Bioavailability and Rhizotoxicity of Cd to Pea (Pisum sativum L.)

摘要

Risk assessment of cadmium (Cd) contamination in soils requires identifying the bioavailable portion of the total Cd, a portion that is determined by environmental conditions such as pH and calcium (Ca) level in soils and by the physiological processes going on in the plant roots. Growth tests in solutions were conducted to develop a terrestrial biotic ligand model to describe uptake and rhizotoxicity of Cd to pea (Pisum sativum L. cv. Lincoln). Inhibition concentration associated with a 50% reduction in root elongation (IC_(50)) values were found to vary with external Ca~(2+) and H~+ activities. Root-bound Ca was found to reach a plateau of about 63 μmol g~(-1) (dry weight) although Ca treatment increased from 0.04 to 2 mmol L~(-1). When experimental treatments (e.g., pH 6, Ca 0.2 to 2 mM) resulted in sufficient Ca supply, dose-response curves relating root elongation to root-bound Cd could be modeled with Weibull equations; IC_(50) values were expressed in terms of root-bound Cd concentration. When the treatments (e.g., pH 4 or 5, Ca 0.04 mM) suggested a low Ca supply, root elongation was more sensitive to Ca content and root-bound Ca concentration became the dominant predictor variable. Cd accumulation was modeled by treating the pea roots as an assemblage of biotic ligands with known siterndensities (Q_(Lj)) and proton binding constants (K_(HLj)). The logK_(Ca) and logK_(Cd) values were established using measured root-bound ion concentrations and solution chemistry. The logK_(Ca) values were negatively correlated to root Ca contents. The logK_(Cd) values were positively correlated to logk_(Ca) values. Explanations for the changing of constants are discussed.