Thermal effect of CO2 on apoplastic ice in rye and oat during freezing

Meristematic tissues from rye (Secale cereale) and oat (Avena sativa) were studied in an isothermal calorimeter at -3 degrees C. When the frozen tissue was placed in the calorimeter, the pressure increased within 4 d to 25 and 9 kPa above ambient pressure in the sample vessels containing crowns of rye and oat, respectively. Concurrently, the thermal output went down to -194 mu W in rye over the 4-d period; this negative thermal activity could be accounted for by ice melting in the plants. When the pressure was released, the output from the calorimeter went from -194 to 229 mu W within 1 h, suggesting that water had frozen in the plants. We propose that CO2 from respiration had dissolved in the water in the plants and caused melting of ice (heat absorption) due to the colligative properties of solutions. When the pressure was released, the CO2 came out of solution and the water froze (heat evolution). These thermal observations were duplicated in a simplified, nonbiological system using a glycol/water mixture that was partially frozen at -3 degrees C.