Ethnobotany

Hevea brasiliensis

Introduction

Rubber is obtained from the milky juice, or latex of various erect or climbing woody plants of the tropics and subtropics. The majority of the rubber plants belong to the Moraceae, Euphorbiaceae or Apocynaceae. Although well over fifty species are available as sources, only a few have been important commercially and at the present time Hevea brasiliensis (Euphorbiaceae) stands preeminent. Rubber is the most recent of the major crops of the world. The Hevea or Para rubber tree (Hevea brasiliensis) normally is the source of from 95 to 98 per cent of the rubber produced throughout the world. Latex occurs in special cells or in a series of special vessels which permeate the bark, leaves and other soft parts of the tree. Usually only the latex from the lower part of the trunk is of importance commercially. Latex is a gummy white liquid full of minute globules. It is a varying mixture of water, hydrocarbons, resins, oils, proteins, acids, salts, sugar, and caoutchouc, the substance used as the source of rubber. The significance of latex to the plant is obscure. It is of some value in the healing of wounds, and it may serve for protection, nutrition, the transport of materials or as a fluid reservoir (Hill 1952).

Ecology

Wild Para rubber grows in the tropical evergreen rain forest of the Amazon basin, often in periodically flooded areas, but larger trees are found on the well-drained plateaux. Most of the planted rubber is grown between 15°N and 10°S where the climax vegetation is lowland tropical forest and where the climate is hot, humid and equable, with temperatures ranging from 74–95°F and a well-distributed rainfall of 75-100 inches or more per annum. Areas with wide temperature ranges and pronounced dry reasons are not ideally suited to rubber, although it is grown under these conditions in southern India and Vietnam. Wickham had collected his seeds on an undulating plateau, but Cross had obtained his from a swampy area. On account of the latter the trees sent to Singapore in 1877 were transferred to the edge of a swamp; those at Kuala Kangsar were planted on moist, well-drained soils which were more suited to Hevea. Although the crop will tolerate a wide range of soils with pH from 4-8, the optimum range in pH 5-6 and it does best on deep, well-drained loams. Shallow, badly-drained or peaty soils should be avoided and lime is deleterious. Much of the rubber in Malaysia is grown on land formerly covered with lowland Dipterocarp forest. It is important in clearing the forest to maintain a ground cover, and this is done by planting leguminous cover crops. Early plantings had usually been cleaned weeded which caused erosion and loss of organic matter (Purseglove 1974).

Distribution

Native to the Amazon region; Brazil, Venezuela, Ecuador, Colombia, Peru, and Bolivia. Introduced to many other tropical regions of the world, as Indonesia, Malaysia, Liberia, India, Sri Lanka, Sarawak and Thailand (Duke & duCellier 1993).

Germplasm

Reported from the South American and, secondarily, the Indonesia-Indochina Centers of Diversity, rubber, or cultivars thereof, is reported to tolerate disease, drought, high pH, insects, laterite, low pH, slopes, virus, and waterlogging. There are many improved varieties and cultivars in areas where Para Rubber is cultivated commercially. These vary in size, productivity of latex, and disease resistance (Duke & duCellier 1993).

Morphological Characters

Morphological characters are based on Purseglove (1974).

1) Structure : A quick-growing tree, rarely exceeding 25 m in height in plantations, but wild trees over 40 m have been recorded. Copious latex in all parts, usually white, sometimes tinged yellow.

2) Roots : Well-developed tap-root, 2.5 m long at 3 years, with laterals 7-10 m long.

3) Trunk, bark and branches : Trunks of seedling trees taper from base and are conical in shape; trunks of buddings are cylindrical. Both seedlings and buddings have periodicity of growth. During the resting stage whorls of scale leaves occur round terminal bud. Terminal buds of main stems of saplings grow rapidly producing long internodes towards the end of which leaves are clustered. Leaves thus appear in tiers. In trees which are old enough, leaves shed, terminal buds of branches grow rapidly and trees are temporarily bare of leaves, a condition known as ‘wintering’; new leaves are then produced at proximal end and inflorescences in axil of scale leaves and lower foliage leaves; new leaves then harden off. Fruits borne on proximal part of long shoots. In Malaya there is usually a ‘wintering’ of all trees after the dry weather at the beginning of year and often a second complete or partial leaf-change in August or September. At other times when there is a short dry spell, trees may change their leaves on one or two branches only.

Bark consists of outer corky layers and greenish cork meristem; then the hard bark which is orange-brown in colour and with large numbers of stone cells which are more numerous towards the periphery, parenchyma and disorganized sieve tubes and a few latex vessels; then comes the soft bark consisting mainly of vertical rows of sieve tubes with some medullary rays and latex vessels which are more numerous nearest the cambium. Thickness of bark and proportion of tissues varies with different clones, and age of tree, whether virgin or renewed bark, whether seedling or budded, and in case of seedling with height from ground. Bark thickness varies from 6.5-15 mm, but is usually 10-11 mm. Complete renewal of bark after tapping takes 7-8 years.

4) Leaves : Spirally arranged, trifoliate, glabrous, with deciduous stipules and 3 extra-floral nectarines at junction of leaflets with petiole, which only secrete nectar on the new flush during flowering. Oldest leaves of flush are larger with longer petioles than those produced at end of flush. Petiole 2-70 cm long, but usually about 15 cm. Leaflets short-stalked, elliptic or obovate, entire, base acute, apex acuminate, dark green above, paler glaucous beneath, veins pinnate with about 20 pairs; lamina 4-50 x 1.5-15 cm, usually about 15 x 5 cm; petiolule 0.5-2.5 cm. Young leaves purple-bronze, becoming green on hardening and turning orange-brown, red or yellow before falling.

5) Flowers : Borne in many-flowered, axillary, shortly pubescent panicles on basal part of new flush; flowers small, scented, unisexual, shortly-stalked, with larger female flowers at terminal ends of main and lateral branches and more numerous smaller male flowers, with 60-80 males to each female flower; calyx yellow, bell-shaped, with 5 narrow triangular lobes; petals absent, male flowers 5 mm long with 10 sessile anthers set in 2 superposed circles of 5 each on central slender column; female flowers 8 mm long with green disc at base and 3-celled, shortly pubescent ovary and 3 white short sessile stigmas. Flowering takes place over a period of about 2 weeks with some male flowers opening first, lasting for 1 day and then dropping, followed by female flowers which remain open for 3-5 days; remainder of male flowers then open.

6) Fruits : Only a small proportion of female flowers set fruit and of these 30-50 per cent fall off after a month and more fall off later. Fruit wall attains full size in about 3 months, when embryo still microscopic; ripens 5-6 months after fertilization. Mature fruit a large, compressed, 3-lobed capsule, 3-5 cm in diameter, with 1 seed per carpel. Mesocarp thin, coriaceous; endocarp woody. Dehisces explosively and noisily with endocarp breaking into 6 pieces and seeds are thrown a distance of about 15 yards.

7) Seeds : Large, ovoid, slightly compressed, shiny, 2-3.5 x 1.5-3 cm, testa grey or pale brown with irregular dark brown dots, lines and blotches. The testa being derived from the female parent and the seed shape being determined by the pressure of the capsule, it is possible to identify the female parent of any seed by its markings and shape and is the most reliable method of identifying ‘clonal’ seed. Endosperm white in viable seeds, turning yellow in older seeds. Weight of seed 2-4g.

Anatomical Characters

General characteristics of family Euphorbiaceae are based on Metcalfe & Chalk (1965):

The hairs are very diverse and include glandular, non-glandular, and stinging types. Extra-floral nectaries are also common. Some of the cells of the epidermis may be sclerosed, mucilaginous, silicified, or papillose in different genera and species. No single type of stoma is very widespread in the family, and their distribution is also variable. The cortex of the axis of the cactoid Euphorbias consists largely of palisade assimilatory tissue, which often increases in thickness through the activity of a peripheral meristem. Stone cells are also common in the cortical region. The pericycle usually includes a varying amount of sclerenchyma, which may be in the form of a continuous or interrupted ring; less frequently it is absent. The phloem often includes isolated or small bundles of fibres, but is seldom stratified into hard and soft portions. The vascular bundles are generally collateral. Even in many of the lianes belonging to the family, secondary thickening proceeds normally. On the other hand, secondary vascular strands arise in the pericycle of a few genera, whilst medullary vascular bundles are known to occur in Ricinus. Intraxylary phloem occurs in a number of genera. Crystals are mostly solitary or clustered, but special types such as sphaerites and styloids also occur. They are sometimes situated in crystal-idioblasts. Secretory elements of various kinds are very widespread. Those which secrete latex have the form of tubes, cells, or sacs, but in some instances tanniniferous and other types of material are secreted in similar elements or in intercellular cavities. Anomalous structure occurs in a few genera.

Clonal Seed

This is an unfortunate term as such seeds are the sexual progeny of budded clones and seed garden progeny is probably a better name. The seeds may be obtained from monoclone plantings of self-fertile clones or by planting a mixture of a small number of clones, which are known to produce high-yielding families. This is usually done in special seed gardens, isolated from other rubber, and on flat land to facilitate harvesting of the seeds. In a polyclone garden the progeny can only have two parents or may be the result of self-pollination; as the male parent is not known the seed is illegitimate. It is usually desirable to obtain as much cross-pollination as possible, as selfing may reduce vigour. The clones used as seed parents should produce 20 lb (pound) or more of dry rubber per tree per annum. The seed harvested rarely exceeds 3 lb per tree per annum. If at some future date good parents can be found which are male- or self-sterile, seeds of guaranteed parentage could then be produced. As clonal seedlings are more variable than budded rubber and their average yield is less, it is desirable to select for vigour in the nursery and the field, and later for yield in the early years of tapping, cutting out until the desired spacing is obtained. Seedlings are cheaper to produce than buddings and can be tapped a year earlier than brown buddings. Legitimate seed may be produced by hand pollination between selected clones, but it is much more expensive to produce and the yield of seed is less (Purseglove 1974).

Germination

Seeds are viable for a short time only, so must be planted as soon after harvesting as possible. Viable seeds germinate in 3-25 days. Germination is hypogeal (Purseglove 1974).

Chemical Composition

Fresh latex consists of a colloidal suspension of rubber particles in an aqueous serum. The content of rubber hydrocarbon varies from 25-40 per cent, with an average of about 30 per cent. It is cis-1,4-polyisoprene with a formula of (C5H8)n. It is manufactured in the plant from carbohydrates and its function in the plant is not known. Although it is an energy-rich compound, there does not seem to be any means of readily releasing this energy within the plant. Non-rubber constituents present in the latex include proteins, resins, sugars, glucosides, tannins, alkaloids and mineral salts. The kernels, which constitute 50-60 per cent of the seed, contain 40-50 per cent of semi-drying oil, which may be used for soap-making. The cake after oil extraction contains about 30 per cent protein. Seeds contain a cyanogenetic glucoside, linamarin, and an enzyme, linase, which hydrolyses the glucoside to hydrocyanic acid (Purseglove 1974).

Utilizations

Sources of Hevea or Para Rubber, obtained by tapping the trunks of the trees. The latex coagulates with the aid of acetic acid, formic acid, and alum. Cured rubber used for all types of rubber products. Seeds are source of Para Rubber seed oil, recommended for manufacture of soap. Although poisonous, seeds can be eaten as a famine food after processing. Boiling removes the poison and releases the oil which can be utilized for illumination. Seeds are sometimes eaten off the ground by cattle. Kernels (50-60% of the seed) contain 40-50% of a semi-drying pale yellow oil, effective against houseflies and lice, and used in soap making, paints, varnishes. Press cake or extracted meal can be cautiously used as fertilizer or feed for stock. Rubber is a big economic actor in the Extractive Reserves Concept, a recently arrived concept proposed by champions of the rainforest. They are attempting to show, and succeeding in showing, that the rainforest is more valuable standing ‘on the hoof’ than ‘on the slab’, i.e., cut down and carved up and replaced with soybean and ‘hamburger’ (cattle) farms which quickly degenerate to relatively useless savannas or wasteland. Granted, for a year or two, the soybean or cattle farm might return a few more dollars per hectare, but over ten years, the Amazonian ‘Extractive Reserve’ is reported to return more dollars from rubber and brazilnuts than it could return from soybeans and cattle (Duke & duCellier 1993).

Nearly 50,000 different products are made directly or indirectly from rubber. Some 70 per cent of rubber production is consumed in the manufacture of tyres, tubes, and other articles associated with the automobile industry; about six per cent is utilised for footwear and nearly four per cent for wire and cable insulation. Other miscellaneous manufactured articles include rubberised fabric, raincoats, household and hospital supplies (such as sheets, hot water bags, surgeon’s gloves, etc.), shock absorbers, washers, gaskets, belts, hoses, sports goods, toys, erasers, adhesives, rubber bands and a host of other auxillary products. Hard rubber, vulcanite or ebonite (highly sulphurised rubber) is used in the electrical and radio engineering industries, for protective lining in chemical plants and also in the fabrication of battery boxes, fountain pens, barrels, tobacco pipes, telephones and combs, etc. Concentrated latex is used for most dipped goods such as gloves, balloons, and contraceptive appliances. Sponge rubber from foamed latex is used in upholstery, i.e. seating, cushions, mattresses, pillows; in life belts and in carpeting. Rubber powder, along with bitumen, is used for road surfacing. Rubber also finds its use in all sorts of military clothing – pressurised suits for aircraft personnel operating at high elevations; frogmen’s suits for divers spending considerable time underwater and insulated suits to keep men in arctic zones warm (Kochhar 1986).

Folk Medicine

Some Indians reportedly drink the dangerous latex of other Hevea species to make themselves stronger (Duke & duCellier 1993).

Biotic Factors

i) Major Pests

About 90 species of fungi are known to attack Hevea trees, the most prevalent ones being the following: Botryodiplodia elactica and B.theobromae, Colletotrichum heveae (leaf spot), Fomes lamaensis (brown root rot), Gloeosporium heveae (die-back), Oidium heveae (powdery mildew) and Pellicularis salmonicolor. It is also attacked by Bacterium albilineans, and parsitized by Loranthus spp. Nematodes isolated from Hevea brasiliensis include: Helicotylenchus cavenessi, H. dihystera, H. erythrinae, Meloidogyne incognita acrita, M. Javanica, Pratylenchus coffeae, and P. Brachyurus. Insect pests include: Scale insects (Aspidiotus cyanophylli and Parasaissetia nigra). White ants cause serious damage to trees at all ages (Duke & duCellier 1993). Termites (Captotermes spp.), and cockchafers (Holotrichia spp.) can be troublesome locally. The giant snail, Achatina fulica can be serious pests to young trees. Various animals can damage the trunks, include: elephants, wild pigs, deer, porcupines, rats and squirrels may cause damage to bark and trunk, particularly when young (Purseglove 1974).

ii) Major Diseases

Leaf disease, consisting of abnormal leaf fall, Gloeosporium leaf disease, powdery mildew, and bird’s eye spot, is controlled by a variety of sprays, including copper oxychloride, sulfur dust and others applied by spray or dusting techniques. Stem disease, consisting of pink disease, stem canker, and dieback, is reduced by brushing on specific fungicides. Panel diseases, classified as black stripe, moldy rot, and panel necrosis, are minimized by spraying or coating specific fungicides. Three types of root disease, classified as white, red, and brown, are controlled by cutting away diseased tissue and applying prophylactic coatings (Duke & duCellier 1993).

Leaf blight, Dothidella ulei P. Henn., is the most serious disease of rubber. It is the major limiting factor to rubber production. The high-yielding Far Eastern clones are extremely susceptible. In the asexual stage on young leaves olive green spots appear on the undersurface and usually result in leaf shedding. If the leaves do not fall a sexual stage is reached on older leaves with small raised black perithecia usually on the upper surface and often bordering holes in the lamina where diseased tissue has died out. It also attacks young stems and fruits. In powdery mildew, Oidium heveae Steinm., circular white colonies occur on lower surface of leaves and cause leaf-fall during refoliation. Black stripe and leaf blight, Phytophthora palmivora (Butl.) Butl., attacks the tapping panel just above the cut (black stripe) and vertical black lines are seen in the wood on scraping away sunken discoloured areas of bark. It is treated with fungicide weekly (not copper) and tapping knives are disinfected. It causes a rapidly spread necrosis killing leaves, branches and pods. Mouldy rot, Ceratocystis fimbriata Ell. & Halst., only infects freshly tapped bark, giving greyish mould just above cut followed by wounds and killing the cambium. It is controlled by stopping or reducing tapping intensity and applying fungicide. Pink disease, Corticium salmonicolor Berk. & Br., causes pinkish mycelial encrustation over the surface of the bark, followed by wilt, die-back, cankers and latex bleeding. It is most serious on 3-4-year-old trees, which can be sprayed with copper. White root rot, Fomes lignosus Kl., is the most serious root disease in Malaya and Indonesia. Rhizomorphs white, flattened, sparsely branched. Sporophores orange brown with white margin. Leaves go yellow and die; die-back of branches occurs followed by death of trees; when these symptoms occur it is too late to save the tree. It is essential to control from planting, followed by regular foliar and collar inspection; dead roots and infected tissue are removed and collars painted with protectant such as Fomac. Red root rot, Ganoderma pseudoferreum (Wakefield) Over. & Steinm. Rhizomorphs reddish skin round root surface; sporophores dark red-brown. It spreads more slowly than white root rot; controlled by removal and isolation of infected material. Brown bast: Initial symptom is increased yield of watery latex, followed by a drying up of part or whole of the tapping cut. Affected parts are darker coloured and cankerous wood growths occur. Some clones are more susceptible than others. The true cause is not known, but it is often considered to be a physiological condition due to overtapping. Total or partial dryness can also result from more than one cause, including root disease, necrosis extending from branch fork wounds, sodium arsenite damage, etc. The affected bark should be excised and, if healthy yielding bark is found lower down, tapping can be continued, but at reduced intensity (Purseglove 1974).

Harvesting

Tapping begins when trees are 5-8 years old, depending on the area, and increases every year until a maximum at about 20 years, then yield sustained for 40-50 years or more. Tapping consists of removal by excision of a thin cut of bark about 1 mm deep at regular intervals, thus opening the latex vessels in the bark, which are arranged in concentric cylinders and run in counter-clockwise spirals up the trunk. Usually the cuts run half-way around the trunk, but may encircle the tree. Several types of cuts are used. Only the basal part (1.3 m) of the trunk is tapped (most latex vessels develop here). Special knifes are used to cut the proper depth and angle. Latex is collected through a small spout fixed in the bark in cups placed at end of cut, large enough to collect one-day’s flow. Trees are tapped early in morning when flow of latex is highest; flow decreases with temperature and usually ceases in about 3 hours. An average tapper can tap 200-300 trees in 3 hours. Then the tapper starts back through the grove and empties the cups into large pails or buckets, sometimes adding a few drops of dilute ammonium solution to prevent coagulation. Rubber yield can be increased treating the bark below the tap with yield-stimulating mixtures containing plant hormones and selective weed-killers with hormone properties, as Stimalax, Eureka, 2,4-D, 2,4,5-T in palm oil. Copper sulphate also enhances latex flow (Duke & duCellier 1993).

Tapping (excision of a thin cut of bark)

Latex is collected through a small spout fixed in the bark in cups

Substitutes for Para-rubber

Besides Hevea, many other trees and shrubs have been exploited from time to time as sources of rubber, but none can rival it. There is probably a wide range of plants (trees, vines, and shrubs) in tropical Africa producing relatively large quantities of rubber. At one time an appreciable proportion of the world’s supply came from this source which was also important during World War II between 1942 and 1945. During a search for substitutes, interest centred first on apocynaceous vines of equatorial Africa, particularly Landolpdia, Landolphia heudelotii A. DC. And Lagos silk, Funtumia elastica (Preuss.) Stapf. The latex was obtained by cutting the plant near ground level, an irresponsible practice that soon destroyed the African rubber boom. Other rubber producing tropical plants of local importance are: Manihot glaziovii (ceara rubber) of the Euphorbiaceae; Castilla elastica (Panama or Castilla rubber) of the Moraceae, indigenous to Central and South America; Ficus elastica (India or Assam rubber) of Moraceae and Cryptostegia grandiflora (rubber vine) of Asclepiadaceae, native to Malagasy Republic (Kochhar 1986).

Our crop residues (such as cassava, chicory, elderberry, lettuce, mint, sunflower, etc.) or weeds (burdock, burnweed, camphorweed, cocklebur, dandelion, dogbane, evening primrose, everlasting, fleabane, goldenrod, groundsel, honeysuckle, horsebalm, jimsonweed, milkweed, moth mullein, oxeye, pricklepoppy, quackgrass, selfheal, sow thistle, sumach, tansy, thistle, velvetleaf, wormseed goosefoot, or yarrow), as alternative sources of rubber (Duke & duCellier 1993).

References

Duke, J.A. & duCellier, J.L. 1993. Handbook of Alternative Cash Crops. CRC Press Inc.

Hill, A.F. 1952. Economic Botany. McGraw-Hill Book Company, Inc.

Kochhar, S.L. 1986. Tropical Crops. Macmillan Publishers Ltd.

Metcalfe, C.R. & Chalk, L. 1965. Anatomy of the Dicotyledons. Vol II. Oxford: Clarendon Press.

Purseglove, J.W. 1974. Tropical Crops Dicotyledons. Longman Group. Ltd.