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.

‘Shingil’ was developed as a processing rice for specialization as a rice flour by means of mutation breeding from ‘Hanareum’ MNU (N-methyl N-nitrosourea) treatment. The bulk population was displayed from M₁ to M₆, followed by pedigree methods from M₇, where line selection was carried out based on the amylose content, with opacity in the endosperm. The result was that ‘Milyang317’, which has a somewhat high amylose content as well as a high ratio of starch opacity, was selected and named as ‘Shingil’ in 2017. ‘Shingil’ is a mid-maturing ecotype with a heading date of August 10, showing resistance to both leaf/panicle blast and rice stripe virus (RSV), but susceptibility to Bph. ‘Shingil’ showed a low viviparous germination rate of 5.2%. The yield capacity of ‘Shingil’ was 745kg/10a over two years based on a regional yield test. The amylose content of ‘Shingil’ is 23.4% with the grain showing opacity in most parts of the endosperm, which is caused by the round particle shape, unlike the polygonal shape of ordinary grains. Thus, ‘Shingil’ could be used as a rice flour source in various processing field and dry milling industries where milling costs could be saved (Registration No. 8019).

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.

Ginseng (Panax ginseng) is a self-fertilized crop, and all ginseng cultivars developed until now have been bred by pure-line selection. Ginseng has few genetic variations and lacks diversity in genetic resources. To obtain genetic resources with useful traits, mutations must be artificially induced. In this study, indehiscent seeds from ginseng landrace were treated with 20 to 400 Gy of gamma rays for 24 hours to determine the optimal dose for mutation breeding. Results showed no significant differences in seed dehiscence rates among the 0 Gy (control) to 80 Gy groups. However, above 100 Gy, the dehiscence rate sharply decreased as the radiation dose increased. Seed development below 40 Gy was superior to that of control but declined rapidly above 60 Gy. The seedling survival rate decreased significantly at 60 Gy (less than 50% compared to that of the control), and most seeds did not survive above 100 Gy. We observed a significant reduction in the growth of seedlings irradiated above 60 Gy. The above results suggest that a suitable gamma-ray dose for inducing mutagenesis in indehiscent ginseng seeds is 40 Gy.

‘Baegilmi’ is an extremely early maturing rice variety that can be harvested within 100 days after transplanting, and also exhibits strong blast resistance and good grain appearance. From a mutant population of ‘Koshihikari’ treated with ethyl methanesulfonate, a promising line, ‘Suweon 559’, was selected through pedigree breeding and yield trials, and subsequently registered as ‘Baegilmi’. According to the 3-year (2012–2014) regional adaptability tests, the average heading date of Baegilmi in ordinary planting was July 14^th, which was 23 and 9 days earlier than that of the check varieties ‘Hwaseong’ and ‘Odae’, respectively. The milled rice yield of ‘Baegilmi’ was 4.53 MT/ha in ordinary planting (83% and 98% of ‘Hwaseong’ and ‘Odae’, respectively). ‘Baegilmi’ had a culm length of 75 cm (10 cm shorter than ‘Hwaseong’), a panicle length of 21 cm (similar to ‘Hwaseong’), and 12 panicles per plant (two fewer than ‘Hwaseong’). The brown rice of ‘Baegilmi’ was slightly more slender than ‘Hwaseong’, with a 1,000 grain weight of 20.6 g and length/width ratio of 1.92. The milled rice of ‘Baegilmi’ was translucent, with a protein content of 8.4% (1.7% higher than ‘Hwaseong’) and an amylose content of 18.6% (similar to ‘Hwaseong’). ‘Baegilmi’ exhibited strong blast resistance, but was susceptible to bacterial blight, viral diseases, and insect pests. The release of ‘Baegilmi’ is expected to provide a useful early-maturing rice variety that can be used in diverse cropping systems in paddy fields (Registration No. 6805).

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.

This study was carried out to develop new zoysiagrass (Zoysia japonica Steud.) cultivar ‘Halla Green 2’ (Grant number: No. 118). To develop a zoysiagrass cultivar with dwarfism by using the mutation breeding method, the wild type control "Gosan" plants were irradiated using a 30 Gy gamma ray source in 2010. Dwarf mutants were selected from the mutated grasses in successive generations. Dwarf mutant lines were identified and a new zoysiagrass variety Halla Green 2 was developed. The plant height of Halla Green 2 was 3.4 and 1.8 times lower than that of Gosan and Zenith, respectively. This cultivar has dwarf characteristics such as shorter sheath, shorter leaf blade, shorter flag leaf, and shorter third internode of stolon compared to those of Gosan and Zenith. Additionally, the sheaths and leaf blades color of Gosan, Zenith and Halla Green 2 were all light green, whereas their stolons were purple, yellow-green and yellow green, respectively. Trichomes(hairs) were visible on both adaxial and abaxial surfaces of the Gosan leaves, whereas only on the adaxial side of the Zenith and Halla Green 2 leaves. The Halla Green 2 grass showed distinguishable morphological traits compared to those of wild type Gosan and Zenith.

To develop a dwarf turfgrass (Zoysia japonica) cultivar with artificial mutation-induced breeding method, the wild type control "Gosan" plants were exposed to a 30 Gy gamma ray source in 2010. The mutant lines showing short height were selected from successive generations. One of the resulting dwarf lines obtained was registered under the cultivar name of “Halla Green 1” (2016). The dwarf phenotype of the Halla Green 1 includes a reduction of the height by 4.5-fold, an increase in leaf and third internode lengths by about 6- and 2.3-fold, respectively, compared to the Gosan, and approximately 2.4-, 3.8-, and 1.5-fold relative to the Zenith, respectively. In addition, the Halla Green 1 had a sheath of darker green coloring compared to the light green Gosan and Zenith. The leaf blades of Gosan, Zenith and Halla Green 1 were all light green, whereas their stolons were purple, yellow-green and light purple, respectively. Trichomes presented on both adaxial and abaxial surfaces of the Gosan’s leaves, and only on the adaxial side of the Zenith’s leaves, but none on the Halla Green 1 leaves. The Halla Green 1 exhibited sufficiently distinct morphological traits when compared with the wild type Gosan and Zenith that the dwarf phenotype enhances its commercial viability.

Kenaf (Hibiscus cannabinus L.) is an annual herbaceous crop of the Malvaceae family. Recently, kenaf is being used in many diversified applications such as pulp, animal feed, bioplastics and cellulosic biofuel etc. A new cultivar 'Jangdae' was developed by mutation breeding using irradiated with 300 Gy gamma-rays. Jangdae has a few distinguishable characteristics such as early flowering, high seed yield and palmate leaf, compared to wild type (Jinju). The fresh matter (FM) and dry matter (DM) yield of Jangdae are similar to those of Jinju, their seed yield (833.3 kg/ha) is approximately 4 times higher than that of Jinju. In addition, the FM and DM yield of Jangdae are 40% higher than those of early-maturing cultivar (C11). We performed the nutritive value of four kenaf cultivars (Jangdae, Jinju, C11 and Hongma300) at 100 day after seeding. The crude protein content of Jangdae, Jinju, C11 and Honma300 were 10.5, 11.0, 9.4 and 10.6%, respectively. The crude fiber, neutral detergent fiber and acid detergent fiber contents were no significant difference among the cultivars. Jangdae, which afforded both a high biomass and seed yield in South Korea, may be useful as potential source of feed and industrial materials.

‘Jungmo1024’ is a blast resistant early maturing rice cultivar with high temperature tolerance during grain filling stage. ‘Jungmo1024’ was derived from a sodium azide treatment on ‘Suweon472’, a high yielding japonica elite line which was latterly registered as ‘Namil’. Comparison with the agronomical traits of ‘Namil’, ‘Jungmo1024’ was uniquely characterized as the induced gained function due to the reduced culm length, increased tiller number, strong blast resistance and especially high temperature tolerance during grain filling stage. The high temperature tolerance of ‘Jungmo1024’ was supported by two years experiments by comparing the head rice ratio produced in ordinary paddy field and green house condition. The heading date of ‘Jungmo1024’ was July 29 in central plain area, which was 9 days earlier than that of ‘Hwaseong’. The milled rice yield performance of ‘Jungmo1024’ was about 4.98 MT/ha in local adaptability test for three years. ‘Jungmo1024’ had 69 cm in culm length, which was 15 cm shorter than that of ‘Hwaseong’, 20 cm in panicle length, 16 in tiller number, and 22.3g in 1,000 grain-weight of brown rice. ‘Jungmo1024’ exhibited strong rice blast resistance, but do not have any clear resistance gene sources against bacterial blight, viral diseases and insect fests. ‘Jungmo1024’, nevertheless, would be a useful rice cultivar could be used as a donor line for the breeding programs for developing southern plane adaptable early maturing rice cultivars with enhanced rice blast resistance, lodging tolerance, and especially high temperature tolerance during grain filling stage.

A new soybean cultivar ‘Wonhyun’, was developed by mutation breeding technique using a 250 Gy gamma ray at Korea Atomic Energy Research Institute (KAERI) in 2010. ‘Wonhyun’ has black seed coat and much better agronomic performance than original variety ‘Paldal’. Their total yield (177.1kg/10a) is much higher than that of ‘Paldal’ (126.9 kg/10a). Also, 100 seed weight of Wonhyun was 27g compared to ‘Paldal’ (13.7g). Contents of 4 essential amino acids such as aspartic acid, glutamic acid, lysine, arginine and unsaturated fatty acid including linoleic and linolenic acid have much higher than ‘Paldal’. This cultivar is good for cooking with rice as improved functional ingredient soybean.