SUMMARYSucrose is the main transported form of carbon in several plant species, including Populus species. Sucrose metabolism in developing wood has therefore a central role in carbon partitioning to stem biomass. Half of the sucrose-derived carbon is in the form of fructose, but metabolism of fructose has received little attention as a factor in carbon partitioning to walls of wood cells. We show that RNAi-mediated reduction of FRK2 activity in developing wood of hybrid aspen (Populus tremula · tremuloides) led to the accumulation of soluble neutral sugars and a decrease in hexose phosphates and UDP-glucose, indicating that carbon flux to cell-wall polysaccharide precursors is decreased. Reduced FRK2 activity also led to thinner fiber cell walls with a reduction in the proportion of cellulose. No pleiotropic effects on stem height or diameter were observed. The results establish a central role for FRK2 activity in carbon flux to wood cellulose.

This article focuses on the effect of pulp bleaching and emerging commercial compatibilizers on physical performance of pulp fiber reinforced poly(lactic acid) (PLA) biocomposites. Industrially bleached and unbleached hardwood kraft pulp fibers are treated with several additive types, and compounded with PLA to fiber content of 30 wt %. After injection molding, the produced biocomposites are evaluated by their mechanical performance and fiber-matrix adhesion. For selected materials, fiber surface and fiber properties are reflected to composite performance by analyzing the compositions, dimensions, and lignin coverage of original fibers, as well as fiber dispersion and dimensions after melt processing. As a conclusion, unbleached kraft pulp fibers provide significant improvement in physical properties of PLA/pulp fiber composites. Of the screened compatibilizers, epoxidated linseed oil has a clear positive effect on performance when bleached kraft pulp fibers are used. The improvements correspond to enhanced fiber-matrix adhesion and differences in remaining fiber length distributions.

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Xylans occupy approximately one-third of the cell wall components in hardwoods and their chemical structures are well understood. However, the microdistribution of xylans (O-acetyl-4-O-methylglucuronoxylans, AcGXs) in the cell wall and their correlation with functional properties of cells in hardwood xylem is poorly understood. We demonstrate here the spatial and temporal distribution of xylans in secondary xylem cells of hybrid aspen using immunolocalization with LM10 and LM11 antibodies. Xylan labeling was detected earliest in fibers at the cell corner of the S₁ layer, and then later in vessels and ray cells respectively. Fibers showed a heterogeneous labeling pattern in the mature cell wall with stronger labeling of low substituted xylans (lsAcGXs) in the outer than inner cell wall. In contrast, vessels showed uniform labeling in the mature cell wall with stronger labeling of lsAcGXs than fibers. Xylan labeling in ray cells was detected much later than that in fibers and vessels, but was also detected at the beginning of secondary cell wall formation as in fibers and vessels with uniform labeling in the cell wall regardless of developmental stage. Interestingly, pit membranes including fiber-, vessel- and ray-vessel pits showed strong labeling of highly substituted xylans (hsAcGXs) during differentiation, although this labeling gradually disappeared during pit maturation. Together our observations indicate that there are temporal and spatial variations of xylan deposition and chemical structure of xylans between cells in aspen xylem. Differences in xylan localization between aspen (hardwood) and cedar (softwood) are also discussed.

A new method is presented for quantitative evaluation of hybrid aspen genotype xylem morphology and immunolabeling micro-distribution. This method can be used as an aid in assessing differences in genotypes from classic tree breeding studies, as well as genetically engineered plants. The method is based on image analysis, multivariate statistical evaluation of light, and immunofluorescence microscopy images of wood xylem cross sections. The selected immunolabeling antibodies targeted five different epitopes present in aspen xylem cell walls. Twelve down-regulated hybrid aspen genotypes were included in the method development. The 12 knock-down genotypes were selected based on pre-screening by pyrolysis-IR of global chemical content. The multivariate statistical evaluations successfully identified comparative trends for modifications in the down-regulated genotypes compared to the unmodified control, even when no definitive conclusions could be drawn from individual studied variables alone. Of the 12 genotypes analyzed, three genotypes showed significant trends for modifications in both morphology and immunolabeling. Six genotypes showed significant trends for modifications in either morphology or immunocoverage. The remaining three genotypes did not show any significant trends for modification.