Mechanism of Short Term FeIII Reduction by Roots : Evidence against the Role of Secreted Reductants

TLDR: The 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.

Abstract: The hypothesized role of secreted reducing compounds in FeIII 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 FeIII reduction when roots were returned to solutions. On the contrary, exposing roots to N2 decreased rates of FeIII reduction. CCCP also decreased rates of FeIII 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 FeIII. However, both acetic acid and EDTA caused rapid decreases in rates of FeIII reduction by the roots. In addition to peanuts, acetic acid also decreased rates of FeIII 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 FeIII reduction by roots of Fe-deficient plants is not due to the secretion of reducing compounds.