Mechanism of Cadmium Toxicity and Tolerance in Crop Plants - Crop Improvement Under Adverse Conditions - page 353

in growth and productivity, and eventually cause the death of plants (Gill and Tuteja
2010 ). ROS are efficiently scavenged by enzymatic (SOD, CAT, APX and GR) and
non-enzymatic antioxidants such as ascorbate (AsA) and glutathione (GSH), which
protect plants against oxidative damage (Mittler et al. 2004 ; Gill and Tuteja 2010 ).
The enzymes SOD and CAT are involved in the detoxification of O 2 ·ˉ and H 2 O 2 ,
respectively, thereby preventing the formation of OH · radicals, whereas, APX and
GR as well as ascorbate (AsA) and glutathione (GSH) are important components
of the ascorbate-glutathione cycle -(AsA-GSH cycle) responsible for the removal
of H 2 O 2 in different cellular compartments (Gill and Tuteja 2010 ). In the present
chapter, we will attempt to understand the response of crop plants to Cd toxicity and
mechanism of Cd tolerance.
2 Uptake, Transport, Accumulation and Localization
of Cd in Crop Plants
Cadmium (Cd) is a widely spread heavy metal, which causes serious environmental
and human health problems due to its high mobility in the soil-root-shoot (grain)
system (Vázquez et al. 2006 ). Cadmium can get absorbed into the plant root system
through some other nutrient metabolic pathways such as zinc, iron, and calcium
(Pence et al. 2000 ; Cosio et al. 2004 ; Vert et al. 2002 ; Nakanishi et al. 2006 ) after
it reaches root cell membranes via the apoplast, including cell wall continuum and
intercellular space. Plants show a differing metal distribution and accumulation pat-
tern among different parts. Most of the Cd that enters the plant system, accumulates
in the roots and only a small portion is translocated to the parts above the ground
(Patel et al. 2005 ; Kovacik et al. 2006 ; Ekmekci et al. 2008; Liu et al. 2007 ; Singh
et al. 2008 ; Gill et al. 2011 ). The actual accumulation of Cd in plant parts depends
on the plant species and soil properties (Fediuc and Erdei 2002 ; Arao et al. 2003 ).
Substantial variability among 99 Pisum sativum genotypes in tolerance to Cd and
uptake of different heavy metals was reported (Belimov et al. 2003 ). Ten times
higher Cd accumulation in the roots than in the parts above the ground was reported
in Hordeum vulgare plants (Vassilev et al. 1998 ). Kovacik et al. ( 2006 ) reported
that Cd accumulation was seven- (60 µM Cd) to eleven- (120 µM Cd) fold higher
in the roots than in the leaves of Matricaria chamomilla , whereas only 6 % of Cd
was accumulated in the leaves of Crotalaria juncea compared to roots with 2 mM
CdCl 2 (Pereira et al. 2002 ). Cadmium was accumulated 1826 times more in the
roots of Allium sativum than the control with the application of 10-2 M Cd and
very low amount was transported to the bulbs and shoots (Jiang et al. 2001 ). Zhang
et al. ( 2000 ) reported significant difference among Triticum aestivum genotypes in
shoot Cd concentration. Significant differences in Cd accumulation and tolerance
were found in Sedum alfredii populations (Deng et al. 2007 ). The Milyang 23 rice
accumulated 10-15 % of the total soil Cd in its shoot (Murakami et al. 2007 ). Dje-
bali et al. ( 2008 ) reported that the roots of Cd treated Solanum lycopersicum plants
accumulated four to five-fold Cd as compared to mature leaves. Liu et al. ( 2007 )