The role of sugar, vitrification and membrane phase transition in seed desiccation tolerance

In a search for the mechanism of desiccation tolerance, a comparison was made between orthodox (desiccation-tolerant) soybean ( Glycine max L.] Merrill) and recalcitrant (desiccation-intolerant) red oak ( Quercus rubra L.) seeds. During the maturation of soybean seeds, desiccation tolerance of seed axes is correlated with increases in sucrose, raffinose and stachyose. In cotyledons of mature oak seeds, sucrose levels are equal to those in mature soybeans, but oligosaccharides are absent. By using the thermally stimulated current method, we observed the glassy state in dry soybean seeds during maturation. Oak cotyledons showed the same phase diagram for the glass transition as did mature soybeans. By using X-ray diffraction, we found the maturation of soybeans to be associated with an increased ability of membranes to retain the liquid crystalline phase upon drying, whereas the mature oak cotyledonary tissue existed in the gel phase under similar dry conditions. These findings lead to the conclusion that the glassy state is not sufficient for desiccation tolerance, whereas the ability of membranes to retain the liquid crystalline phase does correlate with desiccation tolerance. An important role for soluble sugars in desiccation tolerance is confirmed, as well as their relevance to membrane phase changes. However, the presence of soluble sugars does not adequately explain the nature of desiccation tolerance in these seeds.