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International audience; Mediterranean forests face an intensification of droughts caused by ongoing climate change. To improve our understanding of tree and forest responses to increasing drought, we explored over nine years, the effects of a 30% rainfall exclusion experiment on the water potential, sap velocity and primary and secondary growth of two co-occurring species (Quercus ilex and Pinus halepensis) in a French Mediterranean forest. In addition, native embolism was measured after six and nine years of exclusion onset. Water potentials decreased earlier during summer drought for both species in the rainfall exclusion plot, and to a higher extent during the drought peak for Q. ilex, involving earlier stomatal closure and reduced sap velocity. Sap velocity reduction persisted throughout the years in the exclusion plot. Outside summer the water potential difference between predawn and midday was similar between treatment which indicate that reduced water transport efficiency may be linked to decrease hydraulic conductance. Such differences were neither related to differences in xylem embolism, that remained similar between treatments, nor to change in secondary growth. In contrast primary growth measurements indicate that P. halepensis trees, and Q. ilex to a lesser extent, experienced reduction in total leaf areas in response to the rainfall exclusion. Globally, our results suggest that increase drought lead, for both species, to a decrease in sap velocity not mediated by increase embolism but rather by a reduction in primary growth. So far little conclusion can be drawn regarding the competitive advantage of one species over the other in the context of increasing drought related to climate change.

Under natural conditions, plants experience external mechanical stresses such as wind and touch that impact their growth. A remarkable feature of this mechanically induced growth response is that it may occur at distance from the stimulation site, suggesting the existence of a signal propagating through the plant. In this study, we investigated the electrical response of poplar trees to a transient controlled bending stimulation of the stem that mimics the mechanical effect of wind. Stem bending was found to cause an electrical response that we called ‘gradual’ potential, similar in shape to an action potential. However, this signal distinguished from the well-known plant action potential by its propagation up to 20 cm along the stem and its strong dumping in velocity and amplitude. Two hypotheses regarding the mode of propagation of the ‘gradual’ potential are discussed.One sentence summaryPoplar stem bending induces an electrical response with high speed and strong decrement.

Knowledge on variations of drought resistance traits are needed to predict the potential of trees to acclimate to coming severe drought events. Xylem vulnerability to embolism is a key parameter related to such droughts, and its phenotypic variability relies mainly on environmental plasticity. We investigated the structural determinants controlling the plasticity of vulnerability to embolism, focusing on the key elements involved in the air bubble entry in vessels, especially the intervessel pits. Poplar saplings (Populus tremula x alba (Aiton) Sm., 1804) grown in contrasted water availability or light exposure exhibited differences in the vulnerability to embolism (P50) in a range of 0.76 MPa. We then characterized the structural changes in features related to pit quantity and pit structure, from the pit ultrastructure to the organization of xylem vessels, using different microscopy techniques (transmission electron microscopy, scanning electron microscopy, light microscopy). A multispectral combination of X-ray microtomography and light microscopy analysis allowed measuring the vulnerability of each single vessel and testing some of the relationships between structural traits and vulnerability to embolism inside the xylem. The pit ultrastructure did not change, whereas the vessel dimensions increased with the vulnerability to embolism and the grouping index and fraction of intervessel cell wall both decreased with the vulnerability to embolism. These findings hold when comparing between trees or between the vessels inside the xylem of an individual tree. These results evidenced that plasticity of vulnerability to embolism in hybrid poplar occurs through changes in the pit quantity properties such as pit area and vessel grouping rather than changes on the pit structure.

International audience; Abstract Global climatic models predict an increment in the frequency and intensity of drought events, which have important consequences on forest dieback. However, the mechanisms leading to tree mortality under drought conditions and the physiological thresholds for recovery are not totally understood yet. This study aimed to identify what are the key physiological traits that determine the tree capacity to recover from drought. Individuals of a conifer (Pseudotsuga menziesii M.) and an angiosperm (Prunus lusitanica L.) species were exposed to drought and their ability to recover after rehydration monitored. Results showed that the actual thresholds used for recovery from drought based on percentage loss of conductance (PLC) (i.e. 50% for conifers, 88% for angiosperms) do not provide accurate insights about the tree capacity for surviving extreme drought events. On the contrary, differences in stem relative water content (RWCStem) and the level of electrolytes leakage (EL) were directly related to the capacity of the trees to recover from drought. This was the case for the conifer species, P. menziesii, for which higher RWCStem and lower EL values were related to the recovery capacity. Even if results showed a similar trend for the angiosperm P. lusitanica as for the conifers, differences between the two traits were much more subtle and did not allow an accurate differentiation between trees able to recover and those that were not. RWCStem and EL could work as indicators of tree capacity to recover from drought for conifers but more studies are required to confirm this observation for angiosperms.

International audience; & Key message Mechanical acclimation of young poplars (Populus tremula × Populus alba, INRA 717-1B4) submitted to periodic stem bending is mainly driven by compressive strains. Flexure wood and compressive flexure wood exhibit higher mechanical resilience and lower mechanical damage. & Context It is well known that thigmomorphogenesis modulates tree growth and the anatomical structure of wood. However, nothing is known about the mechanical behaviour of the tissues of fresh wood formed under mechanical stimulation. & Aims We investigated the elastic and plastic properties of the fresh wood of young poplar trees (Populus tremula × Populus alba, INRA 717-1B4) submitted to periodic controlled stem bending that mimics the mechanical effect of wind. & Methods For a set of trees, we applied symmetrical bending treatments, which led to the formation of "flexure wood". For another set of trees, asymmetrical bending treatments, including compression (or tension) only, were applied and generated specific wood formation: "compressive flexure wood" and "tensile flexure wood". We investigated the elastic and plastic properties of these woods at the stem and at the local tissue levels. & Results The results revealed that fresh wood formed under compressive treatments is more resistant to damage (damage reduced by 44%) and a higher mechanical resilience (+ 33%), suggesting that this tissue is able to withstand higher mechanical strains than "normal wood". This improvement could explain the higher mechanical strength of the stem to bending (+ 42%). & Conclusion When trees experience repetitive mechanical stimulations, they adjust the plastic plastic behaviour of their wood in a way that improves the mechanical safety. This demonstrates the adaptive benefit of the mechanical acclimation of trees.

Xylem vulnerability to drought-induced embolism did not differ between stems and petioles of four woody species ( Betula pendula , Liriodendron tulipifera , Populus tremula and Olea europaea ). Our results, together with data compiled from published literature, indicate that hydraulic segmentation during drought stress is not consistently driven by difference in vulnerability to embolism between stem and terminal organs. Hydraulic failure and disconnection of distal organs during protracted drought stress is thought to protect large branches or trunks by reducing water loss and restricting the spread of embolism. Hydraulic segmentation and preferential sacrifice of distal organs such as leaves can be driven by two mechanisms: more negative water potentials at the terminal section of the hydraulic pathway and/or by higher vulnerability to xylem embolism of distal organs. Although vulnerability segmentation has been reported in the literature, the generality of this phenomenon is unclear, in part due to the methodological limitations related to direct measurement of xylem vulnerability to embolism in intact plants. The objective of this study was to evaluate vulnerability segmentation between petioles and stems using non-invasive micro computed tomography (microCT). Vulnerability to embolism was measured in leaf petioles and subtending stems of four woody species (Betula pendula R., Liriodendron tulipifera L., Populus tremula L. and Olea europaea L.) with contrasting drought tolerances. In addition, previously published vulnerability data for petioles and stems were compiled from the literature to investigate the commonality of hydraulic segmentation across a wide range of woody species, with the vulnerability curve methodology distinguished. Using non-invasive imaging on intact plants, we found no evidence of hydraulic segmentation between petioles and stems of four angiosperm tree species, regardless of mechanism. Moreover, the literature dataset indicated that little or no difference in vulnerability to embolism is present between petioles and stems when vulnerability curves were constructed using methods specifically measuring the dynamics of xylem tissue during dehydration (e.g. optical visualization, MicroCT). Our results suggest that vulnerability segmentation between stems and distal organs (petioles and leaves) is limited when only xylem tissue is considered. Large differences in vulnerability between stems and leaves are likely to be driven by extra-xylary components, rather than xylem embolism.

Xylem resistance to cavitation is a key parameter in drought resistance of trees. Indeed, water stress tolerant species are less vulnerable to cavitation than water stress intolerant species. At the within species level, phenotypic variability was reported for the xylem cavitation resistance, mainly based on plasticity among environmental growth conditions. This raises the question of genetic and structural determinants of this plasticity. The objective of the present work is to identify the structural determinants controlling plasticity of xylem resistance to cavitation. Young poplars trees were grown in contrasted water availability or light exposure in order to induce acclimation in hydraulic properties. We then characterized the structural modifications of their acclimated xylem at three levels: xylem organization, vessels dimensions and pits shape. We used different microscopy techniques including Transmission Electron Microscope (T.E.M.), Scanning Electron Microscope (M.E.B.) and Light Microscope – on the same individual trees. We specially investigated the intervessel pits that provide hydraulic connections between vessels while they prevent air embolism spreading. Moreover, a local approach using direct observation of embolism spreading at the cellular level via X-ray micro-CT allowed us to explore the structural determinants of vessel resistance to cavitation.

International audience; Key messagePeriodic bending of young poplars increase the wood production whatever their hydric status; especially in the most highly stressed zones; improving the mechanical behaviour of the stem.AbstractThe ability of trees to acclimate the building of their structures to windy conditions under various hydric conditions is essential in the context of the predicted climate changes. In this study, we investigated the biomechanical responses of young poplar trees to periodic controlled bending stimulations that mimic the mechanical effect of trees growing under windy conditions. This treatment was conducted for 5months in well-watered conditions or under hydric stress. Results demonstrate the high impact of thigmomorphogenesis on growth processes, even under the water shortage. While axial growth was reduced by mechanical stimulations and hydric stress, radial growth was strongly increased by the periodic stem bending. The secondary growth was preferentially increased in the direction of highest longitudinal strains leading to the ovalisation of the cross-section. This ovalisation yielded 16%, regardless the hydric condition and generated a huge increase of the bending rigidity of the trees (+212%). Further, we observed a differential growth between the side growing under tension and the side growing under compression. A Finite Element model was built to investigate the mechanical benefits of the anisotropic cross-section shapes. This FE model enlightened the modulation of the spatial stress distribution that lead to a reduction of the stress in the weakest zones of the trunk; suggesting an improvement of the mechanical safety margin of wood. Thigmomorphogenesis acclimation appears as a complex and costly, but necessary process for the long-term mechanical support of the trees, even under hydric stress conditions.

Leaf hydraulic conductance (Kleaf) and capacitance (Cleaf) are among the key parameters in plant-water regulation. Understanding the responses of these hydraulic traits to drought conditions remains a challenge for describing comprehensive plant-water relationships. The ability of an organism to resist and/or tolerate embolism events, which may occur at high negative pressure caused by hydric stress, relies on how well it can sustain a hydraulic system in a dynamic equilibrium. Populus deltoides is a water-saving tree species with a stomatal conductance that declines rapidly with reduced water availability. Under unfavorable conditions, the stomatal control of transpiration is known to be closely coordinated with a loss of plant hydraulic functioning that can ultimately result in hydraulic failure through xylem embolism, notably in leaves. The effects of drought on leaf hydraulics are also related to regulation in water permeases such as the aquaporins. To describe the responses linked to leaf hydraulics under severe drought and rewatering conditions, water-stressed poplars were monitored daily on an ecophysiological and a molecular scale. A structural and expression analysis on a set of aquaporins was carried out in parallel by in situ hybridization analysis and quantitative PCR. In complement, water distribution in water-challenged leaves was investigated using X-ray microtomography. A general depression of leaf hydraulic conductance and relative water content occurred under drought, but was reversed when plants were rewatered. More interestingly, (i) extreme leaf water deficiency led to marked xylem and lamina embolism, but a degree of hydric integrity in the midrib extraxylem territories and the bundle sheath of the minor veins was maintained, and (ii) the sub-tissue water allocation correlated well with an over-accumulation of several PIP and TIP aquaporins. Our multi-facet molecular ecophysiological approach revealed that leaves were able to secure a certain level of hydric status, in particular in cell territories near the “living ribs”, which provided rapid hydric adjustment responses once favorable conditions were restored. These findings contribute to an integrated approach to leaf hydraulics, thus favoring a better understanding of the cell mechanisms involved in tree vulnerability to climate changes.

Centrifuge-based techniques to assess xylem vulnerability to embolism are increasingly being used, although we are yet to reach a consensus on the nature and extent of artefactual embolism observed in some angiosperm species. In particular, there is disagreement over whether these artefacts influence both the spin (Cavitron) and static versions of the centrifuge technique equally. We tested two methods for inducing embolism: bench dehydration and centrifugation. We used three methods to measure the resulting loss of conductivity: gravimetric flow measured in bench-dehydrated and centrifuged samples (static centrifuge), in situ flow measured under tension during spinning in the centrifuge (Cavitron) and direct imaging using X-ray computed microtomography (microCT) observations in stems of two species of Hakea that differ in vessel length. Both centrifuge techniques were prone to artefactual embolism in samples with maximum vessel length longer than, or similar to, the centrifuge rotor diameter. Observations with microCT indicated that this artefactual embolism occurred in the outermost portions of samples. The artefact was largely eliminated if flow was measured in an excised central part of the segment in the static centrifuge or starting measurements with the Cavitron at pressures lower than the threshold of embolism formation in open vessels. The simulations of loss of conductivity in centrifuged samples with a new model, CAVITOPEN, confirmed that the impact of open vessels on the vulnerability to embolism curve was higher when vessels were long, samples short and when embolism is formed in open vessels at less negative pressures. This model also offers a robust and quantitative tool to test and correct for artefactual embolism at low xylem tensions.

The frequencies of free oscillations of plants, or plant parts, depend on their geometries, stiffnesses, and masses. Besides direct biomechanical interest, free frequencies also provide insights into plant properties that can usually only be measured destructively or with low-throughput techniques (e.g., change in mass, tissue density, or stiffness over development or with stresses). We propose here a new high-throughput method based on the noncontact measurements of the free frequencies of the standing plant. The plant is excited by short air pulses (typically 100 ms). The resulting motion is recorded by a high speed video camera (100 fps) and processed using fast space and time correlation algorithms. In less than a minute the mechanical behavior of the plant is tested over several directions. The performance and versatility of this method has been tested in three contrasted species: tobacco (Nicotiana benthamian), wheat (Triticum aestivum L.), and poplar (Populus sp.), for a total of more than 4000 data points. In tobacco we show that water stress decreased the free frequency by 15%. In wheat we could detect variations of less than 1 g in the mass of spikes. In poplar we could measure frequencies of both the whole stem and leaves. The work provides insight into new potential directions for development of phenotyping.

Wood ensures the mechanical stability of the trunk and branches. Its formation is impacted by windy environments: living cells sense mechanical strains and modify the formation of wood accordingly in terms of quantity and quality: these biological responses are called “Thigmomorphogenesis”. We bent young poplars three times a day, five days per week during six months. When a stem is bent, wood endures compressive and/or tensile stresses. Our objective is to determine the impact of these growing conditions on the mechanical properties of wood.

International audience; The plant ability to orient their growth with respect to environmental stimuli such as gravity, is a key factor for survival and acclimation to their environment. In heterogeneous soils, plant roots modulate their growth towards gravity, allowing soil exploration and uptake of water and nutrients. Gravity sensing in roots occurs in specialized cells called statocytes, located in the central root cap, and which contain starch-filled plastids called statoliths. Being denser than the cytoplasm, statolith position depends on the inclination of roots. Root growth reorientation, triggered by statoliths displacement, is based on auxin redistribution in the root apex, inducing differential growth between the root upward and downward sides. At the cell scale, signaling events starting from statoliths displacement and leading to auxin redistribution remain poorly documented. Some signaling molecules such as calcium, reactive oxygen species, nitric oxide and inositol 1,4,5-triphosphate are serious candidates previously shown to be involved within minutes before modification of the expression of auxin-related genes. In order to investigate the effects of statoliths location on the triggering of signaling pathways, a space experiment called GRAVI-2 was set up aboard the ISS in 2014. During the experiment, lentil roots were grown in microgravity conditions in the European modular cultivation system (EMCS) and were then submitted to a period of gravity (9h at 0.01 g or 15 min at 2 g). The analysis of statolith location showed that statoliths are clustered in microgravity. This clusterization is not affected by 0.01g. On the contrary, a period of 5 min at 2g induces a change in statolith location. Thanks to these changes in statolith positioning, cytological analysis were coupled with RNA sequencing of root apex. Transcriptomic analyses clearly show a regulation in gene expression. Our data are among the first involving global transcriptomic analyses of root material grown in microgravity and of root submitted to a gravity-short period after a growth in microgravity. The effects of the statolith location on signaling mechanisms will be discussed.

Background and aims Trees constantly experience wind, perceive resulting mechanical cues, and modify their growth and development accordingly. Previous studies have demonstrated that multiple bending treatments trigger ovalization of the stem and the formation of flexure wood in gymnosperms, but ovalization and flexure wood have rarely been studied in angiosperms, and none of the experiments conducted so far has used multidirectional bending treatments at controlled intensities. Assuming that bending involves tensile and compressive strain, we hypothesized that different local strains may generate specific growth and wood differentiation responses. Methods Basal parts of young poplar stems were subjected to multiple transient controlled unidirectional bending treatments during 8 weeks, which enabled a distinction to be made between the wood formed under tensile or compressive flexural strains. This set-up enabled a local analysis of poplar stem responses to multiple stem bending treatments at growth, anatomical, biochemical and molecular levels. Key results In response to multiple unidirectional bending treatments, poplar stems developed significant cross-sectional ovalization. At the tissue level, some aspects of wood differentiation were similarly modulated in the compressed and stretched zones (vessel frequency and diameter of fibres without a G-layer), whereas other anatomical traits (vessel diameter, G-layer formation, diameter of fibres with a G-layer and microfibril angle) and the expression of fasciclin-encoding genes were differentially modulated in the two zones. Conclusions This work leads us to propose new terminologies to distinguish the 'flexure wood' produced in response to multiple bidirectional bending treatments from wood produced under transient tensile strain (tensile flexure wood; TFW) or under transient compressive strain (compressive flexure wood; CFW). By highlighting similarities and differences between tension wood and TFW and by demonstrating that plants could have the ability to discriminate positive strains from negative strains, this work provides new insight into the mechanisms of mechanosensitivity in plants.

Plants have developed different tropisms: in particular, they re-orient the growth of their branches towards light (phototropism) or upwards (gravitropism). How these tropisms affect the shape of a tree crown remains unanswered. We address this question by developing a propagating front model of tree growth. This model being length-free, it leads to self-similar solutions, independent of the initial conditions, at long time. Varying the intensities of each tropism, different self-similar shapes emerge, including singular ones. Interestingly, these shapes bear similarities with existing tree species. It is concluded that the core of specific crown shapes in trees relies on the balance between tropisms., 11 pages, 7 figures

Torres-Ruiz, J.M. et al... .--11 páginas.-- 6 figuras.-- 1 tabla.-- 63 referencias, Xylem vulnerability to embolism represents an essential trait for the evaluation of the impact of hydraulics in plant function and ecology. The standard centrifuge technique is widely used for the construction of vulnerability curves, although its accuracy when applied to species with long vessels remains under debate. We developed a simple diagnostic test to determine whether the open-vessel artefact influences centrifuge estimates of embolism resistance. Xylem samples from three species with differing vessel lengths were exposed to less negative xylem pressures via centrifugation than the minimum pressure the sample had previously experienced. Additional calibration was obtained from non-invasive measurement of embolism on intact olive plants by X-ray microtomography. Results showed artefactual decreases in hydraulic conductance (k) for samples with open vessels when exposed to a less negative xylem pressure than the minimum pressure they had previously experienced. X-Ray microtomography indicated that most of the embolism formation in olive occurs at xylem pressures below -4.0 MPa, reaching 50% loss of hydraulic conductivity at -5.3 MPa. The artefactual reductions in k induced by centrifugation underestimate embolism resistance data of species with long vessels. A simple test is suggested to avoid this open vessel artefact and to ensure the reliability of this technique in future studies, Was supported by a scholarship from the Spanish Ministry of Economy and Competitiveness (EEBB-I-15-09191). We thank the PSICHE beamline (SOLEIL synchrotron facility, project 20141229). This work was supported by the programme ‘Investments for the Future’ (ANR-10-EQPX-16, XYLOFOREST) from the French National Agency for Research. B.C. was supported by the Australian Research Council (Future Fellowship grant no. FT130101115). R.L. was supported by a Marie Curie fellowship (no. ) of the EU FP7 Programme.

A significant increase in drought events frequency is predicted for the next decades induced by climate change, potentially affecting plant species mortality rates and distributions worldwide. The main trigger of plant mortality is xylem hydraulic failure due to embolism and induced by the low pressures at which water is transported through xylem. As the Mediterranean basin will be severely affected by climate change, the aim of this study was to provide novel information about drought resistance and tolerance of one of its most widely distributed and common genera as a case study: the genus Cistus. Different functional and anatomical traits were evaluated in four co-occurring Cistus species in the Mediterranean Montado ecosystem. Soil water availability for each species was also assessed to evaluate if they show different ecological niches within the area. Results showed physiological and xylem anatomical differences between the four co-occurring species, as well as in the soil water availability of the sites they occupy. Despite the significant differences in embolism resistance across species, no trade-off between hydraulic safety and efficiency was observed. Interestingly, species with narrower vessels showed lower resistance to embolism than those with higher proportions of large conduits. No correlation, however, was observed between resistance to embolism and wood density. The four species showed different water-use and drought-tolerance strategies, occupying different ecological niches that would make them cope differently with drought. These results will allow us to improve the predictions about the expected changes in vegetation dynamics in this area due to ongoing climate change.

GRAVI-2 space experiment: investigating statoliths displacement and location effects on early stages of gravity perception pathways in plant roots. ASGSR meeting

International audience; Angiosperm trees are able to reorient their axes thanks to their capacities to differentiate tension wood on the upper side of stems and branches. In poplar, tension wood fibres develop an extra cell wall layer, named G-layer, responsible for the peculiar mechanical properties of tension wood. G-layer is composed of highly crystalline cellulose microfibrils embedded in a polysaccharide/glycoprotein matrix devoid of lignin. During the G-layer deposition, cellulose microfibrils get oriented parallel to the fibre axis and placed under the state of high tensile stress. Chitinase-like proteins (CTL) have been proposed to play different roles in plant growth and development. Genes encoding CTL have been associated to cellulose synthesis and their mutation causes ectopic deposition of lignin. Transcripts of CTL2 gene were shown to be highly abundant in differentiating tension wood (Déjardin et al., 2004). Its homologue in Arabidopsis is specifically expressed in stems and co-expressed with the CesA genes involved in cellulose synthesis in secondary cell walls (Persson et al., 2005). In our study, we investigated potential function of CTL2 in the formation and mechanical properties of tension wood. Expression of CTL2 was quantified in different tissues and the effects of CTL2 down-regulation have been characterized in GM poplar trees. CTL2 gene appeared highly expressed in xylem with no difference in expression between tension and opposite wood. Downregulation of CTL2 did not seem to impact growth and development of trees in greenhouse conditions. However, stems appeared to be highly breakable in the transverse direction. Anatomical observations of xylem showed altered formation of Glayers in vessel-surrounding fibres in transgenic poplars. Further analyses indicated a substantial increase in wood lignin content. Cellulose crystallinity, microfibril angle and stem mechanical properties are currently assessed. Results will be presented and discussed in the light of present hypothesis regarding CTL2 function in plant development.

In recent years, the validity of embolism quantification methods has been questioned, especially for long-vesseled plants. Some studies have suggested that cutting xylem while under tension, even under water, might generate artificial cavitation. Accordingly, a rehydration procedure prior to hydraulic measurements has been recommended to avoid this artefact. On the other hand, concerns have been raised that xylem refilling might occur when samples are rehydrated. Here, we explore the potential biases affecting embolism quantification for grapevine (Vitis vinifera L.) petioles harvested under tension or after xylem relaxation. We employ direct visualization of embolism through X-ray micro-computed tomography (microCT) to test for the occurrence of fast refilling (artifactually low per cent loss of conductivity (PLC) due to rehydration prior to sample harvest) as well as excision-induced embolism (artifactually high embolism due to air introduction during harvest). Additionally, we compared the response functions of both stomatal regulation and xylem embolism to xylem pressure (Psix). Short-time (20 min) xylem tension relaxation prior to the hydraulic measurement resulted in a lower degree of embolism than found in samples harvested under native tensions, and yielded xylem vulnerability curves similar to the ones obtained using direct microCT visualization. Much longer periods of hydration (overnight) were required before xylem refilling was observed to occur. In field-grown vines, over 85% of stomatal closure occurred at less negative Psix than that required to induce 12% PLC. Our results demonstrate that relaxation of xylem tension prior to hydraulic measurement allows for the reliable quantification of native embolism in grapevine petioles. Furthermore, we find that stomatal regulation is sufficiently conservative to avoid transpiration-induced cavitation. These results suggest that grapevines have evolved a strategy of cavitation resistance, rather than one of cavitation tolerance (diurnal cycles of embolism and repair).

Virot et al. [E. Virot et al., Phys. Rev. E 93, 023001 (2016)] assert that the critical wind speed at which \ensuremath{\geqslant}50% of all trees in a population break is \ensuremath{\approx}42 m/s, regardless of tree characteristics. We show that empirical data do not support this assertion, and that the assumptions underlying the theory used by Virot et al. are inconsistent with the biomechanics of trees.

National audience; Sur plusieurs sites naturels ayant un intérêt patrimonial, la colonisation par des ligneux est observée et pose des questions sur l’évolution des communautés végétales en place. Mieux prévoir ces évolutions et prendre les décisions appropriées implique de mieux connaître les raisons de ces évolutions, et notamment les capacités de colonisation des ligneux. Le site d’étude est une carrière de pouzzolane abandonnée depuis 1980 situé au pied du Puy de la Vache, dans la chaîne des Puys. Dans cette carrière, comme au sommet du Puy, c’est une végétation pionnière herbacée des substrats meubles (scories) qui est installée. On observe aussi une colonisation par le Pin sylvestre (Pinus sylvestris L.), et dans une moindre mesure, par l’épicea (Picea abies (L.) H. Karst) et le bouleau (Betula pendula Roth), de faciès secs et très filtrants, car en pente forte et sur scories. Nous avons analysé les particularités morphologiques et physiologiques des Pins sylvestres colonisant ces faciès, en les comparant à des individus installés à proximité sur des faciès ayant une réserve en eau. Les paramètres physiologiques suivis portent sur l’état hydrique de l’arbre, les échanges gazeux et le fonctionnement hydraulique. Pour ce dernier, nous avons mesuré la résistance à la cavitation qui est un paramètre de tolérance à la sécheresse.

Highlight The dynamics of water fluxes and cavitation events induced by ice formation were analyzed by visualization (X-ray microtomography and MRI), and detected with a microdendrometer and ultrasonic acoustic emissions., Freeze–thaw cycles induce major hydraulic changes due to liquid-to-ice transition within tree stems. The very low water potential at the ice–liquid interface is crucial as it may cause lysis of living cells as well as water fluxes and embolism in sap conduits, which impacts whole tree–water relations. We investigated water fluxes induced by ice formation during freeze–thaw cycles in Juglans regia L. stems using four non-invasive and complementary approaches: a microdendrometer, magnetic resonance imaging, X-ray microtomography, and ultrasonic acoustic emissions analysis. When the temperature dropped, ice nucleation occurred, probably in the cambium or pith areas, inducing high water potential gradients within the stem. The water was therefore redistributed within the stem toward the ice front. We could thus observe dehydration of the bark’s living cells leading to drastic shrinkage of this tissue, as well as high tension within wood conduits reaching the cavitation threshold in sap vessels. Ultrasonic emissions, which were strictly emitted only during freezing, indicated cavitation events (i.e. bubble formation) following ice formation in the xylem sap. However, embolism formation (i.e. bubble expansion) in stems was observed only on thawing via X-ray microtomography for the first time on the same sample. Ultrasonic emissions were detected during freezing and were not directly related to embolism formation. These results provide new insights into the complex process and dynamics of water movements and ice formation during freeze–thaw cycles in tree stems.

Direct visualization shows enhanced embolism of xylem samples when they are collected under tension.

Plant lignocellulosic biomass, mostly composed of cell walls, is one of the largest, mostly untapped, reserves of renewable carbon feedstock on the planet. Energy-rich polysaccharide polymers of plant cell walls can be broken down to produce fermentable sugars used to produce bioethanol. However, the complex structure of plant cell walls, and in particular, the presence of lignin, makes them recalcitrant to enzymatic degradation. Reducing this recalcitrance represents a major technological challenge. Brachypodium distachyon is an excellent model to identify parameters underlying biomass quality of energy grasses. In this work, we identified a mutant line spa1 with a so far undescribed phenotype combining brittleness with increased elasticity of the internodes. Mutant cell walls contain less crystalline cellulose and changes in hemicellulose and lignin quality and quantity. Using a dedicated reactor to follow in real-time, the evolution of straw particle size and sugar release during enzymatic digestion, we show that, despite the increased lignin content, the spa1 mutant has a dramatic reduced recalcitrance to saccharification compared to the WT. These observations demonstrate that other parameters besides lignin content are relevant for the improvement of biomass recalcitrance in energy grasses.

National audience

The objective of this work wa s to investigate the dynamics of embolism formation within a Douglas fir tree ring . Four resistant and four vulnerable 10 year old trees were selected among 50 trees, based on their P 50 . Stem s amples , taken next to those used to obtain the vulnerability to cavitation curves, were collected and submitted to increasing positive pressures, in order to simulate increasing tension caused by water stress in the xylem . Then the conductive surface of the samples was stained and s canned and the images were analyzed. X ray microdensity profiles were obtained on the same samples. The microdensity profiles of the 2011 ring were analyzed in three parts, earlywood, transition w ood and latewood. The dynamics of embolism propagation was observed separately in these three parts. O ur results showed that the initiation and the propagation of the cavitation follow a discrete trend, with at least two successive initiation events: first cavitation initiate s and propagate s rapidly in the latewood . Then, a second cavitation event begin s and spread s in the earlywood and eventually propagates to the transition wood, which remains the last conductive part in the ring before full embolism. W e observed that resistant to cavitation trees showed lower transition wood density than vulnerable to cavitation trees. Estación Experimental Agropecuaria Bariloche Fil: Dalla Salda, Guillermina. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Bariloche. Área de Recursos Forestales. Grupo de Ecología Forestal; Argentina Fil: Fernandez, Maria Elena. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Balcarce. Agencia de Extensión Rural Tandil. Consejo Nacional de Investigaciones Cientificas y Técnicas; Argentina Fil: Sergent, Anne Sophie. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Bariloche. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Instituto de Investigaciones Forestales y Agropecuarias Bariloche; Argentina Fil: Rozenberg, Philippe. INRA. Biologie intégrée pour la valorisation de la diversité des arbres et de la forêt; Francia Fil: Badel, Eric. INRA. Biologie intégrée pour la valorisation de la diversité des arbres et de la forêt; Francia Fil: Martinez Meier, Alejandro. Instituto Nacional de Tecnología Agropecuaria (INTA). Estación Experimental Agropecuaria Bariloche. Área de Recursos Forestales. Grupo de Ecología Forestal; Argentina

MEDLINE:24918205; International audience; Background and Aims Various correlations have been identified between anatomical features of bordered pits in angiosperm xylem and vulnerability to cavitation, suggesting that the mechanical behaviour of the pits may play a role. Theoretical modelling of the membrane behaviour has been undertaken, but it requires input of parameters at the nanoscale level. However, to date, no experimental data have indicated clearly that pit membranes experience strain at high levels during cavitation events.Methods Transmission electron microscopy (TEM) was used in order to quantify the pit micromorphology of four tree species that show contrasting differences in vulnerability to cavitation, namely Sorbus aria, Carpinus betulus, Fagus sylvatica and Populus tremula. This allowed anatomical characters to be included in a mechanical model that was based on the Kirchhoff–Love thin plate theory. A mechanistic model was developed that included the geometric features of the pits that could be measured, with the purpose of evaluating the pit membrane strain that results from a pressure difference being applied across the membrane. This approach allowed an assessment to be made of the impact of the geometry of a pit on its mechanical behaviour, and provided an estimate of the impact on air-seeding resistance.Key Results The TEM observations showed evidence of residual strains on the pit membranes, thus demonstrating that this membrane may experience a large degree of strain during cavitation. The mechanical modelling revealed the interspecific variability of the strains experienced by the pit membrane, which varied according to the pit geometry and the pressure experienced. The modelling output combined with the TEM observations suggests that cavitation occurs after the pit membrane has been deflected against the pit border. Interspecific variability of the strains experienced was correlated with vulnerability to cavitation. Assuming that air-seeding occurs at a given pit membrane strain, the pressure predicted by the model to achieve this mechanical state corresponds to experimental values of cavitation sensitivity (P50).Conclusions The results provide a functional understanding of the importance of pit geometry and pit membrane structure in air-seeding, and thus in vulnerability to cavitation.

Phys. Rev. E ISI Document Delivery No.: AE1BR Times Cited: 0 Cited Reference Count: 33 Capron, M. Tordjeman, Ph. Charru, F. Badel, E. Cochard, H. Amer physical soc College pk; The xylem vessels of trees constitute a model natural microfluidic system. In this work, we have studied the mechanism of air flow in the Populus xylem. The vessel microstructure was characterized by optical microscopy, transmission electronic microscopy (TEM), and atomic force microscopy (AFM) at different length scales. The xylem vessels have length approximate to 15 cm and diameter approximate to 20 mu m. Flow from one vessel to the next occurs through similar to 10(2) pits, which are grouped together at the ends of the vessels. The pits contain a thin, porous pit membrane with a thickness of 310 nm. We have measured the Young's moduli of the vessel wall and of the pits (both water-saturated and after drying) by specific nanoindentation and nanoflexion experiments with AFM. We found that both the dried and water-saturated pit membranes have Young's modulus around 0.4 MPa, in agreement with values obtained by micromolding of pits deformed by an applied pressure difference. Air injection experiments reveal that air flows through the xylem vessels when the differential pressure across a sample is larger than a critical value Delta P-c = 1.8 MPa. In order to model the air flow rate for Delta P >= Delta P-c, we assumed the pit membrane to be a porous medium that is strained by the applied pressure difference. Water menisci in the pit pores play the role of capillary valves, which open at Delta P = Delta P-c. From the point of view of the plant physiology, this work presents a basic understanding of the physics of bordered pits.

Inter-organ communication is essential for plants to coordinate development and acclimate to mechanical environmental fluctuations. The aim of this study was to investigate long-distance signaling in trees. We compared on young poplars the short-term effects of local flame wounding and of local stem bending for two distal responses: (1) stem primary growth and (2) the expression of mechanoresponsive genes in stem apices. We developed a non-contact measurement method based on the analysis of apex images in order to measure the primary growth of poplars. The results showed a phased stem elongation with alternating nocturnal circumnutation phases and diurnal growth arrest phases in Populus tremula × alba clone INRA 717- 1B4. We applied real-time polymerase chain reaction (RT-PCR) amplifications in order to evaluate the PtaZFP2, PtaTCH2, PtaTCH4, PtaACS6 and PtaJAZ5 expressions. The flame wounding inhibited primary growth and triggered remote molecular responses. Flame wounding induced significant changes in stem elongation phases, coupled with inhibition of circumnutation. However, the circadian rhythm of phases remained unaltered and the treated plants were always phased with control plants during the days following the stress. For bent plants, the stimulated region of the stem showed an increased PtaJAZ5 expression, suggesting the jasmonates may be involved in local responses to bending. No significant remote responses to bending were observed.

Under drought conditions, the xylem of trees that conducts ascending sap produces ultrasonic emissions whose exact origin is not clear. We introduce a new method to record simultaneously both acoustic events and optical observation of the xylem conduits within slices of wood that were embedded in a transparent material setting a hydric stress. In this article, we resolved the rapid development of all cavitation bubbles and demonstrated that each ultrasound emission was linked to the nucleation of one single bubble, whose acoustic energy is an increasing function of the size of the conduit where nucleation occurred and also of the hydric stress. We modelled these observations by the fact that water columns in conduits store elastic energy and release it into acoustic waves when they are broken by cavitation bubbles. Water columns are thus elastic, and not rigid, ‘wires of water’ set under tension by hydric stresses. Cavitation bubbles are at the origin of an embolism, whose development was followed in our experiments. Such an embolism of sap circulation can result in a fatal condition for living trees. These findings provide new insights for the non-destructive monitoring of embolisms within trees, and suggest a new approach to study porous media under hydric stress.

Summary Bending of trees causes a transient hydraulic overpressure signal that propagates rapidly along the vascular system in planta. This may be a mechanobiological remote signalling of the mechanical stress., When tree stems are mechanically stimulated, a rapid long-distance signal is induced that slows down primary growth. An investigation was carried out to determine whether the signal might be borne by a mechanically induced pressure pulse in the xylem. Coupling xylem flow meters and pressure sensors with a mechanical testing device, the hydraulic effects of mechanical deformation of tree stem and branches were measured. Organs of several tree species were studied, including gymnosperms and angiosperms with different wood densities and anatomies. Bending had a negligible effect on xylem conductivity, even when deformations were sustained or were larger than would be encountered in nature. It was found that bending caused transient variation in the hydraulic pressure within the xylem of branch segments. This local transient increase in pressure in the xylem was rapidly propagated along the vascular system in planta to the upper and lower regions of the stem. It was shown that this hydraulic pulse originates from the apoplast. Water that was mobilized in the hydraulic pulses came from the saturated porous material of the conduits and their walls, suggesting that the poroelastic behaviour of xylem might be a key factor. Although likely to be a generic mechanical response, quantitative differences in the hydraulic pulse were found in different species, possibly related to differences in xylem anatomy. Importantly the hydraulic pulse was proportional to the strained volume, similar to known thigmomorphogenetic responses. It is hypothesized that the hydraulic pulse may be the signal that rapidly transmits mechanobiological information to leaves, roots, and apices.

Xylem sap is transported in vessels under tension through dead xylem conduits. During drought events, then tension level increases and cavitation of the water column may occur leading to air filled and non functional conduits. Since hydraulic failure impairs the wood productivity and the survival of the tree. In a context of changing climate expecting enhancement of frequency and severity of drought events, there is an increased interest in understanding the mechanism of cavitation phenomena in order to find the anatomical driver that could explain the interspecific variability of VC. Pits represent the most probable air entry point in a vessel. Within angiosperms, the pit structure (pit quality) and the pit area per vessel (pit quantity) are both suggested to explain variation in VC. Pit quality has generally been measured by quantifying the porosity and related thickness of the pit membrane. Here, we propose a mechanical model integrating other morphological parameters of a bordered pit: pit depth, chamber diameter and aperture diameter. These parameters allow us to estimate the quality of a pit from a mechanistic point of view. The model was first used to explain variability in VC for four contrasting species, and was then extended with additional data from literature to perform a broader analysis. . Evidence of residual strains after embolism supports the hypothesis

Times Cited: 0; Xylem cavitation resistance has profound implications for plant physiology and ecology. This process is characterized by a 'vulnerability curve' (VC) showing the variation of the percentage of cavitation as a function of xylem pressure potential. The shape of this VC varies from 'sigmoidal' to 'exponential'. This review provides a panorama of the techniques that have been used to generate such a curve. The techniques differ by (i) the way cavitation is induced (e.g. bench dehydration, centrifugation, or air injection), and (ii) the way cavitation is measured (e.g. percentage loss of conductivity (PLC) or acoustic emission), and a nomenclature is proposed based on these two methods. A survey of the literature of more than 1200 VCs was used to draw statistics on the usage of these methods and on their reliability and validity. Four methods accounted for more than 96% of all curves produced so far: bench dehydration-PLC, centrifugation-PLC, pressure sleeve-PLC, and Cavitron. How the shape of VCs varies across techniques and species xylem anatomy was also analysed. Strikingly, it was found that the vast majority of curves obtained with the reference bench dehydration-PLC method are 'sigmoidal'. 'Exponential' curves were more typical of the three other methods and were remarkably frequent for species having large xylem conduits (ring-porous), leading to a substantial overestimation of the vulnerability of cavitation for this functional group. We suspect that 'exponential' curves may reflect an open-vessel artefact and call for more precautions with the usage of the pressure sleeve and centrifugation techniques.

Présentation orale et résumé; The xylem is a complex tissue that covers different key functions in woody plants: 1) it transports water throughout the plant to replace water lost during leaf transpiration, 2) it confers to trunk and branches their stiffness and flexibility, and 3) it stores water, nutrients and carbohydrates. These functions have set contrasted selective pressures on this tissue and shaped its structure during evolution. Understanding how xylem functions relate to xylem anatomy is a challenging issue. Each function is associated to a set of xylem characteristics that can be antagonistic across functions. It is therefore important to identify the tradeoffs between xylem functions. We will focus here on the tradeoffs between hydraulic traits (efficiency versus safety) and the tradeoffs between hydraulic and mechanical traits. A better understanding of the structural and genomic determinants of each function and the exploration of their genetic variability within and across species has recently brought new light on these questions.

Leaves, the distal section of the soil-plant-atmosphere continuum, exhibit the lowest water potentials in a plant. In contrast to angiosperm leaves, knowledge of the hydraulic architecture of conifer needles is scant. We investigated the hydraulic efficiency and safety of Pinus pinaster needles, comparing different techniques. The xylem hydraulic conductivity (k(s)) and embolism vulnerability (P(50)) of both needle and stem were measured using the cavitron technique. The conductance and vulnerability of whole needles were measured via rehydration kinetics, and Cryo-SEM and 3D X-ray microtomographic observations were used as reference tools to validate physical measurements. The needle xylem of P. pinaster had lower hydraulic efficiency (k(s) = 2.0 × 10(-4) m(2) MPa(-1) s(-1)) and safety (P(50) = - 1.5 MPa) than stem xylem (k(s) = 7.7 × 10(-4) m(2) MPa(-1) s(-1); P(50) = - 3.6 to - 3.2 MPa). P(50) of whole needles (both extra-vascular and vascular pathways) was - 0.5 MPa, suggesting that non-vascular tissues were more vulnerable than the xylem. During dehydration to - 3.5 MPa, collapse and embolism in xylem tracheids, and gap formation in surrounding tissues were observed. However, a discrepancy in hydraulic and acoustic results appeared compared with visualizations, arguing for greater caution with these techniques when applied to needles. Our results indicate that the most distal parts of the water transport pathway are limiting for hydraulics of P. pinaster. Needle tissues exhibit a low hydraulic efficiency and low hydraulic safety, but may also act to buffer short-term water deficits, thus preventing xylem embolism.

Wood provides water transport and mechanical support of trees. Sap is transported under negative pressure in plant xylem conduits, which can be subject to embolism during severe drought. Typically, denser woods show thicker cell walls and stronger mechanical properties. Ten transgenic poplar lines modified for expression of genes involved in lignin metabolism were produced from the female clone 717-1B4 of Populus tremula × Populus alba to test the hypothesis of a possible trade-off between hydraulic and mechanical functions. Poplar lines underexpressed genes encoding for cinnamoyl alcohol dehydrogenase (CAD), cinnamoyl CoA reductase (CCR) and caffeic acid 3-O- methyltransferase (COMT), while new poplar lines underexpressed the CAD genes or overexpressed the MYB308 gene, encoding for a transcription factor repressing the phenylpropanoid metabolism. To maximize the contrast between line behaviors, these plants were grown under two different water regimes, and the impact on their hydraulic traits and xylem properties was analyzed to test for a link between water condition and mechanical and hydraulic properties. Our results show that the resistance to xylem cavitation was lower for the transgenic lines than for the control line 717-1b4 while they show neither a positive nor a negative tendency for the longitudinal Young's modulus between the transgenic lines and the control line. ASOMT10b and ASOMT2b, which possessed a down-regulated expression for all the genes, showed a lower value of the resistance to implosion index (t/b)2. No difference for xylem hydraulic conductivity between the lines was found. The changes in lignin metabolism in these transgenic lines did not affect the water transport, despite the change in the lignin content. Our data on the transgenic poplar lines do not therefore support the mechanical vs. hydraulic trade-off hypothesis and we point out that angiosperm trees have numerous ways to acclimate their internal structure in order to adjust their mechanical properties without hydraulic coupling. Moreover, we observed an acclimation to water stress for P50 but not for the Young's modulus. MYB308-25.1 showed better mechanical properties and vulnerability to cavitation than the control line 717-1b4. Finally, we present evidence that lignins are involved in the vulnerability to cavitation, probably through modifications of pit structure and behavior.

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Mention d'édition : P. Wotjaszek (ed); Mechanosensitive control of plant growth is a major process shaping how terrestrial plants acclimate to the mechanical challenges set by wind, self-weight, and autostresses. Loads acting on the plant are distributed down to the tissues, following continuum mechanics. Mechanosensing, though, occurs within the cell, building up into integrated signals; yet the reviews on mechanosensing tend to address macroscopic and molecular responses, ignoring the biomechanical aspects of load distribution to tissues and reducing biological signal integration to a "mean plant cell." In this chapter, load distribution and biological signal integration are analyzed directly. The Sum of Strain Sensing model S 3 m is then discussed as a synthesis of the state of the art in quantitative deterministic knowledge and as a template for the development of an integrative and system mechanobiology.

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Xylem vulnerability to cavitation differs between tree species according to their drought resistance, more xerophilous species being more resistant to xylem cavitation. Variability in xylem vulnerability to cavitation is also found within species, especially between in situ populations. The origin of this variability has not been clearly identified. Here we analyzed the response of xylem hydraulic traits of Populus tremula xPopulus alba trees to three different soil water regimes. Stem xylem vulnerability was scored as the xylem water potential causing 12, 50 and 88% loss of conductivity (P(12), P(50) and P(88)). Vulnerability to cavitation was found to acclimate to growing conditions under different levels of soil water content, with P(50) values of -1.82, -2.03 and -2.45 MPa in well-watered, moderately water-stressed and severely water-stressed poplars, respectively. The value of P(12), the xylem tension at which cavitation begins, was correlated with the lowest value of midday leaf water potential (epsilon m) experienced by each plant, the difference between the two parameters being approximately 0.5 MPa, consistent with the absence of any difference in embolism level between the different water treatments. These results support the hypothesis that vulnerability to cavitation is a critical trait for resistance to drought. The decrease in vulnerability to cavitation under growing conditions of soil drought was correlated with decreased vessel diameter, increased vessel wall thickness and a stronger bordered pit field (t/b)2. The links between these parameters are discussed.

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The 2-D deformation of bleached plain weave cotton ready-to-wear clothing was measured during adsorption and desorption cycles. A digital X-ray imaging system was coupled with a climatic chamber to control temperature and relative humidity. An image of each sample was recorded for several equilibrium states. The strain along warp (ε cc) and weft (ε ww) directions and the shear deformation (ε wc) were evaluated by image correlation process. The dimensional variations are explained by geometrical consideration of the structure at microscopic (fibers scale) and macroscopic levels (yarns scale). Indeed, the reaction between water vapor molecules and material enlightens two steps. At first, the swelling fibers fill the micropores inside the yarns. Then, the yarns swell and push on their neighbors to fill up the macropores and cause the macroscopic swelling of the overall structure. During the desorption phase, the fibers shrink to create a free space inside the plain weave structure that will be relaxed to find its initial state. The isotropy between the two main directions is explained by the weave symmetry and the similar yarn properties. The shear deformation is related to the cohesion by twist between cotton fibers. This work is more specifically focused on the ironing process ( T = 200°C + steam). The ironing generates flattened yarns and increases their friction, which amplifies the deformation during the first adsorption cycle. However, this effect is cancelled at the end of the first adsorption/desorption cycle with no memory effect of the ironing process.

International audience; In this work, we focus on a thermal insulating panel made of wood fibres. The making process is a non-woven fabric process that generates a high porosity and a very lowdensity material. Thermal properties were first investigated. This paper deals with mechanical properties that are involved in the production line, transport and handling in the construction site. An insulating panel was compressed (33% and 73%) and we followed insitu its internal deformation using X-ray microtomography. The results enlighten a special behaviour that is linked to the low-density structure of the panel. In particular, the fibres are not compressed and the macroscopic deformation of the sample generates only a new organisation of the fibres in the panel. These results are confirmed by morphological measurements that were performed by 3D image analysis.; On s'intéresse à des isolants thermiques à base de fibres de bois utilisés dans le bâtiment, de très forte porosité, élaborés par un procédé textile non tissé. Les propriétés thermiques ont fait l'objet des premières études et les propriétés mécaniques, intervenant lors du transport et de la manipulation, font l'objet de ce travail. Nous avons suivi in situ, grâce à la microtomographie X, les états de déformation interne d'un panneau d'isolation soumis à une sollicitation de compression transverse (33 % et 73 %). Le comportement particulier mis en lumière par l'accès à la déformation locale est relié à la très faible densité du matériau. En particulier, les fibres de bois ne sont pas individuellement mécaniquement comprimées et la déformation macroscopique du matériau n'engendre que leur réorganisation spatiale. Ces résultats sont mis en relation avec des caractéristiques morphologiques de structures évaluées par analyse d'image 3D.

Apple pieces were vacuum-impregnated with either a pectin methylesterase (PME) and calcium solution or with water prior to pasteurization. Pasteurized apple pieces impregnated with PME and calcium showed a significantly higher firmness. Moreover, solid state C-13 NMR spectroscopy of apple cell wall residues revealed an increase of their molecular rigidity. Exogenous PME addition involved a decrease from 82% to 45% of apple pectin degree of methyl-esterification. Microscopic observations of apple slices immunolabelled with antibodies specific for pectins showed that (i) demethyl-esterification was more intense in the cell wall region lining intercellular spaces (demonstrating a key role for these intercellular channels in the enzyme penetration in the tissue during vacuum-infusion) and that (ii) the number of calcium-dimerized deesterified homogalacturonan chains increased. The results corroborate the hypothesis that vacuum-impregnated PME action liberates free carboxyl groups along pectin chains that could interact with calcium, increasing the rigidity of pectins and finally the mechanical rigidity of apple tissue. (C) 2007 Elsevier Ltd. All rights reserved.

A low-density wood fibreboard has been compressed along its transversal direction. The experiment was carried out in ESRF synchrotron (ID19) and X-ray microtomographic images were recorded for each state. Stipulating that the fibreboard is a discontinuous material essentially made of air, that the compression simply re-organises the spatial distribution of the fibres and does not involve their intrinsic mechanical properties, we are able to deduce the material points density variations along the thickness of the panel. Good agreement is achieved between the macroscopic deformation of the sample and the microscopic compression rate evaluation. Then, the modifications of structural parameters are investigated by 3D image analysis. The relationship between the local structure and the behaviour of the wood fibreboard are deduced. Finally, a modelling approach allows the local densification to be predicted and confirms the initial hypotheses about the local behaviour of the material. In particular, polyester bonds are not involved. (c) 2008 Elsevier Ltd. All rights reserved.

This paper deals with the ability of a scientific strategy to compute the shrinkage behaviour of any oak sample, regardless of its origin, density, growth ring, etc. This approach uses the description of the actual oak structure at the annual ring level (i.e. the spatial organisation of the radial ray-cells, fibre, parenchyma areas and large vessels), to evaluate its shrinkage/swelling and elastic properties in the transverse directions. For the shrinkage properties, computed results were compared with experimental values measured on a set of samples depicting a very large diversity of anatomical patterns. The accuracy of our prediction is about 5% in the tangential direction and 20% in the radial direction, which is much better than statistical models over a wide range of variables. These results are discussed and a few microscopic observations with ESEM allow explanation of anomaly points of behaviour to be formulated. Such good results could allow this approach to be used to study the influence of growing conditions or of global changes upon physical wood properties.

We coupled a digital X-ray imaging system with a humid air conditioner. This new configuration allows the shrinkage behaviour of thin samples to be measured. In order to control both the temperature and the relative humidity in the chamber, an air generator was developed which ensures very stable conditions even over several months. The X-ray beam passes through the chamber to the 2D detector. Twelve samples can be placed on a rotating sample holder. The strain field due to the moisture content variations is determined by an image correlation algorithm, which compares X-ray images collected at different moisture conditions. Moreover, inspection by X-ray simultaneously produces complementary data: the local density and the spatial organisation of the tissues within the anatomical pattern. Twelve oak samples, chosen for their wide variability of the anatomical pattern, were characterised using this device. Some models available in the literature are used to predict the swelling. fThe comparison between measurements and prediction is rather poor. The variable anatomical structure of the annual rings permits some explanations for this decrepancy and leads to the conclusion that the complete spatial organisation of the tissues has to be quantified to understand and to predict the behaviour of oak wood.

International audience; This paper proposes two applications of quantitative tomography. Assuming that the attenuation ratio of the x-ray beam throughout the investigated sample can be evaluated with high accuracy, this paper deals with quantitative characterization of glass wool structure. In a first part, we measure the 3D spatial distribution of density in a sample that has been extracted from a glass wool plate. The calibration is performed and an experimental validation of the technique is realized. The heterogeneity of the material is characterized and explained in relation to manufacturing methods. In a second step, we follow the evolution of this distribution during a mechanical test. For this purpose, an original device has been developed in the laboratory in order to perform a tomographic process during a mechanical load. The proposed method compares the density distribution between two different compression steps and determines the evolution of density inside the sample. The local strain is evaluated along the stress direction. These first results allow assumptions about the relationships between structure and properties to be expressed.