Characterization of root boron pools, boron uptake and boron translocation in sunflower using the stable isotopes 10 B and 11 B

Abstract

The B pools in the roots and the characteristics of B uptake and its loading into the xylem were investi-gated in sunflower (Helianthus annuus L.) plants precultured with high (100 M) or low (1 M) ¹¹ B supply. In order to study B fluxes and their dependence on root metabolic activity, short-term treatments with differential ¹⁰ B supply in combination with metabolic inhibition treatments (50 M 2,4-dinitrophenol; root zone temperature of 7˚C) or with no further treatment (control) were carried out. Subsequently, xylem exudate was collected, and roots were harvested and fractionated into two B pools that differed in their water-solubility as well as in their exchangeability. The exchange or release of ¹¹ B initially present during the 3 h treatment was maximal at 18% in the cell wall pool, whilst it was up to 94% in the symplasmic pool. All observed alterations in the cell wall-bound B can be explained by passive processes. Control plants precultured with high B supply showed a linear response of the ¹⁰ B concentrations in the root cell sap and in the xylem exudate to the differential short-term ¹⁰B supply, and this was not affected by the metabolic inhibition treatments. In the control plants precultured with low B supply, the response of the ¹⁰ B concentrations in the root cell sap and xylem exudate to the differential short-term ¹⁰ B supply appeared to be a com-bination of a saturable and a linear component. The metabolic inhibition treatments turned off the saturable compo-nent and the response became linear. In summary, the results suggest that B uptake into the root symplasm, as well as xylem loading, are performed by two transport mechanisms, with the linear components representing B transport by passive diffusion. The saturable components may represent unknown carrier- or channel-mediated transport of B, which is dependent on metabolic energy.