Salicylic Acid-Mediated Abiotic Stress Tolerance - Salicylic Acid: Plant Growth and Development - page 188

suggest that the increase in the oHCA content was induced independently of the
SA biosynthesis, but may play a role in the antioxidative response to cadmium.
The increased endogenous SA levels in the leaves of maize seedlings may be
associated with the oxidative stress observed in the leaves of Cd-stressed plants,
suggesting a role for SA in the response of maize to Cd. At the same time other
authors found no increase in the endogenous SA content as a result of Cd (7 lM),
Cu (3 lM) and Zn (70 lM) treatment, or any difference in the SA content of
sensitive and resistant plants of Salix viminalis L. in the control (Landberg and
Greger 2002 ).
Heavy metal tolerance is often correlated with intracellular compartmentali-
zation (Brune et al. 1995 ). Nickel hyperaccumulation is usually due to a highly
efficient pumping system that transfers the metal to the central vacuole of the shoot
cells, leading to a high level of tolerance to this element (Krämer et al. 2000 ), but it
is clear that a substantial amount of cellular Ni also accumulates outside the
vacuole, suggesting the existence of a cytoplasmic-based tolerance mechanism.
Due to the constitutively enhanced activity of serine acetyl transferase, the glu-
tathione concentration in hyperaccumulating Thlaspi plants is also constitutively
elevated, leading to enhanced tolerance to Ni-induced oxidative stress (Freeman
et al. 2004 ). In a later experiment it was also proved that the glutathione-mediated
Ni tolerance mechanism observed in Ni-hyperaccumulating Thlaspi species is
signalled by the constitutively elevated levels of SA. It was also observed that both
biochemical and genetic manipulations that increase SA in Arabidopsis thaliana
(L.) Heynh plants mimic the glutathione-related phenotypes of the hyperaccu-
mulating Thlaspi, and that these biochemical changes in the non-accumulator are
associated with increased glutathione-mediated Ni resistance. Such observations
suggest that SA may be one of the regulators involved in coordinating certain key
biochemical differences between Ni/Zn hyperaccumulators and non-accumulator
plant species.
2.2 Drought
2.2.1 Drought-Induced Changes in Plants
Plants are exposed to drought stress when there is not sufficient water available, or
when, for some reason, the water present cannot be taken up by the plants, e.g. if
the ground is dry, if there is intense evaporation or severe frost, or if the soil has a
high salt content, leading to strong osmotic water binding (Mishra and Singh
2010 ). Water deficit is a multidimensional stress affecting plants at various levels
of their organization. The first, most sensitive sign of water deficiency is a
reduction in turgor, leading to the retardation of growth processes, especially linier
growth. Nevertheless the effects of stress are not only manifested at the mor-
phological level but also at the physiological level (growth inhibition, stomatal
closure, reduced transpiration rate, decrease in water potential and photosynthetic