Redistribution of cobalt and nickel in detached wheat shoots: effects of steam-girdling and of cobalt and nickel supply

Detached wheat shoots (ear with peduncle and flag leaf) were incubated for 4 d in a solution containing 1 mM RbCl and 1 mM SrCl2 as well as 10, 40 or 160 mu M NiCl2 and CoCl2. The phloem of some plants was interrupted by stea m-girdling the stem below the ear to distinguish between xylem and phloem t ransport. The phloem-immobile Sr flowed mainly to the leaf lamina and to th e glumes via the xylem. The Sr transport was not sensitive to steam-girdlin g. In contrast, the phloem-mobile Rb accumulated during the incubation time mainly in the stem and the leaf sheath. The Rb transport to the grains was impaired by steam-girdling as well as by elevated Ni and Co concentrations in the incubation solution indicating that Rb was transported via the phlo em to the maturing grains and that this transport was affected by the heavy metals. Ni was removed more efficiently from the xylem in the peduncle tha n Co (but far less efficiently than Rb). It became evident that the two hea vy metals can also be transferred from the xylem to the phloem in the stem of wheat and reach the maturing grains via the phloem.