Characteristic Changes in Relaxation Times of Water Protons in Vigna radiata Seedlings Exposed to Temperature Stress

Abstract

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.