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International audience; To grow straight, plants need a motor system that controls posture by generating forces to offset gravity. This motor function in trees was long thought to be only controlled by internal forces induced in wood. We grew artificially tilted seedlings and measured the change in curvature when removing the bark and evidenced a differential mechanical state of the bark on both side of the stem. In several species, bark contributes to the up righting of trees. Combined mechanical measurements and microstructural observations on adults trees enabled us to identify the mechanism responsible for the development of mechanical stress in the bark of plant stems.This mechanism does not result from cell wall maturation like in wood, or from the direct action of turgor pressure like in unlignified organs, but is the consequence of the interaction between wood radial pressure and a smartly organized trellis structure in the inner bark.

International audience

Couche G ou couche tertiaire : quel nom pour la couche interne des fibres du bois de tension ?. 11es Journées du Réseau Français des Parois

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International audience

International audience; A key element of the biomechanical design of trees is their ability to generate largemechanical stresses in wood at the stem periphery. This function is necessary for the tree tocontrol the orientation of its axes, and therefore to grow in height, maintain its branches at anoptimal angle or achieve adaptive reorientations (Fournier et al., 2013). This “maturationstress” appears in wood fibres during their cellular maturation when their secondary cell wallis thickening. In hardwoods, the stress asymmetry is generated by the formation of specificfibres with a very high tensile growth stress on the upper side of the inclined axis. Theresulting tension wood has microstructural features highly different from the normal wood.Tension wood in almost all temperate species has a specific gelatinous cell wall layer, calledG-layer, that is not lignified and highly mesoporous (Chang et al., 2015). It has been shownrecently, at the macroscopic scale, that cellulose tension appears during maturation and issynchronous with the development of this specific layer (Clair et al., 2011). Tropical species,like simarouba, are able to generate lignified tension wood fibres. The mechanisms of stressgeneration in tension wood fibres are not yet well known. The aim of our study is tounderstand these mechanisms and to know the spatial and temporal kinetics of the differentcell wall layers stiffening during maturation, in parallel with tensile stress generation.Contact-Resonance Atomic Force Microscopy, CR-AFM, was used here to address thisquestion on embedded samples (Arnould and Arinero, 2015) using dual resonance frequencytracking (DRFT, Rodriguez et al., 2007). Change in the contact modulus between, and within,each layer of the cell wall has been measured on several radial lines of developing fibres atdifferent steps. In order to understand the obtained results, and to estimate the sensitivity ofthe AFM indentation-like technique to the different cell wall components stiffness, an elasticanisotropic indentation model was used (Vlassak et al., 2003; Jäger et al., 2011). Finally,these mechanical measurements have been compared to topochemical data obtained on thesame fibres.

National audience; Le bois dans l’arbre est un tissu multifonctionnel qui assure diverses fonctions métaboliques (stockage, défense, réparation…) tout en constituant à la fois un système vasculaire (permettant le transport de sève), un système squelettique (fournissant rigidité et résistance à la tige) et un système « musculaire » (lui permettant de contrôler sa forme et son orientation). Cette dernière fonction, qui est sans doute la moins connue et la moins étudiée, a une importance fondamentale pour permettre aux arbres de croître en hauteur. En effet, compte tenu de l’élancement des tiges, leur croissance en hauteur dans le champ de pesanteur est un phénomène intrinsèquement instable, qui résulterait invariablement en un port pleureur si un mécanisme actif de contrôle de l’orientation (gravitropisme) n’était pas en place (Moulia et Fournier 2009). Ce mécanisme est assuré par la différenciation de bois de réaction (bois de tension pour les feuillus, bois de compression pour les résineux) d’un côté de la tige, qui lui permet de changer localement sa courbure, et donc de modifier de façon dynamique sa forme et son orientation. Le mécanisme à l’échelle de la tige est bien compris : les cellules de bois de réaction, lors de leur formation, développent dans leur paroi de fortes contraintes mécaniques (dites contraintes de maturation), et la distribution asymétrique de ces contraintes autour de la tige induit un moment fléchissant qui est à l’origine des variations de courbure (Alméras et Fournier 2009). Cependant, à l’échelle microscopique, les mécanismes restent largement méconnus : qu’est-ce qui, lors de la maturation des parois cellulaires de bois de réaction, génère des contraintes de forte intensité ?Pour le bois de compression, des modèles existent qui permettent de rendre compte de la relation entre fonction et microstructure des parois cellulaires (Alméras et al. 2005). Mais la question reste ouverte pour les bois de tension à couche G, pour lesquels la relation entre la fonction (se contracter dans la direction de la fibre) et les caractéristiques microstructurales (cellulose abondante, très cristalline et orientée parallèlement à l’axe de la fibre) semblent au premier abord paradoxales. Cette présentation a pour objet de faire le point sur cette question, en confrontant les différentes hypothèses formulées dans la littérature à un ensemble d’observations expérimentales (e.g. Clair et al. 2011) et de considération mécaniques, puis de proposer un modèle qui semble compatible avec les résultats les plus récents sur la question.

International audience; The main biological function of reaction wood is to act as “muscle” for trees, enabling them to control their posture and right stems when they are permanently disturbed from their set-point inclination by gravity or winds. The key property to achieve this function is the development of high mechanical stress during the formation of reaction wood cells, called “maturation strains”. Moreover, this function is basically performed through the asymmetric formation of wood (in quantity or quality) around the tree circumference. This asymmetry enables stems to bend upward or to compensate for the downward bending induced by gravity, as an asymmetric force associated to maturation strains creates a bending moment (i.e. one side of the stem “pulls” the other one). We will discuss the traits (cross sectional size and shape, wood features as G fibers or microfibril angle, eccentric growth even in usual or extreme cases as buttresses, total biomass growth and height of center of mass, stem lean and set-point angle) involved to understand how the posture control works. We will give simple formulas to estimate a “muscle efficiency” from biomechanical modeling. We will give practical methods to estimate all the involved traits, especially maturation strains from direct and indirect measurements, in peripheral functional wood or by retrospective dendrochronological analysis. Then, we will illustrate how tree stem tropisms and especially gravitropism, widely studied by physiologists, are key processes in natural ecosystems, through practical examples from tropical rainforest tree communities and regeneration of European mixed beech forests. Using the previous definition of postural control efficiency, we will show that the cross section size is rapidly a limiting factor, opposing the extreme strategies of flexible small stems, which bend easily but also right themselves quickly, to stiff big stems. Lastly, as wood in living trees ensures general storage, defence, vascular and skeletal functions, we will ask general questions about synergies and trade-offs between posture control and other functions.

Colloque avec actes et comité de lecture. Internationale.; International audience; Le bois est un matériau au comportement fortement hygroscope et viscoélastique. Sa microstructure complexe implique de nombreux niveaux d’organisation : un échantillon millimétrique est constitué de plusieurs types de cellules (vaisseaux, rayons et fibres) orientées dans différentes directions ; les fibres, constituant la fraction majoritaire et déterminante pour les propriétés mécaniques, sont des cellules allongées d’une longueur de l’ordre du millimètre et d’une largeur de quelques dizaines de microns ; leur paroi est un multicouche épais de quelques microns ; chaque couche est un matériau composite constitué pour moitié d’une matrice polymérique amorphe, et pour moitié de microfibrilles rigides ayant une orientation spécifique ; les microfibrilles, organisées en agrégats épais de quelques dizaines de nanomètres, sont constituées de cellulose aux deux-tiers cristalline ; les cristaux de cellulose sont larges de quelques nanomètres et faits de longues chaines de cellulose dont le motif élémentaire (lattice) mesure quelques angströms ; la longueur des zones cristallines, l’agencement entre cellulose amorphe et cristalline, et la structuration en agrégats sont mal connus. Les modélisations multi-échelles du comportement mécanique du bois se basent sur l’emboitement de méthodes d’homogénéisation représentant chacun de ces niveaux d’organisation. L’objectif du travail présenté est de vérifier expérimentalement les limites de ces modèles en comparant la déformation macroscopique du bois et la déformation à la plus petite échelle (cristal de cellulose, mesuré par diffraction de rayons X), et ceci pour différents types de comportement (élastique, post-élastique et viscoélastique) et à différentes humidités. Les résultats montrent que les déformations cristallines mesurées sont toujours significativement inférieures à celle prévues sous les hypothèses sous-jacentes aux modèles multi-échelles, notamment quand le bois est humide. Cette différence est interprétée comme résultant de légères approximations (rectitude et longueur infinie des éléments constitutifs) faites par les modèles à chaque niveau d’organisation, dont l’erreur se cumule lors de l’intégration multi-échelle.

International audience; Context: Tree orientation is controlled by asymmetric mechanical stresses set during wood maturation. The magnitude of maturation stress differs between longitudinal and tangential directions, and between normal and tension woods. * Aims: We aimed at evaluating patterns of maturation stress on eucalypt plantation trees and their relation with growth, with a focus on tangential stress evaluation. * Methods: Released maturation strains along longitudinal and tangential directions were measured around the circumference of 29 Eucalyptus nitens trees, including both straight and leaning trees. * Results: Most trees produced asymmetric patterns of longitudinal maturation strain but more than half of the maturation strain variability occurred between trees. Many trees produced high longitudinal tensile stress all around their circumference. High longitudinal tensile stress was not systematically associated with the presence of gelatinous layer. The average magnitude of released longitudinal maturation strain was found negatively correlated to the growth rate. A methodology is proposed to ensure reliable evaluation of released maturation strain in both longitudinal and tangential directions. Tangential strain evaluated with this method was lower than previously reported. * Conclusion: The stress was always tensile along longitudinal direction and compressive along tangential direction, and their respective magnitude was positively correlated. This correlation does not result from a Poisson effect but may be related to the mechanism of maturation stress generation.

The mechanism of growth stress in tension wood is still not fully understood. To characterize the functional contribution of polymers within the cell wall to the generation of growth stress, the ultra-structural arrangements of cell-wall polymers, as well as their orientation, were investigated by Fourier Transform Infrared microspectroscopy (FTIR) along a sequence of cell differentiation from the cambium to the mature wood in tension wood (TW) and normal wood (NW) of poplar.

The ac ve genera on of stress in maturing wood plays an essen al role for the biomechanics of trees. Their main func on is to act as a motor system, enabling the postural control of the tree and adap ve reorienta ons. At the macroscopic level, the mechanism underlying this func on is well-known: the asymmetry of stress magnitude around the tree periphery generates a bending moment that may counteract the bending moment induced by the increasing self load or induce changes in stem curvature. In angiosperm trees, the asymmetry of matura on stress is generally achieved thought the differen a on of tension wood (TW) on one side of the tree. However, the underlying mechanism at the microscopic level (why and how TW develops a tensile stress during its matura on) is not yet fully understood. Actually, the rela on between the structure and func on of tension wood is puzzling: its func on consists in shrinking longitudinally, but one its characteris c structural feature is to have a gela nous layer (G-layer) cons tuted of a large amount of s ff crystalline cellulose oriented almost parallel to the cell axis. This design seems consistent with its func on only if tensile stress results from the shrinkage of cellulose. The paradox lies in the fact that cellulose in wood has a defined crystalline configura on and a great chemical iner a, so that it is difficult to imagine a mechanism by which it would spontaneously shrink a*er its incorpora on into the cell-wall. A revival of interest on this issue during the last ten years yielded a number of experimental results on the fine structure, biochemistry, topochemisty, mechanical behavior and in vivo mechanical state of TW, as well as a number of hypothe c mechanisms that would generate the tension. Our aim is to iden fy which mechanism best describes the real process. This presenta on will briefly review exis ng results and hypotheses, and use micro-mechanical models to discriminate between them. Evidences that the in vivo tensile stress is supported by the microfibrils of the G-layer will be presented, and mechanisms consistent with this fact will be analyzed with respect to their consistency with other observa ons and mechanical considera ons.

The postural control of trees is ensured by the generation of asymmetric longitudinal stresses around the circumference in the newly formed wood layer during the maturation process. In normal and tension wood, these longitudinal (L) tensile stresses are associated with tangential (T) compressive stresses. This study aims at assessing the relationship between L and T maturation stresses, in order to discriminate between hypothetic micromechanical models of maturation stress generation at cell-wall level. In this study, the strain during the release of maturation stress along L and T directions (RLMS and RTMS) were measured at breast height on 5 positions around the circumference of 30 Eucalyptus nitens trees. Experiments were performed in a 17 years-old plantation on trees having DBH within a range of 18-28 cm. Up-right trees as well as tilted trees were selected in order to include various biomechanical situations. These measurements also enable evaluation of the Poisson's ratio of wood. Results show that all trees produce asymmetric RLMS around the periphery but with a high between-trees dispersion of the mean RLMS level: the maximal RLMS of some trees was in several cases found lower than the minimum of some other trees. Analysis of the relation with tree diameter indicates that the higher the growth rate, the lower the mean RLMS in the trees. Moreover, the difference between the maximum and the minimum RLMS within a tree decreased when growth rate increased. Measurements also indicate a clear relationship between the magnitude RLMS and RTMS, even after accounting for the Poisson effect: the higher the tensile longitudinal stress, the higher the tangential compressive stress. This relationship is strong at the between-tree level and could be detected at the within-tree level. These results will be analysed in relation with wood microstructure and the consequences for tree biomechanics will be discussed.

Texte intégral en chinois, résumé en anglais.; Based on the conventional anatomical analysis, the pore structures of poplar tension wood, such as specific surface area, pore volume and pore size distributions were characterized by nitrogen adsorption-desorption method. The shapes of the pores were estimated by nitrogen adsorption isotherms. The results show that poplar tension wood with BET specific surface area of 21.9 m2·g-1, which is 13 times higher than that in opposite wood. Poplar tension wood has intact mesoporosity (pore size between 2-50 nm) with inkbottle and slit shape pore, as well as a certain amount of micropores and macropores. The maximum value of pore size distribution appears at 5 nm. The pore volume with diameter ranging from 4-7 nm makes up 74.4% of the total, and larger than 15 nm, 10.8%. The high amount of mesopores can be attributed to the thick gelatinous layer in poplar tension wood.

Improved knowledge on the way cell wall stiffness changes during the maturation process is of great importance to feed and help theoretical modelling on growth stress generation in trees. This paper outlines one of the most promising techniques to measure viscoelastic properties within the different layers of the developing cell wall based on the so-called Resonant Contact mode using an Atomic Force Microscope. This mode and the required calibrations are briefly described here and an application on chestnut mature tension wood is shown. It demonstrates the ability of the technique for drawing qualitative viscoelastic maps within a cell wall layer. Limitations and necessary development of this method to do quantitative measurements are briefly discussed.

International audience

International audience; The mechanism responsible for unusual mechanical properties of tension wood gelatinous fiber (G-fiber) was investigated. We discussed origins of high tensile growth stress, high drying shrinkage, and rapid increase of Young's modulus due to drying, in association with microscopic structure of gelatinous layer (G-layer). Anatomical, crystallographic, and micromechanical approaches were employed. As the result, it is cleared that G-layer structure as hydro-gel has a key to understand hygromechanical properties in the tension wood G-fiber. The origin of contractive force in maturing G-layer is still unknown, however, the structure and behavior of the swollen hydro-gel may give some key to solve it.

Ce document reprend la structure de mon dossier de candidature au CNRS avec une réflexion sur le contexte et les enjeux de mes recherches suivi d'un aperçu rapide de mes activités de recherche. Ensuite, j'ai voulu montrer l'enchainement logique des travaux qui nous ont conduit à résoudre l'énigme du retrait paradoxal du bois de tension. Cette question qui depuis le début, il y a 10 ans, est présente dans mes recherches trouve aujourd'hui des éléments de réponse. Enfin, mon curriculum vitae donne une idée de ma production scientifique, de mes activités de formation et d'encadrement, de mon implication dans la vie de la recherche localement, au niveau national et international.

Colloque avec actes et comité de lecture. Internationale.; International audience; Les arbres sont capables de contrôler leur forme en induisant des précontraintes dans le bois nouvellement formé. La génération de celles-ci a lieu pendant la phase de maturation des cellules, mais son mécanisme est aujourd'hui encore énigmatique. Des expérimentations en diffraction des rayons X réalisées sur le micro-rayon du synchrotron de Grenoble ont permis de mettre en évidence la mise en tension de la cellulose avant même la formation de la dernière couche de la paroi du bois de tension (la couche G) pourtant jusque là suspectée d'être le moteur de la mise en tension.

Colloque avec actes et comité de lecture. Internationale.; International audience; L’explication de la réorientation des arbres repose sur la compréhension de la différenciation cellulaire lors de la croissance secondaire au niveau du cambium au cours de laquelle la paroi cellulaire va s’épaissir et se rigidifier occasionnant des précontraintes de croissance. Cette étude a pour objectif de mesurer les propriétés mécaniques de la paroi cellulaire par microscopie à force atomique.

Colloque avec actes et comité de lecture. Internationale.; International audience; Les prédictions à long-terme des propriétés viscoélastiques à partir des essais statiques sont en général basées sur l’équivalence temps-température. On se propose de discuter la validité de ce principe et les conditions de son utilisation dans le cas du bois, se basant sur une série d’essais de fluage à température croissante.

Colloque avec actes et comité de lecture. Internationale.; International audience; La production de bois précontraint est un élément-clé du design biomécanique des arbres, mais le mécanisme de mise en place de ces précontraintes reste inconnu. Nous explorons différentes hypothèses sur ce mécanisme en relation avec les paramètres de la microstructure du bois (cellulaire, polylamellé, composite à fibre) et le comportement de ses constituants durant sa formation.

Wood is a material designed by nature to last as long as it is not attacked by biological agents. It can support trees for centuries, and as a technological material it can again sustain loads for considerable periods. It is, as a consequence, a major component of the cultural heritage of many civilisations and the assessment of wood properties from ancient objects is a question of fundamental and practical interest. One major difficulty for such research is the gathering of suitable samples, with well-defined origin, certified dating and permission of publication by conservation administration. The Japanese context, where traditional uses of wood have been maintained for more than 1600 years, offers a unique opportunity to address the question of wood aging. Hinoki (Chamaecyparis obutusa) samples from Horyuji temple in Nara and other famous historical Japanese buildings have been gathered for more than 10 years by RISH, Kyoto University, expanding a collection gathered in the 50ties by Jaro Kohara. The wood age, defined as the time elapsed since wood formation and estimated by dendrochronology cross-checked with 14C measurements, ranged from 600 to 1700 years. The time elapsed since tree felling was also considered whenever possible. The recent wood used for comparison was selected according to similar origin and quality from craftsman viewpoint. The observations made included bending tests in L and R directions, colour measurements using a spectrophotometer, chemical and thermomechanical analysis.

Samples of tension wood and opposite wood were obtained from four species (Iyranthera sagotiana, Ocotea guyanensis, Virola michelii, Sextonia rubra) growing in the tropical rainforest of French Guiana. Dynamic mechanical analysis was performed in the longitudinal dimension with samples in the green "never dried" condition. Temperature and frequency of the tests were regulated to be similar to conditions experienced by the living tree. Tension wood from the species containing a Glayer was found to have higher damping characteristics than opposite wood from the same species whilst no difference was found between the wood types of the non G-layer species. The research thus far does not permit a solid conclusion but speculation into the possible origin of these differences is drawn from the nature of the G-layer matrix.

International audience; Tension wood generates axial tensile stress and tangential compressive stress during its formation, but the mechanism causing this stress is not yet fully understood. A micro-mechanical model is used to test different hypothetic mechanism, based on the assumption that one of the constituents of tension wood undergoes a dimensional change during the formation of the wall. The consistency between the outputs of the model and known facts about tension wood behaviour is used to assess and discuss various hypothetic mechanisms.

International audience; Tension wood is widespread in organs of woody plants. During its formation, it generates large tensile mechanical stress, called maturation stress [1]. This stress performs essential biomechanical functions such as optimizing the stem mechanical resistance, performing adaptive movements and ensuring long-term stability of growing plants. The mechanism generating this stress is not yet fully understood, although various hypotheses have recently been proposed [2-6]. In order to discriminate between these hypotheses, we investigated structural changes in cellulose microfibrils along sequences of xylem cell differentiation in tension and normal wood of poplar.

The domesticated plant known as manioc, or cassava (Manihot esculenta ssp. esculenta Crantz, Euphorbiaceae), is grown commonly as a shrub in many parts of the tropics, including French Guiana and is an important staple for about 700 million people throughout the tropics. Cultivation involves selection and planting in soil of stem cuttings, which subsequently grow as self-supporting shrubs or treelets up to 3 m in height. Shrubs and treelets of manioc are a common sight in many parts of the tropics. Shrubs are subsequently uprooted, and their starch-bearing tuberous roots harvested and processed to make a range of food products. Recent and ongoing phylogenetic research suggests that cassava was derived from a wild species currently referred to as Manihot esculenta subsp. flabellifolia in Brazil and as Manihot tristis (considered synonymous) in French Guiana. Our ecological and biomechanical analyses of these wild taxa indicate a broad plasticity in growth form, with development of mature phenotypes as either self-supporting shrubs in open conditions or as climbing plants in forest understory. We compared growth variation, mechanical architecture and wood microfibril angle of domesticated cassava and its closest wild relatives (considered similar to their common ancestor). Our observatioins included material from a range of habitats and two regions of South America - southwestern Brazil and French Guiana. Domesticated cassava, at least in the studied areas of South America, has a tendency to grow as climbing lianoid growth forms if plantations of cultivated shrubs are abandoned. Field observations and bending tests up to stem failure indicate that stems of domesticated manioc are much more brittle than stems of the putative ancestor. The results suggest that selection during domestication did not noticeably modify the overall ontogenetic trajectory or developmental plasticity of the species, which enable them to grow as either shrub or climber depending on the ecological context, but did change the material properties of the wood of the stem, particularly in terms of failure characteristics and microfibril angle. Studies based on domestication of organisms offer a window of opportunity for studying evolutionary processes - an opportunity that Charles Darwin used to great effect. From a biomechanical point of view, the study has demonstrated a profound influence of domestication on material properties but not on ontogeny. The results have also provided evidence on a phenomenon that we have taken for granted for a number of years but have never been able to test adequately - that certain types of climbing habit must develop stem properties that are flexible and resistant to brittle fracture.

Diversity of vibration properties of ten tropical hardwoods covering wide range of densities and anatomical structures was investigated. Wood from tilted trees was included in the study to examine the intraspecific diversity along with the interspecific one. Moreover, the relation with structural parameters (basic density, microfibril angle) was studied. While the variability of the elastic modulus was in great part explained by the variations of basic density and microfibril angle, damping coefficient was not correlated to structural parameters. This was particularly true for reaction woods and therefore attributed to the intensive loading history likely leading to the formation of microcracks.

International audience; Aiming at modelling tree mechanics, viscoelastic properties of green wood along fibres was investigated through a sequence of creep tests in the temperature range of 308C-708C. The apparent validity of time-temperature equivalency was questioned by discrepancies evidenced in the approximated complex plane (ACP). This paradox was solved by assuming that the temperature not only accelerates the viscoelastic processes but also slightly increases their intensity. This softening effect of the temperature on the compliance was described by a 2nd degree polynomial. Time-temperature dependency fitted very well to the Arrhenius law up to 608C. Based on the ACP, the power law was proposed for modelling creep behaviour in green wood. The method was successfully used for all specimens investigated.

Gravitropism is a major process in vertical growth as it allows the positioning of plant in the gravity field and thus, for trees, it takes part in the light foraging strategies. Without any gravitropism, trees would adopt a weeping habit or fall on the ground due to the increasing with growth supported weight, especially at the advance regeneration stage (sapling stage) in the understorey, where tree are very slender and poorly stiff. Most of the gravitropic studies are made by physiologists in order to understand the process and thus are associated to complex laboratory methods (genetic engineering, anatomical studies, accurate 2D ou 3D measurements of form changes with time). In order to study the ecological relevance and the specific diversity of gravitropic performances in natural forests, we developed a framework that uses different kinds of datas. Synchronical analysis has been done i) at the population level in large sample of trees and ii) at the tree level from destructive measurement of the dissymmetry maturation strains known to be the main motor of the posture control reaction of trees. In both cases, biomechanical models allows to estimate some aspects of the dynamic gravitropic process. Diachronical analysis has been done in natural conditions on permanent plots, or in a more traditional way, by observing in greenhouse experiments the righting movements of plants artificially tilted. In the first case, the problem is the high variability of natural disturbances and the slowness of growth and movements in the dark understorey. The second choice allows a measurement of a capacity (i.e. the ability of the species to react in quite extreme conditions of verticality disturbance but good for growth light and nutrient conditions) that is more easy to analyse, but is maybe not so relevant for estimating the ecological strategy (some species could have developed a high efficience of gravitropic reaction in natural conditions by avoiding the mechanical disturbance or adapting the reaction to slow growth and changing environments in natural conditions). All approaches use the same basic biomechanical modelling, i) to analyse simultaneously the reaction and the disturbance (due to the weight increase) during growth, or ii) to combine different parameters (geometry, growth, weight, maturation strains) involved in shape changes and posture control. They give a classification of the species according to their gravitropic performances. Results on 15 species are discussed, with relation to shade-tolerance or avoidance.

Le phénomène du vieillissement physique caractéristique des polymères semi-cristallins a été décrit dans le bois vert (jamais séché) de trois feuillus tropicaux. L'hypothèse du taux de vieillissement uniforme proposé par Struik pour les polymères synthétiques a été vérifiée. Grâce à la représentation des résultats expérimentaux dans le plan complexe approché le modèle de Maxwell parabolique a été proposé pour décrire le phénomène et utilisé avec succès sur l'ensemble des éprouvettes.

International audience; Six inclined poplar I-69 (Populus deltoids cv. I-69/55) trees were collected for studying the influence of growth stress level on wood properties. Growth stress indicator (GSI) was measured at 8 positions around the periphery of each trunk at breast height and corresponding wood samples were obtained. Wood anatomical, physico-mechanical and chemical characteristics were measured including cell diameter, fibre length, double wall thickness excluding gelatinous layer, lumen diameter after gelatinous layer removal, proportion of wood tissues, basic density, FSP, MOE, compressive strength, shrinkage and chemical composition. Each property was regarded in relation to the growth stress level to be discussed.

Normal wood swells in the longitudinal direction during serial ethanol substitution and this swelling is larger when the MFA is larger. However, tension wood shrinks under these conditions. This was observed both in a species with G-layer and in a species lacking a G-layer. We propose a mechanism explaining this difference in behaviour, based on the differential relaxation of internal stress during molecular substitution.

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International audience; Wood behaviour is characterized by a high sensibility to humidity, swelling when humidity increases and shrinkage during drying. From green condition to ovendry condition, shrinkage ranges from 0.05 % to 0.3 % along the grain direction, however, two cases exist where longitudinal shrinkage starts to be more important: in reaction wood (tension wood of hardwoods and compression wood of conifers) and juvenile wood. In these two types of wood, axial shrinkage can reach 1 % or more. These changes in longitudinal shrinkage can be explained by the wood fibre structure. The knowledge of the wood cell structure, as a multi-layer fibre composite, allows the modelling of the longitudinal shrinkage, considering that the cell wall is reduced to S2 layer. These models give a good understanding of macroscopic axial shrinkage for different values of microfibril angle, for normal, compression and juvenile wood. However, they cannot explain the behaviour of tension wood. Tension wood cell wall is characterised by a very low microfibrils angle. In some species, important changes occur and part of the secondary wall is replaced by a distinct layer. This layer is described as purely or highly cellulosic, highly crystalline and with microfibril angle very low or nil. However, although a very low microfibril angle in G-layer, macroscopic longitudinal shrinkage is high in tension wood. Some years ago, we shown the experimental evidence of the G-layer shrinkage, thinks to the new observation techniques such as Atomic Force Microscopy but we were enable to explain it. Recent observations of some cutting arfefact in G-layer evidenced the weakly bonding of microfibrils in G-layer transversally to the fibre axis. Latter X-ray diffraction experiments proved that cellulose microfibrils d-spacing is very lightly shortened during axial shrinkage compare to macroscopic one. These observations conclude that cellulose was subject to buckling during G-layer shrinkage and cannot be the driving force of G-layer longitudinal shrinkage. The origin of G-layer shrinkage is then still in question. In this study, we concentrate on the characterisation of the non-cellulosique component of the G-layer, its organisation and its behaviour during drying.

Tension wood xylem of arboreal eudicot species often produces unusual wood fiber, called gelatinous fiber (G-fiber). The G-fiber forms a gelatinous layer (G-layer) as the innermost layer of the multi-layered cell wall. The G-fiber generates a high tensile growth stress, which enables the hardwood species to perform the negative-gravitropic behaviors in its inclined shoot. However, the tension wood often causes various obstacles when we use the forest resources as the raw material for the timber products. Examples are the processing defects caused by its abnormal growth stress. Other examples are a high longitudinal Young's modulus and a high longitudinal drying shrinkage; their combined effect causes serious processing defects during the lumbering and drying process, e.g., distortion and cleavage of the sawn lumber, and so forth. Some researchers consider that characteristic properties of the G-fiber should be attributed to the intrinsic behaviors of the G-layer, while others emphasize the role of other lignified layer because the G-layer is often peeled off the lignified layer in the same direction during microtoming. Lately, Clair et al. revealed that detachment of the G-layer is an artifact that is caused by the stress concentration from the microtome blade, and they concluded that the G-layer is strongly attached to the lignified layer even in the oven-dried specimen. This positively supports the idea that the characteristic behaviors of the G-fiber originate from the property of the G-layer. However, the researchers still have no explanation for the generation mechanism of those G-layer properties. In our presentation, we focused our attentions on the moisture dependent changes of the longitudinal Young's modulus and the longitudinal shrinkages in two Quercus acctissima containing the tension wood xylem. And, we revealed the astonishing difference of those properties between G-layer and the lignified wall (= normal wood cell wall). Based on the obtained results, we propose a model of fine structure of the G-layer (and that of the lignified wall) to explain what is going on the drying G-layer.

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Non renseigné

In order to understand what is controlling the magnitude of the longitudinal growth stress in tension wood, an experiment was carried out on poplar tension wood (Populus I4551). Results show that (a) in the region where growth strain value is between 610 and 816 µstrain gelatinous fibres are common, above the value between 1540 and 1935 µstrain almost all fibres contain gelatinous layers; (b) the amount of fibres, the amount of fibres with gelatinous layer (G-Fibres), per unit of tissue area, and the thickness of the gelatinous layers in fibres control mostly the magnitude of growth stress; (c) other possible factors contributing to growth stress are the lignification, crystallinity and microfibril angle in the fibres without G-layer within the region partly containing G-fibres and in G-layers themselves.