**6. Problems and challenges**

#### **6.1 Problems**

448 Genetic Diversity in Plants

Since the IRRDB'81 collection exhibited very poor profiles in agronomical characteristics, especially latex productivity, the chance for direct use of this collection for latex purposes seemed to be impractical although certain accessions could be planted for timber purposes. Regarding widening the genetic base for genetic improvement, several promising IRRDB'81 accessions have been included in hand pollination program in RRIV since 1997. Based on the agronomical and morphological traits as well as the genetic diversity analysis, recently, many attempts have been made to enlarge the genetic base of *Hevea* breeding materials by polycrossing among different genetic resources. In this way, many crosses between maternal

Fig. 9. Genebank documentation program in conservation of *Hevea* germplasm

W and paternal A accessions have been preferably made and the progenies of which are in various phases of evaluation with the expectation that the W x A progenies could combine the good agronomical characteristics from parents while optimizing the genetic variability in this population. In fact, Amazonian accessions were crossed with Wickham high yielding and good set fruit clones such as PB260 and RRIC110, and progenies derived from these hand pollination crosses were disbudded into fields of early selection trials for evaluation of agronomical performances such as latex yield, growth and diseases incidence. In general, these progenies exhibited rather good in girth but very poor in latex productivity. Most of the progenies produced a very little or negligible amount of latex; this agreed closely with the previous finding in Ivory Coast for the progenies from W x A crosses (Clement-Demange et al, 1990). However, some progenies had the production of 1.9 – 3.2 gram/tree/tapping or 106 – 132% of the production of the control clone (i.e. PB 260), and a large number of the progenies showed very good in growth performance with girth at 34 months old after planting ranging from 15.6 – 21.2 cm, or 101 – 161% of that of the control clone. These progenies are being further tested in the small scale clonal trials Because the genetic structure of natural *H. brasiliensis* populations is based upon both hydrographical network patterns and long-range isolation by distance, it is likely that the known genetic diversity represents only part of its true natural diversity (Le Guen et al., 2009). In order to enlarge the genetic resources of *H. brasiliensis*, it is necessary to conduct additional expeditions in other areas that were not yet covered previously, such as Amazonian basin in Colombia, Peru and Bolivia, and the Brazilian states of Pará and Amazonas. Besides, since a very small part of the diversity of *Hevea* germplasm has been collected and conserved in Vietnam, a much greater diversity of the germplasm should be imported from other countries such as Brazil, Malaysia and Ivory Coast. Moreover, it is urgent need to duplicate the *Hevea* germplasm accessions, particularly the IRRDB'81 collection, in all IRRDB country members to prevent the loss of accessions and to increase the genetic resources as raw materials in *Hevea* breeding programs. Additionally, molecular tools may contribute in *ex situ* conservation of *Hevea* germplasm to the sampling, management and development of "core" collections as well as the utilization of genetic diversity. However, the use of such molecular tools is limited in the rubber growing countries including Vietnam due to their cost.

Rubber tree is traditionally propagated through bud grafting on unselected seedlings, which maintains intraclonal heterogeneity for vigour and productivity. Therefore, variation among a bud-grafted population is significant and can influence the growth and productivity levels. Therefore, a great improvement may be expected by using *in vitro* micropropagation. *In vitro* techniques have currently made a commercial impact in rubber, and their propagation systems can circumvent the influence of the stock-scion interactions in *Hevea* clones (Priyadarshan, 2007). Recently, there has been an increasing interest in the induction of somatic embryogenesis in rubber trees. However, successful somatic embryo formation and plant regeneration have been reported by a few researchers in different countries using limited genotypes of *Hevea*. In addition, the frequency of somatic embryo induction was 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 further developed.

#### **6.2 Challenges**

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 conditions.

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 resistance breeding approaches.

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

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.
