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By this in vitro study addressing greenhouse gas (GHG) emissions from soil-plant mesocosms, we suggest a method to investigate the joint effects of environmental conditions, growth of plants, and agricultural soil management. Soils from two long-term agricultural trials in France were placed in climate chambers. The rotation trial was with or without grain legumes, and the tillage trial used plowing or reduced tillage. Environmental conditions consisted of two contrasting temperature regimes combined with ambient (400 ppm) or high (700 ppm) CO2 concentrations in climate chambers. The plant growth went from seeding to vegetative growth. Carbon dioxide gas exchange measurements were conducted in both soil types for a period representing initial plant growth. The CO2 exchange was influenced by the growing plants increasing the mesocosm respiration and gross ecosystem production. The environmental settings had no noticeable impact on the CO2 exchange in the soils from the legume trial. The CO2 exchange from the tillage trial soils exhibited variations induced by the environmental conditions depending on the tillage treatment. The N2O emission measurements in the legume trial soils showed little variability based on rotation, however, in soils with legumes, indications that higher temperatures will lead to more N2O emission were seen.

The triploidy has proved to be a powerful approach breeding programs, especially in Citrus since seedlessness is one of the main consumer expectations. Citrus plants face numerous abiotic stresses including water deficit, which negatively impact growth and crop yield. In this study, we evaluated the physiological and biochemical responses to water deficit and recovery capacity of new triploid hybrids, in comparison with diploid hybrids, their parents (“Fortune” mandarin and “Ellendale” tangor) and one clementine tree used as reference. The water deficit significantly decreased the relative water content (RWC) and leaf gas exchange (Pnet and gs) and it increased the levels of oxidative markers (H2O2 and MDA) and antioxidants. Compared to diploid varieties, triploid hybrids limited water loss by osmotic adjustment as reflected by higher RWC, intrinsic water use efficiency (iWUE Pnet/gs) iWUE and leaf proline levels. These had been associated with an effective thermal dissipation of excess energy (NPQ) and lower oxidative damage. Our results showed that triploidy in citrus enhances the recovery capacity after a water deficit in comparison with diploids due to better carboxylation efficiency, restored water-related parameters and efficient antioxidant system.

End-stage lung diseases may result in death either by oxygenation and carbon dioxide exchange insufficiency or by right heart failure. Concerning the therapeutic options currently available, macroscopic blood oxygenators based on extracorporeal membrane (ECMO) technology are currently used. However, the environment of an intensive care unit is still required. Modern oxygenators need to be exchanged within a couple of weeks because of clotting.In this context, the goal of my PhD was the development of a novel microfluidic device for blood oxygenation, which exhibits a large surface area of gas exchange and can support long-term sustainable endothelialization of blood microcapillaries enhancing its hemo-compatibility for clinical applications. Numerical calculations based on the gas exchange model of Potkay et al. helped to best size the three-layer "blood microcapillary / membrane / gas microchannel" system.I then developed a microfabrication protocol that allows the integration of a thin polymer membrane with a very large surface area, producing robust sealed oxygenators.The gas exchange performances achieved with venous pig blood are remarkable both for unit trilayers, and for stacked structures with a low reduced injection volume, high oxygenation (379 ml O2 / min / m²) at a flow rate high (15 ml/min). These experimental results could be compared to numerical calculations. Finally, with an optimized geometry minimizing shear stress, a sustainable endothelialization protocol in blood capillaries has been proposed.; Le poumon est un organe vital dont les pathologies au stade terminal peuvent induire une insuffisance circulatoire avec une défaillance cardiaque droite secondaire. Concernant les options thérapeutiques disponibles, des oxygénateurs sanguins macroscopiques basés sur la technologie des membranes extracorporelles (ECMO) sont actuellement utilisés au sein d'une unité de soins intensifs. Ces oxygénateurs doivent être remplacés en quelques semaines en raison de la coagulation dans le système.Dans ce contexte, le but de mon doctorat était de développer un dispositif microfluidique pour l'oxygénation du sang, qui présente une grande surface d'échange gazeux et capable de soutenir une endothélialisation durable à long terme des microcapillaires sanguins améliorant son hémocompatibilité pour les applications cliniques.Des calculs numériques basés sur le modèle d’échange gazeux de Potkay et coll. ont permisde comprendre le rôle de chaque paramètre géométrique sur l’échange gazeux et, donc, de dimensionner au mieux le système tri-couches « microcapillaire de sang / membrane / microcanal de gaz ».J’ai ensuite mis au point un protocole de microfabrication qui permet d’intégrer une membrane fine de polymère de très grande surface, et de fabriquer des oxygénateurs robustes et étanches sous pression.Les performances d’échange gazeux mesurées avec du sang veineux de cochon sont remarquables, tant pour les tri-couches unitaires que pour les structures empilées, avec un faible volume d’injection et une oxygénation élevée (379 ml O2/min/m²) à débit élevé (15ml/min). Ces résultats expérimentaux ont pu être comparés aux calculs numériques. Enfin, avec une géométrie optimisée pour minimiser la contrainte de cisaillement, un protocole d’endothélialisation durable dans les capillaires sanguins a été proposé.

Les mesures d'échanges des principaux gaz à effet de serre (CO2, N2Oet CH4) entre le sol et l'atmosphère sont des données importantes dans l'étude du fonctionnement des agrosystèmes. Ces données peuvent être mesurées à haute résolution temporelle grâce à des chambres de mesure automatiques qui constituent actuellement la technique de référence. Les systèmes automatisés commerciaux sont coûteux, ce qui limite le nombre de points de mesure sur un même site, et donc, la capacité des équipes de recherche à mieux intégrer et comprendre la variabilité spatiale de l'activité métabolique du sol et plus généralement les cycles biogéochimiques qui lui sont associés. Dans cette étude, nous proposons les plans pour réaliser soi-même un système multiplexé original de chambres de mesure automatisées. Nous avons opté ici pour une fermeture horizontale, afin de minimiser les effets liés à la surpression lors de la fermeture et/ou passer sous des branches basses. Des résultats de tests de fuite et de fonctionnement en plein champ pendant plusieurs mois sont présentés. Une attention particulière a été apportée au coût du matériel, à la consommation énergétique ainsi qu'à la simplicité de mise en oeuvre. Les informations techniques relatives à la conception, à la fabrication, à l'utilisation de ces chambres et à l'analyse des données de sortie sont détaillées dans cet article, avec les plans, les programmes de pilotage et les codes pour l'analyse de données.

Background and Aims Predicting both plant water status and leaf gas exchange under various environmental conditions is essential for anticipating the effects of climate change on plant growth and productivity. This study developed a functional-structural grapevine model which combines a mechanistic understanding of stomatal function and photosynthesis at the leaf level (i.e. extended Farqhuhar-von Caemmerer-Berry model) and the dynamics of water transport from soil to individual leaves (i.e. Tardieu-Davies model). [br/] Methods The model included novel features that account for the effects of xylem embolism (f(PLC)) on leaf hydraulic conductance and residual stomatal conductance (g(0)), variable root and leaf hydraulic conductance, and the microclimate of individual organs. The model was calibrated with detailed datasets of leaf photosynthesis, leaf water potential, xylem sap abscisic acid (ABA) concentration and hourly whole-plant transpiration observed within a soil drying period, and validated with independent datasets of whole-plant transpiration under both well-watered and water-stressed conditions. [br/] Key Results The model well captured the effects of radiation, temperature, CO2 and vapour pressure deficit on leaf photosynthesis, transpiration, stomatal conductance and leaf water potential, and correctly reproduced the diurnal pattern and decline of water flux within the soil drying period. In silico analyses revealed that decreases in g(0) with increasing f(PLC) were essential to avoid unrealistic drops in leaf water potential under severe water stress. Additionally, by varying the hydraulic conductance along the pathway (e.g. root and leaves) and changing the sensitivity of stomatal conductance to ABA and leaf water potential, the model can produce different water use behaviours (i.e. iso- and anisohydric). [br/] Conclusions The robust performance of this model allows for modelling climate effects from individual plants to fields, and for modelling plants with complex, non-homogenous canopies. In addition, the model provides a basis for future modelling efforts aimed at describing the physiology and growth of individual organs in relation to water status.

This paper presents HydroShoot, a leaf-based functional-structural plant model (FSPM) that simulates gas exchange rates of complex plant canopies under water deficit conditions. HydroShoot is built assuming that simulating both the hydraulic structure of the shoot together with the energy budget of individual leaves is the asset for successfully scaling-up leaf to canopy gas exchange rates. HydroShoot includes three interacting modules: hydraulic, which calculates the distribution of xylem water potential across shoot hydraulic segments; energy, which calculates the complete energy budget of individual leaves; and exchange, which calculates net carbon assimilation and transpiration rates of individual leaves. HydroShoot was evaluated on virtual and real grapevines having strongly contrasted canopies, under well-watered and water deficit conditions. It captured accurately the impact of canopy architecture and soil water status on plant-scale gas exchange rates and leaf-scale temperature and water potential. Both shoot hydraulic structure and leaf energy budget simulations were, as postulated, required to adequately scaling-up leaf to canopy gas exchange rates. Notwithstanding, simulating shoot hydraulic structure was found more necessary to adequately performing this scaling task than simulating leaf energy budget. That is, the intra-canopy variability of leaf water potential was a better predictor of the reduction of whole plant gas exchange rates under water deficit than the intra-canopy variability of leaf temperature. We conclude that simulating the shoot hydraulic structure is a prerequisite if FSPMs are to be used to assess gas exchange rates of complex plant canopies as those of grapevines. Finally, HydroShoot is available through the OpenAlea platform (https://github.com/openalea/hydroshoot) as a set of reusable modules. EEA Mendoza Fil: Albasha, Rami. Institut National de la Recherche Agronomique. LEPSE Montpellier; Francia Fil: Fournier, Christian. Institut National de la Recherche Agronomique. LEPSE Montpellier; Francia Fil: Pradal, Christophe. CIRAD-UMR AGAP; Francia Fil: Chelle, Michael. Institut National de la Recherche Agronomique. Ecosys; Francia Fil: Prieto, Jorge Alejandro. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Mendoza; Argentina. Fil: Louarn, Gaëtan. Institut National de la Recherche Agronomique; Francia Fil: Simonneau, Thierry. Institut National de la Recherche Agronomique. LEPSE Montpellier; Francia Fil: Lebon, Eric. Institut National de la Recherche Agronomique. Unité Mixte de Recherche; Francia

The amount of residual gas trapped in the cylinder of an internal combustion engine has a huge influence on its behavior (combustion, efficiency,emission,..), in particular for spark ignition engines. Nowadays, it is possible to modify this amount, in particular with variable valve train. However, the experimental assessment of residual gas content and its evaluation with numerical simulation are still challenging. The objective of this study is to propose new methodologies to improve these two aspects. A bibliographical survey is first proposed to give state of the art. It gathers the main parameters influencing residual gas content, the effects of residual gas on engine behavior, experimental procedures and numerical models available for residual gas content estimation. An original system is then developed to measure the amount of residual gas with an in-cylinder gassampling triggered at the end of compression stroke. The results, obtained on the whole operating map of a naturally aspirated automotive spark ignition engine, are analyzed with respect to engine rotation speed, load and cam phaser position (intake side). Finally, various modeling of valve overlap with a 0D/1D approach are assessed. The standard “perfect mixing” assumption is not fully satisfactory, so that new assumptions are proposed and tested. A hybrid approach combining “perfect mixing” and “perfect displacement” allows for improved agreement with experiments, after calibration of a model parameter with respect to engine rotation speed and load.; La quantité de gaz résiduels présents dans le cylindre d’un moteur à combustion interne a une influence important sur son fonctionnement (combustion, rendement, émissions,..) particulièrement en allumage commandé. Aujourd’hui, il est possible de modifier cette quantité, notamment grâce à des systèmes de distribution variable. Cependant, la détermination expérimentale de la quantité de résiduels et l’estimation à partir de modèles numériques restent délicates. L’objectif de cette thèse est de proposer de nouvelles méthodologies pour traiter ces deux problématiques.Un point bibliographique est tout d’abord effectué pour dresser un état de l’art. Il recense les principaux paramètres influençant la quantité de résiduels, les effets des résiduels sur le fonctionnement du moteur, les moyens expérimentaux et les modèles disponibles pour en évaluer la quantité. Un système original est ensuite développé pour mesurer la quantité de résiduels à partir d’un prélèvement gazeux effectué dans le cylindre à la fin de la compression. Les résultats ainsi obtenus sur l’ensemble du champ de fonctionnement d’un moteur automobile atmosphérique à allumage commandé sont ensuite analysés en fonction du régime, de la charge et de la position du déphaseur installé sur l’arbre à came d’admission.Enfin, plusieurs modélisations de la phase de croisement des soupapes en approche 0D/1D sont évaluées. L’approche classique de mélange parfait n’étant pas satisfaisante, de nouvelles approches originales sont proposée et testées. Une approche hybride mêlant mélange parfait et déplacement parfait permet d’obtenir des résultats améliorés, après calibration d’un paramètre en fonction du régime et de la charge du moteur.

The amount of residual gas trapped in the cylinder of an internal combustion engine has a huge influence on its behavior (combustion, efficiency,emission,..), in particular for spark ignition engines. Nowadays, it is possible to modify this amount, in particular with variable valve train. However, the experimental assessment of residual gas content and its evaluation with numerical simulation are still challenging. The objective of this study is to propose new methodologies to improve these two aspects. A bibliographical survey is first proposed to give state of the art. It gathers the main parameters influencing residual gas content, the effects of residual gas on engine behavior, experimental procedures and numerical models available for residual gas content estimation. An original system is then developed to measure the amount of residual gas with an in-cylinder gassampling triggered at the end of compression stroke. The results, obtained on the whole operating map of a naturally aspirated automotive spark ignition engine, are analyzed with respect to engine rotation speed, load and cam phaser position (intake side). Finally, various modeling of valve overlap with a 0D/1D approach are assessed. The standard “perfect mixing” assumption is not fully satisfactory, so that new assumptions are proposed and tested. A hybrid approach combining “perfect mixing” and “perfect displacement” allows for improved agreement with experiments, after calibration of a model parameter with respect to engine rotation speed and load.; La quantité de gaz résiduels présents dans le cylindre d’un moteur à combustion interne a une influence important sur son fonctionnement (combustion, rendement, émissions,..) particulièrement en allumage commandé. Aujourd’hui, il est possible de modifier cette quantité, notamment grâce à des systèmes de distribution variable. Cependant, la détermination expérimentale de la quantité de résiduels et l’estimation à partir de modèles numériques restent délicates. L’objectif de cette thèse est de proposer de nouvelles méthodologies pour traiter ces deux problématiques.Un point bibliographique est tout d’abord effectué pour dresser un état de l’art. Il recense les principaux paramètres influençant la quantité de résiduels, les effets des résiduels sur le fonctionnement du moteur, les moyens expérimentaux et les modèles disponibles pour en évaluer la quantité. Un système original est ensuite développé pour mesurer la quantité de résiduels à partir d’un prélèvement gazeux effectué dans le cylindre à la fin de la compression. Les résultats ainsi obtenus sur l’ensemble du champ de fonctionnement d’un moteur automobile atmosphérique à allumage commandé sont ensuite analysés en fonction du régime, de la charge et de la position du déphaseur installé sur l’arbre à came d’admission.Enfin, plusieurs modélisations de la phase de croisement des soupapes en approche 0D/1D sont évaluées. L’approche classique de mélange parfait n’étant pas satisfaisante, de nouvelles approches originales sont proposée et testées. Une approche hybride mêlant mélange parfait et déplacement parfait permet d’obtenir des résultats améliorés, après calibration d’un paramètre en fonction du régime et de la charge du moteur.

Background and Aims Predicting both plant water status and leaf gas exchange under various environmental conditions is essential for anticipating the effects of climate change on plant growth and productivity. This study developed a functional-structural grapevine model which combines a mechanistic understanding of stomatal function and photosynthesis at the leaf level (i.e. extended Farqhuhar-von Caemmerer-Berry model) and the dynamics of water transport from soil to individual leaves (i.e. Tardieu-Davies model). [br/] Methods The model included novel features that account for the effects of xylem embolism (f(PLC)) on leaf hydraulic conductance and residual stomatal conductance (g(0)), variable root and leaf hydraulic conductance, and the microclimate of individual organs. The model was calibrated with detailed datasets of leaf photosynthesis, leaf water potential, xylem sap abscisic acid (ABA) concentration and hourly whole-plant transpiration observed within a soil drying period, and validated with independent datasets of whole-plant transpiration under both well-watered and water-stressed conditions. [br/] Key Results The model well captured the effects of radiation, temperature, CO2 and vapour pressure deficit on leaf photosynthesis, transpiration, stomatal conductance and leaf water potential, and correctly reproduced the diurnal pattern and decline of water flux within the soil drying period. In silico analyses revealed that decreases in g(0) with increasing f(PLC) were essential to avoid unrealistic drops in leaf water potential under severe water stress. Additionally, by varying the hydraulic conductance along the pathway (e.g. root and leaves) and changing the sensitivity of stomatal conductance to ABA and leaf water potential, the model can produce different water use behaviours (i.e. iso- and anisohydric). [br/] Conclusions The robust performance of this model allows for modelling climate effects from individual plants to fields, and for modelling plants with complex, non-homogenous canopies. In addition, the model provides a basis for future modelling efforts aimed at describing the physiology and growth of individual organs in relation to water status.

In Mediterranean mountains, Pinus sylvestris L. is expected to be displaced under a warming climate by more drought-tolerant species such as the sub-Mediterranean Quercus pyrenaica Willd. Understanding how environmental factors drive tree physiology and phenology is, therefore, essential to assess the effect of changing climatic conditions on the performance of these species and, ultimately, their distribution. We compared the cambial and leaf phenology and leaf gas exchange of Q. pyrenaica and P. sylvestris at their altitudinal boundary in Central Spain and assessed the environmental variables involved. Results indicate that P. sylvestris cambial phenology was more sensitive to weather conditions (temperature at the onset and water deficit at the end of the growing season) than Q. pyrenaica. On the other hand, Q. pyrenaica cambial and leaf phenology were synchronized and driven by photoperiod and temperatures. Pinus sylvestris showed lower photosynthetic nitrogen-use efficiency and higher intrinsic water-use efficiency than Q. pyrenaica as a result of a tighter stomatal control in response to summer dry conditions, despite its less negative midday leaf water potentials. These phenological and leaf gas exchange responses evidence a stronger sensitivity to drought of P. sylvestris than that of Q. pyrenaica, which may therefore hold a competitive advantage over P. sylvestris under the predicted increase in recurrence and intensity of drought events. On the other hand, both species could benefit from warmer springs through an advanced phenology, although this effect could be limited in Q. pyrenaica if it maintains a photoperiod control over the onset of xylogenesis.

The exchanges of carbon, water and energy between the atmosphere and the Amazon basin have global implications for the current and future climate. Here, the global atmospheric inversion system of the Monitoring of Atmospheric Composition and Climate (MACC) service is used to study the seasonal and interannual variations of biogenic CO2 fluxes in Amazonia during the period 2002–2010. The system assimilated surface measurements of atmospheric CO2 mole fractions made at more than 100 sites over the globe into an atmospheric transport model. The present study adds measurements from four surface stations located in tropical South America, a region poorly covered by CO2 observations. The estimates of net ecosystem exchange (NEE) optimized by the inversion are compared to an independent estimate of NEE upscaled from eddy-covariance flux measurements in Amazonia. They are also qualitatively evaluated against reports on the seasonal and interannual variations of the land sink in South America from the scientific literature. We attempt at assessing the impact on NEE of the strong droughts in 2005 and 2010 (due to severe and longer-than-usual dry seasons) and the extreme rainfall conditions registered in 2009. The spatial variations of the seasonal and interannual variability of optimized NEE are also investigated. While the inversion supports the assumption of strong spatial heterogeneity of these variations, the results reveal critical limitations of the coarse-resolution transport model, the surface observation network in South America during the recent years and the present knowledge of modelling uncertainties in South America that prevent our inversion from capturing the seasonal patterns of fluxes across Amazonia. However, some patterns from the inversion seem consistent with the anomaly of moisture conditions in 2009.

Soil gas fluxes depend on soil gas concentrations and physical properties of a soil. Taking soil samples for physical analysis into the laboratory strongly modifies soil gas concentrations and also cuts roots that sustain the activity in the rhizosphere. Since microbial processes interact with gas concentrations in soil, we need to study gas transport and production in situ. We developed a method to monitor the transport and production and consumption of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in soils in situ in a two dimensional (2D) profile using tetrafluoromethane (CF4) and sulfur hexafluoride (SF6) as tracer gases and Finite Element Modeling of soil gas transport. Continuous injection of the inert tracer gases and 2D gas sampling in a soil profile allowed for inverse modeling of the 2D profile of soil gas diffusivity. In a second step, the 2D profiles of the production and consumption of CO2, CH4, and N2O were inversely determined. Soil gas concentrations were monitored in a Scots pine stand in South-West Germany during a rain-free week in the fall. The 2D relative (so as to be independent of gas species) soil gas diffusivity profile showed large horizontal variability. Relative soil gas diffusivity was found to be anisotropic with the vertical direction greater by a factor of 1.26. Topsoil moisture decreased slowly over time resulting in an increase in relative soil gas diffusivity. The soil was found to be a source of CO2, and a net sink of CH4 and N2O, with the highest production (CO2) and consumption (CH4, N2O) occurring in the topsoil. The gas concentration and production profiles of CO2 were nearly horizontally homogenous, while those for CH4 showed larger horizontal differences. Net consumption of CH4 and net production of CO2 both increased as the soil dried. This occurred despite reverse trends for these variables in the topsoil (0–8 cm depth) which were more than offset by the underlying soil becoming more active. Sensitivity tests showed that the determination of 2D profiles of soil gas diffusivity and production and consumption of CO2 and CH4 were more reliable than the estimates for N2O because the magnitudes of these for N2O were very low. Our method represents a useful tool for the analyses of soil gas flux heterogeneities and associated microbial processes within soil profiles.

Résumé Les techniques de circulation extracorporelle (CEC) peuvent être utilisées dans les défaillances respiratoires graves des syndromes de détresse respiratoire aiguë (SDRA) avec trois objectifs : 1) assurer une oxygénation satisfaisante en court-circuitant le poumon malade grâce à une circulation veinoveineuse à haut débit ; cette technique assure sans difficulté l’épuration de CO2 ; 2) assurer avant tout une élimination partielle de CO2 dans le but de protéger le poumon d’une ventilation mécanique dangereuse. Des débits sanguins quatre à cinq fois plus faibles sont suffisants avec une circulation veinoveineuse ou artérioveineuse sans pompe ; 3) exceptionnellement, la prise en charge d’une défaillance cardiaque associée peut nécessiter une circulation veinoartérielle à haut débit. Des études physiologiques détaillées et des essais cliniques sont indispensables pour mieux connaître les indications de ces techniques.

Ripening of climacteric fruits is associated with pronounced changes in fruit gas composition caused by a concomitant rise in respiration and ethylene production. There is a discrepancy in the literature since some authors reported that changes in fruit gas compositions differ in attached and detached fruits. This study presents for the first time an overview of pre- and post-harvest factors that lead to variations in the climacteric respiration and ethylene production, and attempts to determine their impacts on fruit composition, i.e., dry matter, total soluble solids content and acidity. The impact of growing conditions such as the fruit position in the canopy and the fruit carbon supply; fruit detachment from the tree, including the maturity stage at harvest; and storage conditions after harvest, i.e., relative humidity and temperature were considered as well as changes in fruit skin resistance to gas diffusion during fruit growth and storage. Results showed that fruit gas composition vary with all pre and post-harvest factors studied. Although all mangoes underwent a respiratory climacteric and an autocatalytic ethylene production, whatever pre and post-harvest factors studied, large differences in ethylene production, climacteric respiration and fruit quality were measured. Results suggested that the ripening capacity is not related to the fruit ability to produce great amount of ethylene. In agreement with precedent studies, this work provided several lines of evidence that gas composition of fruit is related to its water balance. Our measurements indicated that skin resistance to gas diffusion increased after the harvest and during storage. It was so suggested that the faster ripening of detached fruit may be explained in part by changes in fruit water balance and skin resistance to gas diffusion caused by fruit detachment.

International audience; Frequency and intensity of heat waves and drought events are expected to increase in Europe due to climate change. European beech (Fagus sylvatica L.) is one of the most important native tree species in Europe. Beech populations originating throughout its native range were selected for common-garden experiments with the aim to determine whether there are functional variations in drought stress responses among different populations. One-year old seedlings from four to seven beech populations were grown and drought treated in a greenhouse, replicating the experiment at two contrasting sites, in Italy (Mediterranean mountains) and Germany (Central Europe). Experimental findings indicated that: (1) drought (water stress) mainly affected gas exchange describing a critical threshold of drought response between 30 and 26% SWA for photosynthetic rate and C-i/C-a, respectively; (2) the C-i to C-a ratio increased substantially with severe water stress suggesting a stable instantaneous water use efficiency and an efficient regulation capacity of water balance achieved by a tight stomatal control; (3) there was a different response to water stress among the considered beech populations, differently combining traits, although there was not a well-defined variability in drought tolerance. A combined analysis of functional and structural traits for detecting stress signals in beech seedlings is suggested to assess plant performance under limiting moisture conditions and, consequently, to estimate evolutionary potential of beech under a changing environmental scenario.

Grasslands represent canopies with a complex structure where sources and sinks of ammonia (NH3) may coexist at the plant level. Moreover, management practices such as mowing, hay production and grazing may change the composition of the sward and hence the source-sink relationship at the canopy level as well as the interaction with the atmosphere. There is therefore a need to understand the exchange of ammonia between grasslands and the atmosphere better, especially regarding the location and magnitude of sources and sinks. Fluxes of atmospheric NH3 within a grassland canopy were assessed in the field and under controlled conditions using a dynamic chamber technique (cuvette). These cuvette measurements were combined with extraction techniques to estimate the ammonium (NH4+) concentration and the pH of a given part of the plant or soil, leading to an estimated ammonia compensation point (Cp). The combination of the cuvette and the extraction techniques was used to identify the potential sources and sinks of NH3 within the different compartments of the grassland: the soil, the litter or senescent "litter leaves", and the functioning "green leaves". A set of six field experiments and six laboratory experiments were performed in which the different compartments were either added or removed from the cuvettes. The results show that the cuvette measurements agree with the extraction technique in ranking the strength of compartment sources. It suggests that in the studied grassland the green leaves were mostly a sink for NH3 with a compensation point around 0.1–0.4 μg m−3 and an NH3 flux of 6 to 7 ng m−2 s−1. Cutting of the grass did not increase the NH3 fluxes of the green leaves. The litter was found to be the largest source of NH3 in the canopy, with a Cp of up to 1000 μg m−3 NH3 and an NH3 flux up to 90 ng m−2 s−1. The litter was found to be a much smaller NH3 source when dried (Cp=160 μg m−3 and FNH3=35 ng m−2 s−1 NH3). Moreover emissions from the litter were found to vary with the relative humidity of the air. The soil was a strong source of NH3 in the period immediately after cutting (Cp=320 μg m−3 and FNH3=60 ng m−2 s−1), which was nevertheless always smaller than the litter source. The soil NH3 emissions lasted, however, for less than one day, and were not observed with sieved soil. They could not be solely explained by xylem sap flow extruding NH4+. These results indicate that future research on grassland-ammonia relationships should focus on the post-mowing period and the role of litter in interaction with meteorological conditions.

In agroforestry systems, the effect of shade trees on coffee net photosynthesis (A n) has been the object of debates among coffee scientists. In this study, we undertook over 600 coffee A n “spot” measurements under four different artificial shade levels (100, 72, 45 and 19% of full solar irradiance) and analyzed limitations to A n by low light availability (photon flux density, PFD) and stomatal conductance (g s). These gas exchange measurements were carried out during two consecutive coffee growing seasons in a commercial plantation in the Orosi valley of Costa Rica. Levels of A n were related to PFD and g s in order to calculate envelope functions which were used to establish PFD or g s limitations to A n. Under the growing conditions of the present trial, mean leaf A n remained stable for growth irradiance (GI) as low as 45% of full sun and decreased by ~20% at 19% GI. Limitation to A n due to g s was strong in full sun and decreasing with increasing shade levels. On the other hand, limitation due to PFD remained at a similar level for all shade treatments. These different evolutions of limitations of A n by PFD and g s in response to shade explain the absence of a decrease in coffee leaf A n with a shade level up to 55%. Consequently, these results confirm that Arabica coffee is a shade-adapted plant with leaves that can maintain a high photosynthetic performance under low light availability.

The exchange of ammonia between crop canopies and the atmosphere depends on a range of plant parameters and climatic conditions. However, little is known about effects of management factors. We have here investigated the stomatal ammonia compensation point in response to cutting and fertilization of a grass sward dominated by Lolium perenne. Tall grass had a very low NH3 compensation point (around 1 nmol mol−1), reflecting the fact that leaf nitrogen (N) concentration was very low. During re-growth after cutting, leaf tissue concentrations of NO3−, NH4+, soluble N and total N increased along with apoplastic NH4+ concentrations. In contrast, apoplastic pH decreased resulting in largely unaltered NH3 compensation points. Nitrogen fertilization one week after cutting caused the apoplastic NH4+ concentration of the newly emerging leaves to increase dramatically. The NH3 compensation point peaked between 15 and 25 nmol mol−1 the day after the fertiliser was applied and thereafter decreased over the following 10 days until reaching the same level as before fertilisation. Ammonium concentrations in leaf apoplast, bulk tissue and litter were positively correlated (P=0.001) throughout the experimental period. Bulk tissue NH4+ concentrations, total plant N and soil NH4+ concentrations also showed a positive correlation. A very high potential for NH3 emission was shown by the plant litter.

Cinq variétés de niébé (Vigna unguiculata), Bambey 21, Gorom local, KVX61-1, Mouride et TN88-63, cultivées en pots en serre ont été soumises à un déficit hydrique par suspension d'arrosage pendant 14 jours en phase végétative (T1) et 12 jours au stade floraison (T2). Les incidences de ce traitement sur le potentiel hydrique foliaire, les échanges gazeux, le volume racinaire, les teneurs en proline, en amidon et en protéines totales des feuilles, le rendement maximal photochimique (fp0) et les composantes de rendement ont été déterminées. Le potentiel hydrique n'a significativement baissé que chez Mouride et TN88-63 (de -0,55 à -0,92 MPa en moyenne) stressés en floraison, tandis que le volume racinaire, les échanges gazeux, ainsi que la teneur en amidon ont été significativement réduits chez les 5 variétés en conditions de stress aux 2 stades. (fp0) n'a pas été affecté par le stress en phase végétative. En phase floraison il a significativement baissé dès le 6e jour d'application chez Gorom, KVX61-1 et TN88-63 et au 10e chez Bambey 21 et Mouride. Une accumulation significative de la proline due au déficit hydrique a été observée chez les 5 variétés en phase T1 et T2, Mouride et TN88-63 ont les teneurs les plus élevées (respectivement 2,9 et 3,3 mg.g-1 MS) en phase floraison. La teneur en protéines totales n'a pas été significativement modifiée par le stress aux 2 stades. Nos résultats ont montré que les 5 variétés ont évité la déshydratation en baissant la conductance stomatique et la transpiration lors du stress en T1 et T2. L'accumulation de la proline, le maintien de la teneur en protéines totales et la baisse de la teneur en amidon chez les 5 génotypes en conditions de stress aux 2 stades pourraient contribuer au maintien de la turgescence cellulaire. En outre, ces solutés permettraient de protéger l'appareil photosynthétique (PSII) contre la dénaturation notamment durant le stress en floraison. Le nombre de graines par gousse et le nombre de graines par plante ont été réduits en conditions de déficit hydrique, la différence variétale observée a montré que Bambey 21 a été moins affecté que Gorom, TN88-63 et Mouride tandis que KVX61-1 s'est révélée la plus sensible. Bambey 21 s'est montrée tolérante au stress durant les 2 stades, Gorom, Mouride et TN88-63 ont été intermédiaires tandis que KVX61-1 a été la plus sensible.

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International audience; In this study the sensitivity of peach tree (Prunus persica L.) to three water stress levels from mid-pit hardening until harvest was assessed. Seasonal patterns of shoot and fruit growth, gas exchange (leaf photosynthesis, stomatal conductance and transpiration) as well as carbon (C) storage/mobilization were evaluated in relation to plant water status. A simple C balance model was also developed to investigate sink-source relationship in relation to plant water status at the tree level. The C source was estimated through the leaf area dynamics and leaf photosynthesis rate along the season. The C sink was estimated for maintenance respiration and growth of shoots and fruits. Water stress significantly reduced gas exchange, and fruit, and shoot growth, but increased fruit dry matter concentration. Growth was more affected by water deficit than photosynthesis, and shoot growth was more sensitive to water deficit than fruit growth. Reduction of shoot growth was associated with a decrease of shoot elongation, emergence, and high shoot mortality. Water scarcity affected tree C assimilation due to two interacting factors: (i) reduction in leaf photosynthesis (-23% and -50% under moderate (MS) and severe (SS) water stress compared to low (LS) stress during growth season) and (ii) reduction in total leaf area (-57% and -79% under MS and SS compared to LS at harvest). Our field data analysis suggested Psi(stem) threshold of -1.5 MPa below which daily net C gain became negative, i.e. C assimilation became lower than C needed for respiration and growth. Negative C balance under MS and SS associated with decline of trunk carbohydrate reserves -may have led to drought-induced vegetative mortality.

Predicting microbial safety of fresh products in Modified Atmosphere Packaging (MAP) systems implies to take into account the dynamic of O2 and CO2 exchanges in the system and its effect on microbial growth. In this purpose we coupled mathematical models of gas transfer (permeation through packaging and solubilisation / diffusion within food) with predictive microbiology models that take into account the effect of CO2 and O2 partial pressure in headspace and corresponding dissolved concentrations in the food. This mechanistic model was validated in simplified and in real conditions using dedicated challenge-tests performed on poultry meat, fresh salmon and processed cheese, inoculated with either Listeria monocytogenes or Pseudomonas fluorescens. Once validated, this model could be used as a Decision Support Tool in order to optimize the initial packaging atmosphere (level of O2 and CO2) and / or the geometry (ratio headspace volume to food mass). This tool could also be used to identify the packaging gas permeability the most suitable for maintaining the targeted % of gas initially flushed in the pack within a given tolerance. This approach permits a better dimensioning of MAP of fresh produce by selecting the packaging material fitted to “just necessary” (and not by default the most barrier one). The connexion of this model with dedicated databases gathering gas permeabilities of commonly used packaging materials allows us to obtain as output a ranking of the most suitable materials. This tool would be very useful for all stakeholders of the fresh produce chain. A demonstration of this Decision Support Tool is here proposed with the pipeline between mathematical models and related databases.

9 Pags.- 3 Tabls.- 4 Figs., The increase in grape berry ripening rates associated to climate change is a growing concern for wine makers as it rises the alcohol content of the wine. The present work studied the combined effects of elevated CO2, temperature and UV-B radiation on leaf physiology and berry ripening rates. Three doses of UV-B: 0, 5.98, 9.66 kJ m−2 d−1, and two CO2–temperature regimes: ambient CO2-24/14 °C (day/night) (current situation) and 700 ppm CO2-28/18 °C (climate change) were imposed to grapevine fruit-bearing cuttings from fruit set to maturity under greenhouse-controlled conditions. Photosynthetic performance was always higher under climate change conditions. High levels of UV-B radiation down regulated carbon fixation rates. A transient recovery took place at veraison, through the accumulation of flavonols and the increase of antioxidant enzyme activities. Interacting effects between UV-B and CO2–temperature regimes were observed for the lipid peroxidation, which suggests that UV-B may contribute to palliate the signs of oxidative damage induced under elevated CO2–temperature. Photosynthetic and ripening rates were correlated. Thereby, the hastening effect of climate change conditions on ripening, associated to higher rates of carbon fixation, was attenuated by UV-B radiation., This work was supported by Ministerio de Ciencia e Innovación of Spain (MCINNBFU2011-26989), INNOVINE Call FP7-KBBE-2011-6, Proposal N° 311775, Asociación de Amigos de la Universidad de Navarra (grant to J. M-L) and UV4growth COST Action FA0906.

International audience; With the evolution of ecosystem management and the actual climate change we are facing, there is a need to improve our knowledge of carbon (C) balance and more specifically of C allocation in the plants. In our study, we quantified the seasonal variation of gross primary production (GPP, obtained through eddy covariance measurements) and biomass production (BP, the C fixed into the biomass obtained thanks to inventory campaign) for a 60-year-old even-aged beech stand located in North East of France. We also assessed the seasonal evolution of the BP efficiency (BPE=BP/GPP; Vicca et al., 2012) and its potential determining factors for our site. For 2014, we found a net ecosystem exchange (NEE) of -549 gC m-2, corresponding to a C sequestration. This value breaks down between 1089 gC m-2 for the respiration of the ecosystem and -1639 gC m-2 for the GPP. On the same year, our stand built up 461.6 gC m-2 of tree biomass (leaves, trunk, branches, fine roots), leading to an annual BPE of 0.28, which is within the range of value found on other similar sites. There was a large temporal variation of C allocation to the different parts of the tree biomass during the growth season. Our results show that the growth first happened in the trunk and branches -with a peak value of 74.5 gC m-2 month-1 in May - whereas the fine roots biomass production started later (end of July) and reached a maximum at the end of the growth season (28.49 gC m-2 month-1 for September). The BPE varied also during the year from 0.13 in April to 0.31 in August, where the BP was the same than in July but the cumulated GPP was already decreasing. The seasonal variation may be mainly explained by climatic variations, whereas the shift between woody above-ground biomass and fine roots biomass could be explained by the phenology (linked to physiological mechanisms).

In lowland rice production, water-saving irrigation technologies have been developed, but it has rarely been considered that the absence of a ponded water layer could change the field's microclimate due to the different thermal characteristics of water compared to air. At a site in the Senegal River valley, canopy and soil temperature as well as temperature at meristem level and relative humidity inside the canopy were observed in the presence and absence of a ponded water layer in an irrigated rice field. Gas-exchange measurements were conducted at different development stages of three varieties (IR4630, IR64, and Sahel108) sown in bi-monthly intervals and the effects of climatic and microclimatic parameters on stomatal conductance, assimilation rate, and intrinsic water use efficiency were investigated. Minimum soil ( T s min) and meristem temperature ( T M min) were usually lower in the absence of a ponded water layer. Stomatal conductance depended mainly on T s min, T M min, and minimum relative humidity inside the canopy. Assimilation rate was positively correlated with solar radiation, T s min and T M min, but depended mainly on stomatal conductance. Without standing water, stomatal conductance was significantly lower, but reductions could be explained with lower T s min and/or T M min. Nevertheless, T s min and/or T M min were the major determinants of stomatal conductance and assimilation rate, which suggests a pivotal role of root zone temperature on plant growth probably via water uptake and, thus, overall plant water status. Varietal differences were found, with assimilation rate in IR4630 and Sahel108 having been less affected by low temperature than in IR64. When water-saving irrigation measures are applied in irrigated rice, the negative effects of lower soil and meristem temperature in the absence of a ponded water layer in the field on the productivity of rice need to be considered. In regions where night temperatures below 20 °C occur, varieties should be used that are less temperature-responsive, if the effect of cool nights on meristem temperature cannot be mitigated by a ponded water layer.

Potted seedlings from three oak species (Quercus robur, Q petraea and Q rubra), known to present different sensitivities to temporary soil hypoxia, were submitted to two contrasted irradiance regimes in a greenhouse (100% and approximately 35% of full greenhouse irradiance) and shortly after to waterlogging for 2-4 weeks. The experiment was repeated for 2 years. Leaf gas exchange, chlorophyll content and fluorescence, leaf water potential and photosynthetic capacity were recorded several times during the stress. Biomass increment was estimated at the end of the stress, or after 3 weeks of drainage. Despite its late application, after completion of elongation and expansion of leaves, shading significantly reduced growth and modified photosynthetic activity. It generally reduced net assimilation rates and increased chlorophyll contents. Q rubra seedlings behaved differently: growth decreased and chlorophyll was enhanced as in the two other species but net assimilation rates and photosynthetic capacity were higher in the shade than under full irradiance. This surprising effect probably resulted from high irradiance stress, and was not alleviated by improved fertilization during the second year. Waterlogging induced severe disorders in photosynthesis: net CO2 assimilation, stomatal conductance, photosynthetic capacity: chlorophyll content and growth declined. In addition, predawn values of photochemical efficiency of photosystem Il, which are usually close to the optimum value of 0.83, were significantly decreased. The largest dysfunctions occurred in Q rubra seedlings, and the smallest in Q robur, which confirms that the number of disorders in photosynthesis is an accurate estimate of the differential sensitivity to waterlogging among genotypes. Shading of the seedlings interacted with waterlogging and limited the extent of induced damage. In particular, stomatal closure and net CO2 assimilation rates were proportionally less affected under shade. Similarly, predawn leaf water potential presented smaller decreases. The decline of predawn values of photochemical efficiency occured to a much lesser extent under shade. Finally, the behaviour of seedlings during post-waterlogging stress differentiated significantly the species. None of them recovered completely after 3 weeks. Nevertheless, Q robur behaved best, recovering high levels of predawn water potential, slowly reopening stomata and reincreasing net assimilation rates. Q petraea recovered with more difficulties: predawn leaf water potential decreased in several plants after the end of waterlogging, and photosynthesis recovered very slowly. Q rubra did not recover at all and several individuals died after the end of waterlogging stress., Des semis en pots de trois espèces de chêne (Quercus robur, Q petraea et Q rubra), connues pour présenter d'importantes différences de sensibilité à la présence de nappes d'eau temporaires dans le sol, ont été soumis à deux niveaux d'éclairement (100 et 35 % du rayonnement ambiant) dans une serre à Nancy, puis à une période d'ennoyage de plusieurs semaines. Cette expérience a été répétée 2 années de suite. Les échanges gazeux foliaires, les teneurs en chlorophylle, la fluorescence de la chlorophylle, le potentiel hydrique et la capacité de photosynthèse ont été mesurés plusieurs fois au cours de la contrainte. La biomasse accumulée a été estimée à la fin de l'ennoyage, ou après 3 semaines de ressuyage du sol. Malgré son application tardive après l'expansion des feuilles et l'arrêt de croissance en hauteur, l'ombrage a induit des baisses de croissance et d'activité photosynthétique (diminution de l'assimilation nette de CO2 et augmentation des teneurs en chlorophylles). Q rubra a présenté une réponse très différente : alors que la croissance et les teneurs en chlorophylle présentaient les mêmes modifications que dans les autres espèces, l'assimilation nette de CO2 était plus importante à l'ombre qu'en pleine lumière, et la capacité photosynthétique fortement augmentée. Cette réponse anormale révèle que ces semis de Q rubra étaient soumis à un stress lumineux. La fertilisation apportée en deuxième année a permis d'augmenter la photosynthèse, sans toutefois l'amener au niveau des deux autres espèces et sans éliminer la saturation à de faibles éclairements. L'ennoyage a provoqué d'importantes perturbations dans la photosynthèse : l'assimilation nette de CO2 et la conductance stomatique ont été fortement réduites, la capacité photosynthétique et les teneurs en chlorophylles ont baissé ; des limitations de croissance ont également été constatées. De plus, l'efficience photochimique mesurée en fin de nuit, qui est habituellement proche du niveau optimal d'environ 0,83, a baissé significativement. Les perturbations les plus marquées ont été observées sur les semis de Q rubra, et les plus limitées sur ceux de Q robur, ce qui confirme la bonne corrélation existant entre perturbations à court terme de la photosynthèse, et sensibilité globale à l'ennoyage. L'ombrage a permis de limiter et de retarder, dans une certaine mesure, l'apparition de ces perturbations. Enfin, le comportement des semis pendant la phase de ressuyage a permis de différencier fortement les espèces. Aucune n'a retrouvé un fonctionnement optimal après 3 semaines de ressuyage. Q robur a manifesté la meilleure capacité de récupération, alors que Q petraea a présenté de progressives nécroses foliaires, un potentiel hydrique décroissant et des niveaux de photosynthèse très faibles pendant les 3 semaines. Enfin, Q rubra a subi une très forte sénescence foliaire et une mortalité non négligeable au cours du ressuyage.

The latest climate change scenarios predict a rise in mean temperature in Europe of 2 to 4°C for 2099 (IPCC, 2007), associated with extreme climatic events such as severe droughts. Knowing adaptation capabilities of tree species is crucial for understanding their responses and forest ecosystem fate in the near future. Our study object is a species complex inside the Fraxinus genus (ash, Oleaceae). In France, F. excelsior and F. angustifolia are autochthonous, form natural hybrid populations and show remarkable phenotypic and ecological plasticity. This could promote the emergence of new individuals (genotypes) more able to deal with fluctuating environments. Our objective is to characterise the capability of adaptation of different Fraxinus populations, representing the three statuses (F.excelsior, F.angustifolia and hybrids) under abiotic constraints (water constraint). To solve this issue, we examine in a low water constraint experiment (-0.9 MPa) ecophysiological and genetic response, using saplings. A second and more severe water constraint experiment (-4 MPa) was used to investigate ash response to the loss of hydraulic conductivity. The most noticeable result was an intermediate and highly variable behaviour of hybrid ash populations in the two experiments (A, gs, WUEi, PLC) linked with they respective introgression degree (closer to F.excelsior or F.angustifolia). This hybrid trees could be used for foresters as a resource and insurance against catastrophic forest stand decline, for a future climate.; Les derniers scénarios du changement climatique, prévoient une élévation de température (Europe, +2 à +4°C en moyenne en 2099, IPCC, 2007) associée à des épisodes extrêmes, sécheresses sévères par exemple. Connaître les potentialités d'adaptation des espèces forestières s'avère crucial afin de comprendre leurs réponses et le devenir des écosystèmes forestiers, dans un futur proche. Dans ce cadre, nous nous sommes intéressés à un complexe d'espèces du genre Fraxinus, (frêne, Oléacées). En France F. excelsior L., et F. angustifolia, Vahl, sont des espèces autochtones présentant une plasticité phénotypique et écologique remarquable. L'hybridation, suspectée depuis longtemps a été prouvée en conditions contrôlées et naturelles. Les principales zones documentées sont la vallée de la Saône et de la Loire. Cette hybridation entre les deux espèces de frênes européens, pourrait favoriser l'apparition d'individus (génotypes) plus aptes que les espèces parentales à faire face à un environnement changeant. Notre objectif est de caractériser les potentialités d'adaptations de différentes populations de frêne (espèces parentales et de statut hybride) sous une contrainte abiotique (contrainte hydrique). Pour répondre à cet objectif, nous avons testé les réponses à la fois écophysiologiques et génétique de jeunes plants à une contrainte légère (-0,9 MPa). Une seconde expérimentation, centré sur l'écophysiologie a eu pour objet de mesurer la perte de conductivité hydraulique des frênes, sous une forte contrainte (-4 MPa). Le principal résultat de ces travaux est le comportement souvent intermédiaire et très variable des populations de frênes hybrides testés dans ces 2 expérimentations (A, gs, WUEi, PLC), que ce soit en conditions avec ou sans contrainte hydrique. Ce comportement intermédiaire est en lien avec le degré d'introgression respectif des hybrides de frênes (plus proche de F.excelsior ou de F.angustifolia). Ces arbres hybrides pourraient servir de ressources et d'assurance contre des évènements de dépérissement catastrophiques pour les forestiers pour un environnement climatique futur.

Seasonal variations in environmental conditions influence the functioning of the whole ecosystem of tropical rain forests, but as yet little is known about how such variations directly influence the leaf gas exchange and transpiration of individual canopy tree species. We examined the influence of seasonal variations in relative extractable water in the upper soil layers on predawn leaf water potential, saturated net photosynthesis, leaf dark respiration, stomatal conductance, and tree transpiration of 13 tropical rain forest canopy trees (eight species) over 2 yr in French Guiana. The canopies were accessed by climbing ropes attached to the trees and to a tower. Our results indicate that a small proportion of the studied trees were unaffected by soil water depletion during seasonal dry periods, probably thanks to efficient deep root systems. The trees showing decreased tree water status (i.e., predawn leaf water potential) displayed a wide range of leaf gas exchange responses. Some trees strongly regulated photosynthesis and transpiration when relative extractable water decreased drastically. In contrast, other trees showed little variation, thus indicating good adaptation to soil drought conditions. These results have important applications to modeling approaches: indeed, precise evaluation and grouping of these response patterns are required before any tree-based functional models can efficiently describe the response of tropical rain forest ecosystems to future changes in environmental conditions. (Resume d'auteur)

Though the root biomass of tropical rainforest trees is concentrated in the upper soil layers, soil water uptake by deep roots has been shown to contribute to tree transpiration. A precise evaluation of the relationship between tree dimensions and depth of water uptake would be useful in tree-based modelling approaches designed to anticipate the response of tropical rainforest ecosystems to future changes in environmental conditions. We used an innovative dual-isotope labelling approach (deuterium in surface soil and oxygen at 120-cm depth) coupled with a modelling approach to investigate the role of tree dimensions in soil water uptake in a tropical rainforest exposed to seasonal drought. We studied 65 trees of varying diameter and height and with a wide range of predawn leaf water potential (I-pd) values. We confirmed that about half of the studied trees relied on soil water below 100-cm depth during dry periods. I-pd was negatively correlated with depth of water extraction and can be taken as a rough proxy of this depth. Some trees showed considerable plasticity in their depth of water uptake, exhibiting an efficient adaptive strategy for water and nutrient resource acquisition. We did not find a strong relationship between tree dimensions and depth of water uptake. While tall trees preferentially extract water from layers below 100-cm depth, shorter trees show broad variations in mean depth of water uptake. This precludes the use of tree dimensions to parameterize functional models.

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Comparés aux monocultures, les systèmes agroforestiers (SAF) sont censés permettre une meilleure efficience d'utilisation de la ressource et améliorer les services écosystémiques. Cependant, la complexité des interactions se produisant dans les SAF rend délicat la quantification et la décomposition des effets des arbres d'ombrage sur la productivité primaire nette (NPP) de la culture principale. Peu de modèles sont capables d'analyser les effets des interactions entre culture principale et arbres d'ombrage sur les échanges de CO2 et d'eau. En effet, les interactions pour la lumière, l'eau et la chaleur se produisant entre culture et arbres d'ombrage peuvent produire des effets contre-intuitifs sur la photosynthèse, l'efficience d'utilisation de la lumière (LUE), l 'efficience de transpiration et le microclimat. Nous montrons que MAESPA, un modèle 3D mécaniste, peut-être utilisé pour étudier la variabilité de ces processus à des échelles allant de la plante à la parcelle, et de la demi-heure à l'année entière. MAESPA a simulé de manière satisfaisante l'interception de la lumière dans un SAF à base de caféier composé de 2 couches hétérogènes. Des variables modélisées par MAESPA ont été utilisées pour produire de puissantes variables explicatives dans un dispositif expérimental étudiant les déterminants de la NPP aérienne (ANPP) du caféier. Il a été démontré que LUE était deux fois plus élevée pour les caféiers poussant à l'ombre ce qui compensait totalement la diminution de leurs budgets lumineux, résultant en une absence de différence de ANPP entre caféiers de plein soleil et caféiers d'ombrage. MAESPA a aussi simulé de manière satisfaisante les échanges de CO2 à l'échelle du caféier et à l'échelle de la parcelle, lorsque comparés à des mesures d'échanges gazeux dans des chambres plantes entières ou à des enregistrements de flux turbulents au-dessus de la canopée, respectivement. Nous avons utilisé MAESPA pour simuler la variabilité spatiale de la photosynthèse et de LUE. MAESPA a démontré être un modèle robuste pour quantifier les interactions spatiales dans un SAF. Le prochain développement pertinent de cette approche serait de coupler MAESPA avec un modèle d'allocation du carbone dans les organes des plants de caféiers. (Résumé d'auteur)