Relationship between Thermal Transitions and Freezing Injury in Pea and Soybean Seeds.
In an attempt to correlate freezable water with freezing injury, the thermal behavior of pea (Pisum sativum L.) and soybean (Glycine max L. Merr) seed parts at different moisture contents were compared with survival of the seeds when exposed to low temperatures. Thermal transitions between -150 and 10 degrees C were studied using differential scanning calorimetry. In pea, reduction of germinability, after exposure of seeds to temperatures between - 18 and - 180 degrees C, occurred at a constant moisture content (about 0.33 gram H(2)O/gram dry weight) regardless of the temperature; this moisture level was above that at which freezable water was first detectable by differential scanning calorimetry (0.26 gram H(2)O/gram dry weight). In contrast, damage to soybean seeds was observed at progressively lower moisture contents (from 0.33 to 0.20 gram H(2)O/gram dry weight) when the temperature was decreased from -18 degrees C to -50 degrees C. At -18 and -30 degrees C, moisture contents at which damage to soybean seeds was evident were above that at which freezable water was first detectable (0.23 gram H(2)O/gram dry weight). However, at -50, -80, and -180 degrees C, damage was evident even when freezable water was not detectable. The data suggest that, while the quantity of water is important in the expression of freezing injury, the presence of freezable water does not account for the damage.
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U.S. Department of Agriculture, Agriculture Research Service, Ft. Collins, Colorado 80523.