The phenotypic and genetic basis of browsing resistance of Eucalyptus globulus to marsupial herbivores

Significant phenotypic variation in plant resistance to herbivores occurs between and within many plant species. Plant secondary metabolites play a major role in conferring resistance to many herbivores, and intraspecific variation in plant resistance can be influenced by both genetic and environmental variation. The evolutionary and mechanistic processes behind variation in plant resistance in many systems is still unknown, particularly in relation to plant/mammalian herbivore interactions. In this thesis, intraspecific variation in resistance of Eucalyptus globulus to mammaJian herbivore browsing was investigated and the mechanistic and genetic basis of this variation examined. There was significant genetic variation in resistance of E. globulus to browsing by the common brushtail possum (Trichosurus vulpecula) and the red-bellied pademelon (Thylogale billardierii). Browsing damage was assessed on coppiced juvenile foliage of trees of known genetic stock grown in a common environment field trial. There was significant genetic variation in plant resistance amongst geographically distinct populations at the race, locality and family levels. These results were confirmed in a captive feeding trial where there was significant genetic variation in resistance among E. globulus localities (populations). The characteristics responsible for the intraspecific variation in resistance of juvenile coppiced E. globulus foliage were then investigated. There was significant phenotypic variation in foliage chemistry between individual plants. Sideroxylonals, a group of formylated phloroglucinol compounds (FPCs), accounted for the greatest amount of the observed variation in intake of foliage by T vulpecula. Variation in these compounds explained the significant genetic based differences in browsing resistance between different populations within E. globulus. These results may offer some evidence for the potential importance of mammalian herbivores as selective agents on the evolution of plant resistance within Eucalyptus. Hybridisation in plants offers an ideal opportunity to investigate evolutionary processes influencing plant/herbivore interactions. including, for example, the mode of inheritance of plant resistance. The inheritance of resistance to browsing by T. vulpecula and the inheritance of the defensive plant compounds of three year old juvenile E. globulus foliage were investigated using known hybrid progeny (pure parental intra-race hybrids and inter-race F1 hybrids) of two genetically differentiated geographical races (north eastern Tasmanian race and south-eastern Tasmanian race) of E. globulus. The results indicate that the F 1 hybrid was intermediate in resistance, compared to the parental hybrids, suggesting an additive pattern of inheritance of browsing resistance to T. vulpecula. In this foliage the concentration of FPCs was low and the combined effects of condensed tannins and essential oils appeared to explain the pattern of resistance of these hybrids. Effects of variability in plant genotype and in environmental conditions within a population can influence resistance of plants independently and/or can exhibit a genotype-by-environmental interaction. The effects of environmental variability (two nutrient treatments; high and low) on the genetic based expression of defensive chemistry and resistance of two E. globulus genotypes (St. Helens and Blue Gum Hill) to browsing by T. vulpecula was examined. While no significant independent genotype and environmental effects on intake of E. globulus seedling foliage were evident, there was a genotype-by-environment interaction. Genetic differences between the two populations was only expressed under the low nutrient regime. Significant independent effects of genotype and nutrient treatment were observed for primary and secondary leaf chemistry. Finally, the feasibility of using near infrared reflectance spectroscopy (NIR.S) as an accurate tool to rapidly predict the nitrogen content and the resistance to browsing by T. vulpecula of individual E. globulus trees was investigated. Two successful calibration equations were developed (one for nitrogen and one for intake (resistance)) !hat met all statistical requirements, suggesting that these models were useful in predicting nitrogen content and resistance of new E. globulus trees. Independent validation of the nitrogen calibration equation showed that the model was successful at predicting nitrogen content of new E. globulus foliage. However, independent validation of the calibration equation for intake was unsuccessful. We suggest that a change in the defensive chemical profile across seasons, resulted in poor independent validation of the intake calibration model. We discuss the importance of using independent validation when assessing the predictive power of a calibration model and argue that with refinement in specific systems, NIRS can be used as a powerful, rapid tool in predicting resistance of trees to browsing herbivores.