4.3.2 Effects of Cd on Photosynthesis
In many crop species, maize, pea, barley (Popova et al. 2008), mung bean (Vigna
radiata) (Wahid et al. 2008), and wheat (Moussa and El-Gamal 2010), the
evidence showed that photosynthesis was inhibited after both long-term and
short-term Cd exposure. The primary sites of action of Cd are photosynthetic
pigments, especially the biosynthesis of chlorophyll (Baszynski et al. 1980) and
carotenoids (Prasad 1995). Cd caused chlorosis of leaves by decreasing chloroplast
density, and the Cd-induced decrease in pigment content was more powerful at the
leaf surface (stomatal guard cells) than it was in the mesophyll. Besides this, in
Cd-treated leaves, change in cell size and reduction in stomatal density in the
epidermis were observed. This showed that Cd might interfere directly with
chloroplast replication and cell division in the leaf. Cadmium influences two key
enzymes of CO 2 fixation: ribulose-1,5-bisphosphate carboxylase (RuBPCase) and
phosphoenolpyruvate carboxylase (PEPCase). Cd ions damage RuBPCase structure by substituting for Mg ions, which are important cofactors of carboxylation
reactions and thus lower its activity. Cd can also shift RuBPCase activity toward
oxygenation reactions thereby reducing the efficiency of photosynthesis
(Siedlecka et al. 1998). Cd causes an irreversible dissociation of the large and
small subunits of RuBPCase, thus leading to loss of enzyme activity and inhibition
of C 3 cycle (Stiborova 1988; Malik et al. 1992). Cd also affects the water-oxidizing
complex of PSII by replacing the Ca
2+ in Ca/Mn clusters of oxygen-evolving
center (Sigfridsson et al. 2004). Cd induces changes in lipid and fatty acid
composition of membranes and thus produces alterations in the functionality of
membranes (Popova et al. 2009).
4.3.3 Effect of Cd on Mineral Nutrition
Cd has been shown to interfere with the uptake, transport, and use of several
elements (Ca, Mg, P, and K) and water by plants (Das et al. 1997). In sugar beet,
deficiency of Fe in roots induced by Cd was observed (Chang et al. 2003). Uptake
of P, K, S, Ca, Zn, Mn, and B in pea plants was inhibited strongly by Cd exposure
(Metwally et al. 2005). Guo et al. (2007) observed decrease in the concentrations
of P, K, Ca, Mg, Cu, Fe, Mn, Zn, Mo, and B in roots of barley plants when they were
treated with 1.0 μM Cd. Cd also inhibits nitrate reductase activity in the shoots and is
thus responsible for reduced absorption of nitrate and its transport from roots to
shoots (Hernandez et al. 1996). In soybean plants Cd treatments decreased nitrogen
fixation and primary ammonia assimilation in nodules (Karina et al. 2003). Effect of
Cd on mineral nutrition is due to inhibition of H+ATPase in maize root cells. H
+ATPase functions as an ion transporter across the plasmalemma. Cd, which causes
a decrease in activity of H+ATPase, might inhibit absorption of some essential
elements (Astolfi et al. 2005).
368
S. Saini and G. Dhania
In many crop species, maize, pea, barley (Popova et al. 2008), mung bean (Vigna
radiata) (Wahid et al. 2008), and wheat (Moussa and El-Gamal 2010), the
evidence showed that photosynthesis was inhibited after both long-term and
short-term Cd exposure. The primary sites of action of Cd are photosynthetic
pigments, especially the biosynthesis of chlorophyll (Baszynski et al. 1980) and
carotenoids (Prasad 1995). Cd caused chlorosis of leaves by decreasing chloroplast
density, and the Cd-induced decrease in pigment content was more powerful at the
leaf surface (stomatal guard cells) than it was in the mesophyll. Besides this, in
Cd-treated leaves, change in cell size and reduction in stomatal density in the
epidermis were observed. This showed that Cd might interfere directly with
chloroplast replication and cell division in the leaf. Cadmium influences two key
enzymes of CO 2 fixation: ribulose-1,5-bisphosphate carboxylase (RuBPCase) and
phosphoenolpyruvate carboxylase (PEPCase). Cd ions damage RuBPCase structure by substituting for Mg ions, which are important cofactors of carboxylation
reactions and thus lower its activity. Cd can also shift RuBPCase activity toward
oxygenation reactions thereby reducing the efficiency of photosynthesis
(Siedlecka et al. 1998). Cd causes an irreversible dissociation of the large and
small subunits of RuBPCase, thus leading to loss of enzyme activity and inhibition
of C 3 cycle (Stiborova 1988; Malik et al. 1992). Cd also affects the water-oxidizing
complex of PSII by replacing the Ca
2+ in Ca/Mn clusters of oxygen-evolving
center (Sigfridsson et al. 2004). Cd induces changes in lipid and fatty acid
composition of membranes and thus produces alterations in the functionality of
membranes (Popova et al. 2009).
4.3.3 Effect of Cd on Mineral Nutrition
Cd has been shown to interfere with the uptake, transport, and use of several
elements (Ca, Mg, P, and K) and water by plants (Das et al. 1997). In sugar beet,
deficiency of Fe in roots induced by Cd was observed (Chang et al. 2003). Uptake
of P, K, S, Ca, Zn, Mn, and B in pea plants was inhibited strongly by Cd exposure
(Metwally et al. 2005). Guo et al. (2007) observed decrease in the concentrations
of P, K, Ca, Mg, Cu, Fe, Mn, Zn, Mo, and B in roots of barley plants when they were
treated with 1.0 μM Cd. Cd also inhibits nitrate reductase activity in the shoots and is
thus responsible for reduced absorption of nitrate and its transport from roots to
shoots (Hernandez et al. 1996). In soybean plants Cd treatments decreased nitrogen
fixation and primary ammonia assimilation in nodules (Karina et al. 2003). Effect of
Cd on mineral nutrition is due to inhibition of H+ATPase in maize root cells. H
+ATPase functions as an ion transporter across the plasmalemma. Cd, which causes
a decrease in activity of H+ATPase, might inhibit absorption of some essential
elements (Astolfi et al. 2005).
368
S. Saini and G. Dhania