Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 193

increased salt-induced damage (Hao et al. 2012 ). SA promotes seed germination
under high salinity in Arabidopsis. The seed germination of sid2 mutant (SA
induction deficient), which has a defect in SA biosynthesis, is hypersensitive to
high salinity, but the inhibitory effects were reduced in the presence of physio-
logical concentrations of SA (Lee and Park 2010 ).
Salt stress caused a decrease in SA content in Iris hexagona (Wang et al. 2001 ).
A lower concentration of SA was also reported in tomato cell suspension cultures
after NaCl supplement (Molina et al. 2002 ). In maize there were no changes in the
levels of endogenous free and bound salicylic acid during NaCl stress, but the free
oHCA content, a putative precursor of SA, in the leaves increased after 7 days, and
rose dramatically after recovery. Free SA only increased during recovery in the
leaves and roots (Szalai and Janda 2009 ). In contrast, the endogenous free SA
content decreased in soybean under salt stress (Hamayun et al. 2010 ).
2.4 Heat Tolerance
2.4.1 Heat Stress and Thermotolerance
Heat stress is often defined as the rise in temperature beyond a threshold level for a
period of time sufficient to cause irreversible damage to plant growth and devel-
opment. At moderately high temperatures, injuries or death only occur after long-
term exposure. At very high temperatures, severe cellular injury and even cell
death may occur within minutes. Heat stress causes morpho-anatomical, physio-
logical and biochemical changes in plants. Direct injuries due to high temperatures
include protein denaturation and aggregation, and the increased fluidity of mem-
brane lipids. Indirect or slower heat injuries include the inactivation of enzymes in
chloroplasts and mitochondria, the inhibition of protein synthesis, protein degra-
dation and loss of membrane integrity. These injuries eventually lead to starvation,
inhibition of growth, reduced ion flux, and the production of toxic compounds and
ROS (Wahid et al. 2007 ). The homeostasis, content, biosynthesis and compart-
mentalization of hormones are also altered under heat stress (Maestri et al. 2002 ).
The upper developmental threshold, above which growth and development cease
differ for different plant species and for genotypes within species, and also depend
on plant behaviour, which may differ in various environmental conditions.
Plants manifest different survival mechanisms at elevated temperatures,
including long-term evolutionary phenological and morphological adaptation and
short-term avoidance or acclimation mechanisms such as changing leaf orientation,
transpirational cooling, or alterations in membrane lipid composition. The capacity
to survive heat shock varies with the plant species and genotype, and also with the
developmental stage. Plants may have basal thermotolerance in the absence of pre-
adaptation. In addition, plants subjected to mild heat stress may transiently acquire
tolerance to previously lethal high temperatures: this phenomenon is known as
acquired thermotolerance or heat acclimatisation, and is probably an adaptation to