Mobilization of ferric iron from a calcareous soil by plant‐borne chelators (Phytosiderophores)

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Abstract
After labelling a calcareous soil with 59Fe, the mobilization of labelled Fe by root exudates of Fe‐deficient barley plants (Hordeum vulgare L.) was studied in vitro at pH 7.4 in short term experiments of up to 48 h. These root exudates contain chelators (phytosiderophores) which form stable FeIII chelates (ferrated phytosiderophores) . Compared to the control (CaSO4), the mobilization rate of labelled Fe within the first 16 h. was increased by phytosiderophores (10‐5M) by a factor between 10 and 20. At chelator concentrations of 10‐5M, the microbial siderophore, Desferal (ferrioxamine B methane sulfonate), was somewhat more effective than phytosiderophores in mobilization of labelled Fe, whereas the synthetic chelator DTPA (diethylenetriamine pentaacetic acid) was ineffective under these conditions. Decreasing the accessibility of 59FeIII at the soil surface by shaking the labelled soil with increasing phosphate concentrations substantially decreased the amount of labelled Fe mobilized by phytosiderophores. Furthermore, the mobilization of labelled Fe by phytosiderophores was depressed in presence of high NaCl concentrations (20–80 mM). The importance of phytosiderophores compared to microbial siderophores is discussed with respect to mobilization of sparingly soluble ferric Fe in the rhizosphere and in Fe nutrition of graminaceous species. Effect of Phosphate on Iron Mobilization After labelling the soil with 59FeIII (see above) the soil was shaken for 1 h with potassium phosphate solutions (adjusted to pH 7.4) at concentrations between 10‐5 and 10‐3M P. After filtration and repeated washing with distilled water, the soil was transferred to porcelain crucibles for another five successive wetting and drying cycles as mentioned before. After these P treatment the Pcal contents were: 6.8, 7.8, 10.2, 18.9 and 27.3 mg/g soil. For Fe mobilization from these soils the same procedure was applied as described above. Preliminary Experiment for Testing the Dialysis Tubes In preliminary experiment designed to test whether the dialysis tubes allow rapid permeation of the mobilized iron (59Fe phytosiderophores) from inside of the tubes into the external solution, 10 ml of each of the labelled chelates were placed into the dialysis tubes. After 2 h, 93% and 88% of the 59FeEDTA and 59Fe phytosiderophores, respectively, had permeated into the external solution, indicating equilibrium. Within the external solution 89.1 and 88.5% of the 59Fe from EDTA and phytosiderophores, respectively, were absorbed on the resin. All experiments were repeated twice. For each experiment with soil, only freshly labelled soil was used. In all experiments with soils, the distribution pattern of the mobilized Fe was similar between the external solution and the resin.