Éditeur(s) : HAL CCSD
Résumé : International audience
Trees can control their shape and resist gravity thanks to their ability to produce wood under tensile stress. This stress is known to be produced during the maturation of wood fibres but the mechanism of its generation remains unclear. This study focuses on the formation of the secondary wall in tension wood produced in artificially tilted poplar saplings. Thickness of secondary wall layer (SL) and gelatinous layer (GL) were measured from cambium to mature wood in several trees sampled at different times after tilting. Measurements on wood fibres produced before tilting show the progressive increase of secondary wall thickness during the growing season. After the tilting date, SL thickness decreased markedly from normal wood to tension wood while the total thickness increased compared to normal wood, with the development of a thick GL. However, even after GL formation, SL thickness continues to increase during the growing season. GL thickening was observed to be faster than SL thickening. The development of the unlignified GL is proposed to be a low cost, efficient strategy for a fast generation of tensile stress in broadleaved trees.
IAWA journal / International Association of Wood Anatomists.

hal-01229277
https://hal.archives-ouvertes.fr/hal-01229277
https://hal.archives-ouvertes.fr/hal-01229277/document
https://hal.archives-ouvertes.fr/hal-01229277/file/AbediniClair-Iawa-TensionCellWallThickening_preprint_HAL.pdf
DOI : 10.1163/22941932-00000084

Éditeur(s) : HAL CCSD
Résumé : 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.
8th Plant Biomechanics International Conference
Nagoya, Japan

hal-01436113
https://hal.archives-ouvertes.fr/hal-01436113

Éditeur(s) : HAL CCSD Royal Society
Résumé : International audience
Trees control their posture by generating asymmetric mechanical stress around the periphery of the trunk or branches. This stress is produced in wood during the maturation of the cell wall. When the need for reaction is high, it is accompanied by strong changes in cell organization and composition called reaction wood, namely compression wood in gymnosperms and tension wood in angiosperms. The process by which stress is generated in the cell wall during its formation is not yet known, and various hypothetical mechanisms have been proposed in the literature. Here we aim at discriminating between these models. First, we summarize current knowledge about reaction wood structure, state and behaviour relevant to the understanding of maturation stress generation. Then, the mechanisms proposed in the literature are listed and discussed in order to identify which can be rejected based on their inconsistency with current knowledge at the frontier between plant science and mechanical engineering.
Journal of the Royal Society Interface

hal-01392832
https://hal.archives-ouvertes.fr/hal-01392832
https://hal.archives-ouvertes.fr/hal-01392832/document
https://hal.archives-ouvertes.fr/hal-01392832/file/AlmerasClair2016_JRSInterface_Critical%20review%20on%20the%20mechanisms%20of%20maturation%20stress%20generation%20in%20trees_HAL.pdf
DOI : 10.1098/rsif.2016.0550

Éditeur(s) : HAL CCSD Wiley
Résumé : International audience
In order to progress in the understanding of mechanical stress generation, the mesoporosity of the cell wall and its changes during maturation of poplar (Populus deltoides x P. nigra) tension wood (TW) and opposite wood (OW) were measured by nitrogen adsorption–desorption. Variations in the thickness of the gelatinous layer (G-layer) were also measured to clarify whether the mesoporosity change simultaneously with the deposition of the G-layer in TW. Results show that mesoporous structures of TW and OW were very similar in early development stages before the deposition of G-layers. With the formation of the S2 layer in OW and the G-layer in TW, the mesopore volume decreased steeply before lignification. However, in TW only, the decrease in mesopore volume occurred together with the pore shape change and a progressive increase in pore size. The different patterns observed in TW revealed that pores from G-layers appear with a different shape compared to those of the compound middle lamella, and their size increases duringthe maturation process until stabilising in mature wood. This observation strongly supports the hypothesis of the swelling of the G-layer matrix during maturation as the origin of maturation stress in poplar tension wood.
ISSN: 0028-646X

hal-01081834
https://hal.archives-ouvertes.fr/hal-01081834
https://hal.archives-ouvertes.fr/hal-01081834/document
https://hal.archives-ouvertes.fr/hal-01081834/file/Chang_al_Mesoporosity_changes_New-Phyto._2015.pdf
DOI : 10.1111/nph.13126

Éditeur(s) : HAL CCSD Linnean Society of London
Résumé : International audience
The circumscription of Salicaceae has recently been enlarged to include a majority of the species formerly placed in the polyphyletic tropical family Flacourtiaceae. Several studies have reported a peculiar and infrequently formed multilayered structure of tension wood in four of the tropical genera. Tension wood is a tissue produced by trees to restore their vertical orientation, and most studies have focused on trees developing tension wood by means of cellulose-rich, gelatinous fibres, as is known in Populus L. and Salix L. (Salicaceae s.s.). This study aims to determine if the multilayered structure of tension wood is an anatomical characteristic common in other Salicaceae, and if so, how its distribution correlates to phylogenetic relationships. Therefore, we studied the tension wood of 14 genera of Salicaceae as well as two genera of Achariaceae, one genus of Goupiaceae, and one genus of Lacistemataceae, families closely related to Salicaceae or formerly placed in Flacourtiaceae. Opposite wood and tension wood were compared with light microscopy and 3D Laser Scanning Confocal Microscopy. The results indicate that a multilayered structure of tension wood is common in the family except in Salix, Populus, and one of their closest relatives, Idesia polycarpa Maxim. We suggest that tension wood may be a useful anatomical character in understanding phylogenetic relationships in Salicaceae. Further investigation is still needed on the tension wood of several other putatively close relatives of Salix and Populus, in particular Bennettiodendron Merr., Macrohasseltia L.O.Williams and Itoa Hemsl.
ISSN: 0024-4074

hal-01392845
https://hal.archives-ouvertes.fr/hal-01392845
https://hal.archives-ouvertes.fr/hal-01392845/document
https://hal.archives-ouvertes.fr/hal-01392845/file/GhislainEtAl2016_MultylayerTWSalicaceae_Bot_Jsur the_Lin_Soc_HALssfig.pdf
DOI : 10.1111/boj.12471

Éditeur(s) : HAL CCSD Oxford University Press (OUP): Policy B - Oxford Open Option B
Résumé : International audience
To recover verticality after disturbance, angiosperm trees produce " tension wood " allowing them to bend actively. The driving force of the tension has been shown to take place in the G-layer, a specific unlignified layer of the cell wall observed in most temperate species. However, in tropical rain forests, the G-layer is often absent and the mechanism generating the forces to reorient trees remains unclear. A study was carried out on tilted seedlings, saplings and adult Simarouba amara trees – a species known to not produce a G-layer. Microscopic observations were done on sections of normal and tension wood after staining or observed under UV light to assess the presence/absence of lignin. We showed that S. amara produces a cell-wall layer with all of the characteristics typical of G-layers, but that this G-layer can be observed only as a temporary stage of the cell-wall development because it is masked by a late lignification. Being thin and lignified, tension wood fibres cannot be distinguished from normal wood fibres in the mature wood of adult trees. These observations indicate that the mechanism generating the high tensile stress in tension wood is likely to be the same as that in species with a typical G-layer and also in species where the G-layer cannot be observed in mature cells.
ISSN: 0829-318X

hal-01229280
https://hal.archives-ouvertes.fr/hal-01229280
https://hal.archives-ouvertes.fr/hal-01229280/document
https://hal.archives-ouvertes.fr/hal-01229280/file/Roussel-Clair-2015-TreePhysiol-LateLignifGLayerSimarouba_preprintHAL.pdf
DOI : 10.1093/treephys/tpv082