- In vegetable breeding, there are various breeding objectives for consumers, such as eating quality, colour and nutrient content. For producers, there are breeding objectives such as high yields, early maturity and disease resistance. However, there is one more objective in vegetable breeding to consider.
- Even if a variety has a good flavour and high yields and is disease resistant, unless the seeds are stable and can be relied on to produce the same characteristics every year, it cannot become a good variety. Seed is also an agricultural product, and a good variety must have the characteristic of good seed production.
- In recent years, most of the vegetables cultivated over a large part of the world have been F1 hybrid varieties that realize high yields and early maturity through heterosis. Heterosis breeding is only possible if there is an efficient method of producing F1 seed on a large scale. For this, some control of reproduction or pollination is needed.
- In controlling pollination, self-incompatibility (SI) has been used for the Cruciferous crops. These include many important kinds of vegetables, such as cabbage, radish, Chinese cabbage, turnip and broccoli. The study of SI in crucifer crops began in Japan, where it still continues. In 1949, a Chinese cabbage F1 hybrid variety, "Nagaoka Kohai I Go", was produced by Shojiro Ito, and in 1961 a radish F1 hybrid variety, "Harumaki Minowase", was produced by a commercial seed company. Most cruciferous vegetables grown in Japan and in other countries are now F1 hybrid varieties, whose seeds are produced using SI.
- However, SI is not always stable. It can be easily overcome under various external and physiological conditions. Therefore, stable F1 seed production has been the subject of study for many years.
- SI is governed by a series of multiple alleles (referred to as the S-gene) (Bateman 1955). However, there are genetic variations in the level of SI (referred to as LSI). Therefore, it can be assumed that SI is also regulated by genes other than the S-gene.
- There are two major seed production methods using the SI system for crucifer crops. One method is a single cross, in which SI is overcome in parental seeds by treating them with CO2 gas. The other method is a double cross, in which parental seeds are produced using a pair of isogenic lines for the S-gene.
- There are genetic variations in the Reaction Level of Self-Incompatibility (RLSI) to a 4% CO2 gas treatment. Thus, the parental lines in the parental seed production in a single cross have to show a marked reaction to CO2. In F1 seed production on both a single and a double cross, the parental lines have to show a high LSI. It is therefore important to know the genetic relationship between these characteristics.
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