ABSTRACT
Plants that not only have high tolerance of metals in the soil, but also accumulate those pollutants to high concentrations in the shoots, were coined hyperaccumulators (Brooks, Lee et al. 1977). Baker & Brooks defined Cd-hyperaccumulators as species with shoot concentrations of more than 100 mg Cd kg−1 on a dry weight basis and invariably have greater metal concentration in shoots than in roots (Baker and Brooks 1989). To date, scientists have identified approximately 500 trace element accumulating taxa, belonging to at least 100 plant families (Barzanti, Colzi et al. 2011). So far some plants, such as Thlaspi caerulescens J & C. Presl (Brassicaceae) (Baker, Reeves et al. 1994), Sedum
1 INTRODUCTION
As the continuous development of the worldwide industrialization and urbanization, pollution of the environment by heavy metals has become a global problem (Witters, Mendelsohn et al. 2012). Heavy metals that contaminate the soil have the characteristics of high toxicity, persistence, non-biodegradation and bioaccumulation (Yu, Ji et al. 2012). Cadmium (Cd) is one of the most deleterious heavy metals in the present (Dong, Mao et al. 2007). Cd is easily accumulated in plant tissues because of its high plant-soil mobility, then leads to the decrease of crop yield and quality, and threats human health by the accumulative effect of the food chain (Dong, Mao et al. 2007). Cd is detrimental to human health because it accumulates in the human body with a half-life of more than 10 years and it is related to renal tubular dysfunction, pulmonary emphysema and possibly osteoporosis (Majid, Islam et al. 2012).
