Cadmium Uptake Kinetics in Intact Soybean Plants

Abstract

The absorption characteristics of Cd2+ by 10- to 12-day-old soybean plants (Glycine max cv Williams) 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 to 25% of the absorbed fraction at solution concentration of 0.0025 to 0.5 micromolar, and increased to 45% at solution concentration in excess of 0.5 micromolar. The exchangeable fraction represented 1.4 to 32% of the absorbed fraction, and was concentration dependent. Using dinitrophenol as a metabolic inhibitor, the `metabolically absorbed' fraction was shown to represent 75 to 80% of the absorbed fraction at concentration less than 0.5 micromolar, and decreased to 55% at 5 micromolar. At comparatively low Cd concentrations, 0.0025 to micromolar 0.3, root absorption exhibited two isotherms with K2 values of 0.08 and 1.2 micromolar. 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.