In sweet potato tuber, which is a tropical plant, long-term storage leads to loss of water and carbohydrate, thus water mobility was investigated using 1H-NMR spectroscopy. Electrolyte leakage indicated that tubers stored at 15°C for 1 year were partly injured and that frozen-thawed tissues were dead. Nuclear magnetic resonance (NMR) spin-lattice relaxation time (T1) and spin-spin relaxation time (T2) clearly increased with the duration of storage, whereas these values decreased in the dead tissues. Furthermore, Arrhenius plots for T1 and T2 were determined at temperatures ranging from 20 to 0°C in 2.5°C steps. In the fresh tubers, a strong converse temperature dependency was shown in the T2 measurement. On the contrary, there was no temperature dependency in the T2 of the dead tissues. Thus, the existence of inverse temperature dependency reflected tissue viability. Additionally, any change in the T2 of the fresh tubers occurred at about 14°C, which virtually coincided with the storage temperature of 15°C. The slope change in T2 might have responded to a physiological change as a primary event. In conclusion, monitoring water status by NMR could provide early identification of changes in the quality of post-harvest crops; this method shows great promise for use in environmental-stressed crop yield research.
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