Vegetable Seed Production



What is a Vegetable?

Most definitions of vegetables are not botanically based. These definitions are based on usage and are rather arbitrary. A common definition is: A vegetable is a herbaceous plant, or portion of a plant, that is eaten whole or in part, raw or cooked, generally with an entree or in a salad but not as a dessert. Of course there are exceptions to this. Rhubarb and melons are both used commonly as desserts. Since "vegetable" is not a botanical term, some vegetables botanically speaking are also fruits. For example, tomato, pepper, bean, and cantaloupe botanically are fruits but because of the way they are used and produced and for historical reasons they are considered to be vegetables. Fruit is often used in a strictly botanical sense to describe a ripened ovary containing seeds together with adjacent parts that are eaten at maturity. Confusingly, there is also a definition of fruit that is also based on usage and is less botanically oriented. By this definition, a fruit is simply a sweet and edible plant structure consisting of a fruit (in the botanical sense) or a false fruit of a flowering plant usually eaten raw or as a dessert. Many true or botanical fruits are not sweet such as tomato, bean etc. and these are the ones commonly referred to as vegetables. Vegetable crops are usually considered high value crops that are intensively managed and should be contrasted with agronomic crops, which are extensively grown and less intensively managed. Wheat, cotton, and rice are all considered agronomic crops. Some crops like Irish potato may be considered as either vegetable or agronomic crops. Some agronomic crops such as tobacco are intensively managed and of high value but are considered agronomic for historical reasons. Corn and soybeans can be either agronomic or vegetable crops depending on the cultivars grown, the time of harvest, and the end use of the commodity.

Vegetable science, sometimes called olericulture, is one of the most dynamic of the agricultural sciences because of the many different types of vegetables and the diverse methods for growing them. The consumption of fresh vegetables has increased dramatically in recent years in the United States largely due to the realization that vegetables are good for human health and to the greater availability of vegetables throughout the year. There are approximately 100 major vegetable crops and a total of more than 350 vegetable crops traded commercially in the US. Vegetable crops are classified in many ways but the botanical classification system is widely used because it is based largely on flower morphology and uses Latin names that will not change over time. For this class we will emphasize the Latin family, genus, and species names.

There is some confusion about the correct classification of vegetables below the species level. Many vegetables have been classified into different botanical varieties based on their morphological characteristics.

  • Cultivar: contraction for "cultivated variety"; a plant that is clearly distinguished by identical physical characteristics and maintains these characteristics through proper propagation means.

  • Botanical Variety: a ranking of plants below species but above cultivar used to recognize geographic or crop distinctiveness of interbreeding populations, often found in the wild, that have distinctive traits. The ranking of Botanical Variety is equivalent to Subspecies and Group.  

  • Strain: used to designate an improved selection of a cultivar.

  • Lot: a particular batch of seeds that were produced and processed together.

  • Type: used to denote a series of cultivars of a crop that have similar characteristics without reference to genetic or morphological characteristics.

A major problem occurs with the classification of botanic variety. This confusion in nomenclature is due to the fact that molecular genetic studies have shown that major morphological differences may be caused by single or few genes and therefore do not warrant classification into separate botanical varieties.

An example of this would be cauliflower (family: Brassicaceae, genus: Brassica, species: oleracea, botanical variety: botrytis, cultivar: Snowball Y). Molecular genetic studies by Elliot Meyerowitz and other have shown that the a single gene controls the development of undifferentiated flower primordial (curd) and if this gene is expressed in broccoli or cabbage they also produce a curd like cauliflower. Over the past 15 years, there has been a growing consensus among plant biologists that it is invalid to subdivide Brassica species into distinct botanical varieties.

Obviously cauliflower is different from cabbage to make light of these horticultural differences, the Index of Garden Plants by Mark Griffiths, 1994, Timber Press, Portland, Oregon, USA classifies cauliflower as Brassica oleraceae Group Botrytis rather than botanical variety botrytis, which is more common in the older literature. Therefore, the term Group (gp.) is used for convenience by horticulturists to show groups of horticultural significance within a species that were previously classified as separate varieties. Taking the cauliflower example a step further, Snowball is a particular cultivar of cauliflower, Y is a particular strain of Snowball and different production fields of Snowball could be designated as individual lots. A Snowball type would refer to cultivars with the same basic characteristics as Snowball (e.g. a early maturing cultivar that does not require vernalization to develop a curd) and would include all the various strains of Snowball. For example, a cauliflower grower may ask a seed salesman if a cultivar is a "Snowball type" even though it may have a different cultivar name.

What is a seed?

Although a few vegetables like potatoes and sweet potatoes are vegetatively propagated, the vast majority of vegetables are established from seeds. One definition of a seed is "an immature plant in an arrested state". Seeds are one of the true wonders of nature because of their desiccation tolerance, small size, storability, ability to sense the surrounding environment, rapid growth, and persistence. Using quality seeds is a prerequisite for successful vegetable production. There are several classifications of seeds that are used to describe how seeds are pollinated.

Many people now use this term rather loosely to simply refer to any cultivar that is a non-hybrid. A stricter definition is: "a heterogeneous variety of a cross-pollinated crop this is allowed to inter-pollinate freely during seed production." Open-pollinated cultivars will produce plants reasonably true-to-type but by nature there is more natural variation in a open-pollinated than a self-pollinated crop. By the same token an F-1 hybrid will be more uniform than an open-pollinated cultivar of the same crop. Many heirloom cultivars of cross-pollinated vegetables, the cucurbits or radishes for example are open-pollinated.

Self versus Cross-pollinated Crops
Many crops have perfect flowers that contain both male (anthers) and female (pistils) flower parts and therefore self pollinate. Some species have perfect flowers but cross pollinate because flower parts do not develop at the same time, the pollen is not compatible with stigmas on the same plant, or the pollen is sticky and most be moved by bees or other insects to adjacent flowers. Other crops cross pollinate because they have separate male and female flowers or produce separate plants that are male or female

Pure Line
A self-pollinated crop that does not outcross is often referred to as a pure line cultivar because all the seeds were derived by self pollination of a single flower, for example, lettuce or most beans. Pure line seeds are usually uniform and will grow true-to-type from seed.

Increasingly in developed countries vegetable seeds are F-1 hybrids (made by sexually crossing inbred lines) because of their greater vigor, uniformity, and yield potential. Seed companies also like F-1 hybrids because the seeds harvested from the F-1 hybrid plant cannot be successfully used as propagules in the next generation. In other words, commercial seeds grow true-to-type, but their offspring don't. Seeds saved from an F-1 hybrid plant will be the next generation (F-2) and will in most cases produce plants inferior to the parents. Many of the F-1 hybrid seeds used today are still more expensive than the open-pollinated seeds used in the past, but vegetable seeds are still a rather small investment compared to other production costs that growers incur.

There are F-1 hybrids available for both naturally cross-pollinated as well as of self-pollinated crops. Discussion of hybrid production systems will be covered more specifically later. Some crops are not amenable to mass production of hybrid seeds for commercial use. Lettuce, for example, has perfect flowers and each floret produces a single seed so emasculating and hand pollinating each flower is not cost effective. Hybrid seeds are produced in some crops using "tricks" of nature to control the crosses that are made. For example, geneic and cytoplasmic male sterility, genetic pollen incompatibility, and use of plant growth regulating compounds to change floral sex expression are a few of the techniques used to control crosses for the production of F-1 hybrid seeds. In the case of lettuce, none of these techniques have been harnessed for mass production of hybrid seed so all lettuce seeds are open-pollinated at this time.

Most agriculturists recognize it is unwise to cut costs by saving a few dollars on seeds of a substandard cultivar or by purchasing lower quality seeds. Cutting corners on seed costs will generally end up costing even more because of lost revenues from lower yields. Like so many other areas of agriculture, seed handling practices have changed dramatically over the years. For generations, farmers saved their own vegetable seeds and maintained their own cultivars. More recently, seeds were often obtained from local agricultural retailers. Today, vegetable growers are faced with a wide array of seed treatments and cultivars that may be purchased from sources around the world.

In addition to reading the information in this section, please view this PDF file, "A Model for Producing Quality Seed."


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