The goal of this research was to examine how changes in the syringyl to guaiacyl (S:G) lignin monomer ratio will affect the conductive efficiency and resistance to embolism in hybrid poplar. I used two lines of hybrid poplar clone 717 (Populus tremula x P. alba) over-expressing the Arabidopsis gene encoding ferulate-5-hydroxylase (F5H) which resulted in an increased S:G ratio in xylem tissue. The two lines selected for use, F5H37 and F5H64, expressed 88% and 94% syringyl, respectively, compared to 66% syringyl expressed in wild type (WT) plants. To quantify the effects of increased S:G on hydraulic properties, two studies were performed. First, the hydraulic conductivity and resistance to embolism of F5H and wt lines were tested under well watered and water stress conditions. The goal of the second experiment was to examine the response of the F5H and WT lines to winter freeze-thaw stress and spring recovery. Results from this research show that increasing S:G has no significant effect on the hydraulic conductivity of xylem, but lines with increased S:G were observed to be more vulnerable to cavitation. Data for pressure at 75% embolism (P75) showed that F5H64 was approximately 75% more vulnerable to cavitation and in 2009 F5H37 was twice as vulnerable to cavitation as the WT. No significant differences were found between lines in response to drought and winter freeze-thaw stress.
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