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The possibility is examined whether seeds may survive the desiccated state in part by vitrification, or the formation of a glassy state. Embryos excised from viable corn (Zea mays L.) seeds at low moisture contents show a series of low temperature first-and second-order phase transitions in the differential scanning calorimeter. These embryos produce normal seedlings if moistened. The thermal events can be duplicated almost entirely in both extracted lipids and purified commercial com oil. They are therefore associated primarily with these bulk lipids, since membrane phospholipids are present in too small an amount to produce a detectable signal. When the bulk lipids have been extracted, a glass transition appears in the remaining material. At low water contents, it occurs above +400C and systematically falls to below -600C as the water content of the embryo rises to 20%. These data are consistent with our hypothesis that the desiccated state in seeds is a glassy state, and that imbibition of water reduces the glass transition temperature below ambient, allowing biochemical activity to resume.The means by which living materials such as seeds can survive desiccation presents an interesting and difficult problem. As water is removed, the stresses imposed on membranes and other structures may be very large (19), and the maintenance of bilayer organization of membranes may be threatened (3,8). The drying of seeds will lead to enormous concentrations of solutes with an associated threat of crystallization. Yet, the moisture isotherms of orthodox seeds consistently show a region of water "binding" at low water contents (15, 16) which should be absent if substantial crystallization had occurred during drying (7).In this paper we examine the hypothesis that dry seeds exist We have studied embryos from corn seeds by differential scanning calorimetry (DSC2), seeking evidence ofglass signals. We report the existence of glass transitions both in the lipids ofthe embryos and in the nonlipid components. The evidence for glass formation as a function of temperature and water content indicates that corn embryos exist in a glassy state at room temperature when water contents are below 12% g/g dry weight. MATERIALS AND METHODSAll experiments reported were performed in a Perkin-Elmer DSC-4 differential scanning calorimeter with computer-aided data analysis ('TADS'). The thermal head is cooled with liquid nitrogen and the instrument has been modified in several particulars in order to run reliably at low temperatures (18). To obtain a flat baseline, thermal data from the calorimeter were compared with stored data obtained using empty pans ('Scanning Autozero').Unlike the more familiar first-order transitions, no heat of melting is evolved or absorbed when a glass transition is passed. The only distinctive characteristic of these secondorder transitions is a change in heat capacity; in the aqueous glasses we will discuss, this involves mostly a change in the translational mobility of the water in the sample. These changes ar...

ABSTRACTvegetation change. Several hypotheses are advanced to explain this occurrence, including increases in The landward transgression of manrainfall, revegetation of areas cleared for groves into saltmarsh environments in the estuaries agriculture, altered tidal regimes or estuary water of south-east Australia over the last five decades is levels, and increases in nutrient levels and established as a widespread trend through a review sedimentation. of a number of independent studies. This process is occurring in a range of geomorphic settings, and

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