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' frction 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. Cadmium Uptake. Evaluation of the absorption behavior of Cd was performed using 10-to 12-d-old plants. Prior to use, plants were transferred from nutrient solutions to 0.5 mM CaCl2 solutions (pH 5.8) for 12 h to allow for desorption of possible interfering ions from root surfaces. Individual plants were then transferred to fresh 0.5 mm CaCl22 and various concentrations of CdCl2. CaCl2 was employed in all uptake and absorption solutions to provide sufficient Ca2`to maintain membrane permeability (20,22). For absorption periods of 60 min or less, 500 ml volumes were employed; for absorption periods of over 60 min, 1-L volumes were employed to limit reduction of total Cd levels in solution to less than 10% during the experiment.Solutions containing CdCl2 levels from 0.0025 to 5.0 liM were traced with carrier-free`°CdCI2, and adjusted to pH 5.8 with KOH. Following the absorption period, shoots were removed and roots were transferred to 0.5 mm CaCl2 solutions containing unlabeled CdCl2 at concentrations 20-fold higher than treatment concentrations. Roots were routinely desorbed using three changes of solution for a total of 2 h; in preliminary studies, desorbed '"Cd was measured in efflux solutions at 30-min intervals following 5-min wash in 0.5 mM CaC12.

Cadmium accumulation in crops

“…Cataldo et al (1983) observed inhibition of root absorption of Cd and transfer of Cd to shoots by Cu, Fe, Mn and Zn in soybean seedlings exposed to 2.5 × 10 -9 to 5 × 10 -6 M Cd. Detailed kinetic analysis suggested competitive inhibition of Cd absorption and transport by these elements, which Cataldo et al (1983) interpreted as indicating a common transport site or process for Cd, Cu, Fe, Zn and possibly Mn. The uptake of Cd by roots of perennial ryegrass exposed to 9 × 10 -8 M Cd for 3 d was depressed by the addition of Ca, Mn, and Zn (Jarvis et al 1976).…”

“…However, 8.9 × 10 -6 M Cd is a high, probably phytotoxic, level of Cd exposure, and the conclusion that Cd uptake was a passive process (Cutler and Rains 1974) should not be applied to lower, more environmentally relevant concentrations. Other authors of studies with wheat (Smeyers-Verbeke et al 1978), soybean (Cataldo et al 1983), and spruce seedlings (Godbold 1991) have concluded that there was a metabolically active component in total cadmium accumulation by intact plants. In these studies, active accumulation of Cd was observed to occur only at lower Cd concentrations (2.5 to 90 × 10 -9 M) or to be relatively more significant at lower concentrations.…”

“…Other studies at higher, usually toxic, concentrations of Cd also indicate antagonism between Cd and Cu, Fe, Mn and Zn (Thys et al 1991;Jalil et al 1994b;Gussarsson et al 1995). Except for the study of Cataldo et al (1983), the free ion concentrations of Cd, Cu, Fe, Mn and Zn may have varied unintentionally due to competition for the Fe chelator. Nevertheless, the results are generally consistent and support the conclusion (Cataldo et al 1983) that there is a common transport site or other process shared by Cd and other metals.…”

“…Using 5 × 10 -2 M CaCl 2 , Florijn et al (1993) desorbed 16-18% of root-bound Cd from the roots of "shoot Cd excluding" inbred maize lines compared to 3.0 to 5.7% from the roots of "non-shoot Cd excluding" lines exposed to 9 × 10 -8 M Cd in nutrient solution culture. Cataldo et al (1983) exposed soybean seedlings to 109 Cdlabelled solutions with 2.5 × 10 -9 to 5 × 10 -6 M Cd and desorbed the roots with 5.0 × 10 -4 M Ca and a 10-fold excess of unlabelled Cd. They found that 1.4 to 32% of root-bound 109 Cd could be desorbed and that the percentage desorption was lowest for plants exposed to the lowest concentrations of Cd.…”

Interaction and accumulation of manganese and cadmium in the manganese accumulator Lupinus albus

“…However, the effects of Mn starvation reducing Cd uptake and plant concentrations should be noted, indicating that this Mn-Cd interaction was not owed to true cation antagonism. Further, Cd, Cu, Fe, Zn and possibly Mn share a common transport site or process in Cd-treated soybean (Cataldo et al, 1983), tomato (Baszynski et al, 1980) and Phytolacca americana L. (a Mn-Cd hyperaccumulator) (Peng et al, 2008).…”