- Improvement of a tree crop like rubber is relatively slow and laborious. Nearly 30 years are required for recom¬mending a new clone for commercial planting. The phe-nomenal increase in yield of rubber has been achieved after years of repeated selection of high-yielding mother trees followed by their vegetative multiplication, control¬led pollination among high-yielding clones and further selection from among progeny. Current approach is to breed clones with diverse desirable characters such as resistance to biotic and abiotic stresses and utilizing wild germplasm in breeding programmes.
Germplasm
- The spectrum of Hevea germplasm can be broadly classified into those existing in the primary centre of diversity in Brazil and those developed in centres of secondary diversity. Thus, it is a collection of all genotypes that represent the entire genepool, including current popular clones, obsolete clones and wild accessions from the centre of diversity in Brazil. Rubber Research Institute of India, Kottayam, Kerala, maintains a collection of 174 exotic and indigenous clones of Wickham origin in a clone museum. In addition, 4,967 accessions representing the wild Brazilian germplasm collected through germplasm exchange programmes are also maintained. International Rubber Research and Development Board (IRRDB) and Association of Natural Rubber Producing Countries (AN-RFC) are agencies associated with the clone exchange programmes.
- Clones are group of plants produced by vegetative propagation from single trees. All individual trees of a clone possess identical genetic constitution, which is re¬sponsible for the uniformity existing among them. Clones are usually named after the estates, institutes or stations from where they have originated and indicated as abbre¬viations. Based on the type of mother tree, from which the clone is derived, they are classified as:
- Primary clone: Mother tree is of unknown parentage, selection of mother tree is based on superior performance in the existing plantation. Tjir - I, GT I, PB 86, PR 107 and PB 28/59 are primary clones.
- Secondary clone: Mother tree is evolved by controlled pollination between 2 primary clones. RM 600 (Tjir I X PB 86) and RR II 105 (Tjir I X GL - I) are secondary clones.
- Tertiary clone: Mother tree is evolved by controlled polli¬nation in which at least one or both parents are secondary clones. RRIM703 (RRIM 600 x RRIM 500) is an example. In order to obviate the potential risks involved in the monoclone culture, cultivation of a mixture of clones which is categorized as follows is recommended:
- Category I: Clones like RRII 105 (in traditional areas) along with RRIM 600 and GT I (in non-traditional areas) to cover upto 50% of the total area. Other important clones under this category are PB 260, RRII 414 and RRII 430.
- Category II: Clones like RRIM 600, GT 1, PB 28/59, PB 217 and RRIM 703 to cover upto 50% of the total area. Other important clones in this category are RRII 5, RRII 203, RRII 417 and RRII 422.
- Category III: The cultivars under it are divided into 3 categories which can cover upto 15% of the total area in aggregate. They are:
- RRII 5, RRII 203, PB 255, PB 2611 and PB 235
- Tjir I, PB 86, GI 1, PR 107 and RRIM 605
- RRII 50, RRII 51, RRII 52, RRII 109, RRII 116 and RRII 176
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