Page 376 - Biosystems Engineering

Evaluation of Transgenic Wood for Pr oductivity & Quality 353

timber needs a high density and strength. Low-density wood is more
suitable for pulp, paper, and composite production.
The mass density of a substance is its mass per unit volume. A
wood’s structure can be simplified into solid materials (cell walls)
and voids (cell lumens). High-density wood contains thicker cell
walls and smaller lumens than lower-density wood. Our recent
research (Wu et al. 2009) investigated the bulk wood and cell wall
mechanical properties of ten hardwoods with densities varying from
3
0.41 to 1.18 g/cm by nanoindentation and SilviScan as affected by
wood species (density) and microfibril angle (MFA). The elastic mod-
uli of bulk wood and the cell walls of wood were both significantly
different, whereas the hardness of the cell wall was not significantly
different among the ten species. The SilviScan elastic modulus increased
with wood density and decreased with microfibril angle. At the cell
wall level, the elastic modulus and hardness obtained by nanoinden-
tation were more related to the properties of natural libriform fibers.
However, there was no significant trend found for the hardness of
cell wall as affected by either wood bulk density or microfibril angle.
Research by Park et al. (2004) showed that the 54 independent
transgenic poplars produced longer internodes and a larger amount
of cellulose in the secondary xylem than the wild type. The cellulose
content in the tenth internode was also assessed by measuring spe-
cific gravity, which showed relative larger values in the transgenic
than in the wild type. Specific gravity was increased in the transgenic
probably by increasing the density of cellulose, which could result in
an increase in cellulose content. By measuring the tensile Young’s
modulus of the stem via an automatic material-testing machine, Park
et al. (2004) found that the transgenic stem had a lower modulus in
the second internode compared to the wild type, but in the remaining
internodes the modulus was increased gradually from the fourth to
the tenth internodes with the increased length of the internode.
Differences in the location of cell wall fracture after tangential
splitting were observed among three genetic groups of radiate pine
(Donaldson 1995). In a control radiate pine, cell wall fracture occurs
predominantly at the middle lamella/S1 boundary or within the S1
layer, producing large numbers of fines on the fracture surface. On
the other hand, in two genetically select groups of radiate pine, cell
wall fracture occurs predominantly at the S1/S2 boundary, produc-
ing fewer fines on the surface. The differences appear to be related to
reduced lignifications in the inner S1 layer of the genetically select
trees. Observed differences in the type of fracture and its location
were unrelated to cell dimensions. The observed changes in fracture
behavior may be related to differences in the thermomechanical pulp-
ing properties observed among these three groups of trees and may
provide a useful nondestructive screening method for selecting clones
with advantageous pulping properties.