Partitioning of zinc, cadmium, manganese and cobalt in wheat (Triticum aestivum) and lupin (Lupinus albus) and further release into the soil

The uptake and redistribution of the heavy metals zinc, cadmium, manganese and cobalt are relevant for plant nutrition as well as for the quality of harvested plant products. In the experiments reported here, seedlings of wheat (Triticum aestivum L.) and white lupin (Lupinus albus L.) were radiolabelled for 24 h with 65Zn, 109Cd, 54Mn and 57Co via one seminal root (wheat) or via the main root (lupin). Plants were afterwards grown on rhizoboxes containing soil. Samples were collected throughout the experiment and analysed afterwards for their radionuclide contents. A strong retention in the labelled part of the root was observed for 57Co in wheat and lupin and for 109Cd in lupin, while 65Zn and 54Mn were transported to the shoot in both plants. While 65Zn was redistributed via the phloem from older to younger leaves, 54Mn accumulated in the first leaves and no major redistribution within the shoot was observed. 109Cd was present in the shoot of wheat but not in the shoot of lupin. The redistribution of 65Zn, 109Cd, 54Mn and 57Co in the phloem differed between wheat and lupin. The 65Zn content in the wheat roots appearing after the labelling phase represented 34% of the total content in the plant at the end of the experiment and less than 3% remained in the labelled root, while a high percentage of 65Zn was retained in the originally labelled part of the main root of lupin. Smaller quantities of 109Cd, 54Mn and 57Co accumulated in all parts of the root system of wheat and lupin. Nevertheless, heavy metals were found in rhizosphere soil (1–2 mm soil around the roots) and bulk soil (no contact with roots) from both plants. Higher quantities of heavy metals were found in the rhizosphere soil close to the labelled part of the root. 65Zn was present in large quantities in the rhizosphere soil close to all parts of the root system of wheat. For both plants, 65Zn, 109Cd, 54Mn and 57Co were found in the bulk soil indicating that the plant itself might play a role in the redistribution of heavy metals in the soil around its own roots. Phloem-mobile elements may be transported to growing parts of the root system and may reach deeper soil layers. The redistribution of heavy metals in the soil may be in vertical and horizontal directions, at least as far as the root system grows.