O-alpha-D-Galactopyranosyl-(1-->2)-D-chiro-inositol, herein named fagopyritol B1, was identified as a major soluble carbohydrate (40% of total) in buckwheat (Fagopyrum esculentum Moench, Polygonaceae) embryos. Analysis of hydrolysis products of purified compounds and of the crude extract led to the conclusion that buckwheat embryos have five alpha-galactosyl D-chiro-inositols: fagopyritol A1 and fagopyritol B1 (mono-galactosyl D-chiro-inositol isomers), fagopyritol A2 and fagopyritol B2 (di-galactosyl D-chiro-inositol isomers), and fagopyritol B3 (tri-galactosyl D-chiro-inositol). Other soluble carbohydrates analyzed by high-resolution gas chromatography included sucrose (42% of total), D-chiro-inositol, myo-inositol, galactinol, raffinose and stachyose (1% of total), but no reducing sugars. All fagopyritols were readily hydrolyzed by alpha-galactosidase (EC 3.2.1.22) from green coffee bean, demonstrating alpha-galactosyl linkage. Retention time of fagopyritol B1 was identical to the retention time of O-alpha-D-galactopyranosyl-(1-->2)-D-chiro-inositol from soybean (Glycine max (L.) Merrill, Leguminosae), suggesting that the alpha-galactosyl linkage is to the 2-position of D-chiro-inositol. Accumulation of fagopyritol B1 was associated with acquisition of desiccation tolerance during seed development and maturation in planta, and loss of fagopyritol B1 correlated with loss of desiccation tolerance during germination. Embryos of seeds grown at 18 degrees C, a condition that favors enhanced seed vigor and storability, had a sucrose-to-fagopyritol B1 ratio of 0.8 compared to a ratio of 2.46 for seeds grown at 25 degrees C. We propose that fagopyritol B1 facilitates desiccation tolerance and storability of buckwheat seeds.
Stachyose, sucrose, and other non-reducing soluble carbohydrates are associated with the onset of desiccation tolerance during seed development and with seed storability. Mature soybean [Giycine max (L) Merr.l seeds contain several galactosyl cyclitols in addition to sucrose, stachyose, and raffinose, but except for galactinol, the accumulation of these galactosyl cyclitols has not been reported for developing soybean seeds. Fifteen soluble carbohydrates including members of the raffinose, galactinol, galactopinitol, and fagopyritol series were analyzed in extracts from axis and cotyledon tissues of seeds and zygotic embryos matured in planta and in vitro. Galactopinitol A, galactopinitol B, and fagopyritol B1 accumulate in axis tissues of developing soybean seeds in planta in association with the onset of desiccation tolerance, yellowing of axis tissues, and in parallel with stachyose accumulation. Galactopinitol A, galactopinitol B, fagopyritol B1, and stachyose also accumulate in parallel in cotyledons in planta and in axis tissues during in vitro growth of zygotic embryos at 15 and 25°C. Axes of soybean seeds matured at 25°C contained higher concentrations of sucrose, raffinose, o-pinitol, o-chiro-inositol, fagopyritol B1, and total soluble carbohydrates than axes of seeds matured at 18°C. Soybean seeds accumulate mostly galactosyl pinitols and only small amounts of free pinitol. In future work, It would be of interest to determine if gaiactosyl cyclitols may substitute for the role of stachyose in providing desiccation tolerance and prolonged seed storability.
Maturation of white lupin seed (Lupinus albus L. cv. Ultra) at 28°C results in later flowering and reduced seed yield in plants grown from these seed compared with plants grown from seed matured at 13°. The objectives of this study were to determine the changes in seed mass and soluble carbohydrates of white lupin seed as affected by seed maturation temperature. Fourteen soluble carbohydrates were identified and quantified by high resolution gas chromatography of the trimethylsilylimidazole‐derivatization products. Reducing sugars were not detected, and sucrose was 10 to 15% of total soluble carbohydrates in the axis and 12 to 20% in the cotyledons. Seventy to 80% of the total soluble carbohydrates were raffinose, stachyose, and verbascose. In addition to the raffinose series oligosaccharides, four series of galactosyl cyclitols were present including the galactinol series, galactopinitol A series, galactopinitol B series, and fagopyritol B1 series. Seed matured at 28°C accumulated only 53 to 70% as much dry matter as seeds matured at 13<C. Only minor changes in the raffinose series oligosaccharides were observed. Pinitol and the galactose‐containing pinitols were more than doubled by seed maturation at 28°C, but collectively, these compounds are <10% of the total soluble carbohydrates. We conclude that the effect of seed maturation temperature on the composition and concentration of soluble carbohydrates is minimal.
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