Redistribution of nickel, cobalt, manganese, zinc, and cadmium via the phloem in young and maturing wheat

The phloem mobility of heavy metals is relevant to the redistribution of micronutrients and pollutants and, ultimately, to the quality of harvested plant parts. The relative mobility in wheat may vary considerably between different cations. In the study reported here, radio-labeled nickel (Ni), cobalt (Co), manganese (Mn), zinc (Zn) and cadmium (Cd) were introduced into either intact young winter wheat (Triticum aestivum L. cv. Arina) via a leaf flap, or detached maturing shoots via the cut stem. Elements fed into the lamina of the second leaf of 21-day-old plants were translocated to the younger (expanding) leaves and to the roots but not or only in trace amounts to the first (already fully expanded) leaf. The Ni-63 and Zn-61 were exported more rapidly compared with the other heavy metals. Most of Mn-57 was retained in the originally labeled leaf. The peduncle of some maturing shoots was steam-girdled below the ear to distinguish between xylem and phloem transport. This phloem interruption reduced the content of Ni-63 in the ear to about 25%. Intermediate effects were observed for Zn-65, Co-57, and Cd-109. Total Mn-54 accumulation in the ear was hardly affected by steam-girdling, indicating a transport of this element within the xylem to the ear. These results suggest that the relative phloem mobility of Ni and Zn in young wheat plants and in maturing wheat shoots is higher than the mobility of Co and Cd, whereas the mobility of Mn is very low.