Extraorgan freezing in wintering flower buds of Cornus officinalis Sieb. et Zucc.

Abstract. Extraorgan freezing as a mechanism for increasing cold hardiness was shown using flower buds of Cornus officinalis Sieb. et Zucc. Differential thermal analysis (DTA) revealed that florets in flower buds of C. officinalis owed their cold hardiness to deep supercooling and also that slower cooling rates increased the supercooling ability of florets. During slow stepwise cooling (5°C h⁻¹), the water content of florets decreased and that of scales (involucral bracts) increased, which resulted in accumulation of ice within the scales. This was more extensive in early winter and early spring buds than mid-winter ones. Flower buds with silicone oil in the space between florets and scales also showed a similar decrease in water content of florets and an increase in that of scales. This indicated that water migration from the florets to the scales probably took place by way of the peduncles and the receptacle, possibly through their vascular traces, and not directly from the surface of the florets to the ice sink in the form of vapour. Possible mechanisms of extraorgan freezing are postulated along with this finding.