¹H-NMR Method Enables Early Identification of Degeneration in the Quality of Sweet Potato Tubers

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.