Vegetable Seed Production
Vegetables are harvested for some component of their vegetative (celery, carrot) or reproductive (tomato, broccoli) structure. In most cases, the harvest of the market-use crop requires environmental conditions that are very different from those for reproductive and development of a mature seed crop. For example, ambient temperature, photoperiod, length of growing season, and precipitation amount and distribution may vary for the optimum production of market-use and seed crops.
Ambient temperatures have very different effects on vegetables and their end use. Those vegetables that are harvested for market-use such as carrot, onion, radish, brassicas, and lettuce are typically grown in areas with lower temperatures from those required for a seed crop in order to maintain good vegetative growth and minimize bolting. Seed production of these crops often requires higher temperatures to encourage bolting although extremely high temperatures also hinder pollen growth and fertilization which lead to reduced seed number per floret or fruit. This is true in onion, tomato, and lettuce. In contrast, beet and carrot market-use crops have been selected to avoid bolting in mild climates but they still require adequate periods of cool temperatures (vernalization) for flowering and seed production. The Pacific Northwest of the United States provides such sustained cool temperatures and this is the reason why beet and carrot seed production occurs in this region while market-use production of these crops is geographically more diverse.
The requirement of some vegetables for specific photoperiods before induction of flowers also limits those areas where vegetable seed production is successful. This effect is dependent on the latitude and the length of the photoperiod which greatly affect seed yield. For example, onion is a vegetable crop that exhibits a wide diversity in photoperiod response and this influences where seed production occurs. Seed production of long-day onions occurs at latitudes greater than 45o N and short-day onions at latitudes no greater than 35o N. Limited seed production also can be accomplished in glasshouses where supplemental light is provided or light excluded to promote flowering.
Even when appropriate photoperiods are present, temperatures must be suitable for the completion of the entire life cycle of the crop during the growing season. Successful market-use crops such as onion, radish, broccoli, and cabbage are harvested before flowering occurs. As a result, these crops are grown in regions with cooler temperatures not suitable for seed crops which require longer periods and warmer temperatures for maturation of seed.
Precipitation amount and distribution also varies for market-use and seed crops. Many vegetable crops harvested for their vegetative structure require large amounts of early season rainfall for most rapid development. Seed crops, in contrast, demand uniform precipitation during development followed by dry conditions during seed maturation. Under these environments, dependable conditions for seed harvest are possible and pathogen infestations minimized. This is the principal reason much of the vegetable seed production has shifted to the western United States. The Mediterranean climate of the Pacific Northwest assures cool, dry summers and warm, wet winters for optimum development of many biennial seed crops. Southern California and much of Arizona are hot and dry during the summer which is ideal for vegetable seed production. The dry condition of these locations permits seed crops to be irrigated with the proper amount of moisture at the proper time so that optimum seed quality is achieved.
Seed Production
The following represents general requirements and considerations utilized in vegetable seed production.
Tillage. The preparation of a soil site for vegetable seed production differs little from that for fresh market production. Soils should have a high water holding capacity to encourage uniform vegetative growth. Depending on the crop and water availability, planting is done on raised, shaped, or flat planting surfaces which require appropriate soil preparation. In all cases, the seed bed must be uniform because most vegetable seed crops are precision planted and uniform emergence and seedling development necessary for optimum performance.
Planting. The planting of vegetable seeds for seed production is generally done by direct drilling or transplanting of glass house grown seedlings. Biennial vegetable seed production, however, can be achieved using two planting techniques. The "seed-to-seed" method is done by planting biennial seed, allowing the resulting plant to overwinter without being transplanted, and harvesting the seed crop the following season. This method does not permit the selection or roguing of root or other genotypic characters. A majority of carrot and onion seed crops are produced by this method which is less expensive and complex than the "root-to-seed" method. The "root-to-seed" method is done by planting biennial seed, removing the resulting plant from the soil (in some cases stored), replanting, and harvesting the seed crop the following season. The replanted plant is referred to as a "steckling." This method permits the selection or roguing of root or other genotypic characters. It also decreases the time required for varietal development. For example, onion bulbs adapted to dry regions can be planted in the spring in the desert regions of the southwest and then shipped for seed production to central California or the Pacific Northwest for planting in the early fall of the same year. This is the most expensive method of vegetable seed production due to transportation, handling, and replanting costs, but the benefits of checking for minimum genetic quality standards before replanting are important in vegetable seed production.
Vegetable seed crops must be planted so that sufficient vegetative development occurs to support optimum fruit and seed development. In addition, the increasing emphasis on hybrid seed production necessitates planting of male and female lines at times that assure successful pollination, a process known as nicking. This may require that genetically different male and female lines be planted at differing times so that synchronous flowering occurs. Most hybrid broccoli is transplanted and staggered plantings are necessary for the male and female lines to ensure proper nicking. Hybrid cauliflower and squash are direct seeded but because of the differences in time required for the male and female lines to flower, they are planted in a relay fashion where the earlier flowering line is planted first followed by the later flowering line.
Vegetable seed production also is unique from most agronomic crops because vegetables such as beets, carrots, and cabbage are biennials and must develop sufficient vegetative growth prior to cool temperature exposure in order that vernalization successfully induces flower formation the following season. Planting such crops too early causes winter kill or late season pest infestations. Planting too late results in a lack of vernalization which limits flowering and reduces seed yield. In some instances, vegetable seed producers opt not to start the seed crop by seed. Rather, to ensure optimum vegetative growth for vernalization, they plant some developed portion of the vegetative structure. This is commonly done with carrot and beet stecklings and onion bulbs which allows for maximum production of plant biomass before vernalization.
Vegetable seeds also vary greatly in their tolerance to soil temperatures at planting. Pea, radish, and spinach require cool soil temperatures for optimum seedling emergence. Beets, cabbage, carrot, and onion are tolerant of cool soil temperatures although they do better in warmer soils. Squash and melons require warm soil temperatures for optimum seedling emergence. In regions with high summer temperatures, vegetables such as tomato, pepper, eggplant, and cucurbits are planted in the early spring to optimize flowering and seed set in early summer. Later plantings of these seed crops are possible by transplanting young plants previously started in glass houses. This avoids the delayed establishment of plants from direct seedings due to cooler night temperatures and wetter soils encountered in the early spring.
Row spacings and planting densities of vegetable seed crops differ from those for fresh market production. Sufficient space for flower development, air movement to reduce pathogens, unrestricted access to inflorescences by pollinators, mechanical cultivation, and harvest operations are necessary for seed crops. In some cases (e.g., eggplant, pepper, tomato, muskmelon, watermelon), row spacings used for seed production are the same as those for fresh market production. In most cases, row spacings are different for vegetable seed production. In lettuce, the size of the head will be larger than the fresh market head in order that bolting is promoted. Large-head types are planted 25 to 30 cm apart while small-head and loose-leaf types are planted 15 to 20 cm apart. Similar considerations are encountered for cabbage. Broccoli and cauliflower produce sizable seed stalks requiring rows 40 to 60 cm apart. Planting densities are often different as well in vegetable seed crops from fresh market vegetable crops. Beet seed production is accomplished at higher planting densities than beets harvested for their roots. Carrot, celery, and parsnip seed production is planted at densities much less than for root or stalk production. Higher planting densities of onions grown by the bulb-to-seed method reduces lodging of the large seed stalk scapes.
The method (wind or insect) of pollination affects row spacings and planting densities. Vegetables that are wind pollinated include sweet corn and spinach. Insect pollinated vegetables include most of the cole crops, carrot, and onion. Row spacings and planting densities are also important considerations in the production of vegetable seed hybrids. In order to maximize the amount of hybrid seed produced, the optimum ratio of female to male rows must be determined.
Fertilization. Vegetable seed crops have similar nutrient requirements as fresh market crops. Where possible, nutrients that stimulate reproductive development can be supplemented with normal fertility regimes. Boron, for example, increases the number of fruiting sites and seed produced per pod in legumes such as pea and bean. Molybdenum also is essential for optimum nitrogen fixation of legume seed crops. Another approach to stimulating reproductive development in vegetable seed crops is to split the application of fertilizers at planting and before flowering. This avoids luxury consumption of the elements at the time of application, improves crop uptake efficiency, and increases the formation of reproductive structures. Generally, lower nitrogen and phosphorus levels are used in seed production because these encourage continued vegetative growth. Because most vegetable seed crops are grown in rows, fertilizers are typically applied either in bands in the planting row or as a side-dressing beside the row after establishment.
Weed and Pest Control. The management of weeds and pests in vegetable seed production poses additional problems due to their longer growing season not encountered for vegetables grown for market use. This extended growing period necessitates an increase in residual pesticide activity or additional pesticide applications. In some cases, governmental regulations do not permit the use of the same pesticide for a vegetable seed crop that is used for the market use crop. This is because the approval process is directed at the more economically important value crop. Some pesticides which control insects also detrimentally affect insect pollinators essential for cross pollination or hybrid seed production and cannot be used in a seed production field. Such restrictions on the use of pesticides in vegetable seed production imply that the safe use of pesticides will be more difficult in the future and greater reliance on crop rotations and biological control will be necessary.
Insects and the timing of their infestations affect vegetable seed production. The Lygus bug prefers legume crops and safflower as its primary host. However, once these crops are harvested, it commonly flies to adjacent carrot seed fields and rapidly creates epidemic infestations. There, it feeds on developing seed embryos and subsequently reduces seed yield and quality. Cabbage looper is a significant pest of brassicas during vegetative development but does not affect seed stalk and pod development. Thus, control of this pest is important during vegetative growth if maximum yield and seed quality are attained. Aphids affect the harvest and subsequent conditioning of brassica seed crops by the development of "honeydew" which adheres to the seed pods and makes separation difficult.
Vegetable seed crops are susceptible to the same diseases as fresh market crops. However, the need to complete their entire life cycle for seed production means that other diseases may also be encountered and require control. Many significant diseases of vegetable seed crops can be controlled by growing the crop in differing geographic locations. For example, the low relative humidity and high temperatures of the southwestern United States reduce the incidence of the seedborne pathogen Cercospora carotae in carrot seed fields commonly found in the high relative humidity, low temperature Pacific Northwest. The incidence of downy mildew is reduced by growing long-day onion seeds in the low humidity areas of the desert southwestern United States.
Diseases in vegetable seed fields can also be minimized by separating the seed crop from disease pests either physically or over time. Physical separation occurs by producing the seed crop in an isolated region from fresh market crops or from the disease causing organism. Celery seed fields are isolated from celery stalk production fields to reduce the incidence of late blight caused by Septoria. Lettuce seed fields are isolated from leaf and head production fields to reduce the incidence of seed-borne lettuce mosaic virus. Isolation from the disease causing organism is achieved by maintaining weed-free irrigation canal banks and roadsides which serve as alternate hosts or growing the seed crop where the disease causing organism does not exist. To meet phytosanitary requirements for export, onion seed crops must be grown in regions where certain disease and nematodes are certified to be absent. The lettuce mosaic virus is another serious disease that results in unmarketable lettuce heads. To control this disease, an integrated pest management strategy has been established which focuses on the planting of disease-free seed and control of insects and weeds. Disease-free seed is achieved by planting stock seed in glass houses free of such virus vectors as the green peach aphid (Myzus persicae). Commercial seed lots are certified to have not more than one infected seed in 1,000 seeds that have been screened. Other effective measures include control of common weeds that serve as alternate hosts to insect vectors and frequent roguing of lettuce seed fields to remove any plants suspected of carrying the virus.
Some disease causing organisms have relatively short life spans. In these cases, separation of the seed crop from the organism over time reduces disease problems. Alternating cabbage and other cruciferous seed crops in a three year rotation with non-cruciferous crops decreases the incidence of club root caused by Plasmodiophora brassicae. Nematodes can similarly be controlled in cucurbit and solanaceous seed crops by three year rotations with crops not from these families. In other cases, diseases can be controlled by storing the seed for a period during which the disease causing organism dies. An example is soft rot (Erwinia carotovora) on celery seeds which dies after two years of seed storage. Hot water baths or chemical treatments also control diseases. Hot water treatment is used on a variety of vegetable seeds to control a number of seedborne bacteria and fungi (Table 0.01). Many disease causing organisms are carried on the seed coat and most of these are controlled by various topical seed treatments consisting of fungicides, insecticides, and bactericides.
Harvesting, Threshing, and Drying. Vegetable seed crops are divided into two categories based on the type of fruit harvested. Dry-seeded fruits include the brassicas, legumes, and onion and wet-seeded fruits include the cucurbits, melons, and tomatoes. The methods of harvest and extraction of seeds vary dependent on the type of fruit harvested.
Of great concern in dry-seeded fruit vegetable seed production is the potential for loss of yield due to seed shattering when harvesting occurs after physiological maturity. To minimize seed loss, two approaches are employed. Since many dry-seeded vegetables achieve physiological maturity before harvest maturity, they are cut while the plant is still green and placed into rows where the wind further dries the plant and its reproductive structure. This permits more uniform maturation of all dry-seeded fruits and seed quality is enhanced compared to the more costly option of sequential harvest of mature fruits. Later, after the seeds have reached harvest maturity, they are extracted from the seed heads either by hand or mechanically using a combine. The second approach is to visually monitor the development of the dry-seeded fruit and harvest the crop before shattering occurs. For example, onion seeds are very susceptible to shattering once the florets have dried and seed germination and quality is reduced if the scape stalks are cut too early. A common harvest criterion has been to examine umbels for the first mature seeds. In indeterminate crops such as carrot, seeds mature in sequential order with those on the primary umbel maturing first followed by those on secondary and tertiary umbels. Secondary umbels produce about 50% the quantity of pure live seed of the primary umbel. Thus, harvest of this dry-seeded crop is complicated by the umbel maturation sequence. Seeds produced from the first lettuce flowers also tend to be larger and of higher quality than seeds produced from later flowers.
Seed harvest from wet-seeded fruits is often determined by fruit color. Seed quality in pepper as determined by percentage germination and rate of germination is higher in seeds harvested from red fruits compared to those harvested from green fruits. In tomatoes, maximum seed germination is attained when the fruits are at a mature green stage compared to those which are at the red or fully-ripe stages.
After the fruits are determined to be at the correct stage for maximum seed quality, they are gathered and the seed extracted. In some cases, this is done in one mechanical operation where the fruit is picked from the row, conveyed to a crushing mechanism, and the pulp removed from the seed by washing with water in a rotating drum. For high moisture vegetables such as melons and tomatoes, water is periodically drained from the equipment. For low moisture vegetables such as squash and peppers, water is periodically added to the equipment. In most cases, vegetable seed companies mechanically harvest crops such as tomatoes, peppers, and cucumber in the field, transport the fruit to a central extraction plant, and remove the seeds from the fruit. In tomato, cucumber, and some melons, a mucilaginous layer around the seed must be removed prior to cleaning. This can be done by two methods. The first, and most common, is the retention of seeds in the crushed fruit pulp for a few hours to more than a day so that fermentation occurs. This process is closely monitored because the fermentation creates heat and mechanical injury to the seeds if continued too long. The second approach is to add sodium carbonate or hydrochloric acid to the crushed fruit extract to degrade the mucilaginous layer. After the fruit is crushed and any mucilaginous layer removed, the seed is separated from the pulp and passed along a water sluice containing gates along its entire length. Mature high quality seeds sink behind the gates and are separated from the pulp by the running water. After the pulp is removed, the sluice gates are opened and the mature seeds captured on a screen. At this stage, the wet seed must be promptly dried to avoid heating and loss of seed quality with increasing imbibition. To accomplish this, the wet seeds are collected and placed into a spin dryer to remove excess water and the moist seeds are evenly spread on screens and exposed to large volumes of heated air. After drying is completed, the seeds are handled and conditioned in the same way as employed for dry-seeded vegetable crops. Examples of conventional harvest methods for dry- and wet-seeded vegetable seed crops are provided in Table 00.2.
Cleaning and Storage. The physical quality of the seed lot following harvesting, threshing, and drying must be evaluated to determine whether precleaning is necessary. If the seed lot possesses large amounts of inert material such as stones or plant debris, it should be passed over a scalping machine. If the seed possesses undesired appendages which cause the seeds to clump together such as carrot, then it must be passed through a debearder to ensure free-flow of individual seed units. Impact debearders are also used in multigerm beet seeds to split the fruit into single seed units. After precleaning, vegetable seed crops are conditioned in the same way as other seed crops using the same principles and equipment (Chapter 00). Some additional improvements can be made. For example, greater seed density improves the seed quality of lettuce, tomato, and onion crops. This can be accomplished by placing the seed mass into a solution of specific osmotic concentration. The more dense seed fractions are retained and the more buoyant seeds removed. After cleaning is completed, vegetable seeds are stored using the principles of successful seed storage (Chapter 00). Approximate seed moisture contents of vegetable seeds held at differing relative humidities in storage are provided in Table 00.3.
It is not possible to exhaustively study the range of seed
Table 00.2. Conventional harvest methods for dry- and wet-seeded vegetable seed crops based on their mature reproductive structures (courtesy of J. J. Steiner).
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Dry-seeded Crops. Fruits and pericarp dry when mature; seeds can be immediately conditioned by conventional mechanical methods.
Direct Harvest: Mother plants erect, all inflorescences mature and uniformly dry, seeds not easily shattered
beans, peas, sweet corn, carrot
Indirect Harvest: Mother plants not erect or inflorescences unevenly mature and seeds of differing moisture contents or seeds shatter easily once mature
brassicas, carrot, onion, sweet pea, beets, spinach
Wet-seeded Crops. Fruits fleshy and pericarp partly or wholly fleshy; seeds extracted from fleshy pericarp and dried before conventional conditioning.
Direct Harvest: Pericarp dry, berry-like, crop can be direct combined with little modification to harvest equipment
asparagus
Indirect Harvest: Pericarp semi-dry to wet, fruits containing seed gathered by hand or machine, fruit crushed, and seed separated from pericarp
cucurbits, tomato, pepper, eggplant
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production schemes utilized in the production of vegetable seeds. Rather, six of the most important vegetables (onion, tomato, cabbage, lettuce, cucumber, carrot, and sugar beet) are considered. These were selected not only because of their economic value but also because they represent differing botanical families, generally differ in their vegetative/reproductive structure harvest, and illustrate differing seed production practices. Similarities exist in the seed production of related species and many of the principles detailed here can be extended to these crops as well.
Bassett, M. J. 1986. Breeding Vegetable Crops. Avi Publishing, Westport.
George, R. A. T. 1985. Vegetable Seed Production. Longman Press, Essex.