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Callus derived from the cambial area of poplar twigs survived freezing down to −120°C or even the temperature of liquid nitrogen (−196°C), after cold acclimation. Exposure to cool preconditioning temperatures for a length of time before being acclimated at 0°C is essential for enhancing fully the callus hardiness.
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Sakai et al. (1973) studied this question.
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