Iron Uptake-Transport of Soybeans as Influenced by Other Cations

Of the many factors affecting the absorption of iron by plants, the interference of other metal ions appears quite important. Hewitt (8, 9), Nicholas (14,15), and Millikan (12,13) found nickel, cobalt, zinc, and copper to induce iron deficiencies in a number of annual crops. Nicholas found high concentrations of these interfering metals reduced the iron content of the leaves of affected plants. More recently, Brown and Tiffin found that heavy applications of zinc salts applied to a zinc-deficient soil induced iron deficiency in corn after lighter applications corrected zinc deficiency of corn and certain other crops (7). Reuther and Smith (16) showed that continued applications of copper salts and copper fungicides induced iron deficiencies in Florida citrus crops. Brown et al. (4) felt that a high coppermanganese to iron ratio caused iron chlorosis to develop in milo in certain calcareous western soils. Milo grew normally in similar soils where this ratio was lower. High concentrations of Ca in the soil have been thought to be a major causative factor of Fe chlorosis (1, 20) but the effect of this ion in soils is difficult to separate from the effect of soil pH on Fe availability. Recent reviews (1, 2, 20) of the research on Fe nutrition of plants point out that even though the interference of some metals with Fe absorption and metabolism are well known, little is known of the mechanism of these phenomena. The work of Hopkins et al. (10) and Somers and Shive (17) indicated that Mn appeared to oxidize ferrous Fe to the more insoluble ferric form. Millikan (12) found that M,o would to some extent overcome the toxic effects of heavy metals on flax, but did not suggest how it did this. Brown et al. (3, 6) have found that roots of chlorotic Hawkeye soybean (Glycine max) develop the capacity to reduce and absorb ferric Fe more rapidly than roots of green plants. Furthermore, Tiffin (18) has determined that most of the Fe is translocated to the top in the form of Fe malate or malonate. It was thought that an interfering ion (Mn, Cu, Zn, Ca) may take the place of Fe in the malic or malonic acid complexes and that this may be one mechanism whereby other metals could interfere with the translocation of Fe. The purpose of these experiments was to determine whether or not various cations interfered with rapid uptake of Fe by chlorotic soybean plants and, if so, to study the mechanism whereby this interference was accomplished.