Are there two mechanisms for boron uptake in sunflower?

Uptake mechanisms for B have been characterized in young sunflower plants (Helianthus annuus L. cv. Frankasol), precultured with either low B supply (1 mu mol(-1)) or high B supply (100 mu mol L-1), by investigating the distribution of B between cell sap and water insoluble residue in roots and the fluxes of B to the shoots in response to short term changes in B supply. Complementary studies were carried out on soluble B complexes in roots and xylem sap and on the effect of inhibition of metabolic processes in the roots on B uptake. Two mechanisms seem to bring about B uptake into the roots, dependent on B supply during plant culture. When B supply is high, passive diffusion is responsible. With low B supply, a concentration mechanism for B is established, which can be turned off by increasing B supply as well as by the inhibition of metabolic processes by low root zone temperature or 2,4-Dinitrophenol. For building up and maintaining the concentration gradient between the root and the external solution, the formation of B complexes within the cell seems at least partially to be involved. The uptake of B from the external medium into the root is likely to be the key step for translocation of B to the shoot, whereas the mechanism of xylem loading is still unclear.