Characteristic changes in relaxation times of water protons in vigna radiata seedlings exposed to temperature stress

Water protons in hypocotyl tissues from etiolated seedlings of Vigna radiata that were exposed to temperature stress showed characteristic relaxation behaviors for ¹H-NMR. Cold stress treatment (0°C) caused gradual prolongation of NMR relaxation times (T₁). After exposure of tissues to cold stress for 24 h, T₁ returned to the initial value as a result of subsequent incubation at normal temperature (20°C). By contrast, heat stress treatment (40°C) induced a time-dependent decrease in T₁, which did not return completely to the initial value upon subsequent incubation at 20°C after exposure to heat stress for 4 h. We examined changes in various physical factors that influence the response of T₁ to temperature stress, namely, water content and the concentrations of protein, diamagnetic (K⁺, Na⁺, Ca²⁺ and Mg²⁺) and paramagnetic (Mn²⁺ and Fe²⁺) ions in the tissues. From the relationships between T₁ and these factors in vitro, we could not interpret the responses of T₁ to the temperature stress only in terms of a change in water content. A synergistic effect of an Mn²⁺ -protein complex and pH might be essential for the mechanism of changes in T₁ that are due to cold stress. The influence of heat stress on structural water in tissues is discussed in terms of water-protein interactions.