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Articles

Mechanism of Short Term Fe^III Reduction by Roots ¹

Evidence against the Role of Secreted Reductants

Institut für Pflanzenernährung, Universität Hohenheim, Postfach 700562, 7000 Stuttgart 70, West Germany

The hypothesized role of secreted reducing compounds in Fe^III reduction has been examined with Fe-deficient peanuts (Arachis hypogaea L. cv A124B). Experiments involved the exposure of roots to (a) different gas mixtures, (b) carbonyl cyanide m-chlorophenylhydrazone (CCCP), and (c) agents which impair membrane integrity.

Removing roots from solution and exposing them to air or N₂ for 10 minutes did not result in any accumulation in the free space of compounds capable of increasing rates of Fe^III reduction when roots were returned to solutions. On the contrary, exposing roots to N₂ decreased rates of Fe^III reduction. CCCP also decreased rates of Fe^III reduction.

Acetic acid and ethylenediaminetetraacetic acid (disodium salt) (EDTA) impaired the integrity and function of the plasma membranes of roots of Fe-deficient peanuts. That is, in the presence of acetic acid or EDTA, there was an efflux of K⁺ from the roots; K⁺ (⁸⁶Rb) uptake was also impaired. Acetic acid increased the efflux from the roots of compounds capable of reducing Fe^III. However, both acetic acid and EDTA caused rapid decreases in rates of Fe^III reduction by the roots. In addition to peanuts, acetic acid also decreased rates of Fe^III reduction by roots of Fe-deficient sunflowers (Helianthus annuus L. cv Sobrid) but not maize (Zea mays L. cv Garbo).

These results suggest that, at least in the short term, the enhanced Fe^III reduction by roots of Fe-deficient plants is not due to the secretion of reducing compounds.

³ To whom reprint requests should be addressed.

¹ Support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.