Teak wood (Tectona grandis L. f.) is known in the international market due to its natural durability (Class 1), dimensional stability, physical-mechanical properties and aestheticity (Silva et al., 2010; Upadhyay, Eid, & Sankhayan, 2005). Although Mexico does not have official statistical information on teak plantations, production and quality of its wood, interest in this species is growing. This has motivated the establishment of commercial plantations, of which, the majority is between 9 to 20 years old. The states reported with teak plantations are: Campeche, Colima, Chiapas, Hidalgo, Jalisco, Michoacán, Nayarit, Oaxaca, Puebla, Quintana Roo, San Luís Potosí, Sinaloa, Tabasco, Tamaulipas, Veracruz and Yucatán (Secretaría de Medio Ambiente y Recursos Naturales [SEMARNAT], 2016).
In general terms, wood is made up of different cellular structures that, depending on the configuration, perform specific functions in the plant. The study of the structure of wood is a starting point for several studies with which it is intended to know the functioning, associative behavior and relationship with the environment (Bhat & Priya, 2004; Kang, Zhang, & Mansfield, 2004; Verheyden, De Ridder, Schmitz, Beeckman, & Koedam, 2005). Wood has variations, especially when considering the origin, age and quality of the site (Richter et al., 2003; Thulasidas & Bhat, 2012). Some studies indicate that the quality of wood from plantations has differences in heartwood coloring and mechanical properties, compared to that from natural forests (Richter, Leithoff, & Sonntag, 2003; Zobel, 1984).
The precise knowledge of the characteristics of the wood cell types allows to determine the conditions of processing and more pertinent use of forest species. In this context, the objectives of the present study were to describe the organoleptic properties of teak wood and determine the main anatomical characteristics of the inner and outer part of the heartwood, according to age and growth site.
Materials and methods
Selection and collection of samples
A total of ten trees were randomly selected and felled according to the group of origin and age: Campeche (9 and 15 years), Tabasco (15 years) and Chiapas (21 years), obtaining a total of 40 samples. Table 1 describes the climatic conditions of the sampling sites and the characteristics of the plantation.
|Sampling site||Climate||Annual average precipitation (mm)||Annual average temperature (°C)||Type of soil||Tree age (years)||Diameter at breast height (cm)||Tree height (m)|
|Campeche19° 44’ 55.85” N90° 10’ 44.85” O||Warm subhumid||1 300 - 1 500||24 - 28||Orthic Solonchak||9||20||15|
|Chiapas17° 38’ 50.58” N91° 40’ 27.67” O||Warm wet||2 000 - 2 500||26||Pelvic Vertisol||21||33||25|
|Tabasco17° 52’ 29.55” N91° 18’ 28.89” O||Warm subhumid||1 500 - 2 000||26 - 28||Cambisol eutric||15||27||14|
The organoleptic and anatomical characteristics of teak wood heartwood were determined in slices obtained at diameter at breast height. A strip of 1.5 cm wide was cut radially in the center of the slice, along the transverse side. A sample (1.5 x 1.5 cm) was taken in the inner part of the heartwood located just to the first growth ring posterior to the medulla, which was called “1”. The second sample was extracted from the outer part of the heartwood, in the limit with the sapwood, which was called “x”, as shown in Figure 1.
The organoleptic description of wood was made according to that described by Burger and Richter (1991). The cell elements were described as stipulated by the International Association of Wood Anatomists (IAWA, 1989).
For the study of the cell structure, the samples were softened for 6 h using boiling water. Cuts between 10 and 20 μm of thickness of each of the anatomical faces (transversal, radial and tangential) were made with a sliding microtome (Microtomo American Optical model 860, USA) (Burger & Richter, 1991). The slices were observed in a light field microscope (Wild Heerbrugg model M-12 83632, Switzerland) with a video camera attached (Hitachi KP-D51 color, Japan), making 30 measurements per character. The images of the histological sections were captured with Matrox PC-VCR version 02.10.10 (Matrox Graphics Inc. 2002). The cell elements were measured with ArcView GIS 3.2 (Environmental System Research Institute, Inc., 1999).
For the individual measurement of the length and thickness of the fiber cell wall, of the internal “i” and external “x” of the heartwood, splinters were dissociated in a test tube with Franklin's solution (1:2 glacial acetic acid and 30 % hydrogen peroxide) in a water bath at 60 ± 5 °C, until they were soft and whitish. Subsequently, water was added to the tubes, then shaken and decanted, until the reagents used were eliminated. Finally, a double stain with safranin “O” and astral blue was made (Burger & Richter, 1991).
The statistical analysis was performed with STATGRAPHICS® Centurion XV version 15.2.06 (StatPoint Inc., 2007). A unifactorial analysis of variance was carried out, both for age and origin, and multiple range tests of Fischer LSD to establish the statistically significant differences (P < 0.05).
Results and discussion
Qualitative characteristics of wood
Tectona grandis wood has growth rings with well-defined limits; brown to yellow (golden brown) heartwood with pronounced veins; yellowish cream sapwood, different from the heartwood color; greasy surface to the touch and rubber odor. The growth ring is demarcated by the presence of larger vessels (Figure 2 A1) and marginal parenchyma (Figure 2 A2). The wood has annular or semi-annular porosity with solitary vessels (Figure 2 A3) and in groups of two to three (Figure 2 A4), arranged in a radial pattern. The perforation plates are simple.
The tyloses in the vessels are thin-walled (Figure 2 A5). The intervessel pits are alternate, with a diameter (vertical) of 5 to 6 μm and ornamented; vessel-ray pits have distinct borders, similar to intervessel pits (Figure 2 B1). Early wood has scanty vasicentric paratracheal axial parenchyma. The multiseriate rays have (2-)3-4(-5) wide cells (Figure 2 C1); the homocellular rays (Figure 2 D1) are composed of a single cell type (procumbent, Figure 2 D2). There are vascular or vasicentric tracheae commonly present, only in latewood. The fibers have uniformly distributed septa (Figure 2 D3). The fiber pits, for the most part, are restricted to radial walls, simple or with tiny borders (Figure 2 E1). The lumen contains clear, almost transparent, organic substances in the form of small droplets (Figure 2 F1).
Quantitative characteristics of wood
Number of vessels per square millimeter
The results of the number of vessels per square millimeter are shown in Table 2. Although the difference is minimal between the internal and external part, the sum of vessels across the width of the heartwood is important. The wood of 9 year-old-trees had the lowest number of vessels in the external part of the heartwood; after 15 years, the quantity tends to be uniform, so it can be deduced that age has an important influence on this variable. The inner part of the heartwood, regardless of age, tends to generate more vessels than the external part. Apparently, in the internal part, the diameter of the trunk plays an important role in the generation of vessels, since the number decreased as the diameter at breast height increased. Rahman, Fujiwara, and Kanagawa (2007) mention that the number of vessels, together with the proportion of medullary rays, have an important relationship with the density of the wood.
|Origin, age and DAP||Vessels·mm-2||Vessels diameter (µm)||Ray width (µm)||Ray Height (µm)||Fiber length (µm)||wall thickness (µm)||Diámetro de fibras (µm)|
|Campeche: 9 years, DAP = 20 cm||8||6||121.1 a||127.8 a||71.3 a||73.1 b||702.9 b||685.8 b||811.4 a||941.3 a||2.0 a||2.8 a||27.0 b||26.7 a|
|Campeche: 15 years, DAP = 32 cm||7||7||117.3 a||127.3 a||69.9 a||64.2 a||561.2 a||600.2 b||924.1 b||1 133.5 c||3.1 b||3.6 b||25.3 a||27.0 a|
|Chiapas: 21 years, DAP = 33 cm||7||7||104.3 b||122.2 b||64.3 b||67.6 a||551.9 a||570.8 a||1 036.2 c||1 168.2 c||3.2 b||3.7 b||27.6 b||30.4 b|
|Tabasco: 15 years, DAP = 27 cm||8||7||109.2 b||120.5 b||64.2 b||76.0 b||539.8 a||566.6 a||910.4 b||1 053.1 b||3.6 c||4.3 c||25.9 a||26.8 a|
|Internal and external population||7||118.2||68.8||593.3||993.6||3.3||27.13|
The quantity of the Mexican production of teak vessels is within the ranges reported in other parts of the world. Josue and Imiyabir (2011) indicate average values of 6 in ranges of 2 to 12 vessels·mm-2 for early wood, and 8 in ranges of 4 to 15 vessels·mm-2 for 15 year-old-latewood trees from Sabah, Malaysia. On the other hand, Moya, Berrocal, Serrano, and Tomazello (2009) counted 2 to 10 vessels·mm-2 in 13-year-old teak, from Costa Rica.
Bhat, Priya, and Rugmini (2001) assert that trees subjected to high growth stress (slow growth) tend to produce more vessels compared to low stress trees and also point out that irrigation increases the diameter of these. These authors also conclude that the cambium activity may be greater in the early stages of plant development, generating more vessels, which tend to decrease or become uniform as the age of the tree increases up to a certain period. These authors also conclude that the cambium activity may be greater in the early stages of plant development, generating more vessels, which tend to decrease or become uniform as the age of the tree increases up to a certain period.
Table 2 shows the results of vessel diameter. In this characteristic, origin influenced more than age. The results show that the diameter of the vessels was greater in the outer part than in the inner part of the heartwood, with significant statistical difference (P < 0.05) between both areas as shown in Table 3.
|Caracteristics||Area||Campeche(9 year-old)||Campeche(15 year-old)||Chiapas (21 year-old)||Tabasco(15 year-old)||Population||Inner and outer population|
|Vessel diameter (µm)||Inner||121.08 a||117.26 a||104.33 a||109.19 a||112.28 a||118.25|
|Outer||127.85 b||127.29 b||122.16 b||120.46 b||124.22 b|
|Ray width (µm)||Inner||71.26 a||69.89 a||64.34 a||64.23 a||67.48 a||68.81|
|Outer||73.15 b||64.16 b||67.65 b||76.02 b||70.14 b|
|Ray height (µm)||Inner||702.97 a||561.25 a||551.92 a||539.83 a||583.29 a||593.28|
|Outer||685.83 a||600.19 b||570.76 a||566.61 a||603.28 b|
|Fiber length (µm)||Inner||811.39 a||924.13 a||1 036.21 a||910.38 a||917.99 a||993.62|
|Outer||941.28 b||1 133.48 b||1 168.21 b||1 053.10 b||1 069.23 b|
|wall thickness (µm)||Inner||2.02 a||3.08 a||3.21 a||3.64 a||3.04 a||3.32|
|Outer||2.80 b||3.58 b||3.70 b||4.29 b||3.60 b|
|Fiber diameter (µm)||Inner||27.02 a||25.35 a||27.65 a||25.90 a||26.46 a||27.13|
|Outer||26.75 a||27.05 b||30.39 b||26.83 a||27.81 b|
On the other hand, Table 2 shows certain relationship between the number of vessels per square millimeter and diameter, showing that as the number of vessels increases, the diameter tends to decrease, keeping a balance between both components. Bhat et al. (2001) analyzed teak wood from three localities of Kerala (India) and found that the diameter of vessels increases until the age of 20 years, after that, there is a slight decrease until 25 years and then tends to uniformity. These authors obtained vessel diameters with variations from 177 to 186 μm, for fast-growing wood, and 162 to 177 μm, for slow-growing wood. Moya et al. (2009) reported that the diameter of vessels (100 to 195 μm) increased with age, in 13-year-old teak trees from two regions in Costa Rica, and detected an inflection in growth rings between 5 and 7 years. On the other hand, Josue et al. (2011) indicated average values of 228 μm in ranges of 150 to 395 μm for early wood, and 112 μm in ranges of 49 to 289 μm for 15 year-old-late teak wood in Sabah (Malaysia). The diameter of teak wood vessels from the Mexican southeast is lower than those obtained by the aforementioned researchers. This may be due to different causes, among them, Anish, Annop, Vishnu, Sreejith, and Jijeesh (2015) propose that the dimension of the vessels can be attributed to the variation of the proportion of young and mature wood, and to the variation due to the growth stress as a result of silvicultural practices; they also mentioned that the relationship between tree age and vessel diameter is significant (P < 0.05).
The results of rays’ width are shown in Table 2. In contrast, in the external part has not a pattern that defines any trend; 9 year-old trees from Campeche and 15 year-old trees from Tabasco had greater rays width, being statistically equal (P > 0.05), while 15 year-old trees from Campeche and 21 year-old trees from Chiapas formed another group. Therefore it follows that, in the inner part, origin was more important than age. In contrast, in the outer part there is no pattern that defines any tendency; 9 years old trees from Campeche and 15 years old trees from Tabasco had greater width of radios, being statistically equal (P > 0.05), while 15 years old trees from Campeche and 21 years old trees from Chiapas formed another group. Due to this it is inferred that, in the external zone, neither the age nor the origin influences the rays’ width. With respect to the heartwood area, the outer zone exhibited higher values (P < 0.05) of rays’ width compared to the internal part (Table 3).
Medullary rays’ width of teak wood from the Mexican southeast is higher than that reported by other researchers. Rahman et al. (2007) indicated that medullary rays in teak wood from the districts of Sylhet and Rangamti (Bangladesh) were 51 and 59 μm, respectively. Moya, Muñoz, and Berrocal (2010) reported a rays’ width of 58 μm with a range of 23 to 112 μm, without mentioning age and origin of wood. Ypushima-Pinedo et al. (2014) determined the rays’ width of 9 year-old wood from Veracruz and Nayarit, with values of 66 and 60 μm, respectively.
The factors that can affect medullary rays’ width are several. Rahman et al. (2007) mention that the anatomical structure of rays is an individual characteristic that is not affected by growth rate, and that rays dimension and proportion seem to be under genetic control. In contrast, Anish et al. (2015) indicate that the environment plays an important role in rays’ width and height, since a stress medium reduces the growth rate of the cambium. Aloni and Zimmermann (1983) assert that the concentration of auxins, main component that regulates cell divisions of the cambium, decreases when age increases, producing fewer vessels and rays, but with cells with larger diameter and dimensions.
The presence of abundant polyserial medullary rays can have a negative effect on the mechanical properties and tangential contraction of wood (Rahman et al., 2007).
The results of rays’ height are shown in Table 2. The statistical analysis shows that age is determinant in rays’ height of the inner part of the heartwood, because 9 year-old wood from Campeche had lower height than the rest of ages and origins. However, the behavior of the outer part of the heartwood is different, since the origin was conclusive in rays’ height; 9 and 15 year-old trees from Campeche had the highest values and were statistically equal (P > 0.05). The height of the medullary rays between the inner and outer part of the heartwood were statistically similar in the samples (P > 0.05), except for the 15-year-old wood from Campeche (Table 3).
The influence of the environment and age on rays’ height has been recorded by Bhat et al. (2001). These authors mentioned that medullary rays of fast-growing trees are significantly longer than slow-growing ones, since the stress environment reduces the growth rate of the cambium. On the other hand, Anish et al. (2015) indicated that rays’ height shows a significant difference (P < 0.05) with respect to the age of the trees.
Fiber length is reported in Table 2. The results show that, at an older age, the length of the fiber tends to increase. Table 3 shows that there are significant differences in fibers length between the inner and outer part of the heartwood. The relationship of length with age has been recorded in different studies; for example, Kokutse, Adjonou, and Kokou (2009) mention that, in young stands, fiber length increases with age; Bhat and Priya (2004) and Thulasidas and Bhat (2012) make the same statement and point out that maturity is reached between 15 and 25 years depending on the locality; after this age, fiber length does not vary significantly. On the other hand, Moya et al. (2009) conclude that the effect of the type of climate or site quality produces few variations in fibers.
Fiber length affects the properties of paper and strength of wood by improving unification, due to more cell-cell contacts (Via, Stine, Shupe, Chi-Leung, & Groom, 2004).
Fiber cell wall thickness results are shown in Table 2. With significant differences (P < 0.05), the 15-year-old tree wood from Tabasco had the highest values, followed by the group formed by 21 year-old trees from Chiapas and 15 year-old trees from Campeche and, finally, 9 year-old trees from Campeche. This relationship occurred both in the internal and external part of the heartwood. The cell wall of the inner and outer part of the heartwood showed significant differences (P < 0.05) in thickness (Table 3), probably related to the maturation process of the cambium.
Table 2 shows a certain (positive) relationship between fiber length and cell wall thickness of 9 and 15 year-old trees from Campeche and 21 year-old trees from Chiapas, estimating that when fiber length increases, the thickness of the cell wall also tends to increase. However, 15 year-old tree wood from Campeche and Tabasco are statistically different (P < 0.05), deducing that origin plays an important role in the maturation process of the cambium up to a certain age, as stated by different researchers (Bhat & Priya, 2004; Moya et al., 2009; Thulasidas & Bhat, 2012; Zobel & Sprague, 1998).
The relationship of anatomical characteristics, such as fiber length and cell wall thickness, is significant with respect to mechanical properties such as resistance to compression and flexion (Chowdhury et al., 2012; Kiaei & Samariha, 2011; Thulasidas & Bhat, 2012).
Diameter of fibers
Table 2 shows the results of fiber diameter, showing that 21-year-old wood from Chiapas and 9 year-old wood from Campeche had the highest values in the inner part of the heartwood and are statistically similar (P > 0.05). The second group consists of 15-year-old wood from Campeche and Tabasco. The results do not show a pattern defined by age or origin, so it can be considered that there is irregularity in the growth during the first stages of plant development, which is reflected in the disparity of the values. In the case of the external part of the heartwood, the values of 21 year-old wood from Chiapas were the highest and statistically different (P < 0.05) compared to the rest. On the other hand, 9 year-old wood from Campeche and 15 year-old wood from Campeche and Tabasco are statistically similar, which shows that, in the outer part of the heartwood, age plays an important role in the diameter of fibers; as age increases, the diameter of the fibers also increases. Regarding the results of the internal and external part of the heartwood, there is a statistically significant difference (P < 0.05) between both sections of the 15-year-old wood from Campeche and 21 year-old wood from Chiapas, but not for the 9-year-old wood from Campeche and 15 year-old wood from Tabasco.
The diameters of teak fibers, coming from the Mexican southeast, are below those exposed by other researchers. Josue and Imiyabir (2011) reported values of 34.6 μm fiber diameter in 15-year-old wood in Malaysia. Thulasidas and Bhat (2012) evaluated 35-year-old teak from three agroforestry locations in Kerala, India. One of the sites was considered wet, the other dry, and a third of forest plantation; the dry site had smaller fiber diameters (28.98 μm) and statistically different (P < 0.05) from those obtained in humid sites (31.06 μm) and with commercial plantation (30.18 μm).
There is an inverse relationship between the number of vessels per square millimeter and vessels diameter in the heartwood of Tectona grandis wood. It follows that the diameter at breast height of the trunk and age play an important role in generating vessels. Origin acts significantly in the increases of vessels diameter, width of medullary rays of the internal part and height of medullary rays of the external part of the heartwood. As age increases, the length and diameter of fibers increase, being greater in the outer part of the heartwood. Regardless of age and origin, the inner part of the heartwood tends to generate more vessels than the outer part, while this has higher values in diameter of vessels, width of medullary rays and thickness of the cell wall of fibers that the inner part. This is attributed to the maturation of the cambium and proportion of young wood, since wood coming from plantation trees is characterized by its rapid growth.