Longitudinal and Radial Variability of Anatomical Properties, Fiber Morphology, and Mechanical Properties of Fibrovascular Bundle from Indonesian Arenga Longipes Mogea. Sp. Nov Frond

ABSTRACT

This study aimed to characterize the physical, anatomical, fiber morphology, as well as mechanical properties of fibrovascular bundle (FVB) from Arenga longipes fronds, based on radial and longitudinal direction. In this research, the fronds were divided into three parts in longitudinal direction (bottom, middle, and upper) sections, three parts in radial direction (outer-convex and concave, as well as middle and inner) zones. The FVB’s anatomical and fiber morphology were observed by light microscope, as well as mechanical properties were tested using the universal testing machine (UTM). The results showed that the longitudinal direction variability had different properties physically particularly in relation to the FVB diameter and density, anatomically in terms of the ratio of vascular tissue area (VTA) to total transverse area (TTA), as well as in the fiber morphology and mechanical properties. Radially, the outer position had a higher value variability of diameter, density, and mechanical properties, compared to the middle and inner zones, respectively. Furthermore, there were relationships between the density and mechanical properties as well as the ratio of VTA to TTA and mechanical properties. Based on the results, the FVB of A. longipes fronds was concluded to possess properties variability in the radial and longitudinal direction.

摘要

本研究旨在基于径向和纵向，表征阿龙加叶纤维血管束（FVB）的物理、解剖、纤维形态以及力学性能. 在本研究中，叶片在纵向上分为三个部分（底部、中间和上部），在径向上分为两个部分（外部凸起和凹陷，以及中间和内部）. 用光学显微镜观察FVB的解剖结构和纤维形态，并用通用试验机（UTM）测试其力学性能. 结果表明，纵向变异在物理上具有不同的性质，特别是与FVB直径和密度有关，在解剖学上，在血管组织面积（VTA）与总横向面积（TTA）的比率方面，以及在纤维形态和机械性能方面. 径向上，与中间区域和内部区域相比，外部位置的直径、密度和机械性能具有更高的值可变性. 此外，密度与机械性能以及VTA与TTA的比例与机械性能之间存在关系. 基于这些结果，长叶A.longipes叶的FVB在径向和纵向上具有变异性.

KEYWORDS:

• anatomical properties
• arenga longipes
• fibrovascular bundle
• fronds
• mechanical properties
• sugar palm

关键词:

• 解剖特性
• 纤维血管束
• 叶片
• 机械性能
• 糖棕榈

Introduction

Generally, the species of sugar palm in Indonesia is called A. pinnata and very abundant, because it grows naturally and is also cultivated by the community. The area of sugar palm plantations in Indonesia, both cultivated by the community and companies, has an area of 64,244 hectares (Ministry of Agriculture RI 2022). Based on this potential, sugar palm generates numerous fronds, albeit this is currently insufficient for industrial-scale sustainability. The raw material for palm fronds, on the other hand, can be utilized as an adjunct in the manufacturing of pulp and paper, or as a raw material for composites.

According to the Mogea report, the sugar palm is an indigenous tree, with high species diversity, including A. distincta, A. longipes, A. plicata, and A. talamuensis (Mogea 2004). The A. longipes species is cultivated by the community in the Langkat Regency, North Sumatra Province, as one of their daily livelihoods (Fadhilla et al. 2023). Generally, its sap can be used as raw material for making palm sugar and wine. The palm fronds are also used in simple construction for the local community. However, information about the fronds of A. longipes has not been widely reported. This part of the plant has an anatomical structure called a fibrovascular bundle (FVB) which is defined as the tissue comprising sclerenchyma fiber, vascular, and parenchyma tissues functioning as reinforcement, water, and nutrients circulation, as well as storage systems, respectively (Hakim et al. 2019). The study by Hakim et al. (2021) reported that FVB from salacca fronds has characteristic variability of unique vascular tissues, in terms of type and shape. The vascular tissues, in turn, also have relationships with other fundamental characteristics, including FVB diameter, density, and mechanical properties (Hakim et al. 2022).

Several studies on the use of biomass from sugar palm as a raw material for constructions, composite, and biopolymers have been carried out (Sherwani et al. 2021). However, the anatomical, physical, mechanical, and even chemical characterization of A. longipes biomass has not been exploited for its use as a raw material. A few publications have reported the fiber morphological and mechanical characteristics of A. longipes’ FVB. To meet the requirements for modern commercial use of sugar palm biomass and possibilities for pulp as well as paper raw materials, investigations are needed particularly on the characterization of the basic properties. Hakim et al. (2019) stated that the properties variability of FVB in salacca fronds was influenced by the difference in the radial direction.

Numerous studies on FVB properties variability based on species differences have been reported, but no studies investigated the species parts. Therefore, this study aims to characterize the fundamental properties including anatomical, chemical, physical, and mechanical, based on the radial and longitudinal variability of sugar palm (Arenga longipes Mogea. sp. nov) fronds.

Materials and methods

Materials

The main materials used were fronds and FVB of A. longipes Mogea, obtained from Langkat Regency, North Sumatra Province, Indonesia. The fronds were harvested from the upper stem at 10 cm from the bottom section. All the leaves attached were all trimmed leaving only the fronds. Furthermore, the fronds were divided into three positions based on longitudinal direction, namely bottom, medium, and upper sections, as shown in Figure 1a. The sample of each longitudinal section was divided into three sections based on the radial direction, namely the outer zone including convex and concave, the middle, and inner zones, as depicted in Figure 1b.

Figure 1. The illustration of section variability. (a) Longitudinal variation: (a1): bottom section, (a2): medium section, and (a3): upper section. (b) Radial variation: (b1): outer convex zone, (b2): middle zone, (b3): inner zone, and (b4): outer concave zone.

Observation of anatomical and physical properties

For the anatomical observation, the samples were sliced at a thickness of ±15 µm on the cross-section direction using sliding microtome with metal knife as well as without coloring. Subsequently, the observation was carried out using light microscopy (a Zeiss Primostar 3, Germany), equipped with a digital camera (8.3 MPx HD WiFi-camera) as well as 4× and 10× magnification. The imaging analysis software (IC-measure 2.0.0.245) was used to characterize the FVB area. Figure 2 shows that the analyzed areas were total transverse area/TTA comprising sclerenchyma fibers and vascular tissues, non-vascular tissue area/n-VTA namely sclerenchyma fibers, and vascular tissue area/VTA or vascular tissues, based on a method developed by Hakim et al. (2022). Several parameters were also measured to discover the relationship between the anatomical and mechanical properties. These include the ratio of VA to TTA, and n-VA to TTA. Meanwhile, the FVB densities were measured using the method proposed by Hakim et al. (2021), and the diameter was evaluated through a light microscopy (Zeiss Primostar 3, Germany) while analysis was performed with the IC-Measure software version: V.2.0.0.245.

Figure 2. An illustration of area occupation measure of FVB. (a) Total transverse area; (b) Non-vascular tissue area;(c) Vascular tissue area.

Mechanical properties of FVB

The mechanical properties measured were tensile strength and Young’s modulus according to the ASTM D-3379-75 (1989). The FVB of the fronds were discovered to have a moisture content of 8 to 12% wt. They were then cut to a length of about 90 mm ±0.1 mm and fixed on a 30 mm long paper frame using epoxy adhesive (ALF Epoxy adhesive, P.T Alfaglos, Semarang, Indonesia). The measurements were conducted with 25 replicates of single FVB for each longitudinal section including bottom, middle, and upper, as well as radial zones namely outer, middle, and inner. Figure 3 illustrates the mechanical properties analysis conducted using a universal testing machine (UTM Tensilon RTF 1350. Tokyo. Japan), with a 1 mm/min crosshead speed. Before the test, the supporting paper’s middle part was cut out, and then test mounting was carried out according to a method proposed by Hakim et al. (2019).

Figure 3. An illustration of testing the FVB mechanical properties.

Fiber analysis

The FVB of sugar palm fronds was macerated, then the length, fiber, and lumen diameter, as well as the thickness of cell wall at 25 fibers were measured based on the method of IAWA (Alfonso et al. 1989) as reported by Alfonso et al. (1989). Furthermore, the FVB was immersed in a mixture of 30% hydrogen peroxide and glacial acetic acid with a 1 to 1 ratio at a temperature of 60°C to 80°C for 24 h (two times) or until the samples became easily separable into individual fibers. The fibers slurry was washed with 70°C of hot water until the acid odor of fibers slurry was removed. Finally, the fibers slurry was stained with safranin to highlight the thickness of the cell wall and the lumen. The fiber morphology was measured using a Zeiss Primostar 3 light microscopy with 4 times magnification for the fiber length and 10 times for the fiber and lumen diameter. Furthermore, the fiber derivative was calculated based on fiber morphology. The derived values were calculated using the equation (Luce 1970; Malan and Gerischer 1987; Runkel 1949):

$\mathrm{S}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{r}\mathrm{n}\mathrm{e}\mathrm{s}\mathrm{s}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}=\frac{Fiberlength}{Fiberdiameter}$

$\mathrm{F}\mathrm{l}\mathrm{e}\mathrm{x}\mathrm{i}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{c}\mathrm{o}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t}=\frac{Fiberlumendiameter}{Fiberdiameter}\times 100\mathrm{\%}$

$\mathrm{R}\mathrm{u}\mathrm{n}\mathrm{k}\mathrm{e}\mathrm{l}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}=\frac{2\times Fibercellwallthickness}{Fiberlumendiameter}$

$\mathrm{L}\mathrm{u}\mathrm{c}{\mathrm{e}}^{\prime }\mathrm{s}\mathrm{s}\mathrm{h}\mathrm{a}\mathrm{p}\mathrm{e}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{o}\mathrm{r}=\frac{Fiberdiamete{r}^2-Fiberlumendiamete{r}^2}{Fiberdiamete{r}^2+Fiberlumendiamete{r}^2}$

$\mathrm{C}\mathrm{o}\mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{r}\mathrm{i}\mathrm{g}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{t}\mathrm{y}=\frac{Fibercellwallthickness}{Fiberdiameter}$

Statistical analysis

This study uses factorial randomized experimental design for the basic characteristics of sugar palm fronds. Regression lines were used to analyze relationships between anatomical, physical (density, and VTA:TTA ratio) as well as mechanical properties (tensile strength and Young’s modulus), while a Statistical analysis was performed using R-Studio software Version 4.0.0.

Result and discussion

The anatomical structure of A.Longipes fronds

The FVB is built up of several tissues, namely vascular metaxylem and phloem, sclerenchyma fiber, and parenchyma. The anatomical structure of the fronds is shown in Figure 4. The xylem has a metaxylem consisting of 1–2 vessels, while the protoxylem ranges from 3 to 8 vessels. Protoxylem and metaxylem are the main xylems that have developed since the beginning of growth. More specifically, protoxylem is formed by primary growth, while metaxylem is formed completely after primary growth. Metaxylem vessels are the larger diameter transport cells found in the last formed part of the xylem. The metaxylem in A. longipes fronds has an almost spherical shape surrounded by parenchyma and several protoxylems which have smaller diameters. The function of the metaxylem vessel and protoxylem is to transport water and nutrients to all parts of the plant. In contrast to dicot plants, the parenchyma found in palm fronds is spread between FVB, which is present in all radial areas where there is no increase in the number of lateral cells due to the absence of cambium. Furthermore, A. longipes has parenchyma tissue which is lighter in color than the FVB. Apart from these color differences, they also have a special shape due to the presence of sclerenchyma fiber cells which experience secondary thickening of their cell walls. The light color around the FVB depicts parenchyma tissue that has a uniform shape in the form of a sponge or honeycomb. It functions as a storage which contains a lot of sugar/starch. The organizational structure of A. longipes fronds is the same as that of other palms, such as Royal palm (Roystonea regia) and Alexandra king palm (Archontophoenix alexandrae) wherein large portions are occupied by fiber and parenchyma (Aritsara and Cao 2020).

Figure 4. The Cross-section image of sugar palm frond (magnification 10×): a: sclerenchyma fiber; b: metaxylem vessel; c and d: parenchyma cell; e: phloem; f: sclerenchyma sheath protoxylem.

The variability of physical and anatomical properties of FVB

Figure 5 is a cross-sectional image of radial variability at the upper, middle, and bottom of the A.longipes fronds. Firstly, the FVB of bottom section has variability in shapes and sizes, in general, the shape at the bottom section is round and oval with the metaxylem vessel in the center. The convex zone type has an oval shape with the vascular tissue located on one of the edges of the FVB tissue similar to that of A. engleri Becc. Meanwhile, the concave zone type has vascular tissue located in the center resembling the FVB shape of Corypha umbraculifera L (Zhai et al. 2013). The middle and inner zones have an oval shape with the vascular tissue located in the center similar to that of the Salacca zalacca fronds (Hakim et al. 2019). However, the diameter of the FVB decreased from the outer zones toward the inner and core zones, respectively. The outer zone both in the convex and concave has a higher proportion of sclerenchyma fibers area compared to the middle and inner, as shown in Table 1. Physically, the diameter of the FVB decreased from the outer to the middle and inner zones, respectively. This condition affects the density of the FVB as well as the fronds of A. longipes. The density in the outer zone is higher than in the middle and inner zones. Furthermore, the number of FVB also decreases from the outer to the middle and inner zones, respectively. Hakim et al. (2021) reported that the FVB found in the fronds of the zalacca plant also showed the same morphology where the density on the outside was higher than on the inside. The FVB on the periphery has a higher density and more lignin compared to the inner zone due to their larger sizes which in turn provide most of the mechanical strength. Meanwhile, the center of the stem is made up of more parenchyma with widely separated FVB, which has spongy ground tissue without lignin. Similar to bamboo, the density of FVB decreases gradually from the outer to the inner zone, hence, the mechanical properties increase gradually from the outer to the middle and inner zones, respectively (Khantayanuwong et al. 2023).

Figure 5. Cross-sectional image of frond upper, middle and bottom section: (a): outer convex zone, (b): outer concave zone, (c): middle zone, and (d): inner zone. Note: (1) is a representation of 4× magnification, and (2) is a representation of 10× magnification.

Table 1. The variability of anatomical and physical properties on bottom section of fronds.

Properties	Radial section (± SD)
	Outer		Middle	Inner
	Convex	Concave
Dimeter (µm)	52.39 ± 5.68	51.84 ± 6.06	51.52 ± 5.35	47.15 ± 4.79
Density (g/cm^3)	0.34 ± 0.07	0.34 ± 0.06	0.23 ± 0.03	0.18 ± 0.03
Number of fibrovascular bundles per 1 mm^2	3–4	3–4	1–2	1
Areas:
Total Transverse area of FVBs (mm^2)/TTA	0.66 ± 0.07	0.65 ± 0.08	0.65 ± 0.07	0.59 ± 0.06
Vascular tissue area of FVBs (mm^2)/VTA	0.10 ± 0.01	0.12 ± 0.03	0.42 ± 0.06	0.48 ± 0.05
Non-Vascular tissue area of FVBs (mm^2)/n-VTA	0.56 ± 0.07	0.53 ± 0.09	0.22 ± 0.03	0.11 ± 0.02
Ratio of VTA to TTA (%)	15.47 ± 2.25	18.55 ± 5.32	65.40 ± 3.93	81.93 ± 2.99
Ratio of n-VTA to TTA (%)	84.53 ± 2.25	81.45 ± 5.32	34.60 ± 3.93	18.07 ± 2.99

SD: standard deviation.

The middle frond’s anatomical characteristics based on radial variability shown the second level. The anatomical characteristics of this section are not significantly different from the pattern of variability at the bottom. The middle section also has a different size and shape of FVB, while the density pattern is similar to that of the bottom fronds. Radially, the density on the outside is higher than on the middle and inner zones, respectively. Based on the longitudinal direction, this section tends to have a lower density for all radial positions. Table 2 shows the anatomical and physical properties of the frond’s middle section. FVB diameter decreased from the outer to the middle and inner zones, respectively. This pattern is also characteristic of the density due to the smaller number of FVB from the outside to the middle and inner zones. The ratio of VTA to TTA increased from the outer to the middle and inner zones. This phenomenon indicates that the sclerenchyma fibers are majorly composed of vascular tissue. Furthermore, this also causes a difference in the density between the peripheral and the inside zones.

Table 2. The variability of anatomical and physical properties on middle section of frond.

Properties	Radial section (± SD)
	Outer		Middle	Inner
	Convex	Concave
Dimeter (µm)	49.63 ± 4.37	49.26 ± 3.47	49.17 ± 3.32	45.00 ± 4.56
Density (g/cm^3)	0.32 ± 0.03	0.32 ± 0.04	0.20 ± 0.02	0.18 ± 0.03
Number of fibrovascular bundles per 4 mm^2	3–4	3–4	1–2	1
Areas:
Total Transverse area of FVBs (mm^2)/TTA	0.62 ± 0.05	0.62 ± 0.04	0.62 ± 0.04	0.57 ± 0.06
Vascular tissue area of FVBs(mm^2)/VTA	0.10 ± 0.01	0.10 ± 0.01	0.43 ± 0.06	0.51 ± 0.05
Non-Vascular tissue area of FVBs (mm^2)/n-VTA	0.52 ± 0.05	0.52 ± 0.04	0.19 ± 0.06	0.06 ± 0.02
Ratio of VTA to TTA (%)	16.06 ± 1.74	16.15 ± 1.61	69.16 ± 8.53	89.68 ± 3.37
Ratio of n-VTA to TTA (%)	83.94 ± 1.74	83.85 ± 1.61	30.84 ± 8.53	10.32 ± 3.37

SD: standard deviation.

The last level is a cross-sectional image of the frond’s upper section anatomical properties. The shape, size, and number of FVB in the radial direction differ due to the differences in the density value. Radially, the upper section also shows that the density has a decreasing value from the outer to the middle and inner zones. The ratio of VTA to TTA has the same pattern as the middle and bottom sections. The consistent pattern also means that the density decreased along with the radial direction from the outer to the middle and inner zones. Table 3 shows the variability of the physical and anatomical properties of the upper section. The outer sclerenchyma fiber network is thicker and more numerous than the vascular tissue. In addition, the number of FVB is more numerous and denser on the outside than in the middle and inside.

Table 3. The variability of anatomical and physical properties on upper section of fronds.

Properties	Radial section (± SD)
	Outer		Middle	Inner
	Convex	Concave
Diameter (µm)	48.78 ± 3.32	48.59 ± 4.60	48.22 ± 3.81	45.11 ± 3.89
Density (g/cm^3)	0.32 ± 0.04	0.32 ± 0.04	0.22 ± 0.02	0.14 ± 0.01
Number of fibrovascular bundles per 4 mm^2	3–4	3–4	1–2	1
Areas:
Total Transverse area of FVBs (mm^2)/TTA	0.61 ± 0.04	0.61 ± 0.06	0.61 ± 0.05	0.57 ± 0.05
Vascular tissue area (mm^2)/VTA	0.10 ± 0.01	0.10 ± 0.01	0.45 ± 0.06	0.49 ± 0.04
Non-Vascular tissue area (mm^2)/n-VTA	0.51 ± 0.04	0.51 ± 0.06	0.16 ± 0.07	0.07 ± 0.04
Ratio of VTA to TTA (%)	16.16 ± 1.59	16.26 ± 2.24	74.59 ± 10.08	87.21 ± 6.29
Ratio of n-VTA to TTA (%)	83.84 ± 1.59	83.74 ± 2.24	25.41 ± 10.08	12.70 ± 6.29

SD: standard deviation.

Fiber morphology and fiber derivatives

The fiber morphology of FVB from the A. longipes fronds is shown in Figure 6, generally, the fiber dimension in each section has varied values. Furthermore, they have fiber lengths ranging from 1.29 mm on the bottom section and concave zone, to 1.79 mm on the upper section and inner zone, as shown in Figure 6a. The fiber length is longer than some non-wood materials such as jute stick with a value of 0.67 mm, pineapple leaves 1.06 mm, wheat straw 0.97 mm, and cotton stalk 0.9 mm. (El-Sayed et al. 2020). However, it is shorter than jute fiber with a value of 2.02 mm, banana fiber 1.92 mm, and bamboo 2.0 mm (Ferdaus et al. 2021). The fiber length is also longer than that of hardwood/eucalyptus at 0.70–0.84 mm and shorter than softwood with a value ranging from 1.57 to 1.96 mm (Rebola, Ferreira, and Evtuguin 2020).

Figure 6. Histogram of longitudinal and radial variability of fiber morphology. a. Fiber length; b. Fiber diameter; c. Lumen diameter; and d. Fiber wall thickness. (Note: bar chart is representation of the standard deviation, means with the same letter are not significantly different from each other (P > 0.05 ANOVA followed by Duncan multiple range test).

The outer zone including the convex and concave has a higher fiber length value compared to the middle and inner. Furthermore, the concave zone has a higher fiber length value than the convex in each frond longitudinally. The FVB on the outer zone has fiber length values that decreased from the bottom to the middle and upper section, respectively, but this pattern differs in the middle and inner zones longitudinally. According to the International Association of Wood Anatomy (IAWA), reported by Alfonso et al. (1989), the long fiber classification has a length of >1.60 mm, medium 0.90–1.60 mm, and short <0.90 mm. Based on the result, the fiber of A. longipes FVB can be included in the medium to long category. Therefore, for their application as raw material and the formation of pulp sheets, the fibers are easy to flatten and they form a strong bond with the weave, culminating in sheets with high tear and tensile strength.

The fiber width ranged from 14.77 μm on the bottom section and convex zone, to 19.83 μm on the upper section and convex zone, as shown in Figure 6b. The empty oil palm fruit bunches (OPEFB) fiber has a diameter value of 28.15 μm, while A. longipes fiber from the FVB fronds is thinner (Yahya et al. 2019). This value is also comparable with that of bamboo including Bambusa vulgaris at 24.01 μm, B. longispiculata 22.84 μm, Dendrocalamus membranaceus 23.71 μm, and D. asper 24.85 μm, which are thicker than the values obtained in this study (Khantayanuwong et al. 2023). The fiber thickness value of FVB A. longipes is also thicker than that of Melia azedarch (hardwood) which has been evaluated for its characteristics as a raw material for pulp by Megra et al. (2022). According to Megra et al. (2022), the fiber width affects the beating of the pulp and produces a better result due to the penetration of chemicals into empty spaces. Longitudinally, the pattern of the fiber width value on the outer zone is similar to the length. The value of fiber width in the outer zone is higher than in the middle and inner zones. Radially, the fiber width on the middle and upper sections was slightly higher in the middle and inner zones compared to the outer zones.

The lumen diameters ranged from 7.65 μm on the middle section and middle zone, to 9.32 μm on the bottom section and convex zone as shown in Figure 6c. According to the IAWA classification reported by Alfonso et al. (1989), the lumen diameter of fiber <50 μm can be included in the small category. This value is slightly high compared to other non-wood materials, such as Gigantochloa levis and G. scortechhinii of 4.00 μm and 8.66 μm, respectively (Wahab et al. 2020). Furthermore, Jayusman, Hakim, and Jayusman (2021) reported that the hardwood Toona sureni and T. sinensis were found to have a lumen diameter ranging from 2.00 μm to 3.50 μm, which is smaller than the value of A. longipes FVB. A small lumen diameter will make the fiber to have as good tensile strength and flexibility as pulp and paper raw materials. Based on the fronds radial variability results, the middle zone had a slightly higher lumen diameter compared to the outer and inner zones, respectively. This phenomenon is difficult to explain because the pattern is opposite to that of the length and width. Meanwhile, longitudinally, there is no specific trend pattern and almost all positions have the same lumen diameter value.

The fiber wall thickness ranged from 3.39 μm on the bottom section and convex zone to 6.06 μm on the upper section and convex zone as shown in Figure 6d. According to Alfonso et al. (1989), the fiber wall can be classified as thin because it has a lumen that is 3 times wider than the wall thickness. The fiber wall thickness value of FVB from A. longipes fronds is thinner than that of the Nigerian P. carribea (softwood) which ranged from 6.95 to 10.47 μm (Adenaiya and Ogunsanwo 2016). However, in comparison with oil palm fiber, the value is thicker at 2.79 μm (Yahya et al. 2019). Ferdaus et al. (2021) reported that the thickness of the fiber wall affects the tensile strength of the paper, the thicker the fiber wall, the lesser the tensile strength. According to the longitudinal variability of the fronds, the pattern of the bottom section on the outer zones has thicker fiber cell wall values compared to the middle and inner zones, respectively. Radially, the middle and the upper section have thicker fiber values from the outer to the middle and inner zones.

Statistically, the fiber length, fiber width, and cell wall thickness have significant interaction between longitudinal and radial positions. However, there is no significant interaction for the lumen diameter value. The fiber length in the bottom position indicates that the convex and concave sections were not significantly different, but have a significant difference with the middle and inner sections. This pattern was also seen in the middle position, but the upper position was not significantly different for all sections. The fiber width has a similar pattern with fiber length statistic values in both the bottom and middle positions. However, for the upper position, only the inner section has a significant difference. The cell wall thickness in the bottom position indicates the convex and concave sections were significantly different from the middle and inner sections. However, the middle and upper positions were not a significant difference in all sections. According to statistical analysis, the morphology of FVB fiber of sugar palm has a variation in both longitudinal and radial positions.

The fiber derivative values of the FVB fiber dimensions in each position are presented in Table 4. The slenderness ratio (SR) value is the ratio between fiber length and diameter. The average SR value of the FVB fiber at each position ranged from 80.03 ± 20.66 in the upper section and inner zone to 112.19 ± 20.66 in the bottom section and inner zone. These properties will affect the qualities of the pulp and paper made with the fibers. The greater the ratio, the higher the tear strength, the better the weaving power, tensile breaking force, folding resistance, and the fiber become more flexible (Palanisamy et al. 2022). In the paper-making process, fibers with high SR values form good bonds. This is because long fibers play a role in increasing the tear strength of paper.

Table 4. Fiber derivatives of FVB’s of A. longipes frond.

Radial Section	Slenderness ratio	Flexibility Coefficient	Runkel Ratio	Luce’s shape factor	Coefficient rigidity
Bottom section of frond (± SD)
Convex	86.00 ± 22.47	0.39 ± 0.07	1.68 ± 0.55	0.85 ± 0.05	0.31 ± 0.03
Concave	92.74 ± 24.14	0.41 ± 0.05	1.47 ± 0.33	0.83 ± 0.04	0.29 ± 0.02
Middle	83.63 ± 19.73	0.50 ± 0.01	0.99 ± 0.05	0.75 ± 0.01	0.25 ± 0.01
Inner	112.19 ± 20.66	0.48 ± 0.01	1.07 ± 0.05	0.76 ± 0.01	0.26 ± 0.01
Middle section of Frond (± SD)
Convex	95.22 ± 22.54	0.50 ± 0.02	0.99 ± 0.08	0.75 ± 0.02	0.25 ± 0.01
Concave	99.32 ± 24.15	0.51 ± 0.02	0.98 ± 0.08	0.74 ± 0.02	0.25 ± 0.01
Middle	83.90 ± 22.82	0.51 ± 0.02	0.96 ± 0.07	0.74 ± 0.02	0.24 ± 0.01
Inner	87.72 ± 17.19	0.50 ± 0.02	0.99 ± 0.05	0.75 ± 0.01	0.25 ± 0.01
Upper section of frond (± SD)
Convex	87.34 ± 31.16	0.54 ± 0.07	0.89 ± 0.22	0.71 ± 0.08	0.23 ± 0.01
Concave	99.99 ± 29.70	0.51 ± 0.03	0.97 ± 0.12	0.74 ± 0.03	0.25 ± 0.01
Middle	99.07 ± 29.15	0.51 ± 0.04	0.98 ± 0.16	0.74 ± 0.04	0.25 ± 0.02
Inner	80.03 ± 20.66	0.50 ± 0.01	0.99 ± 0.05	0.75 ± 0.01	0.25 ± 0.01

SD: standard deviation.

The average value of the flexibility ratio (FR) at each position ranged from 0.39 ± 0.07 in the bottom section and convex zone, to 0.54 ± 0.07 in the upper section and convex zone. In the pulping process, the fiber must have a high FR value due to the thin-wall thickness and because it is easily deformed. The ability to change shape causes the surface contact between fiber-to-fiber to be more flexible and this leads to better fiber bonds which will produce pulp sheets with good strength. The higher the FR value, the greater the breaking strength and the better the bonding ability of the paper. In addition, the fibers can easily be flattened with high-strength properties (Sadiku and Abdukareem 2019).

The average value of the Runkle Ratio (RR) at each position ranged from 1.68 ± 0.55 on the bottom section and convex zone, to 0.89 on the upper section and convex zone. According to the classification of fiber quality, the RR value can be included in quality class V (Runkel 1949). This means that the FVB fiber of the palm fronds has a thicker cell wall and a narrow lumen diameter. The fibers in the pulp sheets are difficult to flatten and the bonds between the fibers are small. In pulping, a good fiber has a small RR value of less than 1.00, a thin cell wall, and a wide lumen diameter (Sadiku and Abdukareem 2019). These properties make the fiber completely flat on the pulp sheet with a very strong bond. The pulp sheets produced in turn will have high tear and tensile toughness.

The Luce’s shape factor (LCF) is defined as the index for the resistance pulp beating and a high value indicates an increased resistance during the pulping process. The average value at each position ranged from 0.71 ± 0.08 in the upper section and convex zone to 0.85 ± 0.05 in the bottom section and convex zone. This value is higher than that of Macaranga personii at 0.09 and eucalyptus at 0.37–0.42 (Takaeuchi et al. 2016). The fiber with a high LCF value will have a narrower surface area causing the bond area and contact between fibers to reduce. Consequently, the paper sheets produced tend to have low tensile strength and crack resistance.

The average Coefficient Rigidity value of the FVB fronds at each position ranged from 0.23 ± 0.01 in the upper section and convex zone, to 0.31 ± 0.03 in the bottom section and convex zone. It is defined as the ratio of the cell wall thickness to the fiber diameter. This comparison shows a negative correlation to tensile strength as well as a positive correlation to pulp yield and density. In the paper industry, fibers with a CR above 0.75 increase the contact surface because the relationship between lumen and fiber diameter is greater. Therefore, the higher the ratio, the lower the stiffness and the higher the fiber flexibility (El-Sayed, El-Shakhawy, and El-Shakhawy 2020).

Mechanical properties of A. longipes FVB

Figure 7 shows the variability in the mechanical properties of the palm fronds’ FVB based on their longitudinal and radial positions. The results showed a similar trend in each section of the longitudinal direction both in the tensile strength and Young’s modulus. The decreasing trend in both values ranged from the bottom, middle, and top, respectively, as shown in Figure 7a, b. The highest value of tensile strength was found at the bottom of the convex zone, namely 159.50 MPa, while the smallest was obtained at the upper of the inner zone, with 70.69 MPa. Furthermore, the tensile strength value obtained in this study is lower than that of the FVB from the zalacca plant (Hakim et al. 2021). According to Hakim et al. (2022), the tensile strength of the FVB is determined by the area of the sclerenchyma fiber. Based on the radial direction, the outer parts tended to have higher tensile strength values than the middle and inner parts. For all longitudinal sections, the convex section was slightly higher than the concave. In general, the results have the same pattern as those obtained in other studies such as bamboo (Srivaro, Rattanarat, and Noothong 2018), and salacca (Hakim et al. 2019).

Figure 7. Histogram of longitudinal and radial variability of mechanical properties. a. tensile strength; and b. Young’s modulus. (Note: bar chart is representation of the standard deviation, Means with the same letter are not significantly different from each other (P > 0.05 ANOVA followed by Duncan multiple range test).

Statistically, the mechanical properties (tensile strength and Young’s modulus) have significant interaction between longitudinal and radial positions. The tensile strength in the all position (bottom, middle as well as upper) indicates that the convex and concave sections were not significantly different, but they have a significant difference with the middle and inner sections. This pattern was also seen in the Young’s modulus values, that outer sections (convex and concave) have a significant difference with the middle and inner sections, respectively.

The relationships between density and mechanical properties of FVB

Figure 8a, b shows the relationships between the density and mechanical properties of FVB fronds from A. longipes. The regression patterns show that an increase in density leads to better mechanical properties based on the tensile strength and Young’s modulus. This indicates that the density of FVB has a role in influencing mechanical properties. As reported in previous studies, the density of FVB increases from the outer to inner zones in salacca fronds (Hakim et al. 2021). Furthermore, the high density in the outer sections is probably caused by the higher number of FVB in the outer compared to the inner section. Statistically, a similar trend of high R-square values above 80% was observed in tensile strength and Young’s modulus, indicating a high statistical significance for all radial zones. Therefore, it can be concluded that density significantly influences the mechanical properties of FVB from A. longipes fronds and the two variables are strongly correlated. This is probably because the density is influenced by the higher sclerenchyma fibers proportion with thick cell walls dominated by cellulose which plays a role in structural function.

Figure 8. The relationships between density and mechanical properties (a. Tensile strength; b. Young’s modulus) as well as VTA:TTA ratio and mechanical properties (c. Tensile strength: d. Young’s modulus).

The relationships between the ratio of VTA:TTA and mechanical properties

Figure 8c, d shows the relationships between the ratio of VTA to TTA and mechanical properties. According to the graph, there was an opposite pattern of regressions trend with the correlations between density and mechanical properties. The regression patterns indicate that an increase in the ratio of VTA:TTA will lead to a decrease in mechanical properties. This implies that larger non-vascular tissue plays a role in influencing the mechanical properties. The non-VTA is composed of sclerenchyma fibers tissue. Consequently, a greater VTA:TTA ratio will improve the mechanical properties. Santhoshkumar and Bath (2014) reported a decrease in the fiber tissue from the outer to the inner zone in bamboo anatomy. A high VTA:TTA ratio indicates an increase in the porosity which causes a decrease in the proportion of fiber tissue. Statistically, the R-square values for all FVB positions were above 80%, indicating that the VTA:TTA ratio affects the mechanical properties of FVB from A. longipes fronds. This is also supported by the high statistical significance in all the ratio regression equations for all three sections.

Conclusion

This study confirmed the variability in the fiber morphology and mechanical properties of FVB palm fronds. This variability is influenced by the position of the longitudinal parts comprising the bottom, middle, and upper sections, as well as radial including outer convex and concave, middle, and inner zones directions of the fronds. Furthermore, the anatomical and physical properties consisting of number, size, shape, and density of FVB have different properties based on radial and longitudinal directions. Radially, the number and density of FVB tend to be higher on the outer than the middle and inner zones, respectively. Fiber morphology, which includes fiber length and diameter, lumen diameter as well as wall thickness has different properties radially, but there is no significant difference in the longitudinal direction. Several derivative properties indicate that fiber from FVB palm fronds meets the requirements as a raw material for pulp and paper. The ratio of VTA:TTA proves that the density of FVB is different and simultaneously affects the mechanical properties longitudinally and radially. The ratio of VTA:TTA which increases indicates that the density decreases which results in decreased mechanical properties. Other sections of the sugar palm plant, including the bunches and inner stem, are also recommended for use as raw material for pulp and paper as well as bio-composite board.

Highlights

• TheFVB of A. longipes frond has variability properties both longitudinally and radially.

• The fiber morphology of FVB of A. longipes frond is good for raw pulp and paper materials. 

• The variability ratio of vascular tissue area to the total transverse area of FVBof A. longipes influenced the density and mechanical properties of FVB.

Ethical approval

We confirm that all the research activities meet ethical guidelines and adheres to the legal requirements of the study country. The research does not involve any human or animal-welfare-related issues.

Acknowledgements

The authors are grateful to Universitas Sumatera Utara for funding this study through the TALENTA Grant Scheme Application Research, No. 174/UN5.2.3.1/PPM/KP-TALENTA/2022.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was funded by TALENTA Grant, Scheme of Application Research Universitas Sumatera Utara with No. 174/UN5.2.3.1/PPM/KP-TALENTA/2022