Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/2/1995
Publication Date: N/A
Interpretive Summary: Iron (Fe) is an essential nutrient for both plants and humans, and iron deficiency is a worldwide problem for both. Acquiring sufficient iron from alkaline soils is difficult in plants; hence not enough iron is available in plant foods to satisfy an adult human s nutritional requirements when eaten in normal amounts. Thus, we are working to increase our understanding gof the processes plants use to acquire Fe from the soil in order to improv the Fe content of plant foods. In this paper, we studied the role of root- mediated release of organic acids in solubilizing Fe from the soil and thus making it bioavailable for uptake by plant roots. A mathematical computer model for organic acid efflux from roots was employed, in conjunction with laboratory experiments to study the solubilization of Fe hydroxides from the soil. It was found that under most growth conditions, citrate and malate released from roots could provide most of the plants Fe from the soil. However in alkaline soils, the roots must acidify the soil adjacent to the root for this mechanism of Fe solubilization to be effective. This type of study, conducted in conjunction with investigations of the root processes involved in the absorption of the solubilized Fe, should enable us to understand the strategies used by plants to acquire Fe from the environment.
Technical Abstract: 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 uM. 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 value - 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.