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

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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.

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