Wide genetic variation in phenolic compound content of seed coats among black soybean cultivars

Black soybeans have been used as a food source and also in traditional medicine because their seed coats contain natural phenolic compounds such as proanthocyanidin and anthocyanin. The objective of this research is to reveal the genetic variation in the phenolic compound contents (PCCs) of seed coats in 227 black soybean cultivars, most of which were Japanese landraces and cultivars. Total phenolics were extracted from seed coats using an acidic acetone reagent and the proanthocyanidin content, monomeric anthocyanin content, total flavonoids content, total phenolics content, and radical scavenging activity were measured. The cultivars showed wide genetic variation in PCCs. Each of the contents was highly correlated with one another, and was closely associated with radical scavenging activity. PCCs were also moderately associated by flowering date but not associated by seed weight. Cultivars with purple flowers had a tendency to produce higher PCCs compared with cultivars with white flowers, suggesting that the W1 locus for flower color can affect phenolic compound composition and content. Our results suggest that developing black soybean cultivars with high functional phenolic compounds activity is feasible.