6. DISCUSIÓN
6.1. CONSIDERACIONES METODOLÓGICAS PREVIAS
The family Sapindaceae is composed of around 150 genera and 2000 species of trees, shrubs and a few herbs and vines. The species are usually monoecious and are distributed widely in the warm tropics. Among the numerous genera, four related genera and fi ve species are of interest to fruit horticulturists, with litchi and longan the most widely known. The genera Nephelium has 22 species of which two have been commercialized: rambutan and pulasan.
N. lappaceum L. is generally known as rambutan (Indonesia, Malaysia, Philippines, English), litchi chevelu (French), ngoh, phruan and ngo (Thailand) and hong mao dan (China). The three varieties of N. lappaceum L. (vars. lappaceum, pallens and xanthioides) exist on the basis of variation in leafl et characteristics.
N. ramboutan-ake (Labill.) Leenh. (Litchi ramboutan-ake Labill., N. mutabile Blume) is known as pulasan (Malay, English), ngo-khonsan (Thailand), kapalasan (Indonesia) and bulala (Tagalog).
RAMBUTAN
Area of origin and distribution
Rambutan (N. lappaceum L.) is thought to be indigenous to the Malay Peninsula, though its long history of domestication makes its origins diffi cult to ascertain. Rambutan is now well distributed in South-east Asia, and is mostly grown in humid, high-rainfall areas. The characteristic environment of rambutan is one of high rainfall (evenly distributed), high humidity, low evaporation rates and average minimum temperatures above 20°C. The crop is now grown in number of locations outside of its natural distribution, including Central America, Sri Lanka, India, New Guinea, tropical Africa, Hawaii and northern Australia.
7
Ecology
Soil
Rambutan thrives on a wide range of soil types, as long as drainage is good enough to prevent water-logging and supplementary irrigation is available during prolonged periods of dry weather. Trees thrive best on deep clay loam soils and prefer a slightly acid soil (pH 5.0–6.5). Soil types that support lush growth may be counterproductive to regular fruit production, particularly in environments where a check in growth caused by either dry or cool conditions does not occur.
Climate
RAINFALL AND MOISTURE High rainfall and humidity induce good growth in rambutan. Ample annual rainfall is considered to be around 1500–3000 mm.
The tree is relatively tolerant of drier environments, as long as supplementary irrigation is available.
Rambutan fl owering correlates with the end of the dry season, leading to a second smaller crop each year on branches that did not bear fruit in the previous fruiting. A dry period of at least 1 month is essential to initiate rambutan fl owering (Fig. 7.1). The intensity of fl owering appears to be closely associated
Flushing or Flowering (% of Canopy)
Fig. 7.1. Mean rainfall, vegetative fl ushing and fl owering pattern for eight cultivars of rambutan over 4 years at Hilo, Hawaii, which has no marked dry season.
Vegetative fl ushing parallels rainfall, with fl owering occurring following a period of lower rainfall. (Redrawn from Kawabata et al., 2007.)
to the duration of water stress. Rambutan requires adequate moisture, from either rainfall or irrigation, during fruit set and growth (Fig. 7.2). There are numerous stomata on the fruit spinterns and low humidity during fruit development can lead to fruit desiccation.
TEMPERATURE Rambutan has no cold requirement for fl owering and is suited to tropical areas with a temperature range of 22–30°C. Temperatures less than 22°C reduce leaf fl ushing and can extend fruit development to nearly 6 months.
LIGHT INTENSITY AND PHOTOPERIOD There is little information on the response of rambutan to varying light levels. It is thought to be day-neutral.
WIND Windbreaks are necessary as young and mature trees buff eted by the wind establish, grow and yield poorly. Common windbreak species in Australia are found in the genera Acacia, Artocarpus, Casuarina, Melaleuca, Pennisetum and Syzygium.
General characteristics
Tree
The tree grows to 25 m in height and has a straight trunk of up to 60 cm wide, with a dense, usually spreading crown. The evergreen alternate leaves are 40 cm long, pinnately compound and divided into two to four pairs of leafl ets, usually alternate elliptic to oblong–elliptic or rather obovate (Fig. 7.3). Leafl ets are 7.6–23 cm u 3.8–9.0 cm. They are pale when young, becoming medium to dark green in color.
Flowers
Rambutan fl owers are small, less than 6 mm wide, greenish and apetalous.
They are produced in panicles (Fig. 7.3) that are axillary near the branch apex (Allen, 1967). Trees are normally classifi ed into three types based on fl ower type: male trees that produce only staminate fl owers (40–60% of a seedling population); trees with functionally female hermaphrodite fl owers;
and trees with functionally female and functionally male hermaphrodite fl owers (Fig. 7.3). Male trees can have 3000 male fl owers on a panicle, with approximately 500 open on one day. Hermaphrodite trees are classifi ed as
‘female’ and produce two types of fl owers, with about 500 fl owers per panicle and 100 opening per day. Male functional fl owers have well-developed stamens and pistils that fail to function normally; the bifi d stigmas split but remain erect, preventing the exposure of the sticky stigmatic section required for pollination. Female functional fl owers have well-developed pistils with non-functional stamens, reduced to fi ve to seven staminodes. The bifi d stigmas split open and curve downward to expose the sticky stigmatic surface
Chapter 7
Fig. 7.2. The fruiting cycle of rambutan, as affected by fertilization and water availability, under normal conditions (A) and when water stress is used to induce fl owering (B). Soil surface covers and ditches are used to take away rainfall (C). The plastic cover is removed when tree fl owering occurs (D).
Temperature >22ºC High Water, High N
Prune Fertilize Irrigate
No Irrigation Induce Stress
First Flowers Remove Plastic and Irrigate
Fertilize Irrigate Temperature >22ºC
Low Water, High N
Temperature >22ºC Water Stress
Temperature >22ºC High Water
Temperature <22ºC Fertilize
Irrigate
No Flowering Floral Induction
and Development
Floral Induction and Development
Fruit Set and Growth
and remain receptive for 1 day. Nectar production for both fl ower types begins at anthesis.
Hermaphrodite trees with both functional male and female fl owers are more desirable and are most commonly found in some rambutan cultivar selections, with male fl owers in the range 0.5–0.9% (Lam and Kosiyachinda, 1987).
Flower induction, pollination and fruit set
Rambutan is not believed to have a cold requirement for fl owering and is suited to tropical areas with a temperature range of 22–30°C. In Australia, fl owering in the dry tropics (Darwin, 12.5°S) usually follows the onset of cool nights (18–12°C) in July to August (Diczbalis et al., 1996). In the wet tropics, however, fl owering is reported to occur throughout the year regardless of the climate (Watson, 1988) but usually occurs from September to October following a short dry season. In Thailand, a drop in night temperatures of 2–3°C with the onset of the dry season has been suggested as the prompt for fl owering. Multiple regression analysis showed that for every 1°C decrease in Fig. 7.3. Rambutan leaves (A), panicle (B), hermaphrodite fl ower (C), male fl ower (D) and fruit bunch (E) showing the spintern on the pericarp, and one fruit with half the pericarp removed to expose the aril that surrounds the seed.
night temperature, fl ower induction increased by 6.7% (Manakasem, 1995).
Temperatures lower than 22°C reduce leaf fl ushing and can extend fruit development to nearly 6 months.
Management practices in Thailand are directed toward producing a large crop in the May to June period. Cultivars vary in their requirements for induction, leading to early- and late-season bearers. The cultivars ‘Jitlee’ and ‘R137’ are more suited to areas with some water stress than ‘R4’ or ‘R7.’ Foliar-applied paclobutrazol does not produce off -season fl owering. Ringing and covering a narrow area of the bark (<2 cm), then spraying with potassium nitrate (30–40 g/l) 1 month after ringing brings about earlier fl owering of mature trees. The ringing treatment changes the translocation of photosynthetate to the roots and increases starch in the shoot. Ringing can only be performed in alternate years.
Because rambutan fl owering is correlated with the end of the dry season, farmers in Vietnam cover the soil with plastic sheets with ditches at the edge of the canopy to carry away any rainfall (Fig. 7.2). A dry period of at least 1 month is needed to initiate rambutan fl owering, with periods of up to 3.5 months being used. The soil covers are removed when the fi rst fl owers appear on the trees. The overall system used involves harvesting the previous crop, pruning the tree and fertilizing to induce vegetative growth, and covering the soil and leaving the soil cover on until the fi rst fl owers appear. The trees are then irrigated on removal of the soil covers, with a second application of fertilizer at fruit set. The intensity of fl owering is associated with the duration of water stress, and harvesting can be scheduled for every 6 months. This protocol is used to induce rambutan fl owering even during the wet season.
Only 1–2% of fl owers form fruit on a rambutan panicle. Since no pollen is apparent on the hermaphrodite fl ower anthers, pollen transfer or apomictic fruit development may occur. In mixed plantings with no male trees, pollination problems are not seen if fl owering periods overlap. The cultivar ‘Seechompoo’
has low numbers of functional male hermaphrodite fl owers and is notorious for poor fruit set in a pure stand.
Fruit
Usually only one of the two locules in the rambutan ovary develops to form a normal fruit; the other aborts and remains at the base of the fruit.
Occasionally, however, both carpels develop equally to form two normal fruit.
The fruit occur in clusters of 10–18 and are large (25–45 g in weight), ovoid or globose, and about 4.5 cm long and 2.5–3.7 cm broad. The outer skin on the pericarp is 2–4 mm thick and covered with long soft spines (spinterns) (Fig. 7.3) that turn red or yellow when the fruit is ripe.
Early rambutan fruit growth (Fig. 7.4) is dominated by skin growth; the aril is attached to the single large seed and does not begin to grow until around 60 days after anthesis. In some rambutan cultivars the hard testa comes away with the aril, making these cultivars less desirable. The major phase of growth
occurs 50–80 days after anthesis. As with litchi, there is a rapid decline in titratable acidity from more than 1% to less than 0.5% and an increase in total soluble solids from about 13% to 20% in the last 35 days of fruit growth. Total starch declines during the same period and fruit begin to change color.
Cultivar development
Cytogenetics and genetics
The diploid number for rambutan is 2n = 22.
Breeding and selection
Breeding and crop improvement attempts appear to be limited or poorly reported. A large-scale evaluation of F1 hybrids based on two maternal parents (‘R99’ and ‘R134’) and 14 popular Malaysian cultivars was undertaken in Malaysia (Sarip et al., 1996). Six years after its establishment, 50% of the population had fl owered and about 40% were males. Seven percent of the population fl owered 2 weeks earlier than both maternal parents and less than 1% produced high-quality fruit with the combined attributes of good appearance, high recovery and cling-free. This suggests that improved cultivars will continue to come from grower selection and potentially via biotechnology.
Time from Anthesis (Days)
Mass (g)
Color Break
Fig. 7.4. The growth pattern of rambutan cultivar ‘Seechompoo’ fruit and its parts from anthesis. (Redrawn from Wanichkul, 1980.)
The selection criteria for rambutan are similar to those of litchi. Adherence of the aril to the testa is an important criterion. The criteria include:
x Fruit: large, with small seed and a high proportion of edible aril. Bright red or yellow skin color, long shelf-life and ability to retain skin color under storage conditions. Firm fl esh with an acceptable sugar-to-acid ratio and resistance to disease.
x Tree: vigorous, precocious, regular and high yielding. Resistant to water stress, wind, soil salinity, diseases and insects.
Numerous rambutan cultivars (Salma, 1986) with a wide range of characteristics (Table 7.1) are grown in major production areas. Salma (1986) identifi ed and developed a key to 31 cultivars of rambutan grown in Malaysia out of an accession list that exceeds 65. A large number of accessions have been collected in major centers in Australia, Hawaii, Indonesia, Malaysia, Mexico, the Philippines, the Seychelles and Thailand (IBPGR, 1986).
Cultural practices
Propagation
Valmayor et al. (1971) reported that planting from seed results in a high proportion of male-fl owering trees. Seedling trees are less precocious and extremely variable in character, and hence commercial production is based on clonally propagated trees. Vegetative propagation is used to propagate selected Table 7.1. Fruit characteristics of selected rambutan cultivars.
Cultivar Synonyms Vigor
‘Jit Lee’ Deli Medium 30–55 35 20–22 Orange/red,
green tips
‘R9’ Tau po cheng Medium/
large
‘Chompoo’ Seechompoo Dwarf 28–35 40 18–20 Orange
female-fl owering trees. The male tree is seldom found as orchards are usually based on clonal budwood. In rare cases where wood below the bud graft has been allowed to develop, shoots with male fl owers are clearly seen.
SEXUAL The seeds are recalcitrated and rapidly lose viability after removal from the fruit. Seeds germinate in 7–15 days.
ASEXUAL Little success has been reported with rooting rambutan cuttings.
Grafting is diffi cult and a number of methods have been tried (e.g. shield, patch and modifi ed Forkert budding; various approach graft techniques and cleft grafting). Modifi ed Forkert or patch budding appears to be the preferred technique of commercial propagators worldwide. Approach grafting, although much more successful, is cumbersome and time consuming, and reliant on scaff olding surrounding the parent tree. Research and experience suggest that budding or grafting operations should occur during the active growing period (the rainy season). Despite the availability of a range of budding and grafting techniques, the overall success rate is generally low. Therefore, air layering is also utilized.
In air layering young, actively growing shoots are girdled and the wound surrounded with peat or similar moisture-absorbing material until roots emerge. The rooted shoots are removed from the parent tree and established in a pot or directly into the fi eld. This technique is growing in popularity in Australia because of the shortage and expense of budded trees. The long-term survival of air-layered trees has been questioned.
The selection of rootstocks is generally based on seed availability. Little research has been conducted on stock scion relationships. In the Philippines, N. intermediam (bulaba) has been shown to be a successful stock for grafted rambutan (Valmayor et al., 1961). A trial was established in Australia’s Northern Territory to evaluate the production of two cultivars (‘Jitlee ‘and
‘R134’) on eight stocks (‘Jitlee,’ ‘R156,’ ‘Rapiah,’ ‘Rongrien,’ ‘Gula Batu,’
‘R9,’ ‘R37’ and ‘Bogor’). To date, no commercial diff erences in growth and yield performance have been detected (McMahon, personal communication, 2004). Field observations have noted that certain stock/scion combinations are semi-dwarfed. There is a need to explore whether this is due to those specifi c combinations or chance.
Field preparation
Deep ripping may be necessary if the soil is compacted. Liming to pH 5.0–5.5 should be carried out and manure incorporated before planting. Rambutan require well-drained alluvial soils for good development. In South-east Asia, rambutan is frequently interplanted with other tree crops such as durian or duku/langsat. Monoculture orchards are the preferred method of establishment in Hawaii, Australia, Central America and, increasingly, South-east Asia.
Transplanting and spacing
Budded or grafted trees are generally ready for fi eld planting within 12–24 months. Air-layered trees are generally removed from the tree with 3–4 months of girdling and established in pots (3–6 months) prior to fi eld planting. Growers in Australia are fi nding success with the direct fi eld establishment of air-layered trees. Rambutan are planted with 5–10 m between trees and 8–10 m between rows. One male rambutan tree is often planted per 8–10 functional female hermaphrodite trees. Windbreaks are necessary for young and mature trees.
Irrigation
Irrigation is required for rambutan grown for high-value domestic and export markets, as water stress during fl ower and fruit development reduces yields and fruit size. Irrigation is also essential during the vegetative fl ushing stage after harvest, and is withheld during fl oral initiation (Fig. 7.2). Prefl owering water stress, which does not induce leaf wilting, leads to earlier fl owering and improved harvest synchrony without aff ecting yield. High rainfall during blooming can lead to poor fruit set. In the monsoonal environment of northern Australia, irrigated rambutan trees have a shallow root system with 80% of the roots in the top 15 cm and not extending beyond the tree canopy.
Such trees can require irrigation up to three times a week (Diczbalis et al., 1996). In trees grown on sandy soils in hot and dry growing environments, severe leaf loss can occur within 4–10 days of withholding irrigation. The amount of irrigation should, at a minimum, replace that lost by evaporation, particularly from fl owering to harvest (Table 7.2).
Table 7.2. Crop coeffi cient (Kc) for rambutan to estimate the daily water requirement (WR) for different stages of development based on evapotranspiration (ET). WR = Kc u ET (Hiranpradit et al., 1998). Higher Kc values are reported during fruit set and fruit development in the wet/
dry tropics of Australia (Diczbalis et al., 1996).
Stage of development Kc
Vegetative growth 0.6
Floral initiation 0.00/0.60
Floral development 0.75
Fruit setting 0.75
Early fruit growth 0.8
Late fruit growth 0.85
Fruit maturity 0.85
Pruning
Little research has been published on the need for and eff ectiveness of pruning. Early training aims to achieve a strong framework of branches to encourage secondary lateral branches. Annual pruning is designed to remove water sprouts, pest- and disease-infected shoots, dead branches and crossing branches. In some areas the panicle is pruned after harvest to induce vigorous canopy regrowth. The eff ect of pruning on rambutan cropping in the wet/
dry tropics of Australia has been evaluated (Menzel et al., 2000). Pruning delays harvest and yields are 20–40% lower than in non-pruned trees, but on much smaller canopies. The reduction in cropping is greatest when shoot-tipping in June is followed by structural pruning in February. The trial clearly demonstrated that in the wet/dry tropical environment of northern Australia, structural pruning can occur without a complete loss of production the following season. Structural pruning and structural plus shoot-tip pruning treatments both delayed fl owering and harvest dates relative to the control treatments. Pruning treatments also improved the synchronization of harvest.
The ability to manipulate harvest dates and synchronization are potentially important management tools.
Commercial growers in Australia are currently using mechanical pruners to reduce tree size and shape trees to allow machinery access. The eff ect of mechanical pruning on tree productivity depends on the time of pruning and the amount of wood removed. In situations where relatively heavy pruning has taken place, fl owering may be delayed until the following season.
Fertilization
The fertilization practices of commercial rambutan orchards diff er because of diff erences in climate, soil, availability of diff erent kinds of organic and inorganic fertilizers and other factors.
Rambutan is reported to have similar nutritional requirements to litchi.
A hectare of rambutan (70–80 trees/ha) yielding 6750 kg/ha in north Queensland removed 13.6 kg nitrogen, 2.1 kg phosphorus, 12.1 kg potassium, 3.7 kg calcium, 1.9 kg magnesium and 1.3 kg sulfur (Diczbalis, 2002). Fertilizer management in rambutan can be enhanced by the use of soil and leaf analysis (early panicle emergence), with nutrient replacement based on nutrient removal plus losses due to leaching, runoff or volatilization. Fertilizer application needs to correspond with diff ering needs at various stages of the growth cycle. Critical periods are before fl owering and fruit set, several weeks after fruit set and after harvest. Leaf samples for analysis are taken twice a year, once at the end of harvest and again at fl owering, with only phosphorous and potassium applied during fruit set to avoid vegetative growth from nitrogen application. Tentative leaf nutrient standards (north Queensland) at early panicle emergence are:
2.01% nitrogen, 0.21% phosphorous, 0.66% potassium, 1.2% calcium, 0.32%
magnesium, 0.21% sulfur, 485 mg/kg manganese, 102 mg/kg iron, 54 mg/
magnesium, 0.21% sulfur, 485 mg/kg manganese, 102 mg/kg iron, 54 mg/