Effects of Chloride and Co-Contaminated Zinc on Cadmium Accumulation within Thlaspi caerulescens and Durum wheat

Abstract

Cadmium is a potentially toxic heavy metal with no known benefit to humans. Soils are the main source of Cd in plants, and plant-derived foods are the main source of Cd in human diets. Cadmium is present in all soils, usually as a trace constituent. The plant availability of Cd in soil is determined by the characteristics that affect the availability of most trace metals. These characteristics include plant species, the concentration and chemical forms of metal in the soil, pH, organic matter content, clay content, its interactions with other elements, and fertilizer practices. In addition, an increase in Cl concentrations in the soil or soil solution was shown to increase Cd concentration in plants in laboratory studies, such as Swiss chard, sunflower, potato, kenaf, sorghum and wheat (Smolders et al. 1997; Hattori et al. 2006). Under field conditions, Cd concentrations in plants were found to positively relate to Cl concentration in soil as well (Norvell et al. 2000; Wu et al. 2002). Chloride is known to reduce soil sorption of Cd, probably due to the fact that chloride forms relatively strong complexes with Cd depending on solution Cl concentration. Chloro-Cd complexation and the resulting improved diffusion of Cd through the soil to plant roots and possibly increased uptake of Cd-chloro complexes are suspected to be the reasons for the Cl effect on Cd uptake (Mclaughlin et al. 1997). Unlike Cd, Zn is an essential nutrient both for plants and animals, but if present in high concentration in plants and animals it can also be highly toxic. It is generally accepted that Zn status in soils and plants plays an important role in Cd accumulation in crop plants. Generally, Zn application decrease Cd uptake and accumulation in plants (Jiao et al. 2004; Zhang and Song 2006), which may be caused by Cd and Zn competivite adsorption in soils and absorption by roots as well as an interaction in the transport system of plant (Hart et al. 2002). However, the effects of Zn application on Cd uptake and accumulation in plants are not consistent. Nan et al. (2002) showed that increases in Zn application enhanced Cd concentration in wheat or vice versa. Similar observations were also made by other researchers (Wu and Zhang 2002). Most of studies under actual field situations, soil environments are often contaminated with multiple heavy metals, such as Cd, Zn, Pb, etc. The study of the Cd accumulation influenced by the interaction between these ions is of practical importance. Durum wheat is an important and valuable crop in many parts of the world that can accumulate Cd to approach or exceed the toxic concentration limits in some places. Durum wheat is a non-accumulator, but Thlaspi caerulescens is a hyperaccumulator of Cd and Zn that can accumulate more than 100 mg Cd kg in their leaves (Baker et al. 1994). Recently, there has been considerable interest in the use of Thlaspi caerulescens as a green technology for phytoremediation of surface soils contaminated with toxic Q. Liu (&) Institute of Soil Science, Chinese Academy of Sciences, 71 East Beijing Road, Nanjing 210008, China e-mail: qliu@issas.ac.cn