Elsevier

Carbohydrate Polymers

Volume 238, 15 June 2020, 116212
Carbohydrate Polymers

Unmasking the heterogeneity of carbohydrates in heartwood, sapwood, and bark of Eucalyptus

https://doi.org/10.1016/j.carbpol.2020.116212Get rights and content

Highlights

Abstract

In this study, the cellulose and hemicelluloses in heartwood, sapwood, and bark of E. urophylla × E. grandis were comprehensively investigated. The ultrastructural topochemistry of carbohydrates in cell walls was examined in situ by confocal Raman microscopy. Cellulose and alkali-extractable hemicelluloses samples were isolated from different tissues and comparatively characterized by compositional carbohydrate analyses, determination of molecular weights, FT-IR spectroscopy, and XRD and NMR techniques. It was found that among all of the samples, heartwood cellulose had the highest molecular weight as well as the lowest degree of crystallinity. Meanwhile the hemicelluloses in heartwood had higher xylose content, lower degree of branching, slightly lower molecular weights but narrower polydispersity than those in sapwood. The eucalyptus hemicelluloses mainly consisted of (1→4)-β-D-xylan backbone with glucuronic acid side chains. Furthermore, the hemicelluloses isolated from sapwood had a higher degree of substitution with terminal galactose than those isolated from heartwood and bark.

Introduction

Lignocellulosic biomass is an important sustainable and renewable energy resource due to its high abundance and potential for production of biofuels, biomaterials, and renewable chemicals (Isikgor & Becer, 2015). The physicochemical structural and compositional factors of lignocellulose give rise to biomass recalcitrance and result in a relatively low efficiency of lignocellulose bioconversion, motivating the development and use of various pretreatment methods to improve the deconstruction and digestibility of lignocellulosic biomass (Himmel et al., 2007; Sun, Sun, Cao, & Sun, 2016). The primary constituents of lignocelluloses are cellulose, hemicelluloses and lignin that comprise the plant cell walls. Optimizing the utilization of these three components is the core issue for maximizing the value of lignocellulosic biomass (Alonso et al., 2017; Zhang, Wang, Feng, & Pan, 2020). Researches on the accurate analysis of cellulose, hemicelluloses and lignin will enhance the understanding of biomass recalcitrance, and provide theoretical guidance for the further development of industrial biorefining processes.

Cellulose and hemicelluloses are two important carbohydrate polymers in lignocellulosic biomass. Cellulose is a linear polysaccharide consisting of β-1,4-glucan chains that tightly aggregate into microfibrils held together by hydrogen bonds and van der Waals interactions (Chundawat, Beckham, Himmel, & Dale, 2011). Because of the strong intra- and intermolecular hydrogen bonds that arise due to the large number of hydroxyl groups present along the cellulose backbone, a fraction of cellulose chains exist as compact crystalline structures (Sun et al., 2016). Unlike cellulose, hemicelluloses are heteropolysaccharides containing several sugars and uronic acids, and their fine structure varies widely among the different plant species and tissue types. Hemicelluloses can be structurally classified into four classes, including xyloglucans, xylans, mannans, and β-glucans with mixed linkages (Chundawat et al., 2011; Scheller & Ulvskov, 2010). In current biorefinery process, cellulose is regarded as the main processing target, which can be extensively utilized in papermaking, biofuel production, nanomaterials, films, food packaging, biomedical applications, and three-dimensional printing (Dai et al., 2019; Klemm et al., 2018). Despite the utilization of hemicelluloses is hindered by their complicated chemical compositions and structures to some extent, they are still used in biofuels, chemicals (such as furfural, levulinic acid, and xylitol), biopolymers, and pharmaceuticals (Peng, Bian et al., 2012; Peng, Peng, Xu, & Sun, 2012). To optimize the separation processing of cellulose and hemicelluloses and develop high-value products, it is necessary to study their chemical structures and physicochemical properties in depth.

The composition of plant cell walls is tightly controlled in different cell types and in relation to the growth and development. However, the present understanding of the cell walls is far from complete (Somerville et al., 2004). Considering the importance of tuning the suitable process conditions for the respective wood fractions in biorefineries, several previous studies have reported the chemical compositions and particularly the polysaccharides contents of the heartwood and sapwood in different tree species (Bertaud & Holmbom, 2004; Çetinkol et al., 2012; Johansson et al., 2015; Willför, Sundberg, Pranovich, & Holmbom, 2005). Accounting for 5–28 % of trees, bark is considered to be a promising biomass source for biorefineries due to its availability (Pásztory, Mohácsiné, Gorbacheva, & Börcsök, 2016). Several research studies have analyzed the carbohydrates composition in bark, indicating that bark is a potential feedstock for valorization (Lima, Miranda, Knapic, Quilhó, & Pereira, 2018; Miranda, Gominho, Mirra, & Pereira, 2013; Neiva, Araújo, Gominho, de Cássia Carneiro, & Pereira, 2018; Sartori et al., 2016). However, little work has been done for the comprehensive elucidation of the structural differences between the polysaccharides in heartwood, sapwood, and bark based on the properties such as the crystallinity and molecular weights of cellulose and the composition of hemicelluloses. All of these features of cellulose and hemicelluloses will affect their further application. Therefore, it is meaningful to explicitly improve our understanding of cellulose and hemicelluloses in different tissues.

The hybrid Eucalyptus urophylla × E. grandis is one of the most widely planted and utilized eucalypt species in southern China. The fast tree growth, excellent stem form, and reasonable wood density make it a valuable fast-growing wood specie (Chen et al., 2010; Xie et al., 2017). In our previous study, the distribution and structural features of lignin in heartwood, sapwood, and bark of E. urophylla × E. grandis have been reported (Xiao et al., 2019). The present work represents an extension of the previous work, and investigates the cellulose and hemicelluloses in different eucalyptus tissues. The cellulose and alkali-extracted hemicelluloses samples isolated from heartwood, sapwood, and bark were systematically characterized and compared, and the distribution of cellulose and hemicelluloses in cell walls of different wood fractions were monitored by confocal Raman microscopy (CRM). The results obtained in this study are expected to improve the understanding of the cell walls heterogeneity and be useful for the design of strategies for the improved utilization of the cellulose and hemicelluloses in E. urophylla × E. grandis.

Section snippets

Materials

The 4-year-old plantation-grown E. urophylla × E. grandis tree was harvested from Guangxi Province, China. The procedures for the preprocessing and preparation of dewaxed samples from heartwood, sapwood, and bark fractions were the same as in previous work (Xiao et al., 2019). The details on experimental process as well as the total structural carbohydrates and lignin contents of each sample are present in the Supplementary material. All reagents were of analytical grade and used without

Component analysis of eucalyptus and the fractional yield

The main chemical components of E. urophylla × E. grandis in different tissues were described in our previous study (Xiao et al., 2019). As shown in Table S1, it is clear that the contents of carbohydrate polymers vary among heartwood, sapwood, and bark. Based on the current knowledge, the relatively low total carbohydrates content in heartwood is mainly due to the deposition of lignin and extractives during heartwood formation. The cellulose content of sapwoods is generally higher than that of

Conclusions

Based on the CRM spectra, it was concluded that the carbohydrates concentration was ranked as: S > CML > CCML, while the distribution of xylan was relatively uniform in cell walls. Related to the different maturation states of the tissues, the cellulose in heartwood had higher viscosity average DP and molecular weight but lower degree of crystallinity than that in sapwood. All the alkali-extracted hemicelluloses from eucalyptus were formed by a linear backbone of (1→4)-β-D-Xylp substituted with

CRediT authorship contribution statement

Ming-Zhao Xiao: Conceptualization, Methodology, Investigation, Writing - original draft. Wei-Jing Chen: Investigation, Data curation. Xue-Fei Cao: Formal analysis, Writing - review & editing. Yue-Ying Chen: Software, Resources. Bao-Chen Zhao: Investigation, Visualization. Zhi-Hua Jiang: Validation, Visualization. Tong-Qi Yuan: Supervision. Run-Cang Sun: Project administration.

Acknowledgments

The authors are extremely grateful for financial support from the National Key R&D Program of China (2017YFB0307903), the National Natural Science Foundation of China (31971613), the Fundamental Research Funds for the Central Universities (2015ZCQ-CL-02), and Beijing Forestry University Outstanding Young Talent Cultivation Project (2019JQ03005).

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