In this article we will discuss about:- 1. Introduction to Forest Tree 2. Vegetative Propagation Methods in Forest Trees 3. Application 4. Techniques.
Introduction to Forest Tree:
Forest tree breeding aims to increase forest health, quality of trees and stand productivity using combination of different approaches. However, the application of science of genetics in improving yield from forests was quite late with large-scale tree breeding programs being initiated around the world only in the 1950s. Zobel and Talbert (1984) cite 23 papers from 14 countries published in the 1950s that describe tree improvement programs.
ADVERTISEMENTS:
Since then, substantial advances have been made in forest genetics and tree breeding research. Today, there are breeding programs for nearly all commercially important tree species in the world. Genetically improved plantations from such programs have made significant impacts on forest productivity, wood supplies and sustainability of forest resources.
Conventional tree breeding programs comprises several steps. The programme is initiated with provenance selection which aims to select well adapted genotypes to serve as base population. Within the base population, phenotypically superior (plus) trees in natural stands are selected. Trees are selected from these plus trees after the genetic tests for identification of individuals with good genotypes and high combining ability for economic and adaptive traits.
The best parents may be selected, or the best progeny of the best parents may be selected. In order to use selected genotypes for reforestation, production populations are established. Based on genetic tests the seed orchard is rouged. Subsequent generations of tree breeding are initiated by treating the first generation seed orchard as the base population. These individuals are then inter-mated to generate new combinations and a new array of genetic variations to select in the second generation.
Offspring’s are then tested in second generation genetic tests and selections made to go into second generation orchards and a new breeding population. Thus efficient tree breeding requires long-term commitment, selection of appropriate methods and large scale application of results to provide practical benefits.
The incorporation of the improved genotypes that evolve out of the selection, testing and breeding cycle into operational forestry is an essential component of any successful tree improvement programme. The genetically superior plants that result from breeding programs are deployed into operational forestry through mass propagation.
The objectives of breeding cycle and field deployment components of a tree breeding programme are quite distinct. The breeding cycle tries to improve the base population with every cycle, maintaining the broad genetic base, which is essential for any cross breeding crop and has a long vision that spans generations.
ADVERTISEMENTS:
The propagation component tries to utilize the best genotypes available in the operational forestry and has a focus on the current generation. Since, the success of any tree improvement programme is rated based on the realized benefits from the field; these two components will have to work in tandem.
Vegetative Propagation Methods in Forest Trees:
Vegetative Propagation was originally seen as a way to bulk up small amounts of rare and valuable material. Since, vegetative propagation is done from well-differentiated vegetative parts; the propagules maintain physiological condition of the propagated part and genetic consistence of the parent tree.
It is possible to capture and transfer all genetic potential through use of vegetative propagation to the new tree. Vegetative propagation has been widely used in the establishment of clonal seed orchards and clone banks. Exceptional hybrids, sterile hybrids and selected plants can be propagated vegetatively. The original tree from which the parts to be propagated are taken is known as the ortet. Each plant propagated from an ortet is a ramet. A set of ramets from the same ortet is a clone.
Differences observed between trees propagated by the same method are called clonal variation. Methods of vegetative propagation can be broadly classified as macro propagation and micro propagation techniques. There are three main methods of macro propagation used in trees: cuttings, grafts and layers. Latest in vegetative propagation of trees is the micro propagation through tissue culture techniques.
ADVERTISEMENTS:
i. Cuttings:
Cuttings form the earliest and easiest method of vegetative propagation. These are sections taken from the tree and put to root in an appropriate medium. Cutting as a method of propagation is cost-effective compared to other vegetative propagation methods and they result in good plant type. However, they need rooting. Multiplication rate of cuttings are low, need large areas for donor plant hedges and ageing problem complicates clone testing. Some tree species, such as the poplars and willows, are very easy to propagate by stem cuttings.
ii. Grafts:
Grafts are plants obtained by fusing a part from the tree to be propagated with another part which has its own root. This involves the removal of a vegetative part from the parent tree (the scion) and its attachment to a part with its own root (the stock) so that the tissues fuse. Heteroplastic grafts belong to different species, while homoplastic grafts belong to the same species.
ADVERTISEMENTS:
According to the position of the scion, there are top and side grafts, depending on whether the scion is inserted into the top or the side of the stock. Grafting is relatively simple technique and it avoids rooting problems. However, it is expensive and not applicable for mass propagation.
iii. Layers:
Layers are sections of the tree which are induced to root and then separated from the tree. The procedure consists of making a wound in a section of the tree and covering this with a medium which retains moisture (moss, earth, etc.). Healing causes the formation of a callus from which adventitious roots may grow. The most common type of layering is aerial layering in which the covering is done with a medium other than soil and the operation takes place rather high off the ground.
When long, low, flexible branches are available, these can be inserted into the soil and it is known as ground layering. Usually the part propagated is a branch, but in some species interference with the roots can cause them to sprout, constituting a form of layering in that they do not separate from the stem. Layering is relatively simple technique and it avoids rooting problems. However, it is also expensive and not applicable for mass propagation.
Micro Propagation through Tissue Culture:
The latest among the technologies for vegetative propagation are micro propagation through tissue culture techniques. Tissue culture refers to the culture of cells with the potential mitotic activity, in an appropriate medium under aseptic conditions. Micro- propagation using tissue culture method provides unique opportunities to rapidly multiply the elite trees either through organogenesis or through somatic embryogenesis.
In organogenesis, the explants under favorable media give rise to axillary or adventitious shoots which can be separated and rooted under another medium and hardened to plant out to the field. Tissue culture propagation by axillary shoots offers low risks of post-effects. However, the technique needs rooting and is expensive. In vitro propagation by adventitious shoots offers high multiplication rate but this technique also needs rooting. There are risks of post-effects and is an expensive propagation method.
In somatic embryogenesis, embryo like structures arise from the explants or callus which can be germinated as a normal plant under specific media combinations. In somatic embryogenesis, embryos are induced either directly from the explant or from an intervening callus. Somatic embryogenesis is very promising because, the multiplication rate is very high and there is good prospect for automatic handling. However, the method is expensive, there are risks of post-effects, loss of genotypes and certain selection required during the process.
Vegetative propagation has been historically used for maintenance of genetic uniformity, since it offers the fastest way to get improved material into production by reproducing the genetic makeup of the selected individuals. However, consistency in phenotypic effect is not always ensured by vegetative propagation. Some of the vegetative propagation could have somaclonal variation, where the differences have genetic origin.
Propagation methods can introduce effects that are non-genetic, but are confounded within the genetic unit. These non- genetic effects that can be confounded with genetic effects are called C-effects. In connection with vegetative propagation, the C effect can be caused either by differences in donor plant physiology or by differences in the environment during propagation.
Application of Vegetative Propagation in Forestry:
Vegetative propagation of trees was originally used for the species like Poplar and Cryptomeria which reproduced vegetatively under natural condition. By the development of other methods of vegetative propagation namely, cuttings and grafting it was made possible to multiply the selected superior trees for use in seed orchard and also to preserve germplasm in banks. Large-scale propagation from cuttings at costs similar to those of seedlings has been developed in many forest tree species.
Vegetative propagation has been used in forestry for production of quality planting stock. It has been used to develop fast and economical methods of raising superior planting stock in the intensive management of forests. These techniques become important in forestry owing to their potential in propagating superior trees to increase volume and quality of production. It has also been used for propagation of problem trees.
Vegetative propagation could be effectively used in species, which are not able to produce seeds frequently like bamboos. Propagation by vegetative means may be easy, more rapid and economical than by seed in many cases. In some tree species, germination may be poor or slow or there may exist complex dormancy problems or the seed may lose its viability very quickly like in recalcitrant species. Moreover, seedlings of many species grow slowly and take a long time to reach marketable size. In all these cases, use of vegetative multiplication is a more convenient method of propagation.
Since, most of the tree species are cross-pollinated and highly heterozygous, asexual clonal propagation helps in maintaining the genetic characteristics of each species. Because of this property, vegetative propagation methods can be used to develop clonal repository. Genotypes are preserved in such clonal banks. In genetic testing’s, vegetative propagation has been used to evaluate genotypes and their interaction with the environment through clonal testing.
Clonal propagation will also help to conserve and multiply the valuable trees in a centralized area such as in a laboratory or green house for intensive study and breeding. Vegetative propagation has been used for maintenance of genetic gain. It helps to capture maximum genetic gains when used for regeneration in operational planting programme.
In addition to the above benefits, some forms of vegetative propagation can be used for production of disease free plants. In tree species that are often susceptible to some pests and diseases and while some may be partially or entirely resistant. Vegetative approaches like grafting will help to produce resistant clones.
Vegetative propagation can also be used to speed up the reproductive cycle for accelerated breeding and testing. Vegetatively propagated plants are precocious in bearing than seed propagated plants. Early induction of flowering thus induced will help to reduce the rotation of the tree species and also to increase the productivity.
Incorporation of Vegetative Propagation Techniques in Tree Breeding:
Because of the various advantages of vegetative propagation in preserving the genetic characters of the propagated plants, it is used for the following uses in tree breeding programs.
i. Establishment of Clonal Seed Orchards:
One of the objectives of a tree improvement programme is to propagate the interim benefits of the programme at the fastest rate to meet the propagule requirement of operational forestry through appropriate technologies. In advanced breeding programmes seed orchards are established for production of superior quality seeds. A seed orchard is a collection of selected clones or families established in one physical location and then managed to produce genetically improved seed for operational forestation.
Seed orchard are established either through seeds (seedling seed orchards) from open pollinated seed or clones (clonal seed orchard) by the use of grafts, cuttings, air-layered plants, tissue culture plantlets or other methods of vegetative propagation. Clonal seed orchard is generally preferred when vegetative propagation is possible and the seed orchard only serves as a production area for seeds or vegetative propagules. Clonal seed orchard is raised in species in which flowering can be initiated at an earlier age through vegetative propagation. A clonal seed orchard typically contains many clones.
ii. Genetic Testing in a Breeding Programme:
In conventional tree breeding programmes, the usual approach for testing candidates for the next generation involves progeny tests involving progenies by controlled crossings. The candidates must reach the flowering stage, which can take several years in trees. Vegetative propagation allows the candidates to be directly tested as clones, instead of indirectly, as in progeny tests.
Besides saving time, since it does not require trees to reach flowering competence, this strategy also increases the selection intensity, by allowing more candidates to be tested at the same cost. Studies on this subject, with varying breeding parameters, have consistently concluded that clonal candidate testing is efficient.
iii. Deployment of Genetically Superior Families through Bulk Propagation:
Large scale deployment of genetically superior families selected based on genetic tests is one of the common steps in tree improvement programmes. After controlled pollination of selected parents in the breeding programme, families can be mass propagated without knowing the value of each progeny and without identifying the individual progeny. Quite often, it would be expensive to do controlled crosses to obtain large quantities of seed to meet requirement of operational forestry.
In such situations, vegetative propagation using the limited seeds produced through such controlled crosses is often resorted to. Each seed produced through such crossing would represent a different genotype and if 100 seeds are obtained from one cross we will end up with 100 genotypes. If 30 such crosses are made, the lot would contain 3000 genotypes. This mode of using vegetative propagation is called bulk propagation and provides the same genetic gain as if the material was propagated via seeds.