Control of cambial activity and wood formation in Fraxinus americana: Involvement of nuclear genome size, auxin and gibberellin

The overall objective of this dissertation was to investigate internal factors involved in the control of cambial activity and xylem differentiation in Fraxinus americana. The specific objectives were to study: (1) nuclear genome size of ray cambial cells in relation to seasonal wood production; (2) the effects of exogenous auxin (IAA) and gibberellin (GA{dollar}sb3{dollar}) on cambial growth rate and xylem cell differentiation, particularly lignin formation; and (3) the relationship between xylem cell type and the amount and composition of lignin.; Nuclear genome size of ray cambial cells was observed to increase during cambial dormancy and decrease during springtime cambial reactivation, associated with seasonal wood production.; Cell type proportion and lignin content and composition of wood were manipulated in both 1-year-old debudded shoot cuttings and 20-year-old stem segments by exogenous IAA and/or GA{dollar}sb3{dollar} during a growing period. IAA alone or a high IAA/low GA{dollar}sb3{dollar} combination induced vessel-rich wood with a relatively high lignin content and guaiacyl/syringyl (G/S) molar ratio, whereas a low IAA/high GA{dollar}sb3{dollar} combination promoted fibre-rich wood with a lower lignin content and G/S ratio, and improved chemical digestibility. The results show the potential for producing trees of high fibre yield, low lignin content and G/S ratio, and improved chemical digestibility during pulping through manipulation of the phytohormone concentration in the cambial region.; While IAA appears to be a factor controlling the production of lignified vessel elements and axial parenchyma cells, GA{dollar}sb3{dollar} appears to be a factor which promotes the differentiation and lignification of fibres. In addition, the lignins of white ash woods differ from those of synthetic dehydrogenation polymers of coniferyl alcohol and sinapyl alcohol in having aldehydes as precursors in addition to alcohols. Preferential incorporation of aldehydes appears to be controlled by some factor(s) other than IAA and GA{dollar}sb3.{dollar} Furthermore, different xylogenic responses to IAA and especially to GA{dollar}sb3{dollar} arise as a function of cambial age.; These results suggest that nuclear genome size, auxin and gibberellin are involved in the control of wood formation.