The acquisition of cadmium by Lupinus albus L., Lupinus angustifolius L., and Lolium multiflorum Lam.

Several plant species mobilize soil phosphate and cations like Fe and Al by excreting organic acid anions and protons. We investigated the Cd acquisition of mobilizing plant species (L. albus and L. angustifolius cultivars) and Lolium multiflorum: as a non-mobilizing plant species. Two contrasting soils were utilized: an acid humic podzol and a calcareous loess subsoil. The Cd uptake into the shoots was 5 - 10 times higher in Lolium multiflorum compared with the of L. albus and L. angustifolius. There are several reasons for this result: (i) The root length/shoot weight ratio was 2-3 times higher in ryegrass compared with the lupin cultivars, (ii) the translocation of Cd from the root into the shoot is higher in ryegrass compared with lupin, (iii) the Cd uptake is lower in lupin than in ryegrass in the humic podzol and at P deficiency also in the loess soil. In the acid podzol, the Cd concentration of the soil solution under lupin was lower than the control (pots without plants), whereas in the calcareous soil, the reverse relation was found. In white and clue lupin the carboxylate afflux, mainly citrate and malate was by a factor of 10-100 higher than in ryegrass. This can lead to a high rate of complexation of Cd by citrate (similar to 85%) in the rhizosphere soil solution of lupin. These results seem to support the hypothesis that the Cd uptake of the root is restricted by the complexation of Cd by citrate.