Lignins and Abiotic Stresses - Botanical Research - page 243

Populus euramericana and Populus nigra in a FACE experiment ( Luo and

Polle, 2009; Luo et al., 2008 ) as well as in birch ( Mattson et al., 2005 ).
However, different responses were observed in other experiments. Lignin
content was reduced in beech ( Blaschke et al., 2002 ) whereas no modification
was reported in birch, maple, or aspen clones in a FACE experiment
( Kaakinen et al., 2004; Kostiainen et al., 2008 ).

J. NITROGEN STRESS

Plants require mineral nutrients from the soil for their growth. Among these,
nitrogen (N) is the most important inorganic nutrient in plants and it is
generally considered to be the most limiting nutrient for tree growth ( Finzi
et al., 2007 ) and agricultural productivity ( Tonitto et al., 2006 ). Nitrogen is a
major constituent of proteins, nucleic acids, many cofactors and secondary
metabolites ( Maathuis, 2009 ). Therefore, strong variations in nutrient supply,
both deficiency and excess, induce important changes in plant metabolism
with a subsequent impact on biomass, yield and nutritional or wood quality
( Amtmann and Armengaud, 2009 ).
As with elevated CO 2 levels, nitrogen deficiency alters the C/N balance.
Plants growing on low N displayed a shift in shoot/root ratio to roots,
a decrease in amino acids, proteins and chlorophyll and an increase in starch
( Larcher, 2003 ). Nitrogen deficiency typically led to the accumulation of
secondary metabolites including phenylpropanoids. Increased lignin content
has been demonstrated in tobacco stems ( Fritz et al., 2006 ) and in chamo-
mile (Matricaria chamomilla) roots together with a stimulation of PAL
activity ( Kovacik et al., 2011 ). Further information on the interactions
between nitrogen levels and phenylpropanoid metabolism was provided
by a study on tobacco ( Fritz et al., 2006 ). Wild-type tobacco was grown
on low or high nitrate supply and compared with a nitrate reductase-
deficient mutant growing on high nitrate supply. Low-N wild-type plants
were highly lignified compared to high-N wild-type plants. The nitrate
reductase-deficient mutant accumulated large amounts of nitrate and re-
sembled high-N wild-type plants with respect to phenylpropanoid and
lignin levels, but to low-N wild-type concerning amino acids. PAL, 4CL
and HCT transcripts were induced in low-N wild-type plants but not in the
nitrate reductase-deficient mutant. It was concluded that nitrogen deficien-
cy leads to a marked shift of metabolism towards phenylpropanoids and
that stimulation of phenylpropanoid metabolism is triggered by changes of
nitrate levels, rather than downstream nitrogen metabolites. In Arabidopsis,
nitrate deficiency led to a coordinated induction of phenylpropanoid and
shikimate pathways ( Scheible et al., 2004 ).