Partitioning of zinc, cadmium, manganese and cobalt in wheat (<i>Triticum aestivum</i>) and lupin (<i>Lupinus albus</i>) 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 (<i>Triticum aestivum</i> L.) and white lupin (<i>Lupinus albus</i> 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.