Variance investigation on the microstructural characteristics of vessels among three lignocellulosic biomasses

Abstract

Especially, the processing and utilization of biomass-based material is closely related to the vessel, e.g. the flow of vapour and additive. It is conventional that vessels in most plants can influence on water and nutrients transport between adjacent cells, which could just infer to be important in the wood-based panel industries. In this work, a complete characterization of vessels and pits is presented for three conventional biomasses in wood-based panel: poplar (Populus deltoides) (P), moso bamboo (Phyllostachys edulis) (B), and the fruit shell of oil camellia (Camellia Oleifera) (FS_OC). Every material is analyzed by combining several techniques including: light microscopy, scanning electron microscopy and surveying calculations from resin casting. The results show that among the three biomass materials, B has a significantly larger vessel width (164.8 ± 6.0 μm for B, 2.2 ± 6.2 μm for P, 10.0 ± 0.8 μm for FS_OC) and smaller inclination angle of the perforation plates (6.8° for B, 44.7° for P), which is more conductive to improving moisture transfer between the vessels. The vessel length of P varies widely from 676.8 μm to 1025.2 μm, which is related to its seasonal growth. By resin casting analysis, more differences in the morphology and distribution of pits in the vessel walls were observed between the three species. Such as, For B, there are numerous pits between vessel cells, while very few to none between vessel and parenchyma cells or fiber. In addition to pits, B and FS_OC also have spiral thickening structures on their vessel walls. The pit membrane is an elliptical shape in P, while slit-like shape in FS_OC and a combination of both elliptical and slit-like shape in bamboo. The unique microstructural characteristics of vessels is related to the individual plant growth traits, which is the basis for biomass-based material processing and utilization.

Introduction

A vessel is a tubular structure in the xylem, which is an important anatomical feature for plant identification^1,2. As a transport system inside the plants, several differences can be observed among these functional vessels. The main differences depend on the biological characteristics of the plants and their responses to environmental changes³. This is especially the case for the transport of water and nutrients during plant growth⁴. In dicotyledonous plants, such as most deciduous trees, both longitudinal and horizontal transport systems are required for plant growth. In the xylem, vessels play the role of main transport pathways in the longitudinal direction, where the pits are flowed through the transversal ray cell tissues of the transport way. However, in monocotyledonous plants, such as bamboo, the xylem vessels are also the primary channel for water and nutrients transport. Furthermore, due to a lack of procumbent tissues, transverse transport and infiltration are carried out by the pits on the adjacent walls of the tissues’ cells^5–7. But in some biomass byproducts, such as the fruit shell of oil camellia in China, the spherical morphology of the shell is related to the disordered arrangement of vessel tissues⁸. As the biomass raw material, vessel distribution type (ring porous, dispersed-porous etc.) and the size of vessel lumen significantly affect the permeability and the porousness, which are some crucial parameters during the wood drying, and other modification treatments^9,10. However, some literatures cannot be sought out by ourselves, which focus on a comparison between the morphological features among different types of vessel structures including pit shape in vessel wall, pit distribution, pit arrangement, etc. Therefore, in order to take advantage of these biomass in China, the effect of these structural characteristics, being vital transport tissues, need to be elucidated.

For an anatomical structure identification, conventional methods of sectioning and dissociation can only provide limited information on the local structure of objects. Scanning electron microscopy (SEM) can be applied to obtain a finer (ultrastructural) observation of the dissected vessels, but it is difficult to get a complete three-dimensional (3D) image of the internal wall in the vessel tissue. Hence, it becomes very important to examine the internal spatial structure of vessels and understand both the geometry and dimensions of the (sub)structures¹¹. To this end, resin casting was proposed as an option to visualize the real 3D cellular structure. This method was derived from studies on animal and human blood vessels using medical electron microscope^12–15. In recent years, this technique was also used to study plant tissues, which was shown to be highly effective to complement/replace conventional microstructural examinations^16,17. Today, more and more studies are devoted to the mechanisms involved in xylem transport and the formation of pits during tree growth^18–22.

Especially, the utilization of lignocellulosic biomasses in processing industry as value-added products is an important issue to support the zero-emission concept. As well known, the permeability to gas and liquid is very important, that produced suitable physical and mechanical properties of plant-based composite materials. During the processing and utilization process, vessel density, vessel length, lumen diameter, and slip flow through the pit membranes of the axial might be responsible for the permeability to vapors and additives. The main objective of this study is to quantify the basic cell structural characteristics to systematically explain the characteristics related to an efficient transport inside different lignocellulosic biomasses. In particular, the diameter, length, perforated plate, pits and spiral thickening of the vessel and space-structure characteristics of the vessel are accurately investigated. All this information is systematically studied to be optimized to greater advantage for biomass material modification and other biomass-based production.

Materials and methods

Materials

Poplar (Populus deltoides) (P), moso bamboo (Phyllostachys edulis) (B) and fruit shell of oil camellia (Camellia Oleifera) (FS_OC) were selected as the typical examples. To reduce the impact of sampling locations on the results, a more universal sampling method that has been widely accepted by researchers were adopted in this study. The 9-year-old poplar tree of around 20.5 m in height and 13.7 cm in diameter at breast height was sampled at the Taizishan forest farm in Hubei, China. In order to reduce the interference of different sampling locations on the experiment, test materials were taken from the disc at the breast height. The 4-year-old bamboo, with an average diameter at breast height of 14.5 cm, was collected from the campus of Central South University of Forestry and Technology, Changsha Hunan, China. The samples were cut from the center of the bamboo culm at breast height at the internode, in order to ensure relative uniformity and reduce interference due to variations in the bamboo properties²³. Three fresh C. oleifera fruits were collected in Changsha, at the Chinese National Engineering Research Center for Olitea Camellia. The fruit shells were collected during the maturity period from September to October, during which the fruit shell structure was relatively stable. The sample fruits were uniform in size and color and had not been attacked by insects. After removing the endocarp and exocarp carefully, the mesocarp was used for test material. After being cleaned, the selected samples were cut longitudinally to remove the seeds as well as the inner and outer parts of the shell carefully, while the sampling method was presented in a previous work⁸. Sampling was duplicated in each of the sample preparations to validate the repeatability of these measurements. The fresh mature specimens obtained were preserved in a fixative solution (38% formaldehyde, glacial acetic acid, and 70% alcohol with a volume ratio of 5:5:90, coded as FAA) to maintain the original cell morphology.

Light microscopy

For P and B specimens, samples with dimensions of 10 mm × 10 mm × 10 mm were placed in hot water at 80 °C for two weeks until they were completely soaked and sank. Three sections with a thickness of 10–15 μm were cut using a precision sliding microtome (Sledge microtome G.S.L. 1, Switzerland). After rinsing with deionized water for several times, the slices were stained with 1% safranine solution for 2 h, then dehydrated by a series of ethanol solution (50%, 70%, 85%, 95%, 100%). Finally, the treated microsections were fixed on glass slides with Canada balsam of neutral quick-drying glue. For FS_OC, three sections (X, Y and Z) were used to investigate the microstructure for better distinction of the observed shell surfaces⁸. As shown in Fig. 1, the X section represents the observation plane from the endocarp to the exocarp side following seed removal. The Y section represents the observation plane obtained by cutting along the longitudinal line, while the Z section represents the observation plane obtained by cutting along the latitudinal line.

Fig. 1.

FS_OC sample observation planes diagram⁸. (a) longitudinally dissected FS_OC without seeds. (b) Diagram of the longitudinal and latitudinal directions of FS_OC. (c) Diagram of the three observation sections (X, Y, Z) of FS_OC. N = north and S = south direction.

The samples were macerated using a previously reported method^24,25. Samples of each species were split into small sticks with an approximate size of 10 mm (longitudinal) × 5 mm (tangential) × 5 mm (radial) to obtain a representative selection of vessel elements. The sticks were macerated in a solution containing equal parts of glacial acetic acid (99.5%) and hydrogen peroxide (30% solution) and were subsequently heated to 60 °C for 48 h. The maceration process was ended when the color of the sticks turned white and they were easily broken into several parts. The separated vessels were thoroughly washed in distilled water until all the acid was removed. These observations and photography were performed by a light microscope (Leica Microsystems, Milton Keynes, UK).

Scanning electron microscopy

Three types of SEM were used in this study, severally listed as following, 1 - the ultrastructure of all biomasses, 2 - the morphology of vessels in resin casting experiments, 3 - the three-dimensional organization of pit cavity.

The 1st part as following, the sticks of the biomasses were smoothly trimmed and then dried by the supercritical CO₂ to maintain the original form of the cell structure.

In the 2nd part, some samples with dimensions of 5 mm × 5 mm × 5 mm were used for the resin casting experiments. The experimental procedure is composed of five steps: chemical fixation, ethanol dehydration, resin infiltration, polymerization and tissue digestion¹⁷. For P and B samples, the following dehydration for the required ethanol concentrations of 25%, 50%, 70%, 85%, 95% and 100% was respectively kept for 4 h, and the resin infiltration was performed for 6 h for each resin concentration except at 100% resin (24 h). For FS_OC, ethanol dehydration was maintained for 6 h to each ethanol concentration, and every resin infiltration was kept for 12 h for each resin concentration except at 100% resin (24 h). The chemicals list for resin casting is presented in Table 1.

Table 1.

Concentration ratio and processing time of styrene required for resin penetration in lignocellulosic biomasses.

Styrene concentration (%)	Chemicals ratio of resin solution	Processing time (h)
25	Volume100% ethanol : VolumeStyrene = 3:1	12
50	Volume100% ethanol : VolumeStyrene = 1:1	12
75	Volume100% ethanol : VolumeStyrene = 1:3	12
100	Only styrene	24

Here, the following statements of 3rd SEM were presented. The inner wall structure of the vessel tissues, such as pit aperture, pit membrane, and horizontal distances of pit cavity was analyzed using more than 30 vessel pits per biomass. The pits on the resin casting were photographed and measured in combination with the pit characteristics of the vessel wall. The 3D structural characteristics of the pits (Fig. 2) was determined to allow a quantitative analysis of the pit aperture, pit membrane, and depth of pit cavity (cell wall thickness).

Fig. 2.

Structure parameters of bordered pit in the vessels. D_m: pit membrane, D_n: pit aperture and D_p: pit cavity depth.

All samples of lignocellulosic biomasses were firstly coated with platinum for 5 min at 10 mA in a sputter coater (Emitech, Ashford, UK), and respectively observed with a field emission SEM at a voltage of 2 kV (Hitachi Regulis8100, Sigma 300, Japan).

Statistical analysis

To quantitively analyze the morphology, 50 vessel elements per plant were selected randomly to measure the length, width and inclination angle of the perforated plate, and 100 pits per plant were selected randomly to measure the parameter of pit membrane, pit aperture and pit cavity depth. All the measurements of pit, vessel dimensions and inclination angle of the perforated plate were manually carried out with image analysis software Image-Pro Plus 6.0 (National Institutes of Health, Bethesda, MD, USA).

The recorded data of vessel elements and pits were analyzed by the statistical analysis of software SPSS 22.0 (Statistical Package for the Social Sciences, IBM, USA) including the average, the maximum, the minimum, standard deviation and coefficient of variation. One-way ANOVA analyses were performed to test individual vessel element size and pit size of each type of specimen.

To display the morphological characteristics of vessels, the selection of microstructural and ultrastructural images is based on the main features of each section of the material that can be displayed, especially the morphology of vessel in different sections.

Results and discussion

Microstructure observation

Anatomical features of P, B, and FS_OC are shown in Figs. 3, 4 and 5, respectively. Firstly, as Figs. 3, 4 and 5 indicated, the vessel density of P is distinctly maximum among three lignocellulosic biomasses. Hence, it’s could prove that the P’s transportation of water and nutrients have not been restricted in the plenty of xylem conduits (Figs. 3, 4 and 5, compare the vessel distribution in P vs. B and FS_OC). In the transverse section (Figs. 3a and 4a), the vessels of both P and B are mainly of circular or oval shape. As the main transport channel during the process of plant growth, the vessel element’s end walls are connected by single perforated plates and arranged longitudinally to form a specific length of vessel tissue in the longitudinal sections (Figs. 3b,c and 4b,c). However, the vessel cell morphology of FS_OC is more variable in all three sections with irregular polygonal shape in both X and Z sections with a longer vessel canal in the Y section. The size of vessels was more strongly correlated with the individual plant growth traits²⁵. Species with faster growth and larger size have fewer but larger vessels and higher hydraulic conductivity. Compared to P, B has the highest average vessel width (164.8 ± 6.0 μm), which is 1.7 times that of P (92.2 ± 6.2 μm) and 16.3 times that of FS_OC (10.0 ± 0.8 μm) (Table 2). It could be inferred that vessels occupy the mainstream in the B’s vascular bundle as seen in Fig. 4. However, the vessel trait of P shows strong seasonality. The vessel length exhibits a wide range from 676.8 μm to 1025.2 μm for P, which leads to a variation in the cross-sectional diameter of the vessel elements. The vessel diameter also affects the variation of the inclination angle of the perforation plates, which in turn affects the moisture transfer²⁶. The bigger vessel diameter, the lower the inclination angle of the perforation plates, which is more conductive to improving moisture transfer between the vessels. The average inclination angle of P is 44.7°, while the value is only 6.8° for B. According to the above analysis, the P particle has more advantages in board manufacturing with respect to the grain of B and FS_OC, because the permeability of adhesive and the passageway of hydrothermal transport during the fabrication are the direct correlation with major vessel density, longer vessel length, greater lumen diameter, etc. in xylem.

Fig. 3.

The microstructure of poplar (P) for the three observation sections. (a) Transverse section, (b) Radial section and (c) Tangential section.

Fig. 4.

The microstructure of bamboo (B) for the three observation sections. (a) Transverse section, (b) Radial section and (c) Tangential section. MV  metaxylem vessel, PV  protoxylem vessel.

Fig. 5.

The microstructure of Camellia oleifera shell (FS_OC) for the three observation sections. (a) X section, (b) Y section and (c) Z section.

Table 2.

Statistical results for the length and width of the vessel for the three type of samples.

Species	Parameter	Mean/µm	Max/µm	Min/µm	SD/µm	CV/%
Poplar (P)	LV/µm	832.4	1025.2	676.8	99.4	11.9
	DV/µm	92.2	104.8	83.1	6.2	6.7
	APP/°	44.7	54.8	23.3	3.8	8.5
	RV/%	19.6	25.7	15.6	1.6	8.2
Bamboo (B)	LV/µm	360.8	426.6	279.8	35.7	9.8
	DV/µm	164.8	171.5	151.2	6.0	3.6
	APP/°	6.8	14.5	3.9	0.9	13.2
	RV/%	9.4	12.4	6.8	1.1	11.7
Camellia oleifera shell (FS_OC)	LV/µm	383.2	436.0	314.0	46.2	12.0
	DV/µm	10.0	11.0	9.2	0.8	8.7
	APP/°	–	–	–	–	–
	RV/%	1.3	2.1	0.8	0.2	15.4

L_V: Vessel length, D_V: Vessel width, A_PP: Angle of perforation plates, R_VT: Vessel tissue ratio, SD: standard deviation, CV: coefficient of variation. 50 vessel elements per plant were selected randomly to measure the parameter above.

Ultrastructure observation on the vessel wall

The structural morphology of the lateral walls in the vessels is further examined via SEM, which are presented the typical images in Figs. 6 and 7, and 8 for P, B, and FS_OC, respectively. In these samples, it can be observed that the inner wall of the vessels is characterized by the presence of pits and spiral thickening structures. The pit morphology can be classified into two types: oval and narrow slit. Furthermore, the types of pit arrangement can be classified into pairwise column, mutual column or a combination of two aforementioned ways. The inner wall of P’s vessels is mainly characterized by pits, while the inner walls of FS_OC’s vessels is mainly composed of spiral thickening structures. However, B’s vessels are distributed into both pits and spiral thickening structures.

Fig. 6.

Ultrastructure of poplar (P) for the three observation sections. (a) Transverse section, pit in a vessel wall (arrow), (b) Radial section and (c) Tangential section.

Fig. 7.

Ultrastructure of bamboo (B) for the three observation sections. (a) Vascular bundles transverse section, spiral thickening of the protoxylem vessel (arrow). (b) Radial section, an enlarged view shows obvious perforation plate, The inset is a magnification of the composite pits (arrow). (c) Tangential section, perforation plate (arrow). PC  parenchymal cells , FC fiber cells.

Fig. 8.

Ultrastructure of Camellia oleifera shell (FS_OC) for the three observation sections. (a) X section, (b) Y section and (c) Z section. The insets are magnifications of the spiral thickening.

The number of pits in the P’s vessel is mainly in the tangential section compared to the radial section (Fig. 6b,c). B’s bordered pits are narrower slit (Fig. 7b,c), which are arranged in pairs or trapezoidal shape. The presence of narrow pits in B makes it easier for two small pits to form a compound pit (Fig. 7b,c, white arrows). Compound pits are usually present in the vessel cell wall and are characterized by paired membranes of several small pits forming an elongated pit on the luminal side of the vessel^17,23.

Compared these images between the light microscopy and scanning electron microscopy, The general morphology presented by the optical microscopy in Figs. 3, 4 and 5 is limited due to the resolution/magnifying power. However, the detailed structures of vessel cell walls, including pit cavity, spiral thickening, and other cell wall structures, could be directly visualized by SEM. In spite of this, depending on sample preparation, the conventional acquisition of SEM has still constrained possibility to accurately characterize the structural characteristics in the inner vessel wall and vessel lumen. This is especially the case for three-dimensional (3D) analysis of complex structures, such as the stereostructure of vessel. Hence, the technology of resin casting via SEM is going to be successively input in next study.

Space-structure characteristics of the vessels

By resin casting, it can be easier to visualize the internal structure and morphological features of the vessels that cannot be directly observed by standard microscopy (optical and SEM). A comparison of the images (Figs. 9a-b and 10a) for P, B and FS_OC shows that the resins casting of the vessels are composed of several continuous vessel elements and are not separated since the resin was able to pass through the perforated plates. The presence of pits between the cell walls of the vessels allows the resin to form a connected network (Figs. 9c and 11c). For P samples, the vessel element ends have long and short tails extending flat and forward against the next vessel element wall (Fig. 9a,b), which has commonly been considered as an identifying characteristic of wood species in the literature^27,28. Furthermore, a pit structure can be seen on the tail (Fig. 9a, inset), which is not observed in B and FS_OC. This pit structure is often considered to be involved in both horizontal and vertical water transport to some extent.

Fig. 9.

Resin casting of vessels in poplar (P). (a) solitary vessel, the pit of end-tail in vessel (inset). (b) multiple vessels. (c) vessel clusters, magnification of bordered pits (inset). (d) Selective distribution of pits in vessel: the black arrow indicates concentrated pit distribution, while the white arrow identifies numerous pits on the lateral wall.

Fig. 10.

Resin casting of vessel in bamboo (B). (a) Resin casts of vessels in metaxylem. The image shows obvious perforation plate structure. (b) smaller metaxylem vessel element cast. (c) Spiral thickening of vessel elements in protoxylem (arrow). (d) arrow is the pit distribution on the large metaxylem vessel. The heterogeneity of pits adjoining different type of cells. The white arrow identifies numerous pits on the lateral wall close to the parenchyma cells with fewer pits on the wall adjoining the fiber sheath in the black arrow’s direction. (e) Magnification of bordered pit cast.

Fig. 11.

Resin casting of vessel in Camellia oleifera shell (FS_OC). (a) Resin casts of spiral thickening of vessel, (b) Pit from inner cell wall view and (c) Structure of bordered pit pair between vessels.

In both P and B vessel castings, it can be seen that the vessels are in direct lateral connection with different tissues. The pit distribution in the vessel walls is obviously heterogeneous (Figs. 9d and 10d), which is consistent with the literature²⁹. A possible explanation for this phenomenon is that on the one hand the pits are part of the transport system and the presence of a high number of pits improves the efficiency of material transport²³. The vessel cells adjoining other vessel cells display a large number of pits, while in other parts, facing the parenchyma cells or fiber, they have very few to none. On the other hand, the fibers are stiff and provide mechanical support to the stem. So the presence of holes in the cell walls can compromise the supporting ability^11,30. This is in high contrast with the pit distribution observed in the metaxylem vessels of B, while this phenomenon is not seen in FS_CO. A different distribution of vessel pits appears to be the most efficient way to adapt to the dual demands of material transport and mechanical support as a compromise/balance. For P and FS_OC, the presence of depressions on the striatal membrane can be observed, while in B the striatal membrane is protruding which is another significant difference between the samples.

In B samples, smaller metaxylem elements are mainly dispersed in the area between the metaxylem vessel cells and the protoxylem vessel cells. Small metaxylem elements also play an important role in the transport of nutrients. Because the location of small vessel cells in bamboo metaxylem is not the same, it is difficult to find and recognize by conventional anatomical methods²⁶. In this study, a relatively complete structure of small metaxylem vessel casting was obtained (Fig. 10b,c). The casting of small metaxylem vessel (Fig. 10b) is close to the morphology of the metaxylem vessels, except that they have lower diameter (about 20 μm) than those of the latter (about 90 μm). In P casting samples, the presence of a concave surface on the pit membrane is observed, which is not the case for B and FS_OC samples (Figs. 9c, 10e and 11c).

The vessels of FS_OC are smaller in size and grouped with an irregular distribution. By conventional anatomical methods, the vessels having a pit structure are difficult to observe and recognize. By resin casting enables these vessels to be seen which are mainly in the form of spiral thickening (Fig. 11a) with a pit structure in the cell walls (Fig. 11b,c). The pit type can be considered as scalariform and a transition type from scalariform to spiral thickening. The arrangement of the pits is parallel to the long axis of the vessel (Fig. 11b) or in a cross arrangement in the top and bottom pits (Fig. 11c). The connection between the vessels is bordered by pits, and the shape of the pit membrane is mainly a square/rectangle (Fig. 11c).

Table 3 presents the quantitative results by providing the mean, minimum, and maximum values of the pit sizes. The pits can be divided into three classes: a pit membrane in the outer wall of the vessel, a pit cavity, and a pit aperture in the inner wall. The differences between the minimum and maximum of both pit membrane and pit apertures are significant in all three species (Table 3). In Fig. 11, the pit membrane of FS_OC is a rectangle and characterized by R and L associated with the length direction and width direction, respectively. The pit membrane length of FS_OC has the broadest range (2.45 to 7.44 μm) in the R direction. On the other hand, the narrowest range (1.43 to 2.61 μm) is observed in the L direction of FS_OC’s pit membrane. Compared with the 0.9–2.7 μm wide pit apertures and the 1.1–3.8 μm wide pit membrane in bamboo³¹, the widths of the pit apertures of B in this study are much narrower and the pit membrane are much wider. Compared to the data on pit sizes in P, the pit apertures and pit membrane in B have a smaller range. The length of pit apertures and pit membrane is only 0.67–1.75 μm and 7.49–9.78 μm, which is much higher than that of B. The differences in the average of pit cavity depth are significant between all three species. Hence, it is great that the raw materials of wood-based panel are going to be multi-mixed because their different permeability to vapour and additives. It is primarily outstanding that the flexibility in using different types of raw materials has encouraged the utilization of lignocellulosic materials in the domain of particleboard industry.

Table 3.

Structural parameters of the pits in the vessels.

Species	Parameter		Mean/µm	Max/µm	Min/µm	SD/µm	CV/%
Poplar (P)	Dm/µm		8.56	9.78	7.49	0.57	6.6
	Dn/µm		1.33	1.75	0.67	0.10	7.5
	Dp/µm		2.41	3.17	1.68	0.23	8.5
Bamboo (B)	Dm/µm		3.16	6.74	5.74	0.28	8.8
	Dn/µm		0.84	1.23	0.42	0.11	13.9
	Dp/µm		2.69	3.22	1.93	0.32	11.8
Camellia Oleifera shells (FS_OC)	Dm/µm	R	4.20	7.44	2.45	0.35	8.3
		L	1.96	2.61	1.43	0.28	13.6
	Dn/µm		0.50	0.76	0.22	0.02	4.0
	Dp/µm		1.14	2.03	0.65	0.13	11.4

D_m : pit membrane, D_n : pit aperture, D_p : pit cavity depth, SD : standard deviation, CV: coefficient of variation. 100 pits per plant were selected randomly to measure the parameter above.

Conclusion

By combining resin casting with conventional anatomy methods (optical and scanning electron microscopy), the main characteristics of the vessel morphology in lignocellulosic materials, as well as the connection between the vessels and adjacent cells, was fully revealed. In this work, these methods were combined to examine the different types and morphological characteristics of vessel elements of three lignocellulosic biomass: poplar (Populus), moso bamboo (Phyllostachys edulis), and the fruit shell of oil camellia (Camellia Oleifera).

Conventional microanatomical observation (microscopy) was able to identify some vessels which originated from poplar (conventional vessel), bamboo (conventional vessel and spiral thickening vessel), and Camellia oleifera fruit shell (spiral thickening vessel). The largest vessel was observed in bamboo (164.8 μm in average width), while the shortest and smallest vessel was in FS_OC (383.2 μm in average length and 10.0 μm in average width). It was observed that some of the vessels were short and broad similar to discs (e.g. vast vessel in bamboo), while others were slim and elongated similar to sticks (e.g. poplar and Camellia oleifera fruit shell).

But finer structural characteristics of the vessels’ inner wall were more easily determined by a resin casting technique. From this analysis, more differences were observed between the three species. The inner walls of poplar’s vessel were mainly composed of pits, while the inner walls of bamboo’s and Camellia oleifera fruit shell’s vessel were composed of pits and spiral thickening structures. It was observed that the morphology of the pit membrane was composed of two shapes: concave and convex surfaces. While a convex surface of pit membrane was observed in poplar’s vessels, bamboo’s and Camellia oleifera fruit shell’s vessels were mainly composed of concave surface of the pit membrane. The geometry of pit membrane was also different: an elliptical shape in poplar, a slit-like shape in Camellia oleifera fruit shell, and a combination of both elliptical and slit-like shape in bamboo.

Finally, the results obtained in this study open the doors for sample recognition/differentiation based on morphological characteristics. Nevertheless, more work is under way to add more species and refine the morphological analyses to better and more precisely characterize the complex structure of lignocellulosic materials.

Author contributions

Writing – original draft, M.Y. and Z.Y.; Formal analysis, S.C. and X.W.; Methodology, M.Y. and J.H.; Investigation, Z.Y.; Project administration, J.H.; Resources, X.W.; Software, T.L.; Supervision, Y.L. and X.L.; Validation, Y.L.; Visualization, D. R.; Writing – review & editing, M.Y., J.H. and D.R. All authors reviewed the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 32271791 and No. 32171709), and the Forestry science and technology innovation of Hunan Province in China (No. XLK202107-3).

Data availability

No datasets were generated or analysed during the current study.

Competing interests

The authors declare no competing interests.