Genetic and environmental factors affecting the germination of Eucalyptus globulus seeds

Eucalyptus globulus is widely planted in pulpwood plantations in temperate regions of the world, including Australia. Most plantations are established from seedlings rather than clones. Slow and uneven germination of E. globulus seed-lots has been a problem in some nurseries and has been linked with periods of high temperatures. Trials were conducted using commercial and research seed-lots to identify genetic and environmental factors affecting seed germination. Six response traits were studied: proportion germination, proportion of normal seedlings, proportion of germinated seeds which developed into normal seedlings, germination rate, normal seedling development rate, and rate of development from germination to normal seedlings. First, the response of genetically diverse commercial seed-lots across a range of temperatures was studied. Temperatures above 30°C generally delayed 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. The optimum temperature for rate of normal seedling development was between 24.8°C and 25.5°C and for maximum percentage normal seedling development was between 21.2°C and 24.8°C. The cardinal temperatures for normal seedling development were between 9.1°C and 10.5°C at low temperature and between 40.3°C and 41.4°C at high temperature. Despite general consistency in responses there was evidence that seed-lots can exhibit a differential response to temperature, particularly for normal seedling development at the extremes of temperature tested. Second, by sampling from multiple randomised ramets (trees) of maternal genotypes in grafted seed orchards across two sites and two seasons, the extent of maternal genetic regulation at the race and genotype level of seed germination responses was assessed. Maternal genotype had a significant effect on most germination traits but a differential response to temperature was only detected for percent germination and rate of development from germination into normal seedlings. The maternal genotype effect on germination traits varied with sampling season and site, suggesting interaction with environment or harvesting factors. The maternal genotype effect could be explained by maternal race of origin for two traits relating to normal seedling development. Differences between genotypes at 25°C could be used to predict differences at 37°C and germination rate and normal seedling development rate were generally the most correlated between 25°C and 37°C. Third, by using a small diallel crossing design it was shown that both the paternal and maternal parent can affect the germination response, arguing for at least some influence of the nuclear genotype of the embryo. However, the response to high temperature stress was more influenced by the maternal than paternal parent. These results argue that while the nuclear genotype of the embryo may cause variation in the average germination response, it is the maternal genetic effect that affects the response to high temperature stress. This shows that in addition to maternal environment and maturation effects, there may be additive genetic as well as maternal genetic effects on seed germination responses of E. globulus which may affect the proportion and synchrony of seed germination and development. E. globulus has a mixed mating system, and in a separate experiment, comparing seeds from self-pollination, open-pollination and mass-supplementary pollination the germination and development of selfed seeds were poorer than the open-pollinated and mass-supplementary pollinated seeds averaged across temperatures. There was also the suggestion that selfed seeds were more sensitive to high temperature stress. Fourth, the effect of time of seed harvest, kilning temperature at seed extraction and irrigation treatments applied to trees on the germination of E. globulus seeds and the germination response of seeds was tested at 25°C, 32°C and 37°C. Seeds were harvested from three genotypes at 11 months from commencement of flowering (early), 13 months (commercial) and 15 months (late). Kilning temperatures were 30°C, 40°C and 50°C. Irrigation treatments were full irrigation, half irrigation and no irrigation. The kilning temperatures 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 32°C in early harvested seeds compared with that experienced by the commercially and late harvested seeds, suggesting that it has a higher temperature threshold. The effect of harvest date on germination was genotype dependent and it is suggested that variability in seed germination traits between harvest dates may be related to different heat sums which seeds are exposed to during development. The outcomes from this work have significant industry application. Seed germination and seedling development in the nursery could be significantly enhanced by accounting for tree genetics, maternal environment and harvesting factors including seed harvest site, season and age.