The impact of human on the environment since its appearance on the surface of the earth, and this effect was in the form of conflict of human with the environment for survival. However, the problems of environmental pollution have become a serious turning point since the Industrial Revolution and has been a major cause of the rapid deterioration of the natural environment. Heavy metals are among a wide range of pollutants that have caused significant changes in the environment. Environmental polluted by heavy metals are related directly with global Industrialization and urbanization (anthropogenic actions). The Heavy metals are defined as metals that have specific gravity or density higher than 5g/cm³ (Lenntech 2004), or can defined, their atomic weight greater than water, and non-degradable (Huton & Symon, 1986). Example for most common heavy metals are Cadmium (Cd), Lead (Pb), and Mercury (Hg). Which are consider poisonous at low concentration. Other example of heavy metals is considered not toxic at low concentration are Zinc (Zn), Chromium (Cr), and Copper (Cu) (Beak et al., 2006). Mercury has high toxic in vapor form. in contrast, lead and cadmium have more toxicity in their cationic form. however, some plant that are grow in soil polluted with mercury appears less vitality. Generally, the existence and of toxic heavy metals in the plants, lead to change of plant physiological processes that can represented in enzymes denaturation and inactivation, proteins interaction, disruption of functional groups of metabolic important molecules and substitution of essential metal ions from biomolecules and functional cellular (Villiers et al., 2011; Ramesh & Reddy, 2009).
Neurotoxicity, Hepatotoxicity and Nephrotoxicity are happening when these heavy metals created toxicity (Benavides et al., 2005). Heavy metals are occurring and deposit in soil via uses of agricultural fertilizers and industrial wastewater disposal, also in air and water when they are produced from industrial emission or solid waste combustion, Fossil fuel combustion, Smelting and mining (Khan et al., 2008). The concentration of metals, PH and other ions are affected in the chemical form of heavy metals in soil solution (Das et al., 1997). Geologic and anthropogenic activates increase the level concentration of these metals (Macfarlan & Burchett 2001). an excess concentration of the heavy metals has effect in excessive production of reactive oxygen species (ROS) include singlet oxygen (O), superoxide radicals (O?²), hydrogen peroxide (H2O2), and hydroxyl radicals (OH), that are causing oxidative stress in plant environment. (Erdei et al.,2002; Shaw et al., 2004; Nada et al., 2007).
Due to their solubility under physiological condition and absorbability it is easily transferred in to the food chain through accumulation in soil and plant uptake. This may lead to inhibit the growth and productivity of plant or can alter physiological, metabolic, photosynthesis processes and respiration in plants (Hossain et al., 2009,2010; Sharma & Dubey 2007; Dubey 2010). Also, may increase the risk of damage in animal and human health through biomagnification process (Davydova 2005; Javed & Usmani 2011, 2012). a pollution uptake by humans, causes complications in health and therefore leads to serious diseases such as mutagenesis (cancer), teratogenesis and cause damages in cardiovascular, renal and skeletal systems (Radwan & Salama 2006).
The plants are exposed to high toxicity levels of heavy metal, effects may appear in chlorosis of leaf, necrosis, loss turgor or reduction in the rate of seed germination.
1.1.1 Cadmium and its toxicity in animals and plants
Cadmium (Cd) is one of the heavy metals widespread in terrestrial and marine environment, having a density of 8.64g/cm3 and atomic weight is 112.41g/mol1. It was discovered in 1817 by Friedrich Stromeyer and Karl Hermann German chemists from zinc oxide sample (Mahiya et al., 2014). Also, it is considered one pollutants of environment which is easily to contaminate the soil, plant, water and air. This pollutant results from natural resource such as volcanoes and anthropogenic resource like using of phosphate fertilizers in agriculture and processes of industrial (De Michele et al. 2009; Gallegoet al. 2012). It is considered as a toxic metal. Thus, the toxic Cd is found in environment source as ionic salt form with simple anion and cannot found it in pure state in nature.
Due to an absorbed and accumulate of cadmium in various tissue, that is caused damages in plants and animals such as make changes in levels of genetic and physiological and biochemical (Jin et al., 2003; Herbette et al.,2006; Liu et al., 2011; Greco et al., 2012). Moreover, is threat human health, by transferred into food chain (Arao et al., 2003; Sanitá & Gabbrielli 1999; Clemens 2006). This accumulation Causing damage in important vital centres in the human body like liver, which Cd reacted with some types of proteins and builds up in the liver and kidneys. also, can replace the zinc element in some human body enzymes. These enzymes lose their biological potential. Because cadmium is similar to calcium. It deposited with the bones in the form of triphosphate Cd, which damage to the spine (REF). The common disease of itai-itai was discovered in japan resulted from high accumulation of cadmium in the rice (Reilly 2002).
Cadmium have availability to react with carbon dioxide produces cadmium carbonate, Also, reacts with hydrogen chloride produces cadmium chloride. and is oxidised to cadmium oxide in the air (REF)
The presence of toxic Cd in animals is obtained from the surrounding environment (air-soil water) and this leads to poisoning when ingested or inhaled, which increases its concentration in the bloodstream and then translocated to the liver. pigments, PVC products, phosphate fertilizers and pesticide products, that are constantly introduced into the environment, are among the most dangerous environmental contaminants (McDowell 1992; Roman et al., 2002; Miranda et al., 2005; Patra et al., 2005; Swarup et al., 2005; Swarup et al., 2007). 0.5mg/kg is tolerated maximum concentration of Cd in animals feeding (McDowell 1992; NRC 2001). The Cd as is known for its ability to stay inside the body for a long time. For this reason, this mineral was being an interest to many health and environmental authorities in the world (REF).