Cellular Cobalt Fluxes in Roots and Transport to the Shoots of Wheat Seedlings

Macklon, A. E. S. and Sim, A. 1987. Cellular cobalt fluxes in roots and transport to the shoots of wheat seedlings.—J. exp. Bot. 38: 1663–1677. From compartmental analysis of radioisotope elution measurements, concentrations and fluxes of cobalt were estimated for cortical cells of wheat (Triticum aestivum L. cv. Fenman) seedling roots, relative to a complete nutrient solution containing 20 mmol m⊟3 Co2+ . The water free space, Donnan free space and cytoplasm contained little Co relative to the high levels accumulated in the vacuole, resulting in time courses of Co uptake which were essentially linear in continuous light. In light/dark cycles there was almost complete inhibition of uptake during the dark periods. Comparison of estimated concentrations of Co in cytoplasm and vacuole with values to be expected, if passive diffusion alone was operative, indicated that active transport was occurring in an outward direction at the plasmalemma and into the vacuole at the tonoplast. Transport to the shoot, which was unaffected by transpirational water flux through the plant, reached about 10% of root absorbed Co, most of which was stored in the cortical cell vacuoles and removed from the transport pathway. Between 0·02 and 20 mmol m ⊟3 Co in external solution, uptake was linearly related to concentration on a log/log plot, and the proportion of transported Co was constant. The implications of these findings are discussed.