Agriculture Reference
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(ALA) and in the formation of photoactive protochlorophyllide reductase complex.
Horvath et al. ( 1996 ) reported that the photoconversion of protochlorophyllide was
not inhibited, but Cd disturbed Chl molecules' integration in stable complexes. On
the other hand, Greger and Lindberg ( 1986 ) suggested that the lower Chl concen-
trations in plants were a result of the deficiency of Mg and Fe in the leaves of Cd-
treated sugar beet plants. Rai et al. ( 2005 ) and Singh et al. ( 2008 ) reported decreased
Cd and Chl content in Phyllanthus amarus and Vigna mungo plants with increas-
ing Cd concentration, respectively. Ekmekci et al. ( 2008 ) reported that the increase
in Cd concentration caused loss of Car in Zea mays cultivars. Collins et al. also
reported decreased concentration of Car in Arabidopsis plants. Earlier investiga-
tions have demonstrated a marked reduction in the rate of photosynthesis by Cd in
different plant species (Wojcik and Tukendorf 2005 ; Mobin and Khan 2007 ). It has
been reported that Cd affects photosynthesis at various facets, such as Chl metabo-
lism (Padmaja et al. 1990 ), functioning of photochemical reactions (Skorzynska
and Baszynski 1995 ) and the activities of the Calvin cycle enzymes (Krupa 1999 ).
Cadmium-induced reduction in the activity of ribulose 1,5 bisphosphate carboxyl-
ase (Rubisco) has been reported in Hordeum vulgare (Vassilev et al. 2005 ), Caja-
nus cajan (Sheoran et al. 1990a , b ), Triticum aestivum (Malik et al. 1992a ), Pisum
sativum seedlings (Chugh and Sawhney 1999 ), Zea mays (Krantev et al. 2008 ) and
Brassica juncea (Mobin and Khan 2007 ). Wahid et al. ( 2007 ) reported that Cd in-
duced reduction in transpiration rate, stomatal conductance and net photosynthesis
due to reduced CO 2 fixation by Rubisco in Vigna radiata plants. Stomatal closure
to minimize water loss has been identified as an early event in plant response to Cd-
induced water deficiency leading to limitations in carbon uptake by leaves (Barcelo
et al. 1986a , b ; Chaves 1991 ; Poschenreider et al. 1989 ). In addition, conductance
and index of stomata, transpiration and net CO 2 uptake are greatly reduced with
elevated Cd levels in the growth media (Bindhu and Bera 2001 ; Balakhnina et al.
2005 ). Cd interacts with the water balance (Costa and Morel 1994 ) and damages
the photosystem apparatus, particularly in the photosystems I and II (Siedlecka and
Krupa 1996 ). Mobin and Khan ( 2007 ) found that the Cd-induced decrease in net
photosynthetic rate in non-tolerant cultivar (RH30) of Brassica juncea was accom-
panied by an increased transpiration rate and stomatal conductance but in tolerant
cultivar (Varuna), it remained unaltered.
Photosynthesizing plants are naturally prone to oxidative stress because they
have an array of photosensitizing pigments. These pigments produce and consume
oxygen which can easily donate electrons to form ROS. It is reported that 1 % of
the oxygen consumed by the plants is diverted to produce activated oxygen species
like hydroxyl radical (OH), singlet oxygen ( 1 O 2 ) and superoxide radicals (O 2 ·ˉ).
Free radicals and other derivatives of oxygen are inevitable byproducts of biologi-
cal redox reactions. Their production is considered to be a universal and common
feature of living world under natural conditions as a byproduct of respiration and
photosynthesis during electron transport systems of mitochondria and chloroplast.
Their concentration increases under unfavorable conditions. Intracellular structures
like membranes and biomolecules like proteins, enzymes, lipids and DNA have a
high degree of organization that is at the risk of being destructed by these oxidative
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