Abstract
The bark of Pteroceltis tatarinowii is one of the main raw materials for the manufacturing of Xuan paper. In order to guide the production of Xuan paper, the anatomical properties and chemical compositions of the bark and branch wood from P. tatarinowii at different years of age were analyzed in this study. The results from a variation analysis of the anatomical properties indicated that the ratio of the length to width of the bark and branch wood was greater than 30, while the ratio of the lumen diameter to the wall thickness was less than 1. Furthermore, there were significant differences in the length and the ratio of length to width of the bark and branch wood. The results from a variation analysis of the chemical composition indicated that the lignin content of branch wood at various ages (years) was greater than the lignin content of bark. Additionally, the cellulose extractive and pentosan contents of the branch wood was less than the contents in the bark. Based on the analysis of the anatomical and chemical composition, barks that were 2 to 3 years old were the most suitable raw materials for the manufacturing of Xuan paper.
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Variation Analysis of the Difference in Anatomical Properties and Chemical Composition of the Bark and Branch Wood of Pteroceltis tatarinowii at Different Annual Rings
Ying Guan,a,b Hui Gao,a,b,* Hao Shen,a Wenqi Li,a Yule Wu,a and Ran Yang a
The bark of Pteroceltis tatarinowii is one of the main raw materials for the manufacturing of Xuan paper. In order to guide the production of Xuan paper, the anatomical properties and chemical compositions of the bark and branch wood from P. tatarinowii at different years of age were analyzed in this study. The results from a variation analysis of the anatomical properties indicated that the ratio of the length to width of the bark and branch wood was greater than 30, while the ratio of the lumen diameter to the wall thickness was less than 1. Furthermore, there were significant differences in the length and the ratio of length to width of the bark and branch wood. The results from a variation analysis of the chemical composition indicated that the lignin content of branch wood at various ages (years) was greater than the lignin content of bark. Additionally, the cellulose extractive and pentosan contents of the branch wood was less than the contents in the bark. Based on the analysis of the anatomical and chemical composition, barks that were 2 to 3 years old were the most suitable raw materials for the manufacturing of Xuan paper.
Keywords: Pteroceltis tatarinowii; Bark; Branch wood; Anatomical properties; Chemical compositions; Variation analysis
Contact information: a: School of Forestry and Landscape Architecture, Anhui Agricultural University, Hefei 230036 China; b: Wood Quality Improvement and High Efficient Utilization, Key Lab of State Forest and Grassland Administration, Hefei 230036 China; *Corresponding author: huigaozh@163.com.
INTRODUCTION
The Chinese paper industry has grown rapidly since the 1990s, and China is now at the top of the world in terms of the production and consumption of paper and cardboard (Ma et al. 2015). Over the past decades, in order to meet the growing demands for raw materials, considerable efforts have been made to improve the quality of trees, particularly in P. tatarinowii (Fang et al. 2004, 2006; Gao et al. 2007). The utilization of P. tatarinowii by the pulp industry has developed during recent years, and its usage will continue to increase in the future. The reason for this phenomenon is that the bark of P. tatarinowii is the main raw material in the manufacturing of traditional Xuan paper. Xuan paper is one of the most famous traditional inventions in China (Fang et al. 2004). Furthermore, as one of the Four Treasures. It is called the “King of Papers”, due to its thin, dense, durable, non-absorbent, and tough characteristics, as well as its pure white appearance (Gao et al. 2016). In addition, Xuan paper is made via an 18-step process and it is said to maintain its texture for over 1000 y. As the primary raw material of Xuan paper, P. tatarinowii has a long history and a special purpose in Chinese culture.
In the past few decades, scholars (both domestic and overseas) have become interested in handmade Xuan paper, which is peculiar to China. This interest is for the purpose of conservation (Shi and Tao 2013; Wang et al. 2014; Brown et al. 2017), dating (He et al. 2010; Helman-Ważny 2016) or assessment of authenticity (Li et al. 2017), and the exploration of applicable and reliable new, analytical methods (Yang et al. 2011).
P. tatarinowii is the only deciduous leaf arbor of the Ulmaceae, which is one of the tertiary residual plants in China. It is an endangered tertiary relic tree endemic to China with a high ecological and economic value (Fang et al. 2004, 2006). As such, P. tatarinowii has been designated as a national third class protected plant in China. It is distributed sporadically in Anhui, Shandong, Henan, Hubei, and 19 other provinces in China (Li et al. 2015). Due to the widespread vegetation destruction and excessive utilization of resources, the P. tatarinowii was decreased rapidly in recent years (Zhang and Liu 2019). Presently, there are multiple studies on the seed source selection, chemical components, productivity and bark quality, and rheological models of Xuan paper (Zuo et al. 1998; Wang et al. 2000; Fang et al. 2001, 2004; Gao et al. 2007; Gao et al. 2016). Since the properties of wood are affected by the environment, there are drastic variations in the structure and function of the wood. The same phenomenon is also present in the bark and branch wood. Therefore, the anatomical properties and chemical components of the bark and branch wood should be studied. Liu and Hu (1985) and Liu and Qu (1986) reported that the bark quality has a direct influence on the quality of the Xuan paper. The toughness of the P. tatarinowii branch wood has the advantages of a thin cell wall, which leads to high performance in terms of adsorbed ink (Chai et al. 2010). The chemical components of the raw material have an influence on the process, economic benefit, and pollution of the chemical pulping. At this point, a study on the variation of wood chemical components could provide a theoretical basis for the early selection and prediction of the wood properties of pulp forests and for reasonable rotation periods.
Based upon previous literature, multiple studies were focused on the anatomical properties and chemical components of P. tatarinowii that were 2 years to 3 years old (Gao et al. 2007; Li et al. 2012). Thus, in this paper, the anatomical properties and chemical components of the bark and branch wood from P. tatarinowii with an age range of 2 to 6 were studied. Moreover, the variation features in the axial direction were identified, in order to provide a basis for the utilization of P. tatarinowii, as well as to increase the value derived from the bark and branch wood. Therefore, this study could be a theoretical foundation for the high value utilization of P. tatarinowii.
EXPERIMENTAL
Materials
The P. tatarinowii samples, with an age range of 2 years to 6 years were collected on 27th July 2015 from Jing county, Anhui province, China. The collection site was located at approximately 117°57′ to 118°41′E longitude and 30°21′ to 30°50′N latitude, which is a humid subtropical monsoon climate zone. The area has a climate with an annual average temperature of approximately 15 °C, a mean annual precipitation of 1300 to 1600 mm, and an annual evaporation of 1400 to 1600 mm during the growing seasons. The frost-free period is approximately 230 d every year, and the average relative humidity is 79%. The total solar radiation is 484.68 kJ/cm2 to 501.48 kJ/cm2, the sunshine time is greater than 2000 h, and the relative sunshine duration is 42% to 49% (Fang et al. 2002). The collected samples were grown in the limestone highland and in calcareous soils.
Bark Preparation
Different aged branch wood of P. tatarinowii were used in this study. The P. tatarinowii branch was cut and then placed in water for two days. Then the bark and the wood were easily separated from the branch (as shown in Fig. 1).
Fig. 1. Schematic diagram of sampling
Wood Anatomical Properties
Measurement of the fiber length
The P. tatarinowii bark and branch wood were cut into small pieces. The samples were placed in the test tube with acetic acid and hydrogen peroxide (at a 1 to 1 ratio by v/v) for segregation. Then, the test tube was plugged and placed in an oven at 80 °C for 8 h. Finally, the segregated fibers were prepared for the slices and observed under an electron microscope (DS RI-2, Nikon, Tokyo, Japan) to measure the length of the fibers. Approximately 500 fibers from different aged barks and branch wood were measured under a 100-fold microscope.
Preparation of the samples
The slices samples of the bark and branch wood were prepared according to the previous method with a minor modification (Barbosa et al. 2010). In brief, the samples were softened with a hot-cold cycle and then embedded in polyethylene glycol. After drying, the sample was dehydrated with 50%, 75%, 85%, 95%, or 100% alcohol in advance. Next, the samples were transparent treated and sealed; xylene was used for the transparent treatment of dehydrated slices, in order to make the slices clean and transparent. Afterwards, a neutral gum solution was added to the slices. Finally, the slices were placed in an oven at 40 °C. The fiber length, width, lumen diameter, and wall thickness of the bark and branch wood of P. tatarinowii were measured via an electron microscope (Nikon DS RI-2). The morphologies of samples were investigated by field emission scanning electron microscopy using a Hitachi S-3400N II (Hitachi, Japan) instrument at 15 kV. Before SEM observation. Transmission electron microscopy (TEM) images were obtained using transmission electron microscope (TEM, HT7700, Hitachi, Japan) under an acceleration voltage.
Wood Chemical Composition
Alcohol-benzene extractives
Standard GB/T 2677.6 (1994) was referenced to measure the content of the extractive, and the dried P. tatarinowii specimens were ground into 40 to 60 mesh particles for further experimentation. The P. tatarinowii powder was extracted with ethanol/toluene (at a 1 to 2 ratio by v/v) in a Soxhlet apparatus for 6 h and dried in a cabinet oven with air circulation at 60 °C for 16 h.
Holocellulose extraction
Standard GB/T2677.10 (1995) was referenced to extract the holocellulose from the P. tatarinowii samples. First, 2 g of the extracted sample, 65 mL of distilled water, 0.5 mL of glacial acetic acid, and 0.75 g of sodium chlorite (NaClO2) were mixed and heated in a 75 °C water bath for 1 h. Then, 0.5 mL of glacial acetic acid and 0.75 g of sodium chlorite (NaClO2) were continuously added to the reaction. This procedure was repeated three times until the sample became white. Finally, the sample was washed with distilled water to remove the acid, and then washed with acetone three times. The sample was dried at 105 °C overnight to obtain the holocellulose content.
Alpha-cellulose extraction
Using standard GB/T2677.10 (1995), the alpha cellulose content of the samples was obtained. The hemicellulose content was removed using sodium hydroxide. Then, 2 g of holocellulose was transferred to a small breaker, 30 mL of 17.5% NaOH was added, and the sample was mercerize-treated for 45 min in a 20 °C water bath. Afterwards, 30 mL of distilled water was added, and the sample was stirred for 1 to 2 min. The reaction solution was carefully removed into a pre-weighed glass filter. All retentate was filtered through the pre-weighed glass filter by washing it with 9.5% NaOH (3 mL x 25 mL). After washing with distilled water (400 mL), the solution was neutralized via soaking in 2 mol/L of acetic acid for 5 min. Finally, the sample was dried at 105 °C overnight to obtain the alpha cellulose content.
Acid-insoluble lignin extraction and acid-soluble lignin extraction
Using standard GB/T10337 (1989) and standard GB/T2677.8 (1994) the acid-insoluble and acid-soluble lignin content of the samples were obtained, respectively. The extracted sample (1 g) and 15 mL of 72% H2SO4 (10 °C to 15 °C) were added to a 250 mL Erlenmeyer flask. The mixture was heated in a 20 °C water bath for 2 h and was shaken during the water bath. After that, the reaction was transferred to a 1000 mL Erlenmeyer flask and then diluted with water until the concentration of the acid was 3% (approximately 560 mL of distilled water). The Erlenmeyer flask was placed on an electric furnace for 4 h and the total volume of the solution was maintained at 575 mL via the addition of hot water. Then, the sample was filtered and washed until a neutral pH was reached, in order to obtain the acid-insoluble lignin content of the samples.
The filtrate of the acid-insoluble lignin contained a small amount of acid-soluble lignin, which showed characteristic absorption in the ultraviolet region, and its absorption intensity was related to the content of the acid-soluble lignin. The content of acid-soluble lignin was obtained via measuring the absorbance of the filtrate at 205 nm.
Pentosan extraction
Using standard GB/T2677.9 (1994), the pentosan content of the samples was obtained. First, 0.5 g of the P. tatarinowii sample, 10 g of NaCl, and 100 mL of 12% HCl were mixed in a round bottom flask. The temperature of the furnace was adjusted to keep the distillation rate at 30 mL of distillate per 10 min. Next, 30 mL of 12% HCl was added after 30 mL of the distillate was distilled off. This procedure was repeated until the total volume of the distillate reached 360 mL. The distillate was transferred to a volumetric flask and fixed to 500 mL with 12% HCl. Then, 200 mL of the distillate, 150 g of crushed ice, and 25 mL of a mixed solution of potassium bromide and potassium bromate were added to an iodine flask. The mixtures were placed in the dark for 5 min. Finally, 10 mL of a 10% potassium iodide solution was added to the mixture and allowed to stand for 5 min. The pentosan content was calculated via the titration of 0.1 mol/L sodium thiosulfate.
Data Analysis
SPSS 20.0 software IBM (USA) and Microsoft Excel 2010 were used to perform statistical analysis, i.e., T-test, variance, and experimental data. In the analysis of variance, it was assumed that the treatment variances were equal. That was known as homogeneity of variance (HOV). As one of the most commonly used was HOV test, Levene ‘s test was selected for testing whether the variance was equal. The output in the difference T-test for equality of means by SPSS included two rows: equal variances assumed and equal variances not assumed. Levene’s test indicated that the variances were equal (i.e., p-value large), the first row of output was reliable in the results from the actual independent samples T-test. If Levene’s test indicated that the variances were not equal, the second row of output was reliable. The difference between these two rows of output above rely on the way the T-test statistic was calculated.
RESULTS AND DISCUSSION
Analysis of the Anatomical Properties of the Bark and Branch wood of P. tatarinowii at Different Years of Age
The anatomical properties of the bark and branch wood of P. tatarinowii at different years of age are listed in Table 1 and Fig. 2. As depicted in Table 1 and Fig. 2a, the fiber lengths of the bark and branch wood at 3 years were longer than the rest, while the largest ratio of length to width from the bark was also the longest at 3 years from 1 to 6 years. Generally, the fiber length determines the most suitable raw material for the paper industry. The paper properties, such as the tensile strength, optical property, and surface quality are affected by the fiber length (Zhang et al. 2006; Gao et al. 2007). The contact site and binding force of the pulp fibers were directly influenced by the fiber length. In recent work by Gao et al. (2007), it was shown that the quality of Xuan paper can be directly influenced by the fiber length. The fiber aspect ratio is one of the most important factors that affects the quality of the paper. As the fiber aspect ratio is increased, the probability of hydrogen bonds occurring between the fibers is also increased, and the mechanical properties of the paper would be increased. In general, larger fiber lengths and narrower fiber widths (with a length to width ratio greater than 30) were the first choice for pulp and paper production; this led to higher density paper, smoother paper, more uniform formation, etc. (Ma et al. 2015). All the data presented in Table 1 had a ratio of length to width (for both bark and branch wood) greater than 30, which met the prerequisites for paper-making raw materials.
The fiber wall thickness, lumen diameter, and ratio of lumen diameter to wall thickness are the standards for measuring the merit of a papermaking material. Usually, the thinner the cell wall and the larger the lumen diameter, the easier it is for the fiber to collapse during the papermaking process. This leads to the binding ability of the fibers being drastically enhanced. For wood fibers, the ratio of lumen diameter to wall thickness is less than 1, which is classified as the finest papermaking material (Niu et al. 2010). Based on the anatomical analysis (Table 1), it was found that 2- to 3-year-old bark was the most suitable raw materials for manufacturing Xuan paper.
Table 1. Descriptive Statistics of the Anatomical Properties of P. tatarinowii Bark and Branch wood at Different Years of Age
Statistical analysis of the anatomical properties indicated that the coefficient of variation (CV) of the factors were ranked as follows: the ratio of lumen diameter to wall thickness of the bark (0.341) ＞ wall thickness of the bark (0.317) ＞ ratio of length to width of the branch (0.240) ＞ length of the branch (0.171) ＞ width of the branch (0.166) ＞ ratio of the lumen diameter to the wall thickness of the branch (0.156). The CVs of the other parameters were relatively small (CV was less than 0.1).
Fig. 2. Variation in the anatomical properties of P. tatarinowii at different years of age (a: fiber length; b: fiber width; c: ratio of length to width; d: lumen diameter; e: wall thickness; and f: ratio of lumen diameter to wall thickness)
Illustration of the Bark Anatomy
The anatomical properties of 2-year-old to 3-year-old P. tatarinowii bark were reported in this study (as shown in Figs. 3 through 8). Illustrated in the cross-section pictures, the bark of a P. tatarinowii branch was composed of the epidermis, periderms, cortex, and secondary phloem (Fig. 4a). The optical and electron microscopic observations showed that the secondary phloem of the bark primarily consisted of bast fiber cells, sieve tubes, companion cells, axial parenchyma, and ray parenchyma cells. The bast fibers were agglomerated or slightly broken. The individual bast fibers were spindle-shaped cells with sharp ends, and the fiber width was irregular with an uneven thickness. There were multiple forms of fibers, which included the abnormal fibers. The ray parenchyma cells were primarily rectangular, with both single and double rows. Most of the parenchyma cells contained starch granules and red inclusions, which were probably the extractive of the wood such as tannins (Fig. 5b) (Helama et al. 2010). The sieves and parenchyma cells became small clusters scattered among the fibers.
Fig. 3. The anatomy of the 2-year-old to 3-year-old P. tatarinowii bark (LM x 100) (a, b, c, and d: fiber; e: axial parenchyma; f: sieve tube; g: pit; and h: abnormal fiber)
As shown in Fig. 6, the cross-section of the P. tatarinowii bark fiber was elliptical, polygonal, and irregular. However, the cell wall of the fiber was thin with small lumen. This was due to the fact that the cambium cell was continuously proceeding of periclinal division and anticlinal division during the growth of trees, resulting in the formation of secondary phloem outward and xylem inward (Luostarinen and Hakkarainen 2019). Furthermore, the cells of xylem were usually more than those of phloem in the wood. Therefore, phloem fiber was shoved to the bark, and the fiber cell was thin with small lumen. The cell was mostly oval, polygonal and irregular. The capillary tube had stronger liquid absorption with more gaps among microfibrils and microfibrils, resulting in the Xuan paper having good ink absorption (Fig. 6). There were bordered pits on the cell walls, however, there was a low number of total pits.
Fig. 4. Cross section of the 2-year-old to 3-year-old P. tatarinowii bark (LM x 100)