Supercooling ability of rhododendron flower buds in relation to cooling rate and cold hardiness

The freezing process and supercooling ability in flower buds of 11 native Rhododendron species were examined with reference to the cooling rate and cold hardiness by differential thermal analysis. The freezing patterns of the excised whole buds varied with the season: in autumn, buds froze as whole units, while in winter, freezing was initiated in the scales and propagated to each floret. The supercooling ability of florets was enhanced during winter. The freezing patterns in winter buds were strongly influenced by the cooling rate (1 to 30°C/hr). Although the first exotherm in scales occurred at -5 to -10°G and was rate-independent, the occurrence of several floret exotherms shifted considerably to lower subzero temperatures at slower rates. The most reliable cooling rate for testing maximum supercooling ability was l°C/hr. The exotherm in florets of hardier species occurred at -20 to -25°C and at -7 to -20°C for less hardy ones, and were well correlated with their killing temperatures. Water relations within bud tissues in response to freezing are briefly discussed.