Freezing Avoidance Mechanisms by Supercooling in Some Rhododendron flower Buds with Reference to Water Relations

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Excised florets of some hardy Rhodode'idron species did not tolerate freezing at -5℃ when ice-inoculated due to intracellular freezing. Florets in intact December buds, however, could be supercooled to about -30℃. When flower buds of R. japonicum were slowly cooled with daily decrements of 5℃ to temperatures ranging from 0 to -20℃, the exotherm temperatures of the florets drastically decreased. This was accompanied by a decrease in water content of florets and peduncle and an increase in that of scales. The water in florets and the peduncle is thought to migrate to scales and other tissues during the early stages of freezing; the dehydrated floret has a lower freezing point which enhances its supercooling ability and the dehydrated peduncle helps to maintain the supercooled state of the florets. This hypothesis would explain the dependence on the cooling rate of supercooling in Rhododendron flower buds. Water migration within flower buds was observed in other hardy Rhododendron species with some variation in ice formation site and the quantity ofmigrated water. The exotherm temperature of excised florets was inversely proportional to their water content. De-hydration of flower buds by wind at 0℃ also enhanced their supercooling ability. Mechanisms of freezing avoidance by supercooling in Rhododendron flower buds and the relationship of supercooling to freezing tolerance are discussed.
日本植物生理学会の論文