Maturation stress generation in poplar tension wood studied by synchrotron radiation microdiffraction

Tension wood is widespread in the organs of woody plants. During its formation, it generates a large tensile mechanical stress called maturation stress. Maturation stress performs essential biomechanical functions such as optimizing the mechanical resistance of the stem, performing adaptive movements, and ensuring the long-term stability of growing plants. Although various hypotheses have recently been proposed, the mechanism generating maturation stress is not yet fully understood. 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 (Populus deltoides × Populus trichocarpa 'I45-51'). Synchrotron radiation microdiffraction was used to measure the evolution of the angle and lattice spacing of crystalline cellulose associated with the deposition of successive cell wall layers. Profiles of normal and tension wood were very similar in early development stages corresponding to the formation of the S1 layer and the outer part of the S2 layer. Subsequent layers were found with a lower microfibril angle (MFA), corresponding to the inner part of the S2 layer of normal wood (MFA approximately 10°) and the G layer of tension wood (MFA approximately 0°). In tension wood only, this steep decrease in MFA occurred together with an increase in cellulose lattice spacing. The relative increase in lattice spacing was found close to the usual value of maturation strains. Analysis showed that this increase in lattice spacing is at least partly due to mechanical stress induced in cellulose microfibrils soon after their deposition, suggesting that the G layer directly generates and supports the tensile maturation stress in poplar tension wood.     

Atomic force microscopy of pollen grains,cellulose microfibrils,and protoplasts

Summary Atomic force microscopy (AFM) holds unique prospects for biological microscopy, such as nanometer resolution and the possibility of measuring samples in (physiological) solutions. This article reports the results of an examination of various types of plant material with the AFM. AFM images of the surface of pollen grains ofKalanchoe blossfeldiana andZea mays were compared with field emission scanning electron microscope (FESEM) images. AFM reached the same resolutions as FESEM but did not provide an overall view of the pollen grains. Using AFM in torsion mode, however, it was possible to reveal differences in friction forces of the surface of the pollen grains. Cellulose microfibrils in the cell wall of root hairs ofRaphanus sativus andZ. mays were imaged using AFM and transmission electron microscopy (TEM). Imaging was performed on specimens from which the wall matrix had been extracted. The cell wall texture of the root hairs was depicted clearly with AFM and was similar to the texture known from TEM. It was not possible to resolve substructures in a single microfibril. Because the scanning tip damaged the fragile cells, it was not possible to obtain images of living protoplasts ofZ. mays, but images of fixed and dried protoplasts are shown. We demonstrate that AFM of plant cells reaches resolutions as obtained with FESEM and TEM, but obstacles still have to be overcome before imaging of living protoplasts in physiological conditions can be realized.Abbreviations AFM atomic force microscope - FESEM field emission scanning electron microscope - PyMS pyrolysis mass spectrometry - TEM transmission electron microscope     

The glucuronoxylans and the helicoidal shift in cellulose microfibrils in linden wood: Cytochemistryin muro and on isolated molecules

Summary In fibres of wood, the classical S₁ and S₂ layers are connectedvia a transition zone where a helicoidal texture occurs. In order to understand the actual mechanism of cellulose microfibril rotation in this zone, the study of relationship between cellulose and matrix was undertaken cytochemically at the ultrastructural level.Glucuronoxylans,i.e., the main hemicellulose component of hardwood, were studied in cell walls of linden tree. Xylanase-gold complexes were used as a new cytochemical tool to directly and specifically label glucuronoxylans within the wall of fibres. Subtractive localization (KOH or DMSO extraction and PATAg test or shadowing) associated with chemical analysis was carried out as control. The study of isolated glucuronoxylan molecules was undertaken in parallel.Both from direct (xylanase-gold labeling) and indirect techniques (extractions), glucuronoxylans appear preferentially concentrated in the transition zone which overlaps the layers S₁ and S₂. A comparison between KOH and DMSO extraction indicates a difference in accessibility of glucuronoxylans distributed across the whole wall and those located in the transition zone. Isolated molecules have a rodlike aspect and show a tendancy to spatially organize in parallel alignment. Cytochemical labeling of the isolated molecules concerns covalent linkages, vic-glycol groups and acid side groups along the main chain.The preferential localization indicates that in the helicoidal zone glucuronoxylans constitute a thick matrix embedding the cellulose microfibrils in the course of rotation. This data leads to a discussion of how these localized matrix molecules could intervene in the assembly and the twisted morphogenesis of the fibre cell wall.     

Helicoidal self-ordering of cellulose microfibrils in aqueous suspension.

In many skeletal support systems of plants and animals, cellulose, chitin, and collagen occur in the form of microfibrils ordered in a chiral nematic fashion (helicoids). However, these structures remain poorly understood due to the many constituents present in biological tissues. Here we report an in vitro system that attracts by its simplicity. Only one chemical component, cellulose, is present in the form of fibrillar fragments dispersed in water. Above a critical concentration the colloidal dispersion separates spontaneously into a chiral nematic liquid crystalline phase. On drying this phase solidifies into regularly twisted fibrillar layers that mimic the structural organization of helicoids in nature.     

Immunocytochemical characterization of tension wood: Gelatinous fibers contain more than just cellulose

Gelatinous fibers (G-fibers) are the active component of tension wood. G-fibers are unlike traditional fiber cells in that they possess a thick, nonlignified gelatinous layer (G-layer) internal to the normal secondary cell wall layers. For the past several decades, the G-layer has generally been presumed to be composed nearly entirely of crystalline cellulose, although several reports have appeared that disagreed with this hypothesis. In this report, immunocytochemical techniques were used to investigate the polysaccharide composition of G-fibers in sweetgum (Liquidambar styraciflua; Hamamelidaceae) and hackberry (Celtis occidentalis; Ulmaceae) tension wood. Surprisingly, a number of antibodies that recognize arabinogalactan proteins and RG I-type pectin molecules bound to the G-layer. Because AGPs and pectic mucilages are found in other plant tissues where swelling reactions occur, we propose that these polymers may be the source of the contractile forces that act on the cellulose microfibrils to provide the tension force necessary to bend the tree trunk.     

Orientation of cellulose microfibrils in cortical cells of tobacco explants

The deposition of nascent cellulose microfibrils (CMFs) was studied in the walls of cortical cells in explants of Nicotiana tabacum L. flower stalks. In freshly cut explants the CMFs were deposited in two distinct and alternating orientations — all given with respect to the longitudinal axis of the cell —, at 75° and 115°, in a left-handed (S-helix) and right-handed (Z-helix) form, respectively. The CMFs deposited in these orientations did not form uninterrupted layers, but sheets in which both orientations were present. After explantation, the synthesis of CMFs and their deposition in bundles continued. New orientations occurred within 6 h. After 6 h a new sheet was deposited, with orientations of 15° (S-helix) and 165° (Z-helix). The changes could be seen as sudden bends in individual CMFs or in small bundles of CMFs. In the next stage, more CMFs were deposited with these new orientations and the bundles became larger. New orientations arose by a shift towards more longitudinal directions, starting from either the S-helix or the Z-helix form. It was only after an almost longitudinal orientation was reached that the CMFs were deposited in two opposing directions again and a new sheet was formed. Neither colchicine nor cremart influenced the changes in CMF deposition. It is concluded that microtubules do not control CMF deposition in cortical cells of tobacco explants; control of CMF deposition and microtubule orientation occurs by factors related to cell polarity.Abbreviations CMF cellulose microfibril - MT microtubule We thank Professor M.M.A. Sassen and Dr. G.W.M. Barendse (Department of Experimental Botany, University of Nijmegen, Nijmegen, The Netherlands) for helpful discussions and Mrs. A. Kemp for her assistance in the ethylene experiments.     

Deposition and organisation of cell wall polymers during maturation of poplar tension wood by FTIR microspectroscopy

To advance our understanding of the formation of tension wood, we investigated the macromolecular arrangement in cell walls by Fourier transform infrared microspectroscopy (FTIR) during maturation of tension wood in poplar (Populus tremula x P. alba, clone INRA 717-1B4). The relation between changes in composition and the deposition of the G-layer in tension wood was analysed. Polarised FTIR measurements indicated that in tension wood, already before G-layer formation, a more ordered structure of carbohydrates at an angle more parallel to the fibre axis exists. This was clearly different from the behaviour of opposite wood. With the formation of the S₂ layer in opposite wood and the G-layer in tension wood, the orientation signals from the amorphous carbohydrates like hemicelluloses and pectins were different between opposite wood and tension wood. For tension wood, the orientation for these bands remains the same all along the cell wall maturation process, probably reflecting a continued deposition of xyloglucan or xylan, with an orientation different to that in the S₂ wall throughout the whole process. In tension wood, the lignin was more highly oriented in the S₂ layer than in opposite wood.     

Changes in the arrangement of cellulose microfibrils associated with the cessation of cell expansion in tracheids

 The relationship between the cessation of cell expansion and formation of the secondary wall was investigated in the early-wood tracheids of Abies sachalinensis Masters by image analysis and field emission scanning electron microscopy. The area of the lumen and the length of the perimeter of the lumen of differentiating tracheids increased from the cambium towards the xylem. These increases had just ceased in the case of tracheids closest to the cambium in which birefringence was first detected by observations with a polarizing light microscope. Cellulose microfibrils (MFs) deposited on the innermost surfaces of radial walls were not well ordered during the expansion of cells, but well ordered MFs were deposited at the subsequent stage of cell wall formation. The first well ordered MFs were oriented in an S-helix. The well ordered MFs had already been deposited at the tracheids where birefringence was first detected under the polarizing light microscope. These results indicate that the deposition of the well ordered MFs, namely, the formation of the secondary wall, begins before the cessation of cell expansion of tracheids. Therefore, it seems that the expansion of tracheids is restricted by the deposition of the secondary wall because the cell walls become rigid simultaneously with the development of the secondary wall and, therefore, the yield point of cell walls exceeds the turgor pressure of the cell. Received: 3 July 1996 / Accepted: 24 September 1996     

Internodal elongation and orientation of cellulose microfibrils and microtubules in deepwater rice

Excised stem sections of deepwater rice (Oryza sativa L.) containing the highest internode were used to study the induction of rapid internodal elongation by gibberellin (GA). It has been shown before that this growth response is based on enhanced cell division in the intercalary meristem and on increased cell elongation. In both GA-treated and control stem sections, the basal 5-mm region of the highest internode grows at the fastest rate. During 24 h of GA treatment, the internodal elongation zone expands from 15 to 35 mm. Gibberellin does not promote elongation of internodes from which the intercalary meristem has been excised. The orientation of cellulose microfibrils (CMFs) is a determining factor in cell growth. Elongation is favored when CMFs are oriented transversely to the direction of growth while elongation is limited when CMFs are oriented in the oblique or longitudinal direction. The orientation of CMFs in parenchymal cells of GA-treated and control internodes is transverse throughout the internode, indicating that CMFs do not restrict elongation of these cells. Changes in CMF orientation were observed in epidermal cells, however. In the basal 5-mm zone of the internode, which includes the intercalary meristem, CMFs of the epidermal cell walls are transversely oriented in both GA-treated and control stem sections. In slowly growing control internodes, CMF orientation changes to the oblique as cells are displaced from this basal 5-mm zone to the region above it. In GA-treated rapidly growing internodes, the reorientation of CMFs from the transverse to the oblique is more gradual and extends over the 35-mm length of the elongation zone. The CMFs of older epidermal cells are obliquely oriented in control and GA-treated internodes. The orientation of the CMFs parallels that of the cortical microtubules. This is consistent with the hypothesis that cortical microtubules determine the direction of CMF deposition. We conclude that GA acts on cells that have transversely oriented CMFs but does not promote growth of cells whose CMFs are already obliquely oriented at the start of GA treatment.     

Mechanical behavior of fetal dura mater under large deformation biaxial tension

The mechanical behavior of fetal dura mater was investigated by means of a biaxial tension test designed to simulate the constraints imposed on the membrane by the cranial bones. The experimental results are compared with the theoretical results obtained by using two published strain energy functions: one defined by Mooney and Rivlin (MR) and the other by Skalak, Tozeren, Zarda and Chien (STZC). The latter constitutive relations fit the experimental results consistently well. The STZC stiffness values from this series of tests are compared with those from membrane inflation tests performed previously and reported elsewhere by the authors.     

Characterization of cross-links between cellulose microfibrils, and their occurrence during elongation growth in pea epicotyl

The occurrence and chemical nature of the cross-links between cellulose microfibrils in outer epidermal cell walls in Pisum sativum cv. Alaska was investigated by rapid-freezing and deep-etching techniques coupled with chemical and enzymatic treatments. The cell wall in the elongating region of epidermal cells was characterized by the absence of the cross-links, while in the elongated region, the cell wall was characterized by the presence of cross-links. The cross-links remained in the cell wall of the elongated region after treatment with SDS electrophoresis sample buffer and treatment with 4% potassium hydroxide. After treatment with endo-1,4-beta-glucanase, which fragments xyloglucan, the cross-links were remarkably reduced from the cell wall of the elongated region. The endoglucanase treatment also reduced immunogold labeling of xyloglucan in the cell wall. The endoglucanase hydrolysate from the cell wall fraction of the elongated region gave spots of oligosaccharides in thin layer chromatography, which were identical to the spots of xyloglucan oligosaccharides produced by xyloglucanase from both the cell wall fraction and tamarind xyloglucan. These results indicate that the cross-links are made of xyloglucan. We discussed the possibility of cross-links involved in the control of mechanical properties of the cell wall.     

Effect of reaction conditions on the properties and behavior of wood cellulose nanocrystal suspensions

Sulfuric acid hydrolysis of native cellulose fibers produces stable suspensions of cellulose nanocrystals. Above a critical concentration, the suspensions spontaneously form an anisotropic chiral nematic liquid crystal phase. We have examined the effect of reaction time and acid-to-pulp ratio on nanocrystal and suspension properties for hydrolyzed black spruce acid sulfite pulp. Longer hydrolysis times produced shorter, less polydisperse black spruce cellulose nanocrystals and slightly increased the critical concentration for anisotropic phase formation. Increased acid-to-pulp ratio reduced the dimensions of the nanocrystals thus produced; the critical concentration was increased and the biphasic range became narrower. A suspension made from a bleached kraft eucalyptus pulp gave very similar properties to the softwood nanocrystal suspension when prepared under similar hydrolysis conditions.     

Atomistic behavior analysis of Cu nanowire under uniaxial tension with maximum local stress method

This study analyzes the atomistic behaviors of a Cu nanowire (NW) during uniaxial tensile deformation by molecular dynamics simulation. In this work, the maximum local stress calculated method (MLS) is proposed to validly elucidate the plastic behaviors of the Cu NW. Analysis results demonstrate that the pre-tension stress is caused by the intense surface tension, which is an important factor for dislocation emission from surface. The motion of Shockley partials that interact to produce a stair-rod dislocation is determined. Following the dislocation mechanism, deformation twinning is the primary mechanism that dominates the plastic deformation at such a high strain rate. Immediately before fracture, the stress increases markedly since the primary failure mode is atomic bond breakage.     

The presence of cellulose microfibrils in the proteinaceous slime track of Dictyostelium discoideum

Summary Electron microscopy and enzymatic analysis show that the slime track of Dictyostelium discoideum represents a two-phase system with a fibrillar component, most likely cellulose, embedded in an amorphous, protein containing matrix. In contrast the slime from the sorus of the mature fruiting body is devoid of any fibrillar component. Thus, this organism produces two significantly different types of slime material.     

Deposition and reorientation of cellulose microfibrils in elongating cells of Petunia stylar tissue

According to Roelofsen and Houwink's (1953, Acta Bot. Neerl. 2, 218–225) multinet growth hypothesis, microfibrils originally deposited transversely in the cell wall become gradually reoriented towards more axial orientations during cell elongation. To establish the extent of reorientation, microfibrils were studied during their deposition and elongation, using stylar parenchyma and transmitting tissue cells of Petunia hybrida L. At the inner surface of very young cells, microfibrils were deposited in alternating Z- and S-helical orientations. The following sequence in deposition, from the exterior to the interior side of the wall, could be inferred: Axial: 150°–180° (Z-helical), 0°–30° (S-helical); oblique: 110°–150° (Z-helical), 30°–70° (S-helical); transverse: 90°–110° (Z-helical), 70°–90° (S-helical). With the increasing pitch, the density of the deposited microfibrils increased as well, giving rise to an alternating helical texture. During elongation, only transversely S- and Z-helically oriented microfibrils were deposited and all microfibrils underwent a certain reorientation as described in the multinet growth hypothesis. The texture resembled that of young cells and the wall maintained its thickness. The extent of passive reorientation was in agreement with the theoretical calculations made by Preston.Dedicated to Professor Dr. A.B. Wardrop, Melbourne, on the occasion of his 70th birthday     

X-ray microdiffraction reveals the orientation of cellulose microfibrils and the size of cellulose crystallites in single Norway spruce tracheids

The microfibril angle (MFA) distribution and the size of cellulose crystallites in isolated double cell walls of Norway spruce (Picea abies [L.] Karst.) tracheids were determined by synchrotron X-ray microdiffraction using the reflections 200 and 004. Samples were 25 μm thick longitudinal sections of earlywood from annual rings 6–18 of several stems. The asymmetric MFA distributions extended from ?20° to 90°. The mean MFA of tangential cell walls decreased from an average of 24° into 19° from the pith to the bark. The mode of the MFA distribution was about 10° smaller than the mean MFA. The standard deviation of the MFA distribution varied between 18° and 25°. The mean MFA and the mode of the MFA distribution were larger in radial than in tangential cell walls. MFA distributions of mature wood samples exhibited a separate small peak at around 90°. The average width and length of cellulose crystallites varied between 28.9–30.9 Å and 192–284 Å, respectively. Both increased slightly as a function of annual ring number from the pith up to the 15th annual ring. An irrigation–fertilisation treatment of some of the stems resulted in longer cellulose crystallites compared to the untreated stems.     

Gibberellin-induced formation of tension wood in angiosperm trees

After gibberellin had been applied to the vertical stems of four species of angiosperm trees for approximately 2 months, we observed eccentric radial growth that was due to the enhanced growth rings on the sides of stems to which gibberellin had been applied. Moreover, the application of gibberellin resulted in the formation of wood fibers in which the thickness of inner layers of cell walls was enhanced. These thickened inner layers of cell walls were unlignified or only slightly lignified. In addition, cellulose microfibrils on the innermost surface of these thickened inner layers of cell walls were oriented parallel or nearly parallel to the longitudinal axis of the fibers. Such thickened inner layers of cell walls had features similar to those of gelatinous layers in the wood fibers of tension wood, which are referred to as gelatinous fibers. Our anatomical and histochemical investigations indicate that the application of gibberellin can induce the formation of tension wood on vertical stems of angiosperm trees in the absence of gravitational stimulus.     

Mechanical properties of wood disproportionately increase with increasing density

? Premise of study: Prior work using a large data set has shown that the mechanical properties of wood disproportionately increase with increasing wood density across diverse species, e.g., stems composed of denser wood are stiffer and stronger than stems with equivalent cross-sections composed of less dense wood. However, an alternative approach, introducing the precondition of constant construction cost for the same data set, adduces that for any given construction cost, stems composed of lesser dense woods are stiffer and stronger then stems composed of denser woods. ? Methods: We evaluated these two approaches using generic allometric principles and the same large data set. ? Key results: This evaluation shows that construction costs cannot be constant over an entire ensemble of stems composed of different species of wood. For any specified construction cost (denoted by a k-value), only a particular subgroup of stems is addressed. The conclusions derived for this subgroup cannot be generalized to the entire ensemble of stems composed of different species of wood. ? Conclusion: Stems composed of denser wood are, on average as stiff and strong, or stiffer and stronger than stems with equivalent cross-sections composed of less dense wood. Denser wood may have a higher carbon construction cost, but its mechanical benefits likely outweigh the extra cost.