Lycopersicon esculentum (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Lycopersicon esculentum Miller

Protologue: Gard. Dict. ed. 8, n. 2 (1768).

Family: Solanaceae

Chromosome number: 2n= 24

Synonyms

• Solanum lycopersicum L. (1753),
• Lycopersicon lycopersicum (L.) Karsten (1882).

Vernacular names

• Tomato, love apple (En)
• Tomate (Fr)
• Indonesia: tomat
• Malaysia: tomato
• Papua New Guinea: tamato
• Philippines: kamatis (Tagalog), butinggan (Ilocano), ang-angseg (Bontoc)
• Cambodia: peeng pâh
• Laos: khüa sômz
• Thailand: makhua-thet (general), makhua-som (northern)
• Vietnam: cà chua.

Origin and geographic distribution

Tomato originated from the Andean region of South America, in the area now covered by parts of Bolivia, Chile, Ecuador, Colombia and Peru. The related species of cultivated tomato are native and widely distributed in this region. Archaeological and circumstantial evidence (diversity of types and culinary uses, abundance of local names) all suggest that tomato was domesticated in Mexico, outside its centre of origin, and that the most likely ancestor is the primitive cherry tomato (L. esculentum var. cerasiforme (Dunal) Gray). Tomato was introduced into Europe in an already fairly advanced stage of domestication soon after the discovery of the New World. From there, it was taken to other parts of the world at various times: in the 17th Century to China, South and South-East Asia; and in the 18th Century to Japan and the United States. Although initially grown only as a curiosity in Europe because of its erroneous reputation as a poisonous fruit, tomato has now become one of the most important vegetables worldwide.

Uses

Tomatoes are consumed fresh in salads, sauces and as a flavouring ingredient in soups and meat or fish dishes. Tomatoes can also be made into sweetened candies, dried fruits, and even into wine. The economically most important uses of tomatoes are, however, in various processed forms such as purées, juice, ketchup, canned whole and diced fruits. Although tomatoes generally rank low in comparative nutritional value, they outrank all other vegetables in total contribution to human nutrition because so much is consumed in so many different ways.

Production and international trade

Total area of tomato planted annually worldwide is about 2.7 million ha, 80-85% in market gardens, producing an estimated 68 million t. Tomatoes are produced in the open field, under plastic shelter or in greenhouses, dependent on climate and season. Leading tomato producing countries are: Commonwealth of Independent States (formerly Soviet Union) 400 000 ha, China 320 000 ha, United States 180 000 ha, Egypt 145 000 ha, Italy 140 000 ha, Turkey 125 000 ha, India 83 000, Romania 80 000 ha, Spain 65 000 ha and Brazil 55 000 ha. Most of the world trade in tomatoes comes in processed products from the Mediterranean region, the United States and South and Central America. In Indonesia 75% of all market garden tomatoes (29 000 ha) is grown above 400 m altitude. The area used for tomato production in the Philippines is about 18 000 ha, in Thailand 8300 ha and in Malaysia 700 ha.

Properties

100 g of edible fruit portion contains approximately the following: water 94 g, protein 1.0 g, fat 0.2 g, carbohydrates 3.6 g, Ca 10 mg, Fe 0.6 mg, Mg 10 mg, P 16 mg, vitamin A 1700 IU, vitamin B₁ 0.1 mg, vitamin B₂ 0.02 mg, niacin 0.6 mg, and vitamin C 21 mg. The energy value is 80 kJ per 100 g. As may be noted, tomatoes are a good source of vitamins A and C. Both vitamins increase in quantity when the fruits are allowed to ripen on the vine. Immature fruits contain the alkaloid tomatine. The seeds contain 24% of a semi-drying edible oil. The 1000-seed weight is 2.5-3.5 g.

Description

• Variable annual herb, up to 2 m tall or taller.
• Taproot strong, to 0.5 m deep or more, often damaged at transplanting, and a dense system of lateral and adventitious roots. Stem solid, coarsely hairy and glandular.
• Growth habit varies from indeterminate with stems several m long and prostrate when not supported, carrying an inflorescence every 3rd to 4th leaf, to determinate with several short and more erect stems with inflorescences (4-6 per stem) every second leaf and one terminating the shoot apex.
• Leaves spirally arranged with a 2/5 phyllotaxy, imparipinnate, in outline 15-50 cm × 10-30 cm; petiole 3-6 cm long; major pinnae 7-9, opposite or alternate, ovate to oblong, 5-10 cm long, irregularly toothed and sometimes pinnatifid at base; a variable number of smaller pinnae occur between the larger leaflets; leaflets petiolate, covered with (glandular) hairs, producing a characteristic and species-specific odour.
• Inflorescence cymose, normally with 6-12 flowers, but compound inflorescences with 30-100 flowers do occur; flowers regular, about 2 cm in diameter, pendent, bisexual, hypogynous, usually 6-merous; calyx tube short and green with pointed lobes, persistent and enlarging in fruit; corolla rotate, petals yellow, stellate, later reflexed and dropping off after pollination; stamens 6, anthers bright yellow, conically arranged, surrounding the style and prolonged into a sterile beak; superior ovary with 2-9 loculi and fleshy central placenta.
• Fruit a berry, flattened, globular or oblate, smooth or furrowed, 2-15 cm in diameter, green and hairy when young, glabrous and shiny, red, pink, orange or yellow when ripe.
• Seed flattened ovoid, 3-5 mm × 2-4 mm, up to 250 per fruit, light brown and hairy.
• Germination is epigeal. Seedlings have a thin taproot and cordate cotyledons; the first leaves have few leaflets.

Growth and development

Dry seed (5.5% moisture content) will maintain a high viability (90-95% germination) after several years of storage at ambient (18-24 °C) temperatures, provided the seeds have been extracted from fully mature fruits. Seeds germinate within 6 days after sowing at optimum soil temperatures of 20-25 °C, and the first true leaf is formed one week later. About 7-11 leaves are usually formed on the main stem before the apex is transformed into a terminal inflorescence. The main axis is continued by the development of a new stem from the primary axillary bud in the leaf subtending the inflorescence. As the new shoot grows this leaf changes to a position above the inflorescence. The appearance is of continuous growth with internodal inflorescences, but it is actually sympodial. In indeterminate cultivars this process is repeated indefinitely with inflorescences every 3rd to 4th leaf and fruits maturing sequentially over a long period of time. In determinate types it is arrested after 4-6 inflorescences, when the primary axillary bud of the last leaf aborts and the next bud develops into a slower growing shoot with one leaf and a terminal inflorescence. Strong axillary bud development at the base of determinate plant types produces the bushy habit with several stems and a short period of prolific flowering followed by a period when fruit growth is dominant. In processing tomatoes, synchronization of fruit growth and ripening is such that once-over machine harvesting becomes possible.

The first flowering starts under optimum conditions about 5-7 weeks after sowing. L. esculentum is a moderately cross-pollinated species in origin, but most cultivars have become almost exclusively self-pollinated. Bees and bumble bees are the most important pollinating agents and are also increasingly used in glasshouses to stimulate pollen dehiscence.

Pollen tube growth is slow and fertilization takes place 50-55 hours after pollination. Fruits are mature 6-8 weeks later. Adequate seed set is necessary for normal fruit development, but parthenocarpic fruit set occurs in some types, or can be induced by growth regulators.

The duration to peak harvest (50% of the crop) depends on cultivar and season. In the cool season: 90-110 days after transplanting; during the hot season: 60-90 days after transplanting. Fruits may range in weight from 20 g for cherry tomatoes to as much as 300 g in some large-fruited fresh market cultivars. Each fruit contains numerous seeds embedded in its locules, ranging from 50-80 in the cherry tomatoes to as many as 250 in fresh market cultivars.

Other botanical information

The genus Lycopersicon Miller comprises a relatively small number of species and is variously subdivided in literature. A much followed approach is that of Rick, who recognizes two complexes:

• the esculentum complex, including L. esculentum , L. hirsutum Humb. & Bonpl., L. pimpinellifolium (Jusl.) Miller, L. chmielewskii Rick, Kes., Fob. & Holle, L. parviflorum Rick, Kes., Fob. & Holle, L. pennellii (Corr.) D'Arcy, and L. cheesmanii Riley;

• the peruvianum complex, including L. peruvianum (L.) Miller, and L. chilense Dunal.

All species are closely interrelated and native to western South America; they intercross quite easily. L. esculentum can be hybridized with all other species with varying degrees of difficulty.

Within L. esculentum, two botanical varieties are distinguished:

• var. cerasiforme (Dunal) Gray, with fruits 1.5-3 cm in diameter;

• var. esculentum, with fruits > 3 cm in diameter.

Numerous cultivars of tomato exist. They can be variously classified, e.g. according to:

• growth habit: indeterminate, semi-determinate and determinate (bush);

• fruit size: small round (cherry, 30 g; "Moneymaker", 80 g), medium-large round (120-150 g), beefsteak and ribbed (> 200 g);

• fruit shape: round, egg-shaped and elongated ("San Marzano") or flat ("Marmande");

• colour: red, pink, orange or yellow;

• utilization: for fresh market (direct consumption) or processing (high dry matter content and viscosity).

In South-East Asia, many farmers still use local cultivars (landraces). For example in West Java, "Gondol" is a popular highland tomato, possibly derived from a very ancient import of "San Marzano" and reputed for a good taste and a certain tolerance of late blight and other diseases. This cultivar has been replaced by Taiwanese hybrids which, in spite of an inferior taste, have become popular because of the high yield capacity combined with resistance to transport damage. The Indonesian cultivars "Ratna", "Intan" and "Berlian", selected from AVRDC lines with bacterial wilt resistance, have found some acceptance at lower elevations. The Thai cultivars "Seeda" and "Seedathip" are well adapted to the rainy season. It may be expected that in the coming decade landraces and local open-pollinated cultivars will be replaced by F₁ hybrid cultivars.

Ecology

Ideally, tomato requires a relatively cool, dry climate for high yield and premium quality. However, it is adapted to a wide range of climatic conditions. Tomatoes have been grown as far north as the Arctic circle (under protection) down to the hot and humid equator. The optimum temperature range for growth and development is 21-24 °C. Prolonged exposure to temperatures below 12 °C can cause chilling injury. Mean temperatures above 27 °C severely impair growth and fruit set. Destruction of pollen and egg cells occurs when the maximum daytime temperature is 38 °C or above for 5-10 days. Fruit set is also generally poor if the night temperatures are above 21 °C during the few days before and after anthesis. Hot dry winds can also cause flower abortion. Light intensities below 1000 ft-candles retard plant growth and delay flowering. Tomatoes are not sensitive to daylength and set fruits in photoperiods ranging from 7-19 hours.

Tomatoes can be grown in many soil types ranging from sandy loam to clay-loam soils that are rich in organic matter. The ideal soil pH range is 6.0-6.5; higher or lower pHs can cause mineral deficiencies or toxicities. Long periods of flooding are detrimental to tomato growth and development.

Propagation and planting

Tomatoes can be direct-seeded or transplanted in the field. Relatively little seed-drilling or direct sowing into the field is practised in the humid tropics because of adverse growing conditions. In contrast, raising the young transplants in a special nursery enables growers to achieve great seedling uniformity and to check for early diseases and pests. Other advantages of transplanting are the smaller quantity of seed needed and reduced competition of weeds in the field. For raising transplants, 70-90 g of seeds are sown per 250 m² of seed-bed, which is sufficient to provide enough plants for one ha. Many farmers use banana leaf pots for plant raising. When direct-seeded, the sowing rate is about 500-1000 g of seeds per ha. Fertilizer at the rate of 40 g ammonium sulphate, 50 g superphosphate, 30 g potassium chloride and 2 kg compost per 1 m² of seed-bed area should be broadcast and worked into the seed-bed.

The young seedlings require sufficient water to sustain good, healthy growth. A week before transplanting, watering should be reduced to harden the seedlings. Three- to four-week-old seedlings (15-25 cm high with 3-5 true leaves) are ready for transplanting. Seedlings must be thoroughly watered 12-14 hours before they are lifted out of the seed-bed, to avoid excessive damage to the roots. Transplanting should be done in the afternoon or on a still, cloudy day to reduce the transplanting shock, and should be followed by watering. Spacing between plants and distance between rows depends on the cultivar's growth habit and whether the plants are to be supported by stakes or left to grow on the ground. Common configurations are plants spaced 30-60 cm apart in single rows on 1.0-1.4 m wide beds, while in some cases, a double-row bed system is used.

Husbandry

Fertilizers for tomatoes should be fairly rich in phosphorus. Excess nitrogen is associated with fruit puffiness and blossom-end rot and generally causes excessive vegetative growth. Amount and timing of fertilizer applications vary with soil types and cultivars. The following general recommendation (kg/ha basis) can be used as a guide: 60 N, 80 P₂O₅, 60 K₂O and 10 Borax for basal application; one week after transplanting for determinate, and 3 weeks after transplanting for indeterminate types, 60 N and 60 K₂O as side dressing; at 3-5 weeks after transplanting, another 60 N and 60 K₂O as side dressing.

Pruning the lateral shoots of staked indeterminate cultivars is often practised, to produce fruits of good and uniform size. Only one or two stems may be allowed to grow, depending on local practice. The number of fruits per cluster as well as the number of clusters may also be regulated. No pruning and regulation of fruit number and clusters are normally practised on determinate cultivars. Semi-determinate cultivars may be grown either as a determinate or indeterminate crop.

Tomatoes need adequate irrigation during the early plant growth, fruit set and fruit enlargement stages. About 2 cm of water per week is needed under cool conditions; about 7 cm during hot and dry periods. Consistency of water supply to the plants plays a major role in attaining uniform maturity. It also reduces the incidence of blossom-end rot, a physiological disorder normally attributed to calcium deficiency during fruit enlargement.

Competition with weeds, especially in the hot and humid tropics, can be very severe. To control weeds, a pre-emergence herbicide may be applied onto the field beds before transplanting, supplemented by manual weeding and mulching the beds with rice straw.

Diseases and pests

Tomatoes are attacked by many diseases and insect pests. Of about 60 pathogens that attack tomatoes, 15 are considered to be major diseases in the hot and humid tropics. Bacterial wilt (Pseudomonas solanacearum) has often been reported as the most serious handicap for tomato in the tropics. Bacterial spot (Xanthomonas campestris p.v. vesicatoria) is another serious disease. Long-term crop rotation is recommended to control bacterial wilt. There is no effective chemical control. Minimal infection has been reported when tomatoes are grown after lowland paddy rice. The most important control measures are a good drainage, a large dose of organic manure, and mulching in order to avoid rain damage to the roots. Bacterial spot is serious during the rainy season and is most noticeable on fruits but also causes damage to the foliage and stems. It is transmitted through the seed. Spraying with copper fungicides can control this disease fairly well except under heavy infection. Growing resistant cultivars is the best control method for both bacterial diseases but resistance is not universal owing to variable strains of the pathogens.

The most important fungal diseases of tomatoes in the tropics are early blight (Alternaria solani), black leaf mould (Pseudocercospora fuligena), late blight (Phytophthora infestans), leaf mould (Cladosporium fulvum, syn. Fulvia fulvum), powdery mildew (Leveillula taurica), southern blight (Sclerotium rolfsii) and target spot (Corynespora casiicola). Diseases such as Fusarium wilt (F. oxysporum) and Verticillium wilt (V. dahliae) are reported sporadically in the tropical highlands but they are not a problem elsewhere in the tropics. Some fungal diseases like late blight can cause 100% yield loss in the highland tropics where conditions are cool and moist. Spraying with chemicals such as maneb compounds can control most of the above fungal diseases in varying degrees. Field sanitation and proper rotation are also effective control measures. Planting resistant cultivars is the most effective and the cheapest control method, but resistance to late blight is not yet available. Local cultivars often are more or less tolerant of late blight.

Important virus diseases are tomato mosaic, cucumber mosaic, tomato yellow leaf curl, tomato yellow dwarf, and more recently also tomato spotted wilt virus. Depending on the virus, transmission is through direct contact and through insect vectors such as aphids, whitefly and thrips. Early control of the insect vectors and general field sanitation can serve well to control the virus diseases, but resistant cultivars, once available, will be the most effective control method.

Among the insects, the polyphagous tomato fruitworm (Heliothis armigera) is one of the most destructive, causing as high as 70% yield loss due to fruit boring. Synthetic pyrethroids sprayed at the rate of 50-100 g a.i. per ha can control this pest. Tomatoes should not be planted near other alternative hosts like maize and cotton.

Cotton aphid (Aphis gossypii) is a major pest during the dry season. It injures the plants by sucking the sap and by acting as a vector for the cucumber mosaic virus. Spraying with dimethoate and prothiophos is effective to control aphids. Several species of coccinelid beetles and syrphids also act as natural enemies, but they generally appear when the aphid population is already high. Whitefly (Bemisia tabaci) is a serious pest, not only because of its foraging on the tomato plants but also because it acts as a vector of the tomato yellow leaf curl virus. More recently thrips, especially Frankiniella occidentalis, has become a problem as it is the vector for tomato spotted wilt virus.

Root knot nematodes (Meloidogyne incognita and other species) invade the tomato roots and cause galling. Yield losses due to direct infection and indirect losses due to predisposition or breakdown of resistance to other root diseases, such as bacterial wilt, are significant. Nematicides and other soil fumigants can effectively control nematodes but are expensive. The use of resistant cultivars is still the most cost-effective measure, although breakdown of resistance can occur at high temperatures and there is no resistance to M. hapla.

Harvesting

Fresh-market tomatoes are often harvested at the mature-green stage and ripened in transit or in storage before they are marketed. Generally, tomatoes harvested at pre-ripe stages tend to have lower quality (i.e. lower soluble solids, ascorbic acid and reducing sugars) than vine-ripened tomatoes. The nature of the growth and ripening pattern of fresh-market tomato cultivars requires frequent pickings for either mature-green or vine-ripe fruits.

In contrast to the fresh-market or table tomatoes, processing tomatoes are picked fully ripe. In developed countries, harvesting is often by machine. Tomatoes used for pureed products such as soup, juice, and sauce, are left on the vine until over 85% of the fruits are ripe. Those for whole tomatoes are picked while still firm, but often only 65% of the crop may be ready to pick all at once. Growers sometimes spray the crop with ethephon to accelerate the rate of ripening, thereby increasing the percentage that can be harvested at one time.

Yield

The world's average tomato yield was 25 t/ha in 1989. This relatively high average yield is largely due to the extremely high tomato productivity under glasshouse culture in European countries (e.g. 420 t/ha in the Netherlands). Regionally, the average yields (t/ha) in 1989 were: Africa 18; North and Central America 37; South America 28; Asia 19; Europe 38; and Oceania 35. Average productivity in South-East Asia (Indonesia, Malaysia, the Philippines and Thailand) is still rather low at 8-12 t/ha. Seed yields are 100-150 kg/ha for hybrids and up to 300 kg/ha for open-pollinated cultivars.

Handling after harvest

After picking, tomatoes may be moved to a shady place either in the field or at home to prepare them for the market. Properly sorted and graded fruits generally command a better market price than ungraded fruits. The marketable fruits are then packaged in suitable containers, often 20-kg wooden boxes, bamboo baskets, plastic boxes, or other locally available packaging materials. Protection from injury is the chief benefit of proper packaging; it also reduces water loss although this is not generally a serious problem with tomatoes.

The storage life of tomatoes depends on the maturity stage at which they were harvested and on the desired quality of fruits. Quality is highest when completely ripe, whether artificially or on the vine. Ideally mature-green tomatoes should be stored for 7-10 days at 13-18 °C with 85-90% relative humidity so that they will ripen properly. Ethylene is sometimes used to rapidly and uniformly ripen mature-green tomatoes prior to shipping them to the market, but this adversely affects quality.

Colour is the single most important visual parameter of tomato quality. Lycopene development at temperatures above 30°C is generally poor. This is the main reason that tomatoes grown in the hot tropics tend to have a pale red or yellowish colour and are poorly flavoured.

Genetic resources

Many institutional collections of cultivated and wild Lycopersicon species exist throughout the world. Some of these collections have been well described, evaluated and documented for use by tomato scientists worldwide, the most important being the Tomato Genetic Stock Centre at the University of California, Davis, California (United States). A large collection is maintained at the Asian Vegetable Research and Development Center (AVRDC) in Taiwan. Since modern improved cultivars are rapidly replacing the old landraces, the latter should be collected for future breeding purposes.

Breeding

Tomato is one of the best-studied plant species, reflecting its great economic importance. Many important genetic traits have been discovered, evaluated and genetically localized in their respective chromosomes. Tomato has a prolifically marked genome, very useful in genetic and breeding research. Many useful traits have been incorporated by tomato breeders into modern-day cultivars, among which are high yield, disease resistance, improved quality for processing, and stress tolerance. Seeds of both standard (open-pollinated) and F₁hybrid cultivars are now available to tomato growers. New cultivars continue to be developed, offering improvements in one or more traits of interest. When tested in the South-East Asian countries the "international" hybrid cultivars often show disadvantages and weaknesses, which can only be amended by breeding work "in situ". Areas of continuing concern to tomato breeders in South-East Asia are disease resistance (bacterial wilt, Phytophthora and Alternaria), tolerance to environmental stress (such as high temperatures, excess soil moisture due to high rainfall and problem soils, e.g. acid or saline soils), quality (for fresh-market consumption and for processing), and long shelf life (for long distance transport and longevity in storage).

Prospects

Tomato scientists have accomplished a great deal in the past, including improvements in yield, disease resistance, adaptability to machine harvest, processing quality, tolerance of environmental stress, and others. However, more improvements need to be bred into modern-day cultivars. Fortunately, the vast reservoir of genetic variability in the genus Lycopersicon has been barely exploited. Conventional breeding techniques are still expected to be the mainstay of most future improvement programmes. However, the use of biotechnological techniques is rapidly gaining momentum and their impacts are potentially revolutionary. When integrated into existing plant breeding programmes, some of these techniques, such as DNA markers, allow plant breeders to access, transfer and combine genes at a rate, precision and genomic range never before possible with conventional breeding. Indeed, the prospects for further improving the tomato industry through advances in genetic and production techniques are bright.

Literature

• Atherton, J.G. & Rudich, J., 1986. The tomato crop. Chapman & Hall, London/New York, United Kingdon/United States. 661 pp.
• Divinagracia, C.N. & Villareal R.L., 1971. Tomato production in the Philippines. Department of Agricultural Communication, University of the Philippines, College of Agriculture. College, Laguna, the Philippines. 38 pp.
• Gould, W.A., 1983. Tomato production, processing and quality evaluation. Avi Publishing Company, Westport, Connecticut, United States. 445 pp.
• Kuo, C.G. & Lai, S.H., 1979. Suggested cultural practices for tomato. International Cooperator's Guide. AVRDC 79-127. Asian Vegetable Research and Development Center (AVRDC), Shanhua, Tainan, Taiwan. 3 pp.
• Opeña, R.T., 1985. Development of tomato and Chinese cabbage cultivars adapted to the hot, humid tropics. Acta Horticulturae 153: 421-436.
• Opeña, R.T., Kuo, G.C. & Yoon, J.Y., 1987. Breeding for stress tolerance under tropical conditions in tomato and heading Chinese cabbage. In: Chang, W.N., MacGregor, P.W. & Petersen, J.B. (Editors): Improved vegetable production in Asia. ASPAC-FFTC, Taiwan. pp. 88-109.
• Opeña, R.T., Green, S.K., Talekar, N.S. & Chen, J.T., 1990. Genetic improvement of tomato adaptability to the tropics. In: Green, S.K. & McLean, B.M. (Editors): Tomato and pepper production in the tropics. Asian Vegetable Research and Development Center (AVRDC), Shanhua, Tainan, Taiwan. 619 pp.
• Rick, C.M., 1978. The tomato. Scientific American 239: 76-87.
• Villareal, R.L., 1980. Tomatoes in the tropics. IADS Development-Oriented Literature Series. Westview Press, Boulder, Colorado, United States. 174 pp.
• World Information Center for Tomato Processing Industry, 1989. Tomato News. Bernard Bieche Consultant. Avignon Cedex, France. No 7. 23 pp.

Authors

• R.T. Opeña & H.A.M. van der Vossen