Lignin is an important component of secondary cell walls where it constitutes the matrix in which the cellulose is held and provides significant strength, especially in compression. Lignin is also an important component of the middle lamella, where it functions to bond cells together but allows slow movements (creep) to occur. Industrial uses of cellulose necessitate the partial removal of lignin, an energy-expensive step that requires heat production, chemicals and waste-water remediation. This industrial requirement has led to research aimed at decreasing lignin content and/or producing lignin that is easier to extract. Little research has asked how well these lignin-modified trees perform. We studied 14 genotypes ('lines') of hybrid white poplar (P. tremula x P. alba, INRA-France 717-1B4) plus a control line to learn how modification of xylem lignin content affects water transport and biomechanics.

The control line was used to produce 14 independent gene insertions causing downregulation of 4-coumarate:coenzyme A ligase (4CL) in the resulting mutant lines. The 4CL step is early in the phenylpropanoid pathway leading to the synthesis of lignin as well as flavonoid-derived pigments and defensive chemicals. Controls as well as the mutated lines were micropropagated, grown in a greenhouse and transferred to the field, where they were grown for two years. The same transgenic and control lines were grown attached to stakes in the greenhouse for 1 year.

Thioacidolysis lignin content in the stems was highly but non-linearly correlated with 4CL RNA expression. The amount of lignin in the samples was positively correlated with specific conductivity of trunk wood as well as the xylem water tension at which wood loses 50% of its conductivity (P50), MOE, MOR, and trunk height/diameter in the greenhouse. There was a negative correlation between lignin content and proportion of tension wood xylem fibers. All lines had lower average MOE, MOR, and height/diameter than did the controls. In contrast to the above relationships, the buckling safety factor was not correlated with lignin content.

Control poplar wood is white, but pink to reddish wood was commonly observed in fresh-cut transgenic stems, and distinct patches of brown colored wood also occurred. Five of the transgenic lines had more than 25% brown wood in the trunk wood. These lines also tended to have the lowest lignin content. Most traits showed a marked shift in lines with more than 10% brown wood, coinciding with lignin contents below about 80% of the control values. Microscopic examination showed that brown wood contained some vessels that were occluded with dark extractives, explaining the lower specific conductivity in brown wood samples than white wood samples.

In conclusion, this research suggests that even small modifications to the amount of lignin in the secondary xylem can have significant direct or indirect negative impacts on tree performance. The stems of transgenic poplars were more tapered, allowing trees with weaker, more flexible wood to maintain similar buckling safety factors. With lower lignin content P50s were generally depressed, and brown wood caused reductions in wood specific conductivity. The five lines with the lowest lignin contents and greatest quantities of brown wood averaged about 20% the aboveground biomass of the controls, implying that poor water transport clearly lead to reduced growth. There were lines with higher biomass and higher specific conductivity than the controls, although they had lower MOE and MOR. These results show the importance of rigorous phenotyping of transgenic trees under realistic field conditions to help select lines that are both viable physiologically and meet the desired biotechnological aims.