Accumulation of proanthocyanidins and/or lignin deposition in buff-pigmented soybean seed coats may lead to frequent defective cracking

Defective cracking frequently occurs in buff-pigmented soybean seed coats, where proanthocyanidins accumulate and lignin is deposited, suggesting that proanthocyanidins and/or lignin may change physical properties and lead to defective cracking. In the seed production of many yellow soybean (Glycine max) cultivars, very low percentages of self-pigmented seeds are commonly found. This phenomenon is derived from a recessive mutation of the I gene inhibiting seed coat pigmentation. In Japan, most of these self-pigmented seeds are buff-colored, and frequently show multiple defective cracks in the seed coat. However, it is not known why cracking occurs specifically in buff seed coats. In this study, quantitative analysis was performed between yellow and buff soybean seed coats. Compared with yellow soybeans, in which defective cracking rarely occurs, contents of proanthocyanidins (PAs) and lignin were significantly higher in buff seed coats. Histochemical data of PAs and lignin in the seed coats strongly supported this result. Measurements of the physical properties of seed coats using a texture analyzer showed that a hardness value was significantly decreased in the buff seed coats. These results suggest that PA accumulation and/or lignin deposition may affect the physical properties of buff seed coats and lead to the defective cracking. This work contributes to understanding of the mechanism of defective cracking, which decreases the seed quality of soybean and related legumes.