Agriculture Reference
In-Depth Information
suggested that the root tissue was a barrier to Cd uptake and translocation within the
rice plants. The uptake of Cd by plants varies not only among plant species but also
among cultivars (Metwally et al. 2005 ; Grant et al. 2008 ). Li et al. ( 1997 ) reported
significant variation in the grain Cd level of Helianthus annuus , Triticum aestivum
and Linum usitatissimum . In Glycine max plants, about 98 % of the accumulated
Cd is strongly retained by roots and only 2 % is transported to the shoot system
(Cataldo et al. 1983 ). Rice plants absorb Cd from the rooting medium against the
concentration gradient and the localization of absorbed Cd in rice is greater in roots
than in shoots (Shah and Dubey 1997 ). The absorption of Cd by green microalgae,
Chlorella vulgaris , Ankistrodesmus braunii and Eremosphera viridis shows that Cd
is mainly absorbed in the cell wall. In Eichhornia crassipes , Cd was found to accu-
mulate throughout the roots (Hosayama et al. 1994 ). Ammar et al. ( 2007 ) reported
that Cd was found to be mainly accumulated in roots, but a severe inhibition of
biomass production occurred in Lycopersicon esculentum leaves, even at its low
concentration (1.0 µM).
Higher plants can uptake metals from the atmosphere through shoots and leaves,
entering via roots and rhizomes from the soil (Lyubenova and Schröder 2010 ).
Toxicity of metals within the plant occurs when metals move from soil to plant
roots and get further transported and stored in various sites in the plant (Verma and
Dubey 2003 ). The transfer of HMs from soils to plants depends primarily on the
total amount of potentially available or the bioavailability of the metal (quantity
factor), the activity as well as the ionic ratios of elements in soil solution (intensity
factor), and rate of element transfer from solid to liquid phases and to plant roots
(reaction kinetics) (Brümmer et al. 1986 ).
Most of the Cd accumulates in the vacuole within a cell. The fact that Cd is found
in golgi apparatus and endoplasmic reticulum is apparently related to metal secre-
tion through the cell surface and into the vacuole. A small quantity of Cd reaches
nuclei, chloroplast, and mitochondria and exerts toxic effects on these organelles
(Miller et al. 1973 ; Malik et al. 1992a , b ). Localization of Cd appears to be maxi-
mum in roots than in other parts of the plant (Hart et al. 1998 ).
3   Cadmium Toxicity to Crop Plants
The most characteristic symptoms of Cd stress are brown and short roots, chlorosis,
fewer tillers, senescence and reduced plant growth and biomass (Wu et al. 2003 ;
Cosio et al. 2006 ). In Elodea canadensis , a thinner stem, less expanded leaves with
partial bleaching of green tissues and 40 % internode shortening were observed in
response to Cd treatments when compared with control plants (Vecchia et al. 2005 ).
Roots of Pisum sativum plants were more sensitive to Cd toxicity than shoot (Met-
wally et al. 2005 ). Leaf expansion and root growth were inhibited significantly at
high Cd concentrations in Sedum alfredii , and Cd was suggested to suppress cell
expansion and induced senescence (Zhou and Qiu 2005 ). Ekmekci et al. ( 2008 )
reported that increasing Cd concentration significantly reduced the leaf and root
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