Soybean seed coat cup unloading on plants with low-raffinose, low-stachyose seeds

Sucrose, raffinose and stachyose accumulate in soybean [Glycine max L. (Merrill)] embryos during seed maturation. To determine the relationship of plant maternal composition on seed composition, soluble carbohydrates in three 1-cm2 leaf punches at three plant growth stages (R2, R3, R6) and in seed coat cup exudates in planta were analysed at four 30-min intervals on soybean plants (R5) with low-raffinose, low-stachyose (LRS) seeds expressing the mutant stc1 phenotype; low-raffinose, low-stachyose and lowphytin (LRSP1, LRSP2) seeds expressing the mutant mips phenotype; or normal raffinose, stachyose and phytin (CHECK) seeds expressing the Stc1 and Mips phenotype. Leaf sucrose (23.6mgcm22), myo-inositol (9.3mgcm22), D-chiro-inositol (6.7mgcm22), D-ononitol (0.76 mgcm22), D-pinitol (50.1 mgcm22) and total soluble carbohydrates (107.1mgcm22) were not significantly different between phenotypes. D-chiro-Inositol, myo-inositol, D-pinitol and sucrose were unloaded from soybean seed coat cups in planta at decreasing rates over the four sequential periods of sampling. Unloading rates of sucrose and myo-inositol were highest for LRS, D-pinitol was highest for LRSP2, and D-chiro-inositol was not different between LRS, LRSP1, LRSP2 and CHECK. Free cyclitols were 60% of total soluble carbohydrates in leaves and 20% in seed coat cup exudates. Except for sucrose and D-pinitol, seed phenotype had little influence on the composition of compounds unloaded from seed coats to maturing embryos of low-raffinose, low-stachyose seeds. Maternally supplied cyclitols may contribute, in part, to changes in the composition of cyclitol galactosides stored in mature seeds.