Further studies on the influence of BPDS on the uptake of chelated iron by plants

Abstract

Bush bean plants were exposed to ⁵⁹FeEDDHA (2×l0^‐5 M) for 22 hours with and without CaCO₃ in the nutrient solution for pH differential (less than 7 and over 7) and also with combinations of 10^‐4 M BPDS (bathophenanthroline‐disulfonate), 10^‐4 M EDDHA, 10^‐4 M EDTA, 10^‐2 M H₂O₂ and 3×10^‐5 M FeSO₄ and 5×10^‐5 M FeSO₄. The BPDS and excess EDDHA both inhibited accumulation of the labeled iron in various plant parts but not additively. This implies a pe + pH of 10 or less unless the chelated Fe were absorbed intact. The excess EDDHA and EDTA decreased slightly the inhibiting effect of BPDS at both pH values. This implies a pe + pH of less than 10. When sufficient FeSO₄ was applied to chelate all of the BPDS, inhibition of iron uptake still occurred, although the effect was less than with the unchelated BPDS. The high rate of FeSO₄ decreased the inhibition more than did the low rate although high pH may decrease the availability of the residual 2 × 10^‐5M Fe²⁺ . Hydrogen peroxide, an oxidizing agent, decreased iron uptake at the low pH but not at the high. This implies a pe + pH of over 6. EDDHA decreased its inhibiting effect. The H₂O₂ with excess EDDHA led to a high quantity of labeled iron in primary leaves. By far the largest amount of labeled iron associated with roots was with H₂O₂ without EDDHA at both pH values.

In a second experiment in which plants were exposed for 24 h to ⁵⁹Fe, the Fe response mechanism was pronounced at low solution pH and almost absent at pH 8.5 in the presence of HCO₃ ^‐. At pH 4 and pH 6 the BPDS inhibited Fe uptake with and without pretreatment with Fe. At pH 7.5 and 8.5 the effect of BPDS was less pronounced (statistically not different) and independent of the pre‐Fe treatment. Substantial Fe uptake was obtained with 100 μM excess BPDS which implies Fe uptake for less than 10^‐9 M Fe²⁺.

In another experiment for 24 h., labeled Fe uptake was almost equal for FeEDDHA vs Fe(BPDS)₃ at pH 3.5 but was greater for FeEDDHA at pH 7.5. Fe from Fe(BPDS)₃ was available to plants. Excess BPDS inhibited Fe uptake at low pH more than did excess EDDHA (EDDHA stimulated Fe uptake). Iron uptake was much lower at pH 7.5 than 3.5 but excess BPDS inhibited more than did EDDHA at pH 7.5. BPDS equivalent to the FeEDDHA had no real effect on uptake of labeled Fe at low pH and little at high pH.