THERMODYNAMIC ANALYSIS OF THE PHYSICAL STATE OF WATER DURING FREEZINGIN PLANT-TISSUE, BASED ON THE TEMPERATURE-DEPENDENCE OF PROTON SPIN-SPIN RELAXATION

Multi-proton spin-echo images were collected from cold-acclimated wint er wheat crowns (Triticum aestivum L.) cv. Cappelle Desprez at 400 MHz between 4 and -4 degrees C. Water proton relaxation by the spin-spin (T-2) mechanism from individual voxels in image slices was found to be monoexponential. The temperature dependence of these relaxation rates was found to obey Arrhenius or absolute rate theory expressions relat ing temperature, activation energies and relaxation rates. Images whos e contrast is proportional to the Arrhenius activation energy (E(a)), Gibb's free energy of activation (Delta G double dagger), enthalpy of activation (Delta H double dagger), and the entropy of activation (Del ta S double dagger) for water relaxation on a voxel basis were constru cted by post-image processing. These new images exhibit contrast based on activation energies rather than rates of proton relaxation. The te mperature dependence of water proton T-2 relaxation rates permits pred iction of changes in the physical state of water in this tissue over m odest temperature ranges. A simple model is proposed to predict the fr eezing temperature of various tissues in wheat crowns. The average E(a ) and Delta H double dagger for water proton T-2 relaxation over the a bove temperature range in winter wheat tissue were -6.4 +/- 14.8 and - 8.6 +/- 14.8 kT mol(-1), respectively. This barrier is considerably lo wer than the E(a) for proton translation in ice at 0 degrees C, which is reported to be between 46.0 and 56.5 kT mol(-1).