TWIG: A model to simulate the gravitropic response of a tree axis in the frame of elasticity and viscoelasticity, at intra-annual time scale

International audienceTrees are able to maintain or to modify the orientation of their axes (trunks or branches) by tropic movements. For axes in which elongation is achieved but cambial growth active, the tropic movements are due to the production of a particular wood, called reaction wood which is prestressed within the growing tree. Several models have been developed to simulate the gravitropic response of axes in trees due to the formation of reaction wood, all within the frame of linear elasticity and considering the wood maturation as instantaneous. The effect viscoelasticity of wood has, to our knowledge, never been considered. The TWIG model presented in this paper aims at simulating the gravitropic movement of a tree axis at the intra-annual scale. In this work we studied both the effect of a non instantaneous maturation process and of viscoelasticity. For this purpose, we considered the elastic case with maturation considered as an instantaneous process as the reference. The introduction of viscoelasticity in TWIG has been done by coupling TWIG to a model developed for bridges. Indeed from a purely mechanical point of view, bridges and trees are very similar: they are structures which are built in stages, they are made of several materials (composite structures), their materials are prestressed (wood is prestressed during the maturation process as a result of polymerisation of lignin and cellulose to form the secondary cell wall and concrete is prestressed during drying). Simulations gave evidence that the reorientation process of axes can be significantly influenced by the kinetics of maturation. Moreover the model has now to be tested with more experimental data of wood viscoelasticity but it appears that in the range of a relaxation time from 0 to 50 days, viscoelasticity has an important effect on the evolution of tree shape as well as on the values of prestresses

A model to simulate the gravitropic response and internal stresses in trees, considering the progressive maturation of wood

Trees-Struct. Funct. ISI Document Delivery No.: AM5EH Times Cited: 0 Cited Reference Count: 21 Pot, Guillaume Coutand, Catherine Toussaint, Evelyne Le Cam, Jean-Benoit Saudreau, Marc Auvergne Regional Council; European Regional Development Fund This work was supported by a grant from the Auvergne Regional Council and the European Regional Development Fund. Springer New yorkInternational audienceThe developed model of gravitropism takes non-instantaneous maturation of wood into account which enabled to correctly simulate different gravitropic phases and realistic internal stress profiles. A new biomechanical model of tree movement in relation to gravity (gravitropism) is proposed in this study. The modelling of the progressive maturation of wood cells is taken into account, as well as spatio-temporal variations in maturation strains (MS) and mechanical properties. MS were identified using an inverse method that allows the model to fit the gravitropic reaction observed experimentally. For this purpose, the curvature during righting movement, the geometry and the mass distribution of a two-year-old poplar tree was measured. The identified MS are higher than expected, which shows the underestimation of MS by usual measurements. By using the same mechanical parameters and MS as an input, the model gives satisfying results in terms of shape modelling for different trees up to 32 days after tree tilting. The model is able to simulate the latency phase observed in the tree righting movement, and the internal stress profile in the trunk is realistic (low compressive value in the central part of the trunk and zero stress in newly formed cells). The next development of the model will aim to simulate the end of the gravitropic phase in relation with the regulation of MS by the tree

Etude expérimentale à l'échelle intra-cerne de propriétés mécaniques du bois vert de peuplier au cours de sa maturation par des essais de tractions cycliques

Les arbres régulent l'orientation de leurs axes afin d'assurer leur stabilité mécanique et l'acquisition de la lumière. Pour ce faire, ils adaptent les propriétés mécaniques du bois qu'ils créent (formation de bois de réaction). L'objet de ce travail est d'étudier l'évolution des propriétés mécaniques du bois d'arbres inclinés au cours du processus de maturation. Les résultats issus d'une large campagne expérimentale mettent en évidence le comportement mécanique singulier du bois vert (rigidification sous contrainte) ainsi que des phénomènes de maturation complexes

Mechanosensing and thigmomorphogenesis, a physiological and biomechanical point of view

Plant sensitivity to mechanical stimuli is obvious when observing the movements of Mimosa pudica leaflets when they are touched [1] or those of the Venus fly trap [2]. It is now well established that other plants are also sensitive to mechanical stimuli even if they do not exhibit such rapid movements [1]. There is a renewal of interest in mechanical stimuli as very important cues for the control of plant growth [3] and morphogenesis [4,5]. This review focuses on mechanosensing in the case of external mechanical loading and its effect on the growth patterns of plant organs (thigmomorphogenesis). The first part of this paper deals with the responses at the whole plant level and their ecological significance. The second part deals with the perception process, with emphasis on the variable that is perceived by the plant. Knowledge about mechanosensors is not presented in great detail because this area of intensive research has been recently reviewed [6,7]. The third part focuses on transduction, i.e., early responses at the cellular level, and particularly focuses on the importance of the kinetics of loading and the kinetics of cellular responses for the interpretation of experimental results. The fourth part focuses on parameters that regulate the mechanosensing process and points out the importance of quantitative studies. Because thigmomorphogenesis and gravitropism are difficult to disentangle, the review ends with data on gravitropism where mechanosensing is involved. (C) 2010 Elsevier Ireland Ltd. All rights reserved

The Effect of Mechanical Stress on Plant Susceptibility to Pests: A Mini Opinion Review

Plants are subject to multiple pest attacks during their growing cycle. In order to address consumers’ desire to buy healthy vegetables and fruits, i.e., without chemical residues, and to develop environment-friendly agriculture, major research efforts are being made to find alternative methods to reduce or suppress the use of chemicals. Many methods are currently being tested. Among these methods, some are being tested in order to modify plant physiology to render it less susceptible to pathogen and pest attacks by developing plant immunity. An emerging potentially interesting method that is being studied at this time is mechanical stimuli (MS). Although the number of articles on the effect of MS on plant immunity is still not large, it has been reported that several types of mechanical stimuli induce a reduction of plant susceptibility to pests for different plant species in the case of wounding and non-wounding stimuli. This mini review aims to summarize the knowledge available at this time by raising questions that should be addressed before considering MS as an operable alternative method to increase plant immunity for crop protection

Comment les arbres perçoivent les informations mécaniques et d'éclairement pour ajuster les dimensions et la forme de leurs branches ?

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L'arbre est un adepte du taï chi chuan

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Contribution à l’étude du contrôle des dimensions et de la forme des axes aériens par des signaux mécaniques et lumineux chez les ligneux

D’un point de vue écologique les arbres doivent se développer en assurant leur stabilité mécanique d’une part et d’autre part assurer la capture de la ressource lumineuse pour la photosynthèse. D’un point de vue mécanique l’arbre est une structure en croissance i.e. que ses dimensions et son poids propre évoluent avec la croissance qui peut être vue comme des dépôts de matière sur une structure existante. Pour assurer son port (i.e. l’action de se porter) l’arbre doit dimensionner ses axes (tronc et branches) de façon à ne pas s’écrouler sous son propre poids (i.e. ne pas flamber) [C1]. Pour assurer un déploiement spatial assurant l’interception de la lumière, les arbres peuvent être amenés à modifier la forme et l’orientation de leurs axes [A6, C2]. Comme je vais essayer de le montrer dans ce mémoire la perception de leur environnement mécanique permet aux arbres de dimensionner leurs axes par le processus de thigmomorphogénèse. Les réactions tropiques permettent quant à elles aux arbres de contrôler la forme et l’orientation de leurs axes. L’acquisition et le contrôle des dimensions des axes, de leur forme et de leur orientation repose sur un déterminisme génétique d’une part et d’autre par sur un déterminisme lié aux facteurs environnementaux, qui peuvent induire des réactions biophysiques mais aussi des réactions sous contrôle épigénétique. Mon dossier d’Habilitation à Diriger des Recherches porte sur la régulation des dimensions des axes par des signaux mécaniques et sur la régulation de la forme des axes par des signaux mécaniques et lumineux

Etat mécanique de l'arbre, acclimatation des arbres à leur environnement mécanique et sécurité de l'arbre au vent

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