Occurrence and tolerance mechanisms of seed cracking under low temperatures in soybean (Glycine max)

In soybean, occurrence of, or tolerance to, seed cracking under low temperatures may be related to the presence or absence, respectively, of proanthocyanidin accumulation in the seed coat dorsal region. Soybean seeds sometimes undergo cracking during low temperatures in summer. In this study, we focused on the occurrence and tolerance mechanisms of low-temperature-induced seed cracking in the sensitive yellow soybean cultivar Yukihomare and the tolerant yellow soybean breeding line Toiku 248. Yukihomare exhibited seed cracking when subjected to a 21-day low-temperature treatment from 10 days after flowering. In yellow soybeans, seed coat pigmentation is inhibited, leading to low proanthocyanidin levels in the seed coat. Proanthocyanidins accumulated on the dorsal side of the seed coat in Yukihomare under the 21-day low-temperature treatment. In addition, a straight seed coat split occurred on the dorsal side at the full-sized seed stage, resulting in seed cracking in this cultivar. Conversely, proanthocyanidin accumulation was suppressed throughout the seed coat in low-temperature-treated Toiku 248. We propose the following mechanism of seed cracking: proanthocyanidin accumulation and subsequent lignin deposition under low temperatures affects the physical properties of the seed coat, making it more prone to splitting. Further analyses uncovered differences in the physical properties of the seed coat between Yukihomare and Toiku 248. In particular, seed coat hardness decreased in Yukihomare, but not in Toiku 248, under the low-temperature treatment. Seed coat flexibility was higher in Toiku 248 than in Yukihomare under the low-temperature treatment, suggesting that the seed coat of low-temperature-treated Toiku 248 is more flexible than that of low-temperature-treated Yukihomare. These physical properties of the Toiku 248 seed coat observed under low-temperature conditions may contribute to its seed-cracking tolerance.