Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 202

treatment. These results suggested that there is a strong negative correlation
between the growth rate under cold conditions and the levels or perception of SA.
In conclusion, SA has a contradictory role in cold stress tolerance. Although
exogenous SA treatment can protect plants against stress-induced injury, the
treatment itself causes stress to the control plants. Furthermore, SA treatment may
alter the endogenous SA metabolism, either by inducing de novo synthesis (Szalai
et al. 2011 ) or by decreasing and disturbing it (Rakhmankulova et al. 2010 ).
Although cold stress induces an increase in endogenous SA, it seems it is better if
the SA level remains at a low level.Thus,the effect of SA in plants is related to its
initial endogenous content (Yang et al. 2004 ), to changes in this and possibly to
redistribution between free and conjugated forms.
2.6 Other Stresses
2.6.1 Ozone Stress
Ozone is formed through photochemical reactions between nitrogen oxides, carbon
monoxide and hydrocarbons released, primarily, through the burning of fossil fuels
in urban areas (Mauzerall and Wang 2001 ). Ozone production is particularly
favoured in summer months by strong sunlight, high temperature and stagnant
high-pressure systems, and concentrations therefore tend to be at their highest
during the growing season of most of the world's crop plants. Ozone is toxic to
plants and animals because it is a powerful oxidizing agent, which is able to react
directly with lipids and proteins. Such reactions and the decomposition of ozone in
aqueous environments such as the plant apoplast can lead to the production of
other ROS such as the hydroxyl radical, singlet oxygen and H 2 O 2 (Kanofsky and
Sima 1991 ; Mehlhorn et al. 1990 ; Evans et al. 2005 ). The primary site of ozone
interaction with plant cells is the extracellular matrix, where ozone challenges the
antioxidant protection of the cells (Baier et al. 2005 ). Long-term chronic exposure
to ozone can lead to a reduction in growth and crop yield, resulting from the
inhibition of photosynthesis, premature senescence, altered biomass partitioning
and changes to reproductive processes (Black et al. 2000 ; Pell et al. 1997 ; Saitanis
and Karandinos 2002 ; Sandermann 1996 ). Ozone is also able to act as an abiotic
elicitor of plant defence reactions and acute exposure can result in the appearance
of small necrotic hypersensitive response (HR)-like lesions on foliage (Rao and
Davis 2001 ).
Ozone treatment led to the accumulation of SA, the synthesis of PR protein and
the development of virus resistance in tobacco (Yalpani et al. 1994 ). The role of
SA in counteracting ozone stress was also demonstrated in Arabidopsis plants,
where NahG plants were more sensitive to the damaging effect of ozone. The
synthesis of some ozone-induced mRNAs is SA-dependent, so only a few were
found in transgenic plants. Other authors reported that both a deficiency and an
excess of SA caused greater ozone sensitivity (Rao and Davis 1999 ).