Abstract
This research presents the anatomical, chemical, and mechanical properties of fibro-vascular bundles (FVBs) from two species of Salacca (snake fruit) frond: Salaccca sumatrana Becc. and Salacca zalacca Gaert (Voss). The anatomical properties were observed in the cross-section by light microscopy and digital microscopy. The anatomical observation focused on the location of the inner and outer vascular system. In the chemical analysis, FVBs were characterized for cellulose, hemicellulose, Klason lignin, and extractive content. Tensile strength and Young’s modulus were investigated, and the structural implications were considered. The FVBs from salacca frond contained vascular tissue in the cross section had new and different vascular type. Generally, the vascular tissue has a wider area than the sclerenchyma tissue. The FVBs of S. sumatrana and S. zalacca contained 41.75 and 44.60 wt% cellulose, 31.36 and 36.39 wt% hemicellulose, and 27.90 and 33.00 wt% lignin, respectively. The hot water solubility and ethanol-toluene solubility of FVBs of S. sumatrana and S. zalacca showed that extractive content were 2.96 wt% and 5.55 wt%; 18.54 wt% and 25.00 wt.%, respectively. As the diameter of FVBs increased, the tensile strength and Young’s modulus decreased. Increased FVB density will directly increase tensile strength and Young’s modulus. Based on the result, it was concluded that the FVBs of salacca type had significantly different properties compared to other palms’ FVBs, and this study confirmed the correlation between the physical and mechanical properties of the FVBs from salacca frond.
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Anatomical, Chemical, and Mechanical Properties of Fibrovascular Bundles of Salacca (Snake Fruit) Frond
Luthfi Hakim,a,b,* Ragil Widyorini,a Widyanto Dwi Nugroho,a and Tibertius Agus Prayitno a
This research presents the anatomical, chemical, and mechanical properties of fibro-vascular bundles (FVBs) from two species of Salacca (snake fruit) frond: Salaccca sumatrana Becc. and Salacca zalacca Gaert (Voss). The anatomical properties were observed in the cross-section by light microscopy and digital microscopy. The anatomical observation focused on the location of the inner and outer vascular system. In the chemical analysis, FVBs were characterized for cellulose, hemicellulose, Klason lignin, and extractive content. Tensile strength and Young’s modulus were investigated, and the structural implications were considered. The FVBs from salacca frond contained vascular tissue in the cross section had new and different vascular type. Generally, the vascular tissue has a wider area than the sclerenchyma tissue. The FVBs of S. sumatrana and S. zalacca contained 41.75 and 44.60 wt% cellulose, 31.36 and 36.39 wt% hemicellulose, and 27.90 and 33.00 wt% lignin, respectively. The hot water solubility and ethanol-toluene solubility of FVBs of S. sumatrana and S. zalacca showed that extractive content were 2.96 wt% and 5.55 wt%; 18.54 wt% and 25.00 wt.%, respectively. As the diameter of FVBs increased, the tensile strength and Young’s modulus decreased. Increased FVB density will directly increase tensile strength and Young’s modulus. Based on the result, it was concluded that the FVBs of salacca type had significantly different properties compared to other palms’ FVBs, and this study confirmed the correlation between the physical and mechanical properties of the FVBs from salacca frond.
Keywords: Salacca (snake fruit); Fibrovascular bundles; Tensile strength; Type of vascular tissue
Contact information: a: Department of Forest Product Technology, Faculty of Forestry, Universitas Gadjah Mada, Jl. Agro No. 1, Yogyakarta, Indonesia 55281; b: Department of Forest Product Technology, Faculty of Forestry, Universitas Sumatera Utara, Jl. Tri Dharma Ujung No. 1, Medan, Indonesia 20155;
* Corresponding author: luthfi@usu.ac.id
INTRODUCTION
Indonesia has many options for development of the forest community, such as the intensive agroforestry system between the main plant (tree) and the supporting plant (multi purposes tree species). One of the agroforestry systems, which was developed in Indonesia, is the salacca-based agroforestry system. Salacca belongs to family Palmae or Arecaceae, and is a native plant from the Indonesian-Malaysian region (Mogea 1986; Dransfield et al. 2005; Supapvanich et al. 2011; Zumaidar et al. 2014). The salacca fruits are known as salak (snake fruits). In Indonesia, the two popular snake fruit plants cultivated as a supporting plant are salak Sidempuan (Salacca sumatrana Becc.) and salak Pondoh (Salacca zalacca Gaert. (Voss). Both plants are cultivated based on the agroforestry system from Java Island and Sumatera Island. Generally, snake fruits are edible with a sweet taste, but the fronds of the salacca tree are not used optimally. Some previous studies showed that salacca fronds have been used as raw material for particleboard with citric acid-based adhesive (Widyorini et al. 2018; Widyorini et al. 2019) and as the wear component of natural fiber reinforced phenolic (Rohardjo and Ridlo 2019). To optimize utilization as a raw material, however, it is necessary to know the basic properties of the salacca tree fronds. The basic properties of the salacca tree frond from the two species (S. sumatrana and S. zalacca) have not been reported.
The anatomy of 18 species of palm frond has been described by Zhai et al. (2013), and there are many studies on their fibro-vascular bundles structures, including those of coconut trees (Satyanarayana et al. 1982), windmill palm (Trachycarpus fortune) (Zhai et al. 2012), non-wood plant fiber bundles (Munawar et al. 2007), and Nypa palm (Nypa fruticans) (Tamunaidu and Saka 2011). Natural fibers are being developed as a raw material to substitute for wood in natural fiber composites. Natural fiber can replace synthetic fiber as a raw material for a cheaper, renewable, and sustainable alternative (Pickering et al. 2016). Munawar et al. (2007) characterized the morphology, physical, and mechanical properties of seven natural fibers of non-wood that could be used as a raw material of the composite board. There has been research on the structure of monocot anatomy, especially the fiber bundles, vascular bundles, or fibro-vascular bundles. Grosser and Liese (1971) observed bamboo anatomical structures throughout Asia by emphasizing the differences in the structure of vascular bundles.
In addition, Baley (2002) conducted research on the characteristics of the natural fiber of flax as a raw material of the composite board. Flax contains fiber bundles composed of between 10 to 40 fibers bound by lignin and pectin. The tensile strength of flax is approximately 600 to 2000 MPa, which is sufficient as a raw material of the composite board. Furthermore, fiber strands from oil palm empty fruit bunches are similar in structure to those of the leaf-sheath coconut tree (Law et al. 2007).
This study investigated the characteristics of the fronds of salacca, especially fibro-vascular bundles (FVBs) as a natural fiber. The palm tree has FVBs with anatomical properties such us long fiber (sclerenchyma fiber), vascular and parenchyma cells in the stem, leaf sheath in the fruit. The FVB tissue structure depends on the palm species (Zhai et al. 2013). Zhai et al.(2013) characterized 18 leaf-sheath FVBs of palm. The FVBs can be included as part of the natural fiber. Darmanto et al. (2017a,b) separated the salacca frond into fiber and reported that the fiber had a tensile strength of 160 MPa. Furthermore, after alkali, alkali-steaming, and alkali-steam explosion treatment, the fiber can improve in strength to 275 MPa, 220 MPa, and 226 MPa, respectively. Isolation of cellulose from salacca frond fibers by alkali treatment and bleaching with hydrogen peroxide successfully increased α-cellulose content, decreased of lignin content, and increased percentage of index crystallinity (Yudha et al. 2018). Unfortunately, the properties of salacca frond and their relationship with the mechanical properties of FVBs have not been reported. This paper presents the anatomical characteristics, physical properties, mechanical properties, and chemical properties of FVBs from two species of salacca (S. sumatrana and S. zalacca), which were cultivated from the agroforestry system in Indonesia.
EXPERIMENTAL
Materials
The salacca fronds were collected from two different growth locations of the agroforestry system in Indonesia. They were salak sidempuan (S. sumatrana) from Tapanuli Selatan (Province of Sumatera Utara), and salak pondoh (S. zalacca) from Sleman (Province of Yogyakarta). Sumatera Utara Province is located north of the equator, while Yogyakarta province is located south of the equator. The FVBs were obtained from the fronds of 15- to 25-year-old salacca trees. The fronds were hand-picked from the plant at the height of 5 cm from the main stem. Before being soaked in water, the leaf was removed from the main frond and was rinsed in cold water (23 °C) to remove soil and mud. The FVBs were collected after soaking in water for 4 weeks. The FVBs were air-dried to remove excess water and moisture prior to use in further experiments.
Methods
Sample preparation
The specimen preparation for anatomical observation of salacca fronds in this study was based on the methods of Jansen et al. (1998). No more than two fronds of each salacca species were cut approximately 1 to 2 cm2 on three positions of the transverse section. The three positions were observed based on the concept of the inner and outer vascular system developed by Zimmermann and Tomlinson (1972). The inner vascular system is that of the central transverse section, and the outer vascular system is that of the peripheral near cortex. In this research, the outer vascular system was divided into two parts, namely the convex vascular system part and concave vascular system part (Fig. 1A). Cross-section (transversal) orientations were prioritized as the focus of observations. To soften the specimens, the block specimens of the frond were immersed in a boiling water mixture of glycerin and water (1:10 volume ratio) for 2 h until the specimen became supersaturated. Transverse sections of the block specimens (10 to 15 µm thick) were cut using a sliding microtome with a metal knife. The samples were stained with safranin to highlight the area of fibro-vascular bundles.
Fig. 1. A. Pattern of transverse section for anatomical observation: (a) convex vascular system; (b) inner vascular system, and (c) concave vascular system. B: Mounting of FVB on the paper frame for mechanical testing
Observation of anatomical properties
The anatomical properties of FVB included measurement of the number of FVBs per 4 mm2 on the transverse section area, total transverse area of FVB, vascular tissue area, fiber tissue area, the ratio of vascular area to the total area, the ratio of non-vascular tissue area to the total transverse area, and the ratio vascular tissue area to the non-vascular tissue area. These parameters were measured to know relationship between anatomical and physical properties. The anatomical characteristics were observed with a light microscope (Olympus BX 51, Tokyo, Japan) equipped with a digital camera (Olympus DP 70, Tokyo, Japan) and imaging analysis software system (ImageJ; v.1.46r). The cell types, fibers, and vascular tissue localization were observed. Additionally, the number of sclerenchyma fibers and vascular tissue occupying the area of transverse sections were observed. Based on the Zhai et al. (2013) methods, the number of FVBs per 4 mm2 was calculated on the transverse section. In this research, the number of FVBs per 4 mm2 was calculated on three positions of transverse sectional (the convex vascular system, concave vascular system, and inner vascular system). The ratio of vascular tissue area or sclerenchyma fiber to the total transverse sectional area of FVBs was calculated.
Physical and mechanical properties measurement
The FVBs were air-dried (moisture content: 8 to 12 wt%), and 100 replicates were measured. The specimen size was approximately 90 mm in length. The FVBs were fixed on paper frames with a 30 mm gauge length by medium-viscosity epoxy adhesives (ALF Epoxy adhesive, P.T Alfaglos, Semarang, Indonesia) according to the ASTM D-3379-75 (1989). Previously, the diameter of each FVBs was measured using a handheld digital microscope (dino-lite edge 3.0 AM73915MZTL, New Taipei City, Taiwan) and analyzed using dino-lite software V.2.0 at 100 samples of FVBs. The specimens were conditioned at 60% relative humidity and 20°C for a week before mechanical testing. The mechanical properties of FVBs were determined using a universal testing machine (Tensilon RTF 1350, Tokyo, Japan) with a crosshead speed of 1 mm/min. Prior to testing, the middle part of the supporting paper was cut (Fig. 1B).
Chemical content analysis
According to ASTM D 1110-84 (2013), extractives were removed from the oven-dried samples (2 g) by extraction with boiling water for 3 h. The weight loss from this step was defined as hot water extractive solubility. The ASTM D 1105-96 (2013) was used to prepare extractive-free samples. An ethanol-toluene mixture (ratio 1 L:427 mL) was used to remove the extractive of oven dried samples (2 g) by the Soxhlet extraction method for 4 h. For the extractive-free samples, the cellulose (ASTM D 1103-84 2013), hemicellulose (ASTM D 1104-84 2013), and lignin Klason (ASTM D 1106-84 2013) composition was determined. All analyses were conducted in triplicate.
RESULT AND DISCUSSION
Anatomical Properties
Fibrovascular bundles of S. sumatrana
The microscopic characteristic of frond shows the organization of the inner and outer of vascular system of S. sumatrana, as shown in Fig. 2A. The FVBs were different in position on the cross section; they were not uniform in size and shape. Zhai et al. (2013) divided FVBs into three types based on differences in diameter and location within a single frond. Type A is a rounded vascular tissue located in the central region of the FVB; type B is rounded/angular vascular tissue located in the marginal region of the FVB. Type C is aliform vascular tissue in the region of the fibro-vascular bundles. In the present research, the new types of FVBs were found on a different position of the vascular system. The outer vascular system between the convex and concave vascular system had a different shape of FVBs. Fibro-vascular bundles in the convex vascular system of the frond had a round shape, while those in the concave vascular system had an oval shape. The FVBs in the outer vascular system (peripheral area) of frond had wider sclerenchyma fibers than vascular tissue. The shape of FVBs in the inner vascular system of the frond was oval, but the vascular tissue had a wider area than the sclerenchyma fibers.
Vascular tissue is clear, but lumens are not uniform in size and shape in each position. The vascular tissue of FVB of S. sumatrana has several lumens, which consists of a large lumen that is surrounded by a small lumen (Fig. 2A). Generally, the sclerenchyma fibers is around the vascular tissue for each FVB, but the tissue area was different for each transverse sectional of FVBs.
Fig. 2. Transverse sectional image of frond fibro-vascular bundles: A: S. sumatrana, B: S. zalacca, (a) convex vascular system, (b) inner vascular system, and (c) concave vascular system
Fibrovascular bundles of S. zalacca
The transverse sectional of the FVBs of S. zalacca is illustrated in Fig. 2B. According to the classification by Zhai et al. (2013), the FVB of S. zalacca was almost similar with type C, but the aliform pattern was not clear in the convex and inner vascular system. The vascular tissue of the FVB of S. salacca has several lumens, but the lumen was dominated by a wide area. FVBs in the convex vascular system and the inner vascular system of frond had sclerenchyma fibers that did not encircle vascular tissue. Furthermore, the shape of FVB in the convex vascular system was oval. The FVB in the concave vascular system were oval, but the sclerenchyma fibers around the vascular tissue different with the FVB in the convex vascular system. The shape of FVB in the inner vascular system of the frond was round, and the vascular tissue had a wider area compared with the sclerenchyma fibers.
Generally, the salacca vascular tissue had a different type on each position based on wide area of vascular tissue and sclerenchyma fibers. Rüggeberg et al. (2009) reported that the vascular bundles of stem of the Mexican fan palm (Washingtonia robusta) had a different type between stem periphery and cortical zone. They were classified of the vascular tissue type based on cell wall thickness. The vascular tissue type on inner position of salacca frond had a wider area compared with the sclerenchyma fibers, and types D and E tissues were found. These two types of FVB showed correlation to the phylogenetic classification by Dransfield et al. (2005; 2008), as illustrated in Table 1.
Table 1. Type of Vascular Tissue in FVBs of Salacca Compared to FVBs of 14 Genera of palm (Arecaceae) from Zhai et al. (2013)
Note: The table shows the phylogenetic classification of genera of palms (Arecaceae), redrawn from Dransfield et al. (2005; 2008). The gray area is occupied by sclerenchyma fibers and the white area by a vascular tissue.
Anatomical properties of fibro-vascular bundles
The frequency of FVB in S. sumatrana was between 3 to 5 FVB per 4 mm2 in the convex vascular system 1 to 2 FVB per 4 mm2 in the concave vascular system, and 0 to 1 FVB per 4 mm2 in the inner vascular system. Similar to S. sumatrana, the frequency of FVBs in S. zalacca only differed in the convex vascular system of frond, where values were between 3 to 4 FVB per 4 mm2.
The total transverse area of FVBs of the S. sumatrana and S. zalacca was 0.589 ± 0.17 mm2 and 0.455 ± 0.10 mm2, respectively. The difference between FVB and natural fibers (fiber bundles) was found in vascular tissue in FVB, but the vascular tissue was not found in natural fibers. Vascular tissue contains vessels that improve porosity properties. Increasing the lumens (like a vessel in the vascular tissue) increases the porosity and decreases the density of the fiber (Baley 2002; Munawar et al. 2007). Thus, the presence of non-vascular tissue increases density. In this research, vascular and non-vascular tissue areas were calculated. The vascular tissue area of FVB S. sumatrana and S. zalacca were 0.059 ± 0.01 mm2 and 0.054 ± 0.14 mm2, compared with the non-vascular tissue area at 0.531 ± 0.15 mm2 and 0.401 ± 0.09 mm2, respectively. The ratio of vascular tissue to non-vascular tissue of FVB in S. sumatranaand S. zalacca were 11.25 ± 0.87% and 13.83 ± 3.29%, respectively. Based on these results, the density of the FVB in S. sumatrana was higher than in S. zalacca. The ratio of vascular tissue and the total area of FVB in S. sumatrana was lower than in S. zalacca; the ratio of non-vascular compared with the total area of FVB in S. sumatrana was higher than in S. zalacca.
Table 2. Anatomical Properties of Salacca FVBs
Chemical Properties
Table 3 shows the chemical compositions of FVB of S. sumatrana and S. zalacca. Chemical analysis of S. sumatrana FVB indicated that the main components are α-cellulose (44.6%), hemicellulose (31.4%), and lignin (33.0%). For S. zalacca, the main components are α-cellulose (41.8%), hemicellulose (36.4%), and lignin (27.9%). Darmanto et al. (2107) reported that the major components of a single fiber in S. zalacca are cellulose (47.2%), holocellulose (79.1%), and lignin (22.3%). These results were relatively higher in cellulose and lower in lignin compared to the current study, and the hemicellulose was relatively similar to the results obtained here. Tomimura (1992) reported the lignin content of vascular bundles of oil palm trunk was 15.7%, which is comparable to the present result (33.0% for S. sumatrana and 27.9% for S. zalacca). However, the main components of oil palm trunk are 73.1% holocellulose, 41.0% α-cellulose, and 24.5% lignin (Khalil et al. 2008), and Abe et al. (2013) also reported that the main chemistry component of oil palm vascular bundles were α-cellulose (42.5%), hemicellulose (37.6%), and lignin (16.1%). These results were relatively similar to those obtained in this study. The Klason lignin content of fibrovascular bundles from the different 18 species, as determined by Zhai et al. (2013), have a mean value of 29.6%, which is relatively higher than fibrovascular bundles of salacca frond. In addition, Fathi et al. (2014) observed the topochemical distribution of lignin in vascular bundles by UV-microspectrophotometry (UMSP) and stated that there is a relationship between degree of lignification and the tensile strength properties of vascular bundle of coconut wood. Tensile strength increased with a decreasing of lignification.
Furthermore, the chemistry component that soluble in hot water of S. sumatrana and S. zalacca were 55% and 2.96%, soluble in ethanol-toluene were 25.00% and 18.54%, and ash content were 0.89% and 1.28%, respectively. Tomimura (1992) reported that the ash content of vascular bundles of oil palm trunk is 2.2% higher than FVB from salacca frond. Abe at al. (2013) stated the extractive component of vascular bundles of coconut wood is 2.54 % lower than FVBs from salacca frond.
Table 3. Chemical Properties of Salacca FVBs
Physical and Mechanical Properties
The physical and mechanical properties of FVBs of salacca frond are shown in Table 4. The average diameter of FVBs in S. sumatrana and S. zalacca was 0.047 ± 0.14 mm and 0.036 ± 0.01 mm, respectively. The density of FVBs of S. sumatrana (0.46 ± 0.15 g/cm3) was higher than S. zalacca (0.35 ± 0.11 g/cm3). Furthermore, in the natural fiber, such us non wood plant fiber bundles which was described by Munawar et al. (2007), the density of fiber bundles decreases with increasing diameter of fiber bundles. Unfortunately, Baley (2002) reported that flax fiber shows a different relationship, whereby the size of lumens of flax fiber increased with the increasing of diameter. Increasing the lumen size will increase the porosity of the fiber and decrease the density of the fiber. In this research, FVBs of salacca has a vascular tissue that make the porosity. Increasing the porosity of the FVBs will decrease the density.
Table 4. Physical and Mechanical Properties of Salacca FVBs
Typical stress-strain (S-S) curves for FVB from two salacca species are illustrated in Fig. 3. The S-S curve between S. sumatrana and S. zalacca shows breakage at 13.7% and 10.5%, respectively. The breakage strain of single fibro-vascular bundles of salacca showed much higher values than those in non-timber plants, including flax (3.3%), jute (8.2%), seagrass (3.4%), sisal (3 to 7%), abaca/banana (1 to 3.5%), ramie bast fiber, sanseviera leaf fiber, pineapple leaf fiber (2 to 6%) (Satyanarayana et al. 1982; Baley 2002; Munawar et al. 2007); Cocos nucifera L (9%), Butia capitate (Mart.) Becc. (11%), Rhapis excels (Thunb) A. Henry (11%), Corypha umbraculifera L (10%), Trachycarpus fortune (Hook) H. Wendl. (12%), and Sabal umbraculifera Mart. (13%) (Zhai et al. 2013); maize fiber bundles (2.5-3%) (Huang et al. 2016); vascular bundles of royal palm (Roystonea regia) (1.5%) (Wang et al. 2014); Coconut palm wood (1 to 2%) (Fathi, and Frühwald 2014).
Unfortunately, the breakage values were lower than windmill palm (Trachycarpus fortunei) (39.5 to 55.2%) (Zhai et al. 2012); Coconut husk fiber (24%) (Munawar et al. 2007); Syagrus romanzoffiana (Cham.) Glassman (25%), Elaeis guineensis Jacq. (17%), Medemia nobilis (hildebrant and H. Wendl.) Gall. (24%), Phoenix dactylifera L. (25%), Phoenix roebeleniiO’brien. (21%), Caryota monostachya Becc. (40%), Caryota urens L. (62%), and Washingtonia filifera (Linden ex Andre) H. Wendl. ex de Bary. (21%). (Zhai et al. 2013).
The tensile strength FVBs of S. sumatrana and S. salacca were 212.75 ± 97.78 MPa and 193.51± 47.39 MPa, respectively, and the specific tensile strength were 495.69 ± 236.86 MPa and 602.30 ± 215.85 MPa, respectively (Table 4). The values of the tensile strength were lower than fibrovascular bundles of Elaeis guineensis frond (228 MPa) (Zhai et al. 2013); natural fiber, including ramie bast (849 MPa), pineapple leaf (654 MPa), kenaf bast (473 MPa), Sanseviera leaf (562 MPa), sisal leaf (375 MPa), and abaca leaf (452 MPa) (Munawar et al. 2007); flax (1339 MPa), jute (466 MPa), seagrass (573 MPa), sisal (568–640 MPa) (Satyanarayana et al. 1982; Baley 2002; Razera and Frollini 2003; Davies et al. 2007). Interestingly, previous research by Darmanto et al. (2017a,b), reported that the tensile strength without alkali treatment was lower than values the present research, but these values were successfully increased by alkali treatment, alkali-steam treatment, and alkali-steam explosion treatment (275 MPa, 220 MPa, and 225.75 MPa, respectively). In addition, the tensile strength of FVBs of salacca frond were higher than coconut husk fiber (137 MPa) (Munawar et al. 2007) and fibrovascular bundles of 17 genera palm had been investigated by Zhai et al. (2013).
The Young’s modulus of FVBs of S. sumatrana and S. salacca were 3.03 ± 0.91 and 2.25 ± 0.60 GPa, respectively and the specific Young’s modulus were 7.09± 2.5 and 6.93 ± 2.36 GPa, respectively (Table 4). The Young’s modulus of FVBs salacca were lower than those of flax (58 GPa), jute (26.5 GPa), seagrass (19.8 GPa) (Satyanarayana et al. 1982; Baley 2002; Davies et al. 2007); ramie bast (28.4 Gpa), pineapple leaf (27 GPa), kenaf bast (25.1 GPa), sanseiviera leaf (14.0 GPa), abaca leaf (12.9 GPa), and sisal leaf (9.1 GPa) (Munawar et al. 2007). The Young’s modulus of S. sumatrana were higher than FVBs of 17 genera palm were investigated by Zhai et al. (2013), unfortunately, the Young’s modulus of S. salacca were lower than Arenga engleri (2.9 GPa), Butia capitata (2.5 GPa) and Rhapis excelsa (2.9 GPa) (Zhai et al. 2013).
Fig. 3. Typical stress-strain curve of single fibro-vascular bundles of Salacca frond. SS: S. sumatrana and Sz: S. zalacca
Relationships between Physical and Mechanical Properties
The relationship between physical and mechanical properties from S. sumatrana and S. zalacca are illustrated in Figs. 4 and 5. The curves show an increasing trend in the diameter of FVB, with decreasing trends in the tensile strength and Young’s modulus in two fibro-vascular bundles of frond salacca species. Zhai et al. (2013) reported that the tensile strength and Young’s modulus showed a decreasing trend with the increase in the diameter of fibrovascular bundles of 18 genera of palm. These conclusions are consistent with phenomenon that have been reported for fiber bundles of flax (Baley 2002), ramie bast, abaca leaf fiber, and pineapple leaf fiber, sansevieria leaf fiber, sisal leaf fiber, coconut husk fiber, and kenaf bast fiber (Munawar et al. 2007). Zhai et al. (2012) reported that the thick-walled sclerenchyma fibers predominantly contribute to the mechanical properties of fibrovascular bundles in windmill palm, while vessel and phloem tissues tend to reduce mechanical strength.
The relationships between density and mechanical properties showed that the increasing trend of density is directly proportional to the increase in tensile strength and Young’s modulus in two fibro-vascular bundles of frond salacca species. Density of the FVBs influenced the ratio of the vascular tissue to the total transverse area of FVBs. The higher of the ratio of the vascular tissue to the transverse area of FVBs will decrease density, and would be a factor that affects mechanical properties. Zhai et al. (2012) investigated the tensile strength of windmill palm (Trachycarpus fortunei) fiber bundles and its structural implications and reported that the fiber in the fibro-vascular bundles plays a role in increasing mechanical properties, whereas the presence of vessels and phloem tends to reduce mechanical properties.
Fig. 4. Relationships between diameter and tensile strength (P< 0.000 **); density and tensile strength (P< 0.017*); diameter and Young’s modulus (P< 0.033*); and density and Young’s modulus (P< 0.000 **) of single fibro-vascular bundles of S. sumatrana frond
The results indicated that the physical properties (density and diameter) and mechanical properties (tensile strength and Young’s modulus) of the fibrovascular bundles are closely related to the anatomical characteristics. For the S. sumatrana, the relationships between diameter and tensile strength; and between density and Young’s modulus is significantly higher. Unfortunately, the relationship between density and tensile strength; between diameter and Young’s modulus is significantly lower. For the S. zalacca, the relationship between diameter and tensile strength, between density and tensile strength, and between density and Young’s modulus is significantly higher, only the relationship between diameter and Young’s modulus was found not to be significant. In addition, a low R-squared value indicates that the relationships between tensile strength with density and diameter were weak. There were might other factors which influence relationships between tensile strength with density and diameter. We presume that FVBs porosity contributes to it. Furthermore, the FVBs contain vascular tissues which mostly consist of high porosity vessels.
Fig. 5. Relationships between diameter and tensile strength (P< 0.000**); density and tensile strength (P< 0.000**); diameter and Young’s modulus (P< 0.191ns); and density and Young’s modulus (P< 0.000**) of single fibro-vascular bundles of S. zalacca frond
Based on the result of physical and mechanical properties, this research concluded that the tensile strength and Young’s modulus showed a decreasing tendency with an increase in the diameter of the fibrovascular bundles. But, the increase density of the fibrovascular bundles showed that increasing tendency the tensile strength and Young’s modulus. In the future, FVBs can be used as an alternative raw material as a substitute for wood in the manufacture of structural composite boards based on fibrovascular bundles such as oriented composite board.
CONCLUSIONS
- Salacca frond has a different type of vascular tissue between inner and outer vascular system. In the inner position, the shape of vascular tissue of S. sumatrana frond is oval, while shape of vascular tissue of S. salacca frond is round. But, for both of them, the vascular tissue had a wider area than the sclerenchyma tissue.
- In the outer position of S. sumatrana frond, both of convex vascular and concave vascular have a different type of vascular tissue. Fibro-vascular bundles (FVBs) in the convex vascular system of the frond have a round shape, while those in the concave vascular system have an oval shape. The FVBs in the outer vascular system (peripheral area) of frond have wider sclerenchyma tissue than vascular tissue.
- In the outer position of S. zalacca frond, the shape of vascular tissue in the convex and concave vascular system is oval, but the sclerenchyma tissue has different shape between the convex and concave vascular system.
- The major chemical content of FVBs is relatively similar to the previous research about other species of fibrovascular bundles.
- In this research, relationships were found between the physical (diameter and density) and mechanical (tensile strength and Young’s modulus) properties of fibro-vascular bundles of salacca frond.
- The FVBs is recommended to be an alternative raw material in the manufacture of oriented composite board.
ACKNOWLEDGMENTS
The authors are grateful to the Indonesian Endowment Fund for Education (LPDP), Ministry of Finance, Republic of Indonesia, for the doctoral scholarship by BUDI-DN scheme (Awardee No: 20161141030090), and Ministry of Research, Technology and Higher education, Republic of Indonesia. Special thanks to Nafeeza Ayu, till we meet in Jannah.
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Article submitted: May 13, 2019; Peer review completed: August 4, 2019; Revised version received” August 6, 2019: Accepted: August 7, 2019; Published: August 15, 2019.
DOI: 10.15376/biores.14.4.7943-7957