1491

2: Edible fruits and nuts

Citrus maxima (Burm.) Merr.

Interp. Rumph. Herb. Amb.: 296 (1918).

RUTACEAE

2n = 18

Citrus aurantium L. var. grandis L. (1753), Citrus grandis (L.) Osbeck (1757), Citrus decumana L. (1767).

Pummelo, shaddock, pomelo (En). Pamplemoussier (Fr). Indonesia: jeruk besar, jeruk bali. Malaysia: jambua, limau betawi, limau bali. Papua New Guinea: muli. Philippines: lukban, suha (Tagalog, Ilokano). Burma: shouk-ton-oh. Cambodia: krôoch thlông. Laos: kièngz s'aangz, ph'uk, sômz 'ôô. Thailand: som-o (general), ma-o (northern). Vietnam: bu'o'i.

The origin of the pummelo is uncertain. There is little doubt that the species is indigenous in Malesia. It has spread to Indo-China, southern China and the southernmost part of Japan and westwards to India, the Mediterranean and tropical America. However, it remains a fruit of the Orient; neither in India nor further west has it become popular. The best match of cultivars, environmental niches and growing skills appears to be found in Thailand.

The fresh juicy pulp vesicles are eaten out of the hand or in fruit salads; sometimes the juice is extracted. The white inner part of the peel can be candied after the outer peel containing oil glands has been removed. The aromatic flowers are used to make perfume in Vietnam. The wood is used for tool handles. Even if the fruit is of inferior quality, the tree may still be grown for the medicinal applications of leaves, flowers, fruits and seeds, including the treatment of coughs, fevers and gastric disorders.

Data on pummelo are often included with the statistics on other citrus fruits, but South-East Asian countries published the following figures for pummelo, all referring to 1987: Thailand 76 275 t, the total planted area being 15 100 ha, the Philippines 34 750 t and 4480 ha. The fruit is traded in the region and in 1987 Thailand reported exports of 6900 t, Indonesia of 18 t, largely to Hong Kong, Singapore and Malaysia.

The edible segments form only a small fraction of the thick-skinned fruit; Thai sources give the composition per 100 g edible portion as: water 89 g, protein 0.5 g, fat 0.4 g, carbohydrates 9.3 g, vitamin A 49 IU, vitamin B₁ 0.07 mg, vitamin B₂ 0.02 mg, niacin 0.4 mg and vitamin C 44 mg. Naringin is the characteristic glucoside found in the fruit.

Tree, 5—10(—15) m tall, low-branching; branches spreading, spiny (seed propagation) or spineless (vegetative propagation), spines up to 5 cm long; young parts manifestly pubescent. Leaves ovate to elliptical, 5—10(—20) cm x 2—5(—12) cm, base rounded to subcordate, margin entire to shallowly crenate, apex obtusely acute, glandular dotted; petiole broadly winged, up to 7 cm wide, wing obcordate. Inflorescences axillary, with a cluster of a few flowers or a single flower; flowers large, 2—3 cm long in bud, 3—5 cm wide when fully expanded, pentamerous, puberulous; petals creamy-white; stamens 20—25(—35); ovary with 11—16 loculi. Fruit a subglobose to pyriform berry, 10—20(—30) cm in diameter, greenish-yellow, densely glandular dotted; peel 1—3(—4) cm thick; segments with large, pale yellow or pink pulp-vesicles, filled with sweetish juice. Seeds usually few, large, plump, ridged, yellowish, monoembryonic.

The fleshy seed with its thin coat dries out easily and needs near-ideal conditions to germinate; spontaneous seedlings are rare. A seedling progeny consists largely of tall slender trees with long spines, coming into bearing 6—8 years after sowing.
In the tropics the trees flower 2—4 times per year, mainly in conjunction with shoot growth flushes. The main flowering period follows the onset of the monsoon rains, unless it is brought forward by irrigation as in Thailand (bloom in January—February). A load of growing fruit limits fruit set from subsequent blossom periods. As a consequence there is usually little overlap of crops in high-yielding trees, whereas such overlaps are common in shy bearers. Fruit matures 7—10 months after flowering, in Thailand mainly from August to October. Some fruit is available throughout the year. In Java the main harvest months are from April—June, following blossoming in September—October. In Malaysia there are harvest peaks in July—October and in January—February.
The anthers lie against the surface of the stigma and release their pollen before the flower bud opens. Nevertheless most cultivars are self-sterile to a high degree. In the presence of pollinators most cultivars produce seeded fruit. It has been suggested that seediness may also be the result of selfing, provided favourable conditions prevail, e.g. a temperature (25—30°C) high enough for fast pollen tube growth and low enough to delay wilting of the pistils. During the main season fruits contain more seed than those produced out-of-season; the latter are often virtually seedless.

The closely related grapefruit (Citrus x paradisi Macf.) can be distinguished from the pummelo by the following characteristics: fruits usually smaller, subglobose (pummelo often pyriform); the pulp-vesicles are coherent and the juice copious (in pummelo easily separable and juice rather scanty); petioles broadly winged, but not subcordate as in pummelo; seeds white and polyembryonic (in pummelo yellowish and monoembryonic).
The top pummelo cultivars of Thailand are in a league of their own and fairly distinct: small trees with a spreading habit; fruit with thin rind and firm, crisp flesh, not very juicy; fruit often seedless or with shrivelled seed. Examples of widely-known cultivars: 'Kao Hawm', 'Kao Nam Pheung', 'Kao Paen', 'Kao Phuang', and 'Kao Thong Dee'. 'Kao Thong Dee' is pink-fleshed, the others white-fleshed; 'Kao Nam Pheung' and 'Kao Phuang' are pear-shaped, the latter with a prominent neck, whereas the others are more rounded or flattened ('Kao Paen'). The group also includes excellent cultivars which are typical for a single locality, e.g. 'Kao Teang Kwaa' (Chinat Province), 'Som Tha Khoi' (Phichit Province) and 'Som Thubthim' (Nakhon Prathom Province).
Several of these outstanding cultivars have been introduced to other countries, the names (e.g. 'Siamese', 'Bangkok') indicating their origin. However, fruit quality does not seem to be as good as in Thailand where top quality fruit is also largely limited to the 3 provinces mentioned above.
The numerous lesser cultivars in the tropics have more in common with the seedling trees: large, tall or spreading trees bearing large fruit with thick rind and qualities that range from good to poor. Names often refer to a locality rather than strictly denoting a cultivar, e.g. Tambun (Malaysia), jeruk Bali and jeruk Cikoneng (Indonesia). The subtropics have their own cultivars, adapted to lower temperatures. In China the leading cultivars are 'Shatianyou' (in Guangxi) and 'Wendangyou' (in Zhejiang and Fujian).

The pummelo thrives in the lowland tropics. In the production centres of Thailand mean monthly temperatures are about 25—30°C with a few cooler (and dry) months; the dry season lasts for 3—4(—5) months and annual rainfall is about 1500—1800 mm. The crop is not grown commercially above elevations of 400 m.
Pummelo tolerates a wide range of soils from coarse sand to heavy clay. However, the tree prefers deep, medium-textured, fertile soils free from injurious salts. In this connection it is noteworthy that the best orchards in the 3 major pummelo provinces in Thailand are situated on the banks of current and former river courses.

Although many trees in home gardens are raised from seed, the common propagation method in South-East Asia is air layering. When certified virus-tested mother trees become available, budding is recommended. Pummelo seedlings of sufficiently uniform populations can be used as rootstocks. In the Philippines shield budding is already the standard method, using rootstocks of calamandarin (believed to be a hybrid of calamondin and mandarin). Trees are spaced 8—10 m x 6—8 m, depending on vigour, on well-prepared land; they are shaded and watered frequently until they are established. The planting material is cut back, especially if bare-rooted.

In South-East Asia pummelo is grown in home gardens, in mixed citrus orchards and in pure pummelo orchards, the latter for instance in Thailand and the Philippines. Mixing citrus species complicates crop protection in the orchard. A banana intercrop can serve as windbreak, shade and source of early income. In Thailand the areca palm is also found as intercrop, or as a border plant along the ditches where the pummelos are grown on raised beds (Nakhon Prathom Province).
Young trees are pruned to leave 3 main framework branches, the lowest being at least 30—40 cm off the ground. The trees also need some pruning in later years to keep the tree interior open, to make sure that fruit on sagging branches does not touch the ground, and to remove dead wood. Trees that bear well are propped up with bamboo poles.
A cover crop suppresses weeds to some extent, but in the rainy season weeds need to be slashed; early in the dry season the orchard is hoed or treated with herbicide. Mulching under the trees with rice straw or other material is strongly recommended to maintain root growth in the topsoil.
Irrigation is important from before flowering until after harvest to supplement rain. During the subsequent dry period irrigation is delayed until the trees show signs of wilting. It is customary to force early flowering by irrigating the wilting trees, provided the water supply is secure until the rainy season starts again. Forcing the trees to advance the harvest has its limitations, as it is difficult to sustain new shoot growth and flowering during the hot dry months preceding the rains.
Fertilizer requirements of citrus also apply to pummelo, including attention to magnesium and micro-nutrients (Zn, Mn, Cu, B). An annual or biennial dressing with manure forms a good basis. In Nakhon Prathom growers are advised to apply about 5 kg NPK 16—16—16 per tree per year in bi-monthly applications and foliar fertilizer for every new flush. In the last dressings before harvest potassium-rich NPK 13—13—21 is used to improve fruit taste. Elsewhere 2 fertilizer dressings are recommended, the first before flowering and the second 4—6 months later.

Pummelo is particularly susceptible to bacterial canker, also on the fruit, following fruit fly stings. Frequent spraying with copper fungicides in Thailand does not give adequate control. Root rot, gummosis on the trunk and brown rot of the fruit, all caused by Phytophthora fungi, appear to shorten the life of many trees in South-East Asia, even though pummelo is not rated as very susceptible.
All the citrus pests seem to be at home on the pummelo, including the obnoxious leaf miners Phyllocnistis citrella (in Java it has been recommended to protect young trees with a mosquito net!), leaf-eating caterpillars, fruit-boring caterpillar (Citripestis sp.), scales, red mites, fruit flies, nematodes and vermin (rats).

The dull skin of unripe fruit brightens upon ripening as the oil glands in the skin become more prominent and shiny. This change starts near the tip of the fruit; as it progresses towards the stalk, the fruit attains its full taste and is ready to be picked.

Yields greatly depend on cultivar and environment, but there appears to be no reason why the potential yield of pummelo should be lower than for other citrus species. Thai sources put yield at 70—100 fruit per tree per year, equivalent to the 20 t/ha per year reported as a good yield in Malaysia.

In overripe fruit, juice sacs may become coarse and dry ('raw rice' condition). Fruits picked at an early stage of ripening improves somewhat during storage (1—2 months) and are suited for distant markets. With its thick rind the fruit can be shipped across the oceans without cool storage.

During the 1980s several germplasm collection missions were organized in South-East Asia and all countries in the region hold pummelo collections, including 40 accessions in the Institute of Plant Breeding at College, Laguna, the Philippines and some 200 accessions in collections in Thailand, where genetic diversity of the species seems greater than anywhere else.

The challenge for South-East Asia is to produce pummelos in quantities and qualities comparable to Thailand. The challenge for Thailand is to make certified healthy planting material available and to safeguard tree health more effectively and less expensively through an integrated approach to crop protection.
There is great scope for expansion of pummelo production in South-East Asia if quality fruit can be offered at prices that people can afford; this is also the way to develop the export trade.

Chaiwongkeit, D. & Chaireongyod, T., 1988. Som O. [The pummelo]. Bangkok. 76 pp.
Chomchalow, N., 1984. Genetic wealth of pummelos in Thailand. IBPGR Newsletter, Regional Committee for South-East Asia 8(3): 27—29.
Fachzurozi, L., 1978. Apakah benar jeruk besar (Citrus grandis (L.) Osbeck) mulai menghilang? [Is the pummelo really disappearing?]. Buletin Kebun Raya 3(4): 133—136.
Martin, F.W. & Cooper, W.C., 1977. Cultivation of neglected tropical fruits with promise. Part 3: The pummelo. ARS-S-157, U.S. Department of Agriculture, New Orleans. 17 pp.
Ochse, J.J., Soule, M.J., Dijkman, M.J. & Wehlburg, C., 1961. Tropical and subtropical agriculture. Vol. 1. Macmillan, New York, pp. 486—488.

Chawalit Niyomdham

Niyomdham, C., 1991. Citrus maxima (Burm.) Merr.. In: Verheij, E.W.M. and Coronel, R.E. (Editors): Plant Resources of South-East Asia No 2: Edible fruits and nuts. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea