Since the trees in uneven stands differ in age and there is tendency for storied age class, the characters of interest cannot be compared. Selection is generally avoided in uneven aged stands. However, such standsare used in the absence of even aged stands. In mixed stands, trees are found scattered in the whole area and grow under different environments. The selection methods such as regression and base value method are found to be suitable in such cases.

 Selection Methods

As clonal forestry attracts increasing interest and begins to gain acceptance, the safety of clonal plantations is becoming a major concern to the researchers and managers. One approach to find solution of disasters like epidemics is to prescribe the minimum number of clones per location (Hedstrom and Krutzsch, 1982). Heybroek (1978) reported that in a large crowned and long lived organisms like trees, the rules may be different from those of that govern the race between pathogen epidemics and host harvest in short-rotation plants like wheat and barley. Libby (1987) developed three general guidelines for minimum number of clones per locations (i) mixture of large number of clones is as safe as mixture of genetically diverse seedlings, (ii) mixture of few clones is not safe, in fact, the mixtu re of 2 to 4 clones is often worse than monoclonal deployment and (iii) mixture of 7 to 20 clones is as safe as large number of clones.

 General Purpose and Interactive Clones

It is reported that the overall test may record a large and highly significant clone by location interaction. However, when the performance of the clones analyzed individually, it is found that relative rank of many clones stay approx imately constant in many of the locations. Further, some of the clones may change in their ran king position with regard to growth performance from location to location. This leads to two c ontrasting selection and deployment strategies, firstly to select the high performance clones w hich are relatively stable ranked over many of the locations and deploy these for general purpo se clones to the client areas and secondly to select among interactive clones, deploying these clones only to those locations where their performance was out standing. The best performance in each location is often achieved by selecting few of these interactive clones and plant on lar ge scale. In this way, greater genetic diversity can also be maintained for future improvement a nd danger of epidemics can also be avoided.

 Clonal Forestry

Rooting of cuttings or production of few plantlets by vegetative means may be of an academic interest, production of large scale clonal planting material demands development of infrastructure for propagation so that identified biotypes are propagated in economic manner at an appropriate time. Before 1980s, mass propagation of cuttings was mostly confined to Populus and Salix species in temperate, and Cryptomeria in subtropics (Eldridge et al., 1993). However, the success in mass multiplication of eucalypts has given new dimensions to clonal forestry. By 1991, twenty companies in Brazil had operational scale clonal plantation programme producing about 50 million rooted cuttings of Eucalyptus annually for planting in an area of about 45,000 ha, representing nearly 29 per cent of the total 175 million Eucalyptus planted annually (Griffin and Rivelli, 1993). Other important countries where operational scale clonal Eucalyptus plantation programme have been developed include South Africa, France, Portugal and Spain (Lal, 1993).

In India, though considerable work to propagate more than 220 forest species has been carried out, concerted efforts to produce large scale planting material are negligible. Success achieved in clonal planting of Eucalyptus is due to the active involvement of a private enterprise ITC Bhadrachalam Paperboards Limited, Andhra Pradesh (Lal, 1993). Promising clones of Eucalyptus resistant to leaf spot have attained mean annual increment between 16-20 m3ha-1 yr-1 under moderate rainfall conditions at the age of 3 years against 5-6 m3ha-1 yr-1 by seedling forestry (Lal et al., 1994). Mysore Paper Mills, Karnataka, expected to plant about 60 lakh clonal plants annually of hybrid Casuarina, Acacia, Eucalyptus and Carebia honderanus. The farmers of Haryana, Punjab and Uttar Pradesh are planting about 4,000 ha poplars annually under farm forestry projects promoted by WIMCO Limited (Jones and Lal, 1989). The productivity of these plantations varies from 15 to 20 m3 ha-1yr –1.

It has obviously been a limitation in forest tree species to achieve the gains in a short span of time through breeding approaches. Further, there have been limited artificial and selection in forestry, which have not been fully exploited. Under natural selection, the survival strategy has become an over ridging factor with much of the population being poor yielder. For example, in Eucalyptus tereticornis while 91 per cent of the unproductive population contributed to only 72.40 per cent of utilizable biomass, whereas 9 per cent of the productive population contributed to as high as 41 per cent (Gurumurthi et al., 1991). Similarly, in Casuarina equisetifolia the productive male and female populations of just 10 and 5 per cent contributed to 31.68 and 21.11 per cent to the utilizable biomass, respectively (Kumar, 1996). The similar trend has been reported in a number of other important fast growing tree species including Albizia lebbek, Acacia nilotica, Cassia siamea and Prosopis juliflora (Gurumurthi et al., 1991).

In order to enhance the productivity of man made plantations in a short period, it is imperative to adopt clonal forestry by multiplying genetically improved planting stock. Improved planting stock will not only increase the productivity but also positively influence survival, disease resistance and quality. The existing variability in the base population plays a crucial role in screening the superior biotypes, as it is the platform for other components of improvement cycle viz. breeding, propagation and production populations. Implementing clonal forestry on commercial scale need to be economically viable and increase the productivity substantially in short period. A clone development approach and clone-screening curve will be of immense use in operationlization of clonal forestry (Fig. 4).

Fig. 4. Clonal propagation of Dalbergia sissoo in mist chamber

Verma et al. (1994) derived the ‘clone development approach’ through explaining that clone is a product developed with a view of obtaining high yield at reasonable cost with specific end uses. The development effort on production of clones can add specific variants in terms of identifying clones tolerant to drought, salinity and resistance to pests and diseases, and thus can also be planned for site-specific approaches. Carefully planned development efforts are decided during clonal exploration, selection and evaluation to obtain wide range of clones. This approach can extremely be useful in maintaining genetically broad based productive clones and on the other hand eliminating inferior clones on majority count. Such a clone bank can also serve as breeding population in developing hybrids from superior parents. This can also provide opportunities for hybridization of selected individuals for specific requirements or combining ability viz., tolerance to salinity and high productivity. It does not mention any units for either time or effort, as it would vary from species to species depending on manpower, expertise and base population availability. The experience of Aracruz Florestral, Brazil has shown that it would take about eight years in Eucalyptus. Others drawing on the experience of Aracruz can bring down the time required for commercialization. The local research required on exploration, selection, evaluation and field trials need to be carried out. It is also advisable to select locally performing superior individuals and also introduce exotic clones so that high variability is maintained (Fig. 5).

The short-term strategy to increase productivity involves identification and multiplication of elite biotypes. This should run in consonance with long-term strategy of production of new recombinants through hybridization. In India, though a number of clones of various species have been identified and multiplication techniques developed, large-scale availability of quality planting material remained far from adequate.

Fig. 5. Population cycle to achieve productivity gains through clonal forestry

Large-scale production of quality planting material is possible when operational functions of various clonal propagation activities are managed properly. Only about 15 per cent of the clonal material can be used for commercial production. However, converting selected clones in to large scale plantations involves much more than the selection of few individuals based on just performance and rooting. It may be realized however that even if 10 per cent of the clones selected at the research level reach the commercial stage, the yield of man made plantations can be enhanced substantially.

The development effort on production of clones can add specific variants to understand tolerant to drought, salinity and resistance to pests and diseases, which can also be planned for site-specific approaches. Carefully planned development efforts are decided during clonal exploration, selection and evaluation to obtain wide range of clones. This approach can be extremely useful in maintaining genetically broad based productive clones and eliminating inferior clones on majority count. Such a clone bank can also serve as breeding population in developing hybrids from superior parents. The experience of Aracruz Florestral, Brazil has shown that it would take about eight years in Eucalyptus for this process to complete. However, pro-planning and innovative approach can bring down the time required for commercialization substantially.

 References

TREE IMPROVEMENT FOR HIGHER GENETIC GAINS

Tree improvement is a continual process of selection, testing, and breeding to increase the extent to which each generation of improved stock exhibits desirable traits. Besides, it aims at the sustainable management of genetic variation to produce, identify and multiply for the operational planting of well-adapted genotypes of the desired quality. Breeding and selection are core activities of tree improvement programme. Tree improvement programs were started in different parts of the world in early 1970. Since then, progress has been documented in numerous aspects of forest tree productivity ranging from growth and form improvements to increased tolerance to diseases (Zobel and Talbert, 1984; Sedjo, 1999). Despite this progress, there are a number of constraints in tree improvement processes.

Biotechnology is a collection of techniques that can be used to enhance the impact of biological programme (Cheliak and Rogers, 1990). The application of biotechnology in forestry can be grouped into three main areas: tissue culture techniques, use of genetic markers and production of transgenic trees. Integration of these technologies in tree improvement and the way in which each of these technologies can address the problem/enhance the impact of conventional tree improvement are presented here.

Cell and Tissue Culture

To date, the major emphasis in forest biotechnology has been on tissue culture. Strategies supporting large scale utilization of superior genetic materi al need rational integration in tree improvement. Tissue culture broadly refers to techniques of growing plant tissue or parts in nutrient medium under aseptic condition. It involves a set of technique known as micro-propagation- a kind of clonal or vegetative propagation that can produce multiple copies of an elite genotype. It is a means to introduce novel genes as well. Mean to achieve this include Organogenesis and Somatic embryogenesis. In Organogenesis, organ primordial such as buds are initiated on explants with the aid of exogenously applied hormones. From these buds, shoots and roots are induced. Somatic embryogenesis is a method that starts with somatic cell(s) of donor plants and forms new embryo. Micro propagation is now being taken up as large commercial venture involving hundreds of laboratories around the world. Successful protocol now exists for more than hundred forest tree species, hardwoods and softwood (Bajaj, 1986; Mascarhenas and Muralidharan, 1989). Compared to vegetative propagation through cuttings, the higher multiplication rates possible through micro-propagation offer a quicker capture of genetic gains in clonal forestry. One of the major factors limiting early application of this technique in many large-scale plantation programmes is that breeding and selection of desired clones are not sufficiently advanced to take up clonal forestry (Haines, 1994). Current high costs and sophistications could also be impediments to the direct use of micro-propagations for many species.

In Vitro Selection

In vitro selection refers to selection of genotype based on test results using tissue culture under laboratory conditions. The traits for which selection done includes, growth, resistance to stresses caused by agents such as salt, heat, frost, drought, insects, metals and herbicides. Selection can be at the level of cells, pollens, buds, shoots, embryo or whole plant. Screening is done by subjecting cell cultures to causal agent directly (as in high and low temperature, herbicides, etc.) or its analogue toxins are incorporated in growth medium. Useful correlations between in-vitro studies and expression of desirable traits ex vitro, for disease resistance are being reported (Frampton et al., 1983). One particular interesting example has been the selection of Eucalyptus gunnii pollen surviving exposure to cold, which was capable of germination and could be used in control pollination (Boudet and Marien, 1988). For major important traits in trees (in particular, vigor, stem form and wood quality) poor correlation with field response will limit the usefulness of in vitro selection. However, in vitro selection may be of possible interest in forestry for screening disease resistance and tolerance to abiotic stress.

Somaclonal Variation

The term ‘Soma-clonal variation’ has been used to refer to variation, which has been observed in plants regenerated from cell or tissue culture, but in general not from auxiliary bud or shoot tip culture. Variation arises due to pre-existing inter-cellular variations or induced variation during culture (in most cases). This technique would be of interest where insufficient variation is available to provide the level sought for traits or existing variation is not easily used in breeding. For example, herbicide tolerance is not known to exist in angiosperms which would be difficult to achieve through traditional breeding (Mitchler and Hassig, 1988). This technology has limited application in short or intermediate term improvement programme.

Protoplast Fusion, In vitro Embryo Rescue and Haploid Culture

Protoplast fusion comprises removal of cell wall and then fusion of cell contents. The hybrids developed are called somatic hybrids. Protoplast fusion could be important in genetic improvement for which wild relatives constitute extensive and untapped disease resistance gene sources (Haines, 1994). Nucleo-cytoplasmic incompatibility and poor plant regeneration from protoplast limits its application.

In vitro embryo rescue is usually used in fruit trees, to grow embryo that normally would abort due to incompatibility between ovule and embryo development. Mainly used in making crosses across the genera or species. Some early success include Pinus lambertiana X P. armandii (Stone and Duffield, 1950), Populus simonii X P. pyramidallis (Ho, 1987). Its application in tree species is impeded by difficulties in development of protocol.

Haploids are plants with ‘n’ somatic number of chromosomes, developed from male or female gametophyte. Haploid cultures are employed for rapid production of homozygous lines, which are colchicine treated and chromosome doubled, to get true homozygous lines. Mainly useful in self-pollinated crops to get pure lines. Hardwood trees species for which pollen cultures have produced plants are Betula pendula and 13 species or interspecific hybrids of poplars (Cheliak and Rogers, 1990). Induction of haploid doesn’t have immediate application in forest tree species. May be found useful in basic genetic studies of heterosis.

Cryopreservation and In vitro Storage

This comprises the maintenance of cells, tissues or organs in cultures where growth is slowed (e.g. by the reduction of light, temperature or nutrients) or suspended (by immersion in liquid nitrogen). Many technical difficulties are involved, particularly in the subsequent regeneration of plants from the cultures limit its application. Regeneration from cryo-preserved tissues has been induced for more than 70 species, including coconut, rubber, cocoa and coffee, and for several forest tree species. These results have led to hopes that the technologies may have a number of applications in tree improvement. In vitro storage and cryopreservation have little to offer with regard to forest trees except germplasm preservation of threatened tropical tree species. In the longer term, cryopreservation and in vitro storage may have some application as a backup conservation strategy.

Synthetic or Artificial Seeds

Artificial seeds consist of somatic embryos surrounded by a protective coating. It is a potential method to combine the advantage of clonal propagation with low cost and high-volume capabilities of seed propagation. Artificial seed contain nutrients to provide energy required for germination and the hormones may be used as supplement. Sodium or Potassium alginate is commonly used as gel for encapsulation. Liquid cultures with automation make embryo culture system economically feasible. Application in forestry includes, production of superior genotypes early where seed orchards takes 20-30 years for flowering. Millions of somatic embryo can be produced all the year round. In Conifers-Loblolly pine and Norway spruce are successfully tried but with no reports of field planting and establishment. In India, Eucalyptus citriodora and Santalum album synthetic seeds gave four per cent germination in soil (Bapat and Rao, 1989).

Molecular Markers

Reliable information on the distribution of genetic variation is a prerequisite for sound selection, breeding and conservation programmes in forest trees. Genetic variation of a species or population can be assessed by measuring morphological and quantitative characters in the field or by assessing variation at molecular level (DNA or protein) in the laboratory using a molecular marker. Compared to traditionally measured features such as vigor, stem quality and various morphological aspects, molecular marker s offer the advantages of being unaffected by the environment or the developmental stage of the p lant while also being very numerous. These characteristics have led to a number of potential appl ications in tree improvement. This technique have provided useful genetic information to tree imp rovement and are still very important tools for studies of tree genetics and tree breeding progra mmes (Strauss, 1992). Molecular markers are of two classes. One that is derived from direct analy sis of polymorphism in DNA sequences called as Molecular Genetic Markers, while the other derived from studies of chemical product (terpenes) or proteins of gene expression referred as Biochemi cal Markers. Isozymes are commonly used biochemical markers as detectably different enzymes, whi ch catalyze the same reaction (Hamrick et al., 1992). Allozymes are specifically applied to situation where the different forms are the results of allelic polymorphism (Tanksley, 1983). In isozyme analysis, particular enzyme or protein is extracted from plant tissue and different forms are separated by gel electrophoresis on basis of molecular size, shape and electric charge. Allozymes are co-dominant markers that enable us to distinguish a heterozygote from a homozygote at a locus (Wendel and Weeden, 1989).This technique is useful in assessing gene flow, mating system and genetic drift. However, the number of markers closely linked to important loci is small in number. Further, low resolution of conventional gel electrophoresis makes it difficult to detect subtle variation in allozymes. A difficulty in standardizing the protocol hinders its universal application.

With the advent of restriction enzymes and Polymerase Chain Reaction (PCR) the assessment of genetic variation directly at DNA level is possible. RFLPs (Restriction Fragment Length Polymorphism), RAPDs (Randomly Amplified Polymorphic DNAs) AFLP (Amplified Fragment Length Polymorphism) are simple mendalian molecular markers used to assess the polymorphism at gene loci. As these class of markers are based on DNA sequences, offer advantages such as heritable through generations; availability of standard protocol; easily extractable from any plant part; required in small quantity (in nanograms); easy to store and handle. Microsatellites or Simple Sequence Repeats (SSR) are another class of markers that rely on highly variable repetitive DNA sequences composed of short (<6 base pairs) sequences repeated in tandem. These are usually co-dominant and very powerful (Strauss et al., 1992). Disadvantage of microsatellite is the necessary of prior identification of these regions from genomic library. Using it for a new species makes it time consuming and costly task. In recent years, single nucleotide polymorphisms (SNPs), i.e. single base changes in DNA sequence, have become an increasingly important class of molecular marker. The potential number of SNP markers is very high, meaning that it should be possible to find them in all parts of the genome, and micro-array procedures have been developed for automatically scoring hundreds of SNP loci simultaneously at a low cost per sample.

Applications of molecular markers in the tree improvement include-

a. Quantification of genetic diversity

The use of molecular markers for the determination of the extent of variation at the genetic level, within and between populations, is valued in guiding genetic conservation activities. These are aimed at maintaining genetic diversity with respect to traits of both known and unknown importance. And helps in the development of breeding populations for specific end uses with appropriate large genetic variation. In both the cases, the use of molecular markers would circumvent the obstacle to measure variation in terms of quantitative traits, where environmental effects pose difficulty in calculating genetic diversity parameters and requirement for several years before many traits can be measured. For forest trees species, isozymes have been widely used for assessment of among and within population variation in Abies alba, Larix deciduas, Picea abies, Pinus sylvestris (Muller-strack et al., 1992), Picea glauca, Pinus resinosa (Hamrick et al., 1992) and many others. RAPD techniques have been widely used assessment of genetic diversity in mohagony, eucalypts, poplars, spruce (Lakshmikumaran et al., 2001). AFLPs were used to catalogue genetic variation in neem (Lakshmikumaran et al., 2001), eucalypt (Marques et al., 1998). It should be noted that studies on genetic diversity based on molecular markers must be interpreted with caution, due to frequently low correlations with patterns of variation for adaptive traits, which are of major importance in forestry.

b. Genotype verification and delineation

Molecular markers have been widely used for identification of genotypes and applied in taxonomic studies, biological studies and genetic fingerprinting. The use of molecular markers has revolutionized studies of mating systems, pollen movement and seed dispersal. These studies are of considerable practical significance to advanced tree improvement programmes, specifically in population sampling, design and management of seed orchards, estimation of pollen contamination and development of controlled pollination methods (Neale et al., 1992). Germplasm identification, through ‘genetic fingerprinting’, has been used in advanced breeding programmes which rely on controlled crosses or in which the correct identification of clones for large-scale propagation programmes is essential. RAPDs have been used to delineate provenance variation in Pinus lambertiana, P. ponderosa and breeding zones have been designated in Pinus lambertiana, P.ponderosa and Psedotsuga menziesii (Westfall and Conkle, 1992). In Eucalyptus,RAPD markers have been used in genetic analysis of individuals and populations including clone fingerprinting, outcrossing rate estimation and phylogenic relationship studies (Grattapaglia and Sederoff, 1994). In oak, the molecular differentiation between Quercus petraea and Q. robur was evaluated with RAPD markers (Moreau et al., 1994). The results of such studies aids in population sampling, seed orchard design and management, controlled pollination method and clonal forestry programme. For example, the knowledge of gene movement permits, breeder to estimate minimum distance between candidate trees when sampling natural population. Yeh (1989) cited an estimate of 65 m for Picea glauca based on isozyme studies. The assessment of pollen movement is also of major significance in management of seed orchards, in estimating pollen contamination, mating pattern among the clones, inbreeding, etc. Approaches to detection of contamination have been discussed in more detail by Wheeler and Jech (1992). Isozymes have been used to show that 30 per cent or more of Douglas fir and Loblolly pine controlled crosses were not correct (Adams et al., 1988). Molecular markers also used to assess extent of inbreeding and preferential mating systems in seed orchards. Selfing rates of 16 per cent were reported in a Pinus sylvestris orchard and also revealed that only two of 33 clones contributed about 50 per cent of pollen in two consecutive years. Some times tree breeding programme encounters problems in identification of seed lot (provenance, orchard batch, family lot, etc.) or clone. Misidentifications of clones in seed orchard establishment or crossing are very common which could be overcome with the help of markers.

c. Gene mapping and marker-assisted selection (MAS)

Genetic linkage maps can be used to locate genes affecting quantitative traits of economic importance. Quantitative traits, such as wood yield, wood quality or pulp yield, are usually controlled by many genes, termed Quantitative Trait Loci (QTL). By using molecular markers closely linked to, or located within, one or more QTL, information at the DNA-level can be used for early selection. The potential benefits of MAS are greatest for traits that are difficult, time-consuming or expensive to measure (e.g., wood quality traits or pulp yield). Mapping and MAS tend to be used only in species of high economic value and have most potential in clonal breeding programmes . MAS refer to indirect selection on the basis of markers shown to be associated with commercially important genes. Unaffected by the environment or developmental stages, markers offer the possibility of highly effective and early selection, for e.g. selection for wood quality at the young seedling stage (Nance and Nelson, 1989; Neale et al., 1992). Although the possibilities are very attractive, there are limitations that will prohibit application in the short or medium term. Marker analysis is currently too expensive to permit the screening of large populations of seedlings. It could be appreciated in mainly advanced breeding programmes, those for which the creation and maintenance of the appropriate pedigree record is affordable and where clonal forestry is achievable. Linkage maps have been constructed in using RAPD markers in Douglas-fir (Krutovskii et al., 1998). Highly variable SSR markers are also available (Slavov et al., 2004). Traits of major importance in forestry are mainly quantitative but work directed at identification of QTL’s in trees has not been progressed so far except in Populus and in few conifers.Hindrances are large size of genome, scarcity of multigenerational pedigree and long generation (Tulseiram et al., 1992). QTL analyses are highly informative providing information on location of QTL affecting a trait, magnitude of each of QTL, gene action and parental sources of beneficial alleles. This would help in improving the efficiency of selection and detailed understanding of genetic architecture of complex traits. Mapping of QTLs for commercially important traits and adaptive traits in poplars (Bradshaw and Stettler, 1995), Douglas-fir (Wheeler et al., 2004) and Eucalyptus (Grattapagalia, 2004) have been reported. Poplar is the first tree to be genome sequence (Tuskan et al., 2006)

The major current value of molecular markers lies in long-term strategic research. Marker studies are making great contributions to advances in the understanding of basic genetic mechanisms and genome organization at the molecular level. An important emphasis of this work in coming years will be the study of quantitative traits of forest trees, of which a few model species will receive most attention, for example loblolly pine (Pinus taeda), poplar (Populus spp.).

Genetic Engineering

This technique involves insertion of novel genes into a plant or else the modification of existing genes through manipulation of the DNA molecule. This is more popular in agricultural crops where genes for insect, virus and selected herbicide resistance have been incorporated. Improvement of trees by conventional breeding is constrained by their long reproductive cycle and complex reproductive characteristics. Genetic engineering offers an attractive addition to conventional breeding because it permits the transfer of valuable traits into selected genotypes without compromising their desirable genetic background. Recombinant DNA technology has made an easy and an unlimited gene pool with desirable traits for tree breeders. Endogenous gene already present in the tree genome can be modified to improve certain traits, such as fiber quality and quantity, lignin reduction. The genes coding for novel traits which were not available in the genome are inserted to confer entirely new traits such as herbicide tolerance, insect resistance, abiotic stress tolerance, etc. The traits transferred are expected to have a positive impact on economics of plantation in terms of improved growth, reduced rotation, improved woodquality, reduced cost of pest control or confer distinct environmental benefits during wood production or processing like improved pulping, reduced inputs of hazardous chemicals and energy (Strauss et al., 1992).

The two most common transformation methods are Agrobacterium mediated DNA transfer and bombardment with DNA coated micro-projectile called as biolistic method. Between the two, biolistic method is popular because of non-compatibility of Agrobacterium for most of the species. First report of genetic transformation and recovery of transformedplants of a forest tree involved insertion of herbicide tolerance gene into hybrid poplar using Agrobacteruim tumefaciens (Fillati et al., 1987). Routine transformation procedures are available for most of species such as poplar and their hybrids, Eucalyptus, Pinus, Picea, Larix (Llewellyn, 2000). Production of first transgenic conifer was reported a decade ago (Ellis et al., 1993).

Herbicide tolerance has been a major trait for transformation in both agricultural crops and trees as commercially attractive trait because it minimizes the herbicide use and thus cost of cultural operations. Higher and commercially relevant Glyphosate (a herbicide) tolerance in poplar was achieved by using the CP4 gene with a chloroplast transit peptidase sequence (Haines, 1994). Other species that were explored are Eucalyptus grandis (Llewellyn, 2000) and Larix decidua (Shin et al., 1994).

With respect to insect resistance, primarily it is focused on use of delta endo-toxin gene (cry) from soil bacteria Bacillus thuringensis (Bt). Poplar, walnut, white spruce and European larch were tailored with cry genes (Haines, 1994). Under laboratory conditions, high levels of mortality have been reported for Lepidopteron pests on transgenic poplar (McCown et al., 1991). The large-scale multilocation field evaluation of lepidopteron resistant poplar took place in China, where 20,000 clones were planted at in 1995, but no results have been reported to date (van Frankenhuzen and Beardmore, 2004).

Another trait targeted is to confer disease resistance. Several economically important hardwood tree species are susceptible for crown gall disease. Because of high infective nature of disease, economic loss is considerable on various hardwood trees. Method of using antisense oncogenes from causal agent Agrobacterium itself revealed resistance against crown gall in hybrid poplar. Genes coding for phytoelexin manosonone revealed to hold potential for future development of resistance in elm trees (Duchesne et al., 1985). Very few reports have been published so far in hardwood species.

Abiotic stress, a looming world wide problem of climate change, desertification, and salinization are challenges to foresters in near future. Transgenic forest trees are being developed to unveil the stress tolerance mechanism using loblolly pine (Pinus radiata) and poplar as models. Salt tolerance in loblolly pine with genes transformed to regulate osmotic balance has been found successful. Cold-tolerant genes are likely to be of some commercial value for many species, in particular the eucalypts. Transgenic having potential to clean up the environmental pollutants especially heavy metals and organic pollutants have been targeted. Transgenic yellow poplar (Liriodendron tulipifera) with bacterial mercuric reductase gene was reported to convert highly toxic mercury (Hg[II] to less toxic form Hg[0]) (Rugh et al., 1998). Similar approaches are being followed to remove range of organic pollutants such as TNT, RDX, tricholoethyele (Gordon et al., 1998). The advancement of molecular genetics is expected to provide inputs for engineering abiotic stress tolerance in trees, which is a complex phenomenon.

Modification of Wood Quality and Quantity

Lignin modification and reduction are target trait in wood properties for many reasons. The removal of lignin is costly and energy consuming component of pulp and paper production process. One way to improve efficiency of pulping is to genetically reduce quantity or to alter quality of lignin in pulpwood species. Angiosperms with Guaiacyl-syringyl lignin can be easily excluded at less expenditure of chemicals, energy and time when compared to Guaiacyl lignin in conifers (Chiang and Fanooka, 1990). Moreover, conifers are preferred for pulping due to their long and superior cellulose fibers. The changing of Guaiacyl lignin in conifer to Guaiacyl-syringyl lignin types of angiosperm are being tried (Bugus et al., 1991). The most outstanding achievement to date is 45 per cent reduction in structurally normal lignin resulting from anti-sense suppression. Lapieere et al. (1999) found that Cinnamyl alcohol dehydrogenase (CAD) activity increased the incorporation of aldehydes and free phenolic groups that facilitated lignin solubilisation during kraft pulping. CAD anti sense poplar was found to be amenable for easy delignification with higher pulp yields, thereby saving chemical (6 per cent) and increased pulp yield (2-3 per cent).

Prevention of the escape of genes into wild populations is likely to become an important concern, and inducing sterility an early target of genetic engineering work with forest tree species. The major factor limiting application of genetic engineering in forest trees is the lack of knowledge of molecular control of the traits such as growth, adaptation, stem and wood quality. Long rotation for tree limits extensive evaluation before release. Other concerns like effect on non-target insects, stability of expression up to desired level, etc. are also unpredictable.

Conclusion

Biotechnology has potential to enhance tree improvement programme in number of ways. Tissue culture facilitates multiplication of selected genotype more efficiently or more quickly and means of preserving germplasm. With aid of molecular markers, we can gain more insight into nature, organization and control of genetic variation. Genetic engineering allows creating new types of trees by introducing novel traits. Modern biotechnology should be perceived as a new group of tools, means to be used as adjuncts, or complements to conventional technologies in solving problems directed at meeting the needs of human beings. A balance should be maintained between modern biotechnological and conventional research. The development and application of biotechnology should be driven by needs and not by technological capability. The relative costs (financial, social, political or environmental) of the biotechnologies versus the relative benefits (productivity, food security or otherwise) should be thought carefully before taking up the task. The use of modern biotechnologies should be promoted for more efficient solutions to problems, which requires most urgent attention shortly.

References