Cadmium Uptake Kinetics in Intact Soybean Plants

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Abstract
The absorption characteristics of Cd2+ by 10- to 12-day-old soybean plants were investigated with respect to influence of Cd concentration on adsorption to root surfaces, root absorption, transport kinetics and interaction with the nutrient cations Cu2+, Fe2+, Mn2+ and Zn2+. The fraction of nonexchangeable Cd bound to roots remained relatively constant at 20-25% of the absorbed fraction at solution concentration of 0.0025 to 0.5 .mu.M, and increased to 45% at solution concentration in excess of 0.5 .mu.M. The exchangeable fraction represented 1.4-32% of the absorbed fraction, and was concentration dependent. Using dinitrophenol as a metabolic inhibitor, the metabolically absorbed fraction represented 75-80% of the absorbed fraction at concentration less than 0.5 .mu.M, and decreased to 55% at 5 .mu.M. At comparatively low Cd concentrations, 0.0025 to 0.3 .mu.M, root absorption exhibited 2 isotherms with Ks values of 0.08 and 1.2 .mu.M. Root absorption and transfer from root to shoot of Cd2+ was inhibited by Cu2+, Fe2+, Mn2+ and Zn2+. Analyses of kinetic interaction of these nutrient cations with Cd2+ indicated that Cu2+, Fe2+, Zn2+ and possibly Mn2+ inhibited Cd absorption competitively suggesting an involvement of a common transport site or process.