Quantifying liquid water in frozen plant tissues by isothermal calorimetry

An equation to calculate the percentage of water remaining unfrozen at any temperature due to colligative properties of solutions was derived from the freezing point depression equation. The accuracy of the equation was demonstrated with a 0.1 M sucrose solution frozen at temperatures from −0.5 to −6 °C in an isothermal calorimeter. Empirical measurements using latent heat as a measure of the amount of water frozen were within 1% of the expected values calculated from the equation. The extent to which percentages of water freezing in oat crown tissue at varying temperatures follows the expected freezing curve indicates how closely the system follows colligative freezing processes. The freezing curve for non-acclimated crowns followed a colligative freezing pattern more closely than did the curve for crowns from cold-acclimated plants. This suggests that water in crowns from non-acclimated plants may remain unfrozen primarily by colligative means while other mechanisms of keeping water unfrozen are important in cold-acclimated crowns. This may help explain contradictory results of studies that attempt to correlate carbohydrate concentrations with freezing tolerance.