Germination response of Eucalyptus globulus seeds exposed to low and high temperature stress

Low seedling emergence rates of some seed lots of genetically elite Eucalyptus globulus Labill have been reported by nursery operators and hypothesized to be caused by high temperature stress. This study has confirmed that germination rate was sensitive to exposure to high temperatures. The optimum temperature for the germination rate of normal seedlings in this species was between 24.8°C and 25.5°C. At 25°C, 96% of seeds produced normal seedlings after 21 days. The cardinal temperatures for development of normal Eucalyptus globulus seedlings were between 9.1°C and 10.5°C at low temperature and between 40.3°C and 41.4°C at high temperature. The optimum temperature for maximum attainment of normal seedlings was between 21.2°C and 24.8°C. Temperatures above 30°C generally delayed commencement of germination, reduced total germination percentage, caused a high proportion of seed death and suppressed the development of normal seedlings. Low temperatures also delayed germination but caused less seed mortality. Despite general consistency in responses there was evidence that seed lots exhibit a differential response to temperature, particularly for normal seedling development at the extremes of temperature tested.