Cold acclimation in eucalypt hybrids.

We evaluated cold resistance and the capacity for cold acclimation of different Eucalyptus genotypes. Seedlings of half-sib families of E. globulus and hybrids E. gunnii x globulus, E. viminalis x globulus and E. cypellocarpa x globulus were exposed daily for 56 days to a 9-h photoperiod at 14.7 degrees C, followed by 15 h in a dark cold room maintained at 2.5 degrees C with the root system maintained at 8 degrees C to cold harden the seedlings. Unhardened seedlings were maintained at about 16 degrees C during the dark period. Cold acclimation occurred in all families with decreases in the temperature causing 50% mortality (LT(50)) of between 1.5 and 3 degrees C. Both hardened and unhardened plants of hybrid families were more cold tolerant than E. globulus. A significant correlation between LT(50) and leaf osmotic pressure was observed; the increase in osmotic pressure in hardened plants was predominantly a result of an increase in the concentration of soluble sugars. Exotherm peaks were similar in hardened and unhardened plants. These results indicate that cold hardening increased the ability of eucalypts to endure extracellular ice formation. The maintenance of photosynthetic capacity in cold-hardened plants may also play a role in their response to freezing.