Supercooling ability, water content and hardiness of rhododendron flower buds during cold acclimation and deacclimation

The relationship between supercooling ability and water content and killing temperature of flower buds during cold acclimation and deacclimation were studied using R. kiusianum and R. × akebono. The occurrence of multiple floret exotherms and their shift to a narrow range at lower subzero temperatures, as well as the marked decrease of florets water content, were observed as the symptoms of cold acclimation occuring in flower buds from fall to winter, and vice versa in spring buds during deacclimation. In R. kiusianum, the fully acclimated period was from November to March and two months longer than that of R. × akebono. The supercooling ability of the former was about -25°C and about -20°C in the latter. Although the water migration within bud tissues during the freezing process was determined in the acclimated and deacclimated buds for R. × akebono, no significant water changes could be observed, even in the acclimated buds. Thus, it is conceivable that deep supercooling in florets may result not necessarily from water migration from florets and bud axes to scales in response to freezing, but from low water content in situ of cold-acclimated or artificially dehydrated flower buds.