Lignin is a phenolic polymer that is a fundamental part of the structure and function of plants. It provides mechanical support, acts as a hydrophobic insulation for the water conducting elements and is a strong shield against phytopathogens. Lignin is made of variable combinations of three monomers; syringyl (S), guaiacyl (G) and p-hydroxyphenyl (P) units. For the first time, the combined effects of modified lignin composition and gravity on the growth responses and physical properties were studied in genetically altered poplar. We hypothesized that enhancing the syringyl monomer will have an impact on the gravitropic responses and the mechanical and physiological properties of upright and inclined trees. Hybrid poplar clone 717 (Populus tremula x P. alba) was transformed to over express the F5H/Cald5H gene at different levels resulting in varying syringyl to guaiacyl ratios. Wild type poplar and the transgenic lines were either kept upright or inclined (45°) from vertical to induce gravitropic responses (tension wood formation) for three months. Over-expression of F5H in the tilted trees caused reduced growth rate than the upright wild type controls and a reduction in height to diameter ratio. Interestingly, the tilted stems with the highest syringyl lignin ratio responded faster and recovered back to vertical at least 8 days before the tilted wild type trees. Quantification of released growth strain showed less strain in the trees with the highest syringyl lignin percentage potentially due to a decrease in growth strain earlier than the other genetic lines at the end of the gravitropic period. When all stems were collected and mechanically tested while still fresh or oven-dried, four-point bending and compression tests showed similar elastic properties among the different genetic lines when the trees were kept upright. However, when tested until rupture, the over-expression lines had lower modulus of rupture (MOR) indicating an increase in brittleness. To test the effects of the genetic treatment and/or the gravitropic responses on the trees water conductive properties, the maximum hydraulic conductivity was measured on the stems after high pressure flushing to eliminate native emboli. Interestingly, the stems with higher syringyl lignin ratio showed improved water conductive efficiency in the upright stems. Anatomically, it was found that the lines with higher syringyl lignin abundance had larger vessel diameters and higher percent vessel lumen area. Percent total lumen area of vessels and fibers also increased whereas percent fiber area decreased. However, dry wood density was not affected by the lignin modification. Tension wood had higher fiber wall thickness but less percent total lumen area due to the formation of the gelatinous layer in the G-fibers of the tension wood. Moreover, wood samples from the different treatments and lines were analyzed for cell wall biochemical and structural properties. In the tension wood, increases in cellulose crystallinity and microfibril angle were among the expected results. Lignin quantitative analysis showed a slight decrease in total lignin with increasing syringyl fraction. The acid soluble lignin also increased significantly. Carbohydrate analyses indicated higher percent total sugars in tension wood than in opposite or normal wood. Interestingly, increasing syringyl lignin percentage resulted in slight decrease in percent total sugars. Similar results were observed for glucose and galactose percentages. Percent xylose was lower in the tension wood than in the opposite or normal woods. Before a large scale plantation of the genetically altered poplar trees can be assumed, further testing in the field will be required to verify their ability to withstand more complex stresses.
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