Antioxidant Levels in Germinating Soybean Seed Axes in Relation to Free Radical and Dehydration Tolerance

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Abstract
The axis of soybean seeds suffer dehydration injury if they are dried to 10% moisture at 36 h of imbibition, but tolerate this stress if dried at 6 h of imbibition. Deesterification of membrane phospholipids was 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 .ltoreq. 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-h axes (susceptible) increased from 6-31.degree. C. In contrast, those from membrane lipid from 6-h axes (tolerant) increased from 3 to only 8.degree. 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-h imbibed axes in much greater quantities than in the 36-h 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.