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Survival and growth of woody species in the Mediterranean are mainly restricted by water availability. We tested the hypothesis that Mediterranean species acclimate their xylem vulnerability and osmotic potential along a precipitation gradient.We studied five predominant co-occurring Mediterranean species; Quercus calliprinos, Pistacia palaestina, Pistacia lentiscus, Rhamnus lycioides and Phillyrea latifolia, over two summers at three sites. The driest of the sites is the distribution edge for all the five species. We measured key hydraulic and osmotic traits related to drought resistance, including resistance to embolism (psi(50)) and the seasonal dynamics of water and osmotic potentials.The leaf water potentials (psi(l)) of all species declined significantly along the summer, reaching significantly lower psi(l) at the end of summer in the drier sites. Surprisingly, we did not find plasticity along the drought gradient in psi(50) or osmotic potentials. This resulted in much narrower hydraulic safety margins (HSMs) in the drier sites, where some species experienced significant embolism.Our analysis indicates that reduction in HSM to null values put Mediterranean species in embolism risk as they approach their hydraulic limit near the geographical dry edge of their distribution.

International audience; This paper describes an innovative method, and related equipment, developed by the authors to monitor non-invasively historic panel paintings under museum display conditions. This method permits in-depth knowledge about such artworks, allowing us to understand their reactions to climatic variations, and pro- vides objective data on which conservation decisions can be confidently based, since the data are directly obtained from the individual artwork. Since 2004, following the invitation from the Louvre Museum and the C2RMF (National Centre for Research and Restoration of French Museums), the wooden panel on which Leonardo da Vinci painted his Mona Lisa has been studied by an international research team of wood technologists and engineers, including researchers from French and Italian universities and related scientific institutions (Montpellier, Clermont Auvergne, Poitiers, Florence), to understand its mechanical, hygroscopic and shape characteristics and behaviour, to evaluate its present state, and to provide sug- gestions for optimizing its conservation conditions. Non-invasive methods and equipment were therefore devised and implemented to measure (during the annual opening day of its display case) and automat- ically monitor (during the time the display case remains closed) both the deformations that the panel undergoes (mainly produced by the inevitable small climatic fluctuations within the case) and the con- straining forces acting on the panel itself. The method and the related equipment, improved over the years, are based on miniature load cells and displacement transducers, whose outputs are automatically logged at desired time intervals, typically ranging between 30 min for monitoring during the whole year, and a few seconds for manual measurements, calibrations, and other selected events; the stored data can be downloaded both through a cable connection and wirelessly, by means of a specially developed con- nection apparatus. The panel is confined in a climate-controlled display case, which typically is opened only for a few hours once a year. Additionally, close restrictions must be respected, including absolute non-invasiveness, non-interference with the enjoyment of the artwork by the public, and compliance with strict procedures for safe and secure conservation. The implementation of this method has provided significant information about the actual behaviour of the panel during the whole year. Comparing several annual force-deformation curves, their good linearity suggests that no unacceptable stress or deformation has taken place in it, showing that the climatic conditions (air temperature and relative humidity) main- tained in the display case can be considered favourable to the conservation of the artwork. Moreover, based on the collected data, reliable Finite Element Method (FEM) models are being developed and cali- brated, with the aim of describing the mechanical behaviour of the panel and virtually evaluating the risk of damage (including the propagation of an ancient crack) deriving from external conditions or actions to which it would be unthinkable to submit the original historic artwork.

International audience; This study aims to analyze the influence of the fibrils oscillations and connections on the effective hygro-elastic behavior of the wood cell wall. For that, two different models of microstructure describing the cell wall are introduced: the model 1S, for which the fibrils are allowed to oscillate in one direction like a sine curve, and the model 2S, for which the fibrils oscillate in two directions as a helix. The effective hygro-elastic behavior of the cell wall associated with each of the two models is computed by numerical periodic homogenization. Then, we analyze the effects of the fibers oscillations on the effective behavior by making vary some parameters such as the shape ratio of the unit cell, the fibrils volume fraction, or the elastic contrast between the matrix and the fibrils. It is shown that the fibrils oscillations have a significant effect on some effective moduli of the cell wall, such as the effective shear moduli G ̃12 (for both model 1S and 2S) and G ̃13 , G ̃23 (for model 2S only).

International audience; In the context of increasing heat periods and recurrence of droughts, and thus higher soil water depletion, we explored and quantified the role of understorey vegetation in ecosystem evapotranspiration in boreal and temperate forests. We reviewed and analysed about 200 papers that explicitly gave figures of understorey vegetation evapotranspiration relative to different stand features and traits. Understorey vegetation accounted on average for one-third of total ecosystem evapotranspiration during the growing season. Overstorey leaf area index (LAI) is the main variable that drives understorey evapotranspiration through radiation interception. Most data show that below an overstorey LAI of 2–3, the contribution of the understorey vegetation to ecosystem evapotranspiration increases exponentially, following the exponential increase of the climatic demand, i.e. potential evapotranspiration. Different factors have the potential to modulate this effect such as species composition and phenology, root distribution, and interaction with droughts. Consequently, managers must be aware that depending on understorey species present on site and stand structure, understorey vegetation can contribute significantly to a negative stand water balance.

International audience; A numerical FEM (Finite Element Method) model was implemented to represent the mechanical state of the wooden panel of the Mona Lisa, as it is conserved in its exhibition case, and constrained in its auxiliary frame. The model is based on the integration of advanced numerical analysis and various experimental examinations carried out non-invasively on the artwork by the authors during over 15 years. This includes visual, microscopic and X-ray observations together with mechanical measurements and monitoring of panel deformations and constraining external forces. In addition to the development of non-invasive techniques to characterise the mechanical properties of the panel, the FEM model reliably evaluated the strains and stresses generated in the panel by the various actions it experiences. The paper consists of the following parts: (i) a short summary of the experimental measurements and other observations, (ii) a detailed description of the FEM numerical model, of the hypotheses it is based on, and of its advantages and limits, (iii) the main results obtained by running the model. This includes the identification of local strains and stresses, the location of most critical areas, an evaluation of the risk that the existing ancient crack may propagate, and an evaluation of safe ranges for the forces acting on the wooden panel, (iv) the validation criteria for such results, and (v) a discussion about the significance of the mechanical model.

International audience; Thermal treatments can be considered as an accelerated ageing, bringing partly similar changes in properties as naturally aged wood. Thermal treatment was applied on white mulberry (Morus alba L.), a dominant species for making musical instruments from middle-East to Far-East, to investigate the effects on the vibro-mechanical and physical properties of this wood, and the results compared to previously published data on spruce (Picea abies Karst.) as a reference for the soundboard of Western string instruments. Thermal treatment (TT) at 150 °C and 0% of relative humidity was applied to five analogous groups of specimens with five different durations (2.5, 8, 24, 72, 261 hours). Humidity reconditioning of specimens was done to explore the reversibility of TT effects. Physical and vibrational properties such as specific gravity (γ), equilibrium moisture content (EMC), CIELab colorimetric values, specific modulus of elasticity (E'/γ) and damping coefficient (tanδ) in longitudinal (L) and radial (R) directions, have been measured after stabilisation of samples in standard conditions (20 °C, 65% RH), before and after TT and then after reconditioning. Untreated mulberry had a low EMC, very low L/R anisotropy and low E'L/γ, and relatively low tanδ. Weight loss (WL) and CIELab values evolved similarly during TT for mulberry and for previous results on spruce, however, their EMC and vibrational properties were affected differently. This could be explained in part by the low anisotropy of mulberry, and in part by its particular extractives. The parts of irreversible effects, linked to chemical modification or degradation, and of reversible effects, linked to physical configuration, were different between mulberry and spruce. The applied treatments did not bring permanent "improvements" in vibrational properties of mulberry, yet its colour appearance was enhanced.

Wood characteristics of trees grown in agroforestry systems are little studied, even if growth conditions are different from conventional stands. This work aimed to determine the impact of the agroforestry system on the heartwood formation process of hybrid walnut (Juglans regia × nigra) trees, especially the resulting extractive contents. Ethanol and water extractions were successively performed on wood samples taken across the diameter of the trunk of agroforestry (AF) and forest (FC) walnut trees to get the radial distribution of the extractive contents. All the samples were analyzed by NIR-spectroscopy and NIR-hyperspectral imaging. Statistical discriminant models were developed to classify the samples from both different forestry systems, according to their chemical composition. The results indicated no significant differences between the values of extractive contents of AF and FC walnut woods, whatever the radial position. At the intra-tree scale, the quantity of extractives does not increase significantly with the radial position. However, partial least squares-discriminant analysis (PLS-DA) regression models, developed with NIRS measurements, showed that significant chemical differences exist between AF and FC trees, especially for extractives composition and lignin content. This allowed to classify wood specimens from both forestry systems. These results were confirmed by hyperspectral camera analyses.

Vapor Pressure Deficit (VPD, atmospheric drought) and soil water potential (Ψsoil, soil drought) have both been reported to affect terrestrial plant water stress, plant functions (growth, stomatal conductance, transpiration) and vulnerability to ecosystem disturbances (mortality or vulnerability to wildfires). Which of atmospheric drought or soil drought has the greatest influence on these responses is yet an unresolved question. Using a state-of-the-art soil-plant-atmosphere hydraulic model, we conducted an in-silico experiment where VPD and Ψsoil were manipulated one at a time to quantify the relative importance of atmospheric vs soil drought on most critical plant functions. The model simulates the combined effects of soil drought and atmospheric drought on plant water potential (ΨPlant), a physiologically meaningful metric of plant water status driving plant turgor, stomatal conductance, hydraulic conductance or water content, and thus mortality and fire risks. Contrary to expectations, we showed that VPD had a weaker effect than Ψsoil on tree water stress and forest disturbances risk (i.e leaf moisture content). While physiological responses associated with low water stress such as stomatal closure or turgor loss could be driven by both VPD or soil drought, consequences of extreme water stress such as hydraulic failure, leaf desiccation and vulnerability to wildfires were almost exclusively driven by low Ψsoil. Our results therefore suggest that most plant functions are affected by VPD through its cumulative effect on Ψsoil via increased plant transpiration, rather than through a direct instantaneous effect on plant water potential. We argue that plant hydraulics provide a strong foundation for predicting tree and terrestrial ecosystem responses to climate changes and propose a list of explanations and testable hypotheses to reconcile plant hydraulic theory and observations of soil and atmospheric drought effects on plant functions.

International audience; The use of MRI in the plant sciences has traditionally been limited due to the immobility of the devices, and restrictions with regard to sample size and shape. To overcome these limitations and to be able to study plants directly in their natural environment, we evaluated the capacities of a portable, unilateral magnet: The Nuclear Magnetic Resonance Mobile Universal Surface Explorer (NMR-MOUSE), designed by Blümich et al. [1]. This device permits measuring the water NMR signal in a 100-micrometer slice, and by means of a built-in lift, the magnet can be moved to record an NMR profile within a depth of approximately 25-millimeters from the surface of the device. We tested its capacity to measure tree water content by following the dehydration dynamic of thirty cut branches from six different species and two different functional types. We also tested the device on four in-situ trees. There was a linear correlation between the integral of the MRI profiles obtained and the water content of the branches and trees. This correlation was present regardless of tree species or functional type. Using the profiles obtained on the branches, we then tested the capacity of the device to differentiate the conductive tissues, i.e. the xylem and phloem fluxes. The MRI profiles of the branches presented distinct peaks which corresponded to the xylem and phloem tissues, whose location was validated with x-ray microtomography imaging. In conclusion, the NMR-MOUSE is a promising candidate for measuring tree water dynamics in the field.

International audience; The use of MRI in the plant sciences has traditionally been limited due to the immobility of the devices, and restrictions with regard to sample size and shape. To overcome these limitations and to be able to study plants directly in their natural environment, we evaluated the capacities of a portable, unilateral magnet: The Nuclear Magnetic Resonance Mobile Universal Surface Explorer (NMR-MOUSE), designed by Blümich et al. [1] and commercialized by Magritek. This MRI device permits measuring the signal in increments of up to 100-micrometers, and within a depth of approximately 25-millimeters. We tested its capacity to measure tree water content by following the dehydration dynamic of cut branches from six different species and two different functional types. We also tested the device on four in-situ trees. There was a significant linear correlation between the integral of the MRI profiles obtained and the water content of the branches and trees. This significant correlation was present regardless of tree species or functional type. We then tested the capacity of the device to differentiate the conductive tissues, i.e. the xylem and phloem fluxes, within cut branches. The MRI profiles of the branches presented distinct peaks which corresponded to the xylem and phloem tissues, whose location was validated with x-ray microtomography imaging which allows the high-resolution visualization of the tissues within the sample. In conclusion, the NMR-MOUSE is a promising candidate for measuring plant water dynamics in the field.

The building of a tree is the result of wood growth through successive division, expansion and maturation of living cells at the periphery of the trunk and branches. During this process, diameter growth is combined with sapwood pre-stress to allow posture control by the generation of growth forces in the living wood cells. These mechanical aspects of tree building can be characterised at each peripheral position by parameters describing the amount of material produced, wood rigidity and the strain induced by the maturation process. In-situ assessment of maturation strains at the trunk periphery of beech trees, combined with laboratory measurements of ring width (RW), wood density (D) and wood specific modulus (SM), was used to examine biomechanical aspects of juvenility corresponding to young stages of the tree, as well as the correlation or trade-off between sapwood pre-stressing and the generation of forces in the living wood layer used to control tree posture. The radial variations of RW, D and SM, averaged over 86 trees, were close to the “typical radial pattern” of juvenile wood for softwood plantation trees: decrease in RW and increase in D and SM from pith to bark in the juvenile phase. But D only increased in the very first rings, then remained more or less constant. Furthermore, for all three parameters there were many discrepancies in the pattern of variation between trees and even between plots. This is a good indication that the mechanical juvenility of the wood was more related to the biomechanical conditions experienced by the trees in the young ages than to the age of the tree as such (which is the case for fibre length). The level of pre-stress and posture control forces were strongly dependent on the maturation strain as the first explanatory factor. But pre-stress is independent of RW, whereas posture control force is strongly dependent on this growth parameter. This opens the way to trade-offs between these two biomechanical functions of wood fibres.; La construction d'un arbre est le résultat de la croissance du bois par division, expansion et maturation successives des cellules vivantes à la périphérie du tronc et des branches. Au cours de ce processus, la croissance en diamètre est combinée à la précontrainte de l'aubier pour permettre le contrôle de la posture par la génération de forces de croissance dans les cellules vivantes du bois. Ces aspects mécaniques de la construction des arbres peuvent être caractérisés à chaque position périphérique par des paramètres décrivant la quantité de matière produite, la rigidité du bois et la déformation induite par le processus de maturation. L'évaluation in situ des contraintes de maturation à la périphérie du tronc des hêtres, combinée à des mesures en laboratoire de la largeur des cernes (RW), de la densité du bois (D) et du module spécifique du bois (SM), a été utilisée pour examiner les aspects biomécaniques de la juvénilité correspondant aux jeunes stades de l'arbre, ainsi que la corrélation ou le compromis entre la précontrainte de l'aubier et la génération de forces dans la couche de bois vivant utilisée pour contrôler la posture de l'arbre. Les variations radiales de RW, D et SM, moyennées sur 86 arbres, étaient proches du "schéma radial typique" du bois juvénile pour les arbres de plantation de résineux : diminution de RW et augmentation de D et SM de la moelle à l'écorce dans la phase juvénile. Mais D n'a augmenté que dans les tout premiers cernes, puis est resté plus ou moins constant. En outre, pour les trois paramètres, il existe de nombreuses divergences dans le schéma de variation entre les arbres et même entre les parcelles. Cela indique bien que la juvénilité mécanique du bois était davantage liée aux conditions biomécaniques subies par les arbres dans les jeunes âges qu'à l'âge de l'arbre en tant que tel (ce qui est le cas pour la longueur des fibres). Le niveau de précontrainte et les forces de contrôle de la posture dépendent fortement de la contrainte de maturation, qui est le premier facteur explicatif. Mais la précontrainte est indépendante du RW, alors que la force de contrôle de la posture est fortement dépendante de ce paramètre de croissance. Cela ouvre la voie à des compromis entre ces deux fonctions biomécaniques des fibres de bois.

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International audience; There is a gap between mechanical measurement methods under laboratory conditions and those under real conditions of structural monitoring. This paper proposes a method that applies well-established computer vision developments to photomechanics in difficult conditions. It is therefore a technique that is flexible and versatile while maintaining satisfying accuracy. It consists in marker tracking using ArUco fiducial markers as measurement points. It allows locating markers in non-optimal conditions of camera orientation and position. A homography process was used to analyse pictures taken with a view angle. The accuracy of the method was estimated, especially in case of out-of-plane motions, and the impact of the view angle and of the distance between camera and markers on the location error was studied. The method was applied in creep tests to measure crack parameters as well as the transverse expansion of wooden beams. In the application example presented, it enabled to compute distances between markers with only 0.28% of relative error and hence to measure the crack parameters and the long-term shrinkage-swelling of the wooden beams. However, the impacts of brightness variations and camera parameters have not been estimated. This method is very promising when experimental conditions are variable and when multiple measurements are necessary.

International audience; Blossom and twig blight, caused by Monilinia spp., is the main disease in apricot trees. In this study, we installed transparent rain shelters in apricot orchards to study their influence on the modification of the microclimate at the level of the tree canopy and on the reduction in moniliosis damage in twigs. Rain shelters significantly reduced the leaf wetness time measured within the foliage compared to the unsheltered trees (a reduction of between 43% and 67%). However, very few differences were observed in the daily averaged air temperature (up to 6%) and daily averaged air relative humidity (up to 1%). In the first experiment, on the apricot variety Bergarouge® (CEP Innovation, Lyon, France), moniliosis damage on twigs in the absence of phytosanitary protection was reduced by up to 62% for the trees provided with rain protection compared to the trees that did not receive rain shelters. A second experiment, involving five apricot tree varieties, made it possible to verify that fungicide protection could be reduced for the trees protected by rain covers, reducing moniliosis damage on twigs compared to full fungicide protection combined without rain protection. Finally, a third experiment comprising two apricot tree varieties has shown that in organic orchards, rain protection provides protection against moniliosis (twig blight) that is equivalent to an organic farming fungicide protection programme based on the use of copper sulphate and calcium polysulphide.

Grafting has been widely used in horticulture to induce dwarfing and avoid stress-derived limitations on plant growth and yield by affecting plant architecture and leaf functions. However, the respective effects on plant photosynthesis and water use efficiency (WUE) of leaf distribution and functions that depend on both rootstock and scion have not been fully elucidated. This study aimed to (i) clarify the scion × interstock impacts on the variability of leaf photosynthetic traits and WUE, and (ii) decipher the respective effects of leaf distribution and functions on canopy photosynthesis and WUE (WUEc). Leaf gas exchange over light gradients and responses to light, CO2, temperature, and vapor pressure deficit were measured in two apple cultivars, ‘Liquan Fuji’ (‘Fuji’) and ‘Regal Gala’ (‘Gala’), grafted onto rootstocks combined with interstocks: a vigorous (VV, ‘Qinguan’), or a dwarf one (VD, M26). The 3D architecture-based RATP model was parameterized to estimate the canopy photosynthesis rate (Ac), transpiration rate (Ec), and WUEc. Then, virtual scenarios were used to compare the relative contributions of cultivar and interstock to canopy Ac, Ec, and WUEc. These scenarios changed the leaf distribution and functions of either cultivar or interstock. At the leaf scale, VD trees had significantly higher leaf nitrogen per area but a lower maximum carboxylation rate and dark respiration in both cultivars. In parallel with higher leaf stomatal conductance (gs) and transpiration in VD ‘Fuji’ and similar gs in VD ‘Gala’, VD trees showed significantly lower leaf photosynthesis rate and WUE than VV trees. However, lower leaf photosynthetic capacities in VD trees were compensated at the canopy scale, with Ac and WUEc for ‘Fuji’ significantly improved in VD trees under both sunny and cloudy conditions, and for ‘Gala’ significantly improved in VD trees under cloudy conditions compared with VV trees. Switching scenarios highlighted that ‘Gala’ leaf functions and distribution and VD leaf distributions enhanced Ac and WUEc simultaneously, irrespective of weather conditions. Up-scaling leaf gas exchange to the canopy scale by utilizing 3D architecture-based modeling and reliable measurements of tree architecture and leaf functional traits provides insights to explore the influence of genetic materials and tree management practices.

International audience; Increasing temperature and drought can result in leaf dehydration and defoliation even in drought-adapted tree species such as the Mediterranean evergreen Quercus ilex L. The stomatal regulation of leaf water potential plays a central role in avoiding this phenomenon and is constrained by a suite of leaf traits including hydraulic conductance and vulnerability, hydraulic capacitance, minimum conductance to water vapour, osmotic potential and cell wall elasticity. We investigated whether the plasticity in these traits may improve leaf tolerance to drought in two long-term rainfall exclusion experiments in Mediterranean forests. Osmotic adjustment was observed to lower the water potential at turgor loss in the rainfall-exclusion treatments, thus suggesting a stomatal closure at more negative water potentials and a more anisohydric behaviour in drier conditions. Conversely, leaf hydraulic conductance and vulnerability did not exhibit any plasticity between treatments so the hydraulic safety margins were narrower in the rainfall-exclusion treatments. The sequence of leaf responses to seasonal drought and dehydration was conserved among treatments and sites but trees were more likely to suffer losses of turgor and hydraulic functioning in the rainfall-exclusion treatments. We conclude that leaf plasticity might help the trees to tolerate moderate drought but not to resist severe water stress.

25 páginas.- 8 figuras.- 2 tablas.- 66 referencias.- Additional supporting information can be found online in the Supporting Information section at the end of this article., eaf water potential (psi(leaf)), typically measured using the pressure chamber, is the most important metric of plant water status, providing high theoretical value and information content for multiple applications in quantifying critical physiological processes including drought responses. Pressure chamber measurements of psi(leaf) (psi(leafPC)) are most typical, yet, the practical complexity of the technique and of the underlying theory has led to ambiguous understanding of the conditions to optimize measurements. Consequently, specific techniques and precautions diversified across the global research community, raising questions of reliability and repeatability. Here, we surveyed specific methods of psi(leafPC) from multiple laboratories, and synthesized experiments testing common assumptions and practices in psi(leafPC) for diverse species: (i) the need for equilibration of previously transpiring leaves; (ii) leaf storage before measurement; (iii) the equilibration of psi(leaf) for leaves on bagged branches of a range of dehydration; (iv) the equilibration of psi(leaf) across the lamina for bagged leaves, and the accuracy of measuring leaves with artificially 'elongated petioles'; (v) the need in psi(leaf) measurements for bagging leaves and high humidity within the chamber; (vi) the need to avoid liquid water on leaf surfaces; (vii) the use of 'pulse' pressurization versus gradual pressurization; and (viii) variation among experimenters in psi(leafPC) determination. Based on our findings we provide a best practice protocol to maximise accuracy, and provide recommendations for ongoing species-specific tests of important assumptions in future studies., European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 751918-AgroPHYS; Marie Curie Fellowship (FP7-PEOPLE-2013-IOF-624473); Austrian research agency (FWF) project P32203; National Science Foundation (Grant IOS-#1457279)

Acoustic emission analysis is a promising technique to investigate the physiological events leading to drought-induced injuries and mortality. However, the nature and the source of the acoustic emissions are not fully understood and make the use of this technique difficult as a direct measure of the loss of xylem hydraulic conductance. In this study, acoustic emissions were recorded during severe dehydration in lavender plants and compared to the dynamics of embolism development and cell lysis. The timing and characteristics of acoustic signals from two independent recording systems were compared by principal component analysis. In parallel, changes in water potential, branch diameter, loss of hydraulic conductance and electrolyte leakage were measured to quantify drought-induced damages. Two distinct phases of acoustic emissions were observed during dehydration: the first one associated with a rapid loss of diameter and a significant increase in loss of xylem conductance (90%) and the second one with a significant increase in electrolyte leakage and slower diameter changes. This second phase corresponds to a complete loss of recovery capacity. The acoustic signals of both phases were discriminated by the third and fourth principal components. The loss of hydraulic conductance during the first acoustic phase suggests the hydraulic origin of these signals (i.e. cavitation events). For the second phase, the signals showed much higher variability between plants and acoustic systems suggesting that the sources of these signals may be plural, although likely including cellular damage. A simple algorithm was developed to discriminate hydraulic-related acoustic signals from other sources, allowing the reconstruction of dynamic hydraulic vulnerability curves. However, hydraulic failure precedes cellular damage and lack of whole plant recovery is associated to these latter., 6 figures, 1 table, + 2 supplementary figures and 1 supplementary table

International audience; The characterisation of ecological strategies to predict drought response is still lacking for Caribbean dry forest seedlings. This study documents growth characteristics and tolerance to drought via xylem hydraulic and leaf cell properties of three dominant native species of the Caribbean dry forest. Twenty morphological and physiological traits were assessed in Citharexylum spinosum, Guaiacum officinale and Guapira fragrans in greenhouse conditions. The seedlings displayed contrasting growth rates, which were positively correlated with the capacity to quickly develop a large leaf area and root fraction. The three species had a similar xylem tolerance to embolism (P 50 : −4 MPa) but differed in leaf cell tolerance to dehydration, which was negatively correlated with RGR (R 2 > 0.87). The slowest-growing, G. officinale, had high leaf tolerance to cell dehydration due to low Ψ TLP and πo, but displayed a narrow hydraulic safety margin. The leaves of the fast-growing C. spinosum were sensitive to leaf dehydration but exhibited a surprisingly wide stem hydraulic safety margin. G. fragrans had intermediate traits. Our results showed that dry forest seedling growth in similar environments can exhibit distinct carbon growth strategies as well as contrasting water-use strategies, primarily as they relate to drought resistance, due to variation in root development and leaf cell resistance to dehydration. Our study thus provides an approach to estimate species performance under drought conditions.

14 páginas.- 8 figuras.- 1 tablas.- 62 referencias.- Supporting information the online version of the published article http://dx.doi.org/10.1111/nph.18578, Xylem hydraulic failure (HF) has been identified as a ubiquitous factor in triggering drought-induced tree mortality through the damage induced by the progressive dehydration of plant living cells. However, fundamental evidence of the mechanistic link connecting xylem HF to cell death has not been identified yet. The main aim of this study was to evaluate, at the leaf level, the relationship between loss of hydraulic function due to cavitation and cell death under drought conditions and discern how this relationship varied across species with contrasting resistances to cavitation. Drought was induced by withholding water from potted seedlings, and their leaves were sampled to measure their relative water content (RWC) and cell mortality. Vulnerability curves to cavitation at the leaf level were constructed for each species. An increment in cavitation events occurrence precedes the onset of cell mortality. A variation in cells tolerance to dehydration was observed along with the resistance to cavitation. Overall, our results indicate that the onset of cellular mortality occurs at lower RWC than the one for cavitation indicating the role of cavitation in triggering cellular death. They also evidenced a critical RWC for cellular death varying across species with different cavitation resistance. © 2022 The Authors. New Phytologist © 2022 New Phytologist Foundation., This research was supported by La Region Auvergne-Rhone-Alpes ‘Pack Ambition International 2020’ through the project‘ ThirsTree’ 20-006175-01, 20-006175-02 and the Agence Nationale de la Recherche, grant/award no. ANR-18-CE20-0005, ‘Hydrauleaks’

Fuel moisture content (FMC) is a crucial driver of forest fires in many regions world-wide. Yet, the dynamics of FMC in forest canopies as well as their physiological and environmental determinants remain poorly understood, especially under extreme drought.We embedded a FMC module in the trait-based, plant-hydraulic SurEau-Ecos model to provide innovative process-based predictions of leaf live fuel moisture content (LFMC) and canopy fuel moisture content (CFMC) based on leaf water potential (psi Leaf$$ {\psi}_{\mathrm{Leaf}} $$). SurEau-Ecos-FMC relies on pressure-volume (p-v) curves to simulate LFMC and vulnerability curves to cavitation to simulate foliage mortality.SurEau-Ecos-FMC accurately reproduced psi Leaf$$ {\psi}_{\mathrm{Leaf}} $$ and LFMC dynamics as well as the occurrence of foliage mortality in a Mediterranean Quercus ilex forest. Several traits related to water use (leaf area index, available soil water, and transpiration regulation), vulnerability to cavitation, and p-v curves (full turgor osmotic potential) had the greatest influence on LFMC and CFMC dynamics. As the climate gets drier, our results showed that drought-induced foliage mortality is expected to increase, thereby significantly decreasing CFMC.Our results represent an important advance in our capacity to understand and predict the sensitivity of forests to wildfires.

This work aims to model the mechanical processes used by tree branches to control their posture despite their increasing weight loading. The two known options for a branch to maintain its orientation are the asymmetry of maturation stress, including reaction wood formation, and eccentric radial growth. Both options can be observed in nature and influence the stress distribution developed in the branch each year. This so-called "growth stress" reflects the mechanical state of the branch. In this work, a growth stress model was developed at the cross-section level in order to quantify and study the bio-mechanical impact of each process. For illustration, this model was applied to branches of two 50-year-old trees, one softwood Pinus pinaster and one hardwood Prunus avium (wild cherry tree), both simulated with the AmapSim discrete element software. For the wild cherry tree, the computed ouputs highlighted that the eccentricity of radial growth seems to be as efficient as the formation of reaction wood to maintain the postural control despite the increasing gravity. For the pine tree, eccentric radial growth appears to be less efficient than the formation of reaction wood. But although it does not necessarily act as a relevant lever for postural control, it greatly modifies the profile pattern of mechanical stress and could provide mechanical safety of the branch. This work opens experimental perspectives to understand the biomechanical processes involved in the formation of branches and their mechanical safety.

International audience; Advancements in plant Space biology are required for the realization of long-duration exploratoryclass manned missions, where the re-supply of resources from Earth is not feasible for technical and economic constraints. Plants are key organisms in Bioregenerative Life Support Systems (BLSS) for the regeneration of resources (i.e. oxygen production through photosynthesis, water recovery by transpiration, and wastes recycling) and the production of fresh healthy food. Moreover, plants play a role in psychological support for astronauts. The definition of cultivation requirements for the design, realization and successful operation of BLSS must take into account the effects of Space factors on plant growth, development and reproduction. Altered gravitational fields and radiation exposure are the main Space factors inducing changes in gene expression, cell proliferation and differentiation, signaling and physiological processes, with consequences on tissue organization and organogenesis, thus on the whole organisms functioning. In this paper, the main findings of gravityand radiation-related research of the last years are summarized, highlighting the knowledge gaps that is still necessary to fill. A focus on existing facilities as well as requirements for future facilities to achieve fundamental biology goals is reported. Possible future experiments in the short-mediumlong term are proposed to achieve the targets of crewed Space exploration.

International audience; Climate change is having an increasingly rapid impact on ecosystems and particularly on the issue of water resources. The Internet of Things and communication technologies have now reached a level of maturity that allows sensors to be deployed more easily on sites to monitor them. The communicating node based on LoRaWAN technology presented in this article is open and allows the interfacing of numerous sensors for designing long-term environmental monitoring systems of isolated sites. The data integration in the cloud is ensured by a workflow driving the storage and indexing of data, allowing a simple and efficient use of the data for different users (scientists, administration, citizens) through specific dashboards and extractions. This article presents this infrastructure through environmental monitoring use cases related to water resources.

Background Plants have adapted to survive seasonal life-threatening frost and drought. However, the timing and frequency of such events are impacted by climate change, jeopardizing plant survival. Understanding better the strategies of survival to dehydration stress is therefore timely and can be enhanced by the cross-fertilization of research between disciplines (ecology, physiology), models (woody, herbaceous species) and types of stress (drought, frost). Scope We build upon the ‘growth–stress survival’ trade-off, which underpins the identification of global plant strategies across environments along a ‘fast–slow’ economics spectrum. Although phenological adaptations such as dormancy are crucial to survive stress, plant global strategies along the fast–slow economic spectrum rarely integrate growth variations across seasons. We argue that the growth–stress survival trade-off can be a useful framework to identify convergent plant ecophysiological strategies to survive both frost and drought. We review evidence that reduced physiological activity, embolism resistance and dehydration tolerance of meristematic tissues are interdependent strategies that determine thresholds of mortality among plants under severe frost and drought. We show that complete dormancy, i.e. programmed growth cessation, before stress occurrence, minimizes water flows and maximizes dehydration tolerance during seasonal life-threatening stresses. We propose that incomplete dormancy, i.e. the programmed reduction of growth potential during the harshest seasons, could be an overlooked but major adaptation across plants. Quantifying stress survival in a range of non-dormant versus winter- or summer-dormant plants, should reveal to what extent incomplete to complete dormancy could represent a proxy for dehydration tolerance and stress survival. Conclusions Our review of the strategies involved in dehydration stress survival suggests that winter and summer dormancy are insufficiently acknowledged as plant ecological strategies. Incorporating a seasonal fast–slow economics spectrum into global plant strategies improves our understanding of plant resilience to seasonal stress and refines our prevision of plant adaptation to extreme climatic events.

Investigating the responses of plant anatomical traits of trees to drought–rewatering cycles helps us to understand their responses to climate change; however, such work has not been adequately reported. In this study, Ginkgo biloba L. saplings were subjected to moderate, severe, extreme and lethal drought conditions by withholding water according to the percentage loss of hydraulic conductivity (PLC) and rewatering on a regular basis. Samples of phloem, cambium and xylem were collected to quantify their cellular properties including cambium and phloem cell vitality, xylem growth ring width, pit aspiration rates and pit membrane thickness using light microscopy and transmission microscopy. The results showed that the mortality rate of G. biloba saplings reached 90% at approximately P88 (xylem water potential inducing 88% loss of hydraulic conductivity). The onset of cambium and phloem cell mortality might be in accordance with that of xylem embolism. Close negative correlations between xylem water potential and PLC and between xylem water potential and cambium and phloem mortality suggested that xylem hydraulic traits are coupled with anatomical traits under declining xylem water potential. Cambium and phloem cell vitality as well as xylem growth ring width decreased significantly with increasing drought conditions. However, xylem pit membrane thickness, cambial zone width and cambial cell geometry were not affected by the drought–rewatering cycles. The tracheid radial diameter, intertracheid cell wall thickness and tracheid density decreased significantly during both drought conditions and rewatering conditions. In addition to hydraulic traits, cambium and phloem cell vitality can be used as anatomical traits to evaluate the mortality of G. biloba under drought. Future work is proposed to observe the dynamics of pit aspiration rates under drought–rewatering cycles in situ to deepen our understanding of the essential role of bordered pits in the ‘air-seeding’ mechanism.

International audience; Herbaria, which gather collections of plants sometimes collected centuries ago, can contain specimens of species that are now extinct in the wild, or even in cultivation. This chapter presents the experimental approach and the first results obtained for two extinct species, Filago neglecta , an annual herb that used to occur in the open environments of Western Europe, and Astiria rosea , a woody species that was endemic to Mauritius (Mascarene Islands). There are two steps for selection of target species and identification of affine species. The first step was to establish a state of knowledge on the lists of extinct plant species. The second step was to assess the “viability” (ability to produce seedlings) of available seeds, using X-ray microtomography to see the cellular structures inside of the seeds without any invasive and destructive cutting.

National audience; En sol à nappe d’eau perchée temporaire, le système racinaire du chêne est régulièrement soumis à un engorgement prolongé, suivi par des épisodes de sécheresse édaphique pouvant être marqués. En conditions anaérobies sous l’eau, la croissance des racines est inhibée et leur état se détériore rapidement. Au retrait de la nappe d’eau, seules les racines qui étaient hors de l’eau peuvent avoir une reprise de croissance rapide et recoloniser le sol en profondeur. L’état de santé de l’arbre dépend directement de cette potentialité, en interaction avec la précocité du retour de la sécheresse estivale, de la dimension effective du réservoir en eau, mais aussi de la profondeur de prospection des racines et de leur capacité à absorber l’eau aux grandes profondeurs.

Key message Different components of water balance and temperature reduce density and height growth of saplings of Picea abies (L.) H. Karst (Norway spruce), Abies alba Mill. (silver fir) and Fagus sylvatica L. (European beech) in mixed uneven-aged forests in the French Alps and Jura mountains. Ungulate browsing is an additional pressure on fir and beech that could jeopardise the renewal of these species in the future. Context The uncertainty in tree recruitment rates raises questions about the factors affecting regeneration processes in forests. Factors such as climate, light, competition and ungulate browsing pressure may play an important role in determining regeneration, forest structures and thus future forest composition. Aims The objective of this study was to quantify sapling densities and height increments of spruce, fir and beech and to identify dominant environmental variables influencing them in mixed uneven-aged forests in the French Alps and Jura mountains. Methods Sapling height increment and density were recorded in 152 plots, and non-linear mixed models were obtained to establish relations between them and environmental factors known to affect regeneration, namely altitude, slope, aspect, canopy openness, soil characteristics, temperature, precipitation and ungulate browsing. Results Regeneration density, varying from 0 to 7 saplings per m 2, decreased with sapling height and was also negatively affected for spruce by PET, but positively for fir by precipitation and for beech by mean annual soil water content. Height increment reached up to 50 cm annually, increasing with sapling height and canopy openness and decreasing under high maximum summer temperatures for spruce and beech. The statistical effect of different environmental variables varied slightly among species but trends were quite similar. Additionally, ungulate browsing was high, with fir being the most intensely browsed, followed closely by beech, while spruce was rarely browsed. Conclusions All these results suggest that more temperature warming and a decrease in water availability could negatively impact sapling growth and density in the three species, with possible reduction of forest renewal fluxes. The observed increase of ungulate populations leading to increased browsing could be particularly detrimental to fir saplings.

International audience; Plant interactions play a key role in forest ecosystem dynamics. The tallest plants, namely the overstorey trees, are obvious major drivers, particularly in competition for light. This process has already been amply described. However, the role played by lower strata has often been underestimated. In this review, we first briefly recall the role of over- and understoreys in structuring forest microclimate, mostly through light sharing. We then focus on belowground interactions between over- and understorey, where knowledge is more piecemeal, partly because of measurement difficulties. Even so, some studies show that competition for water and nutrients by the overstorey controls the development of understorey vegetation much more than competition for light. The reverse (overstorey limitation by the understorey) has also been encountered, but has been much less well researched. We also address the involvement of mycorrhizae, specifically their role in alleviating overstorey drought stress and contributing to nutrient cycling. We go on to show how another example of key ecosystem engineers, large mammalian herbivores, shape above- and belowground resources and intervene in over- and understorey interactions. In conclusion, for a better understanding of forest dynamics and adapted management, particularly in the context of global climate change, we advocate taking account not only of trees but of all forest components. Belowground processes need more research. The roles of mycorrhizal networks, root exudates, microbiota, and chemical cues need to be further explored to gain a finer understanding of the interactions between over- and understorey.

International audience

International audience

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International audience; Climate change is altering resource availability and conditions that affect plant performance. Plants respond to these changes through environmentally induced changes in phenotype (phenotypic plasticity). Understanding this plasticity is essential to predict and manage the effects of climate change on crops. Within the framework of the RECITAL project, the responses of Lavandin and Fine Lavender to various water constraints were characterized. Lavandin and lavender plants grown with a lower water supply show a modification of hydraulic parameters (stomatal conductance, resistance to vessel embolism) that are critical for drought resistance. The plasticity of these parameters induced by drier growing conditions would give them a better resistance to a future drought.; Les évolutions climatiques modifient la disponibilité des ressources et les conditions qui influent sur la performance des plantes. Les plantes réagissent à ces changements notamment par des modifications du phénotype (plasticité phénotypique) induites par l'environnement. La compréhension de cette plasticité est essentielle pour prévoir et gérer les effets du changement climatique sur les plantes cultivées. Dans le cadre du projet RECITAL, les réponses du Lavandin et de la Lavande fine à diverses contraintes hydriques ont été caractérisées. Des plants de lavandins et de lavande cultivés avec un apport d’eau plus faible montrent une modification de paramètres hydrauliques (conductance stomatique, résistance à l’embolie des vaisseaux) qui sont déterminants dans la résistance à la sécheresse. La plasticité de ces paramètres induite par des conditions de culture plus sèches leur confèrerait une meilleure résistance à une sécheresse à venir.

International audience

International audience

International audience; Wood is a complex cellular material because of its heterogeneity and variability. The first level of variability is that of the species, which controls the organization of the woody plane, which is specific to it. But within the same species, with the same genetic baggage, each tree has its own mode of growth, of wood formation, which are then strongly dependent on the region, the plot, in which it develops but also on the local environment of the individual. Finally, each year, each season imposes its own environmental conditions of growth which will modulate the biological processes. In addition, there are the local specificities of the wood linked to particular developments such as knots, the presence of reaction wood, etc.But, finally, even among those who work with wood on a daily basis, few have the opportunity to spend hours under a microscope to appreciate all this complexity and diversity of structures. And yet, many teachers, in high school, university or in a wood science engineering school, wish to have their students observe these fascinating structures.There is now a theoretically simple solution, a little more complex to implement, which makes it possible to put in the hands of all, an object, exact copy of an anatomical structure of wood in 3D.; Le bois est un matériau cellulaire complexe par son hétérogénéité et sa variabilité. Le premier niveau de variabilité est celui de l’espèce, laquelle pilote l’organisation du plan ligneux, qui lui est spécifique. Mais au sein d’une même espèce, au même bagage génétique, chaque arbre a son propre mode de croissance, de formation du bois, lesquels sont alors fortement dépendant de la région, de la placette, dans laquelle il se développe mais aussi de l’environnement local de l’individu. Enfin, chaque année, chaque saison impose ses conditions environnementales de croissance qui vont moduler les processus biologiques. A cela se rajoutent les spécificités locales du bois liées à des développements particuliers tels que les noeuds, la présence de bois de réaction, etc.Mais, finalement, même parmi ceux qui travaillent le bois quotidiennement, peu sont ceux qui ont l’occasion de passer des heures sous un microscope pour apprécier toute cette complexité et cette diversité de structures. Et nombreux sont pourtant les enseignants, en lycée, à l’université, dans une école d’ingénieur de sciences du bois, qui souhaitent faire observer ces fascinantes structures à leurs élèves.Il existe maintenant une solution théoriquement simple, un peu plus complexe à mettre en oeuvre, qui permet de mettre dans les mains de tous, un objet, copie conforme d’une structure anatomique de bois en 3D.

Forests, although essential for human survival due to their numerous ecosystem services, have been facing a significant dieback caused by intense droughts since the beginning of the Anthropocene. As global climatic models predict an increment in the frequency and intensity of droughts in the near future, which will consequently increase the risk of tree mortality, it is crucial to better understand the physiological bases of drought-induced tree mortality in order to identify physiological thresholds that will allow us to better predict the impact of climate change on forest ecosystems. While xylem hydraulic failure has been recognised as a ubiquitous factor in tree death under drought conditions, fundamental evidence for the mechanistic link between hydraulic failure, cell mortality and tree mortality has not yet been identified. Therefore, the main objective of this thesis was to elucidate this link while identifying key physiological thresholds beyond which tree survival is greatly compromised. Thus, by focusing first on two species, a conifer (Pseudotsuga menziesii M.) and an angiosperm (Prunus lusitanica L.) the thesis work demonstrated that the levels of hydraulic failure previously considered as lethal for trees (P50 for the conifers and P88 for the angiosperms) were not precise enough to define with certainty the fate of a tree after a drought event. However, although the causal link between xylem hydraulic failure and cell death remained unclear, the interaction between these two phenomena appeared to explain the ability of trees to recover from water stress. Thus, by combining the detection of cavitation events using the optical technique, measurements of relative water content at the tissue level, and measurements of cell lysis on an easily accessible organ: the leaf, we demonstrated that the hydraulic failure of the xylem, by causing a drop in relative water content below a critical level, the RWCcrit, caused cell death. Finally, based on the assumption that not all cells were equally important for the survival of trees and that this survival should certainly be linked to the maintenance of the integrity of cells responsible for plant growth, i.e., the meristematic cells, we sought, by working on a conifer species (Abies concolor (Gordon & Glend) Lindl. Ex Hildebr) and an angiosperm species (Fagus sylvatica L.), to show that the meristematic cells were indeed key cells when evaluating the mechanistic link between hydraulic failure and tree mortality. On the latter part, although statistical analyses revealed again the importance of the level of stem relative water content before re-watering for tree survival, preliminary results obtained by transmission electron microscopy showed that, for the same level of stem relative water content, plants with intact meristematic cells were more likely to survive the drought event than the others as soon as they were re-watered.; Les forêts, bien qu’essentielles à la survie de l’Homme du fait de leurs nombreux services écosystémiques, font face à d’importants dépérissements causés par des sécheresses intenses depuis le début de l’Anthropocène. Comme les modèles climatiques globaux prévoient, pour un futur proche, une augmentation de la fréquence et de l’intensité de ces sécheresses qui, par conséquence, accentuera le risque de mortalité des arbres, il apparait alors crucial de mieux comprendre les bases physiologiques de la mortalité des arbres induite par sécheresse afin d’identifier des seuils physiologiques permettant de mieux prédire l’impact de ces changements climatiques sur les écosystèmes forestiers. Si la défaillance hydraulique du xylème a été reconnue comme un facteur omniprésent de la mort des arbres en conditions de sécheresse, des preuves fondamentales du lien mécaniste reliant la défaillance hydraulique, la mortalité des cellules et celle des arbres n’ont pas encore été identifiées. Ainsi, l’objectif principal de cette thèse était d’élucider ce lien tout en identifiant les seuils physiologiques clés au-delà desquels la survie de l’arbre est grandement compromise. Dès lors, en s’intéressant, tout d’abord, à deux espèces, un conifère (Pseudotsuga menziesii M.) et une angiosperme (Prunus lusitanica L.) le travail de thèse a démontré que les niveaux de défaillance hydraulique considérés jusqu’alors comme létaux pour les plantes (P50 pour les conifères et P88 pour les angiospermes) n’étaient pas assez précis pour définir avec certitude le sort d’un arbre après un événement de sécheresse. Cependant, alors que le lien causal entre défaillance hydraulique du xylème et la mortalité cellulaire restait non élucidé, l’interaction entre ces deux phénomènes semblait expliquer la capacité des arbres à récupérer d’un stress hydrique. Ainsi, en combinant, à la fois, la détection des événements de cavitation à l’aide de la méthode optique, des mesures de contenu en eau relatif au niveau tissulaire, et des mesures de lyse cellulaire au niveau d’un organe facilement accessible : la feuille, nous avons mis en évidence que la défaillance hydraulique du xylème, en entrainant une baisse du contenu en eau des tissus en dessous d’un niveau critique, le RWCcrit, provoquait la mort des cellules. Finalement, en partant du postulat que toutes les cellules n’avaient pas la même importance dans la survie des arbres et que cette survie devait certainement être liée au maintien de l’intégrité des cellules responsables de la croissance de la plante, c’est-à-dire des cellules méristématiques, nous avons cherché, en travaillant sur un conifère (Abies concolor (Gordon & Glend) Lindl. Ex Hildebr) et une angiosperme (Fagus sylvatica L.), à montrer que les cellules méristématiques étaient bien des cellules clés lors de l’évaluation du lien mécaniste entre la défaillance hydraulique et la mortalité de l’arbre. Sur cette dernière partie, bien que les analyses statistiques aient à nouveau révélé l’importance du niveau de contenu en eau relatif des tissus de la tige avant réhydratation pour la survie des arbres, des résultats préliminaires obtenus en microscopie électronique à transmission ont montré qu’à niveaux de contenus en eau égaux, des plantes présentant des cellules méristématiques intègres étaient plus à même de survivre à l’événement de sécheresse que les autres et, ce, dès leur ré-arrosage.

International audience; Quantitative measurements of plant gravitropic response are challenging. Differences in growth rates between species and environmental conditions make it difficult to compare the intrinsic gravitropic responses of different plants. In addition, the bending movement associated with gravitropism is competing with the tendency of plants to grow straight, through a mechanism called proprioception (ability to sense its own shape). Disentangling these two tendencies is not trivial. Here, we use a combination of modeling, experiment and image analysis to estimate the intrinsic gravitropic and proprioceptive sensitivities of stems, using Arabidopsis as an example.

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This article presents a numerical analysis of the instability developing in horizontal plane Poiseuille flow, when stratification extends along the direction orthogonal to the plane of shear. Our study builds up on the previous work that originally detected the linear instability of such configuration, by means of experiments, theoretical analysis and numerical simulations (Le Gal et al. 2021). We extend hereafter this former investigation beyond linear theory, investigating nonlinear regimes with direct numerical simulations. We notice a tendency for the flow to lose its vertical homogeneity through a point of secondary bifurcation, due to harmonic resonances, and further describe this symmetrybreaking mechanism in the vicinity of the instability threshold. When departing away from this limit, we observe a series of bursting events that break down the flow into disordered motions driven by localized shear instabilities. This intermittent dynamics leads to the coexistence of localized layers of stratified turbulence surrounded by quiescent regions of meandering waves.

Trees are long-lived organisms that continuously adapt to their environments, a process in which epigenetic mechanisms are likely to play a key role. Via down regulation of the chromatin remodeler DECREASED IN DNA METHYLATION 1 (DDM1) in poplar (Populus tremula 9 Populus alba) RNAi lines, we examined how DNA methylation coordinates genomic and physiological responses to moderate water deficit.We compared the growth and drought response of two RNAi-ddm1 lines to wild-type (WT) trees under well-watered and water deficit/rewatering conditions, and analyzed their methylomes, transcriptomes, mobilomes and phytohormone contents in the shoot apical meristem. The RNAi-ddm1 lines were more tolerant to drought-induced cavitation but did not differ in height or stem diameter growth. About 5000 differentially methylated regions were consistently detected in both RNAi-ddm1 lines, colocalizing with 910 genes and 89 active transposable elements. Under water deficit conditions, 136 differentially expressed genes were found, including many involved in phytohormone pathways ; changes in phytohormone concentrations were also detected. Finally, the combination of hypomethylation and drought led to the mobility of two transposable elements. Our findings suggest major roles for DNA methylation in regulation of genes involved in hormone-related stress responses, and the maintenance of genome integrity through repression of transposable elements.

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International audience

Under climate change, the increasing occurrence of late frost combined with advancing spring phenology can increase the risk of frost damage in trees. In this study, we tested the link between intra-specific variability in bud phenology and frost exposure and damages. We analysed the effects of the 2021 late frost event in a black spruce (Picea mariana (Mill.) BSP) common garden in Québec, Canada. We hypothesized that the timing of budbreak drives the exposure of vulnerable tissues and explains differences in frost damage. Budbreak was monitored from 2015 to 2021 in 371 trees from five provenances originating between 48° and 53° N and planted in a common garden at 48° N. Frost damages were assessed on the same trees through the proportion of damaged buds per tree and related to the phenological phases by ordinal regressions. After an unusually warm spring, minimum temperatures fell to -1.9°C on May 28 and 29, 2021. At this moment, trees from the northern provenances were more advanced in their phenology and showed more frost damage. Provenances with earlier budbreak had a higher probability of damage occurrence according to ordinal regression. Our study highlights the importance of intra-specific variability of phenological traits on the risk of frost exposure. We provide evidence that the timings of bud phenology affect sensitivity to frost, leading to damages at temperatures of -1.9°C. Under the same conditions, the earlier growth reactivation observed in the northern provenances increases the risks of late frost damage on the developing buds. This article is protected by copyright. All rights reserved.

International audience

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International audience

Climate change is challenging the resilience of grapevine (Vitis), one of the most important crops worldwide. Adapting viticulture to a hotter and drier future will require a multifaceted approach including the breeding of more drought-tolerant genotypes. In this study, we focused on plant hydraulics as a multi-trait system that allows the plant to maintain hydraulic integrity and gas exchange rates longer under drought. We quantified a broad range of drought-related traits within and across Vitis species, created in silico libraries of trait combinations, and then identified drought tolerant trait syndromes. By modeling the maintenance of hydraulic integrity of current cultivars and the drought tolerant trait syndromes, we identified elite ideotypes that increased the amount of time they could experience drought without leaf hydraulic failure. Generally, elites exhibited a trait syndrome with lower stomatal conductance, earlier stomatal closure, and a larger hydraulic safety margin. We demonstrated that, when compared with current cultivars, elite ideotypes have the potential to decrease the risk of hydraulic failure across wine regions under future climate scenarios. This study reveals the syndrome of traits that can be leveraged to protect grapevine from experiencing hydraulic failure under drought and increase drought tolerance.

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