Sugar metabolism in germinating soybean seeds: evidence for the sorbitol pathway in soybean axes.

Characterization of sugar content and enzyme activity in germinating soybean (Glycine max L. Merrell) seeds led to the discovery of sorbitol accumulating in the axes during germination. The identity of sorbitol was confirmed by relative retention times on high-performance liquid chromatography and gas liquid chromatography and by mass spectra identical with authentic sorbitol. Accumulation of sorbitol in the axes started on day 1 of germination as sucrose decreased and glucose and fructose increased. Sucrose also decreased in the cotyledons, but there was no accumulation of sorbitol, glucose, or fructose. Accumulation of sorbitol and hexoses was highly correlated with increased invertase activity in the axes, but not with sucrose synthase and sucrose phosphate synthase activities. Sucrose synthase activity was relatively high in the axes, whereas the activity of sucrose phosphate synthase was relatively high in the cotyledons. Ketose reductase and aldose reductase were detected in germinating soybean axes, but not in cotyledons. Fructokinase and glucokinase were present in both axes and cotyledons. The data suggest a sorbitol pathway functioning in germinating soybean axes, which allows for the interconversion of glucose and fructose with sorbitol as an intermediate.