Biology Reference
In-Depth Information
top chambers and the same trend was also observed in more natural condi-
tions such as free air ozone fumigation facilities (
Betz et al., 2009a; Wustman
et al., 2001
). All these results unambiguously demonstrate that the phenyl-
propanoid pathway is upregulated in leaves under ozone exposure and is
therefore probably involved in defence and acclimation mechanisms.
However, the effects of ozone fumigation on lignin content in leaves were
not so clear. No modifications of lignin content were recorded in Pinus
ponderosa (
Tingey et al., 1976
), black cherry (Prunus serotina) and yellow
poplar (Liriodendron tulipifera;
Boerner and Rebbeck, 1995
), Pinus taeda
(
Booker et al., 1996
), soybean (
Booker and Miller, 1998
), cotton (Gossypium
hirsutum;
Booker, 2000
), birch (
Oksanen et al., 2005
), barley (Hordeum
vulgare;
Plessl et al., 2005
)andQuercus ilex (
Baldantoni et al., 2011
). Lignin
content was increased after ozone treatments in sugar maple (Acer saccharum;
Boerner and Rebbeck, 1995
), bahiagrass (Paspalum notatum;
Muntifering
et al., 2000
), poplar (
Caban
´
et al., 2004
), Trifolium spp. (
Muntifering et al.,
2006; Sanz et al., 2005
), beech (
Betz et al., 2009a; Jehnes et al., 2007; Olbrich
et al., 2010
), Echinacea purpurea (
Szantoi et al., 2007
), rice (
Frei et al., 2010,
2011
)andBriza maxima (
Sanz et al., 2011
). These varying results may be
explained by species-specific differences in response to ozone treatment. For
example, conifers never showed increased lignin content. Stimulation of the
phenylpropanoid pathway could in such cases be associated with a modifica-
tion of the pool of soluble phenolic and not necessarily lead to increased
lignification (
Booker and Miller, 1998; Tingey et al., 1976
). Another source
of potential error could be due to the different techniques (Klason, LTGA,
etc.) used to determine lignin content as well as their relative (in)sensitivity
(
Dence, 1992
), especially in the case of weak variations between control and
experimental samples. Nevertheless, the structure of newly synthesized lignin
(following ozone treatment) has been determined in poplar and beech and
both species displayed comparable changes in lignin structure (
Betz et al.,
2009a; Caban´ et al., 2004
). Lignins were enriched in carbon-carbon interunit
bonds and in H units indicating the production of a more condensed lignin
than usual. Moreover, lignified cells were observed in the mesophyll or epider-
mis near the necrotic lesions in the leaf (
Caban
´
et al., 2004
). These results
support the idea that stress lignins are synthesized in response to and in defence
against ozone or ROS excess. Comparable studies on other species are needed
in order to identify a general trend.
The response to ozone has been extensively studied in leaves. This is
understandable since it is mainly this organ that shows clear ozone-induced
damage. Few studies have analysed the ozone response of stems, especially in
trees. An increase in lignin content was observed in poplar and birch fumi-
gated for 3 years in a free air fumigation experiment (
Kaakinen et al., 2004
),
