**7. Conclusions and prospects**

450 Genetic Diversity in Plants

found to be very low and non-synchronous, its germination remains very difficult and thus *Hevea* embryogenic system needs further investigation. There has been no large scale commercial application of tissue culture techniques for mass propagation of clonal *Hevea* as yet, either by microcutting or by somatic embryogenesis. However, there is sufficient progress at the research level to suggest that tissue culture of *Hevea* can and should be

At the scientific and technical levels, challenges are posed by genetic erosion, genetic vulnerability and utilization. Genetic erosion is defined as "the loss of genetic diversity, including the loss of individual genes, and the loss of particular combinations of genes (i.e. of gene-complexes) such as those manifested in locally adapted landraces" (FAO, 1997). There is no consensus on the optimal balance of *in situ* and *ex situ* conservation methods to combat genetic erosion (Fraleigh, 2006). A gradual erosion of the genetic variability of *Hevea* in all of natural rubber plantations was realized in the 1970s. This erosion occurred because most of *Hevea* clones in cultivation were derived from the few surviving seeds collected by Henry Wickham. Therefore, widening the genetic base of *Hevea* in production was seen as a prerequisite to generate new clones with new and valuable characteristics with regard to productivity, disease resistance, and tolerance to many environmental

Genetic vulnerability was described as "the condition that results when a widely planted crop is uniformly susceptible to a pest, pathogen or environmental hazard as a result of its genetic constitution, thereby creating a potential for widespread crop losses" (FAO, 1996). It is known that genetic vulnerability pertains to the level of the crop genetic diversity actually being used. Because of the very narrow genetic base in the commercially cultivated *Hevea* clones, the commercial rubber cultivation, due to their genetic vulnerability, is under a constant threat of attack by native as well as exotic diseases and insect pests. The changes in the weather parameters due to the increasing trend in climate change have further complicated the above issues. Climate change, which is clearly felt in the traditional rubber growing regions of Vietnam, may possibly alter the host-pathogen interactions. This will lead to the emergence of otherwise minor disease, and *Corynespora* leaf fall disease may represents this scenario. This pathogen is rapidly progressing into new areas, thus highlighting the need for stronger and advanced

It is known that the potential uses and values of *Hevea* genetic resources need to be understood by characterizing, evaluating and documenting them. Methods still need to be developed to improve and facilitate productive utilization. Although biotechnological methods are now increasingly available to facilitate productive utilization of *Hevea*

Another set of challenges is posed for taking action. For instance, how the necessary cooperation can be organized among countries and among disciplines, particularly in order to link the conservation and the use of *Hevea* genetic resources, and how the resources which need to address these issues can be mobilized. Besides, cryogenic preservation of

endangered seedling trees is yet another important aspect to be looked into urgently.

germplasm, not all countries have the capacity to use such new technologies.

further developed.

**6.2 Challenges** 

conditions.

resistance breeding approaches.

Vietnam has received a large share of the *Hevea* germplasm collection of the 1981 IRRDB expedition. A total of 2,999 wild accessions belonging to three states of Brazil *viz*., Acre, Rondonia and Mato Grosso are being conserved. Systematic efforts are underway for conservation, characterization, utilization and documentation of these valuable genetic resources. In general, the genetic base of *Hevea* in Vietnam which is prosperous and diversified has been contributing effectively to the long term progress of *Hevea* breeding program in the country. As management of the germplasm is a herculean task, IRRDB'81 accessions have been evaluated in a phased manner. Studies in different sets of this germplasm have been carried out since 1985 onward to access the extent of variability present in the collection for various agronomical characteristics such as latex yield, girth, wood volume, diseases incidence, biotic and abiotic stresses, in order to exploit them in the improvement programs. A large number of IRRDB'81 accessions are now in various evaluation stages. The evaluated IRRDB'81 collection displayed unimproved characteristics of a wild population and was far inferior to the Wickham clones in agronomical performances, especially latex productivity. However, this germplasm had a much broader genetic variability which can help in broadening the narrow genetic base of cultivated rubber and also in developing location specific rubber clones for cultivation in the marginal and non-traditional rubber regions of the country.

The genetic parameters obtained from isozyme and RAPD analyses indicated that the *Hevea* germplasm conserved in Vietnam exhibited large genetic diversity. The biochemical and molecular markers have also shown to be the effective techniques for breeders to manipulate the *Hevea* germplasm. These markers could be used to select the parents with far genetic distance aiming at enlarging the genetic variability in their progenies and could help in checking the genetic variability of the *Hevea* breeding program. Furthermore, information on the structure of genetic diversity could help establish global *Hevea* collections for long-term conservation with minimum maintenance activity, and help define working collections for medium-term targeted utilization and breeding purposes.

The conventional solution to the conservation of *Hevea* genetic resources has been the establishment of *ex situ* genebanks. *Ex situ* conservation is the only effective method for the long-term conservation of *Hevea* germplasm. *Ex situ* conservation may also represent a last resort for many species and varieties including *Hevea* that would otherwise die out as their habitats are destroyed. Moreover, the management of existing *ex situ Hevea* collections is an alternative solution to rubber trees *in situ* genetic preservation (Le Guen et al., 2009). The main benefit of *ex situ* conservation is to provide breeders with ready access to a wide range of genetic materials with useful traits. *Hevea* germplasms have their own share of problems. Although only a small proportion of *Hevea* genetic resources are actually used by breeders, partly because of the inadequate characterization of the accessions, the costs of characterizing, evaluating and cataloguing genetic resource materials need to be carefully considered. Because of the severe limitations faced by large germplasm collection, establishing a core collection of this germplasm is necessary to facilitate a speedy and more efficient evaluation, and to get a collection which is conserved better and exploited more effectively.

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