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Genetic variation in wood chemistry of Eucalyptus globulus


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Poke, FS 2006 , 'Genetic variation in wood chemistry of Eucalyptus globulus', PhD thesis, University of Tasmania.

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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
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 (R2 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 (R2 of 0.67 to 0.99), with the exception of acid-soluble lignin content for
solid wood (R2 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-t~-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.

Item Type: Thesis - PhD
Authors/Creators:Poke, FS
Keywords: Eucalyptus globulus
Copyright Holders: The Author
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Copyright 2006 the author - The University is continuing to endeavour to trace the copyright
owner(s) and in the meantime this item has been reproduced here in good faith. We
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Available for library use only and copying in accordance with the Copyright Act 1968, as amended. Thesis (PhD)--University of Tasmania, 2006. Includes bibliographical references

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