Poplar Cell Walls Research Articles

Preparation of multifunctional flame retardant composite wood by doping poplar cell walls with metal phytates

Preparation of multifunctional flame retardant composite wood by doping poplar cell walls with metal phytates

Insights into the poplar cell wall deconstruction following deep eutectic solvent pretreatment for enhanced enzymatic saccharification and lignin valorization

In this study, a combination of microcosmic and chemical analysis methods was used to investigate deep eutectic solvent (DES) pretreatment effects on cell wall's micromorphology and lignin's dissolution regular, in order to achieve high-performance biorefinery. The atomic force microscope observed that DES pretreatment peeled off non-cellulose components to reduced “anti-degradation barrier”, resulting to improve the enzymatic saccharification from 12.36 % to 90.56 %. In addition, DES pretreatment can break the β-O-4 bond between the lignin units resulting in a decline in molecular weight from 3187 g/mol to 1112 g/mol (0–6 h). However, long pretreatment time resulted regenerated lignin samples repolymerization. Finally, DES has good recoverability which showed saccharification still can reach 51.51 % at 6 h following four recycling rounds and regenerated lignin also had a typical and well-preserved structure. In general, this work offers important information for industrial biorefinery technologies and lignin valorization.

Pretreatment with ChOH/Urea under mild conditions for the efficient deconstruction of poplar cell wall

In this study, DES consisting of Choline hydroxide (ChOH) and Urea (Ur) was used to pretreat poplar under mild conditions (60 ℃). The 72 h saccharification efficiency of the pretreated samples for 10 min was significantly improved (69.97 %), which amounted to a 2.4-fold increase in sugar yield over the raw material (29.89 %). The remarkable enhancement could be correlated to the topochemical and morphological changes occurring in poplar cell walls, including the removal of lignin and hemicelluloses, decreased crystallinity and microstructural disruption. Noteworthy, X-ray diffraction analysis showed that cellulose I underwent a crystallographic transformation from cellulose I to cellulose II during DES pretreatment. Our study demonstrated that ChOH/Ur pretreatment was quite effective in deconstructing poplar cell walls and increasing the enzymatic hydrolysis efficiency of cellulose under mild conditions.

Chemical and Structural Responses to Downregulated p-Hydroxycinnamoyl-Coenzyme A: Quinate/Shikimate p-Hydroxycinnamoyltransferase in Poplar Cell Walls.

Unraveling the impact of lignin reduction on cell wall construction of poplar stems is important for accurate understanding the regulatory role of biosynthetic genes. However, few cell-level studies have been conducted on the changes in lignin, other important cell wall composition, and the structural properties of transgenic poplar stems at different developmental stages. In this work, the content and microdistributions of cell wall composition as well as the morphological characteristics of cells were studied for p-hydroxycinnamoyl-coenzyme A:quinate/shikimate p-hydroxycinnamoyltransferase (HCT) downregulated transgenic poplar 84K (Populus alba × P. glandulosa cl. ‘84k’) at different developmental stages. Results show that the lignin contents of the upper, middle, and basal parts of HCT transgenic poplar stems were significantly decreased by 10.84, 7.40, and 7.75%, respectively; and the cellulose contents increased by 8.20, 6.45, and 3.31%, respectively, compared with the control group. The cellulose/lignin ratio of HCT transgenic poplars was therefore increased, especially in the upper sections, where it was 23.2% higher. Raman results indicate the appearance of p-hydroxyphenyl units (H) and a decrease in the ratio of syringyl/guaiacyl (S/G) lignin monomers in fiber cell walls of HCT transgenic poplars. In addition, microstructure observations revealed that the fiber and vessel cells of the HCT transgenic poplars exhibited thin cell walls and large lumen diameters. Compared with the control group, the cell wall thickness of fiber and vessel cells decreased by 6.50 and 10.93% on average, respectively. There was a 13.6% decrease in the average ratio of the cell wall thickness to the lumen diameter and an increase in fiber length and width of 5.60 and 6.11%, respectively. In addition, downregulation of HCT did not change the orientation of cellulosic microfibrils, but it led to an 11.1% increase of the cellulose crystallinity in cell walls compared to the control poplars. The information obtained herein could lead to a better understanding of the effects of genetic modifications on wood cell walls.

The Study of Bound Water Status and Pore Size Distribution of Chinese Fir and Poplar Cell Wall by Low-Field NMR

With the increasing shortage of timber resources and the advancement of environmental protection projects, many artificial fast-growing forests are planted and used as raw materials in China. There are significant differences in the properties of natural forest wood and artificial fast-growing forest wood, and the properties of wood mainly depend on the change in the status of bound water in the cell wall. In this study, the fiber saturation point (FSP) and pore size distributions within the cell wall of six species of fast-growing forest wood were studied by low-temperature nuclear magnetic resonance (NMR) technology. The effects of species, growth rings, and extractives on the FSP and pore structure were analyzed. The water vapor sorption experiments were performed, and the adsorption isotherms of the samples were fitted through the Guggenheim-Anderson-de Boer (GAB) equation. According to the least-square regression of the adsorption isotherms and combined with the low-temperature NMR experiments, the content and proportion of the different types of bound water were analyzed. The results showed that the average FSP of each Chinese fir was about 40% and that of each poplar was about 35%. There is about a 10% difference between the FSP measured by NMR technology and the adsorption bound water content obtained by adsorption isothermal. The pore size distribution results show that in all samples, the proportion of pores larger than 10.5 nm is very low, about 10%; the proportion of 1.92-10.5 nm pores is about 30%; and the proportion of pores smaller than 1.92nm is more than 50%. This work will be helpful to the study of the wood moisture status and provide reference data for wood modification.

Effect of integrated treatment on improving the enzymatic digestibility of poplar and the structural features of isolated hemicelluloses

Herein, a two-step hydrothermal pretreatment combined with alkali extraction method was applied to deconstruct the poplar cell walls for enzymatic hydrolysis. Results revealed that 88.1 % of hemicelluloses and 77.6 % of lignin were removed during the integrated treatment performed at 180 °C and a maximum enzymatic hydrolysis efficiency of 96.1 % was achieved. Confocal Raman microscopy suggested that the removal of hemicelluloses from cell walls was inhomogeneous, and most hemicelluloses were released from the secondary wall. In addition, 35.2–56.8 % of hemicelluloses were isolated from the integrated treatment. Detailed structural analysis revealed that the water-soluble hemicelluloses possessed more branched structure than the alkali-soluble hemicelluloses and the hemicelluloses isolated from the poplar were mainly composed of a linear backbone of (1→4)-β-d-Xylp with 4-O-Me-α-d-GlcpA attached as side chains. This work provides an efficient pathway to transform poplar into fermentable sugars and hemicelluloses with considerable yield.

Study on the ultrastructure and properties of gelatinous layer in poplar

As a fast-growing commercial wood species, poplar often produces tension wood because of the leaning stems of logs, which has a great impact on the production and utilization. The gelatinous layer (G-layer) was the unique component of tension wood, almost entirely composed of cellulose, which was the main reason for the special performance of the poplar tension wood. This study used the transmission electron microscopy, Raman spectroscopy and nanoindentation to examine the ultrastructure and mechanical properties of every wall layer of the poplar cell wall in order to figure out the ultrastructure and properties of G-layer. In particular, the nanoindentation mapping was combined with manual screening to obtain the mechanical performance data of each layer. The fibers in the tension wood were gelatinous wood fibers with obvious G-layers, and the thickness was 2.53 μm, which was much larger than that of other wall layers. Compared with the wood fiber in the opposite wood, the S1 layer was thicker and the S2 layer was thinner, and the entire cell wall thickness was increased in tension wood. G-layer in tension wood had a very high cellulose content, and the Raman strength of lignin was weak. In general, the micromechanical properties of poplar tension wood were better, which was consistent with its macromechanical properties. The characteristics of G-layer, including high cellulose content, small microfibril angle and high crystallinity, made G-layer own better elastic modulus (14.08 GPa) and hardness (0.45 GPa) than other wall layers in the tension wood. Moreover, G-layer had a high proportion in the tension wood cell wall, so the average longitudinal elastic modulus and hardness of wood fiber cells in the tension wood were also higher than those in the opposite wood.

Time domain-NMR studies of average pore size of wood cell walls during drying and moisture adsorption

The change in the pore size of cell walls affects many physical properties of wood. In this paper, the dynamic changes in the pore size of wood cell walls during drying and moisture adsorption were studied at four relative humidities. The results showed that the average pore size of cell walls of Qingpi poplar (Populus- platyphylla var. glauca) was larger than that of pine wood (Pinus sylvestris var. mongolica Litv.) under the same experimental conditions, and the changes in pore size of Qingpi poplar cell walls were more sensitive to ambient humidity no matter whether during drying or moisture adsorption. Moreover, the average pore size of cell walls at saturated-water state is about 2.5 times of the average pore size of cell walls when woods reached the moisture absorption equilibrium from the oven-dry state. This article is useful for the wood manufacture industry and for wood modification. It is not only important for wood processing and utilization but also for the research on other porous materials.

Clean plasma modification for recycling waste plastic bags: From improving interfacial adhesion with wood towards fabricating formaldehyde-free plywood

It remains a great challenge to manufacture formaldehyde-free plywood, because of the wide application and cost advantage of formaldehyde-based adhesives. Herein, the waste plastic bag was recycled as an alternative to formaldehyde-based adhesives for manufacturing formaldehyde-free plywood. An industrial atmospheric dielectric-barrier-discharge plasma system was used to treat the plastic bag, in order to enhance the interfacial adhesion of the resultant plywood. The results suggested that plasma modification would excite oxygen and nitrogen into energetic particles and further ruptured the raw chemical bonds of plastic bag. Oxygen- and nitrogen-containing functional groups can be grafted, thus enhancing plastic bags’ surface polarity and wettability. As further supported by nanoindentation, the adhesion energy of plasma-modified plastic bag and poplar cell wall was elevated by 19.13% when compared to the unmodified sample, thus suggesting the improved interfacial adhesion and interfacial compatibility. Benefiting from that, formaldehyde-free plywood with a high bonding strength (0.82 MPa) was successfully fabricated. Shear strain in plywood distributed uniformly after plasma modification, suggesting the even distribution of adhesive and the enhanced bearing-load capacity of interface. This study paves a new way to effectively recycle plastic waste as an alternative to traditional adhesives for formaldehyde-free plywood manufacture.

Structural Variations of Lignin Macromolecules from Early Growth Stages of Poplar Cell Walls

The natural lignin macromolecule is structurally complex and heterogeneous polymers. Herein, to comprehend the dynamic structural characteristics of poplar (Populus tomentosa) lignin during the gro...

Exploration of effect of delignification on the mesopore structure in poplar cell wall by nitrogen absorption method

As an important commodity wood material, poplar has porous structure, which is worthy of being investigated. Especially, the lignin concentration is closely related to the formation and change of pores. Although a few studies reported the relation between the pore volume and the lignin removal of the ground wood (Stone and Scallan in J Polym Sci C Polym Symp 11(1):13–25, 1965), no information of the distribution of pore sizes versus the whole delignification process steps was reported. This study initially explored the effect of delignification level on the pore size distribution and pore structure in poplar cell walls using nitrogen absorption measurement. It was found that delignification increased the N2 adsorption amount and specific surface area of poplar. It also caused a large number of mesopores in the cell wall, mainly in the 2–10 nm pore size range, and decreased the average pore size. The whole delignification process can be roughly divided into three major phases, namely, the initial phase, the transitional phase and the stable phase, in which, the change process was not completely uniform. During the delignification, the size of poplar cell wall pores continued reducing, and finally a large number of uniform pores of about 2.1 nm was formed.

Delignification kinetics and selectivity in poplar cell wall with acidified sodium chlorite

Lignin is the major components limiting enzymatic hydrolysis and normally removed by acidified sodium chlorite in laboratory. However, the information on delignification kinetics and selectivity at cellular level as the reaction proceeds is limited. In this work, the dynamic removal of lignin and its monolignol in different cell types of poplar were visualized in real time by confocal Raman microscopy. At the initial stage, substantial reduction in lignin concentration was observed especially in cell corner and compound middle lamella of fiber, but the removal rate slowed down in the following process. Delignification was highly cell-selective that the vessel was the most resistant to delignification followed by ray and then by fiber. Besides, the removal behavior of lignin monomers in ray varied significantly with vessel and fiber. In the whole process, the syringyl units was more prone to being removed than guaiacyl units. This information could lead to a better understanding of the delignification process of lignocellulosic biomass by chlorite treatment.

Unraveling the impact of Pto4CL1 regulation on the cell wall components and wood properties of perennial transgenic Populus tomentosa

Cell wall components and structure impact the physical and mechanical properties of plants, thereby affecting wood applications. Lignin is the most abundant biopolymer after cellulose in the wood cell wall and can be modified by certain lignin biosynthesis enzymes. 4-Coumarate: coenzyme A ligase(4CL) is an important lignin biosynthesis enzyme. To demonstrate the impact of the regulation of Pto4CL1 from poplar on wood properties, we analyzed the composition and anatomy of 5-year-old Pto4CL1-modified poplar cell walls, assessing the density, strength, volume shrinkage, and impact toughness of the transgenic trees. These results showed that the up-regulation of Pto4CL1 increased the lignin content to 46.65% from 33.11% in the control plants, while hydrophilic polysaccharides such as cellulose, hemi-cellulose, and pectin decreased. In contrast, the down-regulation of Pto4CL1 resulted in a reduction in lignin content to 27.39%, and the content of cellulose and hemi-cellulose showed compensatory variation. Raman spectroscopy showed that the change in lignin in the transgenic events was embodied in the deposition and concentration of lignin in the secondary cell wall. Moreover, the increased lignin content caused significantly increased wood strength and slightly increased wood density. In contrast, a reduction in lignin content resulted in a significant decrease in wood strength and a slight decrease in wood density. However, the Pto4CL1-modified trees had similar stiffness to the control group. We also found a significant decrease in volume shrinkage and increase in impact toughness in the low-lignin events. These results indicate that Pto4CL1 regulation alters the chemical composition of plant cell walls and these changes affect the physical and mechanical properties of the wood.

In situ polymerization of 2-hydroxyethyl methacrylate (HEMA) and 3-(methacryloxy)propyltrimethoxysilane (MAPTES) in poplar cell wall to enhance its dimensional stability

Abstract Poplar wood samples (2×2×2 cm3) were vacuum/pressure impregnated in alcoholic solution with 2-hydroxyethyl methacrylate (HEMA) and 3-(methacryloxy)propyltrimethoxysilane (MAPTES) in a mass ratio of 3/1 in the presence of catalytic amounts of azobisisobutyronitrile (AIBN). Because of their good solubility and permeability, the HEMA/MAPTES precursors evenly penetrate the poplar cell wall. The impregnated samples were heated at 75°C for 8 h, followed by a heating period at 103±2°C for 8 h, in the course of which an in situ polymerization occurred in the cell wall. The modified wood was characterized by Fourier transform infrared (FTIR) and Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Moreover, the dimensional stability of the modified wood was measured and found to be considerably improved.

Dynamical assessment of fluorescent probes mobility in poplar cell walls reveals nanopores govern saccharification

BackgroundImproving lignocellulolytic enzymes’ diffusion and accessibility to their substrate in the plant cell walls is recognised as a critical issue for optimising saccharification. Although many chemical features are considered as detrimental to saccharification, enzymes’ dynamics within the cell walls remains poorly explored and understood. To address this issue, poplar fragments were submitted to hot water and ionic liquid pretreatments selected for their contrasted effects on both the structure and composition of lignocellulose. In addition to chemical composition and porosity analyses, the diffusion of polyethylene glycol probes of different sizes was measured at three different time points during the saccharification.ResultsProbes’ diffusion was mainly affected by probes size and pretreatments but only slightly by saccharification time. This means that, despite the removal of polysaccharides during saccharification, diffusion of probes was not improved since they became hindered by changes in lignin conformation, whose relative amount increased over time. Porosity measurements showed that probes’ diffusion was highly correlated with the amount of pores having a diameter at least five times the size of the probes. Testing the relationship with saccharification demonstrated that accessibility of 1.3–1.7-nm radius probes measured by FRAP on non-hydrolysed samples was highly correlated with poplar digestibility together with the measurement of initial porosity on the range 5–20 nm.ConclusionMobility measurements performed before hydrolysis can serve to explain and even predict saccharification with accuracy. The discrepancy observed between probes’ size and pores’ diameters to explain accessibility is likely due to biomass features such as lignin content and composition that prevent probes’ diffusion through non-specific interactions probably leading to pores’ entanglements.

Synergetic Dissolution of Branched Xylan and Lignin Opens the Way for Enzymatic Hydrolysis of Poplar Cell Wall

As the main hemicellulose of poplar, the interaction of xylan with lignin was expected to have profound effect on biomass recalcitrance. In this paper, the dynamic changes of xylan and lignin in poplar cell wall during a mild pretreatment using γ-valerolactone (GVL) was investigated using chemical and microscopic techniques. Synergetic dissolution of branched xylan and lignin from the secondary wall of the fiber cell was found to play a major role in opening the cell wall structure for enzymatic attack. In the case of the removal of xylan and lignin reaching a certain level, β-O-4' cleavage of lignin which destroyed its interaction with hydrophobic cellulose face was found to make great contribution to the enhanced enzymatic hydrolysis. The deep understanding of this process could lead to a new insight into the understanding of the plant cell wall architecture and provide basic information for biomass processing.

Obtaining pure spectra of hemicellulose and cellulose from poplar cell wall Raman imaging data

A comprehensive understanding of the structural and chemical nature of plant cell walls is important from both the perspectives of plant biotechnology and of commercial utilization. The Raman imaging technique is a preferred solution for its ability to offer spatial and spectral information simultaneously. However, the exact spectra of hemicellulose and cellulose are difficult to directly discern from each other due to the strong spectral overlap. In the present study, we report for the first time that the spectra of hemicellulose and cellulose in poplar cell wall Raman imaging can be discriminated by using multivariate analysis. The semi-quantitative concentrations of polysaccharides are identified based on the corresponding Raman images. Results show that cellulose is mostly concentrated in the secondary wall of poplar fibres, whilst the distribution of hemicellulose is almost uniform throughout the cell wall of fibres except for a higher concentration found in the S1 and the outer S2 layer. The xylem ray and the vessel have relatively high hemicellulose concentrations which is comparable to the outer S2 layer of fibres, but the cellulose concentration is relatively low in these two cell types. This work not only contributes to broadening our knowledge about the distribution of polysaccharides in plant cell walls, but also provides a potential strategy to trace their dynamic changes during the bioconversion at a micro-level.

Exploring accessibility of pretreated poplar cell walls by measuring dynamics of fluorescent probes

BackgroundThe lignocellulosic cell wall network is resistant to enzymatic degradation due to the complex chemical and structural features. Pretreatments are thus commonly used to overcome natural recalcitrance of lignocellulose. Characterization of their impact on architecture requires combinatory approaches. However, the accessibility of the lignocellulosic cell walls still needs further insights to provide relevant information.ResultsPoplar specimens were pretreated using different conditions. Chemical, spectral, microscopic and immunolabeling analysis revealed that poplar cell walls were more altered by sodium chlorite-acetic acid and hydrothermal pretreatments but weakly modified by soaking in aqueous ammonium. In order to evaluate the accessibility of the pretreated poplar samples, two fluorescent probes (rhodamine B-isothiocyanate–dextrans of 20 and 70 kDa) were selected, and their mobility was measured by using the fluorescence recovery after photobleaching (FRAP) technique in a full factorial experiment. The mobility of the probes was dependent on the pretreatment type, the cell wall localization (secondary cell wall and cell corner middle lamella) and the probe size. Overall, combinatory analysis of pretreated poplar samples showed that even the partial removal of hemicellulose contributed to facilitate the accessibility to the fluorescent probes. On the contrary, nearly complete removal of lignin was detrimental to accessibility due to the possible cellulose–hemicellulose collapse.ConclusionsEvaluation of plant cell wall accessibility through FRAP measurement brings further insights into the impact of physicochemical pretreatments on lignocellulosic samples in combination with chemical and histochemical analysis. This technique thus represents a relevant approach to better understand the effect of pretreatments on lignocellulose architecture, while considering different limitations as non-specific interactions and enzyme efficiency.

How Does Hemicelluloses Removal Alter Plant Cell Wall Nanoscale Architecture and Correlate with Enzymatic Digestibility?

Thorough understanding of how hemicelluloses removal influences cell wall nanoscale architecture and cellulose digestion is of crucial importance for enabling low-cost industrial conversion of lignocellulosic biomass to renewable biofuels. In this work, delignified poplar cell walls, after various degrees of hemicelluloses removal, were characterized by Fourier transform infrared imaging spectroscopy and atomic force microscopy to evaluate enhancement in cell wall digestibility. There was a gradual decrease in hemicelluloses content with dilute alkali treatment, which resulted in alterations in the nanoscale architecture and crystallinity of cell walls. Removal of hemicelluloses did not disrupt the integrity of microfibrils but resulted in exposure of microfibrils and a decrease in the diameter of microfibrils. X-ray analysis indicated that the increase in crystallinity beyond natural variations in the crystallinity of cellulose was mainly attributable to removal of hemicelluloses. In conclusion, alterations in the architecture and crystallinity of cell walls facilitated enzymatic digestion of delignified poplar, enhancing cellulose conversion from 68.24 to 75.16 %.

In situ micro-spectroscopic investigation of lignin in poplar cell walls pretreated by maleic acid.

BackgroundIn higher plant cells, lignin provides necessary physical support for plant growth and resistance to attack by microorganisms. For the same reason, lignin is considered to be a major impediment to the process of deconstructing biomass to simple sugars by hydrolytic enzymes. The in situ variation of lignin in plant cell walls is important for better understanding of the roles lignin play in biomass recalcitrance.ResultsA micro-spectroscopic approach combining stimulated Raman scattering microscopy and fluorescence lifetime imaging microscopy was employed to probe the physiochemical structure of lignin in poplar tracheid cell walls. Two forms of lignins were identified: loosely packed lignin, which had a long (4 ns) fluorescence lifetime and existed primarily in the secondary wall layers; and dense lignin, which had a short (0.5–1 ns) fluorescence lifetime and was present in all wall layers, including the cell corners, compound middle lamellae, and secondary wall. At low maleic acid concentration (0.025 and 0.05 M) pretreatment conditions, some of the dense lignin was modified to become more loosely packed. High acid concentration removed both dense and loosely packed lignins. These modified lignins reformed to make lignin–carbohydrate complex droplets containing either dense or loosely packed lignin (mostly from secondary walls) and were commonly observed on the cell wall surface.ConclusionsWe have identified dense and loosely packed lignins in plant cell walls. During maleic acid pretreatment, both dense lignin droplets and loosely packed lignin droplets were formed. Maleic acid pretreatment more effectively removes loosely packed lignin in secondary walls which increases enzyme accessibility for digestion.