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trees cultivated for 6 years under elevated CO
2
concentrations. The authors
suggested that this stimulation would give rise to phenolic compounds rather
than lignins (
Kontunen-Soppela et al., 2010
). Arabidopsis growing under
elevated CO
2
also showed increased phenylpropanoid pathway gene expres-
sion. PAL1 and LAC4 were upregulated as were other cell wall-related genes.
Metabolite profiling indicated that levels of most amino acids decreased
under elevated CO
2
except histidine, tryptophan and phenylalanine which
is consistent with an increased flux towards secondary metabolism (
Li et al.,
2008
). Soybean grown for 40 days under elevated CO
2
also displayed
increased expression in genes involved in secondary metabolism, particularly
phenylpropanoid pathway. COMT and CCR were upregulated in leaves and
the overall gene expression profile suggested an increased flux to secondary
metabolism. Nevertheless, the response may differ according to the genotype
as showed by
Cseke et al. (2009)
. In this study, leaf transcription profiles,
physiology and biochemistry were compared between CO
2
-responsive and
unresponsive clones of P. tremuloides grown in long-term FACE experiments.
The physiological responses of these clones were similar (photosynthesis,
stomatal conductance and leaf area index) except for radial growth. Compari-
son of transcriptomic profiles from these two genotypes suggested that they
used different partitioning strategies under elevated CO
2
.TheCO
2
-responsive
clone partitions carbon into pathways associated with active defence and stress
responses, carbohydrate synthesis and subsequent growth, whereas the CO
2
-
unresponsive clone partitions carbon into pathways associated with passive
defence (phenylpropanoid) and cell wall thickening. However, precise deter-
minations of phenolic and lignin content in leaves from both genotypes would
be necessary in order to confirm or infirm these hypotheses.
Most studies concerning the effect of elevated CO
2
on plants have been
focused on source organs, that is, leaves. Modifications in leaf physiology
and biochemistry could also impact on sink organs, for example, stems—
especially in trees. Gene expression profiles of leaves and stems were com-
pared in P. tremuloides grown for 3 years under elevated CO
2
and high N
supply (
Druart et al., 2006
). CO
2
-responsive genes were more abundant in
stems compared to leaves suggesting a higher readjustment to elevated CO
2
levels in stems. Genes involved in shikimate and flavanol synthesis were
upregulated in leaves whereas genes related to phenylpropanoid and lignin
synthesis (C3H, COMT and CAD) were stimulated in stems. Although it was
suggested that lignification was enhanced by elevated CO
2
in stems, no data
were available on lignin content.
The effect of elevated CO
2
on wood lignin content has been assessed
and it appears that responses depend on the species, the season and N
supply. Lignin content was increased in the wood from coppices of both
