In this article we will discuss about:- 1. Introduction to Tree Improvement of Bamboo 2. Genetic Improvement of Bamboo 3. Tree Improvement of Bamboo through Tissue Culture.
Introduction to Tree Improvement of Bamboo:
The genetic resources of bamboos in India show a wide spectrum of variability in terms of species and their distributions range, economic traits and their end-uses. The most important component and genetic and physiological variability also exist in the flowering behaviour of bamboos and their cytology, variation in chromosome number, fertility and germination. There were also lots of variations in morphological and anatomical characters. Recognizing variation is the first step in any tree improvement programme.
Even though, bamboos are very well associated with lives of our people and is known as poor man’s timber yet, no concerted efforts have been made in the genetic improvement of this species. There is a wide gap of knowledge on certain important aspects like, genetics and physiology of flowering, cytogenesis, cross compatibility, pattern, phonology, polyploidy level, biotechnology, wood genetics and genetics of resistance to pest and diseases, which are most important for undertaking genetic improvement of bamboos.
(a) Candidate Plus Clump/ Seed Selection:
Recognizing variation, isolating and packing it with desirable characters are very important stages in any tree breeding programme. Selection based on phenotypic superiority alone can give 5-10% more gains. But it is very important that the species has to be identified first; its end-use has to be known, and the locality where, the plantations are going to be raised prior to selection. Criteria to be used for selection of plus bamboo clumps will vary with end-use.
The important traits for selection in bamboo species are:
1. Erect, straight and / stout culms,
2. Tall and large in size,
3. Thick wall to solid internode
4. Branching mostly at the top and no or less at the bottom
5. Open not contested clump
6. More number of culms per clump
7. More vigorous and erect culms
8. Culms healthy and not infected by disease or insects.
Selection of species, and parameters for selection vary with breeding objectives for example – if it is for quality pulp production, anatomical characters, that are important to fibre length will be given importance; if it is for building construction, mechanical strength and durability of bamboo will be the main criteria.
Planned and intensive survey over different places where, the population of bamboo species located has to be made so, as to select best species, provenance, and individual superior trees. The selected candidate plus clump can be used for conservation in the germless bank, multiplication and testing for various breeding purposes.
Tree improvement work was carried out in 1980 in respect of 8 important Bamboo species viz., Bambusa balcooa, Bambusa nutans, B. pallida, R. tail, B. tuldi, Dendrocalamus giganteus, and D. hamiltonii, Bambusa species in Arunachal Prudish.
The plus bamboo were selected from natural forest and from bamboos grown in the villages. The selected plus bamboos have shown superiority over other bamboo. Selection can also be done during seedling stage itself as, bamboos are reported to be highly allogamous species.
Early seedling vigour and very good growth habit can be used as important index for selection at seedling stage, Kondas et al., (1973) and Banik (1987) reported differences in seedling characters viz., grassy erect, and very erect in Bambusa bambos and B. multiples. In the above types, erect type showed fast growth rate and vigour.
(b) Hybridization:
Hybridization, whether, natural or artificial, is essential to introduce desirable characters of parents in to the offspring. This will also increase available genetic base for selection. Since bamboos are, monocorpic, those to the flower at very long intervals, carrying out hybridization programme in them are very difficult. To take up such a programme the first and foremost step is to induce bamboos to flower regularly.
Since the exact causes including evolutionary causes, if any, are not known, considerable money, time and efforts towards basic research both at national and international levels would be required in deciphering flowering in bamboos.
Some of the methods like, induction of mutation, hormonal treatment to clones and tissue culture technique can be used to achieve uniform flowering. The report by Nadgauda et al., (1990) is a step forward in this direction.
Since it is known that most of the bamboo species are gregarious flowering in nature, all individuals of the particular stock will flower simultaneously. Clumps from different stocks of the same species may be grown together to get constant supply of flowers. The species which flowers sporadically almost every year or the species in which physiological cycle is less can be used for hybridization purpose.
Normally, it is expected that when two species of same genera occurs together natural hybrids should be found if there are no genetic barriers, and they flower at the same time for example Eucalyptus. There is no such report of occurrence of natural hybrids of bamboo, except segregation of seedling characteristics of Bambusa bambos as observed by Kondas et al, (1973).
Ueda et al., (1961) attempted experimental crossings with limited success. Muramatsu 1981 reported successful hybridization in the genus, Sasa (dwarf bamboo) and Pelioblastus in Japan. Controlled hybridization studies were carried out by Zhang, Qing-Fing and Huang Ke-Fu (1985) in China.
For successful hybridization, it is essential to know physiology and morphology of flowers, viz. opening and closing of flower-, growth of stigma, stamen, and time taken by the pollen to germinate and to reach the ovary, length of pollen tube. It is also necessary to know the effective methods to preserve pollen grains.
From the studies, it was found that pollen begins to germinate in about 15 – 20 minute – The pollen can be stored effectively in refrigerator for a month In vitro flowering has also been reported in D. brandsii and B. bambos.
For artificial hybridization among bamboo species, it is very essential to know the genetic distance. Genetic distance is inversely related to success in hybridization programme. For example, in the hybridized combination of Bambusa pervariabilis x Phyllostachys pubescens, 12,000 flowers were pollinated but only 34 seedlings were obtained, none of which produced ideal hybrid.
The hybrids may be intermediate between parents or superior to parents. Hybrid superiority was reported in case of specific gravity in bamboo hybrids. Since bamboo lands itself for vegetative propagation, once hybrid is obtained, it can be multiplied easily.
(c) Polyploidy:
Ploidy breeding can also be used for the genetic improvement in bamboo. The somatic chromosome number in bamboos varies from 48. 54, 72 and 96 with possibility of 2x, 6x, 8x or 12x as basic number. The chromosome number shows that there are natural polyploids available in bamboos.
There is a possibility that aneuploid, triploids and tetraploids may be available in nature itself and it can also be produced artificially in the laboratories. Detailed study is necessary to know, whether the change in ploidy level brings any improvement in bamboo species.
Productions of haploids are also very important as they can give homozygous male and female lines which can be used for breeding. In Japan, seed of Bambusa bambos were treated with colchicines during 1961 by Ueda for its genetic improvement. In India Rao (1969) treated seedlings of D. strictus with colchicine and observed its characteristics. Polyploidy may be a useful technique in breaking the crossing barrier with species having uneven number of chromosomes.
(d) Mutation:
Inducing mutagenesis using physical and chemical mutagens can be used to improve specific traits like flowering, hairiness, disease resistance and in breaking crossing over barriers. Both seed and vegetative material can be used for these studies. Ueda et al., (1961) irradiated the seeds of B. bambos using X-rays.
Genetic Improvement of Bamboo:
Since bamboo flowers after a very long interval, and due to poor viability of seeds, one cannot always rely upon seeds for propagation. Similarly for the propagation and multiplication of superior genotypes use of seed is not recommended as, the young one may not be true to type to its parents. In both cases vegetative propagation is found to be the best. Different parts of the plant viz., rhizome, culms, branches etc. can be used for propagation.
The various methods of vegetative propagation of bamboo are briefly discussed under the following headings:
(a) Planting by Rhizome:
The planting of culms attached with -rhizomes (offset planting) are the best method. This method is not practical for large scale plantation activities. Offset should be planted before rainy season. In general, bamboo species with large diameter culms require larger rhizomes of length about 5 times the basal girth of the culm. This method is successful in bamboo species with thick walls such as Thyrsostachys stamensis and Melocalamus compactyiflorus.
(b) Culm Cutting:
A large number of bamboo species have been propagated by rooting culm cuttings. A culm cutting comprising of 1-3 node section of culm and one year old culm are best suited for propagation. Horizontal planting is found to be the best when compared to oblique planting.
The size of culm cutting and its position on the culm, the age of the culm, the season of planting the cutting, and the species determines the rooting ability of the culm cutting and its survival. Sharma and Kaushal (1985) reported that single nodded culm cuttings taken from basal 1-10 nodes of 6-8 month old culm in the month of March exhibited better rooting and survival in Dendrocalamus hamiltonii. Bulganawar and Satischandra (1990) had successfully raised planting stock of yellow bamboo (Bambusa vulgaris) with growth regulatory substances which promote root formation in bamboo cuttings.
Surendran et al., (1983) reported that treating the cuttings with coumarin, naphthalene acetic acid and boric acid improved rooting in Bambusa bambos. In Bambusa tulda, coumarin treatment promoted rooting sprouting as well as rhizome formation in summer but only, sprouting in rainy season.
(c) Branch Cutting:
Propagation by branch cutting is a useful, practical and effective method for large scale plantations. This is normally done in case of Dendrocalamus asper. Bigger branches have got more potential than smaller ones. Rooting is abundant in rice husk, charcoal medium. Branch cutting of Cephalostachym pergracile are mostly a failure.
(d) Propagation through Layering:
One year old culm is best suited for air layering. Branches are pruned without damaging dormant buds and are half burned with buds in lateral position. About 28% success was achieved by both air and ground layering in mid culm region but it may vary from species to species. April and May are the best period of the year for layering works.
Conservation of Bamboo Genetic Resources:
There are two most serious factors that are contributing to the erosion in the wealth of bamboo germplasm. One is shifting cultivation and another factor is that, some species are over exploited for fuel and cottage industry. To carry out any tree improvement programme, there is a definite need for larger population with abundance variation, and hence conservation of available resources are very essential
Conservation can be divided into:
In – Situ Conservation:
This mainly deals with conservation of bamboo in its natural habitat. This method helps in preserving of inter-specific and intra- specific genetic variability. Conservation of provenance, natural habitats are some of the example in the gene conservation in-situ.
Ex – Vitro Conservation (Outside the Laboratory):
Seed Banks:
Collection of seeds and preserving them are the major work of seed banks. But the seeds of bamboo remain viable for a shorter period only. For e.g., 30 -35 days for Babusa tulda, 55 days for Dendrocalamus longispathus and 65 days for B. arundinacea. Various methods have been developed by scientists to increase the storage life of bamboo seeds. Banik (1987) reported the increased seed viability of B. tulda to 18 months when stored in desiccators over silica gel.
Gupta and Sood (1978) found that storing under suitable temperature and moisture increased the viability to 34 months.
Clone Banks:
These banks are ideal for gene pool conservation as the offspring will be of true to type. Generally rooted cuttings and rhizome are used for conservation. Developing bamboo garden is also a very good approach.
In Vitro Conservation:
Conservation through in vitro means are very recent one. In vitro Culture, work has been started in some Asian countries such as India, Malaysia, Japan and Thailand. Using tissue culture explants can be effectively stored for a longer time. Keeping the material under ultra-low temperature (-196 C) is also a very good method to preserve the germplasm. By cryopreservation we can preserve the materials for any number of years.
The important advantage in this is that genetic nature of material is kept intact. Within a small space a large number of species can be conserved. It is also a quick method of propagation and enables high multiplication rates.
Tree Improvement of Bamboo through Tissue Culture:
Tissue culture has got immense potential in the improvement of bamboo as it helps in:
1. Mass multiplication of selected superior genotypes,
2. Production of homozygous lines through another culture or ovary culture,
3. Hybridization through protoplast fusion,
4. In vitro flowering and pollination.
(a) Micro-Propagation:
The technique of micro-propagation or in vitro vegetative propagation can yield faithful duplicates of an original parent plant. From the selected superior plant nodal segments, 10-15 mm in length were collected from the secondary and tertiary branches. For sterilization of explants generally, HgCl2 is used. They can be inoculated in an artificial medium Tikiya (1984); Jearth (1986); Chaturvedi and Meena Sharma (1985) and Banik (1987). This involves inoculation of the explant on MS medium supplemented with 0.51 mg/l of BAP and 0.1 to 1 mg/l of NAA. Even though shoot induction was successful rooting per cent was very low i.e. 4-15 per cent.
(b) Rhizome Induction:
Inductions of rhizomes are very important as they help in early establishment as well as early culm production. An evaluation of the published literature has shown that the physiology of rhizome formation and the factors leading to the subsequent growth are relatively unknown. Methods have been developed by Usha Rao et al., (1985) to precociously induce rhizomes in plantlets from somatic embryos and seedlings.
(c) Somatic Embryogenesis:
Somatic embryogenesis can be defined as production of a new plant from vegetative cells and that are not the product of genetic fusion. Explants that can be utilized for somatic embryogenesis are viz., Zygotic embryo (fully matured), immature embryo, roots; rhizome; Node; Shoot-tip, Leaf-Sheath base.
With juvenile or tissue culture raised materials of Bambusa arundinacea and Dendrocalamus strictus, callusing starts soon after inoculation on B5 + 2, 4-D. The callus has both nodular and friable regions. On subculture the compact callus gives rise to somatic embryos. These embryoids can be made to bud off several daughter embryoids, each of which, has the ability to give raise to a completely well-formed plant.
Somatic embryogenesis has got 3 major advantages:
1. The embryoids have pre-formed shoot and root poles thus, eliminating the need for a rooting step as with shoots.
2. Multiplication of somatic embryos is very rapid.
3. Maintaining and manipulating embryogenic cultures is easier and quicker and hence, less labour – intensive and costly, than a shoot culture.
Developing techniques for the production of embryogenic suspension cultures deserves the attention. Once the technique is well established and methods developed to differentiate embryoids in these, the way will open up for a truly mass scale production of bamboo plantlets at minimum cost. It would also enable the production of synthetic seeds. So far the attempts made had not yielded any success but continuous efforts are made in this direction.
(d) In – Vitro Flowering:
Flowering has been one of the most difficult problems in the biology of bamboos. Bamboo hybrids had been obtained for the first time by Guangzhou and Fuqiu, (1987). The opening of the bamboos to conventional methods of hybridization is a major advance towards bamboo breeding. A method that has much promise in this regard is the induction of in vitro flowering.
In Delhi University somatic embryos of Dendrocalamus strictus and Bambusa bambos were induced to flower in culture. Nadgauda et al., (1990) also reported flower induction. Rout and Das (1994) induced flowering in Bambusa vulgaris, Dendrocalaus giganteus and Dendrocalamus strictus on half strength MS basal medium supplemented with IBA, GA and sucrose. Flowering can be induced following these methods and hybridization can be carried out successfully.
The seed thus developed, can be further raised in tissue culture for multiple shoots and roots by the method developed by Nadgir et al., (1984) and as followed by Preetha et al., (1991). By rising in vitro flowering, and pollination techniques desirable qualities can be packaged into the hybrid.
Protoplast:
Producing protoplasts from plants has got number of advantages such as isolation of somoclonal variants, Induction of somatic embryogenesis and somatic hybridization. Lots of problem still needs to be solved to develop an effective methodology to produce high yields of developmentally active protoplasts.
The results of Tseng et al., (1975), Huang (1983 and 1988), Dekkers (1989) and the work in the Delhi University had shown serious limitations in the isolation of protoplasts. Progress in this field will definitely lead to break through in protoplasm culture as has happened in rice and other monocots.
Somoclonal variants:
Even though it is normally expected in tissue culture that the plantlets should be true to type but sometimes deviation also noticed which may be due to chemicals in the artificial medium and artificial environment.
These variants can be used in bamboo improvement as some of them may show superiority in desired lines than the true to type plantlets (For e.g. salinity tolerance, pest and disease resistance etc.). Somoclonal variant of Bambusa arundinacea have been isolated in Delhi University. These variations can be artificially induced once the method for protoplasm isolation is standardized.