1493

2: Edible fruits and nuts

Citrus × paradisi Macf.

Hook. Bot. Misc. 1: 304 (1830).

RUTACEAE

2n = 18

Citrus decumana L. var. racemosa Roem. (1846), Citrus grandis (L.) Osbeck var. racemosa (Roem.) B.C. Stone (1985).

Grapefruit (En). Pomelo (Fr). Indonesia: limau gedang. Malaysia: grapefruit. Papua New Guinea: muli (Pidgin). Cambodia: krôôch thlông. Laos: kièngz s'aangz. Thailand: grapefruit. Vietnam: cam, bu'o 'i.

Grapefruit is the only major citrus fruit that originated outside South-East Asia; probably on the island of Barbados (West Indies) around 1750. It is thought to be either an interspecific hybrid of pummelo (Citrus maxima (Burm.) Merr.) x sweet orange (Citrus sinensis (L.) Osbeck) or a hybrid/mutant pummelo. Nowadays it is widely grown everywhere in the tropics and the warmer subtropics, with Florida (United States) and Israel as main producers. In South-East Asia, the tree and the fruit are rarely encountered, pummelo dominating throughout the region.

Grapefruit is a popular breakfast fruit, but it is also converted to juice; in addition, segments are canned. Pulp and molasses are used as cattle feed.

The world production of grapefruit is not known exactly since it is recorded together with pummelo in FAO statistics. However, grapefruit made up at least 90% of the 4 million t of both fruits that were harvested in 1983. World production in 1987 for the two crops was 4.5 million t. International trade is largely directed towards European markets.

The edible portion takes up 30—40% of the fruit. The vitamin A content (440 IU) is higher than in other citrus fruits, but the vitamin C content (36—50 mg/100 g) is relatively low. Unlike sweet orange it contains no hesperidin, but 0.7—0.8% of a bitter glucoside called naringin. This gives grapefruit its peculiar flavour: a combination of bitter, sweet and acid.

Large spreading evergreen tree, 10—15 m tall, with a round top and dense foliage; twigs angular when young, (sub)glabrous.
Leaves smaller than those of pummelo, larger than sweet orange leaves, nearly glabrous; petioles broadly winged but less than in pummelo, wing oblanceolate to obovate; blade ovate, 7—15 cm x 4—8 cm, often crenulate. Flowers axillary, singly or in small clusters, 4—5 cm in diameter, pentamerous; calyx 5-lobed; petals white, smaller than those of pummelo, larger than in sweet orange; stamens 20—25; ovary-cells 12—14. Fruit a berry, mainly borne in clusters, usually globose, 8—15 cm in diameter, greenish-yellow; segments coherent and not easily separable, smaller but containing more juicy pulp-vesicles than in pummelo. Seeds white, smooth, usually polyembryonic.

The confusion about the correct scientific name of the grapefruit emanates from its unknown origin. Some consider it to be a (hybrid) derivative of the pummelo, only deserving a varietal status. Others see it as an independent species, named Citrus x paradisi to indicate a hybrid origin, a view adopted here. An important argument against the grapefruit being an interspecific hybrid is that the progeny of selfed grapefruit does not segregate into orange-like and pummelo-like forms.
Two cultivar groups exist: white-fleshed and pink-fleshed; in both, seeded and seedless cvs occur. The seeded cvs mature in 8 months in a warm climate, the seedless ones need at least 1 month more.
Well known cvs are: 'Marsh' (seedless, white, with high productivity, good quality and colour), 'Duncan' (seeded, white, of superior quality), 'Thompson' (a sport of 'Marsh' with pink flesh) and 'Ruby' (seedless, with deep flesh colour and reddish rind).
Hybrids of mandarin x grapefruit are called tangelos, e.g. 'Minneola' and 'Ugli'. The latter originated in Jamaica and does well in a tropical climate; it is a big, coarse, excellently flavoured fruit with 3 or 4 seeds and an average weight of 700 g. The 'Chironja' from Puerto Rico is a sweet orange x grapefruit hybrid (called an orangelo); it is pear-shaped, mild-flavoured and should also do well in other tropical regions.

Grapefruit needs a warmer climate than sweet orange: 3000 degree-days or more (one degree-day is the difference between the average temperature and 13°C). It should therefore not be grown at elevations above 800 m. At sea level, near the equator, maturity is reached in 8—9 months. At higher altitudes or latitudes it takes 10 months or more.
Most grapefruits grow on sandy soils, but the crop also grows well on heavy clay soils with good drainage.

Grapefruit is seldom used as a rootstock, although it has a good root system, because yields on this stock are low. In the United States 'Troyer' citrange, sour orange and 'Swingle' citrumelo are the main grapefruit rootstocks. In the humid tropics of Surinam, sour orange is the established rootstock; in experiments, trees on King orange and 'Sunki' mandarin stock gave high yields of good quality in contrast to trees on rough lemon and Rangpur lime.
Grapefruit needs wider spacing than sweet orange, e.g. 7 m x 7 m. It requires less N but more K than sweet orange.
The tree is highly sensitive to citrus canker caused by Xanthomonas campestris p.v. citri. It is more sensitive to scab (Elsinoe fawcetti) than sweet orange, but less so than sour orange.
A yield of 40 t/ha per year is possible with good management. In the subtropics harvestable fruit can be 'stored' on the tree right through the winter to extend the market season, but this reduces the next crop. Shelf life is fairly long and can be greatly extended by cool storage at high humidity when the fruit is coated with a fungicide in wax. Chilling injury may occur below 10°C, but where fruit has to be chilled to 0°C to eliminate possible fruit fly infestations, injury can be prevented by preconditioning at an intermediate temperature (16°C for 6 days).

Grapefruit appears to be well-suited to the tropics, including the humid tropics. The problem in South-East Asia is that few people like the bitter taste; there is very little demand for the fruit in the domestic market. There may be better prospects for the mildly flavoured tangelo and orangelo cultivars mentioned above; familiarity with these fruits could pave the way for the true grapefruits.

Hume, H., 1957. Citrus fruits. New York, Macmillan. 444 pp.
Reuther, W., Webber, H.J. & Batchelor, L.D. (Editors), 1967. The citrus industry. Revised edition. Vol. 1. History, botany and varieties. University of California, Division of Agricultural Sciences, Berkeley. pp. 383—385, 533—552.
Samson, J.A., 1977. Problems of citrus cultivation in the tropics. Span 20: 127—129.

J.A. Samson

Samson, J.A., 1991. Citrus × paradisi Macf.. 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