The triploid seedless grape cultivar ‘Heukarong’ was developed in 2002 through artificial hybridization between ‘Black Olympia’ and ‘Campbell Early’ From 2003 to 2013, field trials were conducted under conventional cultivation practices in Chuncheon, Gangwon State, confirming that ‘Heukarong’ possesses strong resistance to both diseases and frost without the need for special protective measures, thereby demonstrating high tolerance to both environmental and biotic stresses. Based on these findings, an intensive three-year comparative evaluation of growth and fruit characteristics between ‘Heukarong’ and the triploid cultivar ‘Sweet Dream’ was conducted beginning in 2014. The results showed that ‘Heukarong’ consistently produced fruit with commercially valuable traits, leading to its final selection in late 2016 for varietal registration. Under standard cultivation, ‘Heukarong’ can produce high-quality fruits averaging 436 g in weight with a soluble solid content of 19.4 °Brix through a single application of gibberellin at full bloom, making it highly competitive in the market. Furthermore, ‘Heukarong’ exhibits strong resistance to frost damage and berry cracking, indicating excellent adaptability and stable productivity even under suboptimal growing conditions (Cultivar Registration No. 7766).

Seedlessness is the most important characteristic of citrus and one of its breeding purposes. Triploids are the most reliable way to secure seedlessness and can be developed through crosses between tetraploids and diploids or between diploids and diploids. We evaluated the characteristics of seed formation and recovery frequency of triploids in interploid crosses in this study. Normal seed formation was the highest (85.0%) in crosses of 2x×2x, followed by crosses 4x×2x and 2x×4x. Partially developed seeds were obtained at the highest rate (35.3%) from crosses of 4x×2x with 0.7 seeds per fruit, followed by 2x×2x and 2x×4x with 0.2 and 0.1 seeds per fruit, respectively. The number of undeveloped seeds per fruit was 1.1, 1.0, and 0.3 from the 4x×2x, 2x×4x, and 2x×2x crosses, respectively, and the lowest frequency was recorded in the 2x×2x cross. Diploid plants were mostly obtained in the 2x×2x cross (98.8%), while 73.0% triploids and 27.0% tetraploids were obtained with 4x×2x, and 14.3% diploids, 57.1% triploids, and 28.6% tetraploids were achieved with the 2x×4x cross. Diploid plants were mostly obtained from normal seeds, while ad triploid and tetraploid plants were mostly obtained from partially developed and undeveloped seeds. The results indicated that crosses of 4x×2x were the most efficient and that in vitro culture should be applied to rescue plants from partially developed and undeveloped seeds in triploid breeding programs.

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.

‘Sweet Dream’(Vitis complex), a new triploid seedless grape cultivar, is developed from the grape breeding program at Gangwondo Agricultural Research and Extension Services in 2009. ‘Sweet Dream’ was obtained from a cross ‘Honey Black’(Vitis complex) and ‘Campbell Early’(Vitis labrusca), a diploid x tetraploid grape cultivar. Characteristics of fruits and vines in ‘Sweet Dream’ were observed from 2006 to 2008, and ‘Sweet Dream’ was finally selected in 2009. Its optimum harvest time in Chuncheon is September 11, 12 days later than ‘Campbell Early’. ‘Sweet Dream’ has strong vine vigor and moderate freezing tolerance, with no bud damage occurring during winter in Chuncheon. The average weight of a cluster is 440g, and its average soluble solids concentration is 16.4 °Brix. It is seedless and has larger fruit size and better fruit quality than ‘Campbell Early’. It, therefore, is expected that introduction of ‘Sweet Dream’ could contribute to the diversification and gentrification of Korean grape cultivars (Registration No. 4359).

In this study, we investigated the fruit and vine characteristics of hypo- and hyper-tetraploid grapes from which a chromosome was added or deleted. The aim of this study was to evaluate the possibility of developing seedless grapes by utilizing the characteristics of hypo- and hyper-tetraploid grapes. Both the hypo- and hyper-tetraploid grapes showed stable vine growth characteristics unlike other aneuploid plants, which displayed abnormal leaf morphology and unstable vine growth patterns, indicating that hypo- and hyper-tetraploid grapes can be used for commercial purpose. The hypo-tetraploid grapes produced 100% seedless fruits, with large berries ranging from 8.5 to 10.5 g, when the grapes were applied with 100 ppm gibberillic acid during their full bloom period. Although the fruits of the hyper-tetraploid grape were larger than those of the hypo-tetraploid grapes, it produced a lower percentage of seedless fruits. We conclude that the characteristics of hypo-tetraploid grapes may be exploited to develop seedless grapes with large berries.