Vegetable Seed Production: Tomato

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Common Name: Tomato Scientific Name: Solanum lycopersicum Family Name: Solanaceae

In addition to reading the information in this section, please view this multimedia presentation on Tomato (Youtube link)

Botany

The tomato is a warm season (frost intolerant), perennial that is grown as an annual in temperate regions. Solanum lycopersicum is self pollinated. Flowers have yellow petals with fused anthers that form a pollen tube around the style and stigma. Flower movement releases pollen from the pollen tube so that it falls onto the stigma. The fruit of the tomato is a large berry. In cross section, there are two to twenty-five locules (chambers containing gel and seeds) with five to nine being the most common.

Structurally, the fruit consists of pericarp, placental tissue, and seeds. The pericarp has a well defined epidermal layer 3 to 4 cells thick with a relatively thick cuticle. Since the number of epidermal cells does not increase during growth, they stretch and become very large. The major part of the pericarp is made up of thin-walled cells which are large and become progressively softer and moister as the fruit ripens. There is a large placenta in the center, its shape depending on the number of locules. The placenta is made up of parenchyma cells which grow as the ovules develop until they completely surround the ovules. At first, the placental tissue surrounding the ovules is quite hard, but as the fruit ripens, the cells become very soft and tend to collapse leaving seeds embedded in a gelatinous tissue.

Soil Nutrition

Tomatoes can be grown on a wide range of soil types from sandy soils to fine textured clay soils. They are intolerant of waterlogged soil conditions and must have adequate drainage. Tomatoes grow well on slightly acid to moderately acid soil conditions (pH 5.5 - 6.8).

A crop rotation cycle of several years between planting members from the family Solanaceae is required if pathogen populations are very high and fumigation is not used. Members of this family should not be planted in the same field in subsequent years.

Isolation

The minimum isolation distance between different cultivars of tomatoes for seed production is relatively short. This is because of the crop's high level of self-pollination. Most countries specify a distance of between 30 and 200 m, the main consideration being that the distance should be sufficient to avoid admixture at harvesting time. The distances between two parental lines grown for the production of a specific F1 hybrid need be no more than 2 m, and less than this if there is a physical barrier such as under greenhouse conditions.

Planting

Tomatoes are direct seeded or established from transplants. Tomatoes for processing in California are either direct seeded or transplanted to improve earliness in the spring. Plasticulture is not used for processed tomato production because the plastic would interfere with the mechanical harvester. Processing tomatoes are often seeded in clumps of 3 seeds per hole with holes spaced 12 to 18 inches apart in a double row on both side of a standard 40 inch bed. The clump of seeds helps overcome soil crusting. No thinning is required because tomato plant spacing is very flexible. A few widely spaced plants will produce the same yield as plants spaced closer together. In the later case, the per plant yield is lower but the higher plant population compensates to produce the same yield as when fewer plants are used.

In other areas where processing tomatoes are grown, such as Ohio, direct seeding may be used for later season planting but significant acreages are grown from transplants. Transplants may be produced in greenhouses, grown under other types of shelters, or grown in outdoor seedbeds. Such production is a major enterprise for southeastern states such as Georgia. Many tomato transplants come from the Florida transplant industry.

The total annual production of transplants in Florida exceeds a billion plants per year. Transplants are generally set in double rows on top of 40 inch beds. The in-row spacing varies among cultivars but is generally from 12 to 24 inches.

Most tomatoes for fresh market are established from transplants.

Plant through plastic mulch is accomplished by punch planters or planters that burn holes in the plastic. Transplants are generally spaced from 18 to 24" apart depending on the cultivar used. For the string weave system stakes are placed after every second plant. String is wrapped around the stem of each plant and the stakes to provide support. Two or three levels of string are generally wrapped per season. For fresh market production, fruit size may be modified by plant spacing. In the fresh market plasticulture system, plastic mulch, stakes and drip tubing are removed at the end of each season.

Irrigation

Tomatoes have extensive deep root systems. In deep soils, tomato roots have been found 2 meters below the soil surface although most roots are found in the top 60 cm of soil. However, it is still recommended that water be applied consistently, particularly during fruit development. The recommendation of 1 inch per week definitely applies to this crop particularly on sandy soils. For fresh market tomatoes grown using plasticulture, drip irrigation is used. Drip irrigation, is also used with organic mulch as well. Drip irrigation provides the most uniform application of water with the least amount of water. Sprinkler irrigation is used for processing tomatoes grown in the midwest although it may lead to increased foliar disease problems. Furrow irrigation is predominately used in California.

Roguing

Plants showing characters adverse to the type to be removed.

1. Before flowering note desirable characters: growth habit and foliage typical of the cultivar; leaf characters, general habit, width. Observe if specific diseases present.
2. Early flowering and first fruit immature general plant habit and characters as defined for "before flowering". Immature fruit according to the cultivar description, e.g. greenback or greenback-free.
3. Fruiting yield characters, fruit quality, shape, color when ripe, relative size.

Seed Harvest

Seeds are extracted from ripe fruits which have either been hand picked into containers or collected by a single mechanized harvester which removes all the fruit from the crop in a single operation. The method of picking adopted locally depends on the scale of the operation and level of technology available in each seed production area. Fruit resulting from hybridization is always hand picked from the female lines whether it is produced in the field or under protection. This ensures that only the fruits which have been cross-pollinated are included. They are identified from either their label or calyx marking. This check, or confirmation, at the time of harvesting fruit is extremely important when hybrid seed is being produced in order to exclude fruit which contains seed produced by natural selfing of the female line.

Seed extraction of small quantities of fruit

The ripe fruits are cut equatorially and the seeds with the gelatinous material surrounding them are squeezed or spooned out into containers. During this process the main fruit walls, pulp, skin and other debris are excluded, The seeds and the gelatinous material are then separated by one of the processes described later.

Seed extraction of large quantities of fruit

The extraction of seeds from large quantities of tomato fruit can be divided into two main types of operation. Firstly, when the seed extraction is the only commercial operation involved, and secondly, when the seed is extracted as an additional product during the processing of tomatoes for purée or juice.

Commercial seed extraction

Here the word "commercial"is referring to the main purpose of the operation and not to the seed purity or class, as this method is frequently used for large-scale production of basic and certified seed as well as commercial seed. This system can be completely mechanized in that the fruit is harvested automatically before passing into a crusher. Alternatively, the fruit is hand picked and either put directly into the crusher as it slowly passes through the field at a speed in keeping with the rate of hand picking, or the picked fruit is transported to a stationary crusher in a seed cleaning yard or special area. The crusher squashes or crushes the fruit and the resulting mixture of gelatinous seed, juice and fruit residues is passed through a screen to separate off the gelatinous seed from the bulk of the remaining material. The crushed material is usually passed into a revolving cylindrical screen which allows the seeds and juice to pass through the mesh, while the fruit debris passes through the cylindrical screen to drop in the field. Alternatively, the debris is collected separately for later disposal if the operation is stationary. The juice and seed mixture is collected in separate containers. These machines are usually purpose built by, or for, the seed company or organization. The separated seed is then finally extracted from the gelatinous material and other materials by a separation process described later.

Combined juice and seed extraction

This system of seed extraction is done in cooperation with a tomato processing factory. The factory line is generally organized to produce purée or juice processed for domestic use. During the operation the purée or juice is separated from the relatively dry residual mixture of seeds, pulp and skins. Special lines of apparatus are used in processing plants which intend to secure the seed in this way. One important feature is that the seeds are not subjected to the high temperatures used during tomato processing, and a purpose designed modification of the normal processing plant is frequently referred to as a "cold takeoff". The apparatus is normally made and sold by the specialist manufacturers of industrial tomato processing equipment.

Close liaison must be maintained between the seed producing and the tomato processing organizations. One important feature is the need to have large batches of fruit of the same cultivar going through the factory and a thorough cleaning technique to avoid admixture of seeds from different cultivars. The system is used for the production of commercial quality seed for large-scale industrial tomato crops. It should not be used where seed of high genetical quality is required for further multiplication. Seed which has been extracted by this process has usually been separated from the mucilage during the industrial process and all that is required is washing to separate the seed from the other debris. When the tomato seed has been extracted from the fruit by one of the foregoing operations or systems, it is then usually necessary to separate the seed by a further wet-upgrading method referred to as "separation" before it can be finally washed and dried. The method of separation will depend on quantity of seed to be processed, possible need to control a specific seed-borne pathogen or virus, and the temperature of the local environment.

Separation by fermentation

The pulp containing the extracted tomato seed is left to ferment for up to three days at about 20-35 °C. But the rate of fermentation will depend on the ambient temperature and may even take up to five days. In warmer areas such as California the fermentation process is usually completed within twenty-four hours. Frequent inspections will determine when the seed's gelatinous coating has broken down. The mixture must be stirred several times a day to maintain a uniform rate of fermentation in the container and to avoid discoloration of the seed. It is usually necessary to cover the containers with muslin to reduce frit or fruit fly activity. The fermentation time must not be extended beyond that required for breaking down the mucilage or else the subsequent seed quality will be affected by premature germination. There are claims that the fermentation process controls seed-borne bacterial canker of tomato.

Separation with sodium carbonate

This method is relatively safe and can be used for small quantities of seed in cooler temperate areas where the fermentation method is not used. The pulp containing the extracted tomato seed is mixed with an equal volume of a 10 per cent solution of sodium carbonate (washing soda). The mixture is left for up to two days at room temperature after which time the seed is washed out in a sieve and subsequently dried. The sodium carbonate method of extraction tends to darken the testa of the seed and is therefore not normally used for commercial seed-lots, but it is used by plant breeders and other workers who are involved in maintaining breeding material and inbred parent lines.

Separation with hydrochloric acid

This method is often favored by large commercial producers as it produces a very bright clean seed sample. The actual dilution rate and duration of treatment used by the major commercial seed producers is usually a closely guarded secret. The hydrochloric acid treatment is often combined with the later stages of fermentation. However, producers of relatively small quantities of tomato seed find that 567 ml of concentrated hydrochloric acid stirred into 10 liters of seed and pulp mixture and left for half an hour is successful. It is very important that the acid is added to the water and pulp, and not the water and pulp to the acid, otherwise a dangerous effervescence will occur. MI workers handling the concentrated and diluted acid solutions must wear appropriate face shields and protective clothing. The hydrochloric acid is damaging to seeds of the compact ("shortjointed") cultivars which are used by some specialized growers in Britain and therefore it should not be used when producing seed of these types.

Cleaning

Tomato seeds which have been extracted by fermentation or acid treatment are washed immediately the extraction time has been completed- This can be done on a small scale by washing in a series of sieves. Large quantities of seed extracted from field crops are usually washed in long water troughs with a fall of 1 in 50.

The trough has riffles at intervals and works on the principle that the seed is denser and sinks while the other fruit debris is floated off or goes through the water trough in suspension. The skill of doing this operation comes with experience and it is always a safeguard to have a suitably sized screen over the wastewater drain in ease of mishaps.

Seed Yield

Figures quoted for tomato seed yield vary significantly from one production area to another and it is not possible to quote an average seed yield.

The differences will vary according to several important features which include:

Different reports quote two main ways of estimating tomato seed yield: (a) according to weight of fruit; (b) according to unit area of plants. In greenhouse production 1 kg of fruit will produce approximately 4 g of seed (approximately 1200 seeds). In field production one rule of thumb is that the seed weight is 1 per cent of the fruit weight, e.g. 1 ton of fruit will produce 1 kg of seed. The expected tomato seed yield in the USA is between 250 and 400 kg of seed per hectare. Workers in Africa report yields from 10 to 50 kg per hectare. The greenhouse tomato cultivars with a 1000 grain weight of 3.3 g tend to have larger seeds than the field or determinate types. The field or determinate types have a 1000 grain weight of 2.5 g. The multiplication factor is not a very reliable method of calculating seed yield in tomatoes because of the points outlined above and the fact that plants are raised in nursery beds before transplanting. However, a multiplication factor of 200 can be used for the indeterminate types, while the determinate types under field conditions have a multiplication factor of 50.

Please view this informative narrated video on the basic aspects of tomato biology and cultivars. (The video will open in a new page.)

Seed Identification: