‘Cheongja5’ is a black seed coat soybean cultivar developed from crossing ‘Milyang181’ and ‘YS1886 (Cheongdu1/Tanbaguro)’ in 2007. Promising lines were selected using the pedigree method from F₃ to F₅. The preliminary and advanced yield trials were conducted in 2013 and 2014, respectively. Regional yield trials (RYT) were conducted in seven regions from 2015 to 2017. ‘Cheongja5’ has a determinate growth habit, oval leaflet shape, brown pubescence, and white flowers. The seed of ‘Cheongja5’ has a black seed coat color with green cotyledon. Flowering and maturing dates were Aug. 3 and Oct. 24, respectively. Regarding the quantitative characteristics, ‘Cheongja5’ has large seed size (37.0 g/100-seed weight) and more pods than ‘Cheongja3’. ‘Cheongja5’ was tolerant to pod shattering in field and indoor RYT tests. ‘Cheongja5’ was resistant to bacterial pustule in the field and soybean mosaic virus (strains G6H and G7H) in the inoculation test. Although its seed quality-related characteristics were almost similar to those of ‘Cheongja3’, it contained more anthocyanin which is a main functional component of black soybean. The mean yield of ‘Cheongja5’ in the RYTs was 343 kg/10 a which was 30% higher than that of ‘Cheongja3’. ‘Cheongja5’ is expected to be widely cultivated as a material for making soy-food that uses black soybean because of its large seed size and high yield potential (Registration No. 7687).

A black soybean cultivar ‘Gyeongheukcheong’ was developed by Gyeongsangbuk-do Agricultural Research and Extension Services. ‘Gyeongheukcheong’ was selected from a cross between ‘RCS640’ and ‘Geomjeongsaeol’ in 2011. The cultivar was evaluated in multiple performance and yield trials in seven locations from 2017 to 2019. ‘Gyeongheukcheong’ has a gray pubescence, light brown pod, and dark green cotyledon, whereas ‘Cheongja 3’ has a brown pubescence, brown pod, and light green cotyledon. The seed weight and maturity of ‘Gyeongheukcheong’ was similar to that of ‘Chungja 3’. ‘Gyeongheukcheong’ was resistant to soybean mosaic virus (strain G6H), moderately resistant to bacterial pustule (caused by Xanthomonas axonopodis pv. glycines), and resistant to root rot. Total chlorophyll content (sum of chlorophyll a and b) in the green cotyledon of ‘Gyeongheukcheong’ was 48.0 μg/g, which was higher than that of ‘Cheongja 3’ (30.1 μg/g). The regional yield trial showed that ‘Gyeongheukcheong’ had a similar or significantly higher average yield than ‘Chungja 3’ in locations with single (similar) and double (19% higher) cropping systems, respectively. ‘Gyeongheukcheong’ represents a consumer-friendly black soybean with dark green cotyledons and high yield potential (Registration No. 9065).

A black soybean cultivar, “Saebaram” with green cotyledons, was developed and released by the Gyeongsangbuk-do Agricultural Research and Extension Services (GBARS) in 2019. It was developed from a single cross between “Seonheuk” and “RCS640” in 2011. The preliminary yield trial and advanced yield trial were conducted at GBARS from 2015 to 2016. The regional yield trial (RYT) was conducted in seven locations from 2017 to 2019 in Korea. “Saebaram” had purple flowers, grey pubescence, light brown pods, a black seed coat, an ellipse seed shape, and a large seed size (42.8 g/100 seeds). The seed size was significantly larger than that of the check cultivar “Cheongja 3” (36.2 g/100 seeds). Moreover, it was moderately resistant to bacterial pustule (caused by Xanthomonas axonopodis pv. glycines) and soybean mosaic virus (strain G6H). “Saebaram” had dark green cotyledons, whereas “Cheongja 3” had light green cotyledons. The total chlorophyll contents (chlorophyll a and chlorophyll b) in the green cotyledons of “Saebaram” and “Cheongja 3” were 76.1 μg/g and 30.1 μg/g, respectively. Furthermore, the RYT showed that “Saebaram” had 287 kg/10a, which was 10% higher than that of “Cheongja 3” (261 kg/10a). This black soybean cultivar with improved yield, large-size seeds, and dark green cotyledons is expected to have a high commercial value for Korean farmers (Registration No. 9155).

In Korea, black soybeans are traditionally consumed after cooking with rice to supplement protein and oil which are lacking in rice. Seed cooking quality including seed traits after cooking with rice, which is important for consumers, were so far not comprehensively considered during the process of breeding. In this study, we first evaluated seed quality after cooking with rice, we tested the correlation between seed cooking quality and seed water absorption ratio, and we attempted to identify the Quantitative trait locus(QTL)/gene using two recombinant inbred line (RIL) populations, i.e., Daepung × Socheong2 and Daepung × Ilpumgeomjeong. Based on phenotype and correlation analyses, the main factors affecting the hardness of soybeans cooked with rice may differ between RIL population. In the Daepung × Socheong2 RIL population, one QTL associated with seed hardness after cooking with rice was identified on chromosome 11, and Glyma.11g049600, encoding peroxidase, is proposed as a candidate gene. In the Daepung × Ilpumgeomjeong RIL population, two QTLs associated with seed hardness after cooking with rice were identified on chromosomes 7 and 19, one QTL related to seed water absorption on chromosome 3, and Glyma.19g092600 encoding pectin methylesterase inhibitor are proposed as candidate genes. This is the first study on soybean cooking quality after cooking with rice, and the locations of four related QTLs were identified. The results will be of use for future development of high-quality black soybean varieties with high consumer preference using molecular breeding methods.

Understanding plant morphological and seed characteristics is an essential step for the utilization of genetic resources to improve targeted traits in soybean breeding programs. The purpose of this study was to evaluate the agronomic traits and seed pigments of 469 black soybean germplasms with green cotyledons to identify useful resources using multivariate analyses, correlation, principal component analysis (PCA), and cluster analyses. The results showed that the range of flowering days from planting were 37.7-71.5 days, harvesting days from planting were 112.3-153.0 days, plant height was 49.6-151.6 cm, and 100-seed weight was 9.1-49.3 g. According to the correlation analyses with agronomic traits, flowering and harvesting days were positively correlated with the plant height, number of nodes, and number of branches. Additionally, it was found that the chlorophyll a and total chlorophyll content had the greatest effect on the green color of the cotyledon. Based on the PCA, 13 quantitative traits were compressed into three components. The first three principal components accounted for 82.5% of the total variance. In this study, black soybean germplasms with green cotyledons were divided into four clusters by the K-means cluster analysis (non-hierarchical procedure), based on the PCA. The cluster analysis showed that chlorophyll a and plant height had a significant effect on the clustering of the black soybean germplasms used in this study. This study provides opportunities to easily select useful genetic resources for the development of new black soybean cultivars.

‘Taecheong’ is a black soybean cultivar developed from crossing ‘YS1593(Milyang136×Tanbaguro)’ and ‘Cheongja2’ in 2005. F₁ and F₂ populations were grown for 2 years and promising lines were selected by pedigree method from F₃ to F₅. The preliminary yield trial (PYT) and advanced yield trial (AYT) were conducted from 2012 to 2013, and a regional yield trial (RYT) in seven regions was conducted from 2014 to 2016. ‘Taecheong’ is determinate, with purple flower, green cotyledon, and black spherical seed. Flowering date and maturing date were August 3 and October 20, respectively. Regarding quantitative characteristics, ‘Taecheong’ has a larger seed size (44.5 g/100-seed weight) than that of ‘Cheongja3’ (35.8 g/100-seed weight). ‘Taecheong’ was tolerant to pod shattering and lodging at the RYT field and indoor test. In addition, ‘Taecheong’ was resistant to bacterial pustule during the field test, and to soybean mosaic virus (G6H strain) during the inoculation test. Although its seed quality-related characteristics were similar to those of ‘Cheongja3’, it contained more anthocyanin in its seed-coat, which is a primary functional component of black soybean. The mean yield of ‘Taecheong’ in RYT was 240 kg/10a, which was 94% of the yield of ‘Cheongja3’. ‘Taecheong’ is expected to be used as raw material for soy products, such as for preparing soybean chocolates, because of its large seed size (Registration number: 7104).