ENRICHMENT IN SPECIFIC SOLUBLE SUGARS OF 2 EUCALYPTUS CELL-SUSPENSIONCULTURES BY VARIOUS TREATMENTS ENHANCES THEIR FROST TOLERANCE VIA A NONCOLLIGATIVE MECHANISM

A cell-suspension culture obtained from the hybrid Eucalyptus gunnii/E ucalyptus globulus was hardened by exposure to lower temperatures, whe reas in the same conditions cells from a hybrid with a more frost-sens itive genotype, Eucalyptus cypellocarpa/ Eucalyptus globulus, were not able to acclimate. During the cold exposure the resistant cells accum ulated soluble sugars, in particular fructose and sucrose, with a limi ted increase in cell osmolality. In 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 afte r a cold acclimation, frost-hardiness of the cells increased after a 1 4-h incubation with specific soluble sugars such as sucrose, raffinose , fructose, and mannitol. Such hardening was also observed for long-te rm cultures in mannitol-enriched medium. This cryoprotective effect of sugars without exposure to lower temperatures was observed in both th e resistant and the sensitive genotypes. Mannitol was one of the most efficient carbohydrates for the cryoprotection of eucalyptus. The best hardiness (a 2.7-fold increase in relative freezing tolerance) was ob tained for the resistant cells by the cumulative effect of cold-induce d acclimation and mannitol treatment. This positive effect of certain sugars on eucalyptus Greeting tolerance was not colligative, since it was independent of osmolality and total sugar content.