CRITICAL-EVALUATION OF ORGANIC-ACID MEDIATED IRON DISSOLUTION IN THE RHIZOSPHERE AND ITS POTENTIAL ROLE IN ROOT IRON UPTAKE

Both experimental extractions and theoretical calculations were undert aken to assess whether organic acid-mediated Fe dissolution could play a significant role in elevating the concentration of Fe-complexes in the rhizosphere, and further, whether this could satisfy the Fe demand s of a plant utilizing ferric reduction to acquire Fe. Using a mathema tical computer model, it was predicted that organic acids released fro m and diffusing away from the root would result in a solution organic acid concentration at the root surface of between 1 to 50 mu M. Over 9 9% of the organic acids lost by the root were predicted to remain with in 1 mm of the root surface. The experimental results indicated that c itrate-mediated Fe dissolution of amorphous Fe(OH)(3), was rapid in co mparison with citrate dissolution of the Fe-oxides, Fe2O3 and Fe3O4. T he rate of citrate and malate mediated Fe-dissolution was dependent on many factors such as pH, metal cations and phosphate saturation of th e Fe(OH)(3) surface. At pH values less than or equal to 6.8, citrate f ormed stable complexes with Fe and dissolution proceeded rapidly. Unde r optimal growth conditions for a plant utilizing a reductive-bound me chanism of Fe acquisition (dicots and non-grass monocots), it can be e xpected that citrate and malate may be able to satisfy a significant p roportion of the plant's Fe demand through the formation of plant-avai lable organic-Fe3+ complexes in the rhizosphere. In high pH soils (pH greater than or equal to 7.0), the plant must rely on other sources of Fe, as citrate-mediated Fe dissolution is slow and Fe-citrate complex es are unstable. Alternatively, the root acidification of the rhizosph ere could allow the formation of stable Fe-organic complexes.