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International audience; Highlights:This paper reviews ways to improve water use efficiency beyond the plant level, across time and space scales, from cells, organs, and plants, to field, farm, and landscape.Abstract:Water scarcity will be one of the main issues of the 21 st century, because of competing needs between civil, industrial, and agriculture use. While agriculture is the largest user of water, its share is bound to decrease as societies develop. Clearly, agriculture needs to become more water efficient. Improving water use efficiency (WUE) at the plant level is important although there is a long way into translating this at the farm/landscape level. As we move up from a cell/organ/plant scale to more integrated scales such as plot, field, farm system, and landscape, other factors need to be considered, including trade-offs, to possibly improve WUE. These include choices of crop variety/species, farm management, landscape design, infrastructure development, ecosystem functions, where human decisions matter. This review is a cross-disciplinary attempt to analyze ways to address WUE at these different scales where metrics of analysis are defined and trade-offs considered. The equations in this perspective paper use similar metrics across scales for an easier connection and are developed to highlight which levers, at different scales, can improve WUE. We also refer to models operating at these different scales to assess WUE. While our entry point is plants and crops, we scale up the analysis of WUE to farm systems and landscapes.

Poplar plantations, widely used for the production of woody biomass, might be at high risk from the climate change-induced increase in the frequency of drought periods. Therefore, selecting improved genotypes, which are highly productive but with a high water use efficiency (WUE), is becoming a major target. The use of automated weighing systems in controlled environments facilitates the estimation of cumulated water loss and whole plant transpiration efficiency (TE). Differences in TE and leaf level intrinsic WUE as well as the contribution of underlying ecophysiological traits were determined in three contrasting P. nigra genotypes. Strong differences in TE among the selected genotypes were congruent with differences in leaf level intrinsic WUE. Our data show that a high total leaf area was overcompensated by a low per leaf area transpiration rate, leading to higher TE in highly productive genotypes originating from cool locations. Nocturnal water loss was relatively low but contributed to variations in TE among genotypes. In response to drought, leaf level WUE increased but not TE, suggesting that carbon losses due to whole plant respiration could offset the drought-induced increase in intrinsic WUE.

Natural selection driven by water availability has resulted in considerable variation for traits associated with drought tolerance and leaf‐level water‐use efficiency (WUE). In Arabidopsis, little is known about the variation of whole‐plant water use (PWU) and whole‐plant WUE (transpiration efficiency). To investigate the genetic basis of PWU, we developed a novel proxy trait by combining flowering time and rosette water use to estimate lifetime PWU. We validated its usefulness for large‐scale screening of mapping populations in a subset of ecotypes. This parameter subsequently facilitated the screening of water use and drought tolerance traits in a recombinant inbred line population derived from two Arabidopsis accessions with distinct water‐use strategies, namely, C24 (low PWU) and Col‐0 (high PWU). Subsequent quantitative trait loci mapping and validation through near‐isogenic lines identified two causal quantitative trait loci, which showed that a combination of weak and nonfunctional alleles of the FRIGIDA (FRI) and FLOWERING LOCUS C (FLC) genes substantially reduced plant water use due to their control of flowering time. Crucially, we observed that reducing flowering time and consequently water use did not penalize reproductive performance, as such water productivity (seed produced per unit of water transpired) improved. Natural polymorphisms of FRI and FLC have previously been elucidated as key determinants of natural variation in intrinsic WUE (δ13C). However, in the genetic backgrounds tested here, drought tolerance traits, stomatal conductance, δ13C. and rosette water use were independent of allelic variation at FRI and FLC, suggesting that flowering is critical in determining lifetime PWU but not always leaf‐level traits., Accelerated flowering time achieved through nonfunctional and weak alleles of flowering time genes FRIGIDA (FRI) and FLOWERING LOCUS C (FLC) reduce water use in Arabidopsis thaliana without detrimentally impacting reproductive output.

There is increasing concern that due to land pressure and the need to maximize income, smallholder coffee farmers are increasingly being forced to cultivate in areas which are considered to be sub-optimal for coffee. Little is known about optimal coffee and tree combinations in these conditions and the degree to which crops and trees compete or are synergistic. In environmental conditions which were sub optimal for coffee cultivation in Nicaragua (1470 mm annual rainfall, 27 °C mean annual temperature and 455 m altitude compared to optima of 2000 mm, 23–24 °C and altitude between 1000 and 1400 m at that latitude, respectively), coffee and shade tree transpiration and soil evaporation were directly and separately measured in agroforestry (AFS) and full sun systems (FS). AFS was found to be a more efficient water user than FS because a greater proportion of rainfall was used by plant transpiration rather than being lost by soil evaporation. Plant transpiration accounted for 83% and 69% of evapotranspiration while soil evaporation represented 17% and 31%, in AFS and FS respectively. In AFS most of the water transpiration was due to coffee (72.5%) and much less by deciduous Tabebuia rosea (19%) and evergreen Simarouba glauca shade trees (8.5%). Furthermore, the study demonstrated the vastly different behaviour in water use by the shade trees. When in leaf, Tabebuia rosea transpired at four to six times the rate of evergreen Simarouba glauca, although crown sizes were similar. Contrasting precipitation between two consecutive years of study demonstrated that competition for water between coffee and shade tree occurred only in a severe dry season when coffee leaf water potential (LWP) reached its lowest values of −2.33 MPa in AFS. It was concluded that in most circumstances there was sufficient water for both coffee and trees, that coffee in AFS was a more efficient user of water than FS coffee, and that evergreen Simarouba glauca was more suitable as coffee shade tree compared to deciduous Tabebuia rosea in the sub optimal environmental condition studied. (Resume d'auteur)

The terrestrial carbon and water cycles are intimately linked: the carbon cycle is driven by photosynthesis, while the water balance is dominated by transpiration, and both fluxes are controlled by plant stomatal conductance. The ratio between these fluxes, the plant water-use efficiency (WUE), is a useful indicator of vegetation function.WUE can be estimated using several techniques, including leaf gas exchange, stable isotope discrimination, and eddy covariance. Here we compare global compilations of data for each of these three techniques.We show that patterns of variation in WUE across plant functional types (PFTs) are not consistent among the three datasets. Key discrepancies include the following: leaf-scale data indicate differences between needleleaf and broadleaf forests, but ecosystem-scale data do not; leaf-scale data indicate differences between C3 and C4 species, whereas at ecosystem scale there is a difference between C3 and C4 crops but not grasslands; and isotope-based estimates of WUE are higher than estimates based on gas exchange for most PFTs.Our study quantifies the uncertainty associated with different methods of measuring WUE, indicates potential for bias when using WUE measures to parameterize or validate models, and indicates key research directions needed to reconcile alternative measures of WUE.

International audience; Water scarcity associated with climate change particularly threatens the sustainability of viticulture in most cultivated, drought prone areas. Breeding grapevine for reduced water use and maintained production (that is high water-use efficiency) is therefore of major interest. This requires a comprehensive knowledge of the physiological impacts of drought which are the most influential on yield and quality. Special attention should be paid to those mechanisms involved in the regulation of water status in plant tissues as the primary parameter affected by drought. Transpiration rate, which has major influence on plant water status, together with water-use efficiency, therefore require special attention in breeding programs. To progress on the determinism of transpiration rate and water-use efficiency in grapevine, we used a F1 progeny made of 188 genotypes from a cross between two widespread cultivars, Syrah and Grenache, well-known for their contrasted water use. We showed the benefits of combining quantitative genetics (for QTL detection) and physiological experiments to study this population both in the vineyard and on potted plants. On the one hand, we developed an original experimental design in the field coupled to geostatistical modelling to take into account the spatial variability of soil water status inherent to vineyard conditions. This helped to identify significant genetic variability for the traits of interest. On the other hand, we combined powerful phenotyping tools on potted plants (high-throughput platform and controlled chambers) to control water deficit conditions and improve QTL detection. First, we found evidence that a dual physiological mechanism controls the decline of leaf water status under drought with a key role for plant hydraulic conductance beside that of stomatal control of transpiration. Contrasted combination of these two controls may lead to more or less efficient maintenance of leaf water status in response to soil drying (i.e. iso- or an isohydric behaviour). An indirect role of abscisic acid on stomatal conductance was also identified, mediated by the downregulation of leaf hydraulic conductance, with a genetic variability which correlated with genetic variation in iso- or aniso-hydric behaviour. We then revealed wide genetic variations in nocturnal transpiration, which correlated with variations in whole plant water-use efficiency (WUE), and identified corresponding genetic and physiological determinants. Lastly, we showed some consistency between QTLs detected for daytime WUE in pots and in the vineyard. Beyond the potential interest of the QTLs detected in this study for breeding prospects, this work demonstrated the interest of quantitative genetics to shed light on ecophysiological and physiological processes.

International audience; There is increasing interest in rapidly identifying genotypes with improved water use efficiency, exemplified by the development of whole plant phenotyping platforms that automatically measure plant growth and water use. Transpirational responses to atmospheric vapour pressure deficit (VPD) and whole plant water use efficiency (WUE, defined as the accumulation of above ground biomass per unit of water used) were measured in 100 maize (Zea mays L.) genotypes. Using a glasshouse based phenotyping platform with naturally varying VPD (1.5 to 3.8 kPa), a 2-fold variation in WUE was identified in well-watered plants. Regression analysis of transpiration versus VPD under these conditions, and subsequent whole plant gas exchange at imposed VPDs (0.8 to 3.4 kPa) showed identical responses in specific genotypes. Genotype response of transpiration versus VPD fell into two categories: 1) a linear increase in transpiration rate with VPD with low (high WUE) or high (low WUE) transpiration rate at all VPDs, 2) a non-linear response with a pronounced change point at low VPD (high WUE) or high VPD (low WUE). In the latter group, high WUE genotypes required a significantly lower VPD before transpiration was restricted, and had a significantly lower rate of transpiration in response to VPD after this point, when compared to low WUE genotypes. Change point values were significantly positively correlated with stomatal sensitivity to VPD. A change point in stomatal response to VPD may explain why some genotypes show contradictory WUE rankings according to whether they are measured under glasshouse or field conditions. Furthermore, this novel use of a high throughput phenotyping platform successfully reproduced the gas exchange responses of individuals measured in whole plant chambers, accelerating the identification of plants with high WUE.

Highlights • A model scaling stoma behaviour at leaf level is proposed. • The model is tested by reproducing natural variations of light intensity. • Stomatal aperture and photosynthesis decrease as a function of sugar accumulation. • Leaf anatomy influences the rapidity of stomatal conductance. • The model dissects intrinsic water use efficiency into traits of interest., Intrinsic water use efficiency (Wi), the ratio of net CO2 assimilation (A) over stomatal conductance to water vapour (gs), is a complex trait used to assess plant performance. Improving Wi could lead in theory to higher productivity or reduced water usage by the plant, but the physiological traits for improvement and their combined effects on Wi have not been clearly identified. Under fluctuating light intensity, the temporal response of gs is an order of magnitude slower than A, which results in rapid variations in Wi. Compared to traditional approaches, our new model scales stoma behaviour at the leaf level to predict gs and A during a diurnal period, reproducing natural fluctuations of light intensity, in order to dissect Wi into traits of interest. The results confirmed the importance of stomatal density and photosynthetic capacity on Wi but also revealed the importance of incomplete stomatal closure under dark conditions as well as stomatal sensitivity to light intensity. The observed continuous decrease of A and gs over the diurnal period was successfully described by negative feedback of the accumulation of photosynthetic products. Investigation into the impact of leaf anatomy on temporal responses of A, gs and Wi revealed that a high density of stomata produces the most rapid response of gs but may result in lower Wi.

UMR AGAP - équipe AFEF - Architecture et fonctionnement des espèces fruitièresData Availability: The relevant DOIs/accession numbers required to access raw data in Zenodo is: http://dx.doi.org/10.5281/zenodo.35030 (doi: 10.5281/zenodo.35030).; Water use efficiency (WUE) is a quantitative measurement which improvement is a major issue in the context of global warming and restrictions in water availability for agriculture. In this study, we aimed at studying the variation and genetic control of WUE and the respective role of its components (plant biomass and transpiration) in a perennial fruit crop. We explored an INRA apple core collection grown in a phenotyping platform to screen one-year-old scions for their accumulated biomass, transpiration and WUE under optimal growing conditions. Plant biomass was decompose into morphological components related to either growth or organ expansion. For each trait, nine mixed models were evaluated to account for the genetic effect and spatial heterogeneity inside the platform. The Best Linear Unbiased Predictors of genetic values were estimated after model selection. Mean broad-sense heritabilities were calculated from variance estimates. Heritability values indicated that biomass (0.76) and WUE (0.73) were under genetic control. This genetic control was lower in plant transpiration with an heritability of 0.54. Across the collection, biomass accounted for 70% of the WUE variability. A Hierarchical Ascendant Classification of the core collection indicated the existence of six groups of genotypes with contrasting morphology and WUE. Differences between morphotypes were interpreted as resulting from differences in the main processes responsible for plant growth: cell division leading to the generation of new organs and cell elongation leading to organ dimension. Although further studies will be necessary on mature trees with more complex architecture and multiple sinks such as fruits, this study is a first step for improving apple plant material for the use of water.

In Mediterranean regions, water scarcity associated with climate change particularly threatens the sustainability of viticulture. Breeding grapevine for reduced water use and maintained production is therefore of major interest. This requires a comprehensive knowledge of the plant physiological responses to drought. In this study we focused on the determinism of transpiration rate as a key trait regulating water status in plant tissues, and on its relationship with water-use efficiency (WUE). We used a F1 progeny from a cross between cultivars Syrah and Grenache and combined powerful phenotyping tools on potted plants submitted to either well-watered or mild deficit conditions with quantitative genetics (for QTL detection) and physiological experiments. Analysis of the genetic control of water status maintenance in the plant, more or less efficient under soil water deficit (i.e. iso- or anisohydric), revealed a dual physiological determinism with a key role for plant hydraulic conductance beside that of stomatal control of transpiration. An indirect role of abscisic acid on stomatal conductance was also evidenced, mediated by the downregulation of leaf hydraulic conductance, with a genetic variability which correlated with genetic variation in iso- or aniso-hydric behaviour. We then revealed wide genetic variations in nocturnal transpiration, which correlated with variations in water use efficiency, and identified corresponding genetic and physiological determinants. In a final switch to the field, we showed consistency between QTLs detected for daytime WUE in pots and in the vineyard. Beyond the potential interest of the QTLs detected in this study for breeding prospects, this work demonstrated the benefits of quantitative genetics to shed light on ecophysiological and physiological processes., La raréfaction des ressources en eau associée au changement climatique menace particulièrement la durabilité de la viticulture en climat Méditerranéen. Pour y faire face, la création ou le choix de cépages économes en eau et suffisamment vigoureux en cas de déficit hydrique se présente comme un levier important. Une compréhension approfondie des mécanismes qui gouvernent le maintien de l'état hydrique par la plante est indispensable pour avancer dans cette direction. Dans ce travail les déterminants génétiques et physiologiques de l'utilisation de l'eau ont été explorés chez la vigne. Une descendance F1, issue d'un croisement entre les cépages Syrah et Grenache, a été soumise à deux scénarios hydriques dans des pots (bonne irrigation et déficit modéré) en combinant de nouveaux outils de phénotypage, une démarche de génétique quantitative (pour la détection de QTLs) et des approches physiologiques. L'analyse de l'architecture génétique du maintien du potentiel hydrique par la plante, plus ou moins efficace en cas de déficit hydrique (i.e. iso- ou aniso-hydrique), a révélé un double déterminisme, impliquant non seulement la régulation stomatique de la transpiration mais également le maintien de la conductance hydraulique à travers la plante. Nous avons démontré l'existence d'une action indirecte de l'acide abscissique sur la fermeture stomatique à travers une diminution de la conductance hydraulique dans la feuille avec une variabilité génétique reliée aux comportements iso- ou aniso-hydriques. Par ailleurs, nous avons mis en évidence une variabilité génétique importante de la transpiration nocturne, liée à celle de l'efficience d'utilisation de l'eau, avec des déterminants génétiques et physiologiques que nous avons identifiés. Au-delà de l'utilité des QTLs détectés pour l'amélioration variétale, les résultats originaux de ce travail démontrent l'intérêt de la génétique quantitative pour progresser dans la compréhension de mécanismes physiologiques.

Ce numéro d'Innovations Agronomiques comprend les articles correspondant aux présentations du colloque « Viticulture et stress hydrique » qui s'est tenu à Montpellier le 17 juin 2014.; National audience; Drought episodes in 2003 and 2011 had huge, detrimental impacts on vineyard production in South of France. In grapevine, vegetative growth is the first function to be influenced by drought with rapid cessation of growth of secondary shoot axes. Additionally, stomatal closure gradually reduces transpiration rate. These responses result in a decrease of photosynthesis rate which partly explains yield losses and quality damages. During the last years, research in ecophysiology evidenced a large diversity in responses to drought, originating from both scions and rootstocks. New methods have been developed for characterising and modelling scions and rootstocks as regard their susceptibility to drought conditions. Model predictions are becoming useful tools to select appropriate scions and rootstocks as a function of likely climate scenarios. Current work should incorporate additional plant modifications either resulting from acclimation of the plant or from management practices. Future works should address the multiyear effects of drought episodes on wine grape production in order to better account for carry-over or cumulative effects of drought on successive years.; Les sécheresses de 2003 et 2011 ont marqué les esprits dans beaucoup de régions viticoles du Sud de la France avec des baisses de rendement et de qualité de la récolte. Chez la vigne soumise à un dessèchement du sol, la croissance végétative est la première fonction affectée avec, en particulier, un arrêt de croissance des rameaux secondaires. De plus, la fermeture des stomates, par lesquels l’eau s’évapore à la surface des feuilles, limite rapidement la transpiration. Ces réactions ont pour conséquence une diminution de la photosynthèse de la plante qui peut expliquer certaines pertes de rendement. Au cours des dernières années, les recherches en écophysiologie ont permis d’identifier une large diversité dans ces réponses du matériel végétal à la contrainte hydrique liée aussi bien au porte-greffe qu’au greffon. De nouvelles méthodes de mesure et de modélisation sont apparues pour classer les cépages et les porte-greffes selon leur sensibilité à la contrainte hydrique. Les premières simulations permettent de raisonner le choix du cépage, du porte-greffe ou de leur mode de conduite en fonction de scénarios climatiques probables. Les travaux en cours devraient déboucher sur une prise en compte encore plus complète des nombreuses modifications mises en place par la plante ou apportées par le viticulteur pour minimiser l’impact de la contrainte hydrique. Le prochain défi à relever sera sans doute d’intégrer les effets pluriannuels de la contrainte hydrique sur l’élaboration de la production afin de mieux comprendre les arrières-effets d’une sécheresse sur le millésime suivant ou les effets cumulés de sécheresses successives.

Biodiversity is known to support and boost a wide range of forest ecosystem functions and services like productivity and resistance against insect pests and diseases. However, whether tree species diversity also promotes water and carbon acquisition and use in forest ecosystems is still unclear. Furthermore, in the current context of global warming, information on how tree species diversity can influence the response of forest ecosystems to extreme climatic events such as drought are urgently needed. In this framework, the objective of my PhD thesis was to determine how tree species diversity influences important functions of the water and carbon cycle including transpiration, carbon isotope composition and water extraction depth at the tree- and ecosystem-scale under contrasting soil water conditions. My work was conducted within the FunDivEUROPE project in a network of permanent forest stands and tree plantations across a North-South gradient in Europe covering a wide range of climatic conditions. I found considerable variability among species or forest types in the response of transpiration and carbon isotope composition at the tree- and ecosystem-scale across Europe. Species diversity did not affect the water and carbon relations of tree species and forest ecosystems under nonlimiting soil water conditions. However, a strong effect of species diversity was observed under drought conditions in some forest types. Based on these data, I discuss the potential mechanisms of species interactions that may explain the observed patterns. I also point out that the influence of species diversity is highly contextdependent, and changes with local environmental and climatic conditions. In terms of forest management applications, I suggest that, at least in some regions, controlling for tree species diversity along with stand density and total basal area could be recommended to help forests adapt to drier conditions.; La biodiversité favorise un grand nombre de fonctions et services écosystémiques des écosystèmes forestiers tels que la production de bois ou la résistance aux attaques d’insectes et aux maladies. Cependant l’impact de la diversité sur l’acquisition et l’utilisation de l’eau et du carbone reste largement méconnu dans ces écosystèmes. De plus, dans le contexte actuel de changement climatique, l’influence de la diversité sur la réponse des écosystèmes forestiers à des événements climatiques extrêmes tels que la sécheresse reste à étudier. L’objectif de ce travail est donc de déterminer l’impact de la diversité en espèces d’arbre sur d’importantes fonctions du cycle de l’eau et du carbone telles que la transpiration, la composition isotopique du carbone ou la profondeur d’extraction de l’eau à l’échelle de l’arbre et de l’écosystème sous des conditions contrastées de disponibilité en eau du sol. Ce travail a été réalisé dans le cadre du projet FunDivEUROPE sur un réseau de parcelles forestières ainsi que dans des plantations expérimentales le long d’un gradient Nord-Sud en Europe afin de couvrir une importante gamme de conditions climatiques. Nos travaux ont montré une importante variabilité de la réponse à la diversité à l’échelle de l’arbre et de l’écosystème en termes de relations hydriques et carbonées à travers l’Europe. La diversité en espèces ne semble pas influencer les relations hydriques et carbonées des espèces et des écosystèmes forestiers dans des conditions non limitantes de disponibilité en eau. Cependant, un fort effet de la diversité a été observé en conditions de sécheresse pour certains types forestiers. A partir de ces résultats, je discute des mécanismes d’interaction entre espèces qui peuvent expliquer les effets observés. Nos données ont montré que l’influence de la diversité en espèces est fortement dépendante du contexte et peut être modifiée par les conditions environnementales locales et les conditions climatiques. En terme de gestion forestière, je suggère que pour certaines régions en Europe, promouvoir la diversité en espèces ainsi que contrôler la densité des parcelles doit être recommandé afin d’adapter les écosystèmes forestiers aux futures conditions climatiques.

Biodiversity is known to support and boost a wide range of forest ecosystem functions and services like productivity and resistance against insect pests and diseases. However, whether tree species diversity also promotes water and carbon acquisition and use in forest ecosystems is still unclear. Furthermore, in the current context of global warming, information on how tree species diversity can influence the response of forest ecosystems to extreme climatic events such as drought are urgently needed. In this framework, the objective of my PhD thesis was to determine how tree species diversity influences important functions of the water and carbon cycle including transpiration, carbon isotope composition and water extraction depth at the tree- and ecosystem-scale under contrasting soil water conditions. My work was conducted within the FunDivEUROPE project in a network of permanent forest stands and tree plantations across a North-South gradient in Europecovering a wide range of climatic conditions. I found considerable variability among species or forest types in the response of transpiration and carbon isotope composition at the tree- and ecosystem-scale across Europe. Species diversity did not affect the water and carbon relations of tree species and forest ecosystems under non-limiting soil water conditions. However, a strong effect of species diversity was observed under drought conditions in some forest types. Based on these data, I discuss the potential mechanisms of species interactions that may explain the observed patterns. I also point out that the influence of species diversity is highly context-dependent, and changes with local environmental and climatic conditions. In terms of forest management applications, I suggest that, at least in some regions, controlling for tree species diversity along with stand density and total basal area could be recommended to help forests adapt to drier conditions; La biodiversité favorise un grand nombre de fonctions et services écosystémiques des écosystèmes forestiers tels que la production de bois ou la résistance aux attaques d’insectes et aux maladies. Cependant l’impact de la diversité sur l’acquisition et l’utilisation de l’eau et du carbone reste largement méconnu dans ces écosystèmes. De plus, dans le contexte actuel de changement climatique, l’influence de la diversité sur la réponse des écosystèmes forestiers à des événements climatiques extrêmes tels que la sécheresse reste à étudier. L’objectif de ce travail est donc de déterminer l’impact de la diversité en espèces d’arbre sur d’importantes fonctions du cycle de l’eau et du carbone telles que la transpiration, la composition isotopique du carbone ou la profondeur d’extraction de l’eau à l’échelle de l’arbre et de l’écosystème sous des conditions contrastées dedisponibilité en eau du sol. Ce travail a été réalisé dans le cadre du projet FunDivEUROPE sur un réseau de parcelles forestières ainsi que dans des plantations expérimentales le long d’un gradient Nord-Sud en Europe afin de couvrir une importante gamme de conditions climatiques. Nos travaux ont montré une importante variabilité de la réponse à la diversité à l’échelle de l’arbre et de l’écosystème en termes de relations hydriques et carbonées à travers l’Europe. La diversité en espèces ne semble pas influencer les relations hydriques et carbonées des espèces et des écosystèmes forestiers dans des conditions non limitantes de disponibilité en eau. Cependant, un fort effet de la diversité a été observé en conditions de sécheresse pour certains types forestiers. A partir de ces résultats, je discute des mécanismes d’interaction entre espèces qui peuvent expliquer les effets observés. Nos données ont montré que l’influence de la diversité en espèces est fortement dépendante du contexte et peut êtremodifiée par les conditions environnementales locales et les conditions climatiques. En terme de gestion forestière, je suggère que pour certaines régions en Europe, promouvoir la diversité en espèces ainsi que contrôler la densité des parcelles doit être recommandé afin d’adapter les écosystèmes forestiers aux futures conditions climatiques

Biodiversity is known to support and boost a wide range of forest ecosystem functions and services like productivity and resistance against insect pests and diseases. However, whether tree species diversity also promotes water and carbon acquisition and use in forest ecosystems is still unclear. Furthermore, in the current context of global warming, information on how tree species diversity can influence the response of forest ecosystems to extreme climatic events such as drought are urgently needed. In this framework, the objective of my PhD thesis was to determine how tree species diversity influences important functions of the water and carbon cycle including transpiration, carbon isotope composition and water extraction depth at the tree- and ecosystem-scale under contrasting soil water conditions. My work was conducted within the FunDivEUROPE project in a network of permanent forest stands and tree plantations across a North-South gradient in Europe covering a wide range of climatic conditions. I found considerable variability among species or forest types in the response of transpiration and carbon isotope composition at the tree- and ecosystem-scale across Europe. Species diversity did not affect the water and carbon relations of tree species and forest ecosystems under nonlimiting soil water conditions. However, a strong effect of species diversity was observed under drought conditions in some forest types. Based on these data, I discuss the potential mechanisms of species interactions that may explain the observed patterns. I also point out that the influence of species diversity is highly contextdependent, and changes with local environmental and climatic conditions. In terms of forest management applications, I suggest that, at least in some regions, controlling for tree species diversity along with stand density and total basal area could be recommended to help forests adapt to drier conditions., La biodiversité favorise un grand nombre de fonctions et services écosystémiques des écosystèmes forestiers tels que la production de bois ou la résistance aux attaques d’insectes et aux maladies. Cependant l’impact de la diversité sur l’acquisition et l’utilisation de l’eau et du carbone reste largement méconnu dans ces écosystèmes. De plus, dans le contexte actuel de changement climatique, l’influence de la diversité sur la réponse des écosystèmes forestiers à des événements climatiques extrêmes tels que la sécheresse reste à étudier. L’objectif de ce travail est donc de déterminer l’impact de la diversité en espèces d’arbre sur d’importantes fonctions du cycle de l’eau et du carbone telles que la transpiration, la composition isotopique du carbone ou la profondeur d’extraction de l’eau à l’échelle de l’arbre et de l’écosystème sous des conditions contrastées de disponibilité en eau du sol. Ce travail a été réalisé dans le cadre du projet FunDivEUROPE sur un réseau de parcelles forestières ainsi que dans des plantations expérimentales le long d’un gradient Nord-Sud en Europe afin de couvrir une importante gamme de conditions climatiques. Nos travaux ont montré une importante variabilité de la réponse à la diversité à l’échelle de l’arbre et de l’écosystème en termes de relations hydriques et carbonées à travers l’Europe. La diversité en espèces ne semble pas influencer les relations hydriques et carbonées des espèces et des écosystèmes forestiers dans des conditions non limitantes de disponibilité en eau. Cependant, un fort effet de la diversité a été observé en conditions de sécheresse pour certains types forestiers. A partir de ces résultats, je discute des mécanismes d’interaction entre espèces qui peuvent expliquer les effets observés. Nos données ont montré que l’influence de la diversité en espèces est fortement dépendante du contexte et peut être modifiée par les conditions environnementales locales et les conditions climatiques. En terme de gestion forestière, je suggère que pour certaines régions en Europe, promouvoir la diversité en espèces ainsi que contrôler la densité des parcelles doit être recommandé afin d’adapter les écosystèmes forestiers aux futures conditions climatiques.

Increasingly severe droughts are expected to negatively impact forest functioning in the future, especially in the Mediterranean region. Favoring mixed species stands has been advocated as a compromise between wood production and biodiversity conservation, but whether such management practices would allow forest ecosystems to acclimate to future climate conditions remains to be addressed. We tested whether the transpiration of Quercus cerris (Qc) and Quercus petraea (Qp) during droughts differ when they grow in pure or mixed forests. We measured sap flux density ( F D ) and leaf carbon isotope composition ( δ 13 C), as a proxy for intrinsic water use efficiency ( WUE int ), in pure and mixed Qc and/or Qp forest plots in Italy during the summer 2012. Qc and Qp showed a drop in F D of 41% and 52% respectively during the summer drought independently of the mixtures, but with different seasonal trends. Qp transpiration response to soil drought did not differ between pure and mixed plots. In contrast, Qc transpiration was reduced by 56% in mixed plots at the maximum of the drought and only by 31% in the pure plot. Furthermore, we observed higher WUE int for Qc in the mixed plot and no change for Qp, supporting the higher water stress intensity in the mixed plot for Qc. Our study illustrated the negative impact of Qp on Qc when these species compete for water resources. We suggest that managing for mixed stands in the Mediterranean region might, in some cases, increase the detrimental effect of drought on species functioning.

Les sécheresses de 2003 et 2011 ont marqué les esprits dans beaucoup de régions viticoles du Sud de la France avec des baisses de rendement et de qualité de la récolte. Chez la vigne soumise à un dessèchement du sol, la croissance végétative est la première fonction affectée avec, en particulier, un arrêt de croissance des rameaux secondaires. De plus, la fermeture des stomates, par lesquels l’eau s’évapore à la surface des feuilles, limite rapidement la transpiration. Ces réactions ont pour conséquence une diminution de la photosynthèse de la plante qui peut expliquer certaines pertes de rendement. Au cours des dernières années, les recherches en écophysiologie ont permis d’identifier une large diversité dans ces réponses du matériel végétal à la contrainte hydrique liée aussi bien au porte-greffe qu’au greffon. De nouvelles méthodes de mesure et de modélisation sont apparues pour classer les cépages et les porte-greffes selon leur sensibilité à la contrainte hydrique. Les premières simulations permettent de raisonner le choix du cépage, du porte-greffe ou de leur mode de conduite en fonction de scénarios climatiques probables. Les travaux en cours devraient déboucher sur une prise en compte encore plus complète des nombreuses modifications mises en place par la plante ou apportées par le viticulteur pour minimiser l’impact de la contrainte hydrique. Le prochain défi à relever sera sans doute d’intégrer les effets pluriannuels de la contrainte hydrique sur l’élaboration de la production afin de mieux comprendre les arrières-effets d’une sécheresse sur le millésime suivant ou les effets cumulés de sécheresses successives., Drought episodes in 2003 and 2011 had huge, detrimental impacts on vineyard production in South of France. In grapevine, vegetative growth is the first function to be influenced by drought with rapid cessation of growth of secondary shoot axes. Additionally, stomatal closure gradually reduces transpiration rate. These responses result in a decrease of photosynthesis rate which partly explains yield losses and quality damages. During the last years, research in ecophysiology evidenced a large diversity in responses to drought, originating from both scions and rootstocks. New methods have been developed for characterising and modelling scions and rootstocks as regard their susceptibility to drought conditions. Model predictions are becoming useful tools to select appropriate scions and rootstocks as a function of likely climate scenarios. Current work should incorporate additional plant modifications either resulting from acclimation of the plant or from management practices. Future works should address the multiyear effects of drought episodes on wine grape production in order to better account for carry-over or cumulative effects of drought on successive years.

Summary 1. In 2001, Loreau and Hector proposed a method to calculate the effect of biodiversity on ecosystem-level properties that distinguished selection effects (SE) from complementarity effects (CE). The approach was designed and has been widely used for the study of yield in mixed-species situations taking into account the relative abundance of each species in ecosystem-level yield. However, complex functional traits commonly used to integrate ecosystem-level properties that cannot be analysed like yield data because the weighted contribution of each species is not determined by its relative abundance. 2. We adapted the original method by clearly identifying ecologically meaningful weighting coefficients to represent species-specific contributions to ecosystem function. 3. We applied the adapted method of analysis to tree foliar carbon isotope composition in an experimental plantation in order to test the influence of species richness on plot water use efficiency (WUEplot). The appropriate weights for the WUEplot of each species are leaf CO2 assimilation rate. 4. We observed a large range of WUEplot and biodiversity effects among plots. The absence of a significant SE on WUEplot indicated that the overall net biodiversity effect was primarily driven by a CE. The net biodiversity effect and CE were mostly negative, suggesting that interspecific interactions resulted in a decrease in the ratio between carbon acquisition and transpiration at the ecosystem level. 5. The application of the method to complex components of ecosystem functioning provides important new insights into the practical and conceptual aspects of functional biodiversity research.

Throughout the world, huge areas have been occupied by monospecific tree plantations in order to fulfil the increasing demand for industrial wood products. The use of mixed-species plantations has recently been advocated as a potential compromise between maintaining high-volume wood production and conserving other ecosystem services. Yet little is known about the impact of tree species mixture on species- and ecosystem-level carbon accumulation and water use in mixed plantations. We combined data on above-ground biomass, xylem sap flux density, and the carbon and oxygen isotope composition of leaves and needles in a boreal plantation to test whether different levels of species mixture would impact tree- and plot-level biomass production, transpiration and water use efficiency. Data were recorded in 22 20-x-20-m plots (7 for transpiration) randomly allocated to either monocultures or to two-, three-, and five-species mixtures of Scots pine (Pinus sylvestris), Norway spruce (Picea abies), Silver birch (Betula pendula), Common alder (Alms glutinosa) and Siberian larch (Larix sibirica). At tree level, for a given species, we found significant differences among mixtures in transpiration and water use efficiency, though species mixture had no impact on above-ground biomass. At plot level, a large variability in functioning among mixtures was observed, but increasing the number of species in the mixture did not enhance productivity, transpiration or water use efficiency. The presence in mixtures of a high performing species like birch brought about changes in the canopy structure which in turn may have led to changes in micro-environmental conditions. Such changes could have contributed to explain differences in transpiration and water use efficiency among mixtures. We concluded that mixing locally-adapted species under non-limiting soil water conditions did not provide any benefit for ecosystem-level carbon accumulation and water use at this young ontogenic stage, but our study does not preclude that some benefits might arise at older stages. (C) 2013 Elsevier B.V. All rights reserved.

International audience; Climatic events predicted for this century should involve drier and warmer summers, such as those that occurred in 2003 and 2005 in France. The higher temperatures could become a limiting factor for the regeneration of Populus nigra L. species. During establishment and development in summer period, seedlings must be able to resist high temperatures (until 57.8°C on sand on a Loire island in July 2009). Therefore, maintaining a high transpiration rate (E) for leaf cooling (and therefore a high stomatal conductance to water vapour (gs)), may be a prerequisite for seedlings to survive under high temperatures. To test this hypothesis, we used bulk leaf carbon isotope discrimination (Δ) as a time-integrated index of gas exchange activity (Farquhar et al. 1989). The objectives of this study were (1) to explore the genetic variability of ∆ among P. nigra seedlings from different female trees coming from contrasting sites and (2) the plasticity in response to two contrasting temperatures. Seeds from 16 open-pollinated females originating from the Loire river (France) and from the Paglia river (Italy) were first grown during 7 weeks in two growth chambers at 25°C. At this time (t0), one chamber remained at 25°C and in the second one, the temperature progressively increased until 43°C until the first symptoms of wilting appeared (t1). At t0, an important genetic variability for ∆ (∆t0 25°C) was measured ranged from 24.0‰ to 28.0‰ in both chambers, but no provenances differences were detected. At t1, an important genetic variability for ∆t1 25°C and ∆t1 43°C was observed ranged from 22.0‰ to 26.0‰ in each chamber and a significant provenance effect was detected. At t1, ∆t1 43°C values were significantly higher (0.5‰) than ∆t1 25°C. Moreover, a significant correlation was detected between the ∆t1 43°C values and the symptoms of wilting seedlings, seedlings totally healthy exhibiting higher ∆t1 43°C values. We conclude, under the hypothesis that ∆ is mainly controlled by gs, as already reported in poplar species (Monclus et al. 2006), maintaining high E and therefore high ∆ may enable seedlings to survive under high temperature.

International audience; Climatic events predicted for this century should involve drier and warmer summers, such as those that occurred in 2003 and 2005 in France. The higher temperatures could become a limiting factor for the regeneration of Populus nigra L. species. During establishment and development in summer period, seedlings must be able to resist high temperatures (until 57.8°C on sand on a Loire island in July 2009). Therefore, maintaining a high transpiration rate (E) for leaf cooling (and therefore a high stomatal conductance to water vapour (gs)), may be a prerequisite for seedlings to survive under high temperatures. To test this hypothesis, we used bulk leaf carbon isotope discrimination (Δ) as a time-integrated index of gas exchange activity (Farquhar et al. 1989). The objectives of this study were (1) to explore the genetic variability of ∆ among P. nigra seedlings from different female trees coming from contrasting sites and (2) the plasticity in response to two contrasting temperatures. Seeds from 16 open-pollinated females originating from the Loire river (France) and from the Paglia river (Italy) were first grown during 7 weeks in two growth chambers at 25°C. At this time (t0), one chamber remained at 25°C and in the second one, the temperature progressively increased until 43°C until the first symptoms of wilting appeared (t1). At t0, an important genetic variability for ∆ (∆t0 25°C) was measured ranged from 24.0‰ to 28.0‰ in both chambers, but no provenances differences were detected. At t1, an important genetic variability for ∆t1 25°C and ∆t1 43°C was observed ranged from 22.0‰ to 26.0‰ in each chamber and a significant provenance effect was detected. At t1, ∆t1 43°C values were significantly higher (0.5‰) than ∆t1 25°C. Moreover, a significant correlation was detected between the ∆t1 43°C values and the symptoms of wilting seedlings, seedlings totally healthy exhibiting higher ∆t1 43°C values. We conclude, under the hypothesis that ∆ is mainly controlled by gs, as already reported in poplar species (Monclus et al. 2006), maintaining high E and therefore high ∆ may enable seedlings to survive under high temperature.

Aims: Genetic variability in grapevine cultivars may influence their strategy to cope with drought through stomatal regulation of transpiration rate. The aim of the present study was to evaluate the stomatal sensitivity of five cultivars (EkigaIna, Grenache. Marselan, Mourvedre. and Syrah) to soil water status and air water vapor pressure deficit (VPD). Methods and results: Leaf gas exchange and canopy light interception efficiency (epsilon(1)) were evaluated through a wide range of predawn leaf water potential (psi(PD)) measurements in a field experiment in Southern France. Additionally. greenhouse experiments were carried out to monitor stomatal response to increasing VPD levels. Ekigaina showed a strong isohydric behavior with the highest decrease in leaf gas exchange in response to soil water stress and VPD. Mourvedre and Grenache showed a similar but relatively less extreme behavior. These three cultivars showed a constant leaf water status during the day through stomatal regulation and a strong decrease in epsilon(1). In contrast, Syrah and Marselan displayed anisohydric behavior as they presented a less sensitive stomatal control. Both cultivars showed fluctuating midday leaf water potential and Marselan was the least affected in tem of c. Conclusion: This study demonstrated that grape cultivars differed in their stomatal response to soil water deficit and VPD. For a given cultivar, a similar stomatal behavior was found in response to both tyro and VPD. Significance and impact of the results: Adaptation to drought and viticulture viability in hot and dry environments could be achieved by identifying and breeding cultivars with drought tolerance traits, Objectifs : La variabilité génétique existante chez les cépages de vigne peut affecter leur stratégie d’adaptation à la sécheresse en régulant la transpiration au travers de la fermeture des stomates. L’objectif du travail a été d’évaluer la sensibilité des stomates de cinq cultivars (Ekigaïna, Grenache, Marselan, Mourvèdre et Syrah) à la disponibilité en eau du sol et au déficit de pression de vapeur (VPD). Méthodes et résultats : Les échanges gazeux des feuilles et l’efficience d’interception du rayonnement (ei) ont été évalués dans une large gamme des valeurs de potentiel hydrique de base (yPD) en conditions de terrain dans le Sud de la France. En outre, une expérimentation complémentaire en serre a été réalisée pour analyser la réponse des stomates face à une augmentation du VPD. L’Ekigaïna a présenté un caractère ioshydrique très marqué avec la plus grande diminution des échanges gazeux en réponse au déficit hydrique du sol et au VPD. Le Mourvèdre et le Grenache ont présenté un comportement similaire bien que moins extrême que l’Ekigaïna. Ces trois cultivars ont montré un potentiel hydrique foliaire constant pendant la journée à cause d’une forte régulation stomatique et une diminution de ei. Par contre, le Syrah et le Marselan, ont montré un comportement anisohydrique avec un control stomatique moins sensible. Les deux cultivars ont présenté une diminution du potentiel hydrique foliaire à midi tandis que le Marselan a été le moins affecté en termes de ei. Conclusions : La présente étude montre que les cultivars présentent des différences dans leur réponse au déficit hydrique du sol et au VPD. Pour un cultivar donné, un comportement stomatique similaire a été observé en réponse au yPD et au VPD. Impacts et signification des résultats : L’adaptation à la sécheresse et la viabilité de la viticulture sur de climats chauds et secs pourrait être atteint par la sélection de cultivars avec de caractères de tolérance.

International audience; Whereas the effects of water and nitrogen (N) on plant Delta C-13 have been reported previously, these factors have scarcely been studied for Delta O-18. Here the combined effect of different water and N regimes on Delta C-13, Delta O-18, gas exchange, water-use efficiency (WUE), and growth of four genotypes of durum wheat [Triticum turgidum L. ssp. durum (Desf.) Husn.] cultured in pots was studied. Water and N supply significantly increased plant growth. However, a reduction in water supply did not lead to a significant decrease in gas exchange parameters, and consequently Delta C-13 was only slightly modified by water input. Conversely, N fertilizer significantly decreased Delta C-13. On the other hand, water supply decreased Delta O-18 values, whereas N did not affect this parameter. Delta O-18 variation was mainly determined by the amount of transpired water throughout plant growth (T-cum), whereas Delta C-13 variation was explained in part by a combination of leaf N and stomatal conductance (g(s)). Even though the four genotypes showed significant differences in cumulative transpiration rates and biomass, this was not translated into significant differences in Delta O-18(s). However, genotypic differences in Delta C-13 were observed. Moreover, similar to 80% of the variation in biomass across growing conditions and genotypes was explained by a combination of both isotopes, with Delta O-18 alone accounting for similar to 50%. This illustrates the usefulness of combining Delta O-18 and Delta C-13 in order to assess differences in plant growth and total transpiration, and also to provide a time-integrated record of the photosynthetic and evaporative performance of the plant during the course of crop growth.

International audience; The effects of drought, enhanced UV-B radiation and their combination on plant growth and physiological traits were investigated in a greenhouse experiment in two populations of Populus cathayana Rehder originating from high and low altitude in south-west China.* In both populations, drought significantly decreased biomass accumulation and gas exchange parameters, including net CO2 assimilation rate (A), stomatal conductance (gs), transpiration rate (E) and photosynthetic nitrogen use efficiency (PNUE). However, instantaneous water use efficiency (WUEi), transpiration efficiency (WUET), carbon isotope composition (δ13C) and nitrogen (N) content, as well as the accumulation of soluble protein, UV-absorbing compounds and abscisic acid (ABA) significantly increased in response to drought. On the other hand, cuttings from both populations, when kept under enhanced UV-B radiation, showed very similar changes, as under drought, in all above-mentioned parameters.* Compared with the low altitude population, the high altitude population was more tolerant to drought and enhanced UV-B, as indicated by the higher level of biomass accumulation, gas exchange, water-use efficiency, ABA concentration and UV-absorbing compounds.* After one growing season of exposure to different UV-B levels and watering regimes, the decrease in biomass accumulation and gas exchange, induced by drought, was more pronounced under the combination of UV-B and drought. Significant interactions between drought and UV-B were observed in WUEi, WUET, δ13C, soluble protein, UV-absorbing compounds, ABA and in the leaf and stem N, as well as in the leaf and stem C:N ratio.* Our results showed that UV-B acts as an important signal allowing P. cathayana seedlings to respond to drought and that the combination of drought and UV-B may cause synergistically detrimental effects on plant growth.; Les effets d'une sécheresse, d'une augmentation du rayonnement UV-B et de leur combinaison ont été étudiés sur la croissance et les caractéristiques physiologiques de plants de deux populations de Populus cathayana Rehder originaires de haute et basse altitude dans le sud-ouest de la Chine.* Dans les deux populations, la sécheresse a réduit de manière significative l'accumulation de biomasse et les échanges gazeux foliaires, y compris l'assimilation nette (A) de CO2, la conductance stomatique (gs), la transpiration (E) et l'efficience photosynthétique d'utilisation de l'azote (PNUE). Toutefois, l'efficience instantanée d'utilisation de l'eau (WUEi), l'efficience de transpiration (WUET), la composition isotopique du carbone (δ13C) et le contenu en azote (N), ainsi que l'accumulation de protéines solubles, des composés absorbant les UV et de l'acide abscissique (ABA) ont présenté une augmentation significative en réponse à la sécheresse. D'autre part, des boutures des deux populations, quand elles ont été conservées sous un rayonnement UV-B renforcé, ont montré des changements de tous les paramètres mentionnés ci-dessus, similaires à ceux induits par la sécheresse.* Par rapport à la population de faible altitude, la population de haute altitude, était plus tolérante à la sécheresse et au renforcement du rayonnement UV-B; cela s'est traduit par le niveau plus élevé d'accumulation de biomasse, d'échanges gazeux, de l'efficience d'utilisation de l'eau, de la concentration d'ABA et des composés absorbant les UV.* Après une saison de croissance, la diminution de l'accumulation de biomasse et des échanges gazeux induite par la sécheresse, était plus prononcée dans le cadre de la combinaison du rayonnement UV-B et de la sécheresse. Des interactions significatives entre sécheresse et UV-B ont été observées pour WUEi, WUET, δ13C, les protéines solubles, les composés absorbant les UV, l'ABA et l'azote des feuilles et des tiges, ainsi que pour le rapport C/N des feuilles et des tiges.* Nos résultats ont montré que les UV-B agissent comme un signal important en permettant aux jeunes plants de P. cathayana de faire face à la sécheresse et que la combinaison de la sécheresse et des UV-B peut causer par synergie des effets néfastes pour la croissance des plants.