IntroductionTop

Teak (Tectona grandis L.f.) is a woody species belonging to the family Lamiaceae, native to Southeast Asia, mainly from Myanmar, Thailand, India, Malaysia and the Lao People's Democratic Republic (Veit, 1996; Figueiredo et al., 2005). It is estimated that the natural forests of the species occupy approximately 29 million hectares and the planted forest area is between 4.35 and 6.89 million hectares (Kollert & Kleine, 2017). The beauty, workability and durability of its multiple-purpose wood, have made teak the most planted tropical wood species in the world (Kollert & Cherubini, 2012).

In Brazil, the teak was introduced in the decade of 1930 and the first commercial plantations started at the end of the decade of 1960, in the city of Cáceres, state of Mato Grosso (Cáceres Florestal, 1997; Schulli & Paludzyszyn Filho, 2010). It is estimated that in 2018, Brazil had a planted area of 93.957 ha, concentrated in the state of Mato Grosso, followed by the state of Pará (IBA, 2019). Factors such as adaptation to the country climatic conditions, availability of suitable land, high productivity and rapid growth, enabled rotation ages from 20 to 25 years (Costa, 2011; Camino & Morales, 2013). The management by high forest systems is performed with 2 to 4 intermediate thinning, with final cut at 20 to 25 years of age (Foelkel, 2013). In Central and South America, a rotation of 20 to 25 years presents a production that varies between 10 and 20 m³ ha^-1year^-1 (Pelissari et al., 2014).

Currently in Brazil plantations with selected clones of teak have superior performance in growth in DBH (diameter at breast height), average cross-sectional area and basal area when compared with the seminal plantations (Miranda, 2013). Thence, the establishment of clonal plantations became the most adopted practice for the species in the country (Costa et al., 2015).

The characterization of cultivated plant genotypes is an important step in breeding programs, germplasm conservation and is essential in the process of protecting cultivars. The increase in planted area and the high value that teak wood has on the international market aroused in many Brazilian foresters and breeders the interest in seeking mechanisms that allow the intellectual protection of the teak clones developed by them.

Brazil is a member country of the UPOV (Union for the Protection of New Varieties of Plants) and the guidelines for the process of crop protection in the country were established by the Ministry of Agriculture, Livestock and Supply by the Cultivar Protection Law, No. 9,456, of April 25, 1997, regulated by the Decree No. 2,366, of November 5, 1997. This law guarantees intellectual property rights to the creators of a new cultivar. According to this law, the protection of a new cultivar must be result of genetic improvement and must be clearly differentiable from other cultivars through characteristics called morphological descriptors that guarantee its distinction, homogeneity and stability (DHS) through successive generations, creating an identity for each genotype. Thus, for each species, morphological descriptors must be selected to guarantee their distinction of different cultivars to be protected (Aviani, 2011; Santos & Pacheco, 2011).

Although it is an important species in the Brazilian economy, teak still does not have minimum descriptors established by the Ministry of Agriculture, Livestock and Food Supply for cultivars protection in the country, as well as in other countries around the world.

Studies related to morphological characterization have already been developed in teak clones at different ages, allowing differentiation among groups of genotypes and origins, besides identifying the genetic variation (Gunaga et al., 2013; Miranda, 2013; Alcântara et al., 2016; Baretta, 2016; Chimello et al., 2017). However, it has not yet been possible to establish a table of morphological descriptors for the differentiation of genotypic based on the morphological characteristics used, as they did not meet the DHS test. Thus, the objective of this study was to evaluate morphological descriptors to differentiate teak clones in plantations, in the southwest region of the state of Mato Grosso, in order to propose morphological descriptors to support the cultivars protection process in Brazil and in the world.

ResultsTop

Selected descriptors for each age

From 41 evaluated characteristics, 26 and 28 of them allowed the distinction among clones, at 29 and 41 months of age respectively (Table S3 and S4 [suppl.]).

These characteristics presented distinction among the clones and homogeneity within individuals of the same clone (got they have the same code). For the two ages, these are the characteristics for the trunk: insertion angle of the branches, intensity of the color gray, brown color intensity of the bark, inner color of bark, suckers, persistence of bark, and bark drop; for the branch: leaf position, tropism, and sprouting; for the apical branch: leaf position; for the crown: density; for the leaf: attitude, and petiole; for the leaf blade: length, width, length/width ratio, pubescence on the lower face, intensity of the color green on the upper face, intensity of green color on the lower face, shape, shape of the tip of the apex, margin, undulation of the margin, venation, and brightness. The characteristics “presence of inflorescence” and “spots in the trunk” appeared only at the older age.

There were characteristics that were not homogeneous and others that did not allow distinction among the genetic materials. These were the characteristics for the trunk: crown habit, number of internodes per linear meter, and distance between nodes in 1 m; for the branch: shape in cross section, and pubescence; for the apical branch: insertion of branches; for the leaf: phyllotaxy, and petiole length; and for the leaf blade: pubescence on the upper face, consistency, shape of basis, main vein, and veins. Therefore, they were discarded as morphological descriptors because they did not meet the DHS test. In table S5 [suppl.], the authors show how these characteristics were estimated and evaluated.

Through the “numeric code” created for each clone, it was possible to observe that some characteristics have the same pattern for some clones. However, all clones could be differentiated by at least one characteristic (Table 4).

 

Table 4. Morphological descriptors code that allows the distinction among the clones of Tectona grandis L.f. at 2 evaluation times and of the 17 morphological characteristics that met DHS at 29 and 41 months of age

 

Genetic similarity and cluster analysis for clones at age 29 months

For the 26 morphological characteristics at the age 29 months, at a similarity distance of 0.5 the formation of f ive groups was observed (Fig. 2a): group one (clones 7, 11, 12, 13, 16, 17 and 18), group two (clone 8), group 3 (clones 3 and 6), group four (clones 1 and 4), and group 5 (clones 2, 5, 9, 10, 14 and 15). Clone 8 presented lower similarity in relation to the other genotypes, with a distance of 0.45. The most similar clones in their morphological characteristics are clones 11 and 13 and clones 2 and 10, presenting similarity of 0.73 (Table S6 [suppl.]).

The clones 11 and 13 differ by the trunk characteristics, intensity of gray color and intensity of the brown color of the bark, the bud, sprouting, leaf blade, and intensity of thee green color on the lower face. Clones 2 and 10 differ by the characteristics of the trunk, inner color of bark and bark drop, and leaf blade shape from of the tip to the apex and venation.

 

Figure 2. Dendrogram of similarity of Tectona grandis L.f. clones, upon the UPGMA clustering method, based on the Jaccard similarity index. The similarity was obtained by evaluating 26 and 28 morphological characteristics for the ages of 29 (a) and 41 (b) months respectively and for the 17 characteristics that exhibited DHS and remained at the same ages of evaluation (c)

 

Genetic similarity and cluster analysis for clones at age 41 months

For the 28 morphological characteristics at age 41 months, it is also observed that all the clones have distinction among them (Fig. 2b). At a similarity of 0.5, it is possible to identify six groups: group one (clones 8 and 14), group two (clones 2, 5, 9, 10 and 15), group 3 (clones 4 and 1) group four (clone 6), group five (clone 3) and group 6 (clones 7, 11, 12, 13, 16, 17 and 18). The genotypes that showed fewer similarities at this age are clones 3 and 6 with a distance of 0.43. Greater similarity occurred between clones 5 and 9, with a distance of 0.75 (Table S6 [suppl.]), which are differentiated by the following trunk characteristics: insertion angle of the branches, color of inner bark, and spots on the trunk; leaf blade: shape from the tip to the apex, and undulation of the margin.

 

Selected descriptors for both ages

From 41 characteristics evaluated, only 17 showed the same behavior for both ages and met the requirements of the DHS test and, thus, can be considered morphological descriptors (Table S7 [suppl.]). These characteristics are related to the trunk: the insertion angle of the branches, color of inner bark and presence of suckers, persistence of bark, and bark drop; the branches: tropism and sprouting in branches; the canopy: density; the leaf: petiole; and the leaf blade: length, width, pubescence on the lower face, shape, shape from the tip to the apex, margin, venation, and brightness.

Just as it happened for each age separately through the “numeric code” created for each clone, it was possible to observe that some characteristics have the same pattern for some clones. However, all clones could be differentiated by, at least, one characteristic (Table 4).

For the 17 characteristics that enabled distinction at both ages, each of them was described below: the insertion angle of the branches was obtuse only for the clone 6, while clones 1, 2, 3, 10, 11, 13 and 15 showed acute angle and the other clones showed the right angle (Fig. 3a-c).

The color of the inner bark of the trunk for clone 5 was whitish; for clones 7, 10 and 14, it was light greenish; clones 11, 12, 13, 16, 17 and 18 had dark greenish coloration; and the other clones were distinguished by the yellowish color (Fig. 3d-g).

The persistence of the bark was low for clones 1 and 4, medium for clones 3, 5, 9 and 15, and the other showed high persistence of the bark. For the bark drop, the clones 1, 2, 3, 4, 5, 9 and 15 were in the form of cracks, and the others had no drop (Fig. 3h-j).

In terms of tropism for the branch, clone 6 showed pendant tropism, clones 4 and 12 were bent, and the other clones showed erect tropism (Fig. 4a-c). For the density of the crown, clones 11, 12, 13, 16 and 17 showed low density; clones 7 and 18 had a medium density; and the other clones showed high density (Fig. 4d-f).The presence of sprouting in the branches was observed for clones 6, 7, 12, 13, 17 and 18 (Fig. 4g). Presence of suckers was observed in clones 7, 11, 12, 13 and 17 (Fig. 4h).

The petiole was present for most of the clones, and was absent for clones 1, 2, 4, 10 and 14 (Fig. 5a,b). The length of the leaf blade was short for clones 1, 2, 4, 5, 8, 9, 10, 14 and 15, and the others had long length. The length of the leaf blade ranged from narrow for clones 1, 2, 4, 5, 9, 10, 14 and 15 to large for the other genotypes. The shape of the leaf blade, for most of the clones, presented an elliptical shape and only clones 1 and 4 showed obovate shape (Fig. 5c,d). The shape of the apex of the leaf blade ranged between mucronate for the clones 1 and 4, cuspidate for clones 5, 7, 8, 10, 14 and 17, and acute for the others (Fig. 5e-g).

The margin of the leaf blade for most clones was whole, and for clones 3 and 6, it was dentate; whilst the venation of the leaf blade for clones 2, 3, 6 and 15 touches the margin of the leaf blade (Fig. 5h,i). Another characteristic that also enabled distinction among clones was leaf brightness, and it was present in nine clones (Fig. 5j,k).

 

Figure 3. Insertion angle: acute (a), right (b) and obtuse (c) branches; color of the inner bark of the trunk: whitish (d), yellowish (e), light greenish (f) and dark greenish (g). Persistence and bark drop of the trunk: low with drop (h), medium with drop (i) and high without drop (j).

 

Figure 4 Tropism: erect branch (a), curved (b) and (c) pending; crown density: low (d), medium (e) and high (f); presence of sprouts in the branch (g); presence of suckers in the trunk (h).

 

Figure 5. Tropism: erect branch (a), curved (b) and (c) pending; crown density: low (d), medium (e) and high (f); presence of sprouts in the branch (g); presence of suckers in the trunk (h).

 

Genetic similarity and cluster analysis for clones using the descriptors with the same pattern at both ages

Based on the clustering analysis for the 17 characteristics considered as morphological descriptors, it is observed the formation of four groups (Fig. 2c) at a similarity of 0.5: group one (clones 7, 11, 12, 13, 16, 17 and 18), group two (clones 3 and 6), group 3 (clones 1 and 4), and group four (clones 2, 5, 8, 9, 10, 14 and 15). The group 1 (clones 7, 11, 12, 13, 16, 17 and 18) grouped most of the clones from Laos at both ages. For the others, it seems that there is some association, with the proximity of some clones from the Solomon Islands. The grouping is also associated with the majority of the clones as to their origin, mainly those from Laos (clones 11, 12, 13, 16, 17, 18) and the Solomon Islands (clones 1, 2, 4, 8, 10, and 15), and is very different among them (Fig. 2c).

According to similarity matrices of morphological characteristics (Table S8 [suppl.]), the genotypes that showed high similarity have specific characteristics which allow the distinction. For example, clones 11 and 13 from Laos are the closest, with similarity of 0.88 and differed only in the characteristic of the branch sprouting. There are also very different genotypes such as clones 1 and 12, which are of the same origin in the Solomon Islands with a similarity of 0.06. These clones share only two characteristics: the margin and venation of the leaf blade.

Clones 3 and 6 from India and Laos, respectively, also showed low similarity, at a distance of 0.52, distinguished by the characteristics of the trunk (insertion angle of the branches, persistence of bark and bark drop) and of the branch (tropism and sprouting).