always release the highest terpene emission rates (e.g. Eucalyptus globulus, Rosmarinus officinalis). Species that do not possess such structures release only non terpenic emissions (e.g. alkanes, aldehydes). Estimation of the potential contribution of P. halepensis and Q. pubescens litter to O3 and secondary organic aerosol (SOA) formation in the atmosphere showed that P. halepensis has a larger potential impact on the global air quality than Q. pubescens. This study will contribute to build future BVOC emissions inventories on leaf litter for their further integration in atmospheric chemistry models.

International audience; Several plant chemical traits (cellulose, tannins and terpenes) have been related to plant flammability. Contrastingly, no study has focused on the relationship between plant flammability and physico-chemical leaf litter traits with a focus on cuticular wax concentration. This study focuses on alkane cuticular waxes because of their relatively low flash point and storage in the cuticle of all vascular plant species. The sclerophyllous species Quercus coccifera is the model species since it is the main shrub species in the Mediterranean basin and all previously investigated sclerophyllous species feature a high cuticular wax content. Litter was collected in a Mediterranean garrigue where Q. coccifera grows under natural drought and recurrent aggravated drought (consisting of 5.5 years of rain restriction). These different drought conditions were expected to imply different alkane wax concentrations since one of the major roles of cuticular waxes is evapotranspiration limitation during drought. Litter flammability was assessed through ignition delay, flame residence time and flame height (assessed using an epiradiator) and gross heat of combustion (using an adiabatic bomb calorimeter). Results showed that the higher cuticular alkane concentrations reached under aggravated drought were associated with an increased leaf litter flammability as expected. These results confirm that all potentially flammable organic metabolites (terpenes as previously reported in other studies, and cuticular alkane waxes) are drivers of vegetation flammability. It is suggested that Q. coccifera flammability (considered as low to moderate), could increase under a drier scenario in the Mediterranean area. We hypothesize that fire severity would accordingly be intensified in shrubs dominated by this sclerophyllous species without necessarily increasing vulnerability of Q. coccifera to fire since this is a resprouter species after fire and is one of the main pioneer species during post-fire vegetation succession.

International audience; 26 Tropospheric ozone is a strong oxidant which affects human health, agricultural yields, and 27 ecosystems functioning. Thus, it is very important to determine ozone formation in order to 28 control air pollution. It is well known that isoprene participates in ozone formation. In this 29 study, we assess the potential impact of climate change in the Mediterranean region on ozone 30 concentration, through drought-related increase or decrease in isoprene emissions after 1 (Short 31 Drought scenario-1 year of 35% annual rain restriction) and 3 (Long Drought scenario-3 32 repeated years of 35% annual restriction) years of drought stress. 33

Isoprene, the main volatile released by plants, is known to protect the photosynthetic apparatus in isoprene emitters submitted to oxidative pressures caused by environmental constraints. Whether ambient isoprene contributes to protect negligible plant emitters under abiotic stress conditions is less clear, and no study has tested if ambient isoprene is beneficial during drought periods in plant species that naturally release negligible isoprene emissions. This study examines the effect of exogenous isoprene (20 ppbv) on net photosynthesis, stomatal conductance and production of H2O2 (a reactive oxygen species: ROS) in leaves of Acer monspessulanum (a negligible isoprene emitter) submitted to three watering treatments (optimal, moderate water stress and severe water stress). Results showed that A. monspessulanum exhibited a net photosynthesis increase (+30%) and a relative leaf H2O2 decrease when saplings were exposed to an enriched isoprene atmosphere compared to isoprene-free conditions under moderate water deficit. Such physiological improvement under isoprene exposure was not observed under optimal watering or severe water stress. These findings suggest that when negligible isoprene emitters are surrounded by a very high concentration of isoprene in the ambient air, some plant protection mechanism occurs under moderate water deficit probably related to protection against ROS damage eventually impeding photosynthesis drop.

Isoprene, the main volatile released by plants, is known to protect the photosynthetic apparatus in isoprene emitters submitted to oxidative pressures caused by environmental constraints. Whether ambient isoprene contributes to protect negligible plant emitters under abiotic stress conditions is less clear, and no study has tested if ambient isoprene is beneficial during drought periods in plant species that naturally release negligible isoprene emissions. This study examines the effect of exogenous isoprene (20 ppbv) on net photosynthesis, stomatal conductance and production of H2O2 (a reactive oxygen species: ROS) in leaves of Acer monspessulanum (a negligible isoprene emitter) submitted to three watering treatments (optimal, moderate water stress and severe water stress). Results showed that A. monspessulanum exhibited a net photosynthesis increase (+30%) and a relative leaf H2O2 decrease when saplings were exposed to an enriched isoprene atmosphere compared to isoprene-free conditions under moderate water deficit. Such physiological improvement under isoprene exposure was not observed under optimal watering or severe water stress. These findings suggest that when negligible isoprene emitters are surrounded by a very high concentration of isoprene in the ambient air, some plant protection mechanism occurs under moderate water deficit probably related to protection against ROS damage eventually impeding photosynthesis drop.

The capacity of a Quercus pubescens forest to resist recurrent drought was assessed on an in situ experimental platform through the measurement of a large set of traits (ecophysiological and metabolic) studied under natural drought (ND) and amplified drought induced by partial rain exclusion (AD). This study was performed during the 3rd and 4th years of AD, which correspond to conditions of moderate AD in 2014 and harsher AD in 2015, respectively. Although water potential (Ψ) and net photosynthesis (Pn) were noticeably reduced under AD in 2015 compared to ND, trees showed similar growth and no oxidative stress. The absence of oxidative damage could be due to a strong accumulation of α-tocopherol, suggesting that this compound is a major component of the Q. pubescens antioxidant system. Other antioxidants were rather stable under AD in 2014, but slight changes started to be observed in 2015 (carotenoids and isoprene) due to harsher conditions. Our results indicate that Q. pubescens could be able to cope with AD, for at least 4 years, likely due to its antioxidant system. However, growth decrease was observed during the 5th year (2016) of AD, suggesting that this resistance could be threatened over longer periods of recurrent drought.

International audience; The Mediterranean region is recognized as a global biodiversity hotspot. However, over the last decades, the cessation of traditional farming in the north part of the Mediterranean basin has given way to strong afforestation leading to occurrence of abandoned agricultural lands colonized by pioneer expansionist species like Pinus halepensis. This pine species is known to synthesize a wide range of secondary metabolites, and previous studies have demonstrated strong allelopathic potentialities of its needle and root leachates. Pinus halepensis is also recognized to release significant amounts of volatile organic compounds (VOC) with potential allelopathic effects that have never been investigated. In this context, the objectives of the present study were to improve our knowledge about the VOC released from P. halepensis needles and roots, determine if these VOC affect the seed germination and root growth of two herbaceous target species (Lactuca sativa and Linum strictum), and evaluate if soil microorganisms modulate the potential allelopathic effects of these VOC. Thirty terpenes were detected from both, needle and root emissions with β‐caryophyllene as the major volatile. Numerous terpenes, such as β‐caryophyllene, δ‐terpinene, or α‐pinene, showed higher headspace concentrations according to the gradient green needles < senescent needles < needle litter. Seed germination and root growth of the two target species were mainly reduced in presence of P. halepensis VOC. In strong contrast with the trend reported with needle leachates in literature, we observed an increasing inhibitory effect of P. halepensis VOC with the progress of needle physiological stages (i.e., green needle < senescent needle < needle litter). Surprisingly, several inhibitory effects observed on filter paper were also found or even amplified when natural soil was used as a substrate, highlighting that soil microorganisms do not necessarily limit the negative effects of VOC released by P. halepensis on herbaceous target species.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN [ADD1_IRSTEA]Dynamiques spatiales d'anthropisation; International audience; Among plant characteristics promoting flammability, terpenes have received far less attention, especially regarding the vegetation surrounding housing. Here, mono-, sesqui-, and diterpenes were screened in live and dead leaves of ornamental species found in Wildland-Urban Interfaces (WUI) of southeastern France. Terpene content and composition were compared among species and between fuel types. Their influence on flammability was assessed through several variables and compared to that of leaf thickness and moisture content. Six of the 17 species examined contained terpenes. Terpene diversity and content differed among species but not between fuel types. Mono-, sesqui-, and diterpenes (especially the highly concentrated compounds) were involved to varying degrees in both leaf and litter flammability. Their effects could be the opposite according to the flammability variable and the fuel type considered. Leaf sesquiterpene content and litter total terpene content had the strongest influence on maximum temperature; the former also mainly drove leaf flaming duration. The other flammability variables were more strongly associated with either moisture content or leaf thickness. Our findings underlined the idea that fire management in WUI must also acknowledge the potential for ornamental species containing terpenes, such as P. halepensis, in affecting fire behaviour.

The capacity of a Quercus pubescens forest to resist recurrent drought was assessed on an in situ experimental platform through the measurement of a large set of traits (ecophysiological and metabolic) studied under natural drought (ND) and amplified drought (AD) induced by partial rain exclusion. This study was performed during the third and fourth years of AD, which correspond to conditions of moderate AD in 2014 and harsher AD in 2015, respectively. Although water potential (Ψ) and net photosynthesis (Pn) were noticeably reduced under AD in 2015 compared to ND, trees showed similar growth and no oxidative stress. The absence of oxidative damage could be due to a strong accumulation of α-tocopherol, suggesting that this compound is a major component of the Q. pubescens antioxidant system. Other antioxidants were rather stable under AD in 2014, but slight changes started to be observed in 2015 (carotenoids and isoprene) due to harsher conditions. Our results indicate that Q. pubescens could be able to cope with AD, for at least 4 years, likely due to its antioxidant system. However, growth decrease was observed during the fifth year (2016) of AD, suggesting that this resistance could be threatened over longer periods of recurrent drought.

Seasonal variations of Q. pubescens physiology and isoprene emission rates (ER) were studied from June 2012 to June 2013 at the O3HP site (French Mediterranean) under natural (ND) and amplified (+30 %, AD) drought. While AD significantly reduced the stomatal conductance to water vapour over the season excepting August, it did not significantly limit CO2 net assimilation, which was the lowest in summer. ER followed a significant seasonal pattern, whatever the drought intensity, with mean ER maxima of 78.5 and 104.8 µgC gDM−1 h−1 in July (ND) and August (AD) respectively. Isoprene emission factor increased significantly by a factor of 2 in August and September under AD (137.8 and 74.3 µgC gDM−1 h−1) compared to ND (75.3 and 40.21 µgC gDM−1 h−1), but no changes occurred on ER. An isoprene algorithm (G14) was developed using an optimised artificial neural network trained on our experimental dataset (ER + O3HP climatic and edaphic parameters cumulated over 0 to 21 days before measurements). G14 assessed more than 80 % of the observed ER seasonal variations, whatever the drought intensity. In contrast, ER was poorly assessed under water stress by MEGAN empirical isoprene model, in particular under AD. Soil water (SW) content was the dominant parameter to account for the observed ER variations, regardless the water stress treatment. ER was more sensitive to higher frequency environmental changes under AD (0 to −7 days) compared to ND (7 days). Using IPCC RCP2.6 and RCP8.5 climate scenarios, SW and temperature calculated by the ORCHIDEE land surface model, and G14, an annual 3 fold ER relative increase was found between present (2000–2010) and future (2090–2100) for RCP8.5 scenario compared to a 70 % increase for RCP2.6. Future ER remained mainly sensitive to SW (both scenarios) and became dependent to higher frequency environmental changes under RCP8.5.

International audience; Prescribed burning is used to reduce fuel hazard but underburning can damage standing trees. The effect of burning on needle terpene storage, a proxy for secondary metabolism, in fire-damaged pines is poorly understood despite the protection terpenes confer against biotic and abiotic stressors. We investigated variation in needle terpene storage after burning in three Mediterranean pine species featuring different adaptations to fire regimes. In two pure-stands of Pinus halepensis Mill. and two mixed-stands of Pinus sylvestris L. and Pinus nigra ssp. salzmanni (Dunal) Franco, we compared 24 h and 1 year post-burning concentrations with pre-burning concentrations in 20 trees per species, and evaluated the relative contribution of tree fire severity and physiological condition (δ 13 C and N concentration) on temporal terpene dynamics (for mono-sesqui-and diterpenes). Twenty-four hours post-burning, monoterpene concentrations were slightly higher in P. halepensis than at pre-burning, while values were similar in P. sylvestris. Differently, in the more fire-resistant P. nigra monoterpene concentrations were lower at 24 h, compared with pre-burning. One year post-burning, concentrations were always lower compared with pre-or 24 h post-burning, regardless of the terpene group. Mono-and sesquiterpene variations were negatively related to pre-burning δ 13 C, while diter-pene variations were associated with fire-induced changes in needle δ 13 C and N concentration. At both post-burning times, mono-and diterpene concentrations increased significantly with crown scorch volume in all species. Differences in post-burning terpene contents as a function of the pine species' sensitivity to fire suggest that terpenic metabolites could have adaptive importance in fire-prone ecosystems in terms of flammability or defence against biotic agents post-burning. One year post-burning, our results suggest that in a context of fire-induced resource availability, pines likely prioritize primary rather than secondary metabolism. Overall, this study contributes to the assessment of the direct and indirect effects of fire on pine terpene storage, providing valuable information about their vulnerability to biotic and abiotic stressors throughout time.

Biogenic volatile organic compounds (BVOC) emitted by plants are a large source of carbon compounds released into the atmosphere where they are precursors of ozone and secondary organic aerosols. Being directly involved in air pollution and indirectly in climate change, it is very important to understand what factors drive the BVOC emissions in order to characterize the atmospheric composition through models. The main algorithms currently used to predict BVOC emissions are mainly light and/or temperature dependent. Additional factors such as seasonality and drought have been also found to influence isoprene emissions, especially in Mediterranean region which is characterized by its drought period in summer. These factors are increasingly included in models but only for the principal studied BVOC which is isoprene but there are still some discrepancies in estimations of emissions. In this study, the main BVOC (isoprene, methanol, acetone, acetaldehyde, formaldehyde and MACR+MVK+ISOPOOH), emitted by Quercus pubescens a tolerant drought Mediterranean species, were monitored with a PTR-ToF-MS over an entire seasonal cycle, under both natural and amplified drought which is expected with climate change. Amplified drought impacted all BVOC by reducing emissions in spring and summer and, in the contrary, by increasing emissions in autumn. Two types of dependence were found: light and temperature dependence (isoprene, MACR+MVK+ISOPOOH and acetaldehyde), and light and temperature dependence during the day and only temperature dependence during the night (methanol, acetone and formaldehyde). Moreover, a methanol burst in early morning and formaldehyde deposition/uptake were also punctually observed which were not assessed by model.

Biogenic Volatile Organic Compounds (BVOC) emitted from plants are the dominant source of reduced carbon chemicals to the atmosphere and are important precursors to the photochemical production of ozone and secondary organic aerosols. Considering the extensive land used for agriculture, cultivated Citrus plantations may play an important role in the chemistry of the atmosphere especially in regions such as the Central Valley of California. Moreover, the BVOC emissions from Citrus species have not been characterized in detail and more species-specific inputs for regional models of BVOC emissions are needed. In this study, we measured the physiological parameters and emissions of the most relevant BVOC (oxygenated compounds, monoterpenes, and sesquiterpenes) for four predominant Citrus species planted in California (Citrus sinensis var. ‘Parent Navel’, Citrus limon var. ‘Meyer’, Citrus reticulata var. ‘W. Murcott’ and ‘Clementine’). We used two analytical techniques to measure a full range of BVOC emitted: Proton Transfer Reaction Mass Spectrometry (PTR-MS) and gas chromatography with mass spectrometry. Methanol, followed by acetone and acetaldehyde, were the dominant BVOC emitted from lemon and mandarin trees (basal emission rates up to 300 ng(C) g(DW)

International audience; We investigated changes in the occurrence of allelochemicals from leachates of different Pinus halepensis organs taking into account the stages of pine stand age (i.e., young 60-years-old). GC-MS analysis of aqueous extracts revealed approx. 59 components from needles and roots. The major constituents were divided into different phyto-chemical groups—phenolics (50%), fatty acids (44%), and terpenoids. Further analyses were carried out to characterize the distribution of allelochemicals in different organs and P. halepensis successional stages. Roots and needles had two distinct chemical profiles, while needle leachates were composed mainly of oxygenated terpenoids (e.g., α-eudesmol, α-cadinol, and α-terpineol). Roots mainly contained fatty acids. Needles from young pine stands had the highest content of monoterpenes, suggesting their role as potential allelochemicals that could help young pine stands to establish. Pooling the different functional chemical groups showed that needles and, to a lesser extent, old roots, had higher chemical diversity than the roots of young and medium-aged pines. The highest diversity in phenolic constituents and fatty acids was in young needles (D chem =2.38). Finally, caffeic acid, a compound that has allelopathic properties was found in aqueous extracts at high concentrations in both young needles and old roots. The role of this compound in mediation of biological interactions in P. halepensis ecosystem functioning is discussed.

Many studies have assumed that plant terpenes favor fire due to their enormous flammability. However, only a few of them, all performed on green leaves, have demonstrated this. In the present work we investigated the question of whether litter terpene content can be used to estimate flammability and temperatures reached during fire. Epiradiator and burn table tests were used to compare flammability of leaf litter of P. pinaster, P. halepensis, P. pinea, C. albidus, C. ladanifer, C. laurifolius and the mixture of litter of P. pinaster with that of the other five species (e.g. P. pinaster + P. halepensis). Tests with burn table showed increasing spread rates and shorter combustion times under higher terpene contents. Flame height was triggered both with higher a terpene content and bed thickness, whereas the percentage of burned biomass was only significantly correlated to bed height. Epiradiator tests indicated that terpene concentration in leaf litter was positively correlated to flame height and negatively correlated to both flame residence time and ignition delay. Flammability was high for P. pinaster, P. halepensis, and hence for P. pinaster + P. halepensis, intermediate for C. albidus, P. pinea and P. pinaster combined with each of these species, and low for C. laurifolius, C. ladanifer and P. pinaster combined with them. Accordingly, their terpene content was high, intermediate and low. We concluded that plants might influence fire intensity, by having stored terpenes in their dead leaves, in addition to having developed traits to survive fire. Thus, a correct management of dead aboveground fuels rich in terpene concentrations, such as those of P. pinaster and P. halepensis, could prove helpful in reducing the hazard of fire.

International audience; Our study deals with ecological parameters that could influence early recruitment including germination and seedling growth with emphasis on the establishment of Pinus halepensis fire free forests in Mediterranean basin. Because plant secondary compounds (terpenoids and/or phenolic compounds) may govern competition, those produced by P. halepensis were investigated in terms of autotoxicity and implication in natural regeneration. In a laboratory experiment, two ecological factors as: (i) soil and litter conditions, (ii) light availability and three allelopathic processes: (i) the successional stage (age of pines), (ii) organ source of allelochemicals, (iii) and dose effect of allelochemicals were tested together in order to understand their interactions in natural regeneration of P. halepensis. The results showed a potential autotoxicity implied in germination rate and seedling growth. This phenomenon of autotoxicity was modulated by the biotic factors studied here: (i) young Aleppo pines exhibited a toxic effect on germination whereas old pines showed the most important effect on seedling growth and (ii) needle extracts presented an important dose effect on germination compared to roots. Autotoxicity was also modulated by environmental factors: (i) autotoxicity effect on germination and growth was more important on sterile soil, bringing to light the key role of microorganisms in this functional process, (ii) litter seems to be an important parameter influencing germination rates by favouring both drought conditions during germination and high mortality rate and (iii) light availability did not influence either germination or growth rate of early recruitment, but is well known to play an important role in saplings stage. Hence, our study showed that autotoxicity is a potential functional process that could influence natural regeneration of P. halepensis, but field conditions studies are necessary. #

International audience; The effects of water deficit stress and plant water potential (w) on monoterpene and sesquiterpene leaf emissions from Rosmarinus officinalis, Pinus halepensis, Cistus albidus and Quercus coccifera were studied over 11 days of water withholding (from t 1 to t 11), after substrates had achieved their field capacity (control pots: t 0). Volatile compounds were sampled from the same twig per plant all throughout the study, using a dynamic bag enclosure system. Volatiles, collected in Tenax TA, were studied by means of GC-FID and GC-MS. Monoterpene emissions of water stressed plants (t 1 –t 11) were either similar to those of control seedlings (R. officinalis and Q. coccifera)or higher (P. halepensis and C. albidus). By contrast, sesquiterpene emissions were strongly reduced or inhibited after four days of water withholding, particularly for R. officinalis, thus altering terpene emission composition. Despite the positive effect of water stress on leaf monoterpene emissions of P. halepensis and C. albidus, the significant correlation between these emissions and w showed a slow decrease of these emissions over long term water deficit periods. This contrasted with the rapid decline of sesquiterpene emissions of R. officinalis according to lower values of w. These results provide an overall picture of the different responses of monoterpene and sesquiterpene emissions to progressive water loss. They also reveal the utility of using w for estimating some emission rates of some species according to drought conditions.

The CANOPEE project aims to better understand the biosphere–atmosphere exchanges of biogenic volatile organic compounds (BVOCs) in the case of Mediterranean ecosystems and the impact of in-canopy processes on the atmospheric chemical composition above the canopy. Based on an intensive field campaign, the objective of our work was to determine the chemical composition of the air inside a canopy as well as the net fluxes of reactive species between the canopy and the boundary layer. Measurements were carried out during spring 2012 at the field site of the Oak Observatory of the Observatoire de Haute Provence (O3HP) located in the southeast of France. The site is a forest ecosystem dominated by downy oak, Quercus pubescens Willd., a typical Mediterranean species which features large isoprene emission rates. Mixing ratios of isoprene, its degradation products methylvinylketone (MVK) and methacrolein (MACR) and several other oxygenated VOC (OxVOC) were measured above the canopy using an online proton transfer reaction mass spectrometer (PTR-MS), and fluxes were calculated by the disjunct eddy covariance approach. The O3HP site was found to be a very significant source of isoprene emissions, with daily maximum ambient concentrations ranging between 2–16 ppbv inside and 2–5 ppbv just above the top of the forest canopy. Significant isoprene fluxes were observed only during daytime, following diurnal cycles with midday net emission fluxes from the canopy ranging between 2.0 and 9.7 mg m−2 h1. Net isoprene normalized flux (at 30 °C, 1000 μmol quanta m−2 s−1) was estimated at 7.4 mg m−2 h−1. Evidence of direct emission of methanol was also found exhibiting maximum daytime fluxes ranging between 0.2 and 0.6 mg m−2 h−1, whereas flux values for monoterpenes and others OxVOC such as acetone and acetaldehyde were below the detection limit. The MVK+MACR-to-isoprene ratio provided useful information on the oxidation of isoprene, and is in agreement with recent findings proposing weak production yields of MVK and MACR, in remote forest regions where the NOx concentrations are low. In-canopy chemical oxidation of isoprene was found to be weak and did not seem to have a significant impact on isoprene concentrations and fluxes above the canopy.

Agriculture comprises a substantial, and increasing, fraction of land use in many regions of the world. Emissions from agricultural vegetation and other biogenic and anthropogenic sources react in the atmosphere to produce ozone and secondary organic aerosol, which comprises a substantial fraction of particulate matter (PM2.5). Using data from three measurement campaigns, we examine the magnitude and composition of reactive gas-phase organic carbon emissions from agricultural crops and their potential to impact regional air quality relative to anthropogenic emissions from motor vehicles in California's San Joaquin Valley, which is out of compliance with state and federal standards for tropospheric ozone PM2.5. Emission rates for a suite of terpenoid compounds were measured in a greenhouse for 25 representative crops from California in 2008. Ambient measurements of terpenoids and other biogenic compounds in the volatile and intermediate-volatility organic compound ranges were made in the urban area of Bakersfield and over an orange orchard in a rural area of the San Joaquin Valley during two 2010 seasons: summer and spring flowering. We combined measurements from the orchard site with ozone modeling methods to assess the net effect of the orange trees on regional ozone. When accounting for both emissions of reactive precursors and the deposition of ozone to the orchard, the orange trees are a net source of ozone in the springtime during flowering, and relatively neutral for most of the summer until the fall, when it becomes a sink. Flowering was a major emission event and caused a large increase in emissions including a suite of compounds that had not been measured in the atmosphere before. Such biogenic emission events need to be better parameterized in models as they have significant potential to impact regional air quality since emissions increase by several factors to over an order of magnitude. In regions like the San Joaquin Valley, the mass of biogenic emissions from agricultural crops during the summer (without flowering) and the potential ozone and secondary organic aerosol formation from these emissions are on the same order as anthropogenic emissions from motor vehicles and must be considered in air quality models and secondary pollution control strategies.

International audience; Volatile organic compounds (VOCs) are key components in atmospheric chemistry. On a global scale, biogenic VOCs (BVOCs) constitute approximately 90 % of global VOC emissions. They participate in photochemical reactions and thus play a major role in the formation of tropospheric ozone and secondary organic aerosols. However, the impact of BVOCs on air quality is still characterized by large uncertainties, partly because emissions are not well quantified. As part of the Canopee project, a field campaign took place at the Oak Observatory of the Observatoire de Haute-Provence (O3HP), with the aim of quantifying BVOC emissions from a forest representative of the Mediterranean region.Measurements of concentrations and emission fluxes were carried out from the branch to the canopy level. Branch-level measurements enabled a better understanding of the environmental and physiological parameters of the plant that govern emissions. Fluxmeasurements above the top of the canopy enabled us to quantify the outgoing flow of BVOCs from the forest entering the troposphere. The results showed that white oak is a strong emitter of isoprene but a weak emitter ofmethanol andmonoterpenes. Isoprene emission fluxes were estimated at 7.2 mg m-2 h-1 (under standard conditions of temperature and solar radiation), thusmaking theO3HP forest one of the strongest isoprene emitting ecosystems worldwide; Les composés organiques volatils (COV) sont des composants clés en chimie atmosphérique. À l'échelle globale, on estime que 10 % ont une origine anthropique et 90 % une origine biogénique. L'impact des COV biogéniques (COVB), notamment sur la formation d'ozone (polluant et gaz à effet de serre) et d'aérosols organiques secondaires (à impact sanitaire et radiatif) en font un objet d'étude d'actualité. Toutefois, l'importance de l'impact des COVB sur la qualité de l'air est encore marquée par de larges incertitudes, en partie parce que les émissions demeurent mal quantifiées. Dans le cadre du projet Canopee, une campagne de mesure a eu lieu à l'O3HP, Observatoire du chêne pubescent à l'OHP (Observatoire de Haute-Provence), avec pour objectif de quantifier les émissions de COVB issues d'une forêt représentative de la région méditerranéenne. Des mesures de concentrations et de flux d'émission ont été réalisées à plusieurs échelles : de la branche à la canopée. L'étude des émissions à l'échelle du de la plante a permis une meilleure compréhension des paramètres environnementaux et physiologiques qui régissent les émissions. Les flux de COVB sortant de la canopée et exportés vers la troposphère ont pu être quantifiés. Les résultats montrent que le chêne blanc est un fort émetteur d'isoprène, mais un faible émetteur de méthanol et de monoterpènes. À l'échelle de la forêt, les flux d'émissions d'isoprène ont été estimés à 7,2 mg m-2 h-1 en conditions standard, ce qui fait de l'O3HP un des écosystèmes les plus fortement émetteurs d'isoprène à travers le monde.

International audience; Prescribed burning (PB) is gaining popularity as a low-cost forest protection measure that efficiently reduces fuel build-up, but its effects on tree health and growth are poorly understood. Here, we evaluated the impact of PB on plant defenses in Mediterranean pine forests (Pinus halepensis andP. nigra ssp. laricio). These chemical defenses were estimated based on needle secondary metabolites (terpenes and phenolics including flavonoids) and discussed in terms of chlorophyll fluorescence and soil nutrients. Three treatments were applied: absence of burning (control plots); single burns (plots burned once); and repeated burns (plots burned twice). For single burns, we also explored changes over time. In P. laricio, PB tended to trigger only minor modifications consisting exclusively of shortlived increases (observed within 3 months after PB) in flavonoid index, possibly due to the leaf temperature increase during PB. In P. halepensis, PB had detrimental effects on physiological performance, consisting of (i) significant decreases in actual PSII efficiency (ΦPSII) in light-adapted conditions after repeated PB, and (ii) short-lived decreases in variable-to-maximum fluorescence ratio (Fv/Fm) after single PB, indicating that PB actually stressed P. halepensistrees. Repeated PB also promoted terpene-like metabolite production, which increased 2 to 3-fold compared to control trees. Correlations between terpene metabolites and soil chemistry were found. These results suggest thatPB impacts needle secondary metabolism both directly (via a temperature impact) and indirectly (via soil nutrients), and that these impacts vary according to species/site location, frequency and time elapsed since last fire. Our findings are discussed with regard to the use of PB as a forest management technique and its consequences on plant investment in chemical defenses.

Orange trees are widely cultivated in Mediterranean climatic regions where they are an important agricultural crop. Citrus have been characterized as emitters of volatile organic compounds (VOC) in chamber studies under controlled environmental conditions, but an extensive characterization at field scale has never been performed using modern measurement methods, and is particularly needed considering the complex interactions between the orchards and the polluted atmosphere in which Citrus is often cultivated. For one year, in a Valencia orange orchard in Exeter, California, we measured fluxes using PTRMS (Proton Transfer Reaction Mass Spectrometer) and eddy covariance for the most abundant VOC typically emitted from citrus vegetation: methanol, acetone, and isoprenoids. Concentration gradients of additional oxygenated and aromatic compounds from the ground level to above the canopy were also measured. In order to characterize concentrations of speciated biogenic VOC (BVOC) in leaves, we analyzed leaf content by GC-MS (Gas Chromatography – Mass Spectrometery) regularly throughout the year. We also characterized in more detail concentrations of speciated BVOC in the air above the orchard by in-situ GC-MS during a few weeks in spring flowering and summer periods. Here we report concentrations and fluxes of the main VOC species emitted by the orchard, discuss how fluxes measured in the field relate to previous studies made with plant enclosures, and describe how VOC content in leaves and emissions change during the year in response to phenological and environmental parameters. The orchard was a source of monoterpenes and oxygenated VOC. The highest emissions were observed during the springtime flowering period, with mid-day fluxes above 2 nmol m−2 s−1 for methanol and up to 1 nmol m−2 s−1 for acetone and monoterpenes. During hot summer days emissions were not as high as we expected considering the known dependence of biogenic emissions on temperature. We provide evidence that thickening of leaf cuticle wax content limited gaseous emissions during the summer.

Terpenoid production (emission and storage) within foliage plays direct and indirect defensive and protective functions for the plant, mediates complex trophic relationships and controls the oxidation capacity of the atmosphere. Both biotic and abiotic conditions alter terpenoid production, with herbivory, light and temperature effects being reasonably well understood. In this manuscript, the state of the science about nutrient effect on terpenoid production is reviewed. The focus is on isoprene emissions and mono- and sesquiterpenoid emissions and concentrations according to fertilizing treatments and their potential interaction with other environmental factors. Ecological, physiological, biochemical and biophysical hypothesis formulated over research investigations are exposed and several points are highlighted as future research perspectives which could help to elucidate the apparent contrasting results.

Biogenic volatile organic compounds (BVOCs), released by practically all plants, have important atmospheric and ecological consequences. Because BVOC-emission measurements are especially tedious, complex and extremely variable between species, two approaches have been used in scientific studies to try to estimate BVOC-emission types and rates from plant species. The first, which has known little success, involves grouping species according to plant-taxonomy criteria (typically, genus and family). The second involves studying the correlation between BVOC content and emission (i.e. how leaf content could be used to estimate emissions). The latter strategy has provided controversial results, partly because BVOCs are amazingly chemically diverse, and, as a result, techniques used to study plant BVOC content, which we review, cannot be equally adequate for all analytes. In order to choose an adequate technique, two patterns must be distinguished. Specifically stored compounds – mainly monoterpenes and sesquiterpenes that dominate the essential oil obtained from a plant – are permanently and massively present in specific storage structures (e.g., secretory cavities, trichomes) of the order of μg/g–mg/g and usually allow emissions to occur during stress periods when terpenes are weakly synthesized. These BVOCs can be studied directly through traditional extraction techniques (e.g., hydrodistillation) and novel techniques (e.g., application of microwaves and ultrasound), and indirectly by trapping techniques involving the collection, within adsorbent material, of BVOCs present in the headspace of a plant. Non-specifically stored compounds (e.g., isoprene, 2-methyl-3-buten-2-ol, and, in species without storage structures, monoterpenes and sesquiterpenes) can only be temporarily accumulated in leaf aqueous and lipid phases in small concentrations of the order of ng/g. As a result, studying their concentration in leaves requires the use of trapping techniques, more sensitive to trace amounts. Unlike for specifically stored BVOCs, knowledge of the concentration of non-specifically stored BVOCs cannot provide any information regarding the emission potential of a species but, instead, provides crucial information to understand why BVOC emissions may be uncoupled from the physiological processes that drive their synthesis. We describe both extracting and trapping techniques and discuss them in terms of the technical choices that may cause losses of thermolabile constituents, chemical transformations, different volatile recoveries and suitability to represent plant content of BVOCs faithfully. The second part of this review addresses technical shortcomings and biological and environmental factors that may alter the correlations between BVOC content and emission from plants.

International audience; We investigated the effect of sewage sludge compost spreading on plant growth and leaf terpene emissions and content of Quercus coccifera, Rosmarinus officinalis and Cistus albidus in a Mediterranean shrubland. Measurements were performed during 3 consecutive summers on 2 different plots treated in 2002 or 2007 with 50 or 100 tons of compost per hectare, corresponding to observations carried out 2 months to 7 years after spreading. A slight nutrient enrichment of soil and leaves (R. officinalis and C. albidus) was observed, especially for phosphorous. Terpene emissions were not affected by compost spreading, although they tended to increase on treated plots after 6 and 7 years for R. officinalis and C. albidus respectively. Terpene content was not affected by any compost treatment. Leaf and stem growth were significantly enhanced by compost spreading after 2 and/or 7 years in all species with little difference between doses. Total leaf biomass on the last growth units was increased by more than 50% in C. albidus and more than 90% in Q. coccifera. The results suggest that compost spreading in Meditteranean shrublands has no or little direct effect on leaf terpene emissions, but indirectly leads to their increase through leaf biomass enhancement. Simulation of terpene emissions at stand level revealed an increase of terpene fluxes ranging between 6 and 13%, depending on the plant species. Overall, compost spreading was assessed to result in an emission rate of 1.1 kg ha−1 y−1 for a typical Q. coccifera shrubland, but can reach 2.6 kg ha−1 y−1 for a typical R. officinalis shrubland.

International audience; Compost spreading in Mediterranean shrublands has no strong short-term effect on Q. coccifera monoterpene emissions at leaf level. a b s t r a c t Monoterpene emissions of Quercus coccifera L. were repeatedly measured during the two years following the spreading of a sewage sludge compost at rates of 50 Mg ha À1 and 100 Mg ha À1 , in a twelve-year-old post-fire Mediterranean shrubland. We also monitored the patterns of change in soil and leaf nutrient content, plant water potential, chlorophyll fluorescence, and plant growth. Compost spreading resulted in weak changes in leaf nutrient content and plant water status, and therefore no significant effect on monoterpene emissions at leaf scale, except during one summer sampling, probably related to advanced leaf maturity with the highest compost rate. However, compost increased plant growth, particularly the leaf biomass. The results suggest that compost spreading in Mediterranean shrublands has no strong short-term effect on Q. coccifera monoterpene emissions at leaf level, but may indirectly increase volatile organic compound fluxes at the stand scale, which may contribute to regional ozone pollution.

Emissions of biogenic volatile organic compounds (BVOCs) are important precursors to both ozone and secondary organic aerosol formation. In this study, we identify and quantify volatile (C(10)) and intermediate-volatility (C(15)) BVOCs stored in and emitted from 22 prominent woody and herbaceous crops with a particular focus on sesquiterpenoids (SQTs), which have presented measurement challenges in previous studies. Monoterpenoids (MNTs) and SQTs were simultaneously emitted from all the crops studied; there were significant correlations between emission rates and leaf content for both MNTs and SQTs and additional correlations between MNTs and SQTs in both emissions and leaf content. Our results suggest that species with high concentrations of stored terpenoids in their leaves, such as those grown commercially for their essential oil content, are likely high BVOC emitters. Emissions from agricultural species were dominated by SQTs at low MNT emission rates (on the order of several tens of ng/(g(DM)*h)), while at higher MNT levels (on the order of several hundreds of ng/(g(DM)*h)), SQT emissions were approximately equivalent. Based on our empirical correlations, we estimate that global SQT emissions are similar to MNT emissions and on the order of 100 Tg yr(-1), which justifies the need for better representation of SQTs in both BVOC emission and atmospheric models.

The Citrus genus includes a large number of species and varieties widely cultivated in the Central Valley of California and in many other countries having similar Mediterranean climates. In the summer, orchards in California experience high levels of tropospheric ozone, formed by reactions of volatile organic compounds (VOC) with oxides of nitrogen (NO x ). Citrus trees may improve air quality in the orchard environment by taking up ozone through stomatal and non-stomatal mechanisms, but they may ultimately be detrimental to regional air quality by emitting biogenic VOC (BVOC) that oxidize to form ozone and secondary organic aerosol downwind of the site of emission. BVOC also play a key role in removing ozone through gas-phase chemical reactions in the intercellular spaces of the leaves and in ambient air outside the plants. Ozone is known to oxidize leaf tissues after entering stomata, resulting in decreased carbon assimilation and crop yield. To characterize ozone deposition and BVOC emissions for lemon ( Citrus limon ), mandarin ( Citrus reticulata ), and orange ( Citrus sinensis ), we designed branch enclosures that allowed direct measurement of fluxes under different physiological conditions in a controlled greenhouse environment. Average ozone uptake was up to 11 nmol s −1 m −2 of leaf. At low concentrations of ozone (40 ppb), measured ozone deposition was higher than expected ozone deposition modeled on the basis of stomatal aperture and ozone concentration. Our results were in better agreement with modeled values when we included non-stomatal ozone loss by reaction with gas-phase BVOC emitted from the citrus plants. At high ozone concentrations (160 ppb), the measured ozone deposition was lower than modeled, and we speculate that this indicates ozone accumulation in the leaf mesophyll.

The use of composted biosolids as an amendment for forest regeneration in degraded ecosystems is growing since sewage–sludge dumping has been banned in the European Community. Its consequences on plant terpenes are however unknown. Terpene emissions of both Rosmarinus officinalis (a terpene-storing species) and Quercus coccifera (a non-storing species) and terpene content of the former, were studied after a middle-term exposure to compost at intermediate (50 t ha −1 : D50) and high (100 t ha −1 : D100) compost rates, in a seven-year-old post-fire shrubland ecosystem. Some chlorophyll fluorescence parameters (Fv/Fm, ETR, Φ PSII ), soil and plant enrichment in phosphorus ( P ) and nitrogen ( N ) were monitored simultaneously in amended and non-amended plots in order to establish what factors were responsible for possible compost effect on terpenes. Compost affected all studied parameters with the exception of Fv/Fm and terpene content. For both species, mono- and sesquiterpene basal emissions were intensified solely under D50 plots. On the contrary leaf P , leaf N levels reached in D50 were partly responsible of terpene changes, suggesting that optimal N conditions occurred therein. N also affected ETR and Φ PSII which were, in turn, robustly correlated to terpene emissions. These results imply that emissions of terpene-storing and non-storing species were under nitrogen and chlorophyll fluorescence control, and that a correct management of compost rates applied on soil may modify terpene emission rate of plants, which in turn has consequences in air quality and plant defense mechanisms.

International audience; Competition is an important factor that has been extensively reported in the Mediterranean area. There is evidence that leaf terpene accumulation may vary between plants growing on calcareous and siliceous soils. In the present study, leaf terpene emissions from potted seedlings of Pinus halepensis, Cistus albidus, and Quercus coccifera, growing under natural environmental conditions on calcareous and siliceous substrates, were studied by using a bag enclosure method. In both substrates, seedlings were potted alone and in intraspecific competition, to examine the effect of substrate type and that of intraspecific competition on terpene emissions. The results showed that competition favored: (i) overall monoterpene and sesquiterpene emissions from Q. coccifera; (ii) overall monoterpene emissions from P. halepensis; (iii) overall sesquiterpene emissions from C. albidus. Substrate type affected terpene emissions to a limited extent and in a species-specific way. Whereas for Q. coccifera, the overall monoterpene emissions and that of Allo-aromadendrene were favored on siliceous substrate, no significant changes were found in emissions from P. halepensis. Only the release of AR-curcumene from C. albidus was higher on siliceous substrate. We also found high variability in terpene emission composition from the study species, particularly for P. halepensis and Q. coccifera. These two species released both monoterpenes and sesquiterpenes, instead of monoterpenes only, as shown in previous studies.

International audience; Little is known about terpene emissions released by plants in response to abiotic factors, except for climate-related factors. Standard emissions (E S) of monoterpenes (E SM) and sesquiterpenes (E SS)of Rosmarinus officinalis, Pinus halepensis and Cistus albidus in siliceous and calcareous sites were examined. Their dependency on some nutrients in these soils was also analyzed. The study was carried out in the south of France at the end of March, when C. albidus exhibited a leaf growth state, while the other two species exhibited a pre-budbreak state. The results revealed that E S of all major monoterpenes released by R. officinalis and E S of a-pinene and a-humulene of P. halepensis were higher in plants growing in calcareous soils. In contrast, for C. albidus, E SM and E S of b-bourbonene and a-humulene were higher in siliceous soils. E SM of all species was mainly correlated with nitrogen (N) and available phosphorous (P A), while dependency on Ca 2+ or K + was variable. None of these nutrients was significantly correlated with E SS , suggesting that sesquiterpene synthesis pathway requires different nutrient supplies. While higher soil nutrient content stimulated E SM of R. officinalis and P. halepensis, it had a negative effect on E SM of C. albidus, probably because C. albidus exhibited a different phenological state. Considering the soil nature, and particularly N and P A as inputs in plant terpene inventories could hence contribute to obtain more accurate terpene estimates.

There is evidence that secondary metabolism may modulate plant interactions and is modified by different biotic stress agents, such as herbivores or pathogens. However, it is poorly understood whether secondary metabolism is altered during competition among plants. The intraspecific and interspecific coexistence of some Mediterranean potted seedlings, namely Rosmarinus officinalis, Pinus halepensis, Cistus albidus and Quercus coccifera was investigated through their terpene accumulation within leaves (except for Q. coccifera, a non-storing species) and terpene emissions (for all species). Competition had both positive and negative effects for both terpene emissions and content, depending on the species a seedling coexisted with. For R. officinalis, terpene concentrations (1.8-cineole and camphor) and terpene emissions (camphene, camphor and overall monoterpenes) were lower when the neighbour species was P. halepensis. For C. albidus, no changes were observed in its content, while the overall sesquiterpene emissions (70% of total emissions) were reduced in all competition conditions, except in intraspecific competition. In the case of P. halepensis, the highest terpene content occurred when it grew with C. albidus, and in intraspecific competition, while its emissions were reduced under these conditions. Only emissions of Q. coccifera showed no significant changes in the different competition treatments.

International audience; Estimation of litter colonization by fungi, using ergosterol, an indicator of fungal biomass, is a reliable way to describe the process of leaf litter decomposition. This litter colonization by fungi is regulated both by exogenous or environmental factors, and endogenous factors, i.e. litter chemistry. In this work, we have examined the effects of some of these factors on litter fungal colonization in a Mediterranean ecosystem, by determining ergosterol content of Quercus coccifera leaf litter. Environmental factors have been studied through the fertility of the soil, by comparing plots amended with two rates of compost and plots without amendment. Results indicated that (i) compost had a significant effect on soil fertility but did not increase ergosterol content of leaf litter and (ii) soil humidity improved leaf litter colonization by fungi. Endogenous factors have been studied through measurements of total phenolic and ergosterol concentrations of seven shrub species leaf litter. We have shown (i) a negative significant correlation between total phenolic compounds and ergosterol concentrations of leaf litter and (ii) a positive significant correlation between total phenolic compound concentrations in green leaves and in leaf litter. We conclude that, in this Mediterranean shrub ecosystem, leaf litter colonization by fungi is controlled by soil moisture and plant leaf litter quality.

International audience; Mediterranean soils are generally low in organic matter and have a low water holding capacity. Moreover, recurrent fires are common in Mediterranean regions and increase the deterioration of these soils. Soil properties and ecosystem resilience after fire could be improved by amendment with compost, which is a source of organic matter and nutrients. A way to determine ecosystem integrity is to study leaf litter breakdown, which is a vital process in the functioning of terrestrial ecosystems and involves fungi and bacteria. In this study, we determined fungal and bacterial biomass associated with decomposing oak leaves (Quercus coccifera L.) in a burnt shrub amended ecosystem during 1.5 years. Three treatments were studied: control, 50 t ha À1 and 100 t ha À1 of co-composted sewage sludge and green wastes. Bacterial biomass was not affected, although moisture, N and P concentrations in litter were increased by the amendment. However, fungal biomass was depressed on plots amended with 100 t ha À1 , and negatively correlated to exchangeable P. For all treatments, both types of microorganisms followed marked seasonal dynamics, with peaks of biomass during the wet periods of the year. #