Several experiments were conducted using barley (Hordeum vulgare L. cv. Minorimugi) grown hydroponically in a medium containing cadmium (Cd) to clarify the role of phytosiderophores (PS) in chelator-assisted phytoextraction of Cd and the influence of iron (Fe) on Cd uptake. In the first experiment, plants were grown for 7 days in media containing 5 mu mol L(-1) Cd sulfate (CdSO(4)), in which the concentration of Fe was 0 or 10 mu mol L(-1). A marked increase in Cd uptake was obtained in plants grown under the Fe-deficient condition compared with those grown under the Fe-sufficient (10 mu mol L(-1)) condition. In the second experiment, plants were grown for 7 days in media containing 5 mu mol L(-1) CdSO(4), in which the concentration of Fe was 10 or 100 mu mol L(-1). As a result, the uptake of Cd in plants grown with excess Fe (100 mu mol L(-1)) was significantly lower than the uptake in plants grown under a normal concentration (10 mu mol L(-1)) of Fe. These results indicated that Cd uptake was affected by Fe concentration in the medium and, in particular, a lower concentration of Fe in the media allowed the influx of Cd into plants. Thus, Fe application to the rhizosphere might be effective in reducing Cd concentration in plants. In the third experiment, the ability of solubilizing PS for insoluble Cd mixed with gelatinous Fe was examined. The results showed that PS could solubilize Cd from sulfide of Cd (CdS) gel. In the fourth experiment, roots of Fe-deficient or Fe-sufficient plants were fed with 0 or 10 mu mol L(-1) mugineic acid (MA), one of the PS, for 4 h in media containing 5 mu mol L(-1) CdSO(4). The uptake of Cd by the plants was not increased with MA. In the fifth experiment, the amount of PS release in Fe-deficient barley suffering from Cd toxicity was measured. Release of PS was partially reduced by 0.05 mu mol L(-1) CdSO(4) and largely reduced by 0.5 or 5 mu mol L(-1) CdSO(4). Based on these results, it was suggested that, when PS were released by roots of grasses to the rhizosphere, PS could mobilize insoluble Cd around the roots. It was also suggested that PS could not convey Cd into the root cells in the form of a PS-Cd complex. Thus, the main role of PS for chelator-assisted phytoextraction of Cd might be the collection of Cd to the rhizosphere, resulting in the enhancement of Cd concentration in the rhizosphere.