Agriculture Reference
In-Depth Information
inhibition (Scott et al., 2004). The application of exogenous SA (Janda et
al., 1999), but also ethylene (Yu et al., 2001) can actually enhance cold
tolerance of plants.
In contrast to drought and salt stress, the role of the phytohormone
ABA in cold stress and acclimation is highly controversial. Initial experi-
ments in
Solanum commersonii
showed a correlation between ABA and
cold acclimation (Chen et al., 1983). Similar ABA accumulation has been
reported from other cold acclimated plants (Lång et al., 1994; Mäntylä
et al., 1995) and accordingly ABA deficient mutants exhibit lower cold
tolerance (Heino et al., 1990). However, as Thomashow (1999) points
out, the observed ABA accumulation in different plants is only transient
and is not correlated to the enduring effects of cold acclimation. As for
ABA deficient mutants, the reduced cold stress tolerance might be rather a
side effect of the general reduced plants' health, caused by impaired ABA
biosynthesis. Thus there are at least two signaling pathways that activate
cold-regulated gene expression, an ABA dependent pathway (Lång et al.,
1994) and an ABA independent pathway (Capel et al., 1997).
13.6 POLLUTANTS
Many natural occurring substances, termed xenobiotics, are toxic for
plants at high concentrations. Human activity contributed substantially to
the accumulation of heavy metals in soils and ground water. Most plants
do not possess specific mechanisms to prevent the excessive uptake of
heavy metals from the soil. Accumulation of heavy metal ions in the plant
can impair membrane integrity, affect enzyme activity and hinder nutri-
ent uptake. Metal ions can generate ROS by auto-oxidation, Haber-Weiss
cycle or Fenton reaction and disturb the redox status of cells.
Metals without redox capacity such as cadmium, mercury and lead
can disturb the antioxidative glutathione pool, activate Ca2+ dependent
systems and iron mediated processes (Pinto et al., 2003). Apart from soil
pollution human activity also increased concentration of air pollutants
such as O
3
, SO
2
, NO, NO
2
, NH
3
, HNO
3
and HF. Through stomata they can
enter the leaves and affect the plant metabolism. Entry of O
3
into the cell
forces the creation of ROS and induces oxidative stress. It is assumed that
the O
3
derived ROS production mimics the signaling pathways following
oxidative burst during a virulent pathogen attack (Rao and Davies, 2001).
