The Solanaceae or nightshade family is a family of 90 genera and at least 2,600 species. This includes five of the most important agricultural crops in the world, tomato (Solanum lycopersicum), potato (S. tuberosum), sweet and chile peppers (Capsicum spp.), eggplant (S. melogena), and tobacco (Nicotiana tabacum), as well as a number of minor crops that are important as ornamentals or as sources of medicine. A number of members of the Solanaceae family have psychoactive alkaloids that have sedative properties in mammalian systems. The etymology of Solanum is thought to be the Latin word solari, which means “to soothe.” Many of the alkaloids in members of this family are also toxic to varying degrees, and some species like jimsonweed (Datura stramonium) or deadly nightshade (S. nigrum) can be lethal. The common name for the family, the nightshades, may be derived from the fact that many of its species are night blooming, with blossoms that only begin to open at sunset.
The incredibly quick ascent of at least four of the five most important crop plants of this family to prominence worldwide is a testament to how quickly humans and their agricultural systems can adapt to newly introduced crops. Tomato, potato, chile pepper, and tobacco are all New World crops, and in less than 400 years they all became commercially vital crops across most of the agricultural societies on Earth. All four of these crops were first carried to the Old World within the first 20 years or so of European conquest near the beginning of the 16th century. While there is scant documentation of the paths these crops traveled once they were introduced to Western Europe, they must have passed quickly from hand to hand, region to region, and rapidly spread via seafaring vessels once they arrived in the Old World.
Commercial seed production of the Solanaceae crops is done in climates where hot daytime temperatures and warm nights contribute to a high rate of fertilization of ovules, rapid endosperm development, and high levels of fruit maturation. All of these factors result in strong yields of seed with high germination rates. Seed crops for peppers, tomatoes, and eggplants are best grown in regions with dry summer weather to minimize the exposure to many diseases of the fruit and foliage that can impact production. Drier climates can also help you avoid potentially devastating seedborne diseases like bacterial spot and bacterial speck. Hybrid seed production of these crops is frequently done in Thailand, Taiwan, Indonesia, and China because they all require hand pollination. However, many agricultural enclaves across the globe still rely on regionally grown seed of well-adapted, non-hybrid varieties that are often culturally significant.
Family Characteristics
Reproductive Biology
The Solanaceae family has flowers that are either conical or funnelform with five petals that are fused or partially fused. The flowers can be quite large, with fused petals, and are quite showy in genera such as Datura, Nicotiana, and Petunia. In the crop plants of the Solanum and Capsicum genera, the flowers are much smaller and the corolla is bell- or wheel-shaped. The petals can be predominantly white (most chiles) to yellow (tomatoes) to purple (eggplants). The flowers are perfect, with stamens that are nestled into the throat of the corolla. The anthers form a cone in the crops of this family with pollen that is shed from slits or pores situated at different points at or near the end of the anthers. The style can be of varying lengths in each of these species, and in some cases it’s long enough to extend beyond the end of the anther cone and be more apt to cross-pollinate, especially in the presence of pollinating insects. (See the “Reproductive Biology” section for each of the crops in the Solanaceae.) The flowers are usually borne in or near the leaf axils, except with eggplant, which bears its flowers opposite from the leaf axils. The fruit is a true berry with pulpy flesh and placental tissue that can be dry, as with peppers, or wet, as in the locules of tomatoes.
‘Serrano’ chile pepper flower (Capsicum annuum).
Life Cycle
Tomatoes, chiles, and eggplants are all tender, warm-season crops. They are all sensitive to frost and require a reasonable amount of heat, depending on the crop type and variety for each species, to mature their fruit. To take advantage of the warm summer growing season, all of these crops are generally started in the greenhouse and transplanted after all danger of spring frost has passed. These crops are usually started under glass 6 to 8 weeks before transplanting to the field. Plants are routinely hardened off to both the cool of the night and to full sun for several days before transplanting.
Climatic Adaptation
These three crops all produce their best seed crops with warm nighttime temperatures that will support pollen tube growth, to ensure a high percentage of fertilization of the embryos in each fruit that is set. Adequate heat units during the day are also important to stimulate rapid fruit growth and maturity. In summary, the best regions to produce seed of these crops need to have: (1) a relatively long frost-free growing season; (2) enough hot weather for good fruit production; and (3) warm nighttime temperatures for maximum seed set (see individual crop sections for specific temperature ranges for each crop).
Seed Harvest
Perhaps the most important aspect of seed harvest for these crops is making sure that the fruit that is harvested for seed is fully ripe, even overripe, but also making sure that it isn’t rotting and moldy to the point where the seed is damaged. In fact, allowing the fruit of all of these crops to go a bit beyond full ripeness may ensure that the seed is fully mature. In most wet-seeded crops the overripe fruit also usually “release” their seed from the flesh much more easily than the slightly underripe or even perfectly ripe fruit.
Fruit of the Solanaceae crops is usually harvested by hand for seed production in order to have human eyes make a final assessment as to whether: (1) the fruit is fully ripe; (2) there are marked levels of disease or rot on a particular fruit or plant; and (3) the fruit from a particular plant is true to type. Harvesters can then reject individual fruit if they’re rotting or not ripe, and they can make a final act of selection if the plant has excessive disease or off-type fruit. There are also some seed-growing operations where the fruit is machine-harvested for seed extraction. However, this one-time-only harvesting method has obvious disadvantages that make it acceptable only to seed companies selling lower-quality seed.
Historically, tomato seed was sometimes harvested in joint tomato juice/tomato seed operations in North America, at a time when most commercial seed was still non-hybrid. This practice is not done with hybrid tomatoes, as they are almost always hand-pollinated and marked, with many instances where non-hybrid fruit is set that is not a result of a cross-pollination. I recommend that all organic Solanaceae seed crops be hand-harvested to ensure their seed quality and genetic integrity.
Seed Extraction and Cleaning
The three main crop types of this family that are grown from seed are all considered wet-seeded crops. Wet-seed extraction means that water is used in either the extraction or cleaning steps, or both. Tomato seed extraction doesn’t usually use water, but water is always used during the cleaning process. The seed extraction and cleaning of both chiles and eggplants often use water for both steps, especially on a larger commercial scale of seed production. However, many seed growers around the world also use a dry-seed extraction technique for eggplants and many kinds of chiles that is described for both crops in their respective sections.
There has been a lively debate as to what the appropriate minimum isolation distances are for the commonly cultivated seed crops of the Solanaceae. Tomatoes, chiles, and eggplants are all considered self-pollinated species. But as is commonly known, all species that are designated as selfers do outcross to varying degrees. Many seed savers have discovered the hard way that growing more than one heirloom tomato or hot chile variety in close proximity to another can easily lead to plants in next year’s progeny that are the result of an obvious cross-pollination event. Also, the rate of crossing that occurs can change significantly under a number of circumstances that are discussed in chapter 13, Isolation Distances for Maintaining Varietal Integrity.
While a number of factors come into play to increase the rate of crossing in all selfers, there are thought to be two major reasons why many growers of the Solanaceae seed crops have noted an increase in the rate of crossing over what has been reported over the past 50 to 80 years in the literature:
1. Many seed growers have noted that pollinating insects visit the flowers of these crops more than is commonly recognized. This is especially true in the biologically rich environments of many of the diversified organic farms, where there is often much more habitat for pollinators and much less use of insecticides.
2. A number of crop types within these three species groups share a morphological trait that makes the plant much more conducive to outcrossing. Hot chiles, heirloom and potato-leaved tomatoes, and most eggplants have styles that grow beyond the end of the anther cone of the flower. This exposes the stigma to the open air, and to the possibility of cross-pollination when insects visit the flowers. Peppers seem to be the most notorious for outcrossing. Under favorable conditions the crossing rate of some hot peppers has been measured to be anywhere from 8% to over 60% by a number of researchers, who were trying to get an idea of just how extensive crossing can be when more than one pepper variety is planted directly adjacent to another.
For this reason the recommendations for a minimum isolation distance for these crops for organic production systems will be increased from the isolation distances that have routinely been published and used in recent years. They will be based, in part, on some of the excellent investigative work done by pollination biologist Jeff McCormack. Jeff has compiled a set of isolation distances in a series of publications that takes into account the environmental reality of organic farms and the fact that the exerted stigma in some crop types of the Solanaceae contributes to higher-than-expected crossing rates. The recommendations that I am making for commercial production of these crops under organic conditions are somewhat stricter than those recommended by Jeff. I firmly believe that, with the exception of modern tomato and sweet pepper varieties, all of the other most commonly grown seed crops of the Solanaceae should have an absolute minimum of 150 ft (46 m) between crops with a barrier present to minimize the chances of a cross when producing commercial seed. This distance should be increased to 300 ft (91 m) when these promiscuous crops are grown in open terrain. (See specific information under “Isolation Distances” for each crop type.)
Eggplant
Eggplant (Solanum melongena) is a heat-loving crop that is grown in many tropical, subtropical, and warm temperate regions across the globe. It is known as aubergine in most of Europe and brinjal in India, where it is one of the most popular cultivated vegetables. It is a native of the South Asian subcontinent, with its center of origin believed by Nikolay Vavilov to be in the Indo-Burma center, which includes modern-day Myanmar and Bangladesh. A number of closely related wild species of Solanum still grow across this area, though all have bitter alkaloids that largely prevent their use. Early efforts in the domestication of eggplant undoubtedly concentrated on eliminating the bitter glycoalkaloids that are shared by most of the relatives of this important vegetable crop. Eggplant traveled to China very early in its history, and its diversity flourished under the selection of many hands across a number of climates and geographic regions. The diversity that developed there is so profound that southern China is recognized as a secondary center of variation for eggplant.
The color of eggplant fruit can vary from white to yellow, orange, green, and purple and can sometimes have streaked pigmentation. The dark purple types can appear almost black when the anthocyanin pigmentation is most intense. The shape of the fruit can range from small oval or round, to long with a club shape, to the large oval or globe-shaped types that are the norm in North America. The common name of eggplant most probably comes from the phenotype of varieties that are white and bear the smaller oval or round types, which are much the same size as a chicken or duck egg.
These eggplant flowers clearly exhibit the exserted stigmas that result in higher rates of cross-pollination.
Eggplant is such an important staple vegetable crop throughout most of tropical and subtropical Asia that locally produced seed of many of the typical, open-pollinated regional varieties is grown locally by small, diversified farmers in many parts of Asia, as well as in other parts of the world that still depend on decentralized agriculture. Hybrid eggplant seed is becoming more ubiquitous, having been used in Europe and North America for decades. Commercial growers across Asia who are producing eggplant as a commercial vegetable for markets outside of their local communities are increasingly using hybrid seed. Hybrid seed is produced in several of the areas where hybrid tomato seed is also produced, including China and India. At the time of this writing there is increasing pressure to produce and use GM hybrid eggplant seed in India, with considerable resistance from many Indians, who fear the potential consequences of this fundamental change in their agricultural sovereignty. South Asia still has an incredible diversity of open-pollinated eggplant varieties that are grown regionally, and there are also wild related species that can potentially cross with the cultivated types. Eggplant can experience considerable cross-pollination, and the danger is that the flow of the transgenes from these GM hybrids could easily spread through both the traditional varieties and the wild populations, which would further spread these transgenes across the landscape.
Crop Characteristics
Reproductive Biology
Eggplant has perfect flowers that are often solitary, or occur in sets of two to five, and are borne opposite the leaves on the main stem of the plant. The six violet petals are fused; the number of anthers that form the anther cone can vary from 6 to 20, depending on the genetic background. The pores that facilitate the release of the pollen are at the end of each anther. The pistil, which has a style surrounded by the anther cone, is usually exerted out beyond the end of the cone in most eggplant types. The pollen released from the anther pores adjacent to the stigma will usually more than saturate the stigma and lead to a high percentage of self-pollination. However, as the style and the stigma are exerted into the open, out of the confines of the cone, there is a much greater chance of cross-pollination when an insect visits and brushes up against the stigmatic surface. This is much like what occurs with heirloom and potato-leaved tomatoes, and it contributes to a higher percentage of outcrossing than what is normally expected in a self-pollinated species.
An eggplant flower with an inserted stigma that will not be as prone to cross-pollination.
Climatic and
Geographic Suitability
Eggplant is solidly in the class of heat-loving crops. Optimum growth and fruit set occur between 72 and 86°F (22 to 30°C), with nighttime temperatures that average at or above 65°F (18°C). Nighttime temperatures below this can cause erratic pollen tube growth, resulting in lower fertilization rates and ultimately lower seed yields. As with all crops, temperatures that are too high at the time of the pollination will also retard pollen tube growth and prevent fertilization and subsequent seed formation. In eggplant this can happen at temperatures above 95°F (35°C). The most favorable climates to grow eggplant seed are regions with a reasonable amount of heat during the day and enough moisture in the air to hold some of the heat through the night. Regions with too high a level of humidity during the growing season should be avoided due to the potential for increased levels of foliar disease.
Seed Production Practices
Soil and Fertility Requirements
Eggplant is able to grow in a wide variety of soils, but the soil must be well drained to avoid root rot problems. This crop has an extensive root system and can develop a deep, strong taproot if grown in a rich loam soil with good tilth. Eggplants are heavy feeders and thrive when ample amounts of well-decayed organic matter or compost are incorporated into the soil. A soil pH of between 5.5 and 6.5 is desirable. In the era before fungicides there were eggplant growers in North America who attested to the practice of raising eggplants on soils with a pH no higher than 6.0 in order to control verticillium wilt (Verticillium dahliae), claiming that this fungus could not thrive under acidic soil conditions. Long crop rotations are advised if verticillium wilt is present.
Growing the Seed Crop
Eggplant seedlings are usually started in the greenhouse or in cold frames. It is important to seed them about 8 weeks before transplanting. Most growers seed them into flats and prick them out into individual cells when they form their first true leaves. Growers need to be careful and not transplant the plants to the field until the warm weather has settled and summer has truly arrived. Eggplant is particularly sensitive to arrested growth; it needs steady, unchecked growth to develop a full frame that will support a high number of fruit and produce good seed yields.
As with other seed-producing members of the Solanaceae, eggplant seed crops are usually grown at the same spacing as the eggplant vegetable crop. Plants can be spaced at 14 to 18 in (36 to 46 cm) between plants, with 30 to 36 in (76 to 91 cm) between rows. If you prefer growing the crop on beds, the space between plants should be increased to 18 to 24 in (46 to 61 cm) and then planted in an offset pattern in rows 24 to 28 in (61 to 71 cm) apart on wide beds. In some cases growers will stake the plants, which can be effective in slowing fruit rots that can occur over the long season that is necessary to mature a seed crop.
Seed Harvest
As with all of the Solanaceae seed crops, the first step in seed harvest is to harvest the truly ripe fruit by hand. This allows you to harvest the ripest fruit possible multiple times if the season allows. Eggplant fruit should be allowed to ripen to full seed maturity. At maturity all eggplant types have fruit that changes colors. The dark purple types first change to a duller shade of purple and then to a dingy brown color. The other common pigmentation types include the lightly streaked Italian purple types, as well as the green, yellow, and white types, all of which turn to some shade of yellow or gold as they reach seed maturity. With all eggplant varieties an abscission layer forms at some stage in the maturation process, allowing the harvest crew to easily pull the fruit from the plants as opposed to cutting them with clippers. To avoid damaging the plant, all ripe fruit that isn’t easily pulled off the plant should be cut off cleanly with clippers.
Eggplant fruit is routinely allowed several days of after-maturing off the plant in a warm, dry place. This enhances the maturity of the seed, usually improving the germination percentage. In dry, long-season climates eggplant fruit can be left on the plant till it has shriveled, which is often done across many dry areas of Asia when harvesting limited amounts of seed for on-farm or local use.
Seed Extraction and Cleaning
When the fruit has after-matured, the seed can be separated from the fruit by two different methods: wet-seed and dry-seed extraction.
Wet-Seed Extraction: Wet-seed extraction is the method most favored for the commercial extraction of relatively large amounts of seed. The overripe fruit is crushed or macerated mechanically in order to free the seed from the pulp in which it is embedded. This eggplant pulp is then run across screens with a stream of water that helps to loosen the seed from the flesh and force it through the screens; most of the pulp stays on top. This can be done with the help of human hands, rubbing the material across the screens. Larger operations will sometimes employ a large cylindrical screen unit that when spinning will force the seed through the screen under centrifugal force. Both this method and the more manual method require water to loosen the seed from the dry eggplant flesh. The seed that passes through the screen can then be separated from the small pieces of flesh and skin using the same decanting process with water that is used in the final stages of tomato cleaning. Fermentation is not necessary in cleaning eggplant seed and in fact may be harmful to the viability of the seed.
Dry-Seed Extraction: The second method is used when the fruit is allowed to dry. In long-season climates where fruit can be fully dried in a combination of on- and off-the-plant maturation, growers have the option of harvesting the seed from completely dried fruit. The fruit are then beaten to split them open, and seed can be picked from them with much hand labor. This method is only appropriate for extraction of relatively small amounts of seed, though there are probably tricks I am not aware of that are still practiced by farmers in South Asia, the homeland of eggplant.
Genetic Maintenance
Several important traits can be checked before the plant begins flowering. As with all crops, seedling vigor and early robust growth are always important traits to select for in organic production systems. Eggplant prospers from increased vigor, as the seed is often planted into less-than-ideal greenhouse conditions. I have seen many cases where farmers have selected for seedling vigor over a number of generations, and the resultant variety appeared to pass more quickly through a stage of susceptibility to the damping-off complex of seedling diseases. Selecting for vigor may be accomplished by eliminating any slower, poorly formed seedlings, after all of the seedlings have germinated and emerged.
The next traits to observe for off-types are stem color, leaf color, leaf shape, and plant stature. Sometimes there will be an obvious difference for one of these characteristics that will warrant the removal of a plant from the population. While genetic variability of this type isn’t necessarily bad in a genetically elastic crop population, it can be a good indicator of an off-type, outcross, or seedling mix. By paying attention to the basic plant characters it is possible to rogue out a potential off-type plant, instead of not recognizing a plant that can potentially spread pollen to other plants after the seed crop is flowering. Once the crop has started to produce fruit, it is possible to eliminate plants with misshapen or off-type fruit. After the fruit has reached vegetable maturity, you can evaluate plants and perform a final selection for fruit shape or size. Fruit color at vegetable maturity should be at its truest shiny luster. This is the best time to eliminate plants with fruit that varies too much from the norm for color, color intensity, or degree of striping for the particular variety you are growing.
Isolation Distances
Eggplant is traditionally thought of as a self-pollinated species, but the morphology of the flower in most eggplant varieties allows for an appreciable amount of outcrossing, especially under organic production practices (see “Isolation Distances” in the Solanaceae chapter introduction). Elegant research conducted in the 1920s by Kakizaki in Japan demonstrates that crossing rates averaging over 6% and reaching as high as nearly 47% occurred in some instances when the progeny of white-fleshed plants that had been interplanted with purple plants was grown the following year. Kakizaki also reported that no crossing occurred at 164 ft (50 m) between these two different-colored types of eggplant.
From reports of crossing that has occurred in organic production of eggplant seed in recent years, I believe it is important to be even stricter than Kakizaki’s research indicates. I recommend that when growing two eggplant varieties it is wise to isolate them by a minimum of 300 ft (91 m) when growing the seed crops in open terrain. This minimum isolation distance can be decreased to 150 ft (46 m) when natural barriers are present. This also fits with Jeff McCormack’s recommendations, which are based on his thorough investigations into the isolation distances of the Solanaceae crops.
Pepper
Peppers or chiles (Capsicum spp.) are a clan of at least 25 different domesticated, semi-domesticated, and wild species fruiting plants that have become among the most important culinary crops in almost all agricultural societies worldwide. Members of the Capsicum genus are native to the tropical and semi-tropical regions of South and Central America. There are probably still new species of this diverse genus that are awaiting discovery as ethnobotanists venture farther into the wild, rugged, and diverse landscapes of this part of the world. With such a wealth of species occupying different niches in these radically varied ecological zones, it is no wonder that early farmers domesticated at least five different species, selecting them for many of the same forms resulting in shapes, colors, and culinary qualities that are surprisingly similar across the species.
The five domesticated species of the Capsicum genus that account for the vast majority of the agricultural pepper production across the globe are:
1. C. annuum, which is by far the most widely cultivated pepper species and includes sweet peppers, cayenne, paprika, and most of the varied hot chiles grown worldwide.
2. C. frutescens is from the lowlands of the Amazon basin and thrives in hot climates. It includes the well-known Tabasco type and the so-called squash peppers.
3. C. chinense, also from Amazonia and closely related to C. frutescens, includes habanero and Scotch bonnet, varieties famous for very high concentrations of capsaicin, the compound responsible for the pungency in peppers.
4. C. baccatum is from low to mid-elevations in Bolivia, and its aji chiles are grown widely in South America, where they are prized for their subtle bouquet and fruit-like flavors that complement their pungency.
5. C. pubescens, which is named for the hairy pubescence of the leaf veins, is native to the mid-elevations of the Andes, making it adapted to cooler weather than other chile species. It is also grown in mountainous areas of Mexico and Central America. Its plants are striking for their beautiful purple flowers and black seed.
Habanero chiles (C. chinense) are among the hottest in the world and include orange- and red-fruited strains.
The domestication of peppers was already quite advanced and had spread across the tropics of the Western Hemisphere when Columbus landed in the Caribbean on his first voyage in 1492. Columbus famously returned to Spain with seed of what he called pimiento, the Spanish word for “pepper,” and declared his trip to find a faster route to the Far East for spices a success. This story is often told as if Columbus were trying to deceive his benefactor, Queen Isabella, by overstating the value of the chiles as being the equivalent of black pepper (Piper nigrum), which was literally worth its weight in gold at the time.
Bell peppers (C. annuum) and other modern sweet peppers are less likely to cross-pollinate than the hot chile types, but they will still cross at higher rates than is normally thought.
In retrospect, the seed of chile peppers was indeed a treasure, as this crop is widely adapted to any climate with hot summers and can be grown in many regions where the woody perennial black pepper would never survive. In Europe chiles were first grown in monastery gardens, but unlike tomatoes their cultivation spread quickly across the tropical and subtropical regions of the Eastern Hemisphere by land and by sea. The adoption and selection of this crop was so extensive across East Asia that the region became a secondary center of diversity for peppers. In fact, when early taxonomists classified the peppers growing in China, they named one of the types S. chinense, assuming that the extensive diversity they found there meant that this type must be native to this part of Asia.
The ‘Peruvian Purple’ chile (C. frutescens) has fruit that changes in color from purple to red when ripe and is mildly hot.
Crop Characteristics
Reproductive Biology
Peppers are perennials that are usually grown as annuals, because they are easily killed by frost. Peppers have perfect flowers that are often borne singly in the axils of the leaves, though some of the less common species can have multiple flowers at each axil. Most commonly the flowers have white petals, though they can exhibit a purplish blush. The normal state is to have five stamens with long blue-tinged anthers that are fused at their base but do not form a unified anther cone like tomatoes, eggplants, or many other wild members of the Solanaceae. The stamens split along their length to release pollen. The pistil has an ovary with two to four carpels, and the style and stigma collectively are a little longer than the stamens. The length of the style and the absence of an anther cone exposes the pepper flowers to a higher incidence of cross-pollination when insects are present.
Subtle differences in these floral traits are used to distinguish the five major species of cultivated peppers; however, they are anatomically very similar.
Unlike most Capsicum annuum chiles, ‘Serrano’ types are closer to the wild with their dark purple anthers and grayish pubescence on their calyx.
There is some debate on the ability of all five of these species to freely cross and produce viable offspring. In general, while there may be some crossing barriers between specific species it should be assumed that all of these species are potentially sexually compatible whenever you grow a seed crop of any of them. Therefore, always observe the minimum isolation distances recommended in the “Isolation Distances” section.
Climatic and
Geographic Suitability
Peppers are generally considered among the “moderate heat-loving” seed crops, requiring a reasonable amount of heat during fruit set and maturation, but definitely a rung below eggplants or melons in their requirement to mature a good seed crop. Importantly, with the large amount of genetic breadth across all of the cultivated peppers, there are undoubtedly differences in the ideal temperatures for seed production among the various types of peppers.
Optimum growth and fruit set occur at temperatures between 68 and 80°F (20 to 27°C), depending on the type and variety of pepper, with nighttime temperatures that are at or above 60°F (16°C) but don’t regularly exceed 72°F (22°C). As with all heat-loving seed crops, temperatures at or near the accepted low temperatures for seed production will often cause erratic pollen tube growth, resulting in lower fertilization rates and ultimately lower seed yields. Temperatures that are too high at the time of the pollination can cause low pollen production and also retard pollen tube growth, preventing fertilization and subsequent seed formation. High temperatures can also cause flower abscission. These heat-related problems can manifest at temperatures that persist above 90°F (32°C). Sweet peppers are generally more sensitive to these heat-related problems than the hot chile types.
Peppers are such a ubiquitous crop throughout most of tropical and subtropical areas of the world that locally produced seed of many of the typical, open-pollinated regional varieties is still grown locally in many decentralized agricultural systems. While many of these regions may not be ideal for pepper seed production, it is possible to produce plenty of seed for regional cropping needs if growers are careful not to allow the fruit to become moldy while ripening or before seed extraction.
Much of the large-scale commercial seed production for pepper seed is done in the moderately dry climates of northern Chile, northern Thailand, the northern Philippines, southern Taiwan, and more recently parts of mainland China and India. Much of this seed is high-value hybrid seed that is produced through emasculation and pollination by hand.
Seed Production Practices
Soil and Fertility Requirements
Peppers thrive on lighter soils such as sandy or silt loams. These soils warm up faster than heavier clay soils, allowing vigorous early growth before flowering that can subsequently support a sizable fruit crop. Peppers need good, balanced fertility and even moisture throughout the season; however, the soil must be well drained to avoid root rot problems.
Peppers will develop an extensive root system in a rich loam soil with good tilth. The root mass will reach to at least 3 ft (0.9 m) in all directions when healthy. Therefore, cultivation needs to be relatively shallow to avoid excessive root damage. While peppers are heavy feeders, excessive levels of nitrogen should be avoided; this can contribute to plants that lodge (topple over) in extreme weather. According to Jeff McCormack, the incidence of lodging under windy conditions in southeastern United States increases when the organic matter content of the soil exceeds 6%. A soil pH of between 5.5 and 6.8 is desirable for peppers.
Growing the Seed Crop
Pepper seedlings are usually started in the greenhouse or in cold frames, but in the most favorable long-season climates they can be direct-seeded to the field. If you’re transplanting, it is important to seed them under cover up to 10 weeks before transplanting; they often need a couple of weeks more than tomatoes or eggplants to produce a stout transplant. Most growers seed them into flats and prick them out into individual cells when they form their first true leaves. It is important to not transplant the plants to the field until the warm weather has settled and summer has truly arrived.
Pepper seed crops are usually grown at the same spacing used when growing the crop as a vegetable. Planting densities can vary, as there is a significant range in the size, height, and shape of plants among the different types and varieties of peppers. Plants can be spaced at 16 to 24 in (41 to 61 cm) between plants depending on the size of the variety, with 30 to 36 in (76 to 91 cm) between rows. If you prefer growing the crop on beds, increase the space between plants to 18 to 24 in (46 to 61 cm) and then plant in an offset pattern in rows 24 to 28 in (61 to 71 cm) apart on wide beds. In some cases growers stake the plants, which can be effective in slowing fruit rots that can occur over the long season necessary to mature a seed crop. This may be especially important when growing a high-value sweet pepper, as sweet types are often more prone to fruit rots than chiles.
Seed Harvest
As with all of the Solanaceae seed crops, the first step in seed harvest is to harvest the truly ripe fruit by hand. This allows you to harvest the ripest fruit multiple times if the season allows. Pepper fruit need to fully ripen to harvest mature, high-germinating seed. At maturity all pepper types have fruit that change color to red, yellow, gold, orange, red, purple, or chocolate brown. The best indicator that they have reached full seed maturity is when they’ve become uniformly pigmented with one of these colors.
At fruit maturity many growers allow the fruit to remain on the plants to overripen past their edible peak, as this ensures that the seed will be fully mature when picked. This practice is usually successful in seasonally dry climates, where there is less chance of fruit rots developing; however, as you gain experience growing pepper seed in any particular climate, you are advised to monitor the crop closely for fruit rot at or near the time of maturity. Any rot that forms can quickly enter the interior cavity of the fruit and damage the seed. Fruit is routinely hand-harvested, and at the time of harvest the crew should eliminate any fruit with noticeable signs of rot. If the seed is not going to be extracted from the fruit within 24 hours, the fruit should be harvested in such a way as to minimize any damage. This is essential, as fruit rots can easily spread through harvested fruit if held for several days and care is not taken.
Seed Extraction and Cleaning
After harvest, pepper seed can be separated from the fruit by two different methods: wet-seed and dry-seed extraction.
Wet-Seed Extraction: The wet-seed extraction method is most favored for the commercial extraction of relatively large amounts of pepper seed. This process begins in a similar fashion to the method used for both tomato and eggplant, where the ripe fruit is either crushed or macerated. The resultant pulp is then run across screens with a stream of water that helps to loosen the seed from the flesh and force it through the screens while most of the pulp stays on top. This can be done in a very low-tech fashion with the help of human hands, rubbing the pulp across the screens. The workers must be very careful in this situation, though, if they’re handling hot peppers. Gloves, respirators, and eye protection should be used in all cases when processing any peppers that have any detectable levels of heat in any hands-on methods of seed extraction. In fact, everyone from the harvest crews to the seed extractors needs to protect all mucosal tissue against particulates from all peppers that contain capsaicin.
Larger pepper seed operations will sometimes employ a large cylindrical screen unit that when spinning will force the seed through the screen under centrifugal force. Both this method and the more manual method require water to loosen the seed from the flesh. The seed that passes through the screen can then be separated from the small pieces of flesh and skin using the same decanting process with water that is used in the final stages of tomato cleaning. The seed can also be run through a sluice like tomatoes to separate it from fragments of the pulp. Fermentation has been used to a limited extent by some growers; most pepper seed producers agree that it not only is unnecessary but may also be harmful to the viability of the seed.
When using the wet-seed extraction method it is important to dry the seed quickly and efficiently as soon it’s as clean as possible from the decanting process. The seed should be spread thinly on screens and placed in a warm spot with good air circulation. Sun-drying is acceptable. Supplemental heat may be necessary under cool conditions at the end of the season, being careful to not heat the seed much above 95°F (35°C). Seed should always be stirred at least twice a day when drying.
Dry-Seed Extraction: The dry-seed method is only used for thinner-fleshed hot chile types, as sweet pepper types usually have a thicker fresh that is much harder to dry without mold forming. Of course, there are also some hot types with thicker flesh that will not dry easily.
‘Corno di Toro’ is an example of a class of thick-walled peppers that are both sweet and mildly hot. These types often exhibit variation for shape and color that can be selected as novel types by the seed grower.
In long-season, arid climates the thin-fleshed hot types can often be left on the plant to partially dry before harvest and then be dried fully in the sun. But in less-than-optimal climates it is best to pick the fruit at or soon after full maturity, spread them thinly across a clean surface with plenty of good air circulation, and dry them in the sun. The fruit must be protected from precipitation; you may need to supply airflow via fans if you’re drying them in a greenhouse, high tunnel, or dryer.
When completely dried and shriveled the dried fruit are then threshed or flailed to free the seed from the pods. As with all of the steps when handling hot chiles, it is very important for any workers to cover all mucous membranes with protective clothing or equipment to guard against any painful inflammation from the capsaicin found in the dried flesh. During the steps of threshing the dried pods and the subsequent cleaning of the seed it is especially important to avoid the copious hot pepper dust that will become airborne using this dry-seed extraction method.
Seed Cleaning: After the seed is dried using either of the above methods, you may need to winnow it or pass it across the appropriate-sized screen to eliminate any pieces of dried pulp that might remain.
Genetic Maintenance
Several important traits can be checked before the plant begins flowering. As with all crops, seedling vigor and early robust growth are always important traits to select for in organic production systems. Selecting for vigor may be accomplished by eliminating any slower, poorly formed seedlings after all of the seedlings have germinated and emerged.
The traits to observe for possible roguing are stem color, leaf color, leaf size, plant stature, and plant height. Sometimes there will be an obvious difference for one of these characteristics that will warrant the removal of a plant from the population. Leaf size is often correlated with the pungency of the variety, as hot varieties usually have significantly smaller leaves than the sweet types. Therefore, it may be possible to eliminate a small-leaved plant that might carry the pungency trait from a large-leaved sweet pepper crop before flowering and prevent this trait from being crossed into multiple sweet plants in the field.
Once the crop has started to produce fruit, it is sometimes possible to eliminate plants with obviously misshapen or off-type fruit. When fruit reach their vegetable maturity you can rogue for fruit shape, size, and color as peppers turn to their true mature color. The color at full fruit maturity should be at its true shade and luster. This is the best time to eliminate plants with fruit that varies too much from the norm for color and color intensity for the particular variety that you are growing.
Peppers may be the most obvious example of a presumed self-pollinated species that should nonetheless always be treated as if there were a high likelihood that they will cross at an appreciable rate. This is especially true under organic production practices (see the “Isolation Distances” section in the Solanaceae family introduction). As explained in the “Reproductive Biology” section earlier, the morphology of the pepper flower lends itself to cross-pollination in the presence of insects. This is especially true of hot chile types as well as the semi-domesticated and wild types of peppers. While the sweet pepper types may not outcross as readily as the pungent types, they are frequently involved in unwanted cross-pollination events in the field.
It should also be repeated that all five of the commonly cultivated species of peppers may be able to cross under certain circumstances and therefore should all be isolated appropriately when producing seed on any of them. Also, anyone growing a seed crop in or near any of the ancestral centers of diversity of peppers, including western South America or much of Central America, should be conscious of any populations of wild Solanum species that may be in the area. There is also potential crossing that can occur with the wild chiltepin pepper (C. annuum var. aviculare), which is traditionally wild harvested and grows in many areas of northern Mexico and the southwestern United States. According to ethnobotanist Gary Nabhan, the range and proliferation of the chiltepin may have increased since the 1980s as farmers in the Sonoran Basin initiated plantings to increase the commercial harvest of this prized chile.
It is surprising that so many seed growers have treated peppers as if they were faithful self-pollinators, because a number of research teams going back to the 1940s have found cross-pollination common when two distinct varieties were planted in close proximity. In a study in New Mexico where plants of a test variety were interplanted with a commercial chile at the normal within-row spacing, Steve Tanksley found an average crossing rate more than 40% of the time, and some plants had up to 90% of seed resulting from cross-pollinations! While this is an extreme example, where plants of two different varieties are literally touching in the field, it reveals how much crossing can actually happen in a self-pollinated species when both the morphological and environmental factors are conducive to it.
I recommend that when growing two hot pepper varieties (or a combination of one hot and one sweet type), it is wise to isolate them by a minimum of 300 ft (91 m) if growing the seed crops in open terrain. This minimum isolation distance can be decreased to 150 ft (46 m) when natural barriers are present for any combination that includes a hot pepper type. If you are growing seed of two sweet pepper varieties in the same season, then it is possible to reduce this to a 150 ft (46 m) minimum isolation distance when growing them in open terrain and 75 ft (23 m) apart when natural barriers are present. This fits with Jeff McCormack’s recommendations, which are based on his thorough investigations into the isolation distances of pepper crops.
Tomato
The cultivated tomato (Solanum lycopersicum) is one of the most widely grown vegetables on Earth. It is among the five most important vegetables of commerce in many agricultural societies. This is remarkable when we consider that this humble fruit was only grown in limited areas of what is now southern Mexico and Guatemala in the early 1500s when invading Spanish conquistadores found it growing in Aztec villages. This fruit was called tomatl, or “swelling fruit,” in the Nahuatl language of the Aztecs, which became tomate for the Spanish. The origin of this cultivated form is mired in mystery, as all of its wild relatives are native to mountainous regions of western South America, from Ecuador and Peru to northern Chile, including two species that are endemic to the Galapagos Islands. For many years the weedy cherry tomato of Mexico, Solanum lycopersicum var. cerasiforme, was thought to be the progenitor of the cultivated type, but recent genetic profiling at Cornell University indicates that this weedy form is in fact a feral mixture between wild and cultivated types.
Tomatoes were brought back to Spain by Hernán Cortés and were grown as a botanical curiosity. Within very few years they made their way to Italy. In 1544 Pietro Andrea Matthioli, a Tuscan physician who studied the medicinal value of plants, described them in an herbal text he was writing and suggested that tomatoes might be edible. In the second edition of this text 10 years later Matthioli first used the term pomo d’oro, golden apple. While many have speculated since that time that the first Italian tomatoes must have been yellow-fleshed, it seems that pomo d’oro was a generic phrase used for all soft tree fruit at the time and wasn’t specific to the color of the fruit. In Italy, the tomato was embraced as food in the poorer southern regions of the peninsula as well as in Sicily before becoming widely grown. There is also some evidence that tomatoes were grown as a vegetable in parts of Spain relatively soon after their introduction, but many parts of Europe only grew the crop as an ornamental for many years, fearing that the fruit was poisonous like many of the wild native members of the Solanaceae. Northern Europeans were late to the party as far as accepting tomatoes as a food plant, with the British and their colonies (and former colonies like the United States) not eating them widely until early in the 19th century.
Tomato seed has historically been produced wherever tomatoes are produced. As tomatoes spread across the globe people easily saved seed from the fruit and adapted the crop to their regional and climatic needs. The specialization of growing tomato seed in ideal environments didn’t start until well into the 20th century. As several seedborne diseases, including fusarium and verticillium wilts, along with bacterial spot and speck, became more prevalent in commercial tomato acreage, it became obvious that growing tomato seed in drier climates and controlling the irrigation after fruit set was definitely desirable for controlling the spread of these diseases via the seed.
Crop Characteristics
Reproductive Biology
The cultivated tomato is a short-lived perennial that is grown as an annual crop under most cultivation systems. The plant has a range of sizes, from extremely small dwarf determinates that can be 10 in (25 cm) tall to some very vigorous indeterminate types that easily reach 72 in (183 cm). Most modern commercial tomato varieties are determinate or vigorous determinates with an inflorescence borne between each leaf and range in size between 18 to 36 in (46 to 91 cm) in height. Determinate types also have terminal flower clusters at the end of each shoot. Indeterminate varieties usually bear an inflorescence every three to four leaves along the length of their shoots, and apical growth continues throughout the season until frost kills the plant.
Each tomato inflorescence usually has between 4 and 12 flowers that are formed and mature sequentially on a raceme. Individual flowers are perfect, with six bright yellow petals that curve outward, away from the flower as the flower matures. The ovary can have anywhere from 2 (especially in cherry types) to 15 or more locules, which contain the ovules. The six stamens have compact fused anthers that form a yellow cone, 0.5 to 0.75 in (1.3 to 2 cm) long, that surrounds the pistil, with its style and stigma that usually terminates within the cone but can occasionally extend slightly beyond the tip of the cone, which has a small opening. The anthers have slit openings on the interior of the cone, and when pollen dehisces it will shower out of these pores with any kind of motion of the flowers, whether from wind or insect visitation.
As the anther cone of the flower usually points downward, the pollen will thoroughly cover the bulbous stigma, it is well within the anther cone as it is with most modern tomatoes, or the cone is exerted out of the tip of the cone as it often is with many heirlooms. The pollen, which is shed over a 2-day period, will usually pollinate its own stigma within the anther cone, supplying the pistil with plenty of pollen to fertilize a full complement of ovules.
The flower of a modern tomato variety (Solanum lycopersicum), with an inserted stigma that is well within the anther cone, resulting in lower rates of cross-pollination.
This flower of a ‘Pruden’s Purple’ heirloom tomato shows the exserted stigma that makes cross-pollination of such varieties by insects much more likely.
However, the stigma is often receptive a day before pollen shed and remains receptive 2 or 3 days after the pollen from its flower has shed. This means that there are opportunities for crossing to occur, especially with the exerted stigma of the older varieties. When the style pushes the stigma out of the end of the anther cone, it is exposed to possible insect activity. While tomato flowers are not visited by a wide number of insect species, they are often visited by several types of bumblebees (Bombus spp.). Bumblebees have a unique way of clinging to the flowers upside down while vibrating their wings rapidly and shaking the pollen out of the cone onto their abdomen. If the stigma is exerted then it is possible that pollen on their abdomen from a previous flower can be transferred to the flower they are currently visiting, producing a cross-pollination. This is obviously much less likely to occur with more modern tomato varieties, which have stigmas that are well encased in the anther cone; other insect pollinators, however, will sometimes pry the flowers open and cause a cross to occur.
Climatic and
Geographic Suitability
Tomatoes can have problems setting seed at temperatures that are too high or too low. At temperatures above 90°F (32°C) and below 60°F (16°C) the pollen of many varieties will be affected and fertilization of ovules will be impeded, both resulting in poor seed set. In extensive experiments with tomato pollination in the 1930s, Ora Smith of Cornell University found that the optimum temperature for pollen to germinate on the stigmatic surface is 85°F (29°C); at 100°F (38°C) or 50°F (10°C) pollen germination was virtually stopped. Smith found that even at favorable temperatures pollen tube growth is slow, taking 2 to 3 days to reach the ovules following pollination. This means that, even if temperatures are favorable at the time of pollination, any temperature swings below 60°F (16°C) or above 90°F (32°C) may severely slow or stop the growth of the pollen tube on its journey to the ovules. Therefore, even when the temperature for pollen tube growth is at or near the optimum during the day, if the temperature drops to lows at or near 50°F (10°C) during the night, any of the pollen tubes that started their journey within the last day or two can stop growing. Alternatively, in hot climates the pollen can germinate and start growing during the cooler temperatures of the morning or evening and then be stifled when hot temperatures approach or exceed 100°F (38°C) in the middle of the day. Once the pollen tube stops it usually will not resume growth. If this happens repeatedly over the course of the several days that the flower is receptive then there is a good chance that most of the embryos won’t be fertilized; hence the fruit won’t “set” and will abort (see chapter 2, Reproductive Biology of Crop Plants).
A potentially worse situation may occur when a tomato seed crop sets a full complement of fruit, but because of less-than-optimum environmental conditions the fruit can have very little seed at harvest. This can happen when the temperatures at or during the pollination and subsequent fertilization of the embryos are either too hot or too cold, as in the previous example, resulting in only a minority of pollen tubes reaching their destination. Some tomato varieties will successfully set fruit with only a fraction of the available embryos becoming fertilized, resulting in a fruit with potentially very few seeds. This often occurs when seed growers attempt to grow the so-called parthenocarpic tomatoes that can be seedless due to their unique ability to set fruit even when temperatures are too cool for most tomato varieties to successfully do so. A number of farmers have been sadly disappointed when growing a parthenocarpic tomato for a seed crop and having a good fruit set, but then finding the crop is barren of seed (or yielding very little) when they crush the ripe fruit to extract it. (See the “Genetic Maintenance” section for possible solutions to this problem.)
Seed Production Practices
Soil and Fertility Requirements
Tomatoes can be grown on all types of soils, but they seem to benefit greatly from growing in a good agricultural loam, ranging from sandy loams to heavier clay loams that are well drained. These soils, if well drained to avoid root rots, can supply continuous moisture and fertility, which promotes good seed yields in tomato. A soil pH of between 6.0 and 7.0 is desirable, and excess fertility, especially nitrogen, should be avoided; it can promote luxuriant foliar growth to the detriment of fruit growth. Phosphorus should be readily available, which is sometimes problematic under organic fertility regimes, especially under cooler-than-optimum temperatures. In most cases high-quality compost with a good humus fraction is adequate to meet the needs of a tomato seed crop.
Growing the Seed Crop
Planting the Crop: The first step in growing a successful seed crop is to produce healthy, disease-free tomato starts. Seed can be sown into greenhouse hot beds or seedling flats; in warmer climes it can be planted into cold frames. Plants are then frequently pricked out from these initial thick plantings and transplanted into flats or individual pots within a couple of weeks at 6 to 10 in (15 to 25 cm) apart on the greenhouse bench. Seed can be sown as early as 10 to 12 weeks before the projected transplant date. However, many growers prefer a transplant that is stocky and no more than 8 to 10 in (20 to 25 cm) tall when setting them out into the field. Producing a stocky transplant that resists getting leggy can be accomplished by subjecting the plants to good, strong air currents on a regular basis over the duration of their time growing under cover. Seedlings should be grown at a moderately rapid rate and hardened off without exposing them to extremes of cold, below 55°F (13°C), or extremes of direct sun when first being moved outdoors. Lastly, starting with seed that is free from seedborne diseases is very important, and appropriate steps to avoid contaminated seed should be strictly adhered to by all tomato seed growers (see chapter 16, Seedborne Diseases).
Crop Spacing: When transplanting to the field tomato plants can be spaced at much the same spacing as when growing the crop as a vegetable. Spacing depends in large part on the harvest methodology and the degree of vigor in the tomato variety being produced. Determinate tomato varieties vary greatly in their size and the extent of the canopy they produce. Some vigorous determinate types are in fact intermediate in their stature between the average determinate types and the indeterminate types. Indeterminate tomato varieties can also vary considerably in their size and stature and will require a range of plant spacings in order to grow the best seed crop.
Upon transplanting into the field plants of determinate varieties are routinely planted anywhere from 14 to 24 in (36 to 61 cm) apart within the row if staked, and often at increased spacing if no support system is used and plants are allowed to sprawl on the ground. The between-row spacing is anywhere from 36 to 72 in (91 to 183 cm) for determinate varieties, depending on how vigorous the variety is that is being grown. Some growers will also plant two rows on a bed with support to increase the population. Compact determinate varieties that don’t require any staking can be grown at even tighter spacing based on their size and stature.
Indeterminate tomato varieties usually require greater spacing and are usually staked for seed production. Staking tomatoes for seed production is always desirable when possible as it minimizes the soilborne diseases that may possibly infect the crop. The spacing between plants within the row for determinate plants is from 24 to 36 in (61 to 91 cm), and the spacing between rows is similar to the spacing for determinate types, though usually starting at 48 in (122 cm) and going up to 72 in (183 cm), depending on the vigor of the crop and the spacing required for your harvest methods.
Seed Harvest
Tomato harvest for seed is done in essentially the same way that it is done for the fresh market. The major difference is that fruit harvested for seed is often picked at full dead-ripe maturity, where fruit harvested for the fresh market is often harvested at various stages of immaturity, even when sold locally. Damage to the fruit at harvest should be minimized to allow for roguing out diseased or rotting fruit at the time of processing for seed (this is also very important if you want to extract an additional product such as sauce or juice from the fruit pulp). While the tomato seed may be at its peak maturity when the fruit is dead-ripe or even a little overripe, it is important to not leave the fruit on the vine to the point where it is rotting, as any excessive rotting can either damage or discolor the seed.
Once the tomato fruit is harvested it is also important to extract the seed in as short a period of time as possible, as the inevitable rotting of the fruit from saprophytes (bacterial or fungal organisms that feed on damaged or dead tissue) will grow quickly, especially on any damaged fruit. To harvest the seed the fruit needs to be crushed and mashed until the seed is largely freed from the locules, which are the cavities in which the seed is borne inside the tomato. In many of the machines used for wet-seeded crops, the seed is then forced through a circular wire mesh screen by centrifugal force. This allows the seed, the juice, and small pieces of pulp to pass through and go into a container but catches larger pieces of pulp and most of the tomato skin. This fruit debris then comes out the end of the cylinder, and if the operation is done with properly maintained food-grade equipment, the pulp can be used for salsa or sauce. Historically there were companies that harvested tomato juice in connection with seed production using specialized equipment. If you are processing small batches of fruit it is possible to simply press the tomatoes through a wire mesh screen that allows the seed to easily pass through, while catching much of the pulp.
Seed Extraction Methods: Tomato seed is enclosed in a gelatinous sac that clings tightly to each seed. The traditional method used to break down and eliminate this sac is to ferment the seed, juice, and pulp with the endemic yeast that occurs naturally on the skin of the fruit. The fermentation of tomato seed is an effective way to separate it from the gel and one that is acceptable under organic farming practices and organic certification. In contrast, most of the conventionally grown tomato seed since the 1970s has been extracted using an acid separation technique. It is often used in conjunction with partial fermentation and has become popular, as it saves time and produces a very clean-looking, light tan–colored seed. In this method a controlled amount of hydrochloric acid is added to the seed pulp, mixed thoroughly, and only allowed to interact with the seed for up to half an hour. However, it is a potentially dangerous process, and policymakers agree that it doesn’t conform to most organic standards worldwide. At the time of this writing it is still the industry norm, but there is debate over whether it should be acceptable for certified organically grown tomato seed.
Seed Fermentation: The fermentation process is a fairly straightforward process using tubs or tanks to hold the seed that is still suspended in the tomato juice and pulp that has been run through a screen. Some authors encourage adding water to this mash, but this dilution is seen as a hindrance to achieving the full potential of the fermentation process. Some tomato seed growers also believe the presence of water increases the likelihood of germination occurring during this process. This may be based on the fact that the juice of the tomato, which is largely from the locules of the fruit, has sprout inhibitors that are diluted when water is added to the mix.
An important factor in encouraging fermentation is to ensure that temperatures are held at between 72 and 80°F (22 to 27°C) during most of the time that the seed is fermenting. The time it takes to achieve full seed extraction using fermentation is in large part dependent on the temperature that the seed mash is exposed to. The fermentation period will only take 2 to 3 days if temperatures are maintained at the upper end of this scale. Most tomato seed producers agree that it is desirable to have this process take no more than 4 days. If the temperature of the fermenting mash remains much below 70°F (21°C) for any appreciable period of time the gel may not fully separate from the seed, and undesirable fungal or bacterial growth can affect seed quality and lower the germination percentage. If the fermentation temperature goes much above 82°F (28°C) for any appreciable length of time during the process, then the viability of the seed can also be lowered. Therefore, fermenting seed in very hot climates will need to be done within a cooled environment. Conversely, in cooler climates it may be necessary to ferment the seed in a greenhouse or heated building.
From the beginning of the fermentation process, the fibrous pulp and the enclosed seed float to the surface of the fermentation vats. As the seed separates from the gelatinous sacs it will sink to the bottom if it is sound, while the pulp, remnant sacs, and any unviable seed will float to the surface. To encourage this separation regular stirring of this mash should occur at least twice a day and sometimes more often in rapidly fermenting batches. This stirring is also very important to discourage the formation of mold on the pulp at the surface of the mash, which can discolor or damage good seed in this floating material.
Fermentation and Seedborne Diseases: Fermentation of tomato seed is often purported to kill a number of seedborne diseases that may affect tomatoes. Unfortunately, the only pathogen that seems to be affected is the bacterium that causes bacterial canker (Corynebacterium michiganense), and the effectiveness of fermentation on it is dependent on the temperature that is maintained and the duration of time that fermentation occurs (see chapter 16, Seedborne Diseases).
Washing Seed: When the seed has fully separated and collected on the bottom of the fermentation vessel, it is time to wash out all of the pieces of pulp that remain. There are at least two methods commonly used for commercial quantities of seed. For both methods the first step in washing the seed is to remove as much of the floating mass of pulp as possible. After a final thorough stirring to release good seed that will sink from the floating mass, it is important to either scoop and discard as much of the pulp as possible, or to slowly and gently add water to the vessel till the top few inches of pulp spills over the lip of the vessel. This latter option must be done quite slowly and gingerly with a low-volume stream of water so as to not disturb the good seed that is on the bottom of the vessel.
The first seed-washing method uses a sluiceway or flume, which is a long narrow trough, built at a slight decline of approximately 1 in 50. This is the same technology used by gold miners during the California Gold Rush of 1849. These seed-cleaning sluices are usually made of either stainless steel or wood, with a trough 12 to 18 in (30 to 46 cm) wide and anywhere from 10 to 25 ft long (3 to 7.6 m). On the bottom of the trough are a series of riffles or crosspieces that are 2 to 3 in (5 to 8 cm) high and run diagonally every 12 to 24 in (30 to 61 cm) along the length of the flume.
As the contents of the fermentation vessel are mixed with water and gently poured into the top of the trough, the heavier, viable seed is caught behind the riffles and the lighter pulp is easily carried down the length of the trough until it is eliminated over the spillway at the end of the sluice. After the riffles have accumulated a good amount of seed, only water is run until all of the pulp and debris has cleared. Then the riffles are taken out and the seed is washed down onto a clean fine-mesh screen at the spillway. Controlling the flow and speed of the water to properly clean tomato seed with a sluice requires practice and experience, and the trough should always be equipped with a fine-mesh screen at the spillway during the cleaning to capture any good seed that may be washed through the system due to error.
The second seed-washing method is useful for smaller batches that are frequently done in varied-sized pails, from 5-gallon (19-liter) buckets to 55-gallon (208-liter) drums. Be sure to be conscious of the previous contents of these vessels. For organic seed producers it is important to remember that any recycled receptacles used for seed cleaning have to meet organic certification standards.
At the end of the fermentation process the floating pulp that has accumulated at the top of these smaller vessels is easily eliminated by tipping and scooping off as much of the pulp as possible before the actual washing of the seed begins. Always make sure you have stirred this pulp one final time, and give the good seed that has been freed a minute to settle to the bottom of the vessel.
After scooping off the bulk of the pulp and debris, it is time to rinse and decant the seed mass in repeated cycles with cool, clear water. This is done by first filling the vessel with water, stirring, allowing the good seed to settle, and then gently decanting off the pulp that is suspended in the water without disturbing the good seed sitting on the bottom of the vessel. At first the liquid will be quite cloudy with debris, but upon repeated cycles the water will get clearer each time. The idea is to stir up the debris every time you add water, then wait long enough to allow the seed to settle, and then pour off the liquid while the pieces of pulp, placenta, and non-viable seed are suspended in the liquid. This process usually needs to be repeated at least 8 to 10 times to eliminate most of the debris before the water runs clear and the seed is clean. The debris at the end of the process is the hardest to decant off as it is the heaviest and will sink almost as fast as the good seed; it takes a deft hand to get these pieces out of the vessel without losing any good seed.
Drying Seed: Wet-seed extraction requires that the seed be dried as soon as possible after cleaning. When processing large quantities of tomato seed it is actually desirable to wring or squeeze the excess moisture out of large seed masses, which can be done after the seed is placed in strong cloth sacks. In fact, tomato seed in these cloth sacks can be run through unheated spin-dryers, then placed on drying screens. Seed racks need to be elevated to allow airflow both below and above the seed, which should be spread out evenly and fairly thinly on the racks. Tomato seed can be dried in direct sunlight as long as the heat at the surface doesn’t exceed 90°F (32°C); higher temperatures can damage the seed. The important factor in drying all seed crops is to always have good airflow. Never hesitate to use fans when good airflow is lacking, even sometimes if you’re drying seed outdoors. As the seed dries on racks, stir it at least twice a day.
In humid climates or during cool, wet weather, tomato seed is often dried with the help of controlled, supplemental heat. In large seed-processing facilities there are often cabinets where seed racks are placed on the top of long wooden or metal open-topped cabinets in rows, then warm, dry air is forced up through the racks. This warmed air must be blown through the cabinet with enough force to reach all of the seed racks along the length of the structure.
Tomatoes are largely a self-pollinated species, so they do not usually exhibit as much genetic variation as many cross-pollinated crops. Because they are a fruiting crop, most of the evaluation selection of the characteristics that distinguish each particular variety is done after the crop has matured fruit. However, a number of traits can be checked before the plant sets fruit.
It is usually possible to determine differences in leaf shape or leaf color when the plants are still in the pots in the greenhouse with only the first few sets of true leaves. When the plants are forming their first flower clusters it is usually quite easy to see the proportion of flowers to leaf nodes. Determinate types usually have one flower cluster per node, while indeterminate types often bear only one flower cluster every third node. Some tomato breeders evaluate tomato fruit shape shortly after flowering has started, by examining the very small, newly formed fruit shortly after fertilization has occurred. The shape of these small immature fruit is essentially the same as what they will become when they mature. Seed growers have used this method to their advantage by growing tomato plants to the point of flowering while still in pots, then selecting for shape before transplanting to the field.
As the fruit attains full maturity it is possible to judge if each plant’s fruit is true to type. Important traits to consider when roguing to type include fruit shape, color, and relative size. Quality traits such as flavor, texture, and juiciness can also be evaluated when the fruit ripens. The fruit of any plant should always be judged collectively and as an average of all of the fruit on that plant, as any one fruit may differ from the others due to the environmental conditions during its period of initial growth.
Determining adequate minimum isolation distances for tomato seed production has become a lively topic, with few people involved in organic seed production wanting to pin their reputations on specific recommendations. The published isolation distances that have been used for years by the conventional seed industry have been discredited with the advent of unacceptable levels of crossing between adjacent tomato crops. There are many factors that contribute to increased cross-pollination in all crops (see chapter 13, Isolation Distances for Maintaining Varietal Integrity). The accepted theory to explain this is that two fundamental differences in organic production methodology increase the likelihood of cross-pollination in this largely self-pollinated species.
The first factor that contributes to higher rates of cross-pollination is that there is often more biological diversity among plant species within their farms. Many organic farmers strive to have more crop diversity in their fields with both a more diverse number of crop species on their acreage at any given time and more active rotations across years. Both of these factors will usually lead to a richer, more diverse cohort of insect species. This is almost always true when a number of these crops are flowering crops that attract insect pollinators. Many organic growers also try to have some percentage of their diverse crop mix flowering at all times to attract beneficial insects.
The second factor is that organic farmers also often use less insecticide than their conventional counterparts. Many insecticides are broad-spectrum compounds that have the effect of killing a wide range of insect species, including the beneficial pollinators. Also, when organic farmers choose to use an insecticide that is certified by organic standards, it is likely that these products will degrade quickly and have less of an overall impact on the populations of beneficial insects on their farms. The potential increase in beneficial pollinators on organic farms from these two factors must certainly increase the number of cross-pollination events in any given season.
Tomato flowers are of two somewhat distinct types:
1. Modern tomato varieties, usually those bred and released after about 1920, have styles that are shorter than their wild ancestors did and that are usually well encased inside the anther cone. This greatly reduces the chances of the stigma coming in contact with any foreign pollen, even if an insect visits the flower.
2. Heirloom types or varieties with wild ancestry often have styles that are long enough to extend past the end of the anther cone, with their stigma clearly exposed to contact with a visiting insect. Many heirloom tomatoes, especially the potato-leaved and older beefsteak types, have this phenotype that can often be crossed, especially by diligent bumblebees that grab the cone from the bottom and push it right up against their abdomens. The tomatoes with wild ancestry—both the currant tomato (S. pimpinellifolium) and most cherry tomatoes, which are derived from the feral cherry of Mexico and Guatemala (S. lycopersicum var. cerasiforme)—also usually have the exerted styles.
The recommended minimum isolation distances between two different tomato varieties for the modern types should be 75 ft (23 m) if the crops are planted out in the open with no barriers and can be reduced to 50 ft (15 m) between them, when the two crops are separated by a significant barrier of the landscape (see chapter 13, Isolation Distances for Maintaining Varietal Integrity, “Physical Barriers”). While these distances are much increased over the isolation distances published in many older seed guides, they are currently being used by a number of the larger conventional seed production companies in Europe.
This sliced ‘Cherokee Purple’ tomato shows nicely the gel-like liquid in the carpels, which serves as the medium and food for the yeast during the fermentation process.
The isolation distance will need to be doubled for the heirloom class of varieties, which includes many of the potato-leaved types and beefsteak types. These varieties have long been known to cross-pollinate at higher rates than the modern types that are more commonly grown for commercial production. In a thorough study of cross-pollination in tomatoes from the 1920s that was based on differences in style length, J. W. Lesley found that varieties with exerted styles could cross-pollinate up to 5% per generation. For this reason it is important to separate any heirloom varieties from other heirlooms or any other tomato varieties by at least 150 ft (46 m) in open terrain, or by 75 ft (23 m) when natural barriers are present.
The flowers of currant tomatoes (S. pimpinellifolium) often have stigmas that are slightly exserted or that are flush with the apex of the anther cone, as seen in this currant tomato. In either case, much cross-pollination can occur, as bumblebees seem to be particularly fond of the currant types.
Jeff McCormack, a pollination biologist who has thoroughly examined the research into cross-pollination in the Solanaceae, believes that the minimum isolation distance used for the currant types and most cherry tomatoes should be at least as great as the minimum distance used between the heirloom types.
‘Chadwick’s Cherry’ produces an abundant crop of large, red “English-style” cherry tomatoes that are prized by many home and small-scale growers for their exceptional flavor.