Frost Resistance of Subalpine Eucalyptus Species. II. Experiments Using the Resistance Index Method of Damage Assessment

Abstract

Frost damage to leaves of Eucalyptus pauciflora seedlings was measured by an electrical resistance ratio method. Leaves were exposed to below-freezing temperatures by a liquid bath apparatus that allowed accurate temperature control.

Unhardened seedlings exposed to an 18/9°C temperature regime with 12 h-photo- and thermoperiod did not harden significantly, the minimum temperature producing 50% leaf death (T₅₀) remaining higher than - 5° An 18/4°C regime lowered the T₅₀ to - 14° in 50 d, but exposure of hardened seedlings to 18/9°C produced rapid dehardening back to a T₅₀ higher than - 5°.

Rapid (1-2°C min^-I) cooling to the frost minimum was much more damaging than was the standard (2-4°C h^-I) rate used in the experiments.