Antioxidant levels in germinating soybean seed axes in relation to free radical and dehydration tolerance

The axis of soybean seeds suffer dehydration injury if they are dried to 10% moisture at 36 hours of imbibition, but tolerate this stress if dried at 6 hours of imbibition. Deesterification of membrane phospholipids has been correlated with the increased permeability and increased lipid phase transition temperatures of membranes from dehydration injured tissues. Deesterification, measured as increased free fatty acid:phospholipid and decreased phospholipid:sterol ratios, occurred primarily when the tissue was in the dry state and did not change significantly (P ≤ 0.05) with increasing imbibition time.

When liposomes were exposed to free radicals in vitro, wide angle x-ray diffraction indicated that the phase transition temperature of liposomes prepared from membrane lipid from 36-hour axes (susceptible) increased from 6 to 31°C. In contrast, those from membrane lipid from 6-hour axes (tolerant) increased from 3 to only 8°C, indicating that the tolerance of free radicals previously observed in these membranes was due to a lipid-soluble component.

Lipid-soluble antioxidants were detected in 6-hour imbided axes in much greater quantities than in the 36-hour imbibed axes. The presence of lipid-soluble antioxidants in the membrane apparently contributes to the free radical tolerance of seed membranes observed during the early stages of germination, and these antioxidants may contribute to the dehydration tolerance of this tissue.