Shoot Cadmium Accumulation of Maize, Sunflower, Flax, and Spinach as Related to Root Physiology and Rhizosphere Effects

Abstract

Plants differ widely in Cd accumulation when grown on Cd contaminated soils. This is because several soil properties interact with several plant properties. Plant factors determining the shoot Cd concentration of plants are the root-shoot ratio [surface area (RA) per unit of shoot dry weight (SDW)], the relative shoot growth rate (RGRS), the Cd total net influx (Int) and the proportion of absorbed Cd that is translocated from the root to the shoot (p). And the Cd influx is furthermore influenced by plant properties like the Cd uptake kinetics and the ability of the root to affect the Cd solubility in the rhizosphere. The aim of this research was to assess the significance of the above mentioned plant properties for the shoot Cd accumulation for different plant species. A growth chamber experiment was carried out with maize, sunflower, flax, and spinach growing on a sandy soil to which 0, 14 and 40 μmol Cd kg-1 were added resulting in a Cd soil solution concentration of 0.04, 0.68 and 2.51 μmol L-1. At the high Cd addition (40 μmol kg-1) SDW of flax and spinach was significantly decreased by 20% and 40%, respectively while maize and sunflower showed no yield depression. The shoot Cd concentration of plants increased in all species linearly with the Cd addition to the soil. The Cd concentration in shoots varied between maize and spinach by a factor of about 30, sunflower and flax were in between. The root-shoot ratio (RA/SDW) varied by a factor of two and the RGRS by a factor of 1.5. The main factor related to different Cd concentrations in shoots was the Cd shoot influx which varied between maize and spinach by a factor of almost 40 at low and by a factor of 26 at high Cd addition. Differences in Cd influx among species may be based on their ability to solubilize Cd in soil or on differences in Cd uptake kinetics. Plants had a strong effect on Cd concentration in soil solution, CLi, i.e. plant growth decreased (maize, sunflower) as well as increased (flax, spinach) CLi. The decrease of CLi at low Cd addition was not related to Cd uptake by the plants, since maize with the lowest Cd uptake showed the strongest decrease of CLi (about 65%), whereas spinach with the highest Cd uptake, showed the least decrease of CLi (about 20%) indicating a Cd immobilization in soil caused from root activity of maize or spinach. In contrast to decreasing, CLi was increased by flax at both Cd levels up to 100% and by spinach by 23% at the high Cd addition to the soil. Plants therefore solubilized more Cd as was taken up by them. However, these effects did not totally explain the observed differences in their Cd influx. Further explanations were sought by using mechanistic modeling to simulate Cd uptake from soil. Modeling allowed including not only the effect plants had on CLi but also their different Cd uptake kinetics. Uptake kinetics was described by the root absorbing power, α, which is the slope of the uptake isotherm, which is almost constant in the low concentration range as found in the soils of this experiment. The simulated Cd influx was always higher than the measured one and for maize and flax it was about 10 times higher. The sensitivity analysis showed that the Cd soil solution concentration, CLi, should be reduced by 90% for maize and by 37% for spinach in order for the simulated Cd influx to become the same as the measured one. The changes of the α value lead to similar results. The sequential Cd fractionation in the soil with and without plants demonstrated that plants affect the Cd binding in soil. For spinach and sunflower 70% of Cd was found in the mobile and easily mobilizable fraction while for flax it was only 55%. In Mn oxides occluded Cd decreased in spinach and sunflower between 5-10% and in organically bound Cd increased in maize and flax about 5%. The other fractions contained only 5% of the total cadmium that was bound to solid phase. However, the effect different plants had on Cd binding in soil was not related to the effect they had had on Cd concentration in the soil solution.