Abstract
Quantitative aspects were investigated and compared for anatomical characteristics among compression wood (CW), lateral wood (LW), and opposite wood (OW) in the stem wood of Ginkgo biloba. Characteristics of each part were observed in the 5th, 10th, and 15th to 20th growth rings via optical and scanning electron microscopy. The crystalline characteristics were measured using an X-ray diffractometer. The microfibril angles (MFAs) were measured using the iodine method applied to the tangential section, as well as from the slopes of the pit apertures. The CW and OW showed similar tracheid lengths in the 5th and 10th growth rings; however, the CW was the shortest in the 15th to 20th growth rings. The CW showed the highest ray height and lowest ray number in each growth ring. The MFAs of the CW were greater than those in the LW and OW from both measurement methods. The MFAs obtained from the iodine method were smaller than that of the MFAs obtained from the pit aperture measurements. The CW had the lowest relative crystallinity in each growth ring, whereas the crystal width of the CW was the smallest in the 5th and 15th to 20th growth rings.
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Quantitative Anatomical Characteristics of Compression Wood, Lateral Wood, and Opposite Wood in the Stem Wood of Ginkgo biloba L.
Byantara Darsan Purusatama and Nam Hun Kim *
Quantitative aspects were investigated and compared for anatomical characteristics among compression wood (CW), lateral wood (LW), and opposite wood (OW) in the stem wood of Ginkgo biloba. Characteristics of each part were observed in the 5th, 10th, and 15th to 20th growth rings via optical and scanning electron microscopy. The crystalline characteristics were measured using an X-ray diffractometer. The microfibril angles (MFAs) were measured using the iodine method applied to the tangential section, as well as from the slopes of the pit apertures. The CW and OW showed similar tracheid lengths in the 5th and 10th growth rings; however, the CW was the shortest in the 15th to 20th growth rings. The CW showed the highest ray height and lowest ray number in each growth ring. The MFAs of the CW were greater than those in the LW and OW from both measurement methods. The MFAs obtained from the iodine method were smaller than that of the MFAs obtained from the pit aperture measurements. The CW had the lowest relative crystallinity in each growth ring, whereas the crystal width of the CW was the smallest in the 5th and 15th to 20th growth rings.
Keywords: Ginkgo biloba L.; Compression wood; Lateral wood; Opposite wood; Quantitative analysis; MFA
Contact information: Department of Forest Biomaterials Engineering, College of Forest and Environmental Sciences, Kangwon National University, Chuncheon 24341, Republic of Korea;
* Corresponding author: kimnh@kangwon.ac.kr
INTRODUCTION
Reaction wood on the lower sides of branches and leaning stems of conifers, also known as compression wood (CW), restores the vertical growth of stems or branches and is recognized by a darkened color on its transverse surface (Barnett et al. 2014). Lateral wood (LW) and opposite wood (OW) occur in the leaning stems or branches alongside CW (Timell 1986; Eom and Butterfield 1997). LW is the intermediate between OW and CW (Timell 1986). The occurrence of CW has been shown to cause several abnormalities in the anatomical characteristics of both qualitative and quantitative aspects (Timell 1986; Eom 1991; Eom and Butterfield 1997, 2001). There are tracheid characteristics in qualitative aspects that appear as helical cavities, excessive lignification in the middle (S2) layer, a thicker cell wall, differently shaped pit borders, an absence of the inner (S3) layer, intercellular spaces, and distorted and bifurcated tracheid tips (Fujita et al. 1979; Yumoto et al. 1983; Timell 1986; Yoshizawa et al. 1987). In comparison, in the quantitative aspects, the CW shows higher microfibril angles (MFAs) in the S2 layer, shorter tracheid lengths, higher ray heights, and lower ray numbers (Onaka 1949; Timell 1986; Eom and Butterfield 1997, 2001).
Ginkgo biloba is an ancient gymnosperm species; therefore, many researchers are interested in studying its anatomical characteristics. Several studies have reported that G. biloba shows characteristics similar to conifers, and yet this species also shows some different characteristics such as varied size of tracheid lumen and a number of idioblasts with druse (Scott et al.1962; Srivastava 1963; Timell 1986).
The quantitative aspects of anatomical characteristics in the CW of G. biloba have been reported in a few studies (Onaka 1949; Eom 1991; Shirai et al. 2016). Onaka (1949) reported that the number of rays in CW was lower than that in normal wood, whereas the ray heights in CW were higher than that in normal wood. Eom (1991) reported that the anatomical characteristics between CW and OW in G. biloba were distinctively different for both qualitative and quantitative aspects. Eom also reported that the CW showed a thicker cell wall, lower number of rays, and higher ray heights than that of the OW, whereas the tracheid length was nearly identical. Shirai et al. (2016) reported that the MFA, air dry density, and lignin content in CW-like reaction wood of G. biloba were higher than that in the OW and LW, whereas the cellulose content was lower than that in the OW and LW. These authors concluded that thenegative gravitropism in the stem wood of G. biloba was similar to that of other conifer species.
In the previous study (Purusatama et al. 2018), the authors investigated and compared the qualitative aspects of anatomical characteristics in CW, LW, and OW in the stem wood of G. biloba. It was reported that CW showed characteristics differing from LW and OW, such as a circular tracheid shape, intercellular spaces, spiral checks on the cell wall, a piceoid pit in the crossfield, slit-like bordered pits, and distorted tracheid tips. Until now, the anatomical characteristics of G. biloba have been mostly investigated using branch wood. Furthermore, there has been a lack of information regarding the quantitative aspects of the anatomical characteristics in CW, LW, and OW. Therefore, the objectives of the present study are to investigate and compare the quantitative aspects of the anatomical characteristics for CW, LW, and OW in the stem wood of G. biloba.
EXPERIMENTAL
Materials
In the present study, 24-year-old Ginkgo biloba wood was obtained from Hongcheon-gun, Gangwon-do, Republic of Korea (37.6970°N, 127.8887°E). Hongcheon-gun had average high temperature from 1.6°C (January) to 30.2°C (August), average humidity from 57.8% (April) to 77.4 % (July), and average rainfall from 397mm (July) to 19.1mm (December). The fundamental information of the experimental wood is shown in Table 1 (Purusatama et al. 2018).
Methods
Microscopy
The CW, LW, and OW samples were defibrillated in Schültz’s solution (Han et al. 1999) for 3 days. The tracheid length was measured randomly in 50 tracheids from the CW in the 5th, 10th, and 15th to 20th growth rings, and then compared with those in the LW and OW from the same growth rings. The number of uniseriate rays was counted in 20 areas of a 0.5 × 0.5 mm2 microscopic screen in tangential sections. The ray height and ray number were randomly measured from 50 uniseriate rays in the 5th, 10th, and 15th to 20th growth rings of CW, LW, and OW.
Table 1. Fundamental Information of Sample Tree
To measure MFAs in the S2 layer of the tracheid wall, tangential sections were prepared from the CW, LW, and OW with a microtome cutting thin slices (20 to 30 µm), and the slices were soaked in Schültz’s solution for 3 days. The thin slices were stained using the iodine staining method (Senft and Bendtsen 1985). The MFAs in the S2 layer were also measured from the slope of the pit apertures and spiral checks in the radial sections of the CW, LW, and OW (Donaldson 1991). To observe all the quantitative characteristics, an optical microscope (Eclipse E600; Nikon, Tokyo, Japan) connected to an image analysis system (IMT i-Solution Lite, Vancouver, Canada) and a scanning electron microscope (JSM-5510; Jeol Ltd., Tokyo, Japan, 15kV) were used.
Crystalline characteristics
An X-ray diffractometer (DMAX2100V; Rigaku, Tokyo, Japan) equipped with a Cu target (λ = 0.1542 nm) was used for measuring the crystalline properties. The relative crystallinity and crystallite width were calculated using Segal’s equation (Segal et al. 1959) and the Scherrer’s equation (Burton et al. 2009), respectively.
Statistical analysis
Differences in the quantitative features among the CW, LW, and OW were statistically analyzed with one-way analysis of variance and post-hoc Duncan’s multiple range tests (SPSS ver. 21, IBM Corp., New York, USA).
RESULTS AND DISCUSSION
Tracheid Length
The tracheid lengths of CW, LW, and OW in the 5th, 10th, and 15th to 20th growth rings of G. biloba are shown in Table 2. The tracheid lengths of CW, LW, and OW were increased significantly with increasing growth rings. The CW and OW had similar tracheid lengths in the 5th and 10th growth rings; however, CW had the shortest tracheid lengths in the 15th to 20thgrowth rings. The LW showed the longest tracheid length in the 5th and 10th growth rings, whereas there was no significant difference between LW and OW in the 15th to 20th growth rings. As reported by Timell (1986) and Tsoumis (1991), tracheid lengths become longer from pith to bark in conifers. Several studies (Onaka 1949; Wardrop and Dadswell 1950; Yozhizawa et al. 1987) have reported that tracheid lengths in CW were considerably shorter than that in normal wood in many conifers. The average tracheid lengths in CW are shorter than that in normal wood because the formation of CW is accompanied by cambial activity with an increase in pseudo-transverse divisions (Yoshizawa et al. 1987).
Table 2. Tracheid Lengths in CW, LW, and OW of Ginkgo biloba
Note: The numbers in parenthesis are the standard deviations. The same capital letters beside the mean value are insignificant at the 5% significance level between the CW, LW, and OW using Duncan’s multiple range tests.
In a previous study, tracheid lengths were similar between the CW and OW in the branch wood of G. biloba (Eom 1991). Tracheid lengths between the CW and OW are similar in the stem wood of radiata pine, whereas LW had the longest tracheid length in the stem wood and branch wood of the radiata pine (Eom and Butterfield 1997). In addition, the OW was the longest tracheid lengths in the stem wood of New Zealand rimu (Dacrydium cupressinum), whereas CW, LW, and OW did not show significant differences in the branch wood (Eom and Butterfield 2001).
Ray Height and Ray Number
The ray height (expressed as number of cells) and ray number of CW, LW, and OW in the 5th, 10th, and 15th to 20th growth rings of G. biloba are shown in Table 3.
Table 3. Ray Height and Ray Number in CW, LW, and OW of Ginkgo biloba
Note: The numbers in parenthesis are the standard deviations. The same capital letters beside the mean value are insignificant at the 5% significance level between the CW, LW, and OW using Duncan’s multiple range tests.
The relationship between ray height and ray number is shown in Fig. 1. The ray heights of CW, LW, and OW were increased significantly with increased growth rings. The ray height was different among the CW, LW, and OW in each growth ring. The CW had the highest ray height in each growth ring, whereas there was no significant difference between the LW and OW. As reported by Kim et al. (2009), the ray height increased from near the pith to approximately the 10th to 20th growth ring and was almost constant from the following rings in Pinus koraiensis and Larix kaempferi. Onaka (1949) reported that CW of G. biloba showed slightly higher ray height compared to normal wood. In addition, Lee and Eom (1988) and Eom (1991) reported that CW had higher ray heights than that of OW in the branch wood of P. koraiensis and G. biloba. Furthermore, the CW in five conifers had higher ray height compared to normal wood as reported by Verrall (as cited in Timell 1986). The CW had higher ray height than that of LW and OW in the stem of radiata pine (Eom and Butterfield 2001).
The ray number significantly decreased with increasing growth rings in the stem wood of G. biloba. The ray number was significantly different among CW, LW, and OW in each growth ring. The CW had the lowest ray number, whereas there was no significant difference between the LW and OW. The ray numbers in P. koraiensis and L. kaempferi were decreased from the pith to the 10th to 20th growth rings and were almost constant for the following rings toward the bark (Kim et al. 2009). The ray number of CW was lower than that of the normal wood in G. biloba and other conifer species (Onaka 1949; Timell 1986). Eom (1991) reported that the ray numbers of CW were fewer than that of OW in the branch wood of the Ginkgo tree. As reported by Ollinma (as cited in Timell 1986), the CW of P.abies had 34 rays /mm2, while the normal wood of P.abies showed 40 rays/mm2. Eom and Butterfield (2001) reported that the ray number of CW was lower than that of LW and OW in the stem wood and branch wood of New Zealand rimu. In several conifers CW showed greater ray number than normal wood. As reported by Verrall (as cited in Timell 1986), CW in Larix laricina, Picea mariana Pinus palustris, Thuja occidentalis, and Thuja plicata showed greater ray number compared to normal wood.
Fig. 1. Relationship between ray height and ray number of CW, LW, and OW in the stem wood of Ginkgo biloba
Microfibril Angle
The MFA was measured from the microfibril orientation revealed by the iodine staining method in the tangential section and from the slopes of the pit aperture and spiral check in the radial section. The light micrographs of the tracheid walls in the tangential sections are shown in Fig. 2. The iodine solution formed a dark crystal between microfibrils and revealed the orientation of the microfibril. As reported in a study by Senft and Bendtsen (1985), iodine solution and nitric acid produce crystals in the cell wall that align with the MFA. The CW showed distinct microfibril orientation and a greater slope of MFA than that of the LW and OW in each growth ring measured.
The scanning electron micrographs of the pit apertures and spiral checks in the radial section of CW, LW, and OW of G. biloba are shown in Fig. 3. The spiral checks in the tangential wall of CW are shown in Fig. 4. The MFA was also measured from the slope of the pit apertures and spiral checks in the tracheid wall in the radial section. As reported in a study by Cockrell (1974), the inner aperture of the pits and striations or splits of latewood tracheids had almost similar slope orientation with the S2 microfibril. Donaldson (1991) measured the MFA by observing single cell walls from bordered or crossfield pit apertures of chemical pulp fibers.
The MFAs of the CW, LW, and OW in the 5th, 10th, and 15th to 20th growth rings of G. biloba are shown in Table 4. Variations in the measured MFA obtained from the iodine staining method and the pit aperture measurements are shown in Fig. 5. The MFA decreased with increasing growth rings in the CW, LW, and OW in the stem wood of G. biloba. The MFAs in the CW, LW, and OW in the 5th growth ring had narrow ranges, whereas the MFAs in the 10th and 15th to 20th growth rings had wide ranges. From the results of the iodine staining method, the MFA of the CW showed a higher value than that of the LW and OW in each growth ring. The MFA from the pit aperture and spiral check of the CW was higher than that from the LW and OW in the 5th and 10th growth rings, whereas the CW showed a higher MFA but no significant difference in the LW and OW in the 15th to 20th growth rings. The MFA from the pit aperture measurement in the radial sections showed mostly higher values than that of the iodine method in the tangential sections.
Table 4. Microfibril Angle in CW, LW, and OW of Ginkgo biloba
Note: The numbers in parenthesis are the standard deviations. The same capital letters beside the mean value are insignificant at the 5% significance level between the CW, LW, and OW using Duncan’s multiple range tests.
Previous studies regarding MFA in CW include Eun and Kim (2008a), who reported that the MFAs of P. densiflora, P. koraiensis, and P. rigida were 16.4°, 14.4°, and 26.2°, respectively. Shirai et al. (2016) reported that the MFA was higher in the lower side (CW-like) of the inclined stem of G. biloba than in the upper side and lateral side. Sahlberg et al. (1997) reported that the CW had a higher fibril angle than that of normal wood in Norway spruce via an X-ray diffraction analysis. The CW showed a higher MFA than that of adult normal wood, whereas the CW had the same MFA as that of juvenile normal wood in Picea abies and Abies alba (Gorisek et al. 1999). Donaldson et al. (2004) reported that the CW had mostly higher MFA than that of OW in P. radiata, except in a few early juvenile rings and the mild CW. Harris (1977) reported that CW was similar to the OW in the stem wood of radiata pine. The MFA in the tangential walls was considerably smaller than that in the radial walls (Preston 1934; Gorisek et al. 1999).