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.

The ‘Halla Green 7’ cultivar is a hybrid of Zoysia matrella and Zoysia japonica. The inflorescences of Z. matrella lines were collected from Jeju Island near the border between Z. matrella and Z. japonica wild plants, respectively. After their harvest, the seeds were sown and various interspecific lines were obtained. In 2021, a line with greening period extending phenotype during the low temperatures of winter was registered under the cultivar name of ‘Halla Green 7’ (Registration No. 245). The main morphological characteristics of the ‘Halla Green 7’ variety are as follows: genetically short length of plant (12.2±0.58 cm), very wide angle of leaf (68.6°), short length of leaf blade (8.38±0.37 cm), medium-leaf (3.29±0.09 mm), and very low leaf sheath (2.18±0.19 cm). The short length of leaf sheath makes it easy to manage the turf grass with low height. A very wide leaf angle has a high photosynthetic efficiency. The ‘Halla Green 7’ cultivar is morphologically classified as Z. matrella×Z. japonica hybrid line by medium-width of leaf blade and the presence of trichomes on the leaf surface. The colors of the leaf blade, leaf sheath, and stolon are green, light green, and light purple, respectively. During the low temperature period of winter, chlorophyll content of the leaves of ‘Halla Green 7’ shows a 10.1-fold from that of the control Z. japonia ‘Yaji’ and about 4.2-fold from that of the control Z. matrella Zm-6. This greening period extending phenotype is expected to contribute to the industrialization of zoysiagrass (Registration No. 245).

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.

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.

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.