Influence of exposure solution composition and of plant cadmium content on root cadmium short-term uptake

The aim was to evaluate the influence of (i) cations potentially present in soil solution and (ii) of plant Cd content on apoplastic and symplastic root uptake of Cd. Hydroponically grown maize (Zea mays) roots were exposed to two Cd concentrations together with a cation five times the strength of the Cd. The influence of three pH and four Ca levels was also assessed. In addition, maize and alpine penny-cress (Noccaea caerulescens) were grown so their tissues contained Cd at three levels when their roots were exposed for 1 h to three Cd concentrations. Maize Cd uptake in the 5 μM Cd solution was reduced by the presence of Pb2+, Cu2+, Co2+, Zn2+, Mg2+ or Mn2+. At a 1 μM Cd concentration, Cu2+, as well as Ca2+ and H+ reduced the Cd uptake. However, the Ca concentration had to be much higher than that of Cd to restrain its uptake. High plant Cd content was responsible for an increase in the apoplastic Cd uptake and a decrease in the symplastic absorption, for both species. Low plant Cd content neither affected the Cd apoplastic uptake whatever the exposure concentration, nor the symplastic uptake in 0.1 μM Cd exposure solution. Moreover, the hyperaccumulator symplastic uptake increased when exposed to 10 μM Cd, but not when exposed to 50 μM Cd. Maize roots showed a decrease in membrane net flux when exposed to 10 μM and 50 μM Cd. Finally, the results suggest that for plants exposed to usual soil solution concentrations, it is mainly calcium and protons which interfere with Cd internalization.

► Mainly calcium and protons interfered with Cd internalization.
► High plant Cd content increased the apoplastic Cd root uptake.
► High plant Cd content decreased the symplastic Cd root absorption.