الفهرس 
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Abstract
The enormous economic importance of broad bean (Vicia faba L.), the increasing human population and the increasing food demand worldwide makes bean genetic improvement necessary at many levels to ensure food security. As an affordable source of essential nutrients, such as proteins, vitamins and minerals, legumes offer a possible solution for meeting the nutritional needs for many 4 vulnerable populations across the globe (Arora 1983).
Heavy metals have been increasingly found in soils due to atmosphere deposition, sludge, sewage irrigation, utilization of metal-containing farmyard manures and fertilizers, and industry and mine residues, resulting in a potential risk for human health when these metals are transferred from crops to the human diet. They play an important role in the environment not as a result of human activity but also as toxic species above certain concentrations (Ngayila et al., 2009). At high concentrations, a number of heavy metals have been reported to inhibit the growth and decrease the productivity of crops (Liu et al., 2003). Among them, cadmium (Cd) is well known as a highly toxic environmental element due to its great toxicity and high mobility from soil to plants and further down the food chain (Vig et al., 2003). It can be incorporated and accumulated by all organisms in large amounts and disturb physiological metabolisms in plants like transpiration, photosynthesis, respiration, and nitrogen assimilation (Chugh et al., 1999; Zhou et al., 2006; Wang et al., 2008). Additionally, Cd is a divalent heavy metal cation (Cd 2+) which is readily taken up and causes phytotoxicity. Cd excess in the environment decreases chlorophyll content (Chen et al., 2011) and growth (Zhou and Qiu, 2005), affects chloroplast function or CO2 fixation ( Seidlecka et al., 1997). Faller et al. (2005) showed that Cd2+ inhibited photoactivation of photosystem II by competitive binding to the essential Ca2+ site. Earlier investigations have demonstrated that the net rate of photosynthesis can be conspicuously decreased with increasing concentrations of Cd (Lakshaman and Surinder, 1999). Studies on the dose-response of plants to Cd stress have shown that stomatal conductance varies with different Cd 2+ concentrations (Chugh et al., 1999). Furthermore, the photosynthetic responses of monocotyledons to a stress factor have been observed as have those of dicotyledons (Drażkiewicz and Baszyński, 2005).
Zn is an important component of many vital enzymes having catalytic, co catalytic and structural role as structural stabilizer for proteins, membranes and DNA binding proteins (Zn-fingers). Zn is found to be involved in many cellular functions such as protein metabolism, gene expression, chromatin structure, photosynthetic carbon metabolism and indole acetic acid metabolism (Cakmak, and Braum, 2001), yet its higher concentrations cause toxicity (Sinhal,2007). Alleviation of Cd induced toxicity by Zn has been observed by Hassan et al. (2005). Zinc is known to have a stabilizing and protective effect on the biomembranes against Cd induced oxidative and peroxidative damage, loss of plasma membrane integrity (Cakmak, 2000) and also alteration of the permeability of the membrane with the help of enzymatic and non-enzymatic antioxidant.
In recent years, the concentration of Zn in soils and the area polluted by it have continually increased due to wastes of nonferrous metallurgy, the chemical industry, and the introduction of high doses of phosphate fertilizers (Zakrutkin and Shishkina, 1997). Zn inputs on soils are related with mining, industrial activities and agricultural practices (Bi et al., 2006). Excessive Zn in plants can delay or diminish the growth (Andrade et al., 2009) and causes leaf chlorosis (Wang et al., 2009). Though Zn plays a vital role in stability of biomembranes and proteins (Cakmak, 2000), Zn deficiency can affect the photochemical processes in the thylakoids, and thus inhibits biophysical processes of photosynthesis. The measurement of Chl fluorescence is a useful tool for quantification of the effect of stress on photosynthesis (Schreiber et al., 1994). The ratio Fv/Fm is one of the fluorescence parameters most widely used to estimate the degree of photoinhibition (Solhaug and Haugen, 1998). Zinc deficiency causes a drastic decrease in chlorophyll content as well as a severe damage to the fine structure of chloroplasts (Chen et al., 2007). Zinc at higher concentrations inhibits plant growth (Sharma et al., 2008), chlorophyll formation (Kaya et al., 2000) and photosynthesis and transpiration rates (Van Assche et al., 1979).
Experiments on the toxicity of mixtures of pollutants may reflect the actual toxicity to ecosystems in a more realistic way than experiments in which toxicants are tested individually (Spurgeon et al. 1994), and several studies have investigated the combined effects of heavy metals on certain plant species. Shute and Macfie (2006) studied the accumulation and distribution of Cd and Zn in combination on soybeans (Glycine max) and assessed the effect of one metal on the bioavailability of the other metal across the range of concentrations added to the soil. However, there still has been very little published information on combined effects of those toxic elements on soybean in the polluted area.
Relationships between Cd toxicity and oxidative stress have been studied in many systems and heavy metal contamination has often been implicated as the root cause of oxidative injury to the plants. The key step in oxidative stress is the production of reactive oxygen species (ROS) which initiate a variety of autooxidative chain reactions on membrane unsaturated fatty acids, producing lipid hydroperoxides and thereby cascade of reactions ultimately leading to destruction of organelles and macromolecules (Shaw et al., 2004). Removal of ROS and cellular homeostasis are regulated by antioxidant enzymes such as superoxide dismutase (SOD, E.C. 1.15.1.1), catalase (CAT, E.C. 1.11.1.6) and peroxidases (POD, E.C. 1.11.1.7) and a complex antioxidant system—the ascorbate–glutathione cycle (AGC) (Zhang and Kirkham,1996) and various other endogenous antioxidants such as ascorbate (AsA) (Horemans et al.,2000), thiols, glutathione (GSH) ( Foyer et al., 2001) and the associated glutathione metabolism enzymes (Nagalakshmi and Prasad , 2001).
Cadmium is often associated with Zn 2+ as a contaminant, up to 5% in the processed Zn 2+-ores of Zn 2+ mines and smelters (Sterckman et al., 2000). Since both Cd 2+ and Zn 2+ belong to group II transition elements with similar electronic configuration and valency, they have many physical and chemical similarities. Biologically, however, these two elements have different properties. The fact that Cd 2+ is a toxic heavy metal and Zn+2 is an essential element makes this association interesting as it raises the possibilities that the toxic effect of Cd 2+ may be preventable by Zn 2+. Interactions between Cd 2+ and Zn 2+ and their transfer in soil–crop systems under field conditions, in solution culture experiments have been reported (Nan et al., 2002).
The aims of this study are to investigate the effect of Cd and or Zn on growth parameters in hydroponics, several parameters of stress such as growth inhibition, lipid peroxidation and antioxidant enzymatic activities were analyzed. Also, the current study was to examine, the modulatory role of Zn on the antioxidant pathways of AGC and associated GSH enzymes in a Cd exposed broad bean plant (Vicia faba L. cv. Nubaria 1).