Effects of excess manganese and metal chelators on micronutrient concentrations in the xylem sap of iron-deficient barley plants

Barley (Hordeum valgare L.) plants were grown hydroponically in a greenhous e for 14 d under Fe-deficient conditions before treatment for 3 h with exce ss Mn (25 mum) and equimolar amounts of plant-borne (phytosiderophores, PS) or synthetic (ethylene diamine tetraacetic acid, EDTA) metal chelators. Th e xylem sap was collected for 3 h and analyzed for PS, Fe, Mn, Zn, Cu, and citrate concentrations. Excess Mn in the feeding medium decreased the conce ntrations of PS, Fe, Mn, Zn, and Cu in the xylem sap. Addition of 25 muM Mn arid an equimolar amount of PS to the feeding medium increased the concent rations of PS, Fe, Zn, and Cu in the xylem sap, while EDTA decreased the co ncentrations of PS and the above nutrients. Excess Mn in the feeding medium increased the Mn concentration in the xylem sap and this increase was more pronounced with the addition of PS to the feeding medium, while EDTA had a depressing effect. These findings suggested that the roots of Fe-deficient barley plants can enhance the absorption and/or translocation of both Mn2 and a PS-Mn complex. Addition of excess Mn to the feeding medium, irrespec tive of chelators, did not affect the xylem citrate concentration, indicati ng that citrate may not contribute to the translocation of metal micronutri ents. In the xylem sap of Fe-deficient barley plants, the concentrations of metal micronutrients were positively correlated with the concentrations of PS.