Jameel Al-Haddad1, Shawn Mansfield2 and Frank W. Telewski1,3

1Department of Plant Biology, Michigan State University, U.S.A. 2Department of Wood Science, University of British Columbia, Canada 3 W.J. Beal Botanical Garden, Office of Campus Planning and Administration, Michigan State University, U.S.A.

Lignin is an important phenolic polymer in the secondary cell walls. These walls are responsible for the mechanical and some physiological features of the woody tissue in plants. In angiosperms, the predominant monomers of lignin are syringyl and guaiacyl. The ratio of these monomers was found to change in response to external stimuli such as wind or gravity. The latter induces the formation of special type of wood called tension wood. The impact that results from artificially modifying the lignin monomeric ratio was investigated in three types of wood tissues in one-year old hybrid poplar clone 717 (Populus tremula x P. alba). The clone was transformed to overexpress the F5H/Cald5H gene to different levels resulting in increased syringyl to guaiacyl ratio (S:G). Wild type poplars and the transgenic lines were left upright or inclined 45 from vertical for three months to induce the gravitropic responses including tension wood formation. Wild type poplar stems had 9.3% and 11.2% increase in percent syringyl in tension wood side than normal or opposite wood, respectively. Increasing syringyl formation increased percent acid soluble lignin 2.3 folds. Cell wall crystallinity was also higher in tension wood than the other types of wood tissues. Both tension and opposite woods had higher percent total sugars. Interestingly, in tension wood a 25.5% increase in percent syiringyl led to 4.7% decrease in percent total sugars and 7.3% decrease in percent glucose. Percent galactose in tension wood was also higher but dropped by 11.2% in response to the lignin monomer modification in the same tissue. Xylose and rhamnose were lower in tension wood then normal wood in wild type stems. Mechanically, the stems modulus of elasticity (MOE) did not change with increased syringyl when tested with 4-point bending or under compression. A decrease in the stems modulus of rupture (MOR) in response to increased S:G ratio was detected. Trees with increased S:G ratio seemed to adjust their stems to gravity faster after inclination. Evaluating the response of these lines to inclination will improve our understanding of the role lignin monomeric composition plays in altering xylem chemical composition and mechanical properties of normal and tension wood.