Boron compartmentation in roots of sunflower plants of different boron status:: A study using the stable isotopes 10B and 11B adopting two independent approaches

The intracellular compartmentation of boron (B) in roots of sunflower plants precultured with 100 muM B (high B) or 1 muM B (low B) was studied using two independent approaches. In the first approach, short-term efflux studies using the stable isotopes B-11 and B-10 were carried out. In roots of high B plants, the calculated concentrations of B (nmol g(FW)(-1)) were 52.6 in the cell wall, 7.5 in the vacuole, 27.1 in the cytosol and 48.0 in the free space. In roots of low B plants, the concentrations of B (nmol g(FW)(-1)) were 43.4 in the cell wall, 2.8 in the vacuole, 17.9 in the cytosol and almost zero in the free space. Although the B supply differed by a factor 100, the B concentrations in the cytosol and the vacuole of low B plants were 66 and 37% of the respective concentrations in high B plants. This suggests an additional role for B in plant metabolism, besides its function in the cell wall. In the second approach, root B pools (cell sap and water-insoluble residue) were determined for comparison, and found to be in good agreement with the results from the efflux study.