Rubber Tree Cultivation and Improvement: Laticifer Ring Count and Latex Yield Assessment of Rubber Species
Abstract
Rubber trees (Hevea spp.), the wilderness trees originating from rainforests of the Amazon, have been domesticated outside South America for more thana century. The trees, specifically of the species Hevea brasiliensis, are now widely established in the tropics especially in Southeast Asia. Nevertheless, opportunities to improve the latex yield productivity of these cultivated rubber trees are limited by their narrow genetic base since they have been descended from a small selection of seedlings derived from seeds collected in the 1800’s. Commercial rubber trees hence face this genetic vulnerability of inbreeding depression that could hamper progress in crop improvement. To explore the feasibility of broadening the genetic base, various Hevea species, viz. Hevea brasiliensis, Hevea benthamiana, Hevea camargoana, Hevea guianensis, Hevea nitida, Hevea pauciflora, Hevea rigidifolia and Hevea spruceana, were assessed for their possible contribution to the genetic improvement of the cultivated rubber trees particularly in latex yield output. The assessment showed that Hevea benthamiana, and Hevea spruceana to be promising in terms of the number of laticifer rings in the bark and latex yield. They are promising candidates for incorporation into the improvement programmes of the rubber tree in Malaysia.
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Introduction
Key inventions in the 1820s introduced novel uses for natural rubber, Charles Macintosh produced the earliest water-resistant rubberised fabrics while Thomas Hancock invented the rubber masticator that could cut and compressed rubber into moulded solid rubber, Hancock further improved the application of natural rubber latex in various surgical tools, water resistant clothing, footwear, hoses, rubber belts, engine components and rubber inflatables (Barlow, 1978; MRB, 2005; MRB, 2009; Priyadashan, 2011). The demand for natural rubber reached new heights following the introduction of vulcanization by Charles Goodyear in 1839, a process where rubber is heated in the presence of sulphur. This stabilises the rubber at both high and low temperatures while being resistant to melting and able to retain its elasticity, such characteristics being requisite for rubber tyres in the automobile industry. The invention of the motorcar and pneumatic tyres in the mid-1800s was an important factor that triggered higher demand for natural rubber latex as a raw material worldwide (Barlow, 1978; Baulkwill, 1989). The unique properties of rubber and its versatility have resulted in its use in a wide assortment of products. The demand for rubber latex has led to extensive cultivation of Hevea brasiliensis not only in Asia (Malaysia, Thailand, Indonesia, India, Myanmar, the Philippines, Cambodia, Vietnam, China and Sri Lanka) but also in Africa (Nigeria, Cameroon and Ivory Coast). In Malaysia, the natural rubber industry caters to most of the livelihoods of smallholding growers, which is made up of mostly senior residents, who live off the cultivated rubber trees many decades ago. Latex is produced in laticifers which are cells fused end to end to form continuous vessels. Laticifers are found in all parts of the rubber tree although tapped latex is derived mainly from those in the bark of the trunk. In cross-section under the microscope, laticifers in the bark appear as concentric rings surrounding the trunk. The roles of latex in rubber trees and other plant species have been suggested by Bealing (1965), Hunter (1994), Rudall (1994), Agrawal and Konno (2009), and Kajii et al. (2014) to be highly associated with: (1) protection against injury; (2) storage of carbon and its derivatives; and (3) storage of water and the regulation of its supply. Various rubber species, viz. Hevea brasiliensis, Hevea benthamiana, Hevea camargoana, Hevea guianensis, Hevea nitida, Hevea pauciflora, Hevea rigidifolia and Hevea spruceana available in Malaysia (Schultes, 1990). However, these rubber species were not fully utilized in the recent rubber improvement programmes in the country. Thus, these rubber species were assessed for their possible contribution to the improvement of the cultivated rubber trees in latex yield output.
Conclusion
H. brasiliensis, H. benthamiana, and H. spruceana showed the highest number of laticifers rings in the bark whereas H. guianensis had the lowest laticifer ring count among the rubber species studied. The number of laticifer rings were strongly correlated with latex yield produced in these Hevea species. Pearson's Correlation Coefficient indicated that increasing number of laticifer rings would have a tendency to accompany by increasing of latex yield. More attention should, therefore, be paid to laticifer ring count in future rubber improvement programmes. However, this characteristic is not a suitable criterion for the early selection in the nursery for young rubber plants less than 18 months old. In young plants, the laticifers are not fully developed and latex yield at this stage can give misleading results. In a nutshell, H. benthamiana and H. spruceana showed promising in term of number of laticifer rings and latex yield (dry rubber weight), apart from H. brasiliensis, to be considered for incorporation into the rubber improvement programmes in Malaysia.
