Genetic variation in wood chemistry of Eucalyptus globulus
Creators: name: Poke, FS
Publisher: name: University Of Tasmania
Year: 2023

description: Eucalyptus globulus is grown in temperate regions throughout the world predominantly for the production of kraft pulp. Kraft pulping involves chemically removing most of the lignin and extractives and some of the hemicellulose from the cellulose fibres. The amount of lignin and extractives in the wood is therefore important. The aim of this study was to strengthen knowledge of genetic variation in the chemical wood properties of E. globulus. Methods were developed for fast and simple assessment of these traits. These will benefit quantitative and molecular approaches to breeding, both of which were explored in this thesis. In addition, the utility of a lignin biosynthesis gene for phylogenetic analysis in Eucalyptus was investigated. Measuring wood chemistry using traditional chemical methods is costly and time consuming. Near infrared reflectance (NIR) analysis was explored as an alternative to these for the prediction of extractives, lignin and cellulose contents from both ground and solid wood samples. Good calibrations were developed for each of these traits with high correlation coefficients (R\\(^2\\) of 0.62 to 0.93) and standard errors of less than 1.37%. All calibrations were validated using a separate set of samples with strong correlations obtained between predicted and laboratory values (R\\(^2\\) of 0.67 to 0.99), with the exception of acid-soluble lignin content for solid wood (R\\(^2\\) of 0.12). NIR analysis was found to be a reliable predictor of the wood chemistry of E. globulus, with solid wood shown to be a good alternative to ground wood samples. Solid wood NIR calibrations were then used to assess within-tree variation in the wood chemistry of E. globulus. Wood chemistry was found to vary within-tree, with extractives content significantly decreasing with height in the tree, but not varying radially. Bark-to-pith variation was observed for cellulose and lignin contents, the former decreasing and the latter increasing. However, for cellulose content this was generally not significant, while for lignin content significance was found between 20 and 60% of tree height. These traits showed no significant height variation. Results indicated that current assessment techniques for wood chemistry, using wood cores extracted at breast height, are effective. Natural variation in the wood chemistry of E. globulus was investigated using NIR predictions. Physical wood traits, growth and decay were also measured. Genetic variation was found for lignin content and decay, with significant locality differences. The only trait to have significant family within locality variation was acid-soluble lignin content, giving a high narrow-sense heritability (0.51 ¬¨¬± 0.26). Family means heritabilities were high for lignin content, extractives content and decay (0.42 - 0.64). This suggested that traditional breeding may be used to improve these traits. Furthermore, the chemical wood traits were highly correlated with each other and with density and microfibril angle. This indicated that selection for the breeding objective traits (basic density, pulp yield and volume) could concurrently produce favourable states in wood chemistry, decay resistance and fibre properties. Having identified natural genetic variation in lignin, molecular variation in the lignin biosynthesis gene, cinnamoyl CoA reductase (CCR), was explored, firstly for its effect on wood properties; and secondly for its use in phylogenetic reconstruction at the sectional level of Eucalyptus. Segregation and quantitative trait loci (QTL) approaches were used to identify the effect of an amino acid substitution at a highly conserved position in CCR, on lignin content, lignin composition (syringyl/guaiacyl ratio) and wood density. The amino acid substitution had no significant influence on these traits nor did CCR collocate with any of the QTL for growth and density found in that cross. The high levels of sequence variation found for CCR in E. globulus suggested it may be useful for testing the monophyly of Eucalyptus sections Exsertaria and Latoangulatae, using section Maidenaria as an outgroup. Latoangulatae and Maidenaria were polyphyletic or paraphyletic, while Exsertaria species formed a clade but included a single Latoangulatae species. Analysis of intragenic recombination, a confounding factor when using nuclear genes for phylogenetic analysis, identified two events involving species from different sections. The occurrence of intragenic recombination may explain the anomalous positions of some species wilhin the phylogenetic tree, and also suggests that levels of linkage disequilibrium will be low, which has implications for association studies. The high level of CCR sequence variation between and within Eucalyptus species suggests that molecular variants may be found in natural populations that will allow selection for improved lignin profiles.