Rubber Tree Cultivation and Improvement: Biological Aspects and the Risk of Inbreeding Depression
Abstract
Rubber trees (Hevea spp.) are among the essential plants cultivated and have contributed to Malaysia’seconomy growth for many decades. Latex harvested from rubber trees is an irreplaceable raw material and accounts for a wide range of uses in tires, tubes, footwear, rubber gloves, and other rubber-based products. There were many attempts to produce ideal rubber tree for increasing latex yield production through the improvement programmes since 1950s. However, the risk of inbreeding depression and the planting materials produced from the chosen parents that are closely related in the improvement programmes is fairly high. Inbreeding depression caused discouraging effects such as uneven bark surface, leaf disease infection, easily damaged by wind blows and eventually reducing the production of latex yield overall. This review highlights the important of biological aspects for latex production in rubber tree and seeing minimizing the risk of inbreeding depression with the necessity of broader genetic base in the rubber tree cultivation and improvement programmes.
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Introduction
Genus of Hevea belongs to the Euphorbiaceae family, mainly climber herbs, shrubs, and trees. It is among the largest families of plants, comprising over 230 genera and 5,700 species. Generally specified in the grasslands, Euphorbiaceae has the most vegetation types, including grain forest trees, weeds, and succulents. In South America, Hevea trees can be found in forests in Brazil, Bolivia, Colombia, Ecuador, French Guiana, Guyana, Peru, Suriname, and Venezuela (Webster and Paardekooper, 1989; Schultes, 1990; Priyadarshan, 2011). The successful transplantation and domestication of rubber trees from the Amazon forest has allowed Malaysia to play an essential role as one of the most critical natural rubber suppliers in the world for the past several decades. According to a report by the Malaysian Rubber Board (MRB) in 2019, the estimated total production of natural rubber in 2019 from smallholdings and estates in Malaysia was recorded to be 639,000 tons. The total export value of natural rubber and rubber products amounted to approximately RM 40 billion (Natural Rubber Statistics 2019, http://www.lgm.gov.my/nrstat). The primary destinations for raw rubber exports in 2019 were China, Germany, Finland, Iran, the USA, and other countries. In recent trends, the high global demand for natural rubber is a positive sign and robust progress for many rubber-producing countries such as Malaysia.
Conclusion
The selection of productive planting materials to produce vigorous trees capable of producing higher latex quantities has become crucial for Malaysian smallholdings, plantations, and rubber growers. For commercial and large-scale planting, the preferable planting materials should generate high latex yield, healthy leaves and canopies, smooth and soft bark surface for tapping, fast bark renewal rate after tapping, straight stem and trunk, vigorous girth growth and high survival rate of rootstock-scion compatibility. Besides, characteristics such as bark thickness, number of laticifers, number of stomata, leaflet position, seeds, and photosynthetic rates were the primary concerns in Hevea improvement. The characteristics that should be found in suitable rubber planting materials and approved by the rubber growers are as follows: (1) High latex yield production; (2) Smooth and soft bark surface; (3) Low susceptibility to leaf diseases; and (4) High tolerance to wind damage. Currently, rubber plant breeders are still trying to improve rubber-planting materials with all the right characteristics into a single model or "Ideotype", while at the same time considering minimizing the effect of “Inbreeding depression” when breeding for a new rubber planting material for commercial planting. Inbreeding depression ought to reduce the vigorous growth and latex yield production with a narrow genetic base. Therefore, Hevea improvement programs necessitate to incorporate broader genetic resources with a selection of materials that includes commercial rubber planting materials derived from H. brasiliensis and other Hevea species that are compatible with crossing or hybridization. The highlights from this research aim to provide opportunities to employ the potential rubber species and genetic resources to select desirable characteristics with the urge to utilize a new source of rootstocks in bud-grafting and multiplication of planting materials in the future.
