A cell-suspension culture obtained from the hybrid Eucalyptus gunnii/Eucalyptus globulus was hardened by exposure to lower temperatures, whereas in the same conditions cells from a hybrid with a more frost-sensitive genotype, Eucalyptus cypellocarpa/ Eucalyptus globulus, were not able to acclimate. During the cold exposure the resistant cells accumulated soluble sugars, in particular fructose and sucrose, with a limited increase i n cell osmolality.I n contrast, the cell suspension that was unable to acclimate did not accumulate soluble sugars in response to the same cold treatment.To an extent similar to that induced after a cold acclimation, frost-hardiness of the cells increased after a 14-h incubation with specific soluble sugars such as sucrose, raffinose, fructose, and mannitol. Such hardening was also observed for long-term cultures in mannitol-enriched medium. This cryoprotective effect of sugars without exposure t o lower temperatures was observed i n both the resistant and the sensitive genotypes. Mannitol was one of the most efficient carbohydrates for the cryoprotection of eucalyptus. l h e best hardiness (a 2.7-fold increase in relative freezing tolerance) was obtained for the resistant cells by the cumulative effect of cold-induced acclimation and mannitol treatment. This positive effect of certain sugars on eucalyptus freezing tolerance was not colligative, since it was independent of osmolality and total sugar content.Many plants become more resistant to freezing temperatures when first exposed to low, nonfreezing temperatures, a process known as cold acclimation or cold hardening. Cell-suspension cultures derived from plants capable of cold acclimation can also be hardened to withstand freezing, but few data are available on low-temperatureinduced cold acclimation in cell suspensions derived from woody plants (Hellergren, 1983;Tremblay et al., 1992;Arora and Wisniewski, 1995;Leborgne et al., 1995b).Biochemical changes associated with cold acclimation include alterations in the concentrations of a wide range of metabolites, including sugars (Hallgren and Oquist, 1990 These complex changes indicate that cellular metabolism undergoes considerable modifications during acclimation, but the cause/effect relationships have not been clearly established. Carbohydrates that are compatible solutes with a demonstrated cryoprotectant activity are often reported to accumulate at low temperatures (acclimation). These include oligosaccharides such as Suc and raffinose, and polyols such as sorbitol and mannitol. Carbohydrates seem to function in two ways that are often difficult to distinguish: osmotic adjustment (a colligative effect) and cryoprotection of cellular structures.Eucalyptus, an evergreen woody plant exhibiting excellent productivity, good fiber quality, and high pulp yield, is grown extensively around the world, mostly for pulp making. However, a major limitation to the expansion of these plantations is the relatively high frost sensitivity of some species. For example, for Eucalyptus globulus, one...