Rambutan leaflet Var. lappaceum
Rambutan flower
1.0 Introduction
The rambutan or hairy litchi is delicious, juicy fruit of fairly good quality which is highly esteemed as its close relative, the pulasan. It is closely related to other edible tropical fruits such as Litchi, Longan, and Mamoncillo. Evergreen rambutan trees with their abundant colored fruit make beautiful landscape specimens. It is a popular back yard fruit tree and propagated commercially in small orchards. The rambutan is a perennial tree cultivated extensively in S.E. Asia for its edible fruits. The white translucent, subacid-sweet flavored aril is edible flesh of the fruit. Rambutan does best on deep, clay-loam or rich sandy soils that are high in organic matter or in deep peat (with good drainage) and not suitable for hilly terrain as they require a lot of moisture. A warm climate is ideal for its growth and high yields. It is best grown commercially within 15 0 of the equator (Nichols and Christie 1993), is adapted to warm tropical climates and sensitive to low temperatures (below 10ºC).
1.1 Name and Botany
1.1.1 Common Names:
English: rambutan
Spanish: rambutan
French: ramboutan, litchi chevelu
German: rambutan
Philippines: rambutan, usan
Cambodia: saaw maaw, ser mon
Thailand: ngoh, phruan
Vietnam: chom chom, vai thieu
• Botanical Synonyms: Nephelium glabrum, N. chryseum and N. sufferrugineum.
1.1.3 General Biology
Tree: Rambutan is a large evergreen tree which can grow up to 24 m in the wild, but usually not more than 45 14 m in cultivation. The medium sized trees have an erect, dense habit with a straight trunk and grow to a height of 8 to 10 m. Grafted cultivars are usually more compact reaching a height of only 3 to 5 m. Rambutan trees are either male (producing only staminate flowers and, hence, produce no fruit), female (producing flowers that are only functionally female), or hermaphroditic (producing flowers that are female with a small percentage of male flowers).
Root system: The root of rambutan trees do not penetrate deep into the soil although the tap root may penetrate deep several meters below. Most of the lateral roots are found growing within the tree canopy relatively near to the soil surface. Seedling trees have more vigorous roots compared to those of clonal cultivars (refer to: www.nt.gov.au).
Leaves: The leaves are petiolate, alternate, pinnate, with 3-11 dull green leaflets arranged alternately or sub-opposite on the rachis, each is 5-15 cm long and 3-10 cm broad, with an entire margin. The leaflets are smooth and green above and pale green and glaucous on the lower surface. The young leaves are soft light green or pink. The rachis of 7-30 cm is rounded, stout, red-brown in color and is strongly thickened at the base, initially pubescent but becoming glabrous later.
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Rambutan leaflet Var. lappaceum |
Rambutan flower |
Inflorescence: The inflorescences of rambutan are erect, widely branched, bearing many flowers produced at shoot tips. The flowers are either staminate male or hermaphrodite. The hermaphrodite flowers may be either basically female with stamens and anthers which do not dehisce, or male with undeveloped stigmas.
Rambutan trees may be classified into tree groups, according to their flower characteristics (Valmayor et al. 1970) as follows:
• Male trees (usually 40-60% of any seedling population), producing only staminate flowers.
• Trees producing hermaphrodite flowers which are functionally male.
• Trees producing hermaphrodite flowers, some of which are functionally female and some are functionally male. This type of tree is most commonly among cultivars whereby the percentage of male flowers is low (in the range of 0.05-0.9%) (Almeyda, et al., 1979; Chin and Phoon, 1982; Tindall, 1994).
Cultivars that produce only functionally female flowers require the presence of male trees. Male trees are seldom found as vegetative selection has favored hermaphroditic clones that produce a high proportion of functionally female flowers and a much lower number of flowers that produce pollen. The flowers are small, 2.5-5 mm, apetalous, discoidal, and borne in erect terminal panicles (clusters) 15-30 cm long, perfect but functionally staminate or pistillate, in axillary or terminal panicles. Up to 100 flowers in each female panicle that may open each day during peak bloom. There are over 3000 greenish-white flowers in male panicles, each with five to seven anthers and a non-functional ovary. Male flowers have yellow nectaries and 5-7 stamens. There are about 500 greenish-yellow flowers in each hermaphroditic panicle. Each flower has six anthers, usually a bi-lobed stigma, and one ovule in each of its two sections (locules) (Free, 1993; Tindall, 1994). The flowers are receptive for about one day but may persist if pollinators are excluded (Tindall, 1994). Flowers which anthesise in the afternoon will fail to complete opening until dawn the following day resulting the majority of male flowers present in the morning. Open flowers do not wilt, but abort on the same day when the filaments have shriveled. Open flowers usually persist on panicles for 7-10 days if insects are excluded. The stigma remains active for a day, becomes dull on the second, and later turns from brown to black. Most if not all flowers open early in the day and 100 flowersmay open in a female panicle each day during peak bloom. Both male and female flowers are faintly sweet scented and have functional nectaries at the ovary base. Female flowers produce 2-3 times more nectar than male flowers. Nectar sugar concentration ranges between 18 and 47 percent and is similar between the flower types (Tindall, 1994). Rambutan flower is an important nectar source for bees in Malaysia (Phoon, 1983). Initial fruit set may approach 25 percent but a high level of abortion contributes to a much lower level of production at harvest (1-3%). The fruit matures 15 to 18 weeks after flowering (Tindall, 1994).
Fruits: Fruits round to ovoid, drupes of 3 to 8 by 2 to 4 cm are pendant-like in a loose cluster of 10-20 fruits with a red, orange or yellow peel covered with hair-like spin terns. The pericarp of this attractive oval-shaped fruit can be red, orange, pink, or yellow in color and is removable by a twist of the hands. The leathery skin is reddish (rarely orange or yellow), and covered with fleshy pliable spines, hence the name rambutan, derived from the Malay word rambut which means hairs. The edible, translucent pearlish white, juicy, crispy, sweet and subacid flavored flesh (sarcotesta) conceals a single seed with a thin, fibrous seed coat (testa) which resembles an almond. The aril is attached to the seed in some commercial varieties, but 'freestone' varieties are available and in high demand. There is usually a single light brown seed which is high in certain fats and oils (primarily oleic and eicosanoic acids) valuable to industry, and used in cooking and the manufacture of soap (Almeyda, et al., 1979; Tindall, 1994).
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Rambutan fruit showing the pearly aril |
Seed structure: Rambutan flower is bicarpellate but only one carpel develops normally bearing a single seed. The seed is oblong or elliptic ovoid and flattened (Watson, 1984).
1.1.4 Flowering and pollination
Flowering behavior: In Malaysia, rambutan flowers flowers twice a year from March to May and again between August to September, usually in response to rain following a dry period (Shaari et al. 1983). Flowering periods differ for other localities. Flowering seasons may show differences due to prevailing micro-climatic factors. Rambutan clones may be early or late bearers and this combined with the variable micro-climates enables fruit to be available throughout the year except from March to May. Most, but not all, flowers open early in the day. Each flowering season usually lasts from 5-15 weeks.
Flower production: Rambutan is a terminal bearer and new shoots are potential sites for future fruiting. Shoots that had previously bear fruit will develop new vegetative flushes soon after harvest. These flushes usually consist of four lateral shoots that arise below the dried panicles. For non-fruiting shoots, new flushes arise from the terminal buds only. The percentage of new shoot formation in non-bearing and previously- bearing twigs are 57% and 22% respectively (Van Welzen and Verheij 1991).
Pollination Requirements: Cross-pollination is a necessity (Chin and Phoon, 1982; Lim, 1984, 1992) because pollen is absent in most functionally female flowers (Zee, 1993). Although apomixis may occur in some cultivars, research has shown that rambutan, like Lychee, is dependent upon insects for pollination (Free, 1993; Zee, 1993). In Malaysia, where only about one percent of the female flowers set fruit, research revealed that no fruit is set on bagged flowers while hand pollination resulted in 13 percent fruit set. These studies further suggest that pollinators may maintain a fidelity to either male or hermaphroditic flowers (trees), thus limiting pollination and fruit set under natural conditions where crossing between male and female flowers is required.
Pollinators: Aromatic rambutan flowers are highly attractive to many insects, especially bees. Those commonly found visiting rambutan flowers include bees (Apis spp . and Trigona spp.), butterflies, and flies ( Eristalis sp. and Lucilia sp.) (Chin and Phoon, 1982; Lim, 1984). Apis cerana colonies foraging on rambutan flowers that produce large quantities of honey. Bees foraging for nectar routinely contact the stigmata of female flowers and gather significant quantities of the sticky pollen from male blossoms. Little pollen has been seen on bees foraging female flowers. Although male flowers open at 0600 h, foraging by A. cerana is most intense between 0800 and 1100 h, tapering off rather abruptly thereafter. In Thailand , A. cerana is the preferred species for small scale pollination of rambutan (Free, 1993; Lim, 1984; Tindall, 1994).
Pollination Recommendations and Practices: Honey bee colonies are placed in rambutan plantations is an important and practical recommendation for assuring adequate pollination and fruit-set. The bees can be present in the orchards continuously throughout bloom. Although no specific number of colonies per hectare of rambutan can be recommended in the absence of more definitive data, strong (>8 frames with bees and brood) colonies should be provided at a minimal rate of one (or the equivalent) per 0.4 ha. From 1965 to 1967, agronomists at the College of Agriculture, University of the Philippines , studied the growth, flowering habits and yield of the Indonesian cultivars, 'Seematjan', 'Seenjonja', and 'Maharlika'. They found that all the 'Seematjan' flowers were hermaphrodite functioning as female (h.f.f.) and that it is necessary to plant male trees with this cultivar. 'Seenjonja' and 'Maharlika' flowers were mostly hermaphrodite functioning females (h.f.f) with a very few hermaphrodite functioning as males (h.f.m.) in the same panicles, and they concluded that, though self-pollination is possible, planting of male trees with these cultivars should improve production.