Tomato Health Management

CHAPTER 1

M. Allen Stevens Department of Vegetable Crops University of California, Davis

Introduction family (Solanaceae), which has many poisonous species, doubts about the edibility of tomato lingered and its acceptance as a safe food did not occur immediately in the colonies.

The ever-­innovative Thomas Jefferson brought tomatoes to his table from Europe and is credited with a role in their acceptance as food. In 1820, a Colonel Johnson supposedly ate a bushel of tomatoes on the steps of the Boston courthouse to prove the safety of the crop.

In the United States, debate developed over whether the tomato is a fruit or a vegetable, and a legal battle on this issue went all the way to the U.S. Supreme Court. In 1893, the Court ruled that “botanically speaking, tomatoes are fruits

The tomato likely originated along the central west coast of South America in the area of Ecuador, Peru, and northern Chile, the region that clearly represents its center of diversity.

Wild tomato species grow in a wide range of habitats, from sea level to high in the Andes Mountains (Figs. 1.1 and 1.2).

Spanish conquistadors probably first consumed the tomato in its area of origin and introduced it into Central America and Europe. The first domesticated tomato, which was grown in Mexico, was probably the semiwild, small-­fruited variety Solanum lycopersicum var. cerasiforme.

Some time later, the tomato was introduced as an ornamental into North America, where it was valued for “pustule-­ removing properties.” Because it belongs to the nightshade

Fig. 1.2. Solanum arcanum (formerly Lyco­ persicon peruvianum var. humifusum) grow­ ing in a rock crevice at Puente Muyuno, Rio Jequetepeque, Cajamarca, Peru. (Courtesy Charles M. Rick Center)

Fig. 1.1. Solanum lycopersicoides growing at an elevation above 11,800 feet (3,600 m) near Putre, Chile. (Courtesy Charles M. Rick Center)

1

2 • Chapter 1 of a vine, just as are cucumbers, squashes, … but in the common language of the people, … these are vegetables, ... usually served at dinner in, with or after the soup, fish or meats which constitute the principal part of the repast, and not, like fruits generally, as dessert.”

Although many cultivars are adapted to Mediterranean growing conditions, today the crop is grown in many areas of the world.

Wild Tomato Species On the basis of compelling DNA sequence evidence, the species name for tomato has been recently changed to Solanum lycopersicum, although many horticulturists still use the name Lycopersicon esculentum. There are now 13 recognized species in the genus Solanum, which replace the previously recognized nine species of Lycopersicon (Fig. 1.3). The 13 species can be divided into those that can be easily crossed with the cultivated tomato and those that can be crossed with the cultivated species only with considerable difficulty (Fig. 1.4).

There are now 13 recognized species in the genus Solanum, which replace the previously recognized nine species of Lycopersicon.

The easily crossed S. lycopersicum complex consists of seven species. S. lycopersicum var. cerasiforme was probably the first domesticated tomato species and is the likely progenitor of the cultivated tomato. Its fruit are about the size of a cherry.

Because some accessions are adapted to wet and humid growing conditions, this species has been used as a source of resistance to diseases that occur in humid areas.

S. pimpinellifolium, the currant tomato, hybridizes with S. lycopersicum under natural conditions. This species has been

Fig. 1.3. Leaves, flowers, and fruit of cultivated tomato (Solanum lycopersicum, center) and several of its wild relatives in Solanum sect.

Lycopersicon (formerly genus Lycopersicon). Clockwise from top left: S. pennellii, S. chilense, S. peruvianum, S. chmielewskii, S. galapa­ gense, S. pimpinellifolium, and S. habrochaites. (Courtesy Charles M. Rick Center)

an important genetic resource for tomato, providing resistance genes to such diseases as Fusarium wilt (Fusarium oxysporum f. sp. lycopersici races 1 and 2), gray leaf spot (Stemphylium solani), leaf mold (Fulvia fulva), bacterial wilt (Ralstonia solanacearum), bacterial canker (Clavibacter michiganensis subsp. michiganensis), and bacterial speck (Pseudomonas syringae pv. tomato).

S. cheesemaniae, a red-­fruited species, is widely distributed in the Galapagos Islands (Fig. 1.5). Despite being closely related to tomato, it has not been an important genetic resource. There was hope that the salt tolerance of this species could be transferred to tomato, but a commercially successful salt-­tolerant cultivar has not been developed.

S. neorickii and S. chmielewskii are closely related green-­ fruited species found mostly in the Andean region of Peru.

Both can be hybridized with tomato with minimal difficulty.

S. chmielewskii accessions with high fruit solids (10–11%) have been crossed with tomato in an attempt to increase fruit solids, but yields of the progeny were reduced. It is an important axiom that any character adversely affecting yield, even to a small degree, has little commercial potential; generally, growers are not compensated for improved quality.

S. habrochaites is a green-­fruited species found at elevations above 10,000 feet (about 3,000 m). Certain wild populations of S. habrochaites are apparently quite resistant to insect pests.

The two forms of this species differ in their ability to hybridize with tomato. S. habrochaites f. typicum is unilaterally incompatible; seed can be obtained only by using tomato as the female. S. habrochaites f. glabratum is reciprocally compatible with tomato. Resistance to red spider mites (Tetranychus cinnabarinus), lepidopterous larvae, and aphids (Myzus persicae) results from a natural insecticide (2-­tridecanone) found at high levels in the glandular hairs of insect-­resistant accessions. The possibility of increasing cold tolerance in tomato by using high-­a ltitude, cold-­tolerant accessions has also been studied. Introgression of both insect resistance and cold tolerance into a commercially acceptable cultivar has been difficult. Resistance to corky root (Pyrenochaeta lycopersici) was introgressed into tomato from this species.

S. pennellii is a green-­fruited species found in hot, dry areas of midaltitude river valleys of Peru. It is easily hybridized with tomato, and the resulting hybrid is easily backcrossed to the cultivated species. There has been interest in using this species as a source of drought tolerance.

Major barriers to hybridization distinguish the two S. peruvianum complex species (S. peruvianum and S. chilense) from the S. lycopersicum complex. Hybridization between the two complexes is not successful without extraordinary efforts, such as embryo rescue. Notwithstanding these difficulties, S. chilense (Fig. 1.6) and S. peruvianum have been the sources of some important traits present in commercial cultivars. S. chilense and particularly S. peruvianum have great genetic variability. One accession of S. peruvianum has more variability than the entire species of S. lycopersicum. Resistance to Tomato yellow leaf curl virus was introgressed from S. chilense, and nematode resistance was successfully introgressed from S. peruvianum. Many successful cultivars carry this nematode resistance.

A brief consideration of the transfer of nematode resistance illustrates the difficulty encountered when a cultivar with a trait available in a widely separated wild species is bred. The resistance in S. peruvianum was discovered in 1941, and the difficult cross with tomato was made shortly there-

Introduction • 3

Fig. 1.4. Representation of crossing relationships among Solanum species. Widths of lines indicate degrees of crossability. Dashed lines indicate cross failures. Dotted lines show where combinations yield progeny only via embryo culture. (Adapted from Rick, 1979; Courtesy Charles M. Rick Center)

Fig. 1.5. Solanum cheesmaniae growing near the shoreline on Bar­tolome Island, Galapagos, Ecuador. (Courtesy John Rick)

after by using embryo rescue. It took almost 40 years of effort before a successful cultivar with resistance to root-­k not nematodes was developed. This extreme time lag was caused by linkage drag from undesirable traits closely linked to the nematode resistance gene (Mi) and the difficulty in developing a reliable procedure for screening for resistance. Repeated backcrossing and use of a tightly linked marker, an acid phosphatase isozyme, eventually narrowed the linkage drag to a manageable level.

Fig. 1.6. Solanum chilense growing at Que­brada Salsipuedes, Arequipa, Peru. (Courtesy Charles M. Rick Center)

4 • Chapter 1 The most successful nematode-­resistant cultivars are hybrids in which the remaining undesirable traits are masked.

The time from a difficult cross to a successful cultivar has been greatly reduced by using molecular techniques that allow tagging of the desired gene and rapid elimination of undesirable genes from wild species.

Breeding and Genetics Cultivar Development Prior to about 1925, cultivar improvement was accomplished by selection within existing cultivars that were heterogeneous or by selecting variation that resulted from mutation, low levels of outcrossing, and recombination. Early cultivar development through hybridization was done almost entirely to improve disease resistance. Plant pathologists were the early hybridizers and led efforts to improve disease resistance. Some were overly optimistic regarding the potential of increased disease resistance. They found that growers were seldom interested in a new cultivar simply because it had improved disease resistance. Even with good disease resistance, a recently developed cultivar had to be as good as existing cultivars with respect to horticultural characteristics.

Early cultivar development through hybridization was done almost entirely to improve disease resistance.

Until the 1970s, most tomato breeding was done at public institutions. Private seed companies made selections within publicly developed, open-­pollinated cultivars to obtain proprietary strains, which were different mostly in name only.

A major exception to this generalization was the processing tomato-­ breeding programs of major food processors.

In the mid-­1970s, seed companies increased their tomato-­ breeding efforts as university programs shifted their atten-

Fig. 1.7. Before development of the mechanical harvester in the late 1950s, all tomatoes were picked by hand. (Courtesy Uni­versity of California, Davis, Department of Biological and Agricul­ tural Engineering)

tion to molecular and cellular biology. By 1990, most public cultivar-­development efforts had disappeared. Today, a few institutions continue germplasm development; these programs often use molecular techniques to facilitate introgression of traits from wild species.

As private investment in tomato breeding escalated, there was increased effort to protect proprietary developments, principally through hybrid development in which the inbred parents were treated as trade secrets. There was some use of plant variety protection, but most seed companies consider this a secondary level of protection. Use of plant variety protection was not widespread because a protected cultivar still could be used in the breeding programs of a competitor.

Efforts to demonstrate hybrid vigor in tomato have not succeeded. However, it does seem clear that there are distinct advantages to hybrids. One advantage is the masking of linkage drag for traits introgressed from wild species; a second advantage is consistency in the breeding process. However, if growing conditions are optimal, an excellent open-­pollinated cultivar will yield as well as a high-­y ielding hybrid. But when weather or growing conditions are less than ideal, a good hybrid with diverse inbred parents will outperform the open-­ pollinated cultivar. This relationship is best illustrated by increases in statewide yields of processing tomatoes in California as the use of hybrids increased.

Most breeding advancements have represented stepwise improvements in cultivar performance, as breeders have continued to improve on their predecessors’ efforts. However, some breeding programs have represented dramatic changes and had a transforming effect on the tomato industry. Perhaps the most striking was the development of a cultivar suitable for mechanical harvest.

When G. C. “Jack” Hanna started the efforts for such a cultivar in the mid-­1940s, no mechanical tomato harvesters were available (Fig. 1.7). He recognized two crucial characteristics of a cultivar suitable for machine harvest: greater fruit firmness to withstand abuse by the machine and concentrated fruit maturity. As Hanna proceeded with the breeding effort, he encountered and eventually overcame

Fig. 1.8. An early prototypic processing tomato harvester (1959).

Jack Hanna (foreground) and Coby Lorenzen (third from left), of the University of California, were pioneers in the development of harvesting equipment. (Courtesy University of California, Davis, Department of Biological and Agricultural Engineering)

Introduction • 5

many unexpected problems. By 1960, he was field-­testing cultivars that eventually revolutionized the industry. Earlier, he had started a collaborative effort with a colleague at the University of California, Davis, the agricultural engineer Coby Lorenzen, which resulted in a mechanical harvester (Figs. 1.8 and 1.9).

In the early 1960s, the cultivar VF 145 was released. This cultivar dominated processing tomato production in California for more than a decade, and its release resulted in a significant shift in processing tomato production in the United States. Growers in California, where the climate is favorable for tomato culture, quickly adopted mechanical harvesting, and large farms with the economical advantage of size soon dominated processing tomato production in the nation (Fig. 1.10). Currently, about 95% of U.S. processing tomatoes are produced in California. Relative to VF 145, modern processing cultivars are high yielding, adaptable, and resistant to many diseases, and they possess desirable quality and firmness characteristics.

There have also been dramatic improvements in fresh-­ market tomato cultivars for field and greenhouse production.

Cultivars for field production have been improved for disease resistance, high yields, and fruit firmness and smoothness.

Cultivars for greenhouse production have greater disease resistance, greater tolerance to cool growing conditions and low light (reducing production costs), and greater fruit firmness.

There also has been continued development of new types, such as cluster and truss tomatoes, which have been popular with consumers.

Hundreds of cultivars are used worldwide, and no single cultivar or group of cultivars dominates worldwide production.

The commercial life of modern tomato cultivars is usually short. Tomato breeders in seed companies have intensified their efforts as a result of rapid advancements in cultivar development. Hundreds of cultivars are used worldwide, and

Fig. 1.9. The harvesting of processing tomatoes changed dramat­ ically following introduction of new cultivars and machines in the early 1960s. (Courtesy University of California, Davis, Department of Biological and Agricultural Engineering)

no single cultivar or group of cultivars dominates worldwide production. Those that are important for a particular use in a growing area are usually replaced within a few years.

Transgenic Cultivars The first transgenic cultivar to be commercialized was a tomato with a long shelf life. It was developed as a joint effort between Calgene, a biotechnology company, and the Campbell Soup Company. Campbell explored the possibility of entering into the fresh-­produce business and decided to develop a cultivar that could be picked ripe but did not rot during handling and marketing. Researchers hypothesized that if the activity of the enzyme polygalacturonase (PG) could be modulated by using antisense DNA technology, the result might be extended shelf life.

Calgene successfully developed the technology, and it was introduced into fresh-­market and processing cultivars.

However, the low-­PG tomato did not produce the desired results. Campbell eventually decided to withdraw from the project. Calgene continued to market the “Flavr Savr” tomato developed with this technology, and Zeneca did the same for processing cultivars. Both efforts were commercial failures.

Genetics and Cytogenetics Tomato is a model for studies of genetic mutants and cytogenetics. In the 1960s and 1970s, hundreds of monogenic mutants were identified, characterized, and mapped with classical genetic techniques. These results were accomplished as a joint effort through the Tomato Genetics Cooperative.

Concomitantly, there was great progress with cytogenetics research. Complete sets of primary trisomics were developed, including telotrisomics, which facilitated mapping of genes to specific chromosome arms. Comprehensive lineage maps of mutants were developed in these efforts.

Cellular Biology Early in the biotechnology revolution, it was discovered that tomato is easily transformed by using Agrobacterium tumefaciens and can be regenerated from transformed single cell cultures. As a result, tomato was a model species for early recombinant DNA research.

Fig. 1.10. A modern harvester of processing tomatoes. A five-person crew can harvest 25 tons (22.7 metric tons) in 40 minutes. (Courtesy Gene Miyao)

6 • Chapter 1 Molecular Biology Tomato is infamous for the lack of polymorphism among cultivar genomes, no doubt because of restrictive genetic bottlenecks during domestication. The introduction of traits from wild species has increased the genetic variability of tomato.

In the 1970s, there was some success using protein polymorphism (isozymes) to tag a few genes. The most notable was a linkage between the Mi gene for nematode resistance from L. peruvianum and a variant of acid phosphatase. However, the level of protein polymorphism was restricted and of limited utility.

As techniques for sequencing DNA improved, the capability to tag genes and differentiate genotypes was greatly enhanced. In the 1990s, genomic linkage maps of random amplified polymorphic DNA (RAPD) were created. A study sponsored by the seed industry demonstrated that it was possible to differentiate cultivars and determine their genetic relationship by using RAPD. As a result of these technologies,

there has been considerable progress in the identification and characterization of important genes that affect plant habit and fruit shape, size, and ripening, along with numerous other quality traits.

Production In 2007, worldwide production of tomato was about 126 million metric tons, and five countries accounted for 80% of production (Fig. 1.11). China was the largest producer, at 39% of the total, followed by the United States, Turkey, India, Italy, Iran, Spain, Brazil, and Mexico. China’s rise as the prominent producer of tomatoes is illustrated in Figures 1.12 and 1.13.

Consumption of tomatoes as processed products far exceeds consumption of fresh-­fruit tomatoes. For example, in the United States, per capita consumption of processed tomato fruit is about four times greater than consumption of fresh fruit: 70 pounds (32 kg) versus 18 pounds (8 kg). Most processed tomatoes are initially manufactured into paste, with 20–40% of the water removed, and then remanufactured into juice, soup, catsup, and other sauces. Paste manufacture is an essential part of tomato processing, since it reduces shipping costs for tomato solids and because high-­solids paste has greater biological stability than low-­solids products.

Composition and Quality

Fig. 1.11. In 2007, five countries accounted for 80% of the world production of tomatoes. (Based on data from FAOSTAT)

Tomatoes make a significant contribution to the nutritional well-­being of humans, since consumption of the crop is relatively high. The principal nutrients are vitamin C (ascorbic acid) and provitamin A (β-­carotene). It has been reported that consumption of the principal pigment, lycopene, may reduce the risk of prostate cancer. Processed tomato products have higher levels of bioavailable lycopene than fresh

Fig. 1.12. Production trends of the five largest tomato-producing countries: 1987–2007. (Based on data from FAOSTAT)

Introduction • 7

tomatoes, since the availability of hydrophobic carotenoid increases after heat processing. Both lycopene and β-­carotene are antioxidants, important components of a healthy diet. The composition of the tomato fruit and important fruit-­quality components are shown in Tables 1.1 and 1.2.

There have been frequent complaints about the quality (especially the flavor) of modern fresh tomatoes. Handling and transporting fruit contribute significantly to this problem. To prevent serious losses during shipping and sales of fruit that are harvested ripe or nearly ripe, most fresh-­market tomatoes produced in the field are picked before they are ripe.

To lower costs, producers reduce the number of harvests,

which results in a significant proportion of immature fruit in a single harvest. Picking fruit before they start to ripen can have an adverse effect on the flavor when they finally ripen.

Another potential contributor to the quality problem can be refrigeration. Tomatoes are sensitive to chilling, and when fruit are stored at temperatures less than 50°F (10°C), flavor is adversely affected.

The rapid increase in the production and consumption of greenhouse tomatoes is likely fueled by the lack of satisfaction with field-­ grown fresh tomatoes. Greenhouse-­grown tomatoes are generally picked with color and have improved handling, since they are a higher-­priced product.

Fig. 1.13. Volume of tomato exports by China. Much of the increase in Chinese production occurred in the northwestern province of Xinjiang. (Based on data from FAOSTAT)

Table 1.1. Approximate composition of ripe fruit of a typical tomato cultivar (per 100 g fresh weight)a Component

Amount

Component

Amount

Total solids Total carbohydrates Sugars Glucose Fructose Sucrose Polysaccharides Cellulose, hemicellulose Pectic compounds Protein Amino acids Gamma aminobutyric Asparagine Aspartic Glutamic Glutamine Serine

6.0 3.9 2.9 1.3 1.5 100.0 1.0 600.0 400.0 300.0 350.0 70.0 10.0 45.0 100.0 60.0 10.0

Ash Calcium Chloride Magnesium Phosphorus Potassium Sodium Organic acids Citrate Malate Lipids Ascorbic acid Carotenoids Lycopene β-carotene Other compounds

500.0 15.0 40.0 10.0 25.0 250.0 5.0 800.0 600.0 200.0 100.0 23.0 4.8 4.5 150.0 20.0

a

g g g g g mg g mg mg mg mg mg mg mg mg mg mg

Adapted from Stevens and Scott, 1988.

mg mg mg mg mg mg mg mg mg mg mg mg mg mg µg mg

8 • Chapter 1 Table 1.2. Range of concentrations of important quality components in ripe fruits of typical tomato cultivars and their contribution to quality a Fruit Component Total solids Sugars Glucose Fructose Alcohol insoluble solids Polygalacturonides Polysaccharides Ash Potassium Phosphorus Calcium Acids Citrate Malate Carotenoids Lycopene β-carotene Volatiles, several hundred

a

Proximate Range 45.0–65.0 mg/g 20.0–37.0 mg/g 7.0–17.0 mg/g 11.0–20.0 mg/g 7.0–25.0 mg/g 5.0–7.0 mg/g

4.5–8.5 mg/g 4.0–7.5 mg/g 0.5–1.5 mg/g 40.0–65.0 µg/g 35.0–60.0 µg/g 3.0–8.0 µg/g 10.0 µg/g

Contribution to Q uality Flavor

Firmness (texture) and shelf life in fresh fruit Consistency (thickness) of processed product Buffering and acid level Buffer Firmness Flavor and processing safety Color, nutritional value Unique flavor

Adapted from Stevens and Scott, 1988.

Growth and Development Plant Growth Tomato vegetative growth is broadly classified as determinate or indeterminate (Fig. 1.14). Determinate cultivars have a restricted flowering period, and growth terminates with flowering. They are used where a concentrated fruit set is desired. All processing cultivars and many open-­field fresh-­ market cultivars have a determinate growth habit. Indeterminate cultivars have nonrestricted flowering and are commonly used for greenhouse production, where a long harvest period

Fig. 1.14. Indeterminate (left) and determinate (right) tomato plants.

is desired. There are large intercultivar growth habit differences within determinate and indeterminate types.

Flowers Tomato flowers are perfect, hypogynous, and regular. Six or more sepals and petals on each flower are common. Prior to anthesis, the corolla is enclosed within the calyx. At anthesis, the calyx separates to expose the petals, which soon turn from pale yellow-­green to yellow. At full opening, the petals are reflexed, exposing the anther cone, which surrounds the style and stigma (Figs. 1.15 and 1.16).

Tomatoes are self-­pollinating. Although numerous male sterile mutants are known, they have not been widely used for hybrid seed production. Lack of attractiveness to pollinators and leaky male sterility contribute to this lack of success.

Pollen is mature at anthesis. The stigma is receptive to pollen a few days prior to anthesis and remains so for about 4 days postanthesis. In open-­field production, pollination occurs naturally. However, in a greenhouse, where the flowers

Fig. 1.15. Tomato flowers with inserted (left) and exerted (right) stigmas.

Introduction • 9

entirely the result of cell enlargement, which provides most fruit growth.

Ripening

Fig. 1.16. Tomato flower and developing fruit. (Courtesy Gene Miyao)

are not shaken by wind or insects, they must be shaken to ensure pollination. Pollen production and dehiscence are adversely affected by temperatures greater than 104°F (40°C), a key factor in poor fruit set during warm weather. Because the position of the stigma within the anther cone is an important factor in fruit set, cultivars with a short style generally set more fruit.

Tomato ripening involves dramatic and rapid changes in color, flavor, and texture and involves both synthetic and degradative reactions. Changes include the breakdown of starch and the production of glucose and fructose, the degradation of chlorophyll, and the production of carotenoids.

Changes in cell walls include a decrease in water-­insoluble polysaccharides and an increase in soluble pectin and simple sugars. A decrease in total acidity with an increase in the citric acid–malic acid ratio also occurs. The volatiles that give tomatoes their distinctive flavor and aroma are produced during ripening.

Several mutations affect tomato fruit ripening. The two most comprehensively studied are rin (ripening inhibitor), which retards ripening, and nor (nonripening), which prevents ripening. Israeli hybrids with rin genes ripen slowly and have extended shelf life. These cultivars have been used extensively in certain markets.

Horticulture

Fruit

Propagation

Tomato fruit consist of epidermal, pericarp, and locular tissue. The outer and inner pericarp is the fleshy part of the fruit; the locular tissues contain gel and seeds. The ratio of pericarp to locular tissues varies greatly among cultivars (Fig. 1.17). Firm-­ fruited cultivars, which are essential for mechanical harvest, have thicker walls and a greater proportion of pericarp tissue. These characteristics result in processed products with greater consistency, because the pericarp tissue is higher in insoluble solids. It is generally believed that fruit with a greater proportion of locular tissue have better flavor, since this tissue has higher levels of citric and malic acids.

Depending on tomato genotype and temperature, 7–9 weeks is required for a fertilized ovule to develop into a ripe fruit, but intercultivar fruit growth rates vary greatly. For example, development time is essentially the same for a cherry tomato (approximately 0.5 ounce [15 g]) and a beefsteak tomato (more than 14.0 ounces [400 g]). Early, slow fruit growth results from cell division. Later, rapid fruit growth is

The tomato plant is grown as an annual, although it can be a perennial. Virtually all commercial tomato cultivars are hybrids. Hybrid seed is produced mostly by hand emasculation and pollination in countries where labor costs are relatively low. Tomato is self-­pollinating, and hybrid seed production takes advantage of stigma receptivity about 2 days prior to anthesis. A common practice is to emasculate closed florets that are approaching full size and then return a day later to pollinate. Seeds are extracted from ripe fruit and fermented to degrade the gelatinous coat. The seeds are then washed and dried at low temperatures.

Fig. 1.17. As tomato cultivars with increased firmness were devel­ oped, dramatic changes occurred in fruit structure and compo­sition.

Firm cultivars generally have lower acid content, lower sugar content, and greater insoluble solids content.

Virtually all commercial tomato cultivars are hybrids.

Tomatoes are best adapted to a Mediterranean climate, where conditions are relatively warm during the day with substantial diurnal fluctuations. Day temperatures greater than 104°F (40°C) or a combination of high day and high night temperatures adversely affect fruit set. Tomato is adversely affected when the temperature drops below 50°F (10°C), and damage occurs at 43°F (6°C).

The time required for development of the crop after planting depends primarily on temperature. Average temperature (heat units) can be used to estimate time from planting to harvest.

Greenhouse Production There have been dramatic increases in greenhouse production in recent years. Europe, especially the Netherlands, has been the leader in development of technologies and

10 • Chapter 1 techniques for greenhouse production of tomatoes. The cost of producing a crop in a controlled environment is high, and it is essential that the resulting product be of superior quality.

Production of Processing Tomatoes In California, all tomatoes grown for processing are produced in open fields on raised beds, where water is commonly applied via furrow irrigation. In the Midwest, water requirements are generally met by ambient rainfall, with little or no supplemental irrigation. Sprinkler irrigation may be used to promote stand establishment and during early plant growth but can promote certain foliar diseases. Drip irrigation is sometimes used where irrigation water is limited. Water-­use efficiency can be at least 200% higher with drip than with furrow irrigation.

Seeding rates vary but can be as low as 3.5 ounces (0.1 kg), or about 28,000 seeds, per acre with higher-­priced hybrid seed. To obtain an adequate stand, cultural practices during germination must be optimized. At the third to fourth true-­ leaf stage, the plants are mechanically thinned to a spacing of about 8 inches (20 cm), or about 14,000 plants per acre.

Cultivars with small, compact vines may be planted in double rows on a bed. As long as plants are less than approximately 24 inches (60 cm) apart, final yield is not affected. However, wider plant spacing may reduce fruit ripening, which is a disadvantage for mechanical harvesting.

The use of transplants to establish a stand of processing tomatoes has increased dramatically in the past decade, despite the greater cost involved compared with direct seeding.

In 2008, more than 70% of processing tomatoes in California were produced by using transplants. Stand establishment is usually more assured when transplants are used and certain other costs, such as weed control, are lower. Growers often use fewer than 7,000 transplants per acre. Yields are generally similar to direct-­seeded tomatoes that have at least twice as many plants per acre.

The use of transplants to establish a stand of processing tomatoes has increased dramatically in the past decade, despite the greater cost involved compared with direct seeding.

Field Fresh-­Market Tomato Production Stands of fresh-­ market tomatoes are almost all transplanted, and a wide range of growing practices are used.

Raised beds with plastic mulch and drip irrigation are common. In some production areas, determinate cultivars are used without the need to train the plants on staked trellises.

This practice is more common in areas that are well suited to tomato production and when production costs must be minimized. The use of indeterminate cultivars and, in some cases, large determinate cultivars that require staking or trellising is more common for out-­of-­season production—­for example, in winter production in Florida and Mexico, which are major

sources of fresh tomatoes for the United States at that time of the year.

Nutrition Tomatoes are tolerant of a wide range of nutrient regimes, but high yields of high-­quality fruit require good nutritional management. Generally, yields are highest at moderate nitrogen levels. Excessive nitrogen causes plants to become vegetative at the expense of flower production.

An adequate phosphorus level is particularly crucial to early plant growth, when root systems are small. In some soils, phosphorus fertilization is not needed but is still commonly used. Excessive phosphorus may increase certain physiological disorders, such as uneven ripening and puffiness.

High yields can be obtained with moderate levels of potassium. However, optimal levels of potassium are needed to reduce fruit puffiness, uneven ripening, and off flavors caused by low acid levels. Optimal levels of calcium reduce the incidence of blossom-­end rot and increase fruit firmness.

Irrigation Four major types of irrigation are practiced in open-­field production: furrow, sprinkler, seepage, and drip. Furrow irrigation is popular where ample irrigation water is available, but it is the least efficient of the irrigation methods. Uniform water application is more easily accomplished with sprinklers than with furrows. Sprinklers are used most often only for stand establishment, since their continued use increases the possibility of the development of certain diseases later in the season. Seepage irrigation is based on manipulation of the water table. A series of canals and ditches allows growers to raise or lower subsurface water to maintain proper watering of crops. This method is particularly common in Florida. Drip irrigation is the most efficient method, and uniformity of application is more easily achieved. The highest tomato yields have generally been achieved by using drip irrigation; however, the costs of establishing a drip irrigation system are high.

Ripening Hormones The use of ethylene and ethylene-­releasing compounds (e.g., ethephon) is common in tomato production and marketing. Growers of processing tomatoes sometimes apply ethephon to a tomato field to speed ripening. If fresh-­market tomatoes are picked before they start to ripen, ripening may be initiated by gassing them with ethylene after field harvesting. Storage life after picking can be managed to some extent with timing of ethylene application.

Cooling and refrigeration of fresh-­market tomatoes to extend shelf life and reduce losses must be carefully managed.

Temperatures below 50°F (10°C) adversely affect ripening and flavor development.

Selected References Adams, P. 1986. Mineral nutrition. Pages 281-­334 in: The Tomato Crop. J. G. Atherton and J. Rudich, eds. Chapman and Hall, New York.

Besford, R. T., and Maw, G. A. 1975. Effect of potassium nutrition on tomato plant growth and fruit development. Plant Soil 42:395-­412.