Short communication Survival of poplar callus at super-low temperatures after cold acclimation'

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. Extensive attempts to induce environmentally typical cold acclimation in callus cultures with Cornus stolonifera and hardy Dianthus have been unsuccessful (1). Tumanov et al. (2) first demonstrated that the callus derived from cherry twigs which had been cultured on a cultural agar medium containing 2 to 6% or 10 to 20 % sucrose became resistant to —20 ° or — 30°C, respectively, when hardened at 2°C and subsequently subjected to subfreezing temperatures for 20 days. However, on the same cultural agar medium containing 20 % sucrose, we observed little or no growth with poplar callus cultured at 26°C. Steponkus and Bannier (3) observed that callus derived from Chrysanthemum morifolium was injured after freezing at —9°C, but became hardy to — 17°C after hardening at 4.5°C for 6 weeks. One of us (A. S.) first demonstrated that winter twigs survived im