Biology Reference
In-Depth Information
gene expression was also correlated with increased lignin content and this
increase was dependant on exposure duration (
Kimura et al., 2003
).
I. ELEVATED CO
2
LL
1
before the indus-
The concentration of CO
2
has increased from 280
m
LL
1
at the present time and is predicted to double in 2100
according to IPCC scenarios (
IPCC, 2007
). The effects of elevated CO
2
on
plants have been extensively studied. In particular, 'Free-Air CO
2
Enrich-
ment' (FACE) systems involving long-term exposures under natural open-air
conditions have provided us with plausible hypotheses of how plants will
respond to higher CO
2
concentrations. Elevated carbon dioxide constitutes a
stress in that it causes profound reorganizations of plant physiology. In
general, high CO
2
reduces stomatal conductance but increases growth, bio-
mass and yield (
Ainsworth and Long, 2005
). Stimulation of growth and
biomass is often linked with higher carbon assimilation (
Ainsworth and
Rogers, 2007
) although the response depends on the species considered.
Trees were more responsive than herbaceous species whereas high CO
2
had
little effect on C4 species. Nitrogen (N) availability was shown to modulate
the response with the CO
2
-induced stimulation of photosynthesis increasing
under high N conditions.
Elevated CO
2
is predicted to increase the concentration of secondary or
structural compounds according to the 'source-sink balance hypothesis'
(
Penuelas and Estiarte, 1998
). Factors such as elevated CO
2
or nutrient stress
induce a relative increase in carbon availability and consequently lead to the
accumulation of carbon-based secondary or structural compounds in source
leaves. Although different results showed a general trend towards increasing
levels of secondary compounds, their chemical nature varies depending upon
the plant species or genotype (
Bidart-Bouzat and Imeh-Nathaniel, 2008
).
Lignin content increased under elevated CO
2
in tree leaves (
Couteaux et al.,
1999; Norby et al., 2001; Porteaus et al., 2009; Staudt et al., 2001
)andin
tobacco (Nicotiana tabacum;
Matros et al., 2006
), although other results, from
long-term FACE studies, have shown no such effects (
Finzi and Schlesinger,
2002; Liu et al., 2009; Oksanen et al., 2005; Parsons et al., 2008
). However, the
effect was shown to be highly dependent on N supply in F. sylvatica (
Blaschke
et al., 2002
). In N-limited plants, leaf lignin content increased under elevated
CO
2
whereas in plants grown with high nutrient supply, the lignin content was
unaffected or decreased by elevated CO
2
.
Transcriptomic analyses of plants grown under elevated CO
2
revealed a
stimulation of the phenylpropanoid pathway. For example, phenylpropa-
noid pathway transcripts accumulated during the month of August in birch
trial era to 380
m
