Background and aims Accumulation of Cd in the shoots of plants grown on Cd contaminated soils shows considerable variation. A previous preliminary experiment established that one major reason for this variation was the rate of Cd influx into the roots (mol Cd cm −2 root s −1 ). However, this experiment did not distinguish between solubilization of soil Cd on the one hand and difference in Cd uptake kinetics on the other. The main objectives of the present study were thus to characterize Cd uptake kinetics of plants continuously exposed to Cd concentrations similar to those encountered in soils. Furthermore we determined the factors responsible for differences in shoot Cd concentration such as net Cd influx, root areashoot dry weight ratio, shoot growth rate and proportion of Cd translocated to the shoot. Materials and methods Maize, sunflower, flax and spinach were grown in nutrient solution with five constant Cd concentrations varying from 0 to 1.0 μmolL −1 . Root and shoot parameters as well as Cd uptake were determined at two harvest dates and from these data Cd net influx and shoot growth rates were calculated.Results and conclusions Cadmium uptake kinetics, i.e. the net Cd influx vs. Cd solution concentration followed a straight line. Its slope is the root absorbing power, α,The α values of spinach and flax were about double that of maize and sunflower (5×10 −6 cms −1 vs. 2.5× 10 −6 cms −1 ). Spinach and flax had a 3-5 times higher shoot Cd concentration than maize and sunflower. The difference in shoot Cd concentration was partly due to the higher Cd influx but also to a higher translocation of Cd from root to shoot and also to a slower shoot growth rate.

In alkaline soils, where most of the P is acid soluble, we hypothesize that species that acidify the rhizosphere such as oilseed rape, are more efficient to use soil P than for example maize. In field experiments, adult maize plants extracted more P per unit of root than young plants. Here we hypothesize that older plants access P fractions that younger plants were not able to. Thus, the aim of this research was to study the P fractions used by maize and oilseed rape growing in an acid sandy and a neutral loamy soil and how plant age might affect it. A special pot system was developed in which P uptake by the plants came from roots that grew freely in soil. To obtain rhizosphere soil, a portion of the roots was concentrated to form a root mat on a planar soil surface covered by a fine nylon mesh. Cutting the soil on the other side of the mesh into slices gave rhizosphere soil at different distances from the root surface. We examined the P fractions used by maize and oilseed rape at three growth stages by measuring the depletion of three inorganic (P i ) and two organic (P o ) P fractions in the rhizosphere. Oilseed rape did not affect the P o fractions in any of the two soils, and maize only in the acid soil. Both species in both soils depleted only the alkali soluble P i fraction. The degree of depletion was between 12-26 %. The acid soluble P i was not depleted by neither oilseed rape nor maize. Plant age had no effect on P fraction depleted or on the degree of depletion.

Cadmium (Cd) is toxic to plants, animals, and humans. However, different plant species growing on the same soil may have very different shoot Cd concentrations depending on properties such as size of the root system, Cd net influx, shoot‐growth rate, Cd translocation from root to shoot, and the ability to affect Cd availability in the soil. To investigate possible reasons for different shoot Cd concentrations maize, sunflower, flax, and spinach were grown on an acid sandy soil (pH<$>_{{\rm{(CaCl}}_{\rm{2}} {\rm)}<$> 4.5, and Corg 2.8%) in a growth chamber with Cd additions as Cd(NO3)2 of none, 14, and 40 μmol (kg soil)–1 resulting in Cd soil‐solution concentrations of 0.04, 0.68, and 2.5 μM. Only the high Cd addition caused a significant growth reduction of flax and spinach. The shoot Cd concentration was up to 30 times higher in spinach than in maize; the other species were intermediate. Of the plant properties studied only the variation of the Cd net influx explained the differences in shoot Cd concentrations. This was due to a decreased (maize, sunflower) or increased (flax) Cd concentration in soil solution or more effective uptake kinetics (spinach).

In a previous experiment, plants were able to immobilize or solubilize Cadmium (Cd) in a sandy acid soil enriched with 40 μmol Cd kg–1, because Cd solution concentration was decreased by maize (Zea mays) and sunflower (Helianthus annuus), and increased by flax (Linum usitatissimum L. ssp. usitatissimum) and spinach (Spinacia oleracea). It is assumed that the equilibrium with Cd fractions in the soil solid phase and the chemical form of Cd in the soil solution were affected. In the present study, the effect of the four plant species mentioned above on Cd binding in soil was investigated by means of a fractionation of soil Cd with a sequential extraction of seven steps. The seven fractions of Cd are operationally defined by the extraction sequence that follows the order of increasing acidity with extractants of different complexing and redox properties. In the unplanted soil, Cd was predominantly present in the exchangeable Fraction I (F. I) and easily mobilizable Fraction II (F. II) (64%). Significant concentrations of Cd were found in F. III (occluded in Mn oxides; 22%) and F. IV (organically bound; 10%). Fractions V (occluded in poorly crystalline Fe oxides), F. VI (occluded in well crystallized Fe oxides), and F. VII (residual fraction) amounted to less than 5% of the total soil Cd concentration. The plants changed the binding of Cd in soil in a different manner. All plants decreased F. I, but F. II was increased by maize and spinach, decreased by flax or remained unaffected by sunflower. Fraction III was not affected by maize and flax, but decreased by sunflower and spinach, and F. IV was not affected by sunflower and spinach, but was increased by maize and flax. These changes of Cd fractions were not related to the changes the plants had caused in total Cd or Cd2+ concentration of the soil solution. These results show that plant species differ in how they affect Cd binding to the soil solid phase, but this effect is not related to how they affect Cd in soil solution. The mechanisms by which plants affect the relationship between the soil solid and liquid phase are still unclear.

Maize, sunflower,flax, and spinach differed in the accumulation of Cd when grown on a Cd contaminated soil. This was mainly due to the different Cd net influx, In, that varied among species by a factor of up to 30. The objective of this study was to find possible reasons for the different Cd In by using a mechanistic model. After 14 days of Cd uptake the model calculated only a small Cd depletion at the root surface, e.g. from 0.22 mumol L(-1) down to 0.19 mumol L(-1) for maize and from 0.48 mumol L(-1) down to 0.35 mumol L(-1)for spinach. Even so the model always overestimated the Cd I(n), for spinach by a factor of 1.5 and for maize by a factor of 10. Only simulating a decrease of C(Li) or the root absorbing power, alpha, by 40% to 90% gave an agreement of calculated and measured I(n),. This may be interpreted as that about 40% in the case of spinach and 90% in the case of maize of the Cd in soil solution were not accessible for plant uptake. The high sensitivity to alpha also shows that not the Cd transport to the root but alpha was limiting the step for Cd uptake.