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This research was conducted to investigate the need for a breeding strategy for Toona sinensis Roem. The initial section examined the assessment of the foundational population derived from community forests during the years 2006–2008. This included an evaluation of genetic diversity, mating system analysis, and the estimation of parameter values for genetic growth. Therefore, the primary aim of T. sinensis breeding program was to enhance productivity. To achieve this goal, fundamental requirements were identified for devising a breeding approach for T. sinensis. The second section discussed the assessment of six key areas, namely (1) breeding goals, (2) access to fundamental and breeding populations, (3) selection and enhancement, (4) genetic testing, (5) family relationship management, and (6) reproduction. The results showed that a considerable influence was exerted on the efficacy of the selection. Furthermore, the importance of formulating clear and focused objectives was analyzed with an emphasis on one or two specific aims. It was crucial to acquire a comprehensive understanding of reproductive biology, gene activity, and the interplay between genotype and environmental factors. Suggestions for T. sinensis included formulation of a breeding strategy, establishment of a dedicated breeding population, creation of seed orchards, distribution of high-quality seeds, and enhancement of productivity within community forests.
Research Center for Plant Conservation, Botanic Garden and Forestry, National Research and Inovation Agency (BRIN), Bogor, Indonesia
Budi Utomo
Faculty of Forestry Silviculture Department, Universitas Sumatera Utara, Medan, North Sumatra, Indonesia
*Address all correspondence to: jayu001@brin.go.id
1. Introduction
Indonesian Toona sinensis Roem is a species widely planted in community forests because of its wide range of uses. The leaf contains carotenes, amino acids, and vitamins, along with extracts of Surenin, Surenon, and Surenolactone [1, 2, 3]. The species exhibits anti-cancer and anti-tumor properties [4], as well as anti-H1N1-Pandemic influenza A virus activity [5]. The organic leaf extract, containing 2-antitrypanosomal terpenoid, shows a good effect on Trypanosoma brucei rhodesiense, with values ranging from 7.18 to 31.25 μg/ml. Moreover, T. sinensis can be cultivated on a substantial scale with minimal growth requirements [6]. The fundamental challenge for the development is to achieve the target by 2025 by increasing the yield from 19 to 30 m3/ha/year and optimizing the economic value of the plant [7]. Increased forest productivity can be achieved through plant breeding [8] and the success of the program is strongly influenced by the selection of appropriate strategy. T. sinensis breeding activities in Indonesia are not fully organized and can lead to duplication of activities and inefficient use of resources and time. The synthesis of results on the species is limited and dominated by the aspect of seed treatment and evaluation of seedling development. Meanwhile, the aspect of propagation is still very limited and scattered in many areas. Research institutes, companies, and universities have contributed to the challenge of assessing its current status. Breeding efforts commenced between 2006 and 2011, including the exploration and acquisition of genetic resources from different large islands. This endeavor resulted in the successful collection of 50 populations of T. sinensis.
Ex-situ conservation management as the basis for breeding program can be applied to assess species, breeding, and genetic processes. Breeding is conducted on naturalized and native populations, accompanied by testing and selection efforts. The evaluation process should focus on objective criteria such as genotype × environment interaction, testing of advanced generations, combination of mating plan, genetic parameters, and selection of plants. In addition, the traits of interest and the costs associated with breeding are also worth considering with the role of biotechnology in livestock [9]. T. sinensis breeding program has progressed through several stages, excluding the selection and testing of advanced generations [10].
Genetic testing through multisite progeny test was carried out in 2011 by testing 100 families per 5 hectares to generate many interactive groups [11]. This information became the basis for the development of T. sinensis using non-interacting families depending on breeding site. In line with the establishment of ex-situ conservation base population and the progression of genetic test breeding, it is important to formulate a comprehensive breeding strategy for T. sinensis.
Conventional plant breeding largely depends on phenotypic selection and breeder’s experience; therefore, the breeding efficiency is low and the predictions are inaccurate [12] so it’s perfect breeding strategy needs to be carried out. Breeding program aims to (1) raise the base and breeding population, (2) reproduce the upgraded genetic material to build a superior population, (3) maintain variation and size in the base and breeding populations, and (4) achieve economic goals.
2.1 Existing base principles
The success of breeding program is largely determined by the ability to meet the 9 (nine) base principles [13] and the evaluation results are as follows:
2.1.1 Evaluation of T. sinensis breeding program
The evaluation of breeding program for T. sinensis has successfully achieved the following: (1) establishment of the first generation of T. sinensis breeding to enhance productivity through the improvement of height, stem diameter, and volume, (2) mastery of base biology on aspects of phenology and reproduction, (3) identification of genetic material sources from community forest stands, strategically selected from diverse populations and geographic origins, (4) exploration of alternative species, Toona sureniMerr. [8, 14] known for its woodworking properties and in line with the Meliaceae family, (5) availability of seedling sources for progeny testing, (6) implementation of genetic testing through multi-site progeny testing, (7) execution of field operations and research activities in several test plots, (8) strong organizational commitment with research and development tasks, and (9) adherence to a program that combines economic value traits with the management of a broad genetic base, facilitating adaptation and development. Compliance with these base principles shows their level of fulfillment, providing a basis for establishing a breeding program for T. sinensis. The objectives of forestry plant breeding programs have been widely reported among others for forest tree improvement [15], evaluation of Genetic diversity and breeding strategies of Azadirachta indica plants [16]. Establishing domestication strategies for forest trees [17] and for population genetics studies in forest tree improvement [18].
2.1.2 Evaluation of breeding program compliance
Objectives are based on the greatest long-term genetic gain achieved through effective selection, based on large and variable-sized populations with relationship control in future breeding generations. The development of base and breeding populations can use progeny test plots supported by moderate genetic diversity and hybridization patterns of the parent plants. The availability of the base population facilitates the selection of broodstock to establish progeny trial orchards for F-1 seed production and vegetative propagation to support clonal production and cultivation. The management of T. sinensis base and breeding populations to maintain genetic diversity can be improved through the transmission of exogenous populations and inter-families (breeding) to produce superior hybrids (T. sinensis Roem > < T. sureni Merr). The research implementation is sustainable, and meeting the set goals in terms of time and budget efficiency is always evaluated and improved.
2.1.3 Short-term goals of Indonesian T. sinensis breeding program
Quality seeds can be produced by converting the first-generation progeny test. Selection in progeny test plots is performed within families (single tree plots) and the representativeness can always be maintained quantitatively to manage the genetic base of the original population. The development of second-generation progeny testing is optimized through the transmission of genetic material from populations outside first-generation testing. These efforts are consistent with the view that the success of breeding program is determined by two important aspects, namely (1) the rapid acquisition of the desired product and (2) long-term retention to prepare a population with a broad base of genetic diversity for next-generation selection program. Even though the concept is not emphasized in short-term program, it is important in the long term [19].
2.1.4 Constraints on the genetic material in the base population
Restricted genetic material in the base population should be predicted by transferring from an outside population or crossing to produce second-generation breeding (Figure 1). The need for selectable genetic material is essential in the cycle of forestry tree breeding program [13, 20]. Cycle diagram of a breeding program through inbreeding and transmission activities has the potential to increase genetic diversity in the base population of the next generation [20]. Maintaining and enhancing genetic diversity is an important foundation of the subsequent cycle of breeding program (Figure 2).
Figure 1.
T. sinensis Roem. Breeding strategy based on all population levels connected in the line (base population, breeding population, propagation population and production population) and on generation I and generation II.
Figure 2.
T. sinensis community forest plantation and mother tree.
The plan for the attainment of desired objectives within breeding program, which considers all the conditions, is referred to as breeding strategy. The execution is termed breeding, while the use of biological processes in propagation such as grafting, controlled pollination, and field testing, is recognized as a breeding strategy. This strategy determines the most effective system for managing different areas of the program considering the constraints of time and resources. Furthermore, [20] explained that the process included (1) specific design, (2) time-matched, and (3) logistical application of all plant propagation components. These are selection screening, testing, breeding maintenance, breeding population development, commercial release, transmission of new genetic resources, and conservation and research efforts.
Evaluation of breeding strategy to be established concerning the aspects of benefits and constraints faced has been carried out on Azadiracta indica [18], Triticum spp. [21], organic plants [16], Pinus pinaster and Eucalyptus [22], Tectona and Eucalyptus [23], as well as drought tolerance mechanisms in plant [24]. Many breeding strategy evaluations are reported using different methods. These include references to the magnitude of genetic gain [18], accomplished through simulation program grounded in gene action [25]. Additionally, there is a use of simulations to evaluate the gains and losses incurred by the applied breeding strategy [26]. The use of variance components to assess the ramifications of alterations in field trial is conducted through different factors such as the number of genotypes, years, locations, and replications [23]. Modeling and simulation of plant breeding strategies has also been published [13]. The examination of alternative strategy representing intensive tree breeding is also conducted [26].
The research focuses on evaluating the compliance of the required elements of T. sinensis breeding strategy. This includes (1) breeding objectives, (2) availability of base and breeding populations, (3) selection and genetic acquisition, (4) genetic testing, (5) relationship management, and (6) reproduction. The evaluation of breeding strategy compliance is the basis for determining T. sinensis by considering existing conditions to ensure the achievement of predetermined targets.
3.1 Breeding objectives review
Breeding objectives of T. sinensis are focused on increasing plant productivity through the observation of important growth traits selected, such as height growth, stem diameter, stem straightness, and volume. The criteria used to increase productivity must be accompanied by improvements in stem straightness for woodworking purposes. Meanwhile, the selection of parent trees obtains plus trees used to build breeding and propagation populations. The initial criteria set for plus trees are to have desirable traits such as maximum stem straightness, fast growth, cylindrical stem, large diameter, small branching with a horizontal angle, pest and disease resistance, and other specific traits according to breeding objectives.
Selection criteria for woodworking can be adjusted specifically for growth traits having a major influence on tree quality. Information based on the evaluation of growth genetic parameters (height, diameter, stem straightness, and volume) can be used as a basis for selection activities. The strategy used to obtain the greatest genetic gain have been carried out through direct, and indirect selection based on multilocation tests. Evaluation of genetic parameters produces information to fulfill the elements of breeding objectives in the preparation of strategy.
3.2 Base and breeding populations
The availability of base and breeding populations has been fulfilled by the progeny test plots, which also fulfill the element of genetic testing. The analysis in the form of progeny tests in advanced-generation program can be used as base population. The presence of genetic material in the progeny test plots reduces operational costs for efficiency purposes because there is no need to repeat activities in the field (Figure 3).
Figure 3.
Ex-situ conservation and progeny test Indonesia Toona sinensis.
The selection for the subsequent generation of breeders is made from base population. In the first generation, T. sinensis population was primarily from unreforested community forests. In the second generation, the base population can be a test plant genetically derived from selected parents of the previous. Breeding population is a group of individuals selected from the base and become the seniors of the next generation. The mating system in a population reproduces additional genetic variation to achieve a continuous increase in the frequency of dominant genes in the base.
The plan developed to achieve the desired goals for the existing conditions is called breeding strategy, and the implementation process is known as breeding method. The biological processes used in propagation, such as grafting, controlled pollination, and field testing are called breeding strategy. The strategy aims to determine the most effective system for managing different areas of the program considering the constraints of time and available resources. Meanwhile, [20] explains that breeding strategy includes (1) specific design, (2) time-matched, and (3) logistical application of all plant propagation components. These include selection screening, testing, breeding maintenance, population development, commercial release, transmission of new genetic resources, and conservation and research efforts.
Evaluation of breeding strategy concerning the aspects of benefits and constraints has been carried out on Azadiracta indica [18], Triticum spp. [23], organic crops [9], Pinus pinaster and Eucalyptus [22], Pinus radiata [13, 27], Tectona and Eucalyptus [16], and drought tolerance mechanisms in crop plants [24]. Many breeding strategy evaluations are reported using different approaches referring to the magnitude of genetic gain [18] through simulation program. This is based on gene action [25], using simulations to assess the gains and losses of breeding strategy applied [26] and variance components to evaluate the changes in field trial selection strategy. This is achieved by varying the number of genotypes, years, locations, and replications [23] and evaluating alternative strategy representing intensive tree breeding [8].
The present research focuses on evaluating the compliance of the required elements of T. sinensis breeding strategy. This includes (1) breeding objectives, (2) availability of base and breeding populations, (3) selection and genetic acquisition, (4) genetic testing, (5) relationship management and (6) reproduction. The compliance of breeding strategy is the basis for determining T. sinensis by considering all existing conditions to ensure the achievement of predetermined targets.
3.3 Breeding objectives review
Breeding objectives of T. sinensis Roem are focused on increasing plant productivity through the observation of important growth traits, including height growth, stem diameter, stem straightness, and volume. The selection criteria used to increase productivity must be accompanied by improvements in stem straightness for woodworking purposes. Selection of parent trees is aimed at obtaining plus trees to build breeding and propagation populations. The initial criteria are to have desirable traits such as maximum stem straightness, fast growth, large diameter, small branching with a horizontal angle, and pest and disease resistance, according to breeding objectives.
Selection criteria for T. sinensis for woodworking can be adjusted especially for growth traits with a major influence on tree quality. Information based on the evaluation of growth genetic parameters (height, diameter, stem straightness, and volume) can be used as a basis for selection activities. The strategy to obtain the greatest genetic gain have been carried out through indirect, and direct selection based on multilocation tests. Evaluation of genetic parameters produces information to fulfill the elements of breeding objectives required many breeding strategy evaluations have been published including simulation of gene action and dryland environment effects [28], based on partial pedigree reconstruction through simulation [29] and Evaluation of testing strategies for plant breeding field trials [23] in the preparation of breeding strategy.
3.4 Base population and breeding population
The availability of base and breeding populations has been fulfilled by the progeny test plots in line with the element of genetic testing. The analysis of breeding population in the form of progeny tests in advanced-generation program can be used as the base population. The presence of genetic material reduces operational costs for efficiency purposes because there is no need to repeat activities in the field.
The base population is used for the selection of the next generation of breeders. In the first generation, T. sinensis populations were primarily obtained from community forests. In the next generation, the base population can be a test plant genetically derived from selected parent plants of the previous generation. A breeding population is a group of individuals selected and become the seniors of the next generation. Breeding population passes T. sinensis species from one generation to another. The mating system in a population reproduces additional genetic variation to achieve a continuous increase in the frequency of dominant genes. Breeding populations typically include 200 or more selected plants.
The development of progeny trials in multiple sites as breeding populations provides T. sinensis from one generation to another. Selection strategy use genetic variation to achieve a continuous increase in the frequency of the best genes within the families. The process of selecting appropriate genetic resources for the establishment of breeding populations includes the utilization of a population management strategy for selection and breeding functions. To generate a breeding population, a minimum of 20–30 individuals is necessary from the population to produce seeds or vegetative seedlings for the establishment of a commercial plant. The opportunity to obtain individuals from progeny test plots is available by selecting within the family using weights adjusted for preferred traits. Multisite progeny test plots can function as base populations and propagate to selectively advance progeny. A breeding population consists of several individuals (20–30) of plants obtained through intensive selection to produce seeds or vegetative sprouts for propagation of commercial plants. Furthermore, it can be in the form of a seed or pruned orchard and the genetic material is used for commercial cultivation. Based on the evaluation, breeding and the production population of T. sinensis are not available, hence the concept is proposed as a target for development in the next selection phase.
3.5 Selection and genetic enhancement
Many breeding programs create multiple traits and these activities require information on several characteristics in the selection process. Based on the results, selection using the index method is a widely developed system for selecting many traits. The use of the combined selection index makes it possible to assign a score to each individual. In addition, the economics of each trait should be considered when constructing an index of selection. The general selection process includes (1) identification of traits to be evaluated, (2) observation of preferred traits in the parent and breeding populations, (3) data analysis, (4) calculation of the indices for each plant, (5) ranking the best candidate trees, (6) analyzing the genetic gain for each trait, and (7) caring for the selected plants [20]. Genetic selection and augmentation are important steps in breeding cycle to determine the success of breeding strategy.
The selection was tested in T. sinensis progeny evaluation using the weights of all traits and values to compile the index. Based on the selection index, the methods of direct, indirect, and combined selection were analyzed. The analysis shows that direct selection has the greatest genetic benefit compared to indirect and combined selection. The implementation of the direct selection on T. sinensis completed the selection elements in the strategy (Table 1).
Traits
Locations
Candiroto
Ciamis
Direct Selection
Indirect Selection
Combined Selection
Direct Selection
Indirect Selection
Combined Selection
Tree Height (m)
12.36
0.39
9.48
1.05
0.10
−0.14
Diameter (cm)
12.23
0.004
9.09
0.77
0.01
−0.01
Stem Form
1.74
0.02
1.43
6.23
0.01
4.85
Tree Volume (×10−3 m3)
28.39
0.02
21.19
2.50
0.01
−0.42
Table 1.
Estimation results of genetic gain based on family selection using direct, indirect, and combined selection methods.
Combined ANOVA generates values based on a single trait, while selection is made using multiple traits. Therefore, indirect selection remains important in generating information on changes in the ranking of families and becomes the basis for determining the families to join breeding program. The genetic gains in this multisite trial were diverse, with the greatest value produced by direct selection, while indirect and combined selection produced lower values for all four tested traits.
3.6 Genetic testing
Progeny testing selects parents based on the performance of the seedling. This selection is useful in providing information on the genetic thinning of seed orchards. The test is an excellent strategy because the mean of the progeny is an estimate of the heritability. Analysis of genetic parameters shows that the multisite progeny test can be used for single-site and multisite analyses. The ability to view heritability, genetic correlation, and amplification information from multisite testing can be confirmed based on single-site analysis. The establishment of progeny trials at two sites fulfilled the genetic testing elements of T. sinensis breeding requirements.
3.7 Relationship management
The element of relationship management was met by analysis of genetic structure and diversity as well as by analysis of reproductive systems. Genetic diversity is an important fundamental principle in terms of ethical aspects when managing stands, ecosystems, and landscapes [17]. Connection between species diversity and genetic diversity [31]. The distribution is a prerequisite for adaptation that determines the long-term stability of individuals, species, and entire ecosystems. Genetic diversity should be assessed in long-term collections, breeding populations, seed orchards, or production populations (Figure 4) [20]. Genetic variation is a prerequisite for future evolution and we stress that gene conservation programs should provide opportunities for future evolution [33].
Figure 4.
Percentage of molecular Variance T. sinensis mother tree [32].
The use of a genetic basis offers the opportunity to select the superior traits necessary for the wood development. Some of the characteristics required for woodworking include a straight stem, optimum height without branches, minimal knotting defects, good machining properties, and beautiful patterns. T. sinensis has a constant density suitable for molding materials, and the characteristics of processing into sawn products, veneer, and furniture are not difficult.
Breeders must understand the extent of inbreeding within the populations. The discovery of the important role of T. sinensis outcrossing has strategic implications for the established experimental plots of progeny and for reinforcing the selection strategy (Table 2).
The out-crossing information in the navigation system shows that T. sinensis has superior hybrid characteristics used to maintain genetic diversity for the next generation. Seed management is expected to avoid the mixing of families with the same relevant population. The impact of mixed breeding on T. sinensis population requires alternative strategy for the program. The main implication is to prepare opportunities for the use of selection schemes including similar populations, hybridization, or repeated selection to achieve recombination. This is genetically engineered by random crossbreeding to achieve sustainable production and genetic progress.
3.8 Propagation method
The reproduction of genetic material can be achieved by propagation and vegetative methods. Vegetative method is selected when (1) propagation of mature individuals is conducted and (2) propagation of immature individuals is performed. Meanwhile, propagation is selected when (1) seeds are highly produced at a relatively young age and (2) vegetative method proves unfeasible.
Assessing the suitability of proficiency in reproductive propagation for T. sinensis is largely mastered from aspects of plant phenology, seed treatment, and generational propagation. Breeding strategy of T. sinensis is largely respected and can serve as the basis for defining the concept. Compliance with the requirements of each component must be ensured, including the anticipation of certain changes due to technical and management factors.
4. Examining the determinants of the selection strategy
Breeding strategy of T. sinensis is inseparable from the many factors that determine the success of the concept, including (1) breeding goal is not too complicated and should be limited to 1 or 2, and (2) the controllability of reproductive biology, (3) mastering information on the role of genes (gene activity) including plus and non-additive traits, (4) the existence of genotype-environment interactions, and (5) potential future. The phenomenon of hybrid superiority can be related to heterozygosity or complementarity. Heterozygousness (hybrid viability) is the result of the role of non-resonant genes while complementarity is the role of additive genes and related to how two or more traits complement each other in a particular environment. Other aspects that play an important role in the success of a breeding strategy are the appropriateness of budgets for breeding activities and production populations as well as the availability of (1) genetic resources, (2) human resources, (3) information, and (4) infrastructure.
The initial assessment was predicated on the factors influencing the efficacy of breeding strategy for T. sinensis. A majority of these factors have been satisfactorily addressed, including the identification of the primary objective. The commencement of breeding initiative centers on augmenting yield. Moreover, the scrutiny of the Reproductive Biology control has yielded valuable insights, accompanied by an analysis of genetic and environmental interactions. The key to the success of urgently identified and implemented breeding strategy is the aspect of understanding the role of genes and aspects of their potential hybrid ability. The advantage has been emphasized by [19] that hybrids can generate gene combinations while paying attention to the diversity of the old hybrids. The potential of T. sinensis hybrid is quite promising, which is supported by the existence of two species, growing naturally, namely T. sinensis Roem and T. sureni Merr.
The first limitation of the information that deserves attention is the degree of pollination and fertilization compatibility produced by the two species in heterozygous hybrids. The steps begin with controlling pollination of two different species and are aided by easy vegetative propagation of superior hybrid clones to develop the operation of Eucalyptus and Populus hybrid programs [20]. F1 hybrids, resulting from crossing between two different species, have become commercial hybrids. F2 hybrids (resulting from F1 hybrids), and F1 hybrids are hybrids of F1 with F1 hybrids one of the parent species. T. sinensis hybrid development program is part of the strategy to increase productivity in the future.
4.1 Selection strategy
The propagation of T. sinensis should be based on an appropriate breeding strategy, supported by a high degree of technological mastery and the application of appropriate breding. Selection plays an important role in determining genetic gain and the process in progeny trials requires a great deal of additional information for the selection to avoid loss of genetic potential. The risk of genetic loss is greater when breeding to improve multi-trait propagation at multiple breeding stages. Furthermore, the selection of multi-trait varieties is strongly influenced by the behavior, traits, and uses of the selected plants. Efforts to obtain suitable selection criteria for multiple preferred traits were particularly important for T. sinensis for furniture. 1st.
Crossbreeding for the simultaneous improvement of multiple traits depends on whether each trait is controlled by a single or group of genes. Independent characters can be selected consecutively or simultaneously. Traits that are negatively correlated must be carefully estimated because an increase equates to a decrease in the level of another. The use of selection weights and indices is consistent with the recommendation of [38] that the use of selection indices to improve multiple traits is a good choice. The complexity of using a single feature as the basis for selection and the bottlenecks at the operational stage can be reduced by the selection index.
The preparation of breeding populations requires information that evaluates the genetic parameters. Base information includes genetic value, correlation, and gain. Heritability is the rate of variation in a population caused by genetic differences between individuals. Therefore, this ratio indicates the extent to which parental traits are passed on to offspring. The level of heredity should also be considered an important factor in the success of selection for genetic traits. The genetic value varies depending on the type and age of the tree. Since the concept is age-dependent, early selection can be used for efficiency purposes. Based on heritability at a location, all traits, such as height, diameter, straight body, and volume, showed higher values than between sites. The genetic values are the basis for calculating the gain using direct, combined, and indirect selections. The results are used as the basis to develop seed production for site-specific selection (propagation) to avoid loss of genetic gain.
4.2 Relationship management strategy
The genetic relationship management concept existing in the base population must be managed, maintained, and maximized in terms of genetic diversity. The selection of parental plants and the construction of breeding populations that will use information on the genetic distance to create a basis4 remains wide-ranging [34, 36]. Every plant breeder must innovate to create new variations and improve breeding to meet target needs [10]. This previous research is strongly supported by the mating pattern of T. sinensis prone to dominant outcrossing [25], where the parent plants resemble each other. Compliance is assessed with elements of T. sinensis, including relationship management and methods, making recommendations for the management of seed orchards.
4.3 Seed production strategy
A short-term strategy could be to use seeds from progeny test plots converted into first-generation seed orchards. Efforts to convert progeny test plots into seed orchards are strongly supported by information on the genetic diversity of Mother plants (0.304). The results of the evaluation of mating system in the progeny test plots showed that T. sinensis tended to engage in outcrossing (93.6%), inbreeding (17.2%) and selfing (6.1%) [30, 32] respectively. The species with high degree of hybridization should be able to maintain their genetic diversity [30, 32].
The requirements to convert the progeny trial pots to the nursery were evaluated and finalized including area, genetic diversity, species mating system, flowering, and fruiting time. The requirement for genetic diversity ensures and takes advantage of the population in adapting to changes in the environment. The conversion of T. sinensis progeny test plots into first-generation seedling orchards offers an advantage as a short-term source of seed through propagation that leaves 1 plant per plot (one-tree plot) in cut-down trees with suboptimal growth rates and lower yields. Cutting trees as part of a breeding operation creates a wider distance for pollinators to transfer pollen from one tree to another for increased outcrossing value. Furthermore, flowering stimulation was applied using existing technology to achieve fruiting information on the genetic diversity of parent plants, which was (H = 0.304) and 0.024 higher than T. sinensis population in West Java [26] and equivalent to those in China 0.333 [39].
There are two alternatives considering the role of genetic diversity in the management of progeny test populations. The first includes progeny test populations as a means of transmitting genetic material for the construction of breeding populations. This is achieved through stringent selection criteria applied to selected trees while ensuring that the requisite woodworking traits are present within the progeny test population. The second option includes leveraging genetic diversity information as the foundation for batch management during progeny testing within the first-generation seedling orchard lineage, with a focus on short-term seed production. The overarching objective is to facilitate the transmission of genetic diversity and yield-related information from parental plants to their progeny during the testing process.
4.4 Population management strategy
The strategy of establishing a foundational population with a restricted genetic pool comprised of 100 families from 10 diverse geographical origins. This necessitates the incorporation of genetic material from external sources into breeding population in response to external influences. The number of families participating in the base population is greater than 200 families that are representative of the population in terms of distribution and range. Information on the distribution of genetic diversity of T. sinensis is more abundant within populations (84%) and between geographical sources 9 and 7%. The genetic resource naming is to gather more resources from geographical sources [32].
The enhancement of genetic diversity of breeding populations beyond transmission into external populations can be achieved through crossing families with large genetic distances as well as hybridization between T. sinensis Roem and T. Sureni Merr species to create superior hybrids. More crossbreeding increases the likelihood of multiple combinations and increases the genetic diversity of breeding population [20]. According to the concept of the selection cycle, increasing the genetic diversity of breeding population and genetic testing are the main factors that are considered complete.
Research on the structure and genetic diversity of T. sinensis progeny showed that the trend maintained the original genetic diversity. However, research is needed to ensure that the original genetic diversity is maintained to ensure the stability of the testing in the next generation of livestock. The decline in genetic diversity may be due to a reduction in the number of effective populations engaged in next-generation selection. Information on the genetic diversity of T. sinensis carries two recommendations, namely (1) assessing the expected genetic gains for important traits and (2) evaluating the mating system of T. sinensis population in the next generation of breeding.
4.5 High-yielding hybrid, clonal, and hybrid strategy
Breeding strategy for T. sinensis should focus on seed manipulation for generational and vegetative propagation methods of plus plants to support clonal production and plantation development in the nucleus. In first-generation breeding, clonal propagation relies on the selected plant and the outcomes. However, second-generation breeding places a greater emphasis on a combination of plants and progeny selection testing, including System II with the integration of genetic material from hybrid plants. The last major section on new technologies highlights on of which is the increasing importance of hybrid breeding in tree improvement programs [40].
One of the options of the cloning method is to optimize the inheritance of maximum characters that are superior to their descendants. The use of cloning in addition to genetic resources and plant growth can be developed to clone superior hybrids. The search for superior hybrids can increase the genetic diversity of populations and potentially create more choices. Indonesia has natural populations of Toona sinensis & Toona sureni that have potential for hybrid programs. Initial identification of physical & anatomical properties of both Toona species has also been carried out [14]. The creation of hybrids promises variety and new breeding opportunities for traits produced by hybrids.
Families with the lowest distance values have the most stable characteristics in the multisite test because the rankings do not differ at each or combined site. A maximum standard deviation value of 10 was set as a reference to identify the least interacting and most stable families in the preliminary period. Furthermore, the large number of families interacting in the multisite trial may be due to the sensitivity of families to environmental fluctuations. Controlling the number of families interacting in Pinus pinaster [41] was performed by excluding from the subsequent selection program. However, applying this approach to testing T. sinensis is not entirely feasible since only 58 of the 100 families were included in the cross-site test. Removal of all interacting families results in a decrease in genetic potential due to the loss of valuable material.
The preservation of families with lower differential values can be considered as a measure of the conservation of genetic material. The identification of higher-order stable families is important for the development of late-stage T. sinensis. Considering the best ranking from 1 to 20 (35% of the 58 families tested multi-site), the 6 families with the highest rank and the lowest interaction value were selected [30].
The multisite trial analysis shows that surname and site have a very strong influence and the sensitivity of the seed produced will be affected by each planting site. White et al. [20] mentioned that one of breeding strategy for logistics implementation was to manage the distribution of commercial products. Therefore, T. sinensis seed distribution for community forest development needs attention. Seeds should be distributed in areas with similar soil and climatic conditions as the seed source. To enhance the productivity of community forests with the use of high-quality seeds during the selection phase, it is important to implement intensive silvicultural practices. High-yielding seeds need to be supported with the right planting site to realize their genetic potential. Most improved materials currently deployed are seed crops from first-generation phenotypic or tested seed orchards, which offer 10–25% gains in yield depending on the selection intensity of parent trees [42].
7. T. sinensis community forest development strategy
Based on T. sinensis breeding strategy in Figure 1, short-term support can be achieved using seed production from the nursery of generation trial seedlings first-grade descendants. This reduces reliance on seed use dependent on sources from identified seedlings. The quality hierarchy of the nursery is determined to be lower than the nursery. Therefore, a short-term program of plant genetic improvement is quite strategic to achieve every T. sinensis yielding 2–3 kg with the number reaching 40,000–60,000 seeds/kg.
The results of examining aspects of the interaction show that family and habitat have a strong influence, hence, it is recommended to distribute seeds by adjusting the soil and climate conditions of the community forestry development area. The development of seed resources based on planning can be considered as adjusting the distribution of community forest culture. Incorporating forest development planning reduces the chance of inappropriate seed transfer and can optimize productivity and growth.
The strategy in Figure 1 exhibit the most favorable ratings and show reduced interaction. Even though the availability of known parent and family plants with the highest-ranking advantages and minimal interaction remains constrained, efforts can be directed toward optimizing their use to bolster clonal propagation, establish cloned seed orchards, and foster the growth of asexual plants. The cloning strategy has been developed through vegetative propagation with promising results. Propagation of T. sinensis gene by vegetative method is under control [43] and the advantages are well-known to breeders. However, the application requires serious economic consideration, availability of human resources, technology, and budgetary support.
For the realization of long-term benefits, it is important to formulate activities aimed at preserving sufficient genetic diversity to anticipate alterations in the reproductive system within the subsequent generation. Knowledge of mating systems can prove invaluable in enhancing established selection methods. A range of strategy may be evaluated and deployed to broaden the genetic foundation of breeding populations for enduring programs, such as crossbreeding to yield superior hybrid offspring.
The high-quality seeds produced from the nursery must be tested against the conditions of the community forest which has a mixed silviculture pattern with irregular tree spacing and spread over a small planting area. Efforts to determine the superior genotypes suitable for community forest soil conditions can be achieved through seed testing under environmental conditions appropriate to the growing site.
Due to the elevated sensitivity in the tested T. sinensis families, the optimal selection approach uses direct selection rather than conducting further testing. Consequently, the transfer strategy for the cultivation should prioritize the use of high-quality seeds from nurseries within community forest development sites possessing soil conditions and climates.
In conclusion, T. sinensis breeding strategy was improved when establishing breeding populations using previously collected base population data. The establishment of seed orchards was based on the location of cultivation development by restricting the distribution of excellent families to designated breeding areas. Furthermore, the development of species community forest was prioritized using seed sources with soil and agro-climate suitable for the development area. The key feature of any successful breeding strategy was providing flexibility to make changes since information formed part of new technology.
The authors are grateful to Prof. Muhammad Na’iem for the valuable suggestions for this research and to Dr. Arif Nirsatmanto for the support and valuable suggestions during the writing process and discussion. The authors are also grateful to the Ministry of Environment and Forestry of Indonesia for providing scientific support on Breeding Program of Indonesian Toona.
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Written By
Jayusman and Budi Utomo
Submitted: 30 August 2023Reviewed: 01 September 2023Published: 21 December 2023
Open access peer-reviewed chapter
