In this study, we investigated the variety-dependent survival of citrus seedlings and analyzed the pomological characteristics of fruits and thorn lengths following irradiation with gamma rays. Scions from the seedlings of the following citrus varieties, ‘Yuzu’, ‘Noeulhyang’, ‘Sarahyang’, ‘Namgam’ (nucellar), and ‘Meiwakumquat’ irradiated with gamma rays at 0, 60, 80, and 100 Gy were grafted onto trifoliate orange. At 6 months after grafting, the survival rates of ‘Yuzu’ and ‘Noeulhyang’ exposed to 100 Gy were 49.5% and 34.8%, and that of ‘Sarahyang’ and ‘Namgam’ (nucellar) exposed to 80 Gy were 51.6% and 46.8%, respectively. ‘Meiwakumquat’ exhibited the lowest survival among the varieties used in this study. Therefore, we classified ‘Meiwakumquat’ as highly susceptible to gamma radiation; ‘Noeulhyang’, ‘Sarahyang’, and ‘Namgam’ (nucellar) as intermediately susceptible; and ‘Yuzu’ as the least susceptible. We assessed the effects of gamma radiation on the pomological traits in irradiated ‘Tambit1ho’ and ‘Yuzu’. Among the 72 irradiated ‘Tambit1ho’ seedlings, the average seed number (number of seeds per fruit) varied between 0 and 18.6, whereas that in the unirradiated seedlings was between 8 and 18. Among the irradiated seedlings, the average seed number was less than 1.0 in eight seedlings. In addition, we observed variations in weight, rind and segment hardness, and sugar and acid contents. The thorn length of the flush in unirradiated ‘Yuzu’ was between 1.3 and 6.0 cm, whereas that of flush in gamma-irradiated seedlings was between 0.1 and 6.1 cm. Among the irradiated ‘Yuzu’ seedlings, the thorn length was less than 0.1 cm in two seedlings.

Mutation breeding through irradiation has been applied to several varieties and genetic resources since the discovery of the use of X-rays for inducing mutations in plants by Stadler in 1928. A heavy ion beam with high linear energy transfer (LET) shows a higher relative biological effectiveness (RBE), and it is more effective in inducing plant mutations than low LET radiations, such as X-rays, gamma rays, and electrons. Since early 1990s, several plant breeding programs in Japan have used heavy ion beams from accelerators. These beams impart a high energy effect on a local target; therefore, they induce a higher number of single and double strand DNA breaks. In addition, they induce a large number of DNA deletions than low LET radiations. Therefore, a heavy ion beam is superior to low LET radiations in terms of induction rate and the mutation spectrum. In Korea, a heavy ion accelerator that can be used for breeding is under construction. However, a large-capacity proton accelerator (KOMAC: Korea Multi-purpose Accelerator Complex) was built recently, and it is a pioneer step in breeding research worldwide. This review summarizes the basic characteristics, successful research achievements, and the prospect of application of high LET accelerator beams in plant mutation breeding.

Research on mutation breeding started in the early 1960s by researchers at the Atomic Energy Research Institute, Rural Development Administration (RDA) and several universities in Korea. The Radiation Agriculture Research Institute (RARI) was established in 1966, and studies of mutation breeding using radiation were actively conducted for a while. RARI was merged into the Korea Atomic Energy Research Institute (KAERI) and RDA in 1973, and radiation breeding research was neglected by the two agencies. In the 1980s, the relevant research department was lost, which resulted in a recession period of radiation breeding research. The Advanced Radiation Research Institute (ARTI), under the KAERI, was established to promote radiation research and the industry in 2005, which led to the activation of radiation breeding research. Then, the Radiation Breeding Research Center (RBRC) at the ARTI was established with support of the Ministry of Agriculture, Food and Rural Affairs in 2013. Recently, the importance of seed and genetic resources has been emphasized in Korea, and many institutes, companies and private breeders are interested in mutation breeding. The RBRC is trying to develop advanced radiation breeding techniques and new genetic resources using mutation techniques combined with bio-tech. This is to deal with the loss of biodiversity due to global climate change and environmental degradation, growing global demand for food and bio-energy, and to strengthen the protection for new plant varieties. Approximately 180 new mutant varieties were developed and registered officially in Korea. Recently, new mutant varieties, especially of flowers and ornamental plants, have quickly increased and are being commercialized, mainly by private company and breeders.

Tocomi-1’, a rice variety with high tocopherol content and a reddish brown color was developed from ‘Dongan’ by a mutation breeding technique using a 120 Gy gamma ray source at the Korea Atomic Energy Research Institute (KAERI). The heading date of this variety was August 12, which was 2 days later than the original variety ‘Dongan’. The culm and panicle lengths of ‘Tocomi-1’ were 80 cm and 20.3 cm, respectively. The number of tillers per hill was 19.1 and the number of spikelets per panicle was 106.3. The ratio of ripened grain was approximately 87.0% and the weight of 1000 grains was 25.4 g. The pericarp of brown rice was reddish brown in color. The total tocopherol content was 1.65 mg/100g, which was higher than that of the original variety (1.09 mg/100g). In addition, the brown rice of ‘Tocomi-1’ contained 8% more total amino acids. The average yield potential of ‘Tocomi-1’ in grain and brown rice was approximately 6.18 kg/10a and 5.15 kg/10a for 3 years, respectively (Registration No. of Plant Protection Wright: 6813).

This study was carried out to compare the survival and mutation rates and mutation spectrum by gamma-irradiation on rooted and unrooted cuttings of three spray type (‘Lovelydia’, ‘Yellowbabe’, and ‘Haetsal’) and two standard type (‘Vital’ and ‘Aqua’) cultivars in roses. Two groups, rooted and unrooted cuttings were gamma-irradiated at 70Gy for 24 hours. The irradiated rooted and unrooted cuttings were planted in a greenhouse, and survival, mutation rates and mutation spectrum were investigated 30 weeks after planting, respectively. As a result, survival and mutation rates of gamma-ray irradiated plants were 16.4%~50.8% and 0~5.1% for unrooted cuttings, and 39.4%~55.1% and 0.7%~7.4% for rooted cuttings, respectively. In conclusion, both survival and mutation rates were a little higher on rooted cuttings than on unrooted cuttings. However, when only survived plants after gamma-ray irradiation were considered, mutation rates were 0~10% and 1.8%~14.1% for unrooted cuttings and rooted cuttings, respectively, showing no significant difference. In addition, diverse variations on color and number of petals or shape of flowers were detected both in plants from rooted and unrooted cuttings, which indicated that there was no significant difference in mutation spectrum between two groups.