Nephelium lappaceum (PROSEA)
- Protologue: Mant. Pl. 1: 125 (1767).
- Family: Sapindaceae
- Chromosome number: 2n= 22
- Nephelium glabrum Cambess. (1829) (also used for N. maingayi),
- Nephelium chryseum Blume (1847),
- Nephelium sufferrugineum Radlk. (1879).
- Nephelium obovatum Ridley
- Rambutan (En)
- Litchi chevelu (Fr)
- Indonesia: rambutan (general), chorogol (Sundanese), kakapas (Sumatra)
- Malaysia: rambutan (general), buah abong (Kenyah, Sarawak), rangalau (Dusan Ranau, Sabah)
- Philippines: rambutan, usan
- Cambodia: saaw maaw, ser mon
- Thailand: ngoh, phruan, ngoh paa (peninsular)
- Vietnam: chôm chôm, vai thiêù.
Origin and geographic distribution
The origin is untraceable because escapes from cultivation blur the original distribution. The species ranges from southern China (Yunnan and Hainan) through the Indo-Chinese region, Malaysia, Indonesia (Sumatra, Java, Kalimantan, Sulawesi) to the Philippines. The plant is cultivated throughout the humid tropics of Asia (from Sri Lanka to New Guinea), and in small numbers in the humid tropics of America, Africa and Australia.
The trees are cultivated for their very popular fruit. The usually juicy sarcotesta around the seed is eaten. The sweet-tasting fruits are consumed fresh, the more sour ones are eaten stewed. The sarcotesta can be canned or used in jam, but loses much of its flavour. Medicinal uses: the fruit is said to be astringent, stomachic and anthelmintic; the roots are used in decoctions for treating fever; the bark as an astringent for disease of the tongue; the leaves are used in poultices for headache. The fruit wall contains a toxic saponin; cases of poisoning are known; however, in Java it is dried and used as a medicine. Dye uses: young shoots are used as a green dye for silk which has already been dyed yellow with turmeric (Curcuma longa L.; Malaysia: kelantan, pattani). The fruit walls are used, together with tannin-rich parts of other plants, to dye silk black after a preliminary red staining (Malaysia: pekan). Leaves are used, together with mud, as an impermanent black dye. Other less important uses: the seed kernel can be used for the production of rambutan tallow, a solid fat similar to cacao butter, which is edible and also used for soaps and candles. The seed itself is edible (after roasting) but is bitter and narcotic. The wood is suitable for general construction. The tree is very ornamental when it fruits.
Production and international trade
Statistics for 1987/1988 give an area of 71 150 ha rambutan in Thailand, the crop of 448 500 t being smaller than the tangerine crop and about as large as that of mango, durian and jackfruit. In Peninsular Malaysia rambutan is also one of the principal fruit trees with 19 500 ha and a crop of 57 000 t in 1987/1988. In both countries production figures do not fluctuate much, whereas in Indonesia light and heavy crops appear to alternate, e.g. 93 300 t in 1985/1986 compared with 199 200 t in 1986/1987. In the Philippines rambutan growing has been much encouraged by introduction of cultivars from Indonesia, mainly in the 20th Century, with the result that rambutan now ranks among the top 10 tree fruits.
In Thailand and Malaysia canning is important. Exports of rambutan from South-East Asia are increasing steadily and exceeded 2000 t of fresh fruit for Malaysia as well as Thailand in 1987. Moreover, Thailand exported about 3500 t canned fruit in that year, mainly to Singapore, Hong Kong and the European Community.
Thai sources give the following composition of the fruit per 100 g edible portion: water 82.9 g, protein 0.9 g, fat 0.1 g, carbohydrates 14.5 g, fibre 1.1 g, vitamin A 4 IU, vitamin C 31 mg. The energy value is 264 kJ/100 g. The seed kernel yields 30-43% of solid fat, remarkable for its high content of arachidic acid (34.7%) and oleic acid (42.5%); after heating, the fat turns into a yellow, pleasant smelling oil. The wood is hard, heavy, red to reddish-white or somewhat brown, and liable to split during drying.
- Tree, fairly large in natural vegetation; clonal trees small, 4-7 m tall and usually with a spreading habit, branching according to Scaronne's architectural model.
- Leaves alternate, paripinnate, up to 6-jugate; leaflets ovate to obovate, 5-28 cm × 2-10.5 cm, usually horizontal, above glabrous or sometimes slightly hairy on the midrib, beneath variably hairy, domatia common to absent, apex truncate to acuminate, nerves slightly to strongly curving, veins scalariform to coarsely reticulate.
- Inflorescences pseudo-terminal to usually terminal.
- Flowers either male (only stamens well developed; trees dioecious) or hermaphrodite (trees monoecious), the latter either effectively female (stamens small, anther not dehiscing) or male (stigma not opening), actinomorphic, whitish, yellowish or greenish; sepals 4-5(-7), nearly free to more than halfway connate, 0.7-2.1 mm long; petals usually absent, sometimes up to 4 reduced ones, not exceeding 1.6 mm; disk complete, hairy or glabrous; stamens (4-)5-8(-9), exserted in males; filament has dense long hairs at least in the basal part; anther dehiscing latero-introrse, lengthwise; pistil 2- or rarely 3-merous, densely hairy, well developed in hermaphrodite flowers; ovary warty; style well-developed; stigma spreading to finally recoiled.
- Fruit an ellipsoid to subglobular schizocarp, up to 7 cm × 5 cm, weighing 20-95 g, usually consisting of only 1 nutlet, yellow to purplish-red, hardly stalked, often finally dehiscing (at least in the apical part), glabrous, usually densely set with filiform, curved, 0.5-2 cm long appendages; wall coriaceous, up to 2.5 mm thick.
- Seed covered by a usually thick, juicy, white to yellow, translucent sarcotesta.
- Seedling: cotyledons remain enveloped by fruit wall; leaves paripinnate, only first pair opposite.
Growth and development
The seeds are short-lived and therefore sown directly after they have been extracted and washed. Germination takes 7-10(-20) days. Seedlings grow much faster if cared for properly. Monoecious trees grown from seed come into bearing after 5-6 years; budded trees after 2-4 years; maximum production is reached after 8-10 years. Growth is rhythmic; in the seedling phase periods of fast and slow growth alternate; in the sapling phase growth comes to a complete standstill after a flush, the last leaves being reduced to mere bracts which are placed well below the naked bud and drop quickly. Axillary buds may emerge during that period, but usually the terminal bud resumes growth, suppressing lateral growth. Consequently, non-flowering twigs grow terminally and become long and slender. The terminal inflorescence puts an end to extension of the axis, and lateral shoots on twigs which have flowered are the main mode of branching for the tree.
The main flowering period occurs during the dry season. Often some of the terminals flower in one or two subsequent waves, up to 6 months later. Well-developed inflorescences on vigorous shoots generally show good fruit set and retention. Bloom may continue for several months (e.g. cv. Lebakbulus), extending the harvest period. The fruit ripens about 110 days after bloom. Typically, fruit ripens well into the rainy season. Since a good crop suppresses flushing, most vegetative growth - both terminally and laterally - occurs in the rainy season, after harvest. The most vigorous of these early shoots (early in the crop cycle) are likely to flower for the next crop.
Rambutan is effectively dioecious. Orchards are planted with female trees which produce a few (0.05-0.5%) functionally male flowers. This appears adequate where Trigonoid bees and other pollinators are active. However, several cultivars (e.g. "Chompoo", "Rongrien", "Simacan") virtually lack male flowers, so that pollinator trees or hormone treatments are required to ensure adequate fruit set.
Other botanical information
Three varieties are recognized by their leaflet characteristics only:
- var. lappaceum : widest above middle, midrib sparsely pilose below, nerves strongly curved. Distribution: Thailand, Malaysia, Sumatra, Java, Kalimantan, the Philippines (Palawan, Basilan) and possibly Seram; commonly cultivated.
- var. pallens (Hiern) Leenh.: widest at or below middle, midrib usually glabrous below, nerves slightly curved. Distribution: ranges from China (Yunnan, Hainan), Thailand, Laos, Cambodia, Vietnam, Malaysia, Sumatra, Kalimantan, the southern Philippines to Sulawesi.
- var. xanthioides (Radlk.) Leenh.: widest at or below middle, midrib densely shortly hairy below, nerves slightly curved. Distribution: Borneo.
Rambutan is a very variable species and therefore difficult to distinguish from other Nepheliums, especially N. ramboutan-ake (Labill.) Leenh. and N. cuspidatum Blume.
Rambutans are traded under various names, which may refer to fruit characteristics, to the centre of production, or to a specific cultivar. Since established trade names have also been given to some cultivars, the names are rather confusing. In Malaysia this problem has been tackled by giving "R" numbers to old and new clones in a selection programme. The selections R3 ("Peng Thing Cheng"), R134, R156 ("Muar Gading"), R160 ("Khaw Tow Bak"), R161 ("Lee Long"), R162 ("Oh Heok") and R170 ("Deli Cheng") are recommended throughout the country; others are more location-specific. Important cultivars in Thailand are "Chompoo", "Rongrien", "Bang Yi Khan", "See Tong" and "Nam Tan Kruad". In Java cultivars "Lebakbulus", "Binjai", "Sitankue", "Rapiah" and "Simacan" have dominated nurseries since the 1930s. In the Philippines introductions from Indonesia ("Simacan", "Sinyonya", "Maharlika") are grown. Important features of the cultivars are thickness, colour, juice content and aroma of the flesh (sarcotesta); whether or not it adheres to the seed ("clingstone" versus "freestone" cultivars); and whether the papery part of the seed coat comes off with the flesh.
Rambutan thrives in humid tropical lowlands (sea-level up to 600 m) within about 17°from the equator. The trees occur in the lower or middle storey in different types of primary and secondary forest, ranging from dry land to swamp. Rainfall usually exceeds 2500 mm per year. Exposure to dry wind leads to browning of the leaf margins; sheltered locations or wind screens are recommended. Deep, well-drained soils of fertile sandy loam or clay loam are preferred. A pH range of 4.5-6.5 is indicated; at higher pH iron and zinc deficiencies are common (chlorosis, leaf yellowing).
Where the dry season is long (e.g. eastern Java), bloom can be relied upon, although the timing varies from year to year. Consequently the harvest may easily be advanced or delayed by a month or more. Even in well-defined monsoon climates, some trees or branches may flower out of season. For trees growing in areas with a bimodal or continuous rainfall distribution, the timing of flowering becomes very erratic; in that case the intensity of bloom appears to be correlated with the duration of water stress.
Propagation and planting
Many trees are still grown from seed, but commercial production comes by and large from clonal trees. Nurseries use the modified Forkert budding, taking seeds from seedling trees to produce the rootstocks. Seeds are pre-germinated and the seedlings are raised in intensive-care beds under shade for about two months before being transplanted to the nursery rows. The stocks are budded within a year; to stimulate active growth, the nursery rows and the mother trees receive water and nitrogen before budding. Budwood is defoliated 10-14 days before use to trigger the development of the axillary buds. Budding before flowering is avoided as it results in many flowering budlings. One hundred germinating seeds yield about 50 good rootstocks; after budding, these produce about 25 saleable plants early in the next rainy season.
Home gardeners often propagate young watershoots by air layering. Such marcots root well, but losses after separation and during field establishment tend to be excessive. Inarching of rooted stocks into twigs of the mother tree is a good but laborious propagation method.
Planting density ranges from less than 100 to about 300 trees per ha (10 m × 10 m to 7 m × 5 m). The actual spacing depends on the vigour of the stock-scion combination and on the growing conditions (soil depth, irrigation). Intercropping is possible in the first few years; later, sole cropping is common.
Trees from seed are often found in home gardens and on the borders of fields. Orchards consist of clonal trees; they may be pure rambutan stands or mixed plantations with durian (Durio zibethinus Murr.) and some langsat (Lansium domesticum Correa) or mangosteen (Garcinia mangostana L.) trees.
Pruning could possibly play an important role in control of tree size. Growers are aware that supplementary irrigation is desirable; water stress after flowering results in low fruit set and reduced sarcotesta development ("flat fruit"), setting back both yield and fruit quality.
Irrigation complicates clean weeding; to save water, to reduce weeding and to improve tree growth, generous organic mulching under the trees is recommended. The crop does not remove much nutrients: according to Malaysian findings 15 kg N, 2 kg P, 11.7 kg K, 5.9 kg Ca and 2.7 kg Mg per ha for a crop of 7300 kg/ha.
Pruning out all shoots which grow in the interior of the tree is common in Thailand and Indonesia. The canopy is not opened up; on the contrary, after harvest, the panicle remains are cut out to stimulate the growth of side shoots at the periphery of the canopy. Consequently, the trees have long bare limbs which extend further and further outwards. This weakens the shoot/root feedback controls and hastens tree ageing, as evidenced by the progressive decline of the lower limbs.
The opposite approach, fairly drastic pruning in the foliated fringe is practised in places near Kuala Lumpur. Especially branches which have fruited are cut out after harvest. Since these are the main source of lateral shoots, which are less likely to flower next time, their removal keeps the canopy open and brings the terminals which are predisposed to flower into prominence. This pruning system keeps the trees small and their branching pattern simple, provided that the remaining twigs indeed produce enough fruit to moderate the tree's reaction to such rigorous pruning. Under these conditions tree spacings of 6 m × 4 m or 5 m × 3 m may be feasible.
In Thai orchards spot treatments with naphtyl acetic acid (NAA) to increase the proportion of male flowers on "Chompoo" and "Rongrien" trees are standard practice. In recent years the results of NAA application have been less reliable and keen growers now interplant their orchards with male trees on tall trunks to improve pollination.
Diseases and pests
No disease control is practised, except sulphur treatments in Thailand against powdery mildew (Oidium nephelii) during bloom to fruit set. Loss of limbs is caused by stem dieback (Thyronectria pseudotrichia). Stem canker (Dolabra nepheliae) disfigures the surface of branches and twigs; the incidence is reduced if the canopy does not impede air circulation.
Pests occur only incidentally, but a moth identified as the cacao pod borer (Acrocercops cramerella) is becoming a regular pest. There is no tested control recipe. Numerous caterpillars and beetles feed on young shoots and inflorescences. Mealy bugs may shelter in fruit panicles; they are cultured by ants, and sooty mould grows on the secreted honey dew. Fruit flies attack only overripe fruit. The fruit is eaten by bats, rodents and monkeys, and a crop may have to be guarded day and night against these visitors.
The fruit is non-climacteric and has to be harvested when ripe. Entire panicles are twisted or cut off the tree using a bamboo pole which is slit at the top or which contains a small knife. Depending on the cultivar the trees may have to be picked twice a week for 2-8 weeks. In Indonesia and Malaysia the fruit is sold as bunched panicles. In Thailand and the Philippines individual fruits are detached before marketing.
Indications of yield vary. A survey in Malaysia found yields from 2-5.6 t/ha per year; statistics on area and production in Thailand in 1987/88 work out to a mean yield of 6.3 t/ha. An excellent orchard near Surat Tani, Thailand, produced 170 kg per tree, 20 t/ha, in the 11th year. Flowering and fruit set promised a heavy crop the next year, but there did not seem to be much room for further increases in yield. Yields for cultivar R168 are reported in northern Queensland to be 88 kg in the sixth year. It is not unusual for individual trees to produce 5-10 kg 2 years after planting. These figures give an idea of maximum yield levels.
Handling after harvest
The fruit travels well if packed properly, but shelf life is only a few days, mainly because the fruit loses weight rapidly and appendages and skin turn black. Keeping the fruits moist and shaded prolongs this period slightly. Shrink-wrapped fruit from Thailand arrive in Europe in excellent condition. Research work suggests that cold storage at 5-10°C and fungicidal treatment may extend the shelf life to several weeks.
The genetic diversity of cultivated rambutan is narrowing now that home propagation has been abandoned in favour of the few cultivars that are available from nurseries. Exploration in remote areas is still bringing further diversity of wild rambutan to light. Seeds are too short-lived to be of use for germplasm conservation. Tree collections exist in all South-East Asian countries: Thailand (Bangkok, Chantaburi), Malaysia (Kuala Lumpur), Indonesia (Lembang, Bogor) and the Philippines (Los Baños). Outside South-East Asia collections are kept in China, Mexico, the United States, the Seychelles and Australia. It is unknown whether other species of Nephelium L. can be crossed with rambutan; likely candidates are N. ramboutan-ake (pulasan) and N. cuspidatum because of their tasty fruit.
Breeding by crossing selected parents has yet to start; all cultivars have been obtained by cloning superior plants. Most cultivars originate from the wild variety lappaceum ; therefore, the scope of the other two varieties (pallens and xanthioides) for cultivar improvement has to be investigated. There is a need to select not only for fruit characteristics, but also for productivity and manageable tree size. These attributes may be linked to number of fruit per panicle and to the number and fruitfulness of laterals emerging on twigs which have borne the previous crop. Testing of a wide selection of potential rootstocks and the cloning of stocks are among the current breeding objectives in Malaysia.
The short-term outlook ranges from bleak for Thailand, the largest producer, to bright for the Philippines and Australia. In spite of relatively high yield levels, rambutan growing in Thailand is declining owing to over-production and low prices. In Malaysia statistics indicate that the area is expanding. On the basis of nursery output, rambutan growing is on the increase in Indonesia, at least in Java. In the Philippines and Australia rambutan is still a minor crop with great potential for expansion. Rambutan can also become an important fruit tree in the humid tropics outside South-East Asia.
Rambutan trees come into bearing quickly; progress in the control of tree size and flowering will make production in orchards attractive through the combined effect of much higher yield levels, lower prices and widening market demand. Thus agronomic improvements open a long-term perspective for expansion.
The scope for out-of-season production and fruit preservation appears to be limited. Fickle yield and poor fruit quality may remain limiting factors for out-of-season production. Moreover, increased trade within the region should leave a very short off-season, because the main harvest period in most of Indonesia (December-April) supplements the peak season in Thailand and Peninsular Malaysia (June-September). This also limits the regional market for processed (canned) rambutan; outside South-East Asia canned produce has to penetrate the established market for canned litchi, which is not easy.
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P.C. van Welzen & E.W.M. Verheij