THE IRON-MANGANESE RELATION IN PLANT METABOLISM

Abstract

Expts. were carried out with soybeans in soln. culture at 3 Fe levels at each of which the Mn concs. were varied through a relatively wide range. Studies were made of the interrelations between Fe and Mn in their effects upon the metabolic processes of the plants. Toxicity and deficiency quantities of the 2 elements in the substrate and in the plants were investigated and their relations to the resulting pathological symptoms were studied. Pathological symptoms produced with excessive Fe are identical with those produced when Mn is deficient, and the symptoms produced with excessive Mn are identical with those produced when Fe is deficient. The ratios of Fe to Mn in the nutrient substrate corresponding to good growth and development of plants free from pathological symptoms fluctuated within a narrow range around 2, regardless of the total concs. of these elements within the limits employed. Fe/Mn ratio values above this effective range invariably produced a specific type of chlorosis resulting from excess Fe, or deficient Mn, or both; ratio values below this effective range invariably produced a specific chlorosis of a different type resulting from excess Mn, or deficient Fe, or both. Ratios of (soluble) Fe to (soluble) Mn, within the tissues, associated with good growth and development of the plants cover the same range of values as do those in the external medium, although the range of effective values may be somewhat wider in the plant tissues than in the substrate. Variation in the ratios of the soluble fractions either above or below this effective range, resulted in specific pathological symptoms. High concs. of soluble Mn in the tissues are invariably associated with low concs. of soluble Fe and vice versa. This suggests the oxidation of ferrous to ferric ions by active Mn, resulting in the inactivation and precipitation of Fe in the form of ferric organic complexes.