Phytoaccumulation and tolerance of Riccinus communis L. to nickel

The phytotoxicity due to nickel (Ni) and its accumulation in castor (Ricinus communis L.) plant of Euphorbiaceae family resulting from its addition from low to very high levels to a swell-shrink clayey soil (Haplustert) was studied in a pot culture experiment. Nine levels of Ni (0, 10, 40, 80, 120, 160, 180, 200, 250 mg Ni kg(-1) soil) were applied. Crop was harvested at 45 days after sowing. At the higher Ni levels, beyond 200 mg Ni kg(-1) soil, reduced growth symptom was recorded. The concentration of Ni in plant parts increased with increasing dose of applied Ni. Nickel concentration in castor root ranged from traces (control) to 455 mg kg(-1) and was directly related to soil Ni concentration. At 200 mg Ni kg(-1) soil, dry matter yield of castor reduced to 10% of control plant. Significant changes were observed in the roots of castor treated with higher levels of Ni against control. The roots treated with Ni showed a decrease in number of cells in the cortex region. It also appeared that the cortex region consisted of elongated parenchymatous cells instead of the normal parenchymatous tissue as in the control plant. Regarding Ni accumulation capacity, castor plant was recorded as an accumulator (alpha = 0.11 and beta = 1.10). A laboratory study was also conducted in the experimental soil to know the different operationally defined fractions of Ni, which control the availability of Ni to castor. Different fractions of Ni present in this soil followed this order: Residual > Fe-Mn oxides > carbonate > organic > exchangeable > water soluble. Overall results depict that castor is a promising species which can be used as a potential plant for phytoremediation of contaminated soils and to improve soil quality and provide economical benefits.