Abstract
Arrhenius plots of dynamic states of water in various crop plant organs exposed to cold or heat stress were studied by using 1H-NMR. In chilling sensitive plants, phase transition in NMR spin-lattice relaxation time (T1) occurred at 7.5°C and at 12.5°C in Vigna radiata and V. mungo hypocotyls, respectively, while that in spin-spin relaxation time (T2) occurred at 13.5°C in sweet potato tubers. On the other hand, phase transition in T1 occurred at 0°C in both leaves and roots of perennial ryegrass and pea epicotyls. Prolongation of T1 was observed after the break point in these species. Furthermore, phase changes in T2 of the ryegrass leaves and roots occurred at -20°C and -10°C, respectively. Judging from electrolyte leakage and viability test as well as T2, both vacuolar and cytoplasmic compartments froze between -20 and -25°C in the leaves while the freezing in the root tissues occurred between -10 and -15°C in ryegrass. Therefore, supercooling ability of the ryegrass organs were especially detected by T2. On the other hand, adaptation to heat stress was clearly shown by T1 while heat-denature was shown by a marked decrease in T2. From these results, the phase transition temperatures were closely correlated their stress sensitivity, especially acclimation to temperatures was detectable by T1 while critical temperature by T2. In conclusion, analysis of Arrhenius plots of T1 and T2 in plant organs provided sensitive and non-invasive way for evaluating both primary and lethal responses to the temperature stresses.
