The effect of time of harvest, irrigation treatments and kilning temperature on Eucalyptus globulus seed germination response to high temperature stress

Low seedling emergence rates of some Eucalyptus globulus seed-lots have been reported and hypothesised to be caused by high temperature stress. This study tested the effects of time of seed harvest, kilning temperature at seed extraction and irrigation treatments applied to E. globulus trees on seed germination traits. Seeds were harvested from three genotypes at 11 months from commencement of flowering (early), 13 months (commercial) and 15 months (late) and kiln dried at 30, 40 or 50C. Irrigation treatments were full irrigation, half irrigation and no irrigation. Six traits describing the proportion and rate of seed germination and early seedling development were studied. The kilning temperature of capsules and irrigation treatments applied to trees had no effect on the germination traits studied. Time of seed harvest affected the rates of seed germination and normal seedling development. There was little delay in germination rate at 32C in early-harvested seeds compared with that experienced by the commercial- and late-harvested seeds, suggesting that it has a higher temperature threshold. However, the effect of harvest date on germination was genotype dependent which may be related to different heat sums to which developing seeds are exposed.