Biology Reference
In-Depth Information
but this observation was not maintained after 5 years (
Kostiainen et al.,
2008
). In one study, ozone was observed to repress the phenylpropanoid
pathway in poplar wood (
Richet et al., 2011
), probably as a result of reduced
cambial growth. However, the relative cell wall lignin content increased due to
ozone-induced reductions in cellulose biosynthesis, thereby modifying the
cellulose to lignin ratio. The stem response seems to correspond to a metabolic
adjustment due to the reorientation of the metabolism to stress acclimation
in leaves, rather than to a specific defence mechanism. It was hypothesized
that the modification of the cellulose to lignin ratio in the stem could allow the
tree to maintain (radial and height) growth while minimizing carbon cost.
More detailed analyses are needed to draw definite conclusions.
G. UV-B RADIATION
UV-B radiation (280-320 nm) is currently reaching the Earth's surface in
larger proportions because of the depletion of stratospheric ozone (
Caldwell
et al., 2003; Frohnmeyer and Staiger, 2003
). Elevated UV-B radiation
causes multitude effects on plants such as reduced growth, leaf thickening,
decreased photosynthesis and DNA damage (
Frohnmeyer and Staiger,
2003
). Among other physiological responses, UV-B exposure induces the
accumulation of UV-B-absorbing compounds such as phenolics in a large
number of plant species (
Caldwell et al., 2003
). Phenylpropanoids such as
hydroxycinnamic acids and their derivatives (sinapate esters) as well as
flavonoids play complementary roles in UV-B protection. The first group
are constitutive (present at leaf emergence), whereas the second group are
important for the adjustment of epidermal screening during leaf development
(
Burchard et al., 2000
). The absorption of UV-B by epidermal localized
aromatic phenolics prevents radiation-induced damage to photosystem II
and chlorophyll degradation. Ambient UV-B radiation reduced leaf elonga-
tion in Antarctic grass (Deschampsia antarctica) when compared with plants
exposed to reduced UV-B (
Ruhland and Day, 2000
). This reduction could be
explained partly by the accumulation of insoluble hydroxycinnamic acids
(p-coumaric, caffeic and ferulic acids) cross-linking the polysaccharide net-
work of the cell wall limiting the expansion of epidermal cells and leaf
elongation (
Ruhland et al., 2005
). UV-B radiation probably also affects cell
wall reticulation and cell expansion in grapevine (V. vinifera) leaves. The
lignin pathway genes PAL, C4H, 4CL and CCoAOMT were all upregulated
in response to UV-B radiation while genes associated with cell wall loosening
were downregulated (
Pontin et al., 2010
). Evidence for lignin deposition in
response to UV-B was provided by a study of leaves of the dune grassland
species (Calamagrostis epigeios). During one growing season, enhanced
