EFFECT OF ROOT EXUDATES ON MOBILIZATION IN THE RHIZOSPHERE AND UPTAKE OF IRON BY WHEAT PLANTS

To examine the effect of root exudates (e.g. phytosiderophores) on iron (Fe) mobilization in the rhizosphere and Fe uptake, wheat seedlings (Triticum aestivum L. cv. Ares) were precultured for 17 d in nylon bags filled with fine sand and provided with nutrient solution without or with Fe (0.1 mM FeEDTA). After the preculture the nylon bags with the roots of the seedlings (central root compartment, RC) were brought in contact for 4 d with nylon bags filled with calcareous soil (SC). In different distance from the RC (0, 2, 4 mm) the calcareous soil on each side of the RC had been amended with Fe rich sewage sludge resulting in an increase in DTPA-extractable Fe from 1.45 to 4.22 mg Fe kg(-1) dry soil. Through the influence of roots in the RC extractable Fe increased in the SC between 2 and 133%. For the untreated soil this relative increase declined with the distance of 0, 2, and 4 mm from the RC from 86, 50, and 41% with Fe adequate plants and from 133, 86, and 35% with Fe deficient plants, respectively. The corresponding values for the sewage sludge treated soil was 13, 2, and 3% with Fe adequate plants and 24, 20, and 1% with Fe deficient plants, respectively. In accordance with the increased Fe solubility in the various SC Fe uptake and growth of wheat were also enhanced during the 4 d treatment. It can be concluded that the higher solubilization and uptake of Fe by the root of the Fe deficient (chlorotic) compared with the Fe sufficient (green) plants is mainly caused by enhanced release of phytosiderophores under Fe deficiency. The increased mobilization of Fe over a distance of up to 4 mm from the RC demonstrates the high capacity of root exudates (e.g. phytosiderophores) for Fe mobilization in the rhizosphere even under non-axenic conditions.