Dealing with Environmental Stresses: Role of Polyamines in Stress Responses - Crop Improvement: New Approaches and Modern Techniques - page 465

instances, plants generate ROS as defence responses to various stresses. If the equi-
librium between ROS production and scavenging is disturbed it is harmful to the
plants (Apel and Hirt 2004 ). The most important ROS scavenging enzymes are
superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione peroxidase
(GPX), and catalase (CAT).
PAs play a role in oxidative stress tolerance by functioning as antioxidants under
some environmental conditions (Groppa et al. 2001 , Chattopadhayay et al. 2002 ;
Kakkar and Sawhney 2002 ). Few examples are, leaf injury of the ozone-sensitive
tobacco cultivar Bel W3 caused by ozone treatments was minimized when put, spd
or spm was applied to the root system (Bors et al. 1989 ) and it was suggested that
the antioxidative effect is due to a combination of their anionic and cationic-binding
properties in radical scavenging function (Bors et al. 1989 ). Løvaas ( 1997 ) reported
that PAs are involved in preventing photooxidative damage. But there are contrdict-
ing evidences regarding lack of antioxidant activity of PAs.
A study using vesicles prepared with mixed soy bean phospholipids showed
that PA mediated inhibition of lipid peroxidation by metal-catalysed the oxida-
tive reactions (Tadolini et al. 1988 ). H 2 O 2 produced as a result of catabolism of
PA by CuAO/DAO and PAO act as a signalling molecule that promotes activa-
tion of defence responses but it can also act as a prooxidant agent (Groppa and
Benavides 2008 ).
MineralDeficiencyandHeavyMetalStress
Both, natural and agricultural ecosystems are most of the times known to have
sub-optimal levels of mineral nutrients (Lynch and Clair 2004 ). Mineral stress con-
ditions cause important, complex, and poorly understood interactions with global
climate change. Factors such as changes in rainfall, temperature, solar radiation and
atmospheric CO 2 concentration have significant impact on soil nutrient status and
soil erosion rates. Inadequate supply of minerals to the plants results in nutritional
disorders manifested by characteristic deficiency symptoms. Presence of excess
minerals in the soil mostly associated with accumulation of heavy metals such as
zinc, copper, cobalt, nickel, mercury, lead and cadmium causing severe toxicity in
plants (Taiz and Zeiger 2006 ). In plants metal toxicity arise from the binding of
metals to sulphydryl groups of proteins inhibiting enzymes activities or altering
protein structure (Van Assche and Clijsters 1990 ), stimulation of ROS formation
leading to oxidation of macromolecules and oxidative stress (Sandalio et al. 2001 ).
The presence of excess minerals in soil also leads to saline conditions in which
plant growth is restricted. Most of the times mineral deficiency and toxic stresses in
plants are not clearly identifiable with other stress conditions making it even more
complicated to predict and find solutions in improving crop yield. Genetic variation
among plants has a significant influence in dealing with mineral stresses. Plants
display variable morphological, physiological and biochemical adoptive responses
in dealing with particular mineral deficient condition and heavy metal toxicity.