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 undertaken 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 demands of a plant utilizing ferric reduction to acquire Fe. Using a mathematical computer model, it was predicted that organic acids released from and diffusing away from the root would result in a solution organic acid concentration at the root surface of between 1 to 50 μM. Over 99% of the organic acids lost by the root were predicted to remain within 1 mm of the root surface. The experimental results indicated that citrate-mediated Fe dissolution of amorphous Fe(OH)₃, was rapid in comparison with citrate dissolution of the Fe-oxides, Fe₂O₃ and Fe₃O₄. The rate of citrate and malate mediated Fe- dissolution was dependent on many factors such as pH, metal cations and phosphate saturation of the Fe(OH)₃ surface. At pH values ≤6.8, citrate formed stable complexes with Fe and dissolution proceeded rapidly. Under optimal growth conditions for a plant utilizing a reductive-bound mechanism of Fe acquisition (dicots and non-grass monocots), it can be expected that citrate and malate may be able to satisfy a significant proportion of the plant's Fe demand through the formation of plant-available organic-Fe³⁺ complexes in the rhizosphere. In high pH soils (pH ≤7.0), the plant must rely on other sources of Fe, as citrate-mediated Fe dissolution is slow and Fe-citrate complexes are unstable. Alternatively, the root acidification of the rhizosphere could allow the formation of stable Fe-organic complexes.