Biology Reference
In-Depth Information
collapsed xylem cells, an important reduction in aerial biomass, modified tree
architecture and reduced height. Further in-depth analyses carried out on the
same plant material revealed that reduced lignification strongly affected hydrau-
lic and mechanical wood properties. Xylem vulnerability to embolism appeared
strongly increased and resulted, in the most affected field-grown trees, to nearly
90% shoot dieback and 25% tree mortality, whereas these events were not
observed in the wild-type trees or in the same transgenic trees, when grown in
the greenhouse (
Voelker et al., 2011a
). Further analysis of hydraulic architecture
on these trees showed that the dependence of xylem resistance to embolism was
bound to adequate wood mechanical properties, provided by normal (or near-
normal lignin content). In the patches of coloured wood which probably had
very low-lignin content, water transport in vessels appeared strongly impeded
due to the collapse and more importantly the occlusion of these vessels by
phenolic deposits and tyloses (
Kitin et al., 2010
). With regard to mechanical
properties, the strong reduction in lignins also resulted in a major decrease
in wood strength and stiffness even though wood density remained comparable
to that of control wood. Finally, low-lignin trees were shown to contain up to
threefold more tension wood (
Voelker et al., 2011b
).
Wood collected from these field-grown 4CL-downregulated trees was eval-
uated for bioethanol production: surprisingly, despite their strong lignin
reduction and higher cellulose content, pretreated wood did not exhibit any
improvement in saccharification efficiency (
Voelker et al., 2010
). This set
of experiments with 4CL-downregulated trees clearly demonstrated that
promising results primarily generated by the evaluation of greenhouse-grown
4CL-downregulated trees (
Hu et al., 1999
) were not confirmed on trees grown
under natural field conditions, that is, submitted to regular environmental
stress. Decreased tree fitness and increased mortality appeared to be linked
to a threshold in lignin content making the efficiency of this strategy question-
able (strong reduced lignin content) for making wood more amenable to new
industrial uses such as biofuel production.
Stout (2011)
evaluated for two growing seasons the productivity of a
number of transgenic low-lignin Populus trichocarpa trees (Nisqually-1)
with low 4CL activity planted as short rotation coppice at three different
sites in North Carolina, across a range of environmental conditions. In this
experiment, 4CL-downregulated trees exhibited both reduced lignin contents
(up to 50% as observed elsewhere) and increased S/G ratios. The tree biomass
productivity among trials was extremely variable with more than 10-fold
variations between extreme sites; a high field site
transgenic line interaction
was evidenced, confirming the need to evaluate transgenic trees at multiple
field sites, in the same way as for novel cultivars derived from conventional
breeding programs.
