Thermodynamic analysis of the physical state of water during freezing in plant tissue, based on the temperature dependence of proton spin‐spin relaxation

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Abstract
Multi‐proton spin‐echo images were collected from cold‐acclimated winter wheat crowns (Triticum aestivumL.) cv. Cappelle Desprez at 400 MHz between 4 and −4 °C. Water proton relaxation by the spin‐spin (T2) mechanism from individual voxels in image slices was found to be mono‐exponential. The temperature dependence of these relaxation rates was found to obey Arrhenius or absolute rate theory expressions relating temperature, activation energies and relaxation rates, Images whose contrast is proportional to the Arrhenius activation energy (Ea), Gibb's free energy of activation (ΔG‡), and the entropy of activation (ΔS‡) for water relaxation on a voxel basis were constructed by post‐image processing. These new images exhibit contrast based on activation energies rather than rules of proton relaxation. The temperature dependence of water protonT2relaxation rates permits prediction of changes in the physical state of water in this tissue over modest temperature ranges. A simple model is proposed to predict the freezing temperature kof various tissue in wheat crowns. The averageEaand ΔH‡for water protonT2relaxation over the above temperature range in winter wheat tissue were −6.4 ± 14.8 and −8.6 ± 14.8kj mol−1, respectively. This barrier is considerably lower than theEafor proton translation in ice at 0°C, which is reported to be between 46.0 and 56.5 kj mol−1