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chronic exposure. In the different studies described below, ozone varied
between ambient levels (30-50 ppb) to high levels (600 ppb) and exposure
times ranged from 3 h to several years. In addition, plant age varied between
3 weeks and 4 years. Ozone increased PAL enzyme activity in Pinus sylvestris
( Rosemann et al., 1991 ), parsley (Petroselinum crispum; Eckey-Kaltenbach
et al., 1994 ), soybean (Glycine max; Booker and Miller, 1998 ), grape (Vitis
vinifera; Sgarbi et al., 2003 ), poplar (Populus spp.; Caban´ et al., 2004; Di
Baccio et al., 2008 ) and Centaurea jacea ( Francini et al., 2008 ). 4CL activity
was also increased in soybean ( Booker and Miller, 1998 ) and CAD activity
increased in response to ozone in Picea abies ( Galliano et al., 1993a; Heller
et al., 1990 ), parsley ( Eckey-Kaltenbach et al., 1994 ), soybean ( Booker and
Miller, 1998 ), P. sylvestris ( Zinser et al., 1998 ) and poplar ( Caban ´ et al.,
2004; Di Baccio et al., 2008 ). Transcript abundance of genes encoding
phenylpropanoid pathway enzymes responded in the same way suggesting
that ozone-induced lignification involved transcriptional regulation. For
example, PAL transcript levels were more abundant following ozone treat-
ment in parsley ( Eckey-Kaltenbach et al., 1994 ), A. thaliana ( Sharma and
Davis, 1994 ), birch (Betula pendula; P¨¨kk¨nen et al., 1998; Tuomainen
et al., 1996 ), poplar ( Koch et al., 1998; Wustman et al., 2001 ) and C. jacea
( Francini et al., 2008 ). Increased transcript levels were also observed for 4CL
in parsley ( Eckey-Kaltenbach et al., 1994 ) and for COMT in poplar ( Koch
et al., 1998 ) and beech (Fagus sylvatica; Jehnes et al., 2007; Olbrich et al.,
2005 ). CAD transcripts increased in P. abies ( Galliano et al., 1993b ),
P. sylvestris ( Zinser et al., 1998 ) and poplar ( Caban´ et al., 2004 ). Transcrip-
tomic studies indicated that PAL, 4CL, C3H, CCoAOMT and CCR
responded to ozone treatment in rice (Oryza sativa; Frei et al., 2011 ),
COMT in beech and Arabidopsis ( Olbrich et al., 2005, 2009; Tosti et al.,
2006 ) and CCR in birch ( Kontunen-Soppela et al., 2010 ). The responses of
the phenylpropanoid metabolism can be both fast ( Koch et al., 1998; Sharma
and Davis, 1994 ) and substantial ( Caban´ et al., 2004 ). Within an experi-
ment, induction levels were correlated with ozone concentrations ( Caban ´
et al., 2004; Galliano et al., 1993a; Rosemann et al., 1991 ). Stimulation of the
phenylpropanoid pathway was often maintained during the whole period of
ozone exposure ( Caban ´ et al., 2004; Galliano et al., 1993b ) and could even
continue after the end of the treatment ( Tuomainen et al., 1996 ). Interesting-
ly, the shikimate pathway, which supplies substrates to the phenylpropanoid
pathway, was also found to be upregulated by ozone treatment ( Betz et al.,
2009a,b; Caban´ et al., 2004; Guidi et al., 2005; Janzik et al., 2005 ). This
observation suggests a strong coordination between primary and secondary
metabolism to provide substrates to the phenylpropanoid pathway. Most of
the above results were obtained from plants cultivated in controlled or open
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