**2. An overview of radish breeding**

### **2.1 Origin and distribution**

Radish is an annual vegetable in the Cruciferae family. From the Mediterranean to the Black Sea, the genus Raphanus was separated into the Raphanus DC. and Hesperidopsis Boiss sections. Each part has six species (*R. sativus, Raphanus raphanistrum, R. microcarpus, R. rostratus, R. landra,* and *Rumex maritimus*) and one species (*Raphanus aucheri*) [19]. Kitamura [19] also proposed that *R. sativus* may be grown in the Mediterranean region by natural or artificial hybridization of *R. landra* with *R. maritimus*. Banga [20] and Hida [21] proposed that four wild species (*Raphanistrum raphanistrum*, *R. maritimus, R. landra*, and *R. rostratus*) may have aided in the evolution of radish. Panetsos and Baker's [22] study on wild *R. sativus* and *R. raphanistrum* confirmed the species differentiation. The wild *R. sativus* has a white or partially purple flower on a white background as well as a delicate, fairly thick pod made up of spongy parenchyma. *R. raphanistrum*, on the other hand, bears yellow blooms

#### *An Update on Radish Breeding Strategies: An Overview DOI: http://dx.doi.org/10.5772/intechopen.108725*

and slender, robust pods. The ripe pods disintegrate into bits. The aforementioned two species flourished in California in intermediate forms that may have arisen from natural hybridization. The F1 plants produced by artificially crossing the two species demonstrated intermediate features, including chromosomal configurations of 1IV + 7II at initial metaphase (MI) of pollen mother cells (PMCs) and fertility of 50% in both pollen production and seed setting. Based on these findings, it was proposed that the F1 plants had reciprocal translocations in one pair of chromosomes. When *R. sativus* wild type was spontaneously backcrossed with cultivated radish, some progeny lacked quadrivalent chromosomes and had a high seed setting rate. It was therefore suggested that gene flow (or introgression) from *R. raphanistrum* into radish cultivars be encouraged. Eber [23, 24] found comparable findings in their study of hybrids between the wild *R. raphanistrum* and the cultivated *R. sativus* in France. In F1 hybrids of *R. sativus* and *R. raphanistrum* L. ssp. *landra*, Kato and Fukuyama [25] revealed correct chromosomal organization of 9II during meiosis and robust seed setting. Based on these findings, it was hypothesized that *R. raphanistrum* might undergo chromosomal reconstruction. Harberd [26] proposed that the genus *Raphanus* be classified as a cytodeme based on chromosomal number, chromosome layout at MI in PMCs, and fertility studies. This finding was corroborated by Prakash [27]. Tsunoda [28, 29] thought that all wild radish species belonged to R. raphanistrum and evolved around the Mediterranean-Black Sea coast. *R. raphanistrum* was common in Russia and the New World, but it was not found in China, Japan, or India [30, 31]. *Raphanus* was recently separated into two species, *R. sativus* and *R. raphanistrum*, the latter of which contains additional wild species as *R. raphanistrum* subspecies [31]. *R. sativus* var. *hortensis* f. *raphanistroides* Makino [19], also known as Hama-daikon or *R. raphanistrum* ssp. *maritimus* [31], grew wild along the East Asian shoreline. Another kind of wild radish, Nora-daikon or No-daikon, thrived in areas far from the sea. It was thought that these wild radishes were the result of cultivated radishes escaping [19, 32–34] or the migration of weedy radishes tainted with cereals such as wheat and oat. Numerous research, however, support the first point of view. Germplasm resources for understanding the origins of farmed radish and improving the radish crop include Hama-daikon and Nora-daikon. Molecular studies of DNA and genomes, in addition to morphology, ecology, and cytogenetics, may provide insight into the origin, differentiation, and domestication of radish.
