Rubber Tree Cultivation and Improvement: Rootstock-Scion Compatibility between Hevea Species and Cultivated Planting Materials
Abstract
Rootstocks have a clear effect on rubber tree growth and development during the seedling and immature stages. However, the exploration of Hevea species as rootstocks is relatively uncommon in the general practices in the cultivation and improvement programmes in Malaysia. Hevea species were tested in this research including Hevea brasiliensis, Hevea benthamiana, Hevea camargoana, Hevea guianensis, Hevea nitida, Hevea pauciflora, Hevea rigidifolia and Hevea spruceana. This research examined the successful bud-grafted percentage between scion and rootstock of different Hevea species and cultivated planting materials. The results demonstrated that rootstock-scion of H. benthamiana-PB 260 achieved the highest successful bud-grafted percentage at 94.5%, followed by H. nitida-RRIM 2001 (93.8%), H. nitida-PB 350 (92.3%) and H. pauciflora-PB 260 (90.8%). The lowest successful bud-grafted percentage came from H. benthamiana-RRIM 2025 at 51.1 %. Therefore, the exploration of Hevea species as potential rootstocks based on the successful bud-grafted percentage between rootstock-scion and their compatibility could be applied as a speed indicator for rubber nurseries to produce high quality planting materials.
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Introduction
Among the famous events in the history of natural rubber is the massive amount of natural rubber produced by rubber plantations to satisfy the high demand during The Industrial Revolution. Natural rubber is a chain of polymers that exhibits high resilience, impact resistance, elasticity, stretchy strength, as well as low heat swelling during manufacturing processes. This is attributable to the unique molecular structure of the rubber, which is difficult to be matched by synthetic rubber derived from the petroleum sources. The use of natural rubber can be seen in various domestic and industrial products nowadays. This started in 1876 when Henry Wickham collected about 70,000 rubber seeds (H. brasiliensis) near the Tapajos River in Brazil, and attempted to sow them in the United Kingdom. This small population of sowed and germinated seedlings were later transported to Ceylon (Sri Lanka) and Singapore in 1876. Eventually, 22 seedlings survived during the transportation journey and arrived in Kuala Kangsar, Malaya (Malaysia) in 1877, since then these seedlings have formed the genetic base of rubber trees in the country (Baulkwill 1989; Barlow 1978; Loadman 2005, MRB 2005; Priyadashan, 2011). Prior to the 1950s, seedlings that germinated from rubber seeds were widely accepted as cultivated planting material, either by rubber plantations or by smallholdings. For example, the seeds obtained from established experimental gardens at Rubber Research Institute of Malaysia (RRIM) have been accepted as being good in quality and recommended for extensive planting (Heuser 1932; RRIM 1957: Ng, 1983; Ong and Shamsul, 2013). In general, seedling trees would generate variable yield and unpredicted characteristics such as growth rate, canopy density, branching habit, bark thickness, wind damage tolerance, disease tolerance etc., whereas cultivated planting materials recurrently showed uniformity on the mentioned characteristics.
Conclusion
The combinations rootstock-scion of H. benthamiana-PB 260, H. nitida-PB 2001, H. nitida-PB 350 and H. pauciflora-PB 260 showed high successful bud-grafted percentage that should be focused by rubber plant breeders because these neglected Hevea species have the potential to contribute to a higher level than they currently do. From a crop improvement perspective, the genetic potential of Hevea specie is massively determined by the combination of genes that they have contained. Quantitative Trait Loci (QTLs) analysis and Marker-Assisted Selection (MAS) were introduced in rubber improvement programmes in many decades ago, in attempts to verify the purity of rubber planting materials, selection of parentage, population diversity analysis, and increase desired traits for commercial purposes. However, there exist no successful MAS to detect genes make-up for rootstock-scion compatibility in rubber trees. This is because of the low power of QTL detection, whereas complex traits such as callus cells multiplication and vascular cambium tissue development that alleged to associate with a series of QTLs. In short, the successful bud-grafted percentage between rootstock-scion, regardless of Hevea species or commercial planting materials, could be applied as a speed indicator for rubber nurseries to produce high quality planting materials.