K.K.Vinod
Para rubber tree (Hevea brasiliensis)
belong to Euphorbiaceae. It is the major source of natural rubber in the world.
The genus Hevea occurs naturally through out the Amazon basin and in parts of
Matto Grosso, Upper Orinoco and the Guianas (Schultes, 1970). Hevea brasiliensis (2n = 36) is a
monoecious perennial tree belonging to Euphorbiaceae. There are ten species
recognised under this genus, H.
brasiliensis, H.
benthamiana, H. camporum, H. camargoana, H. nitida, H. pauciflora, H. guianensis, H. microphylla, H. spruceana and H.
rigidifolia. These species are highly cross-pollinated and putative hybrids
of natural intercrossing between the species occur in the natural habitat. Experimental
crosses between species show no hybridisation barriers (Webster and
Paardekooper, 1989).
Genetic resources
Though there are few reports of
introduction of rubber plants to other areas of the world, all of them lack
evidence to substantiate such claims. Recorded history of introduction the
rubber from its natural habitat starts with the collection of rubber seeds by
H.A. Wickham near Santarem during 1872. In 1876, Wickham collected about 70,000
seeds of H. brasiliensis from near Boim on the Rio Tapajoz and from the
well-drained undulating areas near Rio Madeira and despatched them to Royal
Botanic Gardens at Kew, England. About 2800 seedlings raised at Kew, 2397 were
despatched to Sri Lanka and few to Malaysia, Singapore and Indonesia. Virtually
all the rubber trees cultivated in Asian countries originated from this
collection.
After Wickham, there were several attempts
to introduce H. brasiliensis to Asian countries, but these
introductions were confined to very few in number did not contribute much to
the introduced genetic base. Later wild species and few cultivated cloned were
introduced to Rubber Research Institute of Malaysia from Brazil. Besides few
other organised attempts by International Rubber Research And Development Board
(IRRDB), a major collection of wild Hevea Germplasm was carried out in 1981 by
IRRDB team of scientists from major member countries, in collaboration with
Brazil collected 64736 seeds from the states of Acre, Rondonia and Mato Grosso
and budwoods from 194 high yielding trees free of major diseases like abnormal
leaf fall (Phytophthora sp.)
and South American leaf blight (SALB) caused by Microcyclus ulei (Ong et al., 1983). Of these
collections, India introduced about 9000 accessions from Malaysia, of which
about 6000 are surviving and being conserved in ex situ gardens. Besides, India
also introduced 127 commercial clones from other countries where Hevea is grown
on commercial scale.
Apart from H. brasiliensis, SALB affects
only three more species, H.
benthamiana, H. guianensis and H.
spruceana (Langford, 1945;
Chee and Holliday, 1986). Others species are free from infection. Eleven
physiological races are reported to be identified for this disease. Though H. brasiliensis is totally affected by this disease
immune reactions are shown by some H.
benthamiana, H. spruceana and H.
pauciflora derivatives. Sporadic
attempts to use these immunities, by crossing with H. brasiliensis had produced
clones with transient and non-durable resistance (Simmonds, 1989). Evaluation
of wild Hevea germplasm for sources for biotic and
abiotic stresses are in progress at major rubber growing countries.
Crop improvement
The rubber tree is introduced into
cultivation very recently. The genetic base of the cultivated germplasm is very
narrow, converging to a very few seeds collected from upper Amazon basin near river
Tapajos in Brazil. Though rubber tree is infected by many diseases, the
plantation industry suffers mainly from only one really devastating pathogen
causing South American leaf blight (SALB). This disease in still confined to
American sub-continent and all the rubber growing areas of South and Southeast
Asia are free from it. However, it is already proved that none of the
cultivated varieties in Asia are resistant to this disease when tested at
various locations of South America. However, in recent years, another disease Corynespora leaf fall (CLF) has gained importance
for wiping out entire RRIC 103 plantations of Sri Lanka, which had almost
gained its name SALB of Asia.
Clonal selection is the most important
procedure followed in breeding rubber. Clones can be evolved at any stages of
different breeding steps. Usually selective hybridisation of promising parents
is done among themselves and also with wild germplasm lines. The progenies are
directly selected from seedling nurseries and cloned for further evaluation. Also,
natural seedling population or half-sib population are also screened for
desirable characters including resistance. Susceptible and poor performing
clones are generally discarded. Polycross gardens comprising of pre-potent clones
are also utilised and the selection is generally exercised in the polyclonal
seedling orchards, even at the stage of maturity. The selections are directly
carried forward for clonal evaluation and selection. A general scheme of rubber
improvement is provided in Fig.
A general scheme of Hevea breeding
The major thrust of quality improvement in Hevea does not orient to the quality of
rubber, but on the quality of secondary products like Hevea wood. Rubber trees produce enormous
quantity of semi-hardwood at every replanting cycle. The shrinking availability
of natural timber from the forests has made this so valuable. The wood on
appropriate chemical treatment could be used as a best substitute for timber
for furniture making and for similar uses. The treated wood is now being used
to produce very high quality furniture, panel boards, house-hold articles and
for flooring purpose.
Owing to the growing importance of rubber
wood, the improvement in the direction of developing timber – latex clones is
in progress. The clones combining better yield, high vigorous growth, short
life span, high quality strong wood, free from diseases and with good branching
habits are preferred.
Rubber tree is a prolific producer of
honey. Honey is produced on extrafloral nectaries located on the Hevea leaves. High honey production can give
additional income to plantation sector, and population that yield more honey
are preferred.
Though rubber products are being used for
so many decades, recently, the problem of latex protein allergy has emerged
mainly in America. The allergy is reported to be caused by the proteins present
in the latex, which are found in traces in the finished products. However, the
allergy reported is mainly of type IV allergy of cutaneous origin. There are
several processing methods available for the deproteinization of the rubber
products. However, it would be better to look out for genotypes, which do not accumulate harmful proteins in the latex.
Biotechnology rubber crop improvement
The one area of biotechnology with clear
applications in rubber is that of molecular markers. Marker-assisted selection
offers prospects of accelerating the process of long term breeding objectives
offered by the conventional approaches. An important first step towards developing
linked markers is the construction of a linkage map. Maps have been published
for rubber (Seguin et al, 1996). A map allows the selection of markers which
are evenly distributed over the genome, thus enhancing the probability of
finding markers linked to quantitative trait loci (QTL). Molecular markers that
are linked to these QTL will co-segregate with the genes involved in desirable
traits and could be used efficiently to follow introgressions and accumulation
of favourable traits during recombination cycles.
Besides, interfering with the biochemical
pathway are also being contemplated, in producing many biomolecules including
antibiotics. However, the lesson is that a single, apparently simple, change in
a synthetic pathway may have unexpected side effects. Successful transformation
programmes have been those where the transformation work is integrated into a
conventional breeding programme, allowing individuals with the required
phenotype to be developed from a range of transformants.
References
Chee, K.H. and Holliday, P. (1986) South
American leaf blight of Hevea rubber. Malaysian Rubber Research and Development
Board, Kuala Lumpur. 50p.
Langford, M.H. (1945) ) South American
leaf blight of Hevea rubber trees. USDA Technical Bulletin, 882: 31.
Ong,S.H., Ghani, M.N.A. and Tan, H. 1983.
New Hevea germplasm: Its introduction and potential. Proceedings of the RRIM
Planters Conference, Kuala Lumpur. pp. 3-17.
Seguin, M., Besse, P., Lespinasse, D.,
Lebrun, P., Rodier-Goud, M., and Nicolas, D., 1996. Hevea molecular genetics.
Plantations, Recherche, Développement 3(2): 77-87
Shultes RE (1970) The history of taxonomic
studies in Hevea. Bot. Rev., 36: 197-276.
Simmonds, N.W. 1989. Rubber Breeding. In:
Rubber (C.C. Webster and W.J. Baulkwill, eds). Longman Scientific and
Technical, U.K. pp. 85-124.
Webster,C.C and Paardekooper. 1989. Botany
of the rubber tree In: Rubber (C.C. Webster and W.J. Baulkwill, eds). Longman
Scientific and Technical, U.K.
