REDUCTION OF STRUCTURAL IRON IN SELECTED IRON-BEARING MINERALS BY SOYBEAN ROOT EXUDATES GROWN IN AN IN-VITRO GEOPONIC SYSTEM

Research on the reduction of iron (Fe) by plant-root exudates has been conducted using hydroponic solutions containing Fe salts or chelates. These solutions, however, fail to reflect the true soil environment b ecause plants derive their majority requirement from the solid Fe(III) sources. An in vitro Geoponic system (IVGS) is developed to study the reduction of Fe-bearing clay minerals, i.e., Upton and SWa-1 (smectit e), and Si-containing amorphous Fe oxide by soybean-root exudates. Sur face sterilized soybean seeds, [Glycine mat: (L.) Men.] cv. Williams ( marginally susceptible to Fe chlorosis), were germinated in presterili zed glass culture tubes containing semi-solid agar media (sucrose free ) and Fe minerals, These tubes were placed in an incubator programmed for a white-fluorescent light cycle for 16 h and temperature setting o f 25+/-2 degrees C. After 15 d of plant growth, the system was analyze d for Fe2+ and total Fe. The amount of structural Fe reduction was 0.0 12, 0.095 and 0.182 mmol/g for Upton, SWa-1, and Si-containing amorpho us Fe oxide samples, respectively. The reduction of structural Fe in t he Fe containing minerals was likely caused by phenolic root exudates which oxidized to diquinones.