Differences in Chemical Constituents between Dalbergia oliveri Heartwood and Sapwood and Their Effect on Wood Color

The purpose of this study was to characterize and quantify the chemical constituents of heartwood and sapwood of Dalbergia oliveri extract in order to investigate the chemical components that determine the formation of heartwood’s color. In this work, the types of pigments in heartwood and sapwood extract were analyzed using UV-Visible (UV) Spectrophotometer, and the main pigment components of heartwood and sapwood extract were identified and quantified using ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS). The results showed that the difference in content of the main components between heartwood and sapwood of Dalbergia oliveri was slight, and the lignin structure between heartwood and sapwood is basically identical; flavonoid pigments were found to be the primary chromophoric components of heartwood and sapwood extract. However, a total of 21 flavonoids were identified in heartwood and sapwood, of which the unique substances to heartwood were vitexin, isorhamnetin, and pelargonidin, and the content of isoliquiritigenin, formononetin, and biochanin A were 253, 37, and 583 times higher in the heartwood than in the sapwood, respectively, which could be the main pigment components affecting the significant color difference between heartwood and sapwood of Dalbergia oliveri. These results will provide a foundation for revealing the underlying mechanism of color difference between heartwood and sapwood and provide a theoretical basis for wood coloring.


Introduction
Dalbergia oliveri Gamble ex Prain (D. oliveri) belongs to the genera of Dalbergia, a family of Fabaceae, and is commercially known as Burmese tulipwood, which is mainly distributed in Myanmar, Thailand, Vietnam, Laos, etc. It was included in the International Union for Conservation of Nature (IUCN) Red List of Threatened Species in 1998 [1]. Sapwood and heartwood are yellow and light red or reddish brown, respectively. Compared to sapwood, heartwood has texture, strong natural durability, fragrance, and potential medicinal value [2][3][4]. Therefore, it has always been used as a material for high-class furniture, which has been loved by people. As market demand increased, many of the original sources of rosewoods have been indiscriminately harvested and drastically reduced. As a result, the price of rosewood increased rapidly. As an example, Dalbergia bariensis timber originating from Vietnam is 3478.3-3913.0 USD/m 3 , while imported Dalbergia bariensis timber is 1787.0 USD/m 3 [5]. To cater for changing market needs, fast-growing and abundant plantation timber (eucalyptus, poplar) has become an alternative in China because of its short rotation period and low price, but the drawbacks are poor decay resistance effect and monotone color of the wood [6,7]. Color affects the visual aesthetics and decorative qualities of wood and wood products, which is an important indicator to evaluate the ornamental and commercial value. Natural dyes are used in textile dyeing and food coloration due to their fewer toxins, UV resistance, and antibacterial properties [8,9]. It is worth noting that natural dyes can be an effective solution to the problem of monotonous wood color, but their few sources, high cost, and poor water fastness have restricted the  Table 1. It can be seen that the content of holocellulose in heartwood of D. oliveri was 8.54% lower than that in sapwood. The content of α-cellulose in heartwood was 3.18% higher than that in sapwood, and the Klason lignin content in heartwood was slightly higher than that in sapwood, by 0.34%. The acid-insoluble lignin was slightly higher in heartwood. The acid-soluble lignin was just lower than sapwood. The results for holocellulose and α-cellulose in sapwood and heartwood are consistent with those reported in the literature for Eucalyptus globulus (21.1-24.1% and 43.4-46.1%), but Dalbergia oliver has a higher content than that of Eucalyptus globulus [20]. The results were similar to those of other studies, including the Klason lignin content in sapwood and heartwood of Quercus faginea (28.1% and 28.6%, respectively), Quercus Laurina (25.1% and 25.5%, respectively), and Quercus crassifolia (24.9% and 25.2%, respectively) reported in the literature [21,22]. Lignin macromolecules contain chromophoric groups such as benzene rings, carbonyl vinyl groups, and auxochrome groups such as hydroxyl, carboxyl, and ether bonds, which can give wood its color. Therefore, the difference in the functional group structure between heartwood and sapwood requires further characterization and analysis to explore its effect on color.

Characterization of Klason Structure
As shown in Figure 1, the peak positions of the infrared spectra of Klason lignin in heartwood and sapwood of D. oliveri were basically identical. It showed that the chemical functional groups of Klason lignin in heartwood and sapwood were basically the same. The absorption peaks at wavelengths 1606, 1504, 1456, and 1419 cm −1 belong to the vibrational absorption of the aromatic skeleton. The absorption peaks at 1268, 1026, and 868 cm −1 were assigned to guaiacyl units. The absorption peaks at 1314, 1216, and 1112 cm −1 were from the syringyl unit. This shows that the heartwood lignin of D. oliveri is mainly composed of guaiac-based units and syring-based units, which is the same type of lignin reported in Dalbergia odorifera [23].  As shown in Figure 1, the peak positions of the infrared spectra of Klason lignin in heartwood and sapwood of D. oliveri were basically identical. It showed that the chemical functional groups of Klason lignin in heartwood and sapwood were basically the same.
The absorption peaks at wavelengths 1606, 1504, 1456, and 1419 cm −1 belong to the vibrational absorption of the aromatic skeleton. The absorption peaks at 1268, 1026, and 868 cm −1 were assigned to guaiacyl units. The absorption peaks at 1314, 1216, and 1112 cm −1 were from the syringyl unit. This shows that the heartwood lignin of D. oliveri is mainly composed of guaiac-based units and syring-based units, which is the same type of lignin reported in Dalbergia odorifera [23]. The main factors affecting the color of wood are lignin and wood extracts [24]. The difference in lignin content and functional group structure between heartwood and sapwood was not obvious. Therefore, it is speculated that lignin may not be the main reason for the obvious color difference of D. oliveri. Previous studies had found that in several larch species, a* (reddish) and L* (brightness) parameters were highly correlated with wood extractive content, while b* (yellowish) parameters were closely related to cell wall chemical composition (cellulose, hemicellulose, and lignin) [25]. This indicated that the extract may be the main reason for the obvious color difference between heartwood and sapwood when the heartwood is red in color. The main factors affecting the color of wood are lignin and wood extracts [24]. The difference in lignin content and functional group structure between heartwood and sapwood was not obvious. Therefore, it is speculated that lignin may not be the main reason for the obvious color difference of D. oliveri. Previous studies had found that in several larch species, a* (reddish) and L* (brightness) parameters were highly correlated with wood extractive content, while b* (yellowish) parameters were closely related to cell wall chemical composition (cellulose, hemicellulose, and lignin) [25]. This indicated that the extract may be the main reason for the obvious color difference between heartwood and sapwood when the heartwood is red in color.

UV-Visible Spectroscopy of D. oliveri Extracts
As shown in Figure 2, there were two peaks between 200-290 nm for the alcoholic extracts of heartwood and sapwood petroleum ether layer, and the peaks were near 210 and 285 nm, respectively. It indicated that the characteristic peaks of aromatic compounds exist in heartwood and sapwood of the petroleum ether layer extract, and the content of heartwood was higher than that of sapwood. Heartwood and sapwood water layer alcohol extracts had absorption peaks near 230, 265, and 330 nm in the ultraviolet region, but heartwood had a stronger absorption peak near 330 nm, while it was a shoulder peak in the sapwood. It shows that the chromophoric system between heartwood and sapwood in the water layer extract is roughly the same. It all contains chromophoric groups, such as benzene rings and conjugated double bonds, while the content of chromophoric groups in heartwood was higher. Heartwood and sapwood of the alcoholic extract of the ethyl acetate layer had absorption peaks near 225 and 270 nm. In addition, the heartwood had a weak and broad absorption peak at 400-600 nm, while sapwood had no absorption peak in the visible region. This indicates that chromogenic groups such as benzene rings exist in both heartwood and sapwood, while there was a long conjugate chain or polycyclic aromatic hydrocarbons at 400-600 nm in heartwood [26]. Therefore, heartwood had different chromogenic groups from sapwood.
As shown in Figure 2, there were two peaks between 200-290 nm for the alcoholic extracts of heartwood and sapwood petroleum ether layer, and the peaks were near 210 and 285 nm, respectively. It indicated that the characteristic peaks of aromatic compounds exist in heartwood and sapwood of the petroleum ether layer extract, and the content of heartwood was higher than that of sapwood. Heartwood and sapwood water layer alcohol extracts had absorption peaks near 230, 265, and 330 nm in the ultraviolet region, but heartwood had a stronger absorption peak near 330 nm, while it was a shoulder peak in the sapwood. It shows that the chromophoric system between heartwood and sapwood in the water layer extract is roughly the same. It all contains chromophoric groups, such as benzene rings and conjugated double bonds, while the content of chromophoric groups in heartwood was higher. Heartwood and sapwood of the alcoholic extract of the ethyl acetate layer had absorption peaks near 225 and 270 nm. In addition, the heartwood had a weak and broad absorption peak at 400-600 nm, while sapwood had no absorption peak in the visible region. This indicates that chromogenic groups such as benzene rings exist in both heartwood and sapwood, while there was a long conjugate chain of polycyclic aromatic hydrocarbons at 400-600 nm in heartwood [26]. Therefore, heartwood had different chromogenic groups from sapwood. The difference in wood color is associated with filled or deposited extracts in the cell wall [27]. Fen et al. [28] conducted an extensive targeted metabolic analysis of Taxus L heartwood and sapwood extracts and found significant differences in flavonoid substances, and 71 flavonoid compounds related to wood color were identified in heartwood using liquid chromatography-electrospray ionization-mass spectrometry (LC-EI-MS) technology. Flavonoids usually have obvious absorption peaks around 230-290 nm or 300-350 nm, and anthocyanins have two peaks between 200 and 700 nm, one at 500-600 nm in the visible light region, and the other is around 275 nm in the UV region. But most phenols have no absorption at 500 nm [29]. Thus, it is preliminarily determined that the main chromophoric substances in heartwood and sapwood of D. oliveri are flavonoid pigments. However, there are anthocyanins in heartwood, but not in sapwood, and the content of chromophoric groups in heartwood is also higher than that in sapwood. It may be the main reason for the obvious difference in color between heartwood and sapwood, and will be further identified by UHPLC-MS. The difference in wood color is associated with filled or deposited extracts in the cell wall [27]. Fen et al. [28] conducted an extensive targeted metabolic analysis of Taxus L heartwood and sapwood extracts and found significant differences in flavonoid substances, and 71 flavonoid compounds related to wood color were identified in heartwood using liquid chromatography-electrospray ionization-mass spectrometry (LC-EI-MS) technology. Flavonoids usually have obvious absorption peaks around 230-290 nm or 300-350 nm, and anthocyanins have two peaks between 200 and 700 nm, one at 500-600 nm in the visible light region, and the other is around 275 nm in the UV region. But most phenols have no absorption at 500 nm [29]. Thus, it is preliminarily determined that the main chromophoric substances in heartwood and sapwood of D. oliveri are flavonoid pigments. However, there are anthocyanins in heartwood, but not in sapwood, and the content of chromophoric groups in heartwood is also higher than that in sapwood. It may be the main reason for the obvious difference in color between heartwood and sapwood, and will be further identified by UHPLC-MS.

Flavonoids
Compounds 1 (tr = 4.68) and 2 (tr = 5.38) were identified as orientin and vitexin, bo flavonoid monosaccharide carbon glycosides, which were relatively stable between the B, and C rings and undergo cleavage mainly through the breaking of their glycosi bonds. Taking orientin as an example, its excimer ion peak was m/z 449 ([M+H] + ), wit possible molecular formula of C21H20O11. The MS 2 characteristic fragment ions at m/z 4 m/z 413, m/z 353, m/z 329, and m/z 299 are consistent with the data reported in the literatu for this compound [30], and its possible cleavage pathway was shown in Figure 3.   [31], and its possible cleavage pathway was shown in Figure 3. Diosmetin had an excimer ion peak at m/z 301 ([M+H] + ), and its molecular formula was probably C 16 H 12 O 6 . The MS 2 characteristic fragment ions at m/z 286, m/z 269, m/z 241, m/z 153, and m/z 134, which are consistent with the data reported in the literature for this compound [32], and its possible cleavage pathway was shown in Figure 3.

Isoflavones
Compounds 3 (tR = 5.52), 7 (tR = 8.46), and 14 (tR = 9.96) were identified as daidzin, ononin, and glycitin, all of which were isoflavone monosaccharide glucoside compounds, and the main cleavage mode is the loss of one molecule of glucoside followed by the loss of neutral molecules such as H 2 O, CO 2 , and CO, etc. Taking ononin as an example, the excimer ion peak at m/z 431 ([M+H] + ), which upon fragmentation yields the product ions at m/z 269, m/z 237, m/z 181, and m/z 137, which are inferred from the literature [33] and the database, and its possible cleavage pathway was shown in Figure 4.
(RDA) cleavage. Taking scrophulein and diosmetin as examples, the scrophulein's molecular formula was probably C17H14O6, and the excimer ion peak in positive ion mode was m/z 315 ([M+H] + ), which upon fragmentation yields the product ions at m/z 300, m/z 283, m/z 255, and m/z 168, as reported in the literature [31], and its possible cleavage pathway was shown in Figure 3. Diosmetin had an excimer ion peak at m/z 301 ([M+H] + ), and its molecular formula was probably C16H12O6. The MS 2 characteristic fragment ions at m/z 286, m/z 269, m/z 241, m/z 153, and m/z 134, which are consistent with the data reported in the literature for this compound [32], and its possible cleavage pathway was shown in Figure 3.

Isoflavones
Compounds 3 (tR = 5.52), 7 (tR = 8.46), and 14 (tR = 9.96) were identified as daidzin, ononin, and glycitin, all of which were isoflavone monosaccharide glucoside compounds, and the main cleavage mode is the loss of one molecule of glucoside followed by the loss of neutral molecules such as H2O, CO2, and CO, etc. Taking ononin as an example, the excimer ion peak at m/z 431 ([M+H] + ), which upon fragmentation yields the product ions at m/z 269, m/z 237, m/z 181, and m/z 137, which are inferred from the literature [33] and the database, and its possible cleavage pathway was shown in Figure 4.   Figure 4. Furthermore, the retention time of this compound matches well with that of biochanin A standard.

Flavonol
Compound 6 (tR = 8.14) had an excimer ion peak of m/z 317 ([M+H] + ), and its molecular formula is presumably C 16 H 12 O 7 . The MS 2 fragment at the m/z values of 285, 257, 229, and 153 were inferred from the literature [34], and the database identified the compound as isorhamnetin, and its possible cleavage pathway is shown in Figure 5.
was probably C15H10O4, and the MS characteristic fragment ions at m/z 227, m/z 199, m/z 181, m/z 137, and m/z 119. Combined with the retention time of the reference substance and the database, the compound was identified as daidzein, and its possible cleavage pathway is shown in Figure 4. In the positive ion mode, the excimer ion peak of biochanin A is m/z 285 ([M+H] + ), the molecular formula is probably C16H12O5, and the MS 2 characteristic fragment ions at m/z 257, m/z 229, m/z 197, and m/z 137, and its possible cleavage pathway is shown in Figure 4. Furthermore, the retention time of this compound matches well with that of biochanin A standard.

Flavonol
Compound 6 (tR = 8.14) had an excimer ion peak of m/z 317 ([M+H] + ), and its molecular formula is presumably C16H12O7. The MS 2 fragment at the m/z values of 285, 257, 229, and 153 were inferred from the literature [34], and the database identified the compound as isorhamnetin, and its possible cleavage pathway is shown in Figure 5.
ion peak of daidzein in positive ion mode was m/z 255 ([M+H] + ), the molecular formula was probably C15H10O4, and the MS 2 characteristic fragment ions at m/z 227, m/z 199, m/z 181, m/z 137, and m/z 119. Combined with the retention time of the reference substance and the database, the compound was identified as daidzein, and its possible cleavage pathway is shown in Figure 4. In the positive ion mode, the excimer ion peak of biochanin A is m/z 285 ([M+H] + ), the molecular formula is probably C16H12O5, and the MS 2 characteristic fragment ions at m/z 257, m/z 229, m/z 197, and m/z 137, and its possible cleavage pathway is shown in Figure 4. Furthermore, the retention time of this compound matches well with that of biochanin A standard.

Flavonol
Compound 6 (tR = 8.14) had an excimer ion peak of m/z 317 ([M+H] + ), and its molecular formula is presumably C16H12O7. The MS 2 fragment at the m/z values of 285, 257, 229, and 153 were inferred from the literature [34], and the database identified the compound as isorhamnetin, and its possible cleavage pathway is shown in Figure 5.

Chalcone
Compound 19 (tR = 11.13) had an excimer ion peak of m/z 257 ([M+H] + ), and its molecular formula is presumably C 15 H 12 O 4 . The MS 2 characteristic fragment ions of m/z 239, m/z 211, m/z 137, and m/z 119, which are inferred from the retention time of the reference substance and the database. The compound was identified as isoliquiritigenin, and its possible cleavage pathway is shown in Figure 7.

Chalcone
Compound 19 (tR = 11.13) had an excimer ion peak of m/z 257 ([M+H] + ), and its molecular formula is presumably C15H12O4. The MS 2 characteristic fragment ions of m/z 239, m/z 211, m/z 137, and m/z 119, which are inferred from the retention time of the reference substance and the database. The compound was identified as isoliquiritigenin, and its possible cleavage pathway is shown in Figure 7.

Anthocyanin
Compound 12 (tR = 9.61) had an excimer ion peak of m/z 271 ([M+H] + ), and the molecular formula was presumed to be C15H10O5, with MS 2 characteristic fragment ions at the m/z values of 239, 211, 151, and 137, which are inferred from the literature [35] and the database. The compound was identified as pelargonidin, and its possible cleavage pathway is shown in Figure 8.

Different Pigment Components between Heartwood and Sapwood of D. oliveri
The unique pigment components of sapwood of D. oliveri were daidzin, ononin, and glycitin, all of which were isoflavone glycosides, and their color state was white, while the unique pigment component of heartwood was vitexin (flavonoid carbon glycosides) and isorhamnetin (flavonol). Its color state was yellow. Pelargonidin (anthocyanin) was red, and sapwood has no anthocyanin pigment, which is consistent with the results of UV spectroscopy (see Table 3). Table 3. Different chemical constituents between D. oliveri heartwood and sapwood.

Structure
Compound Color   sapwood   isoflavone  daidzin  white  isoflavone  ononin  white  isoflavone  glycitin  white   heartwood   flavonoid  vitexin  yellow  flavonol  isorhamnetin  yellow  anthocyanin pelargonidin red This research confirmed that the main factor affecting plant color was the content and type of pigment molecules [36]. Wang et al. [37] reported that anthocyanins are the main

Anthocyanin
Compound 12 (tR = 9.61) had an excimer ion peak of m/z 271 ([M+H] + ), and the molecular formula was presumed to be C 15 H 10 O 5 , with MS 2 characteristic fragment ions at the m/z values of 239, 211, 151, and 137, which are inferred from the literature [35] and the database. The compound was identified as pelargonidin, and its possible cleavage pathway is shown in Figure 8.

Chalcone
Compound 19 (tR = 11.13) had an excimer ion peak of m/z 257 ([M+H] + ), and its molecular formula is presumably C15H12O4. The MS 2 characteristic fragment ions of m/z 239, m/z 211, m/z 137, and m/z 119, which are inferred from the retention time of the reference substance and the database. The compound was identified as isoliquiritigenin, and its possible cleavage pathway is shown in Figure 7.

Anthocyanin
Compound 12 (tR = 9.61) had an excimer ion peak of m/z 271 ([M+H] + ), and the molecular formula was presumed to be C15H10O5, with MS 2 characteristic fragment ions at the m/z values of 239, 211, 151, and 137, which are inferred from the literature [35] and the database. The compound was identified as pelargonidin, and its possible cleavage pathway is shown in Figure 8.

Different Pigment Components between Heartwood and Sapwood of D. oliveri
The unique pigment components of sapwood of D. oliveri were daidzin, ononin, and glycitin, all of which were isoflavone glycosides, and their color state was white, while the unique pigment component of heartwood was vitexin (flavonoid carbon glycosides) and isorhamnetin (flavonol). Its color state was yellow. Pelargonidin (anthocyanin) was red, and sapwood has no anthocyanin pigment, which is consistent with the results of UV spectroscopy (see Table 3). This research confirmed that the main factor affecting plant color was the content and type of pigment molecules [36]. Wang et al. [37] reported that anthocyanins are the main

Different Pigment Components between Heartwood and Sapwood of D. oliveri
The unique pigment components of sapwood of D. oliveri were daidzin, ononin, and glycitin, all of which were isoflavone glycosides, and their color state was white, while the unique pigment component of heartwood was vitexin (flavonoid carbon glycosides) and isorhamnetin (flavonol). Its color state was yellow. Pelargonidin (anthocyanin) was red, and sapwood has no anthocyanin pigment, which is consistent with the results of UV spectroscopy (see Table 3). This research confirmed that the main factor affecting plant color was the content and type of pigment molecules [36]. Wang et al. [37] reported that anthocyanins are the main coloring substances in the magnolia of purplish red and purple, while white and yellow-green magnolias contain no anthocyanins. In addition, anthocyanins have been reported to be the main pigments in red fruits [38,39]. Flavonoids and flavonols can form stable and complex pigment complexes with anthocyanins as copigments [40]. The color of pelargonidin was red, which was consistent with the color of D. oliveri. As flavonoid carbon glycosides and flavonols, vitexin and isorhamnetin have carbonyl auxochrome groups located at the 7 and 4 positions in their own structures, which can promote electron transfer and rearrangement, deepening the color of the compound. In addition, it can also combine with anthocyanins to form stable pigments [41]. In conclusion, pelargonidin, vitexin, and isorhamnetin can be used as potential markers for the color difference between heartwood and sapwood of D. oliveri.

Differences in the Content of Main Chemical Components between Heartwood and Sapwood of D. oliveri
It can be seen from Table 4 that the content of heartwood pigment components liquiritigenin, genistein, isoliquiritigenin, formononetin, and biochanin A were 293.6505 ug/g, 161.8764 ug/g, 4787.8213 ug/g, 2133.5930 ug/g, and 2000.0880 ug/g. The content in sapwood was 31.7227 ug/g, 58.3365 ug/g, 18.9532 ug/g, 57.3800 ug/g, and 3.43030 ug/g, and the content of pigment in heartwood was 9, 3, 253, 37, and 583 times higher than that in sapwood, respectively. The results showed that the content of the pigment components isoliquiritigenin, formononetin, and biochanin A in heartwood was significantly higher than that in sapwood. In another study conducted by Qiu et al., the content of compounds in heartwood and sapwood of teak (Tectona grandis) was significantly different, which was consistent with the results obtained in this paper that the content of chemical components in heartwood was greater than that in sapwood [42]. Isoliquiritigenin belongs to the chalcone, and its color state was yellow. It is an important substrate for the synthesis of other flavonoids and can form anthocyanins through a series of transformations by chalcone isomerase [43]. Formononetin and biochanin A are isoflavones, and their color state is yellow. There are carbonyl, methoxy, and other auxochromic groups related to wood color in the structure. Therefore, isoliquiritigenin, formononetin, and biochanin A may be the main pigment components to cause the color difference between heartwood and sapwood.

Materials and Preparation of Extracts
Dalbergia oliver shavings were taken from Laoyongchun Hongmu Furniture Factory, Nanning, China. It was identified by the Guangxi University Testing Centre as Dalbergia oliver heartwood and sapwood. They were then ground and passed through a 0.3 mm sieve, and these samples were used for the determination of the main chemical composition.
First, 1 g of dried wood powder was accurately weighed into a conical flask, and 30 mL 70% (v/v) of ethanol was added to it. Then, ultrasonic extraction was performed for 30 min; this process was repeated one more time. Subsequently, the extract was filtered and evaporated until dry by rotary evaporation under a vacuum to obtain the concentrated solution. The suspension solution was prepared by adding distilled water to the concentrate. After that, extraction was performed using petroleum ether and ethyl acetate, in that order. Three solutions were collected, concentrated, and evaporated to obtain the extract of the sample. These samples were used for extract analysis.
The dried sample (0.05 g) was precisely weighed and added to an 80% (v/v) methanol solution (5 mL), following which the extraction was performed by sonication for 40 min (40 kHz). The extraction solution (0.1 mL) was diluted 10 times with methanol and passed through a 0.22 µm microporous filter membrane, and then placed in a chromatography sample bottle. These samples were used to extract UHPLC-MS analysis.

Determination of Chemical Composition
Determination of α-cellulose and hemicellulose content was measured by reference to the Chinese national standards according to GBT 2677.10-1995, "Fibrous raw material-Determination of Holocellulose" and GBT 744-1989, "Pulps-Determination of α-cellulose", respectively.
The determination of lignin was carried out in accordance with the National Renewable Energy Laboratory (NREL) NREL/TP-510-42618, Determination of structural carbohydrates and lignin in biomass.

Fourier Infrared Spectroscopy (FTIR) Analysis of Lignin
The spectral characteristics of the lignin were determined using Fourier infrared spectroscopy (Thermo Fisher Scientific, Waltham, MA, USA). The sample was carefully ground, and 1-2 mg of the sample was ground with 0.1 g potassium bromide. The mixture was crushed into a preform. The analysis was performed in the measurement range of 400-4000 cm −1 with 32 cumulative scans.

UV Spectroscopic Analysis
The extract was measured using a UV spectrophotometer (Shimadzu Corporation, Shanghai, China). The analysis was ensured in the measurement range of 190-1100 cm −1 . The resolution and sampling time of the instrument was set to 3.3 nm and 1 ms, respectively. Spectral data for the sample was collected and analyzed.

Ultra-High Performance Liquid Chromatography-Mass Spectrometry (UHPLC-MS) Analysis
The chemical composition of the samples was identified by UHPLC-MS (Thermo Fisher Scientific, Waltham, MA, USA). The chromatographic column was ACQUITY UPLC BEH C18 (2.1 mm × 50 mm × 1.7 µm). The ratio of formic acid and water was 1:1000 in mobile phase A. Methanol was chosen as mobile phase B. Gradient elution conditions were as follows: 0-15.0 min, 5% B; 15.0-18.0 min, 5-80% B; 18.0-18.1 min,100-5% B; 18.1-21 min, 5% B. The flow rate was 0.3 mL·min −1 . The column temperature was 30 • C. The injection volume was 1 µL. The electrospray ionization was 300 • C. The transfer capillary temperature was 320 • C. The spray voltage in positive ion mode was 3.0 kV. The scan modes were selected as Full MS and Full MS/dd-MS 2 , respectively. The primary and secondary scan resolutions were 70,000 and 17,500. The flow rates of sheath gas and auxiliary gas were 206 and 69 kPa. The mass range was 100-1000 Da.
The content of the characteristic flavonoid component was determined by the external standard method. The standard working curve of flavonoids was established by the ratio of peak area and sample concentration.

Data Analysis
A t-test for the parameters of the chemical composition of the wood was run in SPSS package ver.24.0.0.0. The identification of chemical components uses the XIC Manager module in the Compound Discoverer 3.1 software (Thermo Fisher Scientific, Waltham, MA, USA). for automatic screening and analysis of accurate relative molecular masses, combined with relevant literature, mzCloud Mass Spectral Library database, compound secondary mass spectrometry fragmentation rules, and retention times of chemical standards to infer their possible chemical structures.

Conclusions
The content of the main components in the heartwood and sapwood of D. oliveri was not obvious, and heartwood and sapwood were weakly acidic. The lignin content was similar between heartwood and sapwood, and the functional group structure was basically identical. The main color-emitting components of heartwood of D. oliveri are flavonoids and anthocyanins, and for sapwood they are flavonoids. The pigment components in heartwood and sapwood of D. oliveri were identified, and their main components were quantified. A total of 21 flavonoid compounds were identified from heartwood and sapwood, among which vitexin, isorhamnetin, pelargonidin, isoliquiritigenin, formononetin, and biochanin A may be the main pigment components affecting the color difference between heartwood and sapwood.