Frost Resistance in .Eucalyptus nitens (Deane & Maiden) Maiden: Physiological Aspects of Hardiness

Abstract

The effects of photoperiod, temperature regime and differential root and shoot temperatures on frost resistance were examined in both hardening and dehardening Eucalyptus nitens seedlings. Frost resist- ance was primarily determined by measuring the relative loss of electrolytes from frosted leaf discs. This method of assessing frost resistance compares favourably with the frosting of whole seedlings since the critical temperatures producing 50% leaf death in whole seedlings and the leakage of 50% of cellular electrolytes (T50) agreed to within 0.6°C after 14 and 28 days' hardening.

Unhardened seedlings had T50 values of - 3.4°C. Seedlings hardening over a 56 day period, at night temperatures of 3°C for 16 h and days of 13-25°C, steadily increased in frost resistance to T50 values of -7.5°C. Reducing the daily amount of hardening temperature from 16 to 8 h produced seedlings that were over 1.0°C less frost resistant at the end of 42 days' hardening. On the other hand, seedlings exposed to a constant 3°C, day and night, suffered considerable water stress and were unable to harden beyond -5.8°C, even after 77 days. Heating seedling roots to between 6 and 18°C throughout the 3°C regime maintained high plant water status but did not confer any increased frost resistance.

Rates of dehardening in well hardened seedlings increased with increasing day and 1 or night tempera- ture in the range 6-20°C. Seedlings exposed to 14 h nights at 6.0°C, just 3°C warmer than the hardening temperatures used, were unable to maintain T50 levels of -7.5°C and dehardened by 2.5°C over a 3-week period. The dehardening rate of leaves from seedlings exposed to an air temperature of 18°C day and night was significantly reduced if roots were exposed to 3°C.