Primary response in chilling sensitive crop leaves evaluated by Arrhenius plots of ¹H-NMR relaxation times

Temperature dependency of T₁ of water protons in leaf tissues of chilling-sensitive crops was investigated by pulse nuclear magnetic resonance (NMR) spectroscopy. Arrhenius plots of the T₁ of tomato and cucumber leaves indicated biphasic with temperature drop from 20 to 0°C. "Breaks" in Arrheius plots of T₁ of fresh leaves in tomato and cucumber occurred at about 12.5°C and 10°C, respectively, while those of chilling-insensitive clone of gloxinia indicated no break point during the same process. Furthermore, thermal hysteresis of T₁ was observed during a slow cool-warm cycle in these crop leaves. The relation between the restoration process of T₁ upon warming and the degree of chilling injury is summarized as follows: 1) a slight prolongation of T₁ or no hysteresis was observed when chilling injury did not occur; 2) a marked prolongation of T₁ was observed when injury occurred; 3) a marked shortening of T₁ occurred from the earlier stage of the warming process when tissues were severely injured. In conclusion, Arrhenius plots of T₁in leaves provide a sensitive and non-invasive information for evaluating plant response to chilling stress in crops.