Sugar Release in Populus Tree Variants Affected by Lignin Content

Fast-growing trees, such as those from the genus, Populus, are considered as potential (lignocellulosic) feedstock sources for "second generation" biofuels. Thus, research on improving the processability of fast-growing trees for biofuel production is one area of active research. As with the other types of lignocellulosic feedstocks (such as grasses), the reduction of "biomass recalcitrance" is a major challenge for improving the processability for biofuel-ethanol production. The lignin content and the syringyl-to-guaiacyl (S/G) ratio in biomass have been recently used to quantitatively assess biomass recalcitrance. Syringyl (S) and guaiacyl (P) are two of three major structural units of lignin (the third structural unit is p-hydroxyphenyl (P)). The S/G ratio is a quantity has been found to correlate with many variables associated with biomass recalcitrance.

Scientists from the Oak Ridge National Laboratory, and the National Renewable Energy Laboratory (United States) report that the sugar release in Populus tree variants may be affected by both lignin content and the S/G ratio. They found that for pretreated samples with S/G ratios less than 2.0, a strong negative correlation between sugar release and lignin content was found. For pretreated samples with S/G ratios higher than 2.0, "sugar release was generally higher, and the negative influence of lignin was less pronounced". They also found that glucose release was correlated with both lignin content and S/G ratio, but xylose release depended on the S/G ratio alone.

Some samples with average values of lignin content and S/G ratios were observed to exhibit "exceptional sugar release", prompting the researcher to conclude that "factors beyond lignin and S/G ratio influence recalcitrance to sugar release". A critical need for deeper understanding of cell wall structure may be needed "before plants can be rationally engineered for reduced recalcitrance and efficient biofuels production".