The Importance of Ferrous Iron in Plants and Soils

The distribution of iron in plants has been studied by many investigators, the foremost being Molisch (6) who paid little attention to the state of oxidation of the iron. He did mention, however, that the State of oxidation of "inorganic" iron in plant tissue could be ascertained by treating the tissue with the well-known reagents, ferrocyanide and ferricyanide, and then immersing it in a 5 per cent solution of HC1. Molisch differentiated two types of iron in plant tissue, that which could be detected directly with these reagents and that which could be detected only after prolonged pretreatment with a saturated solution of KOH. He called the former "loosely bound or inorganic" and the latter "masked or organic" iron. Macallum (4) cautioned against the use of KOH solution since it often contains iron and is prone to dissolve iron from glass vessels. He used ammonium hydrogen sulfide for the detection of "masked" iron in plant and animal tissues. Stoklasa (9) found that iron in the "organic" form is concentrated in the embryo and endosperm. Jones (3) believed that iron was present in plants mainly in the ferric state and found that the lower plants and animals contained more "inorganic" iron than the higher ones. The state of oxidation of iron was determined by the writer in kernels and leaves of corn and in the leaves of dandelion, Sudan grass, and burr oak. The presence of ferrous iron in kernels of corn was demonstrated in the following manner: A test tube was filled two-thirds full of oxygen-free distilled water, and C02 was bubbled in to replace the air, after which a small crystal of potassium ferrocyanide and a drop of concentrated HC1 were added. Several halved corn kernels were dropped into the test tube which was then completely filled with oxygen-free distilled water. The test tube was then tightly stoppered in such a way as to allow the escape of all air bubbles. A similar trial was made with a second tube using ordinary distilled water without the exclusion of air. After standing several hours,.Prussian blue developed in the cotyledons of kernels in the second test tube indicating the presence of ferric iron, but no color appeared in the cotyledons of kernels in the former case. If the iron present in the kernel had originally been in the ferric state, it might possibly have been reduced so rapidly after solution in the HC1 that no Prussian blue could be formed. This does not appear to be the case, however, since ferric chloride in an acid solution is not reduced instantly by the reducing substances of the plant tissues. Further tests for ferrous and ferric iron in plant tissue were made as follows: Corn leaves in various stages of maturity, Sudan grass, dandelion, and burr oak leaves were torn up, placed in a strong flask, and covered with a solution of one-half per cent potassium ferricyanide. Enough HC1 was then added to give a half normal to normal solution. The flask was connected to a suction pump and evacuated for five minutes to facilitate penetration of the reagent into the tissues, after which the leaves were left in the reagent at atmospheric pressure for a period