Transport of cadmium via xylem and phloem in maturing wheat shoots: Comparison with the translocation of zinc, strontium and rubidium

The toxic heavy metal cadmium is taken up by plants and may contaminate harvested parts of agricultural crops. In the experiments reported here, cadmium was introduced together with markers for phloem (rubidium) and xylem (strontium) transport, either into intact shoots via a flap below the flag leaf node, or into detached shoots via the cut stem. Cadmium introduced into intact plants was redistributed during maturation from the peduncle and the flag leaf lamina to the grain. In detached shoots, some cadmium was removed from the transpiration stream, as judged from the comparison of shoots steam-girdled below the ear and of controls with an intact phloem in the peduncle. A minor quantity of cadmium was transported to the grain via the phloem in control shoots while a high percentage of this element was retained in the peduncle. The cadmium content of the grain increased in response to the increased cadmium concentrations in the feeding solutions (0.1 to 10 mu M). The cadmium content of the grain was slightly lower when zinc (> 10 mu M) was introduced al the same time as cadmium (1 mu M). (C) 1995 Annals of Botany Company