Effect of bicarbonate on iron reduction by soybean roots1

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Abstract
Release of reducing compounds by soybean (Glycine max (L.) Merr.] roots has been identified as an adaptive response mechanism to iron‐deficiency conditions which result in chlorosis. These compounds facilitate the conversion of Fe+3 to the metabolically active Fe+2 form, allowing for increased uptake by roots in solution culture experiments. Degree of chlorosis is closely associated with HCO3 concentration; however, the relationship between that ion and root reduction potential apparently has not been studied. We examined the effect of HCO3‐ on root reduction potential of ten commercially‐grown soybean cultivars known to differ in chlorosis expression in the field. Root reduction potential was measured spectrophotometrically at 594 nm on samples of nutrient solution containing reduced Fe+2 . Plants were grown with 5 mM NaHCO3 or in HCO3 ‐‐free solutions. Averaged over cultivars, 0.205 umoles Fe+3 were reduced in the HCO3 ‐‐free solutions while only 0.009 umoles Fe+3 were reduced in the solutions containing HCO3 ‐. No significant differences were observed among cultivars for root reduction potential within either HCO3 ‐ treatment. Results from this study suggest that HCO3 ‐ may inhibit iron absorption by limiting the ability of roots to release reducing compounds which make available Fe+2 in the soil solution. This may partially explain the role of HCO3 ‐ in reducing chlorosis.